Role of a Dual Splicing and Amino Acid Code in Myopia, Cone Dysfunction and Cone Dystrophy Associated with L/M Opsin Interchange Mutations

Purpose: Human long (L) and middle (M) wavelength cone opsin genes are highly variable due to intermixing. Two L/M cone opsin interchange mutants, designated LIAVA and LVAVA, are associated with clinical diagnoses, including red-green color vision deficiency, blue cone monochromacy, cone degeneration, myopia, and Bornholm Eye Disease. Because the protein and splicing codes are carried by the same nucleotides, intermixing L and M genes can cause disease by affecting protein structure and splicing. Methods: Genetically engineered mice were created to allow investigation of the consequences of altered protein structure alone, and the effects on cone morphology were examined using immunohistochemistry. In humans and mice, cone function was evaluated using the electroretinogram (ERG) under L/M-or short (S) wavelength cone isolating conditions. Effects of LIAVA and LVAVA genes on splicing were evaluated using a minigene assay. Results: ERGs and histology in mice revealed protein toxicity for the LVAVA but not for the LIAVA opsin. Minigene assays showed that the dominant messenger RNA (mRNA) was aberrantly spliced for both variants; however, the LVAVA gene produced a small but significant amount of full-length mRNA and LVAVA subjects had correspondingly reduced ERG amplitudes. In contrast, the LIAVA subject had no L/M cone ERG. Conclusions: Dramatic differences in phenotype can result from seemingly minor differences in genotype through divergent effects on the dual amino acid and splicing codes. Translational Relevance: The mechanism by which individual mutations contribute to clinical phenotypes provides valuable information for diagnosis and prognosis of vision disorders associated with L/M interchange mutations, and it informs strategies for developing therapies.