Improving Diagnostic Detection Strategies of Choroideremia

Abstract

Choroideremia is an X-linked monogenic inherited retinal disease. It affects males starting in their teenage years with night blindness followed by progressive vision loss starting in the peripherals and ending with total vision loss late in life. It is estimated that 1 in 50,000 individuals are affected by this genetic disorder. Female carriers are usually asymptomatic although some may present with an uneven degenerative fundus. With clinical gene therapy trials underway in locations across the world, the molecular diagnosis of choroideremia is increasingly important. In order for a patient to be enrolled in a gene therapy trial, an unequivocal diagnosis of choroideremia is a prerequisite. In the past, research labs performed immunoblot analysis to demonstrate a lack of Rab Escort Protein-1 (REP-1) in patients phenotypically suggestive of choroideremia. Now, exon sequencing of the causative gene, CHM, is a more cost effective and reliable diagnostic tool. However, the sensitivity of exon sequencing is not 100% and occasionally individuals will lack expression of the REP-1 protein, but present no identifiable defect in the coding sequence of the gene. It is important to investigate such patients and examine for further alterations. In this study, we discovered a novel splice mutation, c.1245-521A>G, in two unrelated individuals. In addition, we report, the first inversion mutation in CHM, c.-839_49+5528inv. A fourth patient was determined to be a CHM phenocopy; he bore no mutations in the CHM gene and was eventually found to be REP-1 positive. He instead underwent whole exome sequencing (WES) with both his parents to determine candidate genes for his ocular choroideremia-like phenotype. In conclusion, novel non-coding mutations were determined to be responsible for choroideremia observed in three individuals. Further work is required on the possible pathogenicity of the candidate genes identified in the fourth patient in order to determine the variant responsible for his unique ocular phenotype. These findings suggest the importance of sequencing non-coding regions of the CHM gene in order to improve diagnostic sensitivity and provide molecular diagnoses to patients looking for enrolment in a gene therapy trial.