Biallelic mutations in the autophagy regulator DRAM2 cause retinal dystrophy with early macular involvement.

Abstract

Purpose: To investigate the molecular basis of an atypical adult-onset retinal dystrophy with initial macular degeneration in a consanguineous British family of Pakistani origin where the affected members develop central visual loss in the third decade of life.

Methods: Genetic analysis was performed by autozygosity mapping, with Affymetrix 250K arrays, and whole-exome sequencing (WES) using SureSelectXT Human V4 target enrichment reagent followed by paired-end sequencing on the HiSeq2000. Mutation screening was performed by interrogating existing WES data and/or Sanger sequencing. Confocal immunofluorescence microscopy was completed with polyclonal antiserum on mouse eye sections.

Results: Homozygosity mapping with genomic DNA from seven affected family members identified 2 common regions; a 10.1Mb interval on chromosome 1 and a 2.9Mb region on chromosome 7. WES of the index case identified only one homozygous variant after the filtering process that overlapped with the regions; a single-base deletion in DRAM2 (DNA-damage regulated autophagy modulator protein 2), c.140delG, p.Gly47Valfs*3, that segregated with the disease phenotype and was absent from 159 ethnically matched controls. Sanger sequencing of DRAM2 identified one subject with compound heterozygous changes, c.494G>A, p.Trp165* and c.131G>A, p.Ser44Asn. A gene-based, case-control association study using WES on a group of 18 probands with recessive macular dystrophy and 1,917 controls, highlighted DRAM2 as the most significantly enriched mutated gene; one subject was homozygous for c.362A>T, p.His121Leu and another was compound heterozygous for c.79T>C, p.Tyr27His and c.217_225del, p.73_75del. The clinical features and course of retinal degeneration were highly similar amongst affected cases from the four families. DRAM2 is a transmembrane protein that has previously been localized to lysosomes and has been implicated in inducing autophagy. Immunohistological analysis on retinal cross-sections showed DRAM2 localisation to photoreceptor outer segments where lysosomes are absent and suggests a novel role for DRAM2 in the retina.

Conclusions: We have shown biallelic missense, nonsense and frameshift mutations in DRAM2 cause retinal dystrophy with early macular involvement. Our findings suggest that DRAM2 is essential for photoreceptor survival and further studies should provide insights into its role in photoreceptor outer segments.