What is LCA?
|1.||Definition of LCA|
Leber’s Congenital Amaurosis (LCA) is a rare, hereditary disorder that leads to retinal dysfunction and visual impairment at an early age – often from birth. Of all the retinal degenerations, LCA has the earliest age of onset and can be the most severe.
LCA bears the name of Dr. Theodore Leber (1840-1917), a German ophthalmologist, who first described the condition in 1869. Congenital means "a condition existing since birth, usually hereditary," and Amaurosis refers to any condition of blindness or marked loss of vision, especially loss of vision in which there is little or no change in the appearance of the eye itself. This is why LCA eyes usually look normal upon initial examination.
LCA is sometimes confused with another condition termed Leber’s Hereditary Optic Neuropathy (LHON) that also leads to visual impairment. However, LCA is a separate and distinct disease.
The birth prevalence of LCA is two to three per 100,000 births. The condition is the most common cause of inherited blindness in childhood and constitutes more than 5% of all retinal dystrophies. LCA accounts for the cause of blindness in more than 20% of children attending schools for the blind.
The clinical signs of a disease are collectively called the phenotype. Besides vision loss, other signs of LCA are nystagmus (roving eye), sluggish or nonexistent pupillary response and, in some cases, eye rubbing (oculo-digital reflex). In a smaller number of cases, there can be lens opacity (cataract), cornea abnormality (keratoconus), aversion to light (photophobia), hearing impairment and possibly developmental delays. Retinal blood vessels can become thin and narrow and there can be pigmentary changes that an Ophthalmologist can see within the eye.
A key feature of LCA is an abnormally low electrical response of the retina. This can be measured by the Ophthalmologist using a method called Electroretinography. In this procedure, the retina is stimulated by light and the electrical response pattern is recorded on an electroretinogram (ERG) and compared with ERG responses from normal subjects.
Some LCA types are progressive in that they become more severe with age and some are stationary in that there is little change noted with time.
Summary on OTX2 and CABP4 Phenotypes,
by Dr. Gerald Chader, Doheny Retina Institute
OTX2 gene codes for a protein that is a "homeobox protein". It is a transcription factor that controls basic processes of early embryonic development. In this case, neural tissue development is affected. A recent publication links an OTX2 mutation with rapid RPE cell degeneration with slower photoreceptor degeneration.
CABP4 protein is thought to be important in normal retinal bipolar cell signaling. Classically, a CABP4 mutation leads to a form of night blindness, CSNB2. However, a Dutch group that includes Frans Cremers and Anneke den Hollander have reported that CABP4 mutation leads to a "congenital cone-rod synaptic disorder" in a Dutch family. The patients have reduced visual acuity and abnormal color vision but no night blindness. Thus, it seems that mainly the cones are affected with no night blindness (CSNB2) in this family. The name "congenital cone-rod synaptic disorder" was proposed rather than CSNB2.
Per Dr. Eric Pierce, University of Pennsylvania School of Medicine: The Nijmegen group has reported that mutations in CABP4 cause primarily cone dysfuntion (congenital cone-rod synaptic disorder). The quality of of phenotype information is important.
Note: Although OTX2 and CABP4 cause a retinal degeneration the mutations are not really LCA.
In general, the term Genotype refers to the full genetic constitution of an individual. In a specific disease process, the &ldquoDisease Genotype&rdquo generally refers to the specific gene (or genes) whose mutation causes the disease. LCA can best be thought of as a grouping of hereditary diseases within a larger grouping of diseases called Retinitis Pigmentosa (RP). RP is a family of hereditary diseases of different causes whose common end point is retinal degeneration and loss of vision . Thus, LCA is just one special form of RP. This is an important concept, especially in considering treatments for LCA that may originally be designed for types of classical RP. It is estimated that forms of LCA comprise about 5% of all known hereditary retinal degenerative diseases. At present, 19 different gene mutations are known which lead to different forms of LCA. These are listed below along with short descriptions.
The retina, as the brain, is part of the Central Nervous System. It is a thin, transparent tissue that is attached inside the back part of the eye. Its main function is to capture light images, begin their processing and pass them down the optic nerve to the brain. Structurally, the retina is stratified, i.e., most cells are in distinct bands or layers. In fact, one can think of the retina as a layer cake.
Following is a short description of the important cells types of the retina--
The most important cell is the photoreceptor neuron. Its main function is to capture the light energy in a visual image and convert it to an electrical response. This is done is a specialized part of the photoreceptor cells called the outer segment. In the outer segment is concentrated the visual proteins (pigments) like rhodopsin. These are the proteins that actually capture the light energy. Once the photoreceptor neuron converts the photic energy to an electrophysiological signal, it passes this signal on to secondary neurons in the next layer of the retina (e.g., bipolar cells) and ultimately to the brain.
There are two main types of photoreceptor cells in most animal retinas. These are called rods and cones. Rod cells are, as the name implies, rod-shaped. They are designed to mainly function in dim light and in peripheral vision. Cone cells are more cone shaped. They serve in central vision, bright-light vision and in color vision. There is a concentration of cone cells in a highly specialized, region of the retina called the macula. Most of our central and sharp vision uses macular cone cells.
Interestingly, the photoreceptor cells point towards the back of the eye, necessitating light to pass through all the other retinal layers before striking the photoreceptors.
Retinal Pigment Epithelium (RPE) Cells:
Juxtaposed to the layer of photoreceptor cells is a single cell layer of RPE cells. Perhaps, think of them as frosting on the retinal “cake”. They are tightly intertwined with the outer segments of the photoreceptor cells. The RPE cell layer functions in maintaining proper function of the photoreceptor cells which are thought to have the highest metabolic activity of any cell type in the human body. Thus, RPE cell bring nutrients and oxygen to photoreceptor cells and remove waste products. RPE cells also are heavily pigmented (melanin granules), allowing for capture of stray light. Last but not least, RPE cells are intrinsic to vision in that they participate in the visual cycle with photoreceptor cells. They store the vitamin A (retinoids) needed in vision and also contain enzymes that chemically alter vitamin A to forms used in photoreceptor outer segments in the visual process. When RPE cells are not functioning properly, photoreceptor cells are usually quickly affected resulting in retinal degeneration.
On the other side of the RPE cell layer from the photoreceptors is a dense network of blood vessels called the choroid. It is from this blood vessel system that RPE cells get the nutrients to pass on to photoreceptor cells.
Other Retinal Cell Types:
Beneath the photoreceptor cells are several stratified cell layers. Within these layers are secondary neurons such as bipolar cells, amacrine cells and ganglion cells. These cells are all connected through structures called synapses. The function of these cells is to begin the processing and integration of the visual signals. These signals are finally passed to the brain through the optic nerve. The optic nerve consists of many long, thin processes (axons) of ganglion cells.
|6.||Molecular Biology and Genetics Primer|
To understand the hereditary nature of LCA and its causes, one has to understand the basic principles of how any trait, good or bad, is genetically passed on. All cells of the body have a central organelle called a nucleus. In the nucleus is a very long strand of genetic information called DNA. DNA is organized into coiled structures called chromosomes. Functionally, DNA is divided into specific areas - called Genes - which act as templates for individual proteins. It is estimated that humans have 30-70,000 separate genes encoded on their DNA – collectively called the human genome.
To make a specific protein (e.g., one important in the visual process) within a cell, signals in the nucleus activate the gene. The gene functions as a bluepring, coding for a specific messenger molecule called messenger RNA (mRNA). This secondary blueprint moves out of the nucleus and ultimately directs the formation of the specific protein. Some genes are activated to only function at certain times of development or are specific to only certain cell types of the body. Hence, the opsin gene (visual protein) is present in the DNA of all cell types of our body but only will be activated to produce the opsin protein in photoreceptor cells of the retina.
