Monoallelic ABCA4 Mutations Appear Insufficient to Cause Retinopathy: A Quantitative Autofluorescence Study

PURPOSE: To investigate the effect of ABCA4mutation status on lipofuscin-related quantitative autofluorescence(qAF) in humans and on bisretinoid accumulation in mice. METHODS: Genotyped parents (n = 26; age 37-64 years) of patients with biallelic ABCA4-related retinopathy underwent in-depth retinal phenotyping including qAF imaging as a surrogate measure for RPE lipofuscinaccumulation. In addition, bisretinoids as the main components of autofluorescent lipofuscinat the ocular fundus were quantified in Abca4-/-, Abca4+/-, and wild-type mice. RESULTS: Index patients showed a retinal phenotype characteristic for ABCA4-related retinopathy, including increased qAF levels. In contrast, qAF measures in carriers of only one ABCA4mutation were not different from age-matched controls in this sample, and there was no difference between truncating and missense mutations. Also, none of these carriers presented an abnormal phenotype on conventional imaging. One parent with ABCA4-related retinopathy and increased qAF carried an additional ABCA4mutation, explaining the phenotype under a recessive disease model ( pseudodominance). Biochemical analysis in the mouse model revealed direct downstream products (A2PE-H2, at-RALdimer-PE) of the ABCA4substrate N-Ret-PE to be similar in wild-type and Abca4+/- mice. Both bisretinoids were 12- to 18-fold increased in Abca4-/- mice. Levels of A2E and A2PE in Abca4+/- mice were in between those measured in wild-type and Abca4-/- mice. CONCLUSIONS: This study indicates that carriers of monoallelic ABCA4mutations are phenotypically normal. However, biochemical analysis in the Abca4-deficient mouse model suggests detectable effects of one mutation in ABCA4on the molecular level. The findings may have implications for therapeutic approaches such as gene replacement therapy.