The farnesylated small GTPase RAB28 is mutated in autosomal recessive cone-rod dystrophy

Abstract

Purpose: The majority of genetic causes for autosomal recessive (ar) cone-rod dystrophy (CRD) are currently unknown. Therefore, we employed a combined approach of homozygosity mapping and exome sequencing to identify new genes for arCRD.

Methods: In a German arCRD family with three affected siblings, homozygosity mapping was performed using Affymetrix 250K SNP microarrays. DNA samples of two affected individuals underwent exome sequencing using Agilent’s SureSelect Human All Exon v.2 Kit on a SOLiD4 sequencing platform. Sanger sequencing of the RAB28 gene was performed in 617 additional unrelated individuals with CRD or cone dystrophy, and in families with conspicuously large homozygous regions assessed in SNP data available through the European Retinal Disease Consortium. Identified mutations were screened in ethnically matched controls. Ophthalmic examinations included ERG, perimetry, OCT, FAF, and fundus photography.

Results: Exome sequencing revealed a homozygous nonsense mutation in RAB28 (c.565C>T;p.Q189*) in all three affected individuals of the German arCRD family. In addition, a homozygous nonsense mutation (c.409C>T;p.R137*) was identified in two affected members of a consanguineous arCRD family of Moroccan Jewish ancestry. Both mutations were not identified in 176 and 118 ethnically matched controls, respectively. The five affected individuals of both families presented with hyperpigmentation in the macula, progressive loss of the visual acuity, atrophy of the retinal pigment epithelium, and severely reduced cone and rod responses on the electroretinogram.

Conclusions: RAB28 encodes a member of the Rab subfamily of the RAS-related small GTPases. Alternative RNA splicing yields three predicted protein isoforms with alternative C-termini, which are all truncated by the nonsense mutations identified in this study. Opposed to other Rab GTPases which are generally geranylgeranylated, RAB28 is predicted to be farnesylated. Interestingly, mutations in AIPL1, encoding a chaperone of farnesylated proteins, were previously found in individuals with ar Leber congenital amaurosis and autosomal dominant CRD. Analogous to the function of other RAB family members, RAB28 might be involved in ciliogenesis and/or opsin transport in photoreceptor cells. This study reveals a crucial role for RAB28 in photoreceptor function, and suggests that mutations in other Rab proteins may also be associated with retinal dystrophies.

Keywords: 695 retinal degenerations: cell biology • 539 genetics • 660 proteins encoded by disease genes