Genetics and Disease Expression in the CNGA3 Form of Achromatopsia: Steps on the Path to Gene Therapy

Abstract

Purpose: Achromatopsia (ACHM) is a congenital, autosomal recessive retinal disease that manifests cone dysfunction, reduced visual acuity and color vision, nystagmus, and photoaversion. Five genes are known causes of ACHM. The present study took steps toward performing a trial of gene therapy in ACHM by characterizing the genetics of ACHM in Israel and the Palestinian Territories and analyzing retinal function and structure in CNGA3 ACHM patients from the Israeli-Palestinian population and US patients with other origins.

Design: Case series study.

Participants: Patients with clinically suspected ACHM, cone dysfunction phenotypes, and unaffected family members were included. The protocol was approved by the local institutional review board and informed consent was obtained from all participants.

Methods: Genetic analyses included homozygosity mapping and exome sequencing. Phenotype was assessed with electroretinography (ERG), optical coherence tomography, psychophysics, and photoaversion testing.

Main outcome measures: Single nucleotide polymorphism microarray, exome analysis, DNA sequence analysis, visual function testing including ERG, and photoaversion.

Results: We identified 148 ACHM patients from 57 Israeli and Palestinian families; there were 16 CNGA3 mutations (5 novel) in 41 families and 5 CNGB3 mutations (1 novel) in 8 families. Two CNGA3 founder mutations underlie >50% of cases. These mutations lead to a high ACHM prevalence of ∼1:5000 among Arab-Muslims residing in Jerusalem. Rod ERG abnormalities (in addition to cone dysfunction) were detected in 59% of patients. Retinal structure in CNGA3 ACHM patients revealed persistent but abnormal foveal cones. Under dark- and light-adapted conditions, patients use rod-mediated pathways. Photoaversion was readily demonstrated with transition from the dark to a dim light background.

Conclusions: Among Israeli and Palestinian patients, CNGA3 mutations are the leading cause of ACHM. Retinal structural results support the candidacy of CNGA3 ACHM for clinical trials for therapy of cone photoreceptors. Efficacy outcome measures would include chromatic light-adapted psychophysics, with attention to the photoreceptor basis of the response, and quantitation of photoaversion.