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Lori S Sullivan ,  Daniel C Koboldt ,  Sara J Bowne ,  Steven Lang ,  Susan H Blanton ,  Elizabeth Cadena ,  Cheryl E Avery , Richard A Lewis ,  Kaylie Webb-Jones ,  Dianna H Wheaton ,  David G Birch ,  Razck Coussa ,  Huanan Ren ,  Irma Lopez , Christina Chakarova ,  Robert K Koenekoop ,  Charles A Garcia ,  Robert S Fulton ,  Richard K Wilson ,   George M Weinstock ,  Stephen P Daig er | Investiga tive Ophthalmology & Visual Science | 2014 Sep 4 | 55 (11) | Pgs. 7147-58 | doi: 10.1167/iovs.14-15419 Abstract Purpose: To identify the cause of retinitis pigmentosa (RP) in UTAD003, a large, six-generation Louisiana family with autosomal dominant retinitis pigmentosa (adRP). Methods: A series of strategies, including candidate gene screening, linkage exclusion, genome-wide linkage mapping, and whole-exome next-generation sequencing, was used to identify a mutation in a novel disease gene on chromosome 10q22.1. Probands from an additional 404 retinal degeneration families were subsequently screened for mutations in this gene. Results: Exome sequencing in UTAD003 led to identification of a single, novel coding variant (c.2539G>A, p.Glu847Lys) in hexokinase 1 (HK1) present in all affected individuals and absent from normal controls. One affected family member carries two copies of the mutation and has an unusually severe form of disease, consistent with homozygosity for this mutation. Screening of additional adRP probands identified four other families (American, Canadian, and Sicilian) with the same mutation and a similar range of phenotypes. The families share a rare 450-kilobase haplotype containing the mutation, suggesting a founder mutation among otherwise unrelated families. Conclusions: We identified an HK1 mutation in five adRP families. Hexokinase 1 catalyzes phosphorylation of glucose to glucose-6-phosphate. HK1 is expressed in retina, with two abundant isoforms expressed at similar levels. The Glu847Lys mutation is located at a highly conserved position in the protein, outside the catalytic domains. We hypothesize that the effect of this mutation is limited to the retina, as no systemic abnormalities in glycolysis were detected. Prevalence of the HK1 mutation in our cohort of RP families is 1%. Introduction Retinitis pigmentosa (RP) is a group of inherited dystrophic disorders of the retina leading to profound loss of vision or blindness. The clinical hallmarks of RP are night blindness, followed by progressive loss of peripheral vision, often culminating in complete blindness. Clinical findings of RP include “bone spicule” pigmentary deposits, retinal vessel attenuation, and characteristic changes in the electroretinograms (ERG). 1   Retinitis pigmentosa affects approximately 1 in 4000 people in the United States, Europe, and Japan. Retinitis pigmentosa is exceptionally heterogeneous with many different genes implicated, many different disease-causing mutations in each gene, and varying clinical presentations even among members of the same family. 2 For nonsyndromic RP, mutations in 24 genes are known to cause autosomal dominant RP (adRP); 45 genes cause recessive RP (arRP), and 3 genes cause X-linked RP (summarized in RetNet, https://sph.uth.edu/retnet/ [in the public domain]) .  The genes found to date as causes of adRP do not fall into a single functional category but include a diverse range of retinal functions, including components of the phototransduction cycle ( RHO , GUCA1A , RDH12 ); pre-mRNA processing factors ( PRPF3 , PRPF4 , PRPF6 , PRPF8 , PRPF31 , SNRNP200 ); structural proteins ( PRPH2 , ROM1 ); ciliary proteins ( RP1 , TOPORS ); transcription factors ( NRL , CRX , NR2E3 ); and a seemingly random assortment of other genes ( BEST1 , CA4 , FSCN2 , IMPDH1 , KLHL7 , RPE65 , SEMA4A ). With current techniques, we can identify likely disease-causing mutations in approximately 75% of patients with adRP (Daiger SP, manuscript in preparation, 2014). While mutations in some known genes may be missed, a number of additional adRP genes remain to be identified. In our University of Texas (UT) adRP cohort, mutations have been found in 205 of 265 families, leaving 60 (23%) with potentially novel adRP genes.  