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Choroideremia (CHM) is characterized by progressive degeneration of the choroid, retinal pigment epithelium (RPE), and neural retina. The incidence of CHM is estimated at 1 in 100,000. Typically, affected males develop nightblindness in their teenage years, followed by progressive constriction of visual fields and complete blindness by middle age. Fundus changes initially consist of pigmentary stippling and fine atrophy of the RPE in the posterior and equatorial regions. There is also focal atrophy of the choriocapillaris and the larger choroidal vessels around the optic disc and in the equatorial area. Later, the atrophy of the choroid, RPE and retina spreads from the midperiphery inwards and from the disc outwards, whereas the macula is relatively spared. The fundus has a white appearance. The disease it transmitted as an X-linked recessive trait and female carriers generally show no serious visual impairment, but they do have conspicuous fundus abnormalities such as pigment changes in the periphery closely resembling the fine mottling that is characteristic of the initial stages of the disease in males. Most frequently, clinical signs of CHM in females can be attributed to skewed X-inactivation. Exceptionally, homozygosity or disruption of the CHM gene by X-autosome translocations may also result in manifestation of the disease in females. The gene for CHM has been localized to Xq21.2. A closely related gene, the CHM-like (CHML) gene, has been identified on chromosome 1q. The CHM and CHML genes encode Rab escort protein-1 (REP1) and -2 (REP2) respectively, which are essential for post-translational lipidation (prenylation) and subcellular localization of intracellular protein trafficking regulators designated Rab GTP-binding proteins. Chorioretinal degeneration is thought to result from deficient prenylation of Rab proteins in the choroid and/or retina. No treatment is currently available but ongoing studies are focusing on gene therapies. Management is based on low-vision aids. |