As with all biological mechanisms, changes occur. Occasionally, the genetic building blocks of the DNA in the nucleus can be altered or mutated such that the basic blueprint is changed and thus the resultant protein is changed (mutated). Mutations can be good or bad. A mutation that allows for improved function of an important protein enzyme is probably good while a mutation that disables the enzyme could be very bad. Sometimes the mutation is so severe that the protein is not synthesized at all. In a hereditary disease like LCA, a DNA mutation can cause a protein that is important in the visual process to malfunction or not function at all, leading to visual impairment or blindness.
Hereditary Nature of LCA:
All forms of RP, including individual types of LCA are hereditary diseases, i.e., they are passed down through the generations within families. Hereditary diseases, in general, come in three major genetic modes of inheritance called dominant, recessive or X-linked. Most of the forms of LCA are inherited in a recessive manner although one form has a dominant mode of inheritance. Sometimes, people carry only one mutated gene, the other being normal. In the case of recessive genetic diseases, these people are called “carriers” since they carry the gene and can pass it on to their children but they themselves do not show the signs of the disease.
|7.||Gene Mutations that Cause LCA|
To date, 19 genes have been identified whose mutations lead to forms of LCA. Three other areas have been identified on human chromosomes in which an LCA gene resides but has not been specifically identified. Mutations in these genes do not always cause LCA. For example, mutations in some areas cause other retinal degenerations that have characteristics different from LCA. Investigators believe that not all LCA genes have been found and that there are several yet to be identified. Following is a listing of the known genes whose mutations can cause LCA. More information on these genes and genes whose mutations cause other types of retinal degeneration can be obtained on an excellent website, &ldquoRetNet&rdquo maintained by Dr. Steven Daiger.
1) CRX Cone-Rod Homeobox - LCA7 The protein product of the gene is known to control the synthesis of several functionally important genes in the retina such as opsin, the visual protein. It is thus very important in proper development of the retina. A specific CRX mutation will result in a dominantly inherited form of LCA while another mutation results in the more usual recessive form. LCA cases with CRX mutations are very rare. One study reports that CRX mutations are thought to cause 1-3% of LCA cases another study on a different group of patients yields a figure of 0.6%. CRX gene mutations are associated with other retinal dystrophies as well as LCA.
2) AIPL1 Aryl Hydrocarbon Receptor - LCA4 Interacting Protein-Like 1 gene. The AIPL1 protein product is found in rod photoreceptor cells. Its function is yet unknown but may be involved in directing proper structure (folding) of important photoreceptor proteins. AIPL1 mutations account for 5-10% of recessive LCA cases as reported in one study and 3.4% in another.
3) CRB1 Crumbs Homologue 1 - LCA8 This gene mutation was first seen to cause retinal degeneration in the eye of the fruitfly, Drosophila. The human gene is similar (homologous) to that in the fruitfly and a mutation also causes LCA-like vision loss. Other mutations in the CRB1 gene cause other retinal degeneration phenotypes such as a recessive form of RP. The function of the protein is unknown but is thought to be involved in development of retinal neurons. In the fruitfly, the protein probably functions in maintaining proper cell-cell interactions. In the human, one study estimates that CRB1 mutations account for 9-13% of LCA cases, another study reports a figure of 10%.
4) GUCY2D Retinal Guanylate Cyclase - LCA1 Guanylate Cyclase is a protein enzyme that makes a critical messenger in photoreceptors called cyclic GMP that is a major intermediate in the light-dark visual cycle. A Guanylate Cyclase mutation leads to an abnormal cyclic GMP concentration, inducing dysfunction and degeneration of the photoreceptor cell. In a strain of chickens, an analogous mutation in the guanylase cyclase protein also leads to severe, early (LCA-like) visual loss. In one study, GUCY2D mutations are reported to account for 10-20% of LCA cases another study reports 21.2%.
5) LRAT Lecithin Retinol Acyltransferase - LCA14 The LRAT protein is an enzyme that is important in vitamin A metabolism in the visual process, catalyzing the first step in the visual cycle. The enzyme is specifically found in retinal pigment epithelial (RPE) cells. RPE cells adjoin retinal photoreceptor cells and partner with the photoreceptor cells in the visual process as discussed above. LRAT mutations profoundly disturb the normal chemical transformations of Vitamin A that are intrinsic in the visual cycle thus leading to photoreceptor cell dysfunction. Prevalence estimates of LRAT mutations are unavailable.
6) RPE65 Retinal Pigment Epithelium 65 - LCA2 Like LRAT, the RPE65 protein is specifically expressed in retinal pigment epithelial (RPE) cells. It also is important in Vitamin A metabolism in the visual cycle. An excellent canine (Briard) model exhibiting a mutation in the RPE65 gene has been identified. Gene therapy studies on this model are in progress preliminary to human clinical trials that will test replacement of the RPE65 gene in the human eye. In one study, RPE65 mutations are reported to cause 6-16% of LCA cases another study reports 6.1%.
7) RDH12 Retinol Dehydrogenase 12 - LCA13 The RDH 12 protein, like the LRAT and RPE65 proteins, is involved in chemical transformations of vitamin A (retinol) in the visual cycle. Unlike LRAT and RPE65, however, it is selectively found in retinal photoreceptor cells, probably cone photoreceptor cells. Mutation of RDH12 leads to a severe, progressive form of LCA with extensive macular atrophy. In the human, RDH12 mutations are reported to account for about 4% of LCA cases.
8) RPGRIP1 RPGR-Interacting Protein 1 - LCA6 THE RPGRIP1 protein is actually a member of a closely related family of proteins that, as the name implies, interacts with a protein named RPGR. RPGRIP1 and RPGR are localized in photoreceptor cell outer segments in the human. Here, the interacting proteins appear to be vital in transport processes into the outer segment. Disruption of this transport process would be expected to lead to retinal degeneration. In the human, one study reports that RPGRIP1 mutations account for 4-6% of LCA patients another study gives a figure of 4.5%
9) TULP1 Tubby-like Protein 1 - LCA15 The human TULP1 protein is very similar (homologous) to a protein previously identified in the mouse whose mutations lead to several problems including early progressive retinal degeneration. The protein is thought to function in facilitating the transport of important proteins like opsin to where they function in the photoreceptor outer segment. In a singl study, TULP1 mutations are reported to cause 1.7% of LCA cases. Some mutations in the TULP1 gene can lead to LCA while others lead to retinal degeneration that is of an RP phenotype (1). A number of clinical reports are in the scientific literature describing the characteristics of the degeneration in specific families &ndashSuranamese (2), Algerian and Dominican (3). A good mouse model has been developed and characterized (4). It demonstrated an early-onset retinal degeneration but seems to be normal in other regards. The availability of the model would allow for testing of different types of therapy in the future. References: 1)Schorderet MA, Chachoua L, Boussaiah M, Nouri MT, Barthelmers D, Borrurat D, Munier FL. Novel TULP1 mutation causing Leber Congenital Amaurosis or early onset retinal degeneration. Invest. Ophthalmol. Vis. Sci. 2007,48:5160-7. 2)Den Hollander AI, van Lith-Verhoeven JJ, Arends ML, Strrom TM, Cremers,FP, Hoyng CB. Novel compound heterozygous YULP1 mutations in a family with severe early-onser retinitis pigmentosa. Arch. Ophthalmol. 2007,125:932-5. 3)Banerjee P, Kleyn WK, Kmowles JA, Lewis CA, Ross BM, Parano E. Kovats SG, Lee, JJ, Penchazadeh GK, Ott J, Jacobson, SG, Gilliam TC. TULP1 mutation ion two extended Dominican kindred with autosomal recessive retinitis pigmentosa. Nature Gen. 1998,18:177-9. 4)Ikeda S, Shiva N, Ikeda A, Smith RS, Nusinowitz S, Yan G, Lin TR, Chu S, Heckenlively JR, North MA, Naggert JK, Nishima PM, Duvao MP. Retinal degeneration but not obesity is observed in nulkl mutants of the tubby-like protein1 gene. Hum. Mol. Genet. 2000,9:155-63.