Click here to read entire article References Heckenlively JR. Retinitis Pigmentosa . London: J.B. Lippincott; 1988. Daiger SP. Identifying retinal disease genes: how far have we come, how far do we have to go? Novartis Found Symp . 2004; 255: 17–27, discussion 27–36, 177–178. Sullivan LS Bowne SJ Birch DG Prevalence of disease-causing mutations in families with autosomal dominant retinitis pigmentosa: a screen of known genes in 200 families. Invest Ophthalmol Vis Sci . 2006; 47: 3052–3064. Churchill JD Bowne SJ Sullivan LS Mutations in the X-linked retinitis pigmentosa genes RPGR and RP2 found in 8.5% of families with a provisional diagnosis of autosomal dominant retinitis pigmentosa. Invest Ophthalmol Vis Sci . 2013; 54: 1411–1416. Bowne SJ Sullivan LS Avery CE Mutations in the small nuclear riboprotein 200 kDa gene (SNRNP200) cause 1.6% of autosomal dominant retinitis pigmentosa. Mol Vis . 2013; 19: 2407–2417. Bowne SJ Sullivan LS Mortimer SE Spectrum and frequency of mutations in IMPDH1 associated with autosomal dominant retinitis pigmentosa and Leber congenital amaurosis. Invest Ophthalmol Vis Sci . 2006; 47: 34–42. Sullivan LS Bowne SJ Reeves MJ Prevalence of mutations in eyeGENE probands with a diagnosis of autosomal dominant retinitis pigmentosa. Invest Ophthalmol Vis Sci . 2013; 54: 6255–6261. Sullivan LS Bowne SJ Seaman CR Genomic rearrangements of the PRPF31 gene account for 2.5% of autosomal dominant retinitis pigmentosa. Invest Ophthalmol Vis Sci . 2006; 47: 4579–4588. Gire AI Sullivan LS Bowne SJ The Gly56Arg mutation in NR2E3 accounts for 1-2% of autosomal dominant retinitis pigmentosa. Mol Vis . 2007; 13: 1970–1975. Bowne SJ Sullivan LS Gire AI Mutations in the TOPORS gene cause 1% of autosomal dominant retinitis pigmentosa. Mol Vis . 2008; 14: 922–927. Click here to see all references

S P Daiger ,  L S Sullivan ,  S J Bowne | Clinical Genetics | 23 May 2013 | Volume 84, Issue 2 | Pages 132-141 | doi.org/10.1111/cge.12203 Abstract Retinitis pigmentosa (RP) is a heterogeneous set of inherited retinopathies with many disease-causing genes, many known mutations, and highly varied clinical consequences. Progress in finding treatments is dependent on determining the genes and mutations causing these diseases, which includes both gene discovery and mutation screening in affected individuals and families. Despite the complexity, substantial progress has been made in finding RP genes and mutations. Depending on the type of RP, and the technology used, it is possible to detect mutations in 30-80% of cases. One of the most powerful approaches to genetic testing is high-throughput 'deep sequencing', that is, next-generation sequencing (NGS). NGS has identified several novel RP genes but a substantial fraction of previously unsolved cases have mutations in genes that are known causes of retinal disease but not necessarily RP. Apparent discrepancy between the molecular defect and clinical findings may warrant reevaluation of patients and families. In this review, we summarize the current approaches to gene discovery and mutation detection for RP, and indicate pitfalls and unsolved problems. Similar considerations apply to other forms of inherited retinal disease. To purchase the publication, click here

Michael Danciger, Vickie Heilbron, Yong-Qing Gao, Dan-Yun Zhao, Samuel G. Jacobson, and Debora B. Farber | Mol. Vis. | Volume 2 (10) | 1996 |  molvis.org/molvis/v2/a10/ Based on average estimates of the prevalence of non-syndromic  retinitis pigmentosa  (RP) at 1/4,000, there are approximately 1.5 million people in the world with this inherited, progressive, degenerative disease of the retinal photoreceptor cells which often leads to blindness. About 50% of these cases are inherited in an autosomal recessive manner (AR). With the approach of screening the exons of candidate genes in large numbers of unrelated ARRP probands, mutations associated with disease have been found in several candidate genes expressed in rod photoreceptors at very low frequency:  RHO , encoding rhodopsin, 1/126 patients screened;  PDE6B (click to continue reading) Based on average estimates of the prevalence of non-syndromic retinitis pigmentosa (RP) at 1/4,000, there are approximately 1.5 million people in the world with this inherited, progressive, degenerative disease of