Translational bypass therapy using ataluren to treat nonsense-mediated choroideremia

Abstract

Purpose: Choroideremia is an X-linked recessive chorioretinal dystrophy with an incidence of 1:50,000. Over one-third of choroideremia patients harbour nonsense mutations in the CHM gene, hence translational bypass therapy may be a viable treatment option. Novel small molecule drugs such as ataluren can suppress disease-causing premature stop mutations, thus partially restoring the functional protein. The choroideremia zebrafish (chm-ru848) is the only animal model with a nonsense mutation in the chm gene and displays similar ocular phenotypic features to patients. The aim of this investigation is to identify a safe and effective treatment for choroideremia.

Methods: Mutant chm-/- embryos were treated with 10 µM ataluren from 10 hours post-fertilization. The chm mutation causes embryonic lethality (mean survival 4.8 ± 1.0 days); survival was determined following ataluren treatment (n=63). Retinal ultrastructure was examined using electron microscopy at day 6 and 10. Levels of oxidative stress and apoptosis in the retina were measured using dihydroethidium staining and TUNEL assays, respectively. The presence of functional rep1 protein was assayed by western blot analysis and in vitro prenylation tests. To complement this work, ataluren-mediated readthrough was also assayed on fibroblasts derived from choroideremia patients with nonsense mutations.

Results: Ataluren-treated chm-/- embryos lived 2.0 fold longer (mean survival 10.1 ± 1.6 days); p<0.0001. There was significant rescue of retinal degeneration at day 6 and 10, with reduced levels of apoptotic cell death and oxidative stress in treated chm-/- embryos. In ataluren-treated CHM-/y fibroblasts, although western blot analysis was not sensitive enough to detect a REP1 protein, preliminary results suggest a partial restoration of prenylation function.

Conclusions: This study is the first step towards a ‘proof-of-concept’ for using ataluren as translational bypass therapy for treating nonsense-mediated choroideremia. Affected males develop night blindness in their childhood with progression to complete loss of vision in late adulthood. Thus, early administration could arrest disease progression and even prevent this form of genetic blindness. It may also be applicable to other inherited eye disorders with a similar genetic aetiology.

Keywords: 696 retinal degenerations: hereditary • 503 drug toxicity/drug effects • 539 genetics