10) CEP290 - LCA10 Centrosomal protein of 290 kDa is a protein that in humans is encoded by the CEP290 gene. This gene encodes a protein with 13 putative coiled-coil domains, a region with homology to SMC chromosome segregation ATPases, six KID motifs, three tropomyosin homology domains and an ATP/GTP binding site motif A. The protein is localized to the centrosome and cilia and has sites for N-glycosylation, tyrosine sulfation, phosphorylation, N-myristoylation, and amidation. Mutations in this gene have been associated with Joubert syndrome and nephronophthisis, and recently with a frequent form of LCA, called LCA10. The presence of antibodies against this protein is associated with several forms of cancer.
11) LCA5 Lebercillin - LCA5 The lebercillin protein gets its name from "Leber" and the fact that it is found in the "cilium" area of the photoreceptor cell. The cilium connects the photoreceptor inner segment where proteins like rhodopsin are synthesized and the outer segment where they are utilized in the visual process. Lebercillin apparently forms functional complexes with a number of other proteins in the connecting cilium. A lack of lebercillin disrupts these complexes and protein transport in the cilium. The result is a retinal degeneration. LCA mutations lead to early and severe retinal degeneration with nystagmus. Recent studies on two young patients with LCA5 mutations, however, indicate that photoreceptors are fairly well maintained in the central retina. LCA5 mutations account for 1-2% of LCA cases.
12) IMPDH1 gene - LCA11 The IMPDH1 gene is the blueprint to synthesize the protein called Inosine Monophosphate Dehydrogenase 1. IMPDH1 is an important enzyme in the body that functions in the formation of the compound guanine which is a building block of DNA. Although protein is expressed in many tissues, it it particularly high in retina. This and the fact that there are unique "isoforms" of the IMPDH1 protein in the retina may explain why only the retina demonstrates pathology in IMPDH1 mutations. IMPDH1 mutations lead to a dominant form of LCA. Mutations in other parts of the IMPDH1 gene can lead to dominant Retinitis Pigmentosa.
13) RD3 gene - LCA12 The RD3 protein is highly expressed in the retina, particularly in photoreceptor cells. In the photoreceptor cell, recent work (2010) shows that the RD3 protein is needed to ensure proper transport of a critical enzyme, guanylate cyclase (GC), from where it is synthesized in the photoreceptor cell body, through the cilium into the outer segment portion of the cell. Normal functioning of guanylate cyclase is essential in the Visual Process. Without the RD3 protein, the GC enzyme does not get to the outer segment and the Visual Process stops, leading to photoreceptor cell degeneration. There are excellent mouse and canine models of lCA12. In a mouse model of RD3 mutation, loss of the RD3 protein causes a rapidly progressing LCA disease process. Mutations of the RD3 gene in humans causes a recessive form of LCA. Although the RD3 mouse model of retinal degeneration has been known for many years (1993),it was only in 2006 when the gene mutation causing the disease process was identified by a large consortium of investigators (1). The RD3 protein seems to perform many important functions in the retina Molday and coworkers (2) have recently shown that it is critical for synthesis of a signaling molecule in the photoreceptor cells called cyclic GMP,lack of which could lead to photoreceptor cell death. In the mouse, a variable phenotype is observed with siblings with the exact same mutation exhibiting different levels of degenerative severity. Danciger and colleagues (3) have begun to catalog genetic modifiers for this effect, i.e., genes/alleles that influence the inherited degenerative process. Although preclinical therapeutic experiments are yet to start on the RD3 mutation, excellent rodent and canine models (4) are available that are similar to humans with the RD3 mutation. References: 1)Friedman JS, Chang B, Kannabrian C, Chakravoa C, Sing HP, Hawes, NL, Branham K, Othmanb M, Fillippova E, Thompson DA, Webster AR, Andreasson S, Jacobson, S, Bhattacharya SS, Heckenlively JR, Swaroop A. Premature truncation of a novel protein RD3, exhibiting subnuclear localization, is associated with retinal degeneration. Am. J. Hum genet. 2006,79:159-70. 2)Molday AS, Molday RS. RD3, the protein associated with Lener Congenital Amaurosis type 12 is required for guanylate cyclase trafficking in photoreceptor cells. Proc. Natl. Acad. Sci. 2010,107:158-63. 3)Danciger M, Ogando D, Yang H, Matthes MT, Yu N, Abern K, Yasumura D, Williams RW, LaVail MM. Genetic modifiers of retinal degeneration in the rd3 mouse. Invest. Ophthalmol. Vis. Sci. 2008,49:2863-69. 4)Kukekova AV, Goldstein O. Johnson, JL. Richardson MA, Pierce-Kelling SE, Swaroop A, Friedman, JS, Aguirre, GD, Acland, GM. Canine RD3 mutation established rod-cone dysplasia type 2 (rcd2) as ortholog of human and murine rd3. Mamm, Genone 2009,20109-123.
14) SPATA7 gene - LCA3 This is one of the more recent genes (2009) to be reported whose mutations lead to a form of LCA. Other SPATA7 mutations can lead to juvenile RP. Although the SPATA7 protein is expressed in the retina, its subcellular localization within the cell has not been determined. Similarly, the function of the protein in the retina is not known. A clue is obtained from the known importance of the protein spermatogenesis, ergo the name "Spermatogenesis-Associated-Protein7". Since cilial structures are important in protein transport in both spermatogenesis and vision, the connecting cilium of the photoreceptor cell would be an obvious place to look for SPATA7. An excellent review of the spectrum of SPATA7 mutations and associated LCA phenotypes has recently been published by Kaplan, Rozet and their colleagues (2010). Mutations in the human SPATA7 gene causing LCA were only reported in the scientific literature in 2009 (1). Since then, a few publications have described the screening of SPATA7-specific patients within the LCA population ( 1.7% of cases of childhood retinal dystrophy), the genetic spectrum of SPATA7 mutations and the delineation of the associated disease phenotype . Even though there is severe visual loss in infancy, some preservation of photoreceptor structure has been described in the central retina (2). This gives hope for successful therapy in restoring at least some visual function in an appropriate animal model and ultimately in the human. References: 1)Wang H, Den Hollander AI, Moavedi Y, Abuimitti A, Li Y, Collin RW, Hoyng CB, Lopez I. Abboud EB, Al.Rajhi AA, Bray M, Lewis RA, Lupski JR, Mardon G, Kpenekoop RK, Chen R. Mutations in SPATA7 cause Leber Congenital Amaurosis anmd juvenile retinitis pigmentosa. Am. J. Hum. Genet. 2009,84:380-7. 2)Mackay DS, Ocaka LA, Borman AD, Sergouniotis OI, Henderson RH, Moradi P, Robson AG, Thompson DA, Webster AR, Moore AT. Screening of SAPATA7 in patients with Leber Congenital Amaurosis and severe childhood-inset retinal dystrophy reveals disease-causing mutations. Invest. Ophthalmol. Vis. Sci. 2011,54:3032-8.