the retinal photoreceptor cells which often leads to blindness (1,2,3). About 50% of these cases are inherited in an autosomal recessive manner (AR). With the approach of screening the exons of candidate genes in large numbers of unrelated ARRP probands, mutations associated with disease have been found in several candidate genes expressed in rod photoreceptors at very low frequency: RHO , encoding rhodopsin, 1/126 patients screened (4); PDE6B , encoding the beta-subunit of rod cGMP-phosphodiesterase, 4/88 patients (5); PDE6A , encoding the alpha-subunit of rod cGMP-phosphodiesterase, 2/173 patients (6); and CNCG , encoding the alpha-subunit of the rod cGMP-gated channel, 3/173 patients (7). With the approach of linkage analysis in large families, two ARRP loci have so far been discovered: 1q31-q32.1 (8,9) and 6p, distal to RDS-peripherin (10). Also, linkage analysis of an informative consanguineous family led to the discovery of a second homozygous RHO mutation (11). ARRP tends to appear most often in small nuclear families that by themselves are not informative enough to yield significant linkage data, and screening all the exons of candidate genes like PDE6A and PDE6B (each with 22 exons) in large numbers of unrelated probands is costly and time consuming. Therefore, we have taken the approach of studying candidate genes in small nuclear ARRP families with a double screening protocol. Linkage analyses of markers close to the loci of the candidate genes are performed first, and any families where a gene locus clearly does not segregate with disease are ruled out from further study of that gene. DNAs of the probands from the remaining families (where the gene locus cannot be ruled out from segregating with disease) are then screened for mutations in the exons of the candidate gene by SSCPE (single strand conformation polymorphism electrophoresis) and DGGE (denaturing gradient gel electrophoresis). Any exonic variants found are sequenced directly and analyzed within the corresponding family to see if they appear to segregate with disease. With this approach we studied 24 families with inherited retinal degenerations (14 with typical RP) for mutations in the genes PDE6B, MYL5 , PDE6C, CNCG , RHO, ROM1 and RDS-peripherin . We have reported two typical ARRP families where the affecteds uniquely inherited compound heterozygous mutations in the PDE6B gene (12). With a similar approach, homozygous mutations also have been found in PDE6B in the affecteds of two other ARRP families (13,14). Click here to read more References Kumar-Singh, R, Farrar GJ, Mansergh, F. Kenna, P. Bhattacharya, S, Gal, A, and Humphries, P, Exclusion of the involvement of all known retinitis pigmentosa loci in the disease present in a family of Irish origin provides evidence for a sixth autosomal dominant locus (RP8). Hum. Mol. Genet. 7 (1993) 875-878 . Bundey, S and Crews, SSJ, A study of retinitis pigmentosa in the City of Birmingham. I Prevalence. J. Med. Genet. 21 (1984) 417-420. Boughman, JA, Conneally, PM and Nance WE, Population genetic studies of retinitis pigmentosa. Am. J. Hum. Genet. 32 (1980) 223-235. Rosenfeld, PJ, Cowley, GS, McGee, TL, Sandberg, MA, Berson, EL and Dryja, TP, A null mutation in the rhodopsin gene causes rod photoreceptor dysfunction and autosomal recessive retinitis pigmentosa. Nature Genet. 1 (1992) 209-213. McLaughlin, ME, Sandberg, MA, Berson, EL and Dryja TP, Recessive mutations in the gene encoding the beta-subunit of rod phosphodiesterase in patients with retinitis pigmentosa. Nature Genet. 4 (1993) 130-134. Huang, SH, Pittler, SJ, Huang, X, Oliveira, L, Berson, EL and Dryja, TP, Autosomal recessive retinitis pigmentosa caused by mutations in the a subunit of rod cGMP phosphodiesterase. Nature Genet. 11 (1995) 468-471. Dryja, TP, Finn, JT, Peng, Y-W, McGee, TL, Berson, EL and Yau, K-W, Mutations in the gene encoding the a subunit of the rod cGMP-gated channel in autosomal recessive retinitis pigmentosa. Proc. Nat. Acad. Sci. USA 92 (1995) 10177-10181. van Soest, S, Ingeborgh van den Born, L, Gal, A, Farrar, GJ, Bleeker- Wagemakers, LM, Westerveld, A, Humphries, P, Sandkuijl, LA and Bergen, AB, Assignment of a gene for autosomal recessive retinitis pigmentosa (RP12) to chromosome 1q31-q32.1 in an inbred and genetically heterogeneous disease population. Genomics 22 (1994) 499-504. Leutelt, J, Oehlman, R, Younus, F, Ingeborgh van den Born, L, Weber, JL, Denton, MJ, Qasim Mehdi, S and Gal, A, Autosomal recessive retinitis pigmentosa locus maps on chromosome 1q in a large consanguineous family from Pakistan. Clin. Genet. 