A rare, debilitating and untreatable gene disorder, choroideremia (CHM) is caused by a defective gene on the X chromosome. Mike was in his early 40s when he experienced a rapid diminishment in his visual field and visual acuity. Having been born with CHM, he knew that blindness was approaching and his independence was slipping away. Then, in 2011, he was invited to participate in a gene therapy trial for CHM.

The fault in the CHM gene – which leads to a reduction in Rab escort protein 1 (REP1) typically manifests symptomatically in the late teens or early 20s, initially affecting peripheral vision, then gradually encroaching on the central vision. With a prevalence of one in 50,000 (Zinkernagel and MacLaren, 2015), it is a rare disease, and part of a spectrum of genetic eye diseases grouped under the term “rod-cone dystrophies”. Similar genetic eye diseases such as retinitis pigmentosa, with which CHM is often confused (Hiland, 2018), have now become the most common cause of untreatable blindness in young people (Nuffield Department of Clinical Neurosciences, 2018).