Assessment of the relationship between CHM gene expression and rate of disease progression in choroideremia

Abstract

Purpose : As gene therapy for choroideremia has demonstrated promising early clinical outcomes, it is becoming increasingly important to develop biomarkers that might help to identify patients at risk of rapid disease progression. Skin fibroblasts can be used as an ex vivo model of the disease since the CHM gene is ubiquitously expressed. This project explores potential correlation between gene expression in choroideremic fibroblasts and disease progression rate.

Methods : Fourteen choroideremia patients (with point mutations, minor deletions/insertions and >1 exon alterations) underwent autofluorescence (AF) imaging and a skin biopsy (UK national research ethics approval 15/WA/0087). Dermal tissue from all patients and 14 controls were cultured in vitro and primary fibroblasts were isolated and propagated for up to eight weeks, and characterized by indirect immunofluorescence staining against vimentin. Fibroblasts RNA was extracted at passage 2 and cDNA synthesized for gene expression analysis. The expression of CHM and 3 other genes involved in the prenylation machinery, CHML, RABGGTB and RAB27A was analysed by quantitative PCR. The levels for REP1 protein expression in each sample were also detected by western blot using antibodies against both the N- and the C-terminus of the protein.

Results : The areas of AF in the choroideremia patients were plotted against age, thereby allowing estimation of the individual disease progression rate against the mean rate of disease progression obtained from a large cohort of choroideremia patients. As expected the majority of choroideremia patients showed lower levels of CHM than controls, but no clear association was identified between gene expression and estimated disease progression rate. One patient with a novel non-contiguous duplication of exons showed similar levels of CHM to controls. None of the other genes analysed showed an association between expression and estimated disease progression rate. Analysis of REP1 protein levels showed no protein expression or low, residual expression for some of the mutations assessed.

Conclusions : The data showed no obvious correlation between CHM-related gene expression and estimated disease progression rate. This suggests that REP1 protein expression level may be more relevant clinically, or alternatively other potential modifier genes are involved in determining choroideremia disease progression.

This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.