47 (1995) 122-124. Knowles, JA, Shugart, Y, Banerjee, P, Gilliam, TC, Lewis, CA, Jacobson, SG and Ott, J, Identification of a locus, distinct from RDS-peripherin, for autosomal recessive retinitis pigmentosa on chromosome 6p. Hum. Mol. Genet. 3 (1994) 1402-1403. Kumaramanickavel, G, Maw, M, Denton, MJ, John, S, Srikumari, CR, Orth, U, Oehlman, R and Gal, A, Missense rhodopsin mutation in a family with recessive RP. Nature Genet. 8 (1994) 10-11. Danciger, M, Blaney, J, Gao, Y-Q, Zhao, DY, Heckenlively, JR, Jacobson, SG and Farber, DB, Mutations in the PDE6B gene in autosomal recessive RP. Genomics 30 (1995) 1-7. Bayes, M, Giordano, M, Balcells, S, Grinberg, S, Vilageliu, L, Martinez, I, Ayuso, C, Benitez, J, Ramos-Arroyo, MA, Chivelet, P, Solans, T, Valverde, D, Amselem, S, Goosens, M, Baiget, M, Gonzalez-Duarte, R and Besmond, C, Homozygous tandem duplication within the gene encoding the beta-subunit of rod phosphodiesterase as a cause for autosomal recessive retinitis pigmentosa. Human Mutation 5 (1995) 228-234. Valverde, D, Solans, T, Grinberg, S, Balcells, S,Vilageliu, L, Bayes, M, Chivelet, P, Besmond, C, Goosens, Gonzalez-Duarte, R and Baiget, M, A novel mutation in exon 17 of the beta-subunit of rod phosphodiesterase in two RP sisters of a consanguineous family. Human Genet. 97 (1996) 35-38.

S. Goyal ,  M. Jäger ,  P.N. Robinson ,  V. Vanita | Clinical Genetics | 21 July 2015 | Vol. 89, Issue 4 | pgs. 454-460 | https://doi.org/10.1111/cge.12644 Abstract Nonsyndromic retinitis pigmentosa (RP) is genetically highly heterogeneous, with >100 disease genes identified. However, mutations in these genes explain only 60% of all RP cases. Blood samples were collected from 12 members of an autosomal recessive RP family. Whole genome homozygosity mapping and haplotype analysis placed the RP locus in this family at chromosome 14q31.3. Whole-exome sequencing (WES) in proband revealed a mutation in TTC8 , which was flagged as most likely candidate gene by bioinformatic analysis. TTC8 is mutated in Bardet–Biedl syndrome 8 (BBS8), and once reported previously in a family with nonsyndromic RP. Sequencing of amplified products of exon 13 of TTC8 validated c.1347G>C (p.Gln449His), a novel change that affects the final nucleotide of exon 13 and might deleteriously affect splicing. This mutation segregated completely with the disease in the family and was not observed in 100 ethnically matched controls from same population. This represents second report of a TTC8 mutation in nonsyndromic RP, thus confirming the identity of TTC8 as causative gene for RP51. Click here to buy article

Bernardo V Alvarez, Eranga N Vithana, Zhenglin Yang, Adrian H Koh, Kit Yeung, Victor Yong, Haley J Shandro, Yali Chen, Prasanna Kolatkar, Paaventhan Palasingam, Kang Zhang, Tin Aung, Joseph R Casey | Investigative Ophthalmology & Visual Science  | Aug 2007 | Vol.48 | 3459-3468 | doi.org/10.1167/iovs.06-1515 Abstract Purpose The autosomal dominant retinitis pigmentosa (adRP) gene on chromosome 17, region q22 (RP17), was recently identified as a glycosylphosphatidylinositol membrane-anchored zinc metalloenzyme (protein CAIV), highly expressed in the choriocapillaris of the eye and undetectable in the retina. Only two missense mutations have thus far been identified in the gene CA4 . Functional analysis of these mutations demonstrated that retinal disease may result from perturbation of pH homeostasis in the outer retina, after disruption of CAIV and sodium bicarbonate cotransporter 1 (NBC1)–mediated bicarbonate transport. CA4 was screened in a panel of patients with RP, to expand the mutation spectrum of this novel adRP gene and understand its pathogenic mechanism. Methods A total of 96 patients with simplex RP and adRP of Chinese ethnicity were screened for mutations in the eight coding exons of the CA4 gene by bidirectional sequencing. Functional consequences of CA4 mutations on the NBC1-mediated bicarbonate transport were studied by