求教

凤凰涅盘

哪位战友知道 RDH12  这个致病基因的最新研究状况，望知情者不吝赐教

YESCHONG

楼主是这个基因出问题了吗

BOBOm

这个基因在50多个已发现的突变基因中没有呀，难道是新发现的？

凤凰涅盘

3# BOBOm
不是
这是很久以前就发现的基因
http://www.rp-china.org/viewthread.php?tid=354&extra=page%3D1
阿富汗农村重建方案的基因研究概况：目前已经能够检测到26个与阿富汗农村重建方案相关的致病基因，其中较多见的是：先令甲（10.0％）， CRB1（6.5％），磷酸二酯酶6A条（6.0％），PDE6B（6.0％），ABCA4（2.9％），CNGA1（2.30％），RPE65基因（2.0％），档成分股（1.0％），它们约占阿富汗农村重建方案的32.7％，其他导致阿富汗农村重建方案的相关致病基因还包括切克尔，CNGB1，MERTK，旅客定座记录，RDH12，RLBP1，凹陷，TULP1，商品研究局，先令3A条等，但是它们在阿富汗农村重建方案中的突变率尚不清楚[3,4]（ASPERBIOTECH：www.asperophthaimics.com; RETNET： www.sph.uth.tmc.edu）。

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http://www.tfrr.org/index.php?m=19
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.


2. Prevalence
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.


3. LCA Phenotype
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.


4. LCA Genotype
In general, the term Genotype refers to the full genetic constitution of an individual. In a specific disease process, the “Disease Genotype” 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, 16 different gene mutations are known which lead to different forms of LCA. These are listed below along with short descriptions.


5. Retinal Anatomy
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--

Photoreceptor Cells:
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.

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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.

Genes:
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.

Mutations:
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, 17 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, “RetNet” 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%

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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 –Suranamese (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.

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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 – 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.

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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 – 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:


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.

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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.