Effect of phytanic acid on cultured retinal pigment epithelium: an in vitro model for Refsum's disease.

Refsum's disease (heredopathia atactica polyneuritiformis) is an autosomal recessive retinitis pigmentosa syndrome caused by the excessive deposition of phytanic acid in ocular tissues. It is thought that phytanic acid causes retinal degeneration either by interfering with vitamin A metabolism in the retinal pigment epithelium or by altering photoreceptor cell membrane structure. Efforts to elucidate the molecular mechanism of phytanic acid's retinal toxicity have been hampered by the rarity of human pathological specimens and by the inability to reproduce the disease in living animal models. In this study, an in vitro model for Refsum's disease was established by exposing cultured human and bovine retinal pigment epithelial cells to phytanic acid bound to bovine serum albumin at concentrations comparable to levels found in affected humans. Ultrastructural studies show that these cells exhibit morphological changes consistent with those observed in pathological specimens from patients with Refsum's disease. Biochemical assays of retinoid metabolism by cell membranes from control cells and from cells exposed to 200 microM phytanic acid demonstrate that the ability to esterify retinol and to isomerize all-trans retinoids to 11-cis retinoids remains intact despite the deposition of large amounts of phytanic acid. The work described here is strong evidence against the hypothesis that phytanic acid inhibits vitamin A metabolism in the retinal pigment epithelium, and it demonstrates the potential use of cultured retinal pigment epithelial cells in modeling this and other degenerative diseases of the retina.