Mutations in RDH12 encoding a photoreceptor cell retinol dehydrogenase cause childhood-onset severe retinal dystrophy.

We identified three consanguineous Austrian kindreds with 15 members affected by autosomal recessive childhood-onset severe retinal dystrophy, a genetically heterogeneous group of disorders characterized by degeneration of the photoreceptor cells. A whole-genome scan by microarray analysis of single-nucleotide polymorphisms (ref. 2) identified a founder haplotype and defined a critical interval of 1.53 cM on chromosome 14q23.3-q24.1 that contains the gene associated with this form of retinal dystrophy. RDH12 maps in this region and encodes a retinol dehydrogenase proposed to function in the visual cycle. A homozygous 677A-->G transition (resulting in Y226C) in RDH12 was present in all affected family members studied, as well as in two Austrian individuals with sporadic retinal dystrophy. We identified additional mutations in RDH12 in 3 of 89 non-Austrian individuals with retinal dystrophy: a 5-nucleotide deletion (806delCCCTG) and the transition 565C-->T (resulting in Q189X), each in the homozygous state, and 146C-->T (resulting in T49M) and 184C-->T (resulting in R62X) in compound heterozygosity. When expressed in COS-7 cells, Cys226 and Met49 variants had diminished and aberrant activity, respectively, in interconverting isomers of retinol and retinal. The severe visual impairment of individuals with mutations in RDH12 is in marked contrast to the mild visual deficiency in individuals with fundus albipunctatus caused by mutations in RDH5, encoding another retinal dehydrogenase. Our studies show that RDH12 is associated with retinal dystrophy and encodes an enzyme with a unique, nonredundant role in the photoreceptor cells.

MERTK arginine-844-cysteine in a patient with severe rod-cone dystrophy: loss of mutant protein function in transfected cells.

PURPOSE: Mutations in the MERTK gene are responsible for retinal degeneration in the Royal College of Surgeons (RCS) rat and are a cause of human autosomal recessive retinitis pigmentosa (RP). This study reports the identification and functional analysis of novel MERTK mutations to provide information regarding whether they are causative of severe rod-cone degeneration in a young patient. METHODS: MERTK missense variants identified by single-strand conformational polymorphism (SSCP) and sequence analysis were introduced into expression constructs and used to transfect HEK293T cells. Recombinant protein expression was assayed with anti-MERTK and anti-phosphotyrosine antibodies. Protein turnover was assayed in pulse-chase studies of 35S-methionine incorporation. Transcript levels were determined by quantitative RT-PCR. RESULTS: Three MERTK sequence variants were identified in a patient with rod-cone dystrophy: R722X in exon 16 and R865W in exon 19 on the paternal allele and R844C in exon 19 on the maternal allele. The R844C sequence change affects an evolutionarily conserved amino acid residue and was not detected in unaffected individuals. In transfected HEK293Tcells, wild-type (wt) and W865 MERTK were expressed at equivalent levels and present in the plasma membrane, stimulated tyrosine phosphorylation, and induced significant rounding of the cell bodies. In contrast, C844 MERTK was expressed at low levels and did not stimulate tyrosine phosphorylation. In addition, the relative stability of C844 MERTK was significantly less than wt in assays of protein turnover. At age 13, the patient had 20/60 and 20/200 acuities, tunnel vision of 5 degrees centrally, and a far temporal peripheral crescent bilaterally, and ERGs were nondetectable. The fundi showed bull's-eye macular atrophy and widespread RPE thinning. CONCLUSIONS: The present study reports the identification of R844C, the first putative pathogenic MERTK missense mutation that results in severe retinal degeneration with childhood onset when in compound heterozygous form with a R722X allele. The loss of function of C844 MERTK is probably due to decreased protein stability.