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Choroideremia and retinal gene therapy: initial findings from a phase 1/2 clinical trial

Choroideremia is an X-linked recessive disease that leads to blindness due to mutations in the CHM gene, which encodes the Rab escort protein 1 ( REP1 ).
Researchers have assessed the effects of retinal gene therapy with an adeno-associated viral ( AAV ) vector encoding REP1 ( AAV.REP1 ) in patients with this disease.

In a multicentre clinical trial, six male patients ( aged 35-63 years ) with choroideremia were administered AAV.REP1 ( 0.6-1.0×1010 genome particles, subfoveal injection ).
Visual function tests included best corrected visual acuity, microperimetry, and retinal sensitivity tests for comparison of baseline values with 6 months after surgery.

Despite undergoing retinal detachment, which normally reduces vision, two patients with advanced choroideremia who had low baseline best corrected visual acuity gained 21 letters and 11 letters ( more than two and four lines of vision ).
Four other patients with near normal best corrected visual acuity at baseline recovered to within one to three letters.
Mean gain in visual acuity overall was 3.8 letters.

Maximal sensitivity measured with dark-adapted microperimetry increased in the treated eyes from 23.0 dB at baseline to 25.3 dB after treatment ( increase 2.3 dB ).

In all patients, over the 6 months, the increase in retinal sensitivity in the treated eyes ( mean 1.7 ) was correlated with the vector dose administered per mm2 of surviving retina ( r=0•82, p=0.04 ).

By contrast, small non-significant reductions ( p greater than 0.05 ) were noted in the control eyes in both maximal sensitivity ( -0.8 dB ) and mean sensitivity ( -1.6 dB ).

One patient in whom the vector was not administered to the fovea re-established variable eccentric fixation that included the ectopic island of surviving retinal pigment epithelium that had been exposed to vector.

The initial results of this retinal gene therapy trial are consistent with improved rod and cone function that overcome any negative effects of retinal detachment.
These findings lend support to further assessment of gene therapy in the treatment of choroideremia and other diseases, such as age-related macular degeneration, for which intervention should ideally be applied before the onset of retinal thinning. ( Xagena )

Maclaren RE et al, Lancet. 2014; Epub ahead of print