15) MERTK gene The MERTK protein is a "receptor tyrosine kinase" enzyme that is expressed in many tissues but quite highly in Retinal Pigment Epithelial (RPE) cells. It is thought to be involved in a process called phagocytosis in which some cells engulph and degrade other cells or portions of them. For example, RPE cells phagocytize shed tips of photoreceptor outer segments (OS)in a normal process that renews the outer segments. With a MERTK mutation, the RPE cell can no longer phagocytize the shed OS tips and there is a buildup of "OS garbage" between the retina and the RPE cells. Photoreceptor cell degeneration is the result. MERTK mutations account for only a small percentage of LCA cases. Other MERTK gene mutations have been reported to lead to RP or sever rod-cone dystrophy. MERTK mutations &ndash For many years, the RCS rat has been used as a model in Retinitis Pigmentosa studies. The MERTK mutation in Retinal Pigment Epithelial cells makes them incapable of phagocytizing shed tips of photoreceptor outer segments that normally occurs on a daily basis. There is a resultant buildup of a debris layer between the photoreceptor and RPE cells and rapid photoreceptor degeneration. Besides RP, MERTK mutations can also cause a rare form of LCA. Recently, for example, Moore and his colleagues in London have described novel mutations in the MERTK gene that are associated with childhood rod-cone dystrophy (1). Investigators such as Dr. Ali and coworkers have demonstrated that AAV-mediated gene transfer can slow photoreceptor loss in the RCS rat model of retinal degeneration (2,3). Morphologically, there is a decrease in debris buildup demonstrating at least partial restoration of function in the RPE cells. The number of remaining photoreceptor cells was also higher in the treated vs. control retinas. This success could pave the way for human trials in the future. References: 1)Mackay DS, Henderson RH, Sergouniotis OI. Moradi P, Holder GE, Waseem N, Bhattacharya SS, Aldahmesh MA, Alkuraya FS, Meyer B, Webster AR, Moore AT. Mol. Vis. 2010,16:367-77. 2)Smith AJ,Schlichtenbrede FC, Tschernetter M, Bainbridge JW Thrasher AJ, Ali RR. AAV-mediated gene transfer slows photoreceptor loss in the RCS rat model of retinitis pigmentosa. Mol. Ther. 2003,8:188-95. 3)Tschernetter M, Schlichtenbrede FC, Howe S, Balaggan KS, Munro PM, Bainbrisge JW, Thrasher AJ, Smith AJ, Ali RR. Long-term preservation of retinal function in the RCSD rat model of retinitis pigmentosa following lentivirus-mediated gene therapy. Mol. Ther. 2005,12:694-701.
16) IQCB1 / NPHP5 gene This is one of the latest genes to be identified (2010) whose mutations lead to a form of LCA. The protein appears to be important in functioning of both retina and kidney. In the retina, gene mutations lead to a "ciliopathy", i.e., where the cilium of the photoreceptor cell dysfunctions. The LCA condition caused by problems with IQCB1 gene can also be associated with severe kidney problems called nephronophtisis. New work (2011) indicates that rod photoreceptor loss are severely affected edarly in the disease process but that cone photoreceptor cells are less severely affected. Because these IQCB1 patients are at hight risk of developing kidney failure, all new LCA patients should be screened for IQCB1 mutations. If found, patients should be closely monitored for kidney function. An animal model is being studied that might lead to gene therapy for this form of LCA as well as to the NPHP6/CEP290 form.
17) KCNJ13 gene &ndash LCA16 Inwardly Rectifying Potassium Channel subunit- The outside cellular membranes of many neurons have channels (pores) that will specifically allow passage of small molecules like sodium or potassium. These are important in maintaining a normal balance of these molecules within the cells and often are involved in the generation of neuronal electrical currents. Often these channel receptors consist of several protein subunits, one specific one of the potassium channel is the Kir7.1 subunit whose gene (KCNJ13) has the mutation causing an abnormal Kir7.1 protein that leads to this particular form of LCA. Mutations in the KCNJ13 gene lead to early onset vision loss. This suggests both impaired retinal development and progressive retinal degeneration. The degeneration involves both rod and cone photoreceptor pathways. ·This form of LCA is just one of a family of diseases caused by other mutations in genes for potassium channel proteins that affect other organs. Mutations in this gene have only recently been reported to cause LCA (1). The KCNJ13 gene codes for a protein that is important in the regulation of cellular potassium. Phenotypically, patients demonstrate an early-onset retina degeneration and it is postulated that the KCNJ13 gene protein product is important in retinal development and maintenance of function. References: 1)Sergounitis P, Davidson AE, Mackay DS, Li Z, Yang X, Plagnol V, Moore AT, Webster AR. Recessive mutations in KCNJ13 encoding an inwardly rectifying potassium channel subunit cause Laber Congenital Amaurosis. Am. J. Hum. Genet. 2011,89:
18) NMNAT1 gene (LCA9)  The NMNAT1 gene codes for an important enzyme. It was found in 2011 to be neuroprotective against injury of neuronal axons in the peripheral nervous system (1). It has also been reported that this gene regulates the growth and morphogenesis of neurons. More recently, several groups of investigators have reported that mutations in this gene can lead to early retinal degeneration (2-5). Kopenekoop and his co-workers also report that &ldquoall individuals with NMNAT1 mutations also have macular colobomas which are severe degenerative entities of the central retina (fovea) devoid of tissue and photoreceptors.&rdquo (2). References: 1) Verghese PB, Sasaki Y, Yang D, et al. Nicotinamide mononucleotide adenyl transferase 1 protects against acute neurodegeneration in developing CNS by inhibiting excitotoxic-necrotic cell death. Proc. Natl. Acad. Sci. 2011108:19054-9. 2) Koenekoop RK, Wang H, Majewski J wt al. Mutations in NMNAT1 cause Leber congenital amaurosis and identify a new disease pathway for retinal degeneration. Nat. Genet. 201244:1035-9. 3) Chiang PW, Wang, Chen, Y et al. Exome sequencing identifies NMNAT1 mutations as a cause of Leber congenital amaurosis. Nat. Genet. 201244:972-4. 4) Perraulot I, Hanein S, Zanlounghi X et al. Mutations in NMNAT1 cause Leber congenital amaurosis with early-onset macular and optic atrophy. Nat. Genet. 201244:975-7. 5) Falk MJ, Zhang Q, Nakamaru-Ogiso E et al. NMNAT1 mutations cause Leber congenital amaurosis. Nat. Genet. 201244:1040-5.
19) DTHD1 gene - A recent scientific publication has implicated a new gene in LCA. Specifically, a mutation in a gene called DTHT1 has been shown to cause a form of LCA that accompanies a &ldquomild-moderate form of non-specific muscle dystrophy&rdquo*.  Most of the types of LCA reported to date are classified as & ldquonon-syndromic & rdquo in that the gene mutations do not cause other problems in the body. Here though, DTHD1 mutations are called & ldquosyndromic & rdquo since they lead to both LCA and muscle dystrophy.  This appears to be a very rare form of LCA and has been reported by Dr. L. Abu-Safieh and coworkers*. More work needs to be done to characterize the gene mutation and determine how it results in the disease processes.  * published in Genome Research, vol 23, pp:236-247, 2013
|8.||Future Treatments and Cures|
1) Clinical Considerations
Before any therapy for LCA is considered, the state of the patient’s retina must be determined. Although there is usually severe visual impairment from birth in LCA patients (i.e.low vision), there is little information on the morphological integrity of the photoreceptors or other layers of the retina. Fundus (back of eye) examination of LCA newborns often reveals the retina to be “fairly normal” in appearance although histopathological examination of a single prenatal, embryonic retinal sample demonstrated significant cell loss and other pathological changes.
Dr. J. Kaplan and her associates have examined numerous LCA patients and, in spite of the many genotypes, have phenotypically characterized the disease process into two subtypes, LCA1 and LCA2. The LCA1 group appears to have more severe manifestations of the disease while those falling into the second group retained more function and thus might be considered to be better candidates for treatment and ultimate sight restoration. The bottom line is that a thorough clinical examination of the patient must be performed to determine if they are indeed a candidate for a particular therapy.
For therapies such as Gene Therapy or Pharmamaceutial (neurotrophic) Therapy to be effective, enough photoreceptors need to remain alive and treatable. Luckily, there is a redundancy of photoreceptor cells in the human retina such that only a smaller percentage is needed for fairly good vision. In a similar vein, it is cone cells that are spared longer in most retinal degenerations and it is these cells that are most important for sharp and bright light vision in the human. Significant sight restoration therefore can theoretically occur even when most of the rod cells have degenerated with only a small number of cone cells present – hopefully in the macula.
