Identification of a complex allele in IMPG2 as a cause of adult onset vitelliform macular dystrophy

Abstract

Purpose : Inherited retinal diseases are a group of clinically and genetically heterogeneous disorders with ~270 genes involved. IMPG2 is associated with adult onset vitelliform macular dystrophy (VMD). Here, we investigated two unrelated VMD patients in order to identify the underlying genetic cause.

Methods : Whole-exome sequencing identified a rare putative causal complex allele consisting of c.3023-15T>A and c.3023G>A (p.(Gly1008Asp)) in IMPG2 in both individuals. To assess its effect, in vitro splice assays using midigenes were conducted in HEK293T and further characterization of patient-derived iPSC differentiated towards to photoreceptor precursor cells (PPCs) was performed through qPCR and RT-PCR studies.

Results : Clinical assessment showed a typical VMD-features in both patients. The results of the midigene splice assays showed that the complex allele mainly affects splicing by the generation of an in-frame deletion of 48 nt and also the skipping of multiple exons. In contrast, patient-derived PPCs showed a significant increase of out-of-frame transcripts lacking one or multiple exons compared to control. Overall, control PPCs consistently showed low levels of aberrantly spliced IMPG2 transcripts that were highly elevated in patient-derived PPCs. These differences were even more obvious upon inhibition of nonsense-mediated decay with cycloheximide.

Conclusions : We report a heterozygous complex allele in IMPG2 causative for adult-onset VMD in two unrelated individuals with mild visual loss and bilateral vitelliform lesions. The predicted causal missense mutation c.3023G>A, located in the consensus splice acceptor site, enhances the splicing effect of the upstream variant c.3023-15T>A, leading to the generation of aberrant transcripts that decrease the full-length IMPG2 levels. These results suggest an haploinsufficiency mechanism of action and highlights the importance of using different models to functionally assesses splicing defects.

This abstract was presented at the 2022 ARVO Annual Meeting, held in Denver, CO, May 1-4, 2022, and virtually.