Characterization of the cone-rod dystrophy retinal phenotype caused by novel homozygous DRAM2 mutations. (October 2019)

• Record Type: Journal Article
• Authors: Abad-Morales, Víctor; Burés-Jelstrup, Anniken; Navarro, Rafael; Ruiz-Nogales, Sheila; Méndez-Vendrell, Pilar; Corcóstegui, Borja; Pomares, Esther
• Is Part Of: Experimental eye research. Volume 187(2019)
• Publication Date: 2019-10
• Subjects: Cone-rod dystrophy -- DRAM2 -- Genotype-phenotype correlations -- Inherited retinal dystrophy -- Macula -- Gene expression -- Mutation -- Next-generation sequencing; BCVA best-corrected visual acuity -- CD cone dystrophy -- CRD cone-rod dystrophy -- DRAM2 DNA Damage Regulated Autophagy Modulator 2 -- FAF fundus autofluorescence -- ffERG full-field electroretinoghraphy -- IRD inherited retinal dystrophy -- MD macular dystrophy -- NMD nonsense-mediated mRNA decay -- OCT optical coherence tomography -- ORF open reading frame -- PTC premature termination codon -- RP retinitis pigmentosa -- RPE retinal pigment epithelium -- RT reverse transcription -- STGD Stargardt disease -- WES whole exome sequencing -- WT wild-type; Ophthalmology -- Periodicals; Eye -- Periodicals; Œil -- Périodiques; Ophthalmology; Periodicals; Electronic journals; 612.8405
• DOI: 10.1016/j.exer.2019.107752
• Languages: English
• ISSNs: 0014-4835

Abstract

Abstract: Cone-rod dystrophies (CRD) are a group of Inherited Retinal Dystrophies (IRD) characterized by the primary involvement of cone photoreceptors, resulting in the degeneration of the central retina, or macula. Although there are more than 55 CRD genes, a considerable percentage of cases remain unsolved. In this context, the present study aimed to describe and characterize the phenoptype and the genetic cause of 3 CRD families from a cohort of IRD cases. Clinical evaluation in each patient was supported by a complete ophthalmological examination, including visual acuity measurement, fundus retinography, fundus autofluorescence imaging, optical coherence tomography and full-field electroretinography. Molecular diagnoses were performed by whole exome sequencing analyzing a group of 279 IRD genes, and cosegregation of the identified pathogenic variants was confirmed by Sanger sequencing. Three novel homozygous mutations in the autophagy gene DRAM2 were identified as the molecular cause of disease in the three families: c.518-1G>A, c.628_629insAG and c.693+2T>A. Clinical data revealed that the 3 patients presented a shared CRD phenotype with adult-onset macular involvement and later peripheral degeneration, although the age of onset, evolution and severity were variable. In order to characterize the transcription effects of these variants, mRNA expression studies were performed. The results showed alterations in the DRAM2 transcription, including alternative splicing forms and lower levels of mRNA, which correlated with the phenotypic variability observed between patients. For instance, frameshift mutations were related to a less severe phenotype, with circumscribed mid-peripheral involvement, and lower levels of mRNA, suggesting an activation of the nonsense-mediated decay (NMD) pathway; while a more severe and widespread retinal degeneration was associated to the inframe alternative splicing variant reported, possibly due to a malfunctioning or toxicity of the resulting protein. Following these findings, DRAM2 expression was assessed in several human tissues by semi-quantitative RT-PCR and two isoforms were detected ubiquitously, yet with a singular tissue-specific pattern in retina and brain. Altogether, although the unique retinal phenotype described did not correlate with the ubiquitous expression, the retinal-specific expression and the essential role of autophagy in the photoreceptor survival could be key arguments to explain this particular DRAM2 phenotype. Highlights: Three novel DRAM2 pathogenic mutations were identified to cause autosomal recessive adult-onset cone-rod dystrophy. The identified mutations alter the DRAM2 expression in correlation with the observed phenotypes. Frameshift mutations were related to lower levels of DRAM2 mRNA and milder mid-peripheral retinal degeneration. Inframe alternative splicing transcripts resulted in a more severe and widespread retinal involvement. Two DRAM2 isoforms are ubiquitously expressed in all human tissues, with a specific pattern detected in retina and brain.