measuring bicarbonate fluxes in HEK293 cells cotransfected with NBC1 and CA4 mutant cDNAs. Results Thirteen sequence alterations were identified, including a novel mutation within exon 3 of CA4 (R69H) in a patient with simplex RP. R69H was not found in 432 normal chromosomes. R69H CAIV impaired NBC1-mediated pH recovery after acid load. Conclusions A novel mutation has been identified in CA4 that provides further evidence that impaired pH regulation may underlie photoreceptor degeneration in RP17. This study indicates that, as with European patients with RP, mutations in CA4 also account for ≤1% of Chinese patients with RP. Click here to read original article

Lina Zelinger , Eyal Banin, Alexey Obolensky, Liliana Mizrahi-Meissonnier, Avigail Beryozkin, Dikla Bandah-Rozenfeld, Shahar Frenkel, Tamar Ben-Yosef, Saul Merin,  Sharon B. Schwartz ,  Artur V. Cideciyan , Samuel G. Jacobson, Dror Sharon |   American Society of Human Genetics (AJHG) |  2011 Feb 11 | 88(2) | 207–215 |  doi: 10.1016/j.ajhg.2011.01.002 Abstract Retinitis pigmentosa (RP) is a heterogeneous group of inherited retinal degenerations caused by mutations in at least 50 genes. Using homozygosity mapping in Ashkenazi Jewish (AJ) patients with autosomal-recessive RP (arRP), we identified a shared 1.7 Mb homozygous region on chromosome 1p36.11. Sequence analysis revealed a founder homozygous missense mutation, c.124A>G (p.Lys42Glu), in the dehydrodolichyl diphosphate synthase gene ( DHDDS ) in 20 AJ patients with RP of 15 unrelated families. The mutation was not identified in an additional set of 109 AJ patients with RP, in 20 AJ patients with other inherited retinal diseases, or in 70 patients with retinal degeneration of other ethnic origins. The mutation was found heterozygously in 1 out of 322 ethnically matched normal control individuals. RT-PCR analysis in 21 human tissues revealed ubiquitous expression of DHDDS . Immunohistochemical analysis of the human retina with anti-DHDDS antibodies revealed intense labeling of the cone and rod photoreceptor inner segments. Clinical manifestations of patients who are homozygous for the c.124A>G mutation were within the spectrum associated with arRP. Most patients had symptoms of night and peripheral vision loss, nondetectable electroretinographic responses, constriction of visual fields, and funduscopic hallmarks of retinal degeneration. DHDDS is a key enzyme in the pathway of dolichol, which plays an important role in N -glycosylation of many glycoproteins, including rhodopsin. Our results support a pivotal role of DHDDS in retinal function and may allow for new therapeutic interventions for RP. Main Text Retinitis pigmentosa (RP; MIM 268000 ) is the most common inherited retinal degeneration, with an estimated worldwide prevalence of 1:4000. 1–3 The disease is highly heterogeneous and has several patterns of inheritance. At present, 35 genetic loci have been implicated in nonsyndromic autosomal-recessive RP (arRP), most of which account for a few percent of RP cases each. Although many of the early identified arRP genes were excellent candidates for the disease when mutated, mainly because of the function of the encoded protein (e.g., PDE6A 4 [MIM 180071 ] and PDE6B 5 [MIM 180072 ]), a large proportion of the recently identified genes were not a priori considered as candidates and were identified through whole-genome linkage or homozygosity mapping followed by mutation screening of a large number of genes in the linked intervals (e.g., EYS 6 [MIM 612424 ] and SPATA7 7 [MIM 609868 ]). Through those studies, a new class of genes encoding proteins with housekeeping-like function (e.g., IDH3B [MIM 604526 ] for arRP 8 and splicing factors for autosomal-dominant RP 9,10 ) have been identified and provided new insight into processes that result in retinal degeneration. The reason for the retina-specific phenotype caused by mutations in these genes is still unclear. The Ashkenazi Jewish (AJ) population was established by Jews who originated in the Middle East and migrated to Europe, initially settling in Germany (the “Ashkenaz” region) at or before the 4 th century. The AJ lived in closed communities in European countries and developed a unique culture and language (named Yiddish, which is based on a few different