Thioredoxin reductase activity in Hermansky-Pudlak syndrome: a method for identification of putative heterozygotes

Recent studies indicate that membrane-associated thioredoxin reductase (TR) is a possible regulator of melanin biosynthesis via the inhibition of tyrosinase by reduced thioredoxin. In normal individuals, the levels of TR activity in skin correlate linearly to the Fitzpatrick classification of skin type, being lowest in type I skin and highest in skin type VI. In this study, TR was measured in 3-mm skin biopsies in Hermansky-Pudlak syndrome (HPS) patients and their relatives. Forty-five individuals from seven Puerto Rican kindreds were tested, including 12 homozygotes, nine obligate heterozygotes, and 24 unclassified individuals. In addition, seven separate nonkindred HPS patients were tested. With one exception, TR activity was markedly decreased in 18 homozygotes. TR activity was decreased in eight obligate heterozygotes and in 12 unclassified kindred members, whereas 10 subjects had normal TR activity when compared to the expected activity of their skin type. Four individuals were excluded from the analysis because of inadequate controls for their age group or immunosuppressive treatment for kidney transplant. The results indicate that decreased TR activity assayed in 3-mm skin punch biopsies is a useful method for detecting carriers of the HPS gene.