Thus, there should be testing of the retina both morphologically and functionally to determine its state of degeneration. ERG and similar techniques can be used to determine any remaining functioning. A relatively new technique called Ocular Coherence Tomography (OCT) can be used to determine the thickness and integrity of the retina. Testing should not be done as a prelude to inclusion/exclusion to a clinical trial or treatment regimen. With regular testing of the structure and function of the retina, a better estimate can be made as to the progression of the disease and the possible condition of the retina when a therapy is available.
Thorough clinical testing should also be done of parents, other children and grandparents if possible. Subtle signs of degeneration may be detected in parents, possibly giving clinical clues as to the disease process.
2) Genotyping and DNA Banking
A small blood sample should be taken from the LCA patient as early as possible. From this sample, DNA can be prepared and tested for the known gene mutations described above. Along with efforts to find new LCA genes, the FRR is supporting the creation of a central repository, a “DNA Bank”, for such DNA samples. In this way, not only can immediate testing be performed but, if they are negative for the currently known LCA mutations, the samples can be retested at a later date for newly discovered gene mutations.
Genotyping is important since, if the mutation is actually identified, it lines up the patient for Gene Therapy when it becomes available. It also immediately gives the Ophthalmologist a much better view as to how that specific type of LCA usually progresses over time and the patient’s family can be better advised.
If possible, DNA samples should also be taken from family members. This makes the mutational analysis easier for the genetics investigator, especially in searching for a new gene mutation.
3) Gene Therapy
One of the best possibilities of a treatment and perhaps even a cure for LCA could come from Gene Therapy – more correctly, Gene Replacement Therapy. Simply, if there is a mutated gene that produces a malfunctioning protein or no protein at all, replacement with a normal gene in the proper cell type should result in synthesis of a normal protein, hopefully at a proper level with subsequent restoration of visual function. As outlined above though, Gene Therapy can only be successful if target cells (e.g., photoreceptor cells) are alive.
Before going to human clinical trial with any therapy, it is necessary to demonstrate proof of the efficacy (Proof of Principle) of a potential treatment as well as relative safety. For one form of LCA, the RPE65 mutation, we are lucky to have excellent rodent and canine models that also have RPE65 gene mutations and demonstrate early and severe vision loss. Extensive work has now been done on Gene Replacement Therapy in the canine model and the results are very promising as reported by Dr. Gustave Aguirre and his consortium of collaborators. After replacing the RPE65 gene in target RPE cells, significant vision is restored to all the animals. These results seem to be long term in that all of the dogs treated almost 5 years ago now yet have functional vision. Importantly, the therapy appears to be relatively safe with very few side effects noted and no long term negative effects. At least three other groups (USA, England and France) are preparing for clinical trials for RPE65 Gene Replacement. Similarly, Proof of Principle for Gene Therapy in a mouse model of LCA has been obtained by Dr. Tiensen Li and his coworkers. He has demonstrated the efficacy of replacing the RPGRIP gene in the retina of affected animals. This work will hopefully lead to successful gene therapy for human patients with this particular gene mutation. Thus, progress in effecting long term treatment of forms of LCA has been excellent. Human clinical trials are being planned that should be the models for all forms of LCA.
4) Pharmaceutical Therapy
What are the options while waiting for the gene mutation to be found prior to Gene Therapy or another treatment to be made available? In particular, is there a way to slow down the course of the disease, preserving photoreceptors until a permanent form of sight restoration is available? One option is Pharmaceutical Therapy.
Pharmaceutical Therapy can be defined as the use of any drug or natural agent to slow down the course of a retinal degenerative disease process. This method does not deliver a “cure” as theoretically Gene Therapy could but rather, a slowing down or even halting of the degenerative process. This affords the potential of many years more of functional vision to the patient.
Over the last few years, agents, drugs, natural growth factors, etc. have been identified that protect neuronal tissue against insult. Collectively, these are called neurotrophic agents or neuron-survival agents. The hallmark of their action is that they prolong the life of neuron cells such as photoreceptor cells. In many animal models of retinal degeneration, these different agents have been shown to substantially delay photoreceptor cell death, allowing for not only a longer period of vision but, in some cases, improved vision during this time.
Many agents have been shown to have neurotrophic activity in retinal degeneration. This field has been pioneered by Dr, Matthew LaVail who has compared activities of these agents in many animal models of retinal degeneration. One of the most successful is Ciliary Neurotrophic Factor (CNTF). Another example is a unique natural factor called Rod-Derived Cone Viability Factor. Dr. Jose Sahel and his collaborators have identified and characterized this protein which is particularly effective in slowing cone loss in retinal degeneration.
CNTF, is currently in clinical trial for forms of Retinitis Pigmentosa. The Phase 1 Study (safety) has been successfully completed and the Phase 2 Study (efficacy) is soon to commence. If successful, it should afford the first treatment for a rare human retinal degenerative disease and be available for general patient application in a few years. Since LCA is a special form of RP as described above, it is probable that many LCA patients could benefit from this type of treatment. Thus, while searching for the LCA gene in a particular patient, thought should be given to preserving the retinal photoreceptors as best as possible through Pharmaceutical Therapy.
5) Photoreceptor Cell Transplantation
A theoretically simple and appealing possibility for treatment of any photoreceptor degenerative disease is photoreceptor cell transplantation. In this technique, normal photoreceptor cells (sometimes with adjoining RPE cells) are surgically removed from donor eyes and transplanted into the photoreceptor space of the diseases retina. Most often, sheets of photoreceptor tissue are used, a process made easier because of the relatively flat, layered nature of the retina.
Retinal grafts have already been shown to survive and partially function in animal experiments. Drs. SriniVas, Magdalene Siler and Eugene de Juan have convincing experiments in rodent RP models that, after retinal transplantation, effective albeit limited connections are made with appropriate brain areas. Moreover, such transplantation actually promotes survival of remaining host retinal cells following transplantation probably because neurotrophic factors are elaborated in a tissue subjected to trauma such as the transplantation process. Thus, in addition to preserving vision through this neurotrophic effect, if methods can be developed to enhance the formation of functional connections between the graft and the host, retinal transplantation holds the potential for restoring vision to patients blind from advanced photoreceptor degenerations.
What is the main challenge? Studies from several laboratories have demonstrated that transplanted donor retinal tissue can survive in the subretinal, photoreceptor space but evidence that the transplanted tissue integrates with the host and forms functional synapses has been more limited. However, the group cited above has shown specific and enhanced retinal and brain electrical responses after photoreceptor cell transplantation. Importantly, an FDA-approved trial for photoreceptor transplantation is currently underway by Dr. Norman Radtke in Louisville, KY.
Future work in this area of research needs to concentrate on improving the integration of graft photoreceptor cells with secondary retinal neurons in the host tissue. However, it is now clear that the process of graft – host integration can, in fact, be modulated. Investigators in the field have been gaining in knowledge and expertise and it is hoped that these advances can be applied to the overall question of transplant functionality with a final, positive result in the human.
6) Stem Cell Therapy
Stem Cells are primitive, multipotential cells that have the intrinsic potential of developing into any cell type of the body, e.g., retinal photoreceptor cells. Stem cells are, of course, found in embryonic tissues. They also are present in many (if not all) adult tissues. In structures close to the adult human retina, true retinal progenitor cells (stem cells) have been identified and are currently being studied by several groups of researchers.
Stem cell therapy holds huge promise for replacing cells in the body, for example, those lost through a degenerative process such as inherited retinal degeneration. Specifically for LCA, stem cells could be transplanted into the photoreceptor space, differentiate and functionally take the place of the dead photoreceptors.