languages, including German, Hebrew, and Aramaic). After the Holocaust, the population size dropped from about 8.8 million individuals to only 2.8 million, and AJ immigrated out of Europe, mainly to the United States and the emerging state of Israel. AJ currently constitute the largest Jewish ethnic group in both countries. A large amount of effort was directed to study the genetic structure of the AJ population, in the context of other Jewish ethnic groups and Middle Eastern populations, at the Y chromosome, 11,12 mitochondrial, 13 and genomic 14,15 levels. Although consanguineous marriages are relatively uncommon among AJ (1.5% and rapidly declining), 16,17 most individuals who are affected by a rare AR disease in this ethnic group are homozygous for the disease-causing mutation, mainly because of a high rate of intracommunity marriages. 17 Therefore, genetic analysis of hereditary diseases in the AJ population, via homozygosity mapping, can be highly efficient. To read entire article, click here

Dunja Lukovic , Ana Artero Castro , Ana Belen Garcia Delgado , María de los Angeles Martín Bernal , Noelia Luna Pelaez , Andrea Díez Lloret , Rocío Perez Espejo , Kunka Kamenarova , Laura Fernández Sánchez , Nicolás Cuenca , Marta Cortón , Avila Fernandez , Anni Sorkio , Heli Skottman , Carmen Ayuso , Slaven Erceg , Shomi S. Bhattacharya  | Scientific Reports | Human iPSC derived disease model of MERTK-associated retinitis pigmentosa | 11 August 2015 | https://doi.org/10.1038/srep12910 Abstract Retinitis pigmentosa (RP) represents a genetically heterogeneous group of retinal dystrophies affecting mainly the rod photoreceptors and in some instances also the retinal pigment epithelium (RPE) cells of the retina. Clinical symptoms and disease progression leading to moderate to severe loss of vision are well established and despite significant progress in the identification of causative genes, the disease pathology remains unclear. Lack of this understanding has so far hindered development of effective therapies. Here we report successful generation of human induced pluripotent stem cells (iPSC) from skin fibroblasts of a patient harboring a novel Ser331Cysfs*5 mutation in the MERTK gene. The patient was diagnosed with an early onset and severe form of autosomal recessive RP (arRP). Upon differentiation of these iPSC towards RPE, patient-specific RPE cells exhibited defective phagocytosis, a characteristic phenotype of MERTK deficiency observed in human patients and animal models. Thus we have created a faithful cellular model of arRP incorporating the human genetic background which will allow us to investigate in detail the disease mechanism, explore screening of a variety of therapeutic compounds/reagents and design either combined cell and gene- based therapies or independent approaches. Introduction Retinitis pigmentosa (RP; OMIM 268000) with a prevalence of 1 in 3,500 individuals is the most common form of hereditary retinal disorder affecting the working age group. RP is characterized by progressive dysfunction and death of mainly the rod photoreceptor cells (PR) of the retina however in some cases retinal pigment epithelium (RPE) cells are also involved, often resulting in permanent blindness. So far 54 genes have been implicated in this disease coding for proteins involved in a myriad of functions such as phototransduction signaling cascade, retinoid cycle, cell-cell adhesion or the cytoskeleton 1 . The disease is inherited in all there Mendelian forms, the autosomal recessive (arRP) being the most common with over 50% of cases. Largely due to the high genetic heterogeneity and unavailability of disease tissue, pathology of the disease remains elusive. Patient-derived induced pluripotent stem cells (iPSCs) provide an unprecedented opportunity to recapitulate disease pathogenicity without the need for genetic manipulation and creation of gene targeted animal models. Human iPSCs, similar to embryonic stem cells (ESC), can be expanded indefinitely in vitro and differentiated into any type of mature cell in the human body, without the ethical and immunogenicity issues associated with ESC 2 . These cells are also valuable for developing therapeutic strategies, drug toxicity screens and development of disease