Molecular Genetic Elucidation of Inherited Retinal Diseases

Abstract

Molecular Genetic Elucidation of Inherited Retinal Diseases Inherited retinal dystrophies are characterized by gradual loss of vision due to underlying degeneration of light sensitive photoreceptors or the surrounding retinal pigment epithelium. Clinical subtype of RD called Retinitis pigmentosa involves progressive degeneration of Rod photoreceptor cells earlier in life than the cones cells, and represents one of the most common forms. As rod cells are sensitive to dim light the patients first experience night vision problems but might eventually end up with complete blindness as the degeneration prevails. Of the inherited forms of RP the autosomal recessive inheritance pattern is most common and accounts for more than 50% of cases. RP is genetically heterogeneous, where arRP is at the extreme as mutations in about 40 genes are known to cause same phenotype. Despite large number of genes associated with arRP these genes only explain about 60% of the cases and hence the further research is warranted to find out the missing pieces. In three families, missense mutations were identified in TULP1, which include p.(Thr380Ala), p.(Arg482Gln), and p.(Lys489Arg). In three arRP families protein- truncating mutations were identified in ABCA4 p.(Gln2220*), AIPL1 p.(Trp278*) and CERKL p.(Arg283*). In three families novel missense mutations were identified in different genes, which include CNGA1 (p.(Gly433Asp), EYS (p.(Asp2767Tyr) and PDE6A (p.(Arg544Trp). A novel splice site mutation (c.2493-2A>G; p.(?), was identified in CNGB1 in a family with arRP. In five other unrelated arRP families, previously reported mutations were identified, which include two families with a missense mutation p.(Glu150Lys) in RHO, two families with mutations p.(Arg44Gln) and p.(Ser121Leufs*6) in RPE65, and one family with a mutation p.(Thr745Met) in CRB1. In an X-linked RP family the causative mutation p.(Glu809Glyfs*25) was identified in RPGR. A splice site mutation c.488-1G>A; p.(?) in IQCB1 was identified in a family with syndromic RP. Two families with fundus albipunctatus were identified to have a frameshift p.(Val305Hisfs*29) and a missense p.(Met253Arg) mutation in RDH5 gene. xiiIn an arRP family Exome next generation sequencing (NGS) helped to identify a plausible pathogenic variant c.3269G>A; p.(Arg1090Gln) in SNRNP200, a gene previously found to be mutated in autosomal dominant RP (adRP). We hypothesize that the mutation identified in the Pakistani arRP family represents a hypomorphic variant but further experiments are warranted to prove the causality of the mutation. An overlapping homozygous region was identified that was shared between all the affected individuals of two arRP families. The region encompasses CLRN1, a gene previously found to be mutated in Usher syndrome type III. Two novel missense mutations p.(Pro31Leu) and p.(Leu154Trp) were identified, which were proven to be pathogenic. Hence a novel genotype-phenotype correlation was established for CLRN1. Taking together, sequence variants were identified in 32 of 41 (78%) families of our Pakistani RD cohort, which very likely explains the retinal phenotypes. In addition, we were able to identify nine potential novel arRP loci, which are likely to harbor novel retinal disease gene.