Significant problems of efficacy and safety remain to be overcome, however. For example, only partial differentiation towards a photoreceptor phenotype has been shown for stem cells by vision researchers. Although various biochemical markers unique to photoreceptors (e.g., the visual protein opsin) can be induced in the stem cells, a truly mature morphological and biochemical phenotype as well as light capture and synaptic functionality have yet to be demonstrated. Similarly, before human trials can start, significant safety issues need to be addressed. Stem cells, by definition, have virtually unlimited capacity for multiplication, a facet shared by cancer cells. It will be necessary in the future to demonstrate that stem cells can be managed once implanted in the eye such that they do not continue to grow in an uncontrolled manner.
In summary, stem cell therapy has great potential for treating retinal degeneration. This approach has the possibility of not only replacing dead photoreceptor cells but of allowing for reconstruction of the entire retina in the more severe retinal degenerations where secondary neurons degenerate as well as photoreceptor neurons. Much basic work needs to be done though before this promise is fulfilled.
7) Electronic Prosthetic Devices (the Chip)
Great progress is being made in work on electronic prosthetic devices - the “artificial retina” or “chip”. Animal and human testing is being done at many sites in the USA and in several countries such as Germany, Japan, Ireland, Belgium, Australia and Korea. In the USA, one company (Optobionics) is now 4 years into a Clinical Trial and another (Second Sight) is planning to begin a Trial in the relatively near future.
In a retinal degeneration, the prosthetic device would essentially take the place of the lost photoreceptor cells. Functionally, the photoreceptor cell captures the photic (light) energy and converts this energy to a chemical and then electrical signal and transmits this signal to secondary retinal neurons for processing and transmission to the brain. The retinal prosthetic device has been designed to fulfill all these functions. First, a small camera most probably attached to patient’s eyeglasses would capture the visual images. The camera would send these images to the prosthetic device that had previously been implanted in the eye - attached to the remaining, secondary neuronal cells of the retina. In several subsequent steps, the initial light signal is converted into an electrical signal that is transferred from the chip to the secondary neurons and ultimately to the brain. This internal device consists of an array of electrodes that directly signals and electrically excites secondary retinal neurons.
There has been great progress in chip development over the last few years. Yet, the challenges in producing a functional sight-restoring prosthetic device are significant. It appears, for example, that data from the Optobiobics Company and their academic collaborators demonstrate that their subretinal chip is itself ineffective in improving vision. Rather, it appears that the device acts to induce an “injury response” – eliciting the elaboration of endogenous neurotrophic factors. These neurotrophic factors stimulate remaining neurons to perform better, i.e., “sight restoration”. As of a few months ago, the “improvement” in the patients originally seen after implantation of the Optobiobics chip seemed to be fading with time. The bright side of these essentially negative data is that it now may be that any chip implantation (possibly acting as an “insult” to the retina) could lead to the production of neurotrophc factors. This serendipitous finding could form the basis of enhanced photoreceptor activity after chip implantation.
Another very important finding from the Optobionics clinical trial is that chip implantation appears to be a safe procedure. Few negative effects of implantation were detected allowing for human testing to proceed in a confidant manner.
As mentioned above, several prosthetic device projects are underway across the world. Other than the Optobionics work, one of the most advanced is that mounted by Dr. Mark Humayun (USC Medical School, Los Angeles, CA) and his collaborators in conjunction with a company called Second Sight. Preliminary animal and human testing has been successfully completed and a clinical trial is planned for the near future. The device used by the Humayun/Second Sight consortium has 16 electrodes in contact with the retina. It is a robust electrical device of a different design from the Optobionics device. Indications from a limited number of human implants indicate a high degree of safety and even some improvement in vision using this device. Devices with many more (64, 128, 1000, etc) electrodes are being tested in the laboratory, devices that could lead to a high degree of visual restoration in LCA patients.
|9.||Summary and Conclusion|
In the last few years, much progress has been made in understanding the physical characteristics and progression (phenotypes) of the different types of LCA as well as the gene mutations (genotypes) causing the disease process. Mutations in 19 different genes are now known to cause different forms of LCA. A number of modes of therapy are in different stages of development. In particular, a clinical trial on the neurotrophic agent CNTF is already in progress (Pharmaceutical Therapy) while a human trial using Gene Replacement Therapy for the RPE65 LCA mutation began in 2008. Transplantation and stem cell therapy hopefully will afford treatments in the future. Similarly, electronic prosthetic devices show great promise &ndash one type already in clinical trial and another to soon to begin FDA-approved testing in RP patients.
|10.||August 2012 -Update: Is Anyone Working on the Specific Gene Mutation in our Family, by Dr. Chader
Is Anyone Working On the Specific Gene Mutation in Our Family?
Dr. Jerry Chader, Chairman, Scientific Advisory Board, FRR
Even though it is a very rare grouping of related diseases, committed researchers and clinicians are making a strong effort to find the causes and the cures for LCA. As of today, 18 different genes have been identified whose mutations cause LCA. It is estimated that these gene mutations account for about 70% of all LCA cases. Several animal models have been identified or genetically constructed that have the same gene mutations found in humans with LCA. Researchers are using these to test for efficacy and safety for treatments that could be used in human clinical trials. Following are a number of LCA gene mutations on which progress is being made in preclinical testing and mutations in which clinical trials have already started.
Selected scientific references are given at the end, grouped by genetic mutation.
A)Clinical Trials in Progress:
1) RPE65 mutations (LCA2) – Three laboratories (Drs. Ali, Jacobson, Bennett and their coworkers) have been working on gene replacement therapy for over 3 years now. Previously, preclinical studies in animal models exhibiting mutation in the RPE65 gene were very successful. In the current clinical trials, results for safety and efficacy for all the groups is good (1). These trials continue on more patients and, importantly, on younger patients in hopes of gaining an even better visual improvement with treatment. Recent evidence from Bennett et al. demonstrates brain (visual cortex) responses after gene therapy suggesting that gene therapy “resulted in … sustained and improved visual ability” … “despite severe and long-term visual impairment”(2). Preliminary results from the Bennett group show that treating the other eye of patients already treated in one eye is safe and effective (3).
2) LRAT and RPE65 mutations – Mutations in these genes lead to the synthesis of defective proteins (either the LRAT or the RPE65 protein) that are very important in the photoreceptor visual cycle in maintaining proper vitamin A metabolism. Without proper functioning of these proteins, the visual cycle stops, vision is dramatically degraded and photoreceptor cell degeneration results. A clinical trial is in progress by Dr. Rob Koenekoop in conjunction with the QLT company that is testing a synthetic retinoid (i.e., type of vitamin A compound) in a small number of patients that will circumvent the genetic mutation. To date, there has been significant improvement in a number of visual parameters including visual field and visual acuity in the treated subjects.
Preclinical Testing that Precedes a Clinical Trial:
1) CRB1 mutations – CRB1 mutations are found in 10-15% of LCA cases. Significant phenotype variability is seen in CRB1 patients (1). It has recently been shown that additional modifying factors (besides primary mutations in the CRB1 gene could be responsible for the differential phenotypes (2). Preclinical work by Dr. Jan Windhols of the Institute for Neuroscience in the Netherlands and Dr. John Flannery at UC Berkeley in the USA could lead to new treatment regimes for this form of LCA. These investigators are developing new forms of vectors that can be used in gene therapy techniques. Vectors are modified viruses that carry therapeutic genes into target cells. They are made safe for use by removing intrinsic viral genes which are responsible for replication of the virus before the therapeutic gene is inserted. One particularly good candidate for gene therapy use is the AAV virus vector system. In other important studies, work in an animal model with a CRB1 mutation has shown that cone and rod photoreceptor cells can be transplanted in the retina. This has the potential to replace the lost photoreceptor cells (3).
2) CRX mutations – The CRX protein is critical for photoreceptor cell development and acts closely with another photoreceptor protein NRL (1). Mutant CRX proteins can lead to LCA. An animal model of CRX mutation has been produced in which cell replacement therapy is being tested. Drs. Lamba and Reh are doing experiments to see if human embryonic stem cells (hESCs) can be used to replace retinal neurons after damage and degeneration in the Crx-deficient mouse (2). Early results are very promising in that the stem cell therapy has been able to restore some function in the test retinas. Importantly, a company specializing in stem cell therapy is interested in taking this work to human clinical trial if the animal experiments show definitive safety and efficacy.
