OCA2 deficiency enhances TPC2 channel activity to reduce melanosomal pH and pigment production.

Albinism is a heterogeneous inherited disorder with at least 21 known causative genes. Clinically, interpreting the potential digenic or oligogenic effects of the variants is a big challenge. We identified digenic mutations in a Chinese individual with oculocutaneous albinism, carrying a loss-of-function mutation in the Cl- (chloride) channel gene OCA2 (c.808-3C>G) and an unreported gain-of-function mutation in the Na+/Ca2+ channel gene TPCN2 (p.Ala24Val), which was confirmed by patch-clamp analysis. We further demonstrated that Cl- bilaterally modulated TPC2 activity by patch clamping: cytosolic high Cl- inhibited but luminal high Cl- enhanced TPC2 channel activity. Using CRISPR/Cas9-mediated knockout cell models and knockin mouse models, we confirmed that OCA2 modulated TPC2 activity by influencing melanosomal pH and pigment production. Mice mimicking double heterozygotes for the OCA2 loss-of-function p.Val443Ile and the TPCN2 gain-of-function p.Arg210Cys exhibited synergistic hypopigmentation in both fur and retina, phenocopying the albinism patient with digenic loss-of-function OCA2 and gain-of-function TPCN2 heterozygous mutations. This study provides experimental evidence supporting an oligogenic mode of inheritance in albinism, pinpoints melanosomal ion homeostasis as a key pathogenic mechanism and potential therapeutic target, and establishes a generalized research framework for elucidating gene-gene interactions in heterogeneous inherited disorders, especially for the channelopathies.