Myosin 6 Is Required for Iris Development and Normal Function of the Outer Retina

PURPOSE. To determine the molecular basis and the pathologic consequences of a chemically induced mutation in the translational vision research models 89 (tvrm89) mouse model with ERG defects. METHODS. Mice from a G3 N-ethyl-N-nitrosourea mutagenesis program were screened for behavioral abnormalities and defects in retinal function by ERGs. The chromosomal position for the recessive tvrm89 mutation was determined in a genome-wide linkage analysis. The critical region was refined, and candidate genes were screened by direct sequencing. The tvrm89 phenotype was characterized by circling behavior, in vivo ocular imaging, detailed ERG-based studies of the retina and RPE, and histological analysis of these structures. RESULTS. The tvrm89 mutation was localized to a region on chromosome 9 containing Myo6. Sequencing identified a T -> C point mutation in the codon for amino acid 480 in Myo6 that converts a leucine to a proline. This mutation does not confer a loss of protein expression levels; however, mice homozygous for the Myo6(tvrm89) mutation display an abnormal iris shape and attenuation of both strobe-flash ERGs and direct-current ERGs by 4 age weeks, neither of which is associated with photoreceptor loss. CONCLUSIONS. The tvrm89 phenotype mimics that reported for Myosin6-null mice, suggesting that the mutation confers a loss of myosin 6 protein function. The observation that homozygous Myo6(tvrm89) mice display reduced ERG a-wave and b-wave components, as well as components of the ERG attributed to RPE function, indicates that myosin 6 is necessary for the generation of proper responses of the outer retina to light.