ARR3 variant-induced cone mosaicism alters cone subtype composition and disrupts phototransduction.

Heterozygous variants in ARR3, encoding cone arrestin, have emerged as a frequent cause of early-onset high myopia with a unique X-linked female-limited inheritance pattern. However, the mechanistic basis for this unusual anti-X-linked pattern is still unclear. Developmental expression profiling in mice demonstrated robust Arr3 expression in the retina from postnatal day 14 onward, with localization confined predominantly to outer segments of cones marked by red/green opsins, including a subset co-labeled with both red/green and blue opsins. Retinal flatmounts from Arr3 mutation knock-in mice and Arr3 knockout rats revealed a mosaic pattern of Arr3 expression in heterozygous individuals. Retinal single-cell RNA sequencing revealed significant shifts in cone subtype proportions in Arr3 +/- rats, with a marked reduction in M/S cones and a corresponding increase in S cones. Among differentially expressed genes, Pde6h was the only transcript altered in M/S cones across both Arr3 +/+ vs. Arr3 +/- and Arr3 -/0 vs. Arr3 +/- comparisons but not in Arr3 +/+ vs. Arr3 -/0 . These findings suggest that heterozygous Arr3 deficiency induces cone mosaicism that may mimic retinal defocus-like signals during phototransduction, potentially driving the development of high myopia under this distinctive inheritance model.