Molecular Genetics of Choroideremia

Abstract

Abstract Choroideremia (CHM) is an X‐linked disorder that causes progressive loss of vision. Affected males initially present with nyctalopia (night blindness) and experience progressive loss of peripheral vision until they are legally blind by middle age. Degeneration continues until there is a complete loss of vision. CHM is caused by mutations in the CHM gene, which encodes Rab escort protein (REP) 1. Molecular genetic techniques have identified more than 100 mutations affecting in the CHM gene and all but one results in the absence of REP1 protein. The degenerative process of CHM initially affects rod photoreceptors and the retinal pigment epithelium (RPE), then progresses to involve the choroid and cone photoreceptors. The exact pathophysiological mechanism of this degradation has not yet been elucidated. There is no established treatment for CHM available at this time but gene therapy trials are currently underway. Key Concepts: Choroideremia is a progressive degenerative disorder affecting the photoreceptor, retinal pigment epithelium and choroid layers of the retina. Mutations in the CHM gene result in the absence of REP1 protein, causing low levels of prenylation of RAB proteins. Confirmatory testing for CHM can be done by direct sequencing of the CHM exons or through immunoblot analysis for CHM protein expression. Zebrafish and mouse CHM knock‐out models have significant limitations due to the lack of viability. A conditional knock‐out mouse model has been developed and used in pre‐clinical testing of CHM therapies. Successful techniques in gene therapy for Leber congenital amaurosis‐2, another single gene disorder of blindness, are currently being applied to choroideremia.