BACKGROUND: Oculocutaneous albinism type 1 (OCA1), caused by pathogenic 
variations in the tyrosinase gene (TYR), is the most frequent and severe form of 
hypopigmentary disorder worldwide. While OCA1A manifests as a complete loss of 
melanin pigment, patients with OCA1B show residual pigmentation of the skin, 
hair and eyes. Limited experimental evidence suggests retention of TYR in the 
endoplasmic reticulum (ER) causes OCA1 pathogenesis. However, a comprehensive 
functional analysis of TYR missense variations and correlation with genotype is 
lacking.
OBJECTIVES: Functional characterization of nonsynonymous tyrosinase variants in 
patients with OCA1 reported in the Albinism Database, dbSNP and the published 
literature, and an attempt to correlate them with reported and predicted 
phenotypes.
METHODS: Thirty-four reported missense variants of TYR were subcloned by 
site-directed mutagenesis, and the dual-enzyme activities of the variant 
proteins were compared with the wild-type. The degree of ER retention was also 
checked for each of the variants through endoglycosidase H (Endo H) digestion 
followed by immunoprecipitation and densitometric analysis.
RESULTS: Functional studies revealed one reported OCA1A variation with nearly 
100% enzyme activity, 10 OCA1B variants lacking any enzyme activity, eight 
nonsynonymous single-nucleotide polymorphisms (SNPs) with ~30-70% of enzyme 
activity, and three SNPs that completely lacked activity altogether. The Endo H 
assay corroborated these results.
CONCLUSIONS: Loss of enzyme activity of TYR variants was completely in agreement 
with ER retention across all variants examined. The results of the assay clearly 
established that determination of the biological activity of identified variants 
in patients with OCA is essential to correlate the identified suspect genotype 
with the obvious phenotype of the disease.