ERG abnormalities in relation to histopathologic findings in vitiligo mutant mice.

The vitiligo, mivit, mutation has several prenatal and perinatal effects on development of the retinal pigment epithelium, and later, leads to extensive, progressive degeneration of photoreceptor cells in the neural retina of homozygous affected mice. The aim of the present study was to determine by functional criteria how early can abnormalities be detected in the neural retina. Electroretinograms (ERGs) were correlated with histopathological findings in the same animals. Congenic homozygous mutants, heterozygotes, and homozygous wild-type mice were studied at 2, 3, 6, 24 and 56 weeks of age, the same animals being tested serially at the three older time points. The nontested eye of each animal was embedded in Epon and sectioned at 1 micron for light microscopic study. ERG recordings from vitiligo homozygotes differed from heterozygous and wild-type mice, but the latter two groups did not differ from each other. As early as two weeks of age, homozygous mutants showed a significant reduction of rod dominated maximum ERG a-wave and b-wave amplitude. ERG b-wave sensitivity (sigma) was significantly reduced, and ERG implicit times were delayed for homozygous mutants at 3 (a-wave) and 6 (b-wave) weeks of age. This is the first study to report reduced and delayed ERG a-waves and b-waves in this animal model, like the early functional abnormalities in human retinitis pigmentosa, and also the first to show short and disoriented rod outer segments, beginning retinal separation from the pigment epithelium, and a few macrophage-like cells already present in the subretinal space at 2 weeks of age (in three of four homozygous mutant eyes examined). Given these early functional and structural abnormalities in the neural retina, it remains to be determined whether the mi gene targets the retinal pigment epithelial cell, the photoreceptor cell, or both.