3) Lebercillin (LCA5) mutations – Mutations of the Lebercillin protein can cause the very rare LCA5 type of LCA. Den Hollander and her colleagues (1) identified the mutation and showed that lebercillin is a critical protein in the “cilium” area of the photoreceptor cell. A consortium of investigators is now working to see if gene therapy could be used to restore function in patients with the very rare lebercillin mutation. These investigators include Drs Frans Cremers, Anneke den Hollander and Ronald Roepman in the Netherlands, Dr. Patsy Nishina and Dr. Jean Bennett in the USA and Dr. Rob Koenekoop in Canada. In preclinical work, the group has successfully produced an excellent animal model of LCA5 using sophisticated techniques of molecular biology and is currently testing to see if gene therapy can replace the mutated gene with a normal copy and restore function in affected young mice. If successful, human trials are planned. In this regard, Jacobson and coworkers (2) have shown that, in the human, LCA patients “had evidence of retained photoreceptors mainly in the central retina”. This will make it easier to treat human LCA5 patients once clinical trials begin.
4) GUCY2D (LCA1) mutations – Guanylate Cyclase (GUCY2D) is an enzyme in the photoreceptor cell that produces a critical signal molecule called cyclic GMP. GUCY2D mutations account for about 20% of the LCA population. Based on excellent work on a chicken model of the GUCY2D mutation by Dr. Semple-Rowland at the Univ. of Florida, she has demonstrated restoration of functional vision in these animals using a novel form of gene therapy (1). Two other studies by the groups of Ali et al. (2) and Hauswirth et al. (3)) have used mouse models to demonstrate functional and behavioral restoration of vision using gene replacement of the guanylate cyalase-1 (Gucy2d) gene. For humans, cone photoreceptors are the most important type of photoreceptor cell since they, not rods, mediate sharp-, bright-light and color vision. Cone cell preservation was observed in these studies on the mouse, indicating probably success in vision preservation in human testing in the future. Long-term preservation of cone cell function was observed throughout this time and even significant restoration of cone function. This gives good evidence that there should be not only efficacy in the human but a long-term effect. In other studies, work in an animal model with a Guanylate Cyclase mutation has shown that cone and rod photoreceptor cells can be transplanted in the retina. This has the potential to replace the lost photoreceptor cells (4).
5) CEP290 and IQCB1mutations – Patients with the CEP290 mutation (NPHP6) constitute the largest percentage of LCA patients and have a problem in function of the cilium area of the photoreceptor cell (1). The cilium is a major site of protein transport in the photoreceptor cell. A closely related “ciliopathy” is NPHP5 caused by mutations in the IQCB1 gene. Luckily, Stone, Jacobson and their coworkers (2) found that patients with NPHP5 mutations retain a significant amount of intact photoreceptor architecture in the important cone-rich central portion of the retina even though most exhibit poor visual function. Yzer et al (3) recently reported that some patients with CEP290 mutations demonstrate “distinct extra-ocular findings” so it is important that newly-diagnosed LCA patients be examined for these other problems. In a fish model of the disease generated by investigators including Dr. Ed Stone, vision was restored in the CEP290-mutant model using gene therapy (4). This work and that by other investigators suggests that cone photoreceptors should be the main gene therapy target in the NPHP5 mutations as well as the CEP290/NPHP6 mutations when clinical trials begin. A similar conclusion was reached after studies in a special mouse model. Cideciyan and coworkers (5) found that this model showed “substantially retained cone photoreceptors with disproportionate cone function loss” making the likelihood of successful gene therapy in patients with CEP290 and IQCB1 mutations very good.
6) RPGRIP-1 mutations – RPGR-interacting protein-1 is essential for normal rod photoreceptor outer formation (1). It is localized in the specialized area called the “cilium” of the photoreceptor cell and lack or dysfunction of RPGRIP1 leads to a form of LCA. Dr. Tiensen Li and coworkers at the Harvard Medical School, Boston, MA have produced a good mouse model of this form of LCA and used gene therapy for replacement of the RPFRIP-1 gene (2,3). After therapy, production of the RPGRIP-1 protein was restored in the photoreceptor and the protein was transported to its normal position in the cilium. Moreover, there was better photoreceptor cell survival and preservation of morphology and electrical function in the treated animals. Drs. Jacobson, Stone and their coworkers have closely examined human subjects with RPGRIP1 mutations and found that such patients have “treatment potential” in that these patients maintained relatively good preservation in the central portion of their retinas, the area most important in humans for good visual acuity (4).
7) AIPL1 mutations – Drs. Li, Ali and other workers have intensively studied mouse models with APL1 mutations that reflect the clinical spectrum of human APL1 phenotypes, for example, AIPL1 patients who have different rates of degeneration. They have established that different viral vector systems are effective in APL1 gene replacement. Specifically, there was photoreceptor cell preservation and restoration of cellular function in animals treated by gene therapy (1-3). As with the RPGRIP-1 patients cited above, clinical investigators have evaluated patients known to have APL-1 mutations for their potential for successful gene therapy. In contrast to the RPGRIP-1 patients though, significant macular degeneration was observed although some sparing was found in the peripheral retina. Drs Ali, Moore and coworkers report “a degree of retinal structure and functional preservation” in AIPL1 patients (4). Drs. Simonelli and coworkers in Italy have also evaluated Italian patients with APL1 mutations and feel that future human gene therapy of such patients should be productive based on their finding and on the success of such therapy in the mouse model (5).
8) MERTK mutations – For many years, the RCS rat has been used as a model in Retinitis Pigmentosa studies. The MERTK mutation in RPE cells in the RCS rat makes them incapable of phagocytizing (engulfing and digesting) shed tips of photoreceptor outer segments that normally occurs on a daily basis in all higher animals. There is a resultant buildup of a debris layer between the photoreceptor and RPE cells and rapid photoreceptor degeneration. Besides RP, MERTK mutations can also cause a rare form of LCA. Recently, for example, Moore and his colleagues in London have described novel mutations in the MERTK gene that are associated with childhood rod-cone dystrophy (1). Investigators such as Drs. Ali, Hauswirth and others have demonstrated that AAV-mediated gene transfer can slow photoreceptor loss in the RCS rat model of retinal degeneration (2-4). Morphologically, there is a decrease in debris buildup demonstrating at least partial restoration of function in the RPE cells. The number of remaining photoreceptor cells was also higher in the treated vs. control retinas. This success could pave the way for human trials in the future.
9) IMPDH1 mutations - Depending on the area of the gene affected, IMPDH1 mutations can cause either LCA or a form of autosomal dominant RP classified as RP10. This spectrum of mutations and disease phenotypes has been well characterized by investigators such as Dr. Steve Daiger at the University of Texas in Houston (1). TX. Dr. Pete Humphries and his coworkers in Dublin, Ireland have applied gene therapy to a mouse model of RP10 and found it to prevent photoreceptor degeneration and to preserve neuronal synapse conductivity (2). It has yet to be demonstrated though that this work is applicable to the LCA disease phenotype.
10) RDH12 mutations – There are many members of the retinol dehydrogenase (RDH) family of protein enzymes. They participate in vitamin A molecular conversion in the retina. Biological work indicates that the specific RDH12 member of this family protects the retina from oxidative damage with disruption (mutation) of the gene and protein leading to retinal degeneration (1,2). A “knockout” mouse has been engineered in which the RDH12 gene is disrupted (3) so the biological effects of deleting the gene and protein can be studied. With this model available, gene replacement therapy can also be performed to test for safety and efficacy of the procedure before attempting human clinical trials.
