ABCA4, a member of the ATP-binding cassette (ABC) transporter superfamily, is preferentially located along the rim region of rod and cone photoreceptor outer segment disc membranes. It uses ATP-binding and hydrolysis energy to transport N-retinylidene-phosphatidylethanolamine from the lumen to the cytoplasmic leaflet of disc membranes, clearing all-trans-retinal and excess 11-cis-retinal from photoreceptor cells to prevent toxic retinoid compound accumulation. This process is crucial for the visual cycle [2].

Loss-of-function mutations in ABCA4 cause autosomal recessive Stargardt macular degeneration (STGD1) and related retinopathies. These range from early-onset and fast-progressing cone-rod dystrophy and retinitis pigmentosa-like phenotypes to very late-onset, mostly mild diseases sometimes confused with age-related macular degeneration. Over 1200 disease-causing mutations of various types and severities have been identified in the ABCA4 locus [1]. In a German cohort of 335 STGD1 patients, 148 pathogenic or likely pathogenic mutations were found, including 48 novel ones, along with six risk-modulating common variants [3]. A Chinese cohort study of 129 patients with ABCA4-associated retinal dystrophies reported 35 novel mutations and found genotype-phenotype correlations, such as patients with two "null" variants having an earlier onset age and reaching legal blindness earlier [4].

In conclusion, ABCA4 is essential for the normal function of the visual cycle by preventing the build-up of toxic retinoid compounds in photoreceptor cells. Studies on ABCA4-associated diseases, especially those related to its loss-of-function mutations, have enhanced our understanding of the molecular mechanisms underlying these retinal disorders, which may contribute to the development of therapeutic strategies for treating ABCA4-related retinopathies.