models, in addition to providing a source for cell transplantation therapy. RPE cells and photoreceptors (PR) have been successfully generated from iPSCs (iPSC-RPE and iPSC-PR respectively) by various groups in stepwise differentiation protocols mimicking retinal development by introducing Wnt signaling inhibitors (DKK1), Nodal antagonist Lefty A, Notch pathway inhibitor (DAPT-gamma secretase inhibitor), or IGF-1 3 , 4 , 5 . In contrast, only RPE cells have been generated spontaneously in overgrown iPSC/ESC cultures without the addition of exogenous factors, since derivatives of neuroectoderm appear by default in non-induced cultures 6 , 7 . Generated RPE cells in these studies display a fully mature phenotype and physiological activity in vitro such as phagocytosis, secretion of vascular endothelial growth factor (VEGF) and pigment epithelium-derived factor (PEDF) and epithelial barrier formation. Cellular models of hereditary retinal dystrophies have been successfully created in vitro in Best disease and RP where patients’ fibroblasts were reprogrammed to iPSC and then converted to RPE 8 , 9 or photoreceptor cells 10 , expressing the disease phenotype. iPSC- derived RPE (iPSC-RPE) cells have also been shown to have a protective effect when injected sub-retinaly into the Royal College of Surgeons (RCS) rats 11 and RPE65-defective mice 12 . Moreover, iPSCs have met clinical-grade requirements 13 as a source of RPE grafts and have recently been injected in patients affected by the exudative form (wet-type) of age-related macular degeneration (AMD) 14 . It has been argued that in this form of AMD the dysfunction and loss of RPE cells is the main cause of visual impairment in the elderly. Mer tyrosine kinase receptor (MERTK) belongs to the Tyro3/Axl/Mer (TAM) receptor tyrosine kinase family of proteins distinguished by a conserved intracellular kinase domain and extracellular adhesion molecule-like domain. TAM receptors regulate a variety of processes such as cell proliferation/survival, adhesion, migration, inflammatory response, in a cell- microenvironment- and ligand- specific manner 15 . In previous studies MERTK was found to be disrupted in RCS rats 16 , 17 , a classic model for retinal degeneration inherited as an autosomal recessive trait and found to cause early-onset retinitis pigmentosa in patients 18 . RPE cells fail to phagocytize the shed outer segment (OS) material of PR, a circadian activity performed by RPE cells which serves to renew the damaged lipid and protein components of light exposed PR, while new membranous discs are formed (disc biogenesis) and inserted in the basal part of the OS. As a result, RCS rats exhibit OS associated debris accumulation in the subretinal space, abnormal OS length, eventually leading to the onset of PR degeneration by the P20 stage. Usually complete degeneration occurs by P60. Similar phenotype is observed in merkd mice 19 indicating that the RPE phagocytic defect is the underlying molecular mechanism of disease in humans carrying MERTK mutations. Indeed, the reduced retinal thickness and debris detected in the sub-retinal space in patients harboring the MERTK –splice-site -mutation resembles the observed phenotype in the RCS rat 20 . The distinctive clinical presentation of RP is the only disease manifestation of patients harboring MERTK mutations without any systemic disease or defects of phagocytosis by macrophages, indicating a specialized function of this protein in the RPE cells. In contrast to the detailed clinical understanding of the disease, the mechanism by which MERTK acts during the phagocytosis remains partially unveiled. Outer segments are known to bind to the integrin receptor αvβ5 21 followed by focal adhesion kinase (FAK) activation in the apical membrane of RPE 22 while the MERTK activation occurs via Gas6/Protein S, TUB, TULP1 ligand binding 23 , 24 . The latter is thought to activate autophosphorylation at tyrosine Y-749, Y-753 and Y-754 in the tyrosine kinase domain, which in turn activates the molecular cascade targeting actin or non-muscle myosin II to coordinate the cytoskeletal rearrangements necessary for phagocytic ingestion 25 . Click here to read entire article References Hartong, D. 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