11) LRAT and RPE-65 - In preclinical, animal studies, a small compound named TUDCA has been used on a mouse model of LRAT. Zhang et al. (4) found that systemic injection of TUDCA could prevent the death of cone photoreceptor cells. Cones are particularly important in human vision as it is this type of photoreceptor cell that serves for sharp, central and color vision. Although not tested on animal mutants of RPE-65, it is hoped that a similar positive effect would be obtained in this LCA model.
1) RD3 mutations (LCA12) - Although the RD3 mouse model of retinal degeneration has been known for many years (1993), it was only in 2006 when the gene mutation causing the disease process was identified by a large consortium of investigators (1). The RD3 protein seems to perform many important functions in the retina Molday and coworkers (2) have recently shown that it is critical for synthesis of a signaling molecule in the photoreceptor cells called cyclic GMP, lack of which can lead to photoreceptor cell death. In the mouse, a variable phenotype is observed siblings with the exact same mutation exhibiting different levels of degenerative severity. Danciger and colleagues (3) have begun to catalog “genetic modifiers” for this effect, i.e., genes/alleles that influence the inherited degenerative process. Although preclinical therapeutic experiments are yet to start on the RD3 mutation, excellent rodent and canine models (4) are available that are similar to humans with the RD3 mutation.
2) SPATA7 mutations – Mutations in the human SPATA7 gene causing LCA were only reported in the scientific literature in 2009 by a large consortium of investigators (1). Since then, a few publications have described the screening of SPATA7-specific patients within the LCA population (1.7% of cases of childhood retinal dystrophy), the genetic spectrum of SPATA7 mutations and the delineation of the associated disease phenotype. Even though there is severe visual loss in infancy, some preservation of photoreceptor structure has been described in the central retina by Moore and colleagues (2). This gives hope for successful therapy in restoring at least some visual function in an appropriate animal model and ultimately in the human. Dr. Rui Chen at the Baylor College of Medicine is working on an animal model with hopefully preclinical gene replacement therapy studies to follow. Two clinical research groups have screened SPATA7 patients to better understand the phenotypic changes in the disease (3, 4). Although patients were found to have “severe vision loss” in infancy, they “may have some preservation of the photoreceptor structure in the central retina” (4) opening the door to therapeutics treatment.
3) TULP1 mutations – Some mutations in the TULP1 gene can lead to LCA while others lead to retinal degeneration that is of an RP phenotype (1). A number of clinical reports are in the scientific literature describing the characteristics of the degeneration in specific families –Suranamese (2), Algerian and Dominican (3). A good mouse mode has been developed and characterized (4). It demonstrated an early-onset retinal degeneration but seems to be normal in other regards. The availability of the model would allow for testing of different types of therapy in the future.
4) KCNJ13 mutations – Mutations in this gene have only recently been reported to cause LCA (1). The KCNJ13 gene codes for a protein that is important in the regulation of cellular potassium. Phenotypically, patients demonstrate an early-onset retina degeneration and it is postulated that the KCNJ13 gene protein product is important in retinal development and maintenance of function.
5) IQCB1 mutations – Estrada-Cuzcano et al. have studied the IQCB1 gene and found that the mutations are associated not only with retinal degeneration but sometimes with other serious conditions – Senior-Loken Syndrome, for example in which there can be kidney problems. The authors advise that “all LCA patients be screened for IQCB1 mutations to follow more closely for the possibility of kidney disease.
6) OTX2 mutations – Moore and colleagues (1) have described a very rare condition in which a mutation in the OTX2 gene causes early retinal dystrophy. Other problems such as in pituitary function were also noted.
7) NMNAT1 mutations (LCA9) – The NMNAT1 gene codes for an important enzyme. It was found in 2011 to be neuroprotective against injury of neuronal axons in the peripheral nervous system (1). It has also been reported that this gene regulates the growth and morphogenesis of neurons. More recently, several groups of investigators have reported that mutations in this gene can lead to early retinal degeneration (2-5). Kopenekoop and his co-workers also report that “all individuals with NMNAT1 mutations also have macular colobomas which are severe degenerative entities of the central retina (fovea) devoid of tissue and photoreceptors.” (2).
LRAT and RPE-65
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2) Ashtari, M, Cyckowski, LL, Monroe JF et al. The human visual cortex responds to gene therapy-mediated recovery of retinal function. J. Clin. Invest. 2011:121:2160-8.
3) Bennett, J, Ashtari, M, Wellman, J. et al. AAV2 gene therapy re-administration in three adults with congenital blindness. Sri.Transl Med 2012:8120re15.
4) Zhang T, Baehr W, Fu Y. Chemical chaperone TUDCA preserves cone photoreceptors in a mouse model of Leber Congenital Amaurosis. Invest. Ophthalmol. Vis. Sci. 2012
1) Henderson RH, Mackay DS, Li Z, Sergouniotis P, Russel-Eggitt, I, Thompson DA, Robson, AG, Holder, GE, Webster AR, Moore, AT, Phenotypic variability in patients with retinal dystrophies due to mutations in CRB1. Br. J. Ophthalmol. 2011 95:811-7.
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3) Lakowski J., Baron, M., Bainbridge J, Barber AC, Pearson RA, Ali RR, Sowden JC. Cone and rod photoreceptor transplantation in models of the childhood retinopathy Leber congenital amaurosis using flow-sorted Crx-positive donor cells. Hum Mol Genet 2010:194545-59.
1) Nichols LL, Alur RP, Boobalan E et al. Two novel CRX mutant proteins causing autosomal dominant Leber congenital amaurosis interact differently with NRL. Hum. Mutat. 2010:
2) Lamba DA, Gust J, Reh T Transplantation of human embryonic stem cell-derived photoreceptors restores some visual function in Crx-deficient mice. Cell Stem Cell. 20094:73-9.
1) Den Hollander, AI, Koenekoop, RK, Mohamed MD et al. Mutations in LCA5 encoding the ciliary protein lebercillin cause Leber congenital amaurosis. Nat Genet 20739:889-95.
2) Jacobson SG, Aleman TS, Cideciyan AV et al. Lener congenital amaurosis caused by lebercillin (LCA5) mutation: retained photoreceptors adjacent to retinal disorganization.
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3) Boye SE, Boye SL, Pang J et al. Functional and behavioral restoration of vision by gene therapy in the guabylate cyclase-1 9 (GC1) knockout mouse. PLoS1 20105:e11306.
4) Lakowski J., Baron, M., Bainbridge J, Barber AC, Pearson RA, Ali RR, Sowden JC. Cone and rod photoreceptor transplantation in models of the childhood retinopathy Leber congenital amaurosis using flow-sorted Crx-positive donor cells. Hum Mol Genet 2010:194545-59.
CEP290 and IQCB1
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2) Stone EM, Cideciyan AV, Aleman TS et al. Variations in NPHP5 in patients with nonsyndromic Leber congenital amaurosis and Senior-Loken syndrome. Arch Ophthalmol 2011129:81-7.
3) Yzer, S. den Hollander AI, Lopez I et al. Ocular and extra-ocular features of patients with Leber congenital amaurosis and mutations in CEP290. Mol Vis 2012:18:412-25.,
4) Baye LM, Patrinostro X, Swaminathan S et al. The N-terminal region of centrosomal protein 290 (CEP290) restores vision in a zebrafish model of human blindness. Hum Mol Genet 201120:1467-77.
5) Cideciyan AV, Rachel RA, Aleman, TS et al. Cone photoreceptors are the main targets o for gene therapy of NPHPS5 (IQCB1) and NPHP6 (CAP290) blindness: generation of an all-cone Nphp6 homomorph mouse that mimics the human retinal ciliopathy. Hum Mol Genet 2011:20:1411-23.
1) Won J, Gifford E, Smith RS et al. RPGRIP1 is essential for normal rod photoreceptor outer segment elaboration and morphogenesis. Hum Mol Genet 200918:4329-39.
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