title: Functional assessment of tyrosinase variants identified in individuals with albinism is essential for unequivocal determination of genotype-to-phenotype correlation.
abstract: 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.
  
  
  OBJECTIVE
  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.
authors.affiliation: Molecular & Human Genetics Division, CSIR-Indian Institute of Chemical Biology, Kolkata, 700032, India.
authors.forename: M
authors.initials: M
authors.lastname: Mondal
authors.affiliation: Molecular & Human Genetics Division, CSIR-Indian Institute of Chemical Biology, Kolkata, 700032, India.
authors.forename: M
authors.initials: M
authors.lastname: Sengupta
authors.affiliation: Molecular & Human Genetics Division, CSIR-Indian Institute of Chemical Biology, Kolkata, 700032, India.
authors.forename: K
authors.initials: K
authors.lastname: Ray
mesh_terms: D016115:Albinism, Oculocutaneous / Q000235:genetics*; D005838:Genotype; D057809:HEK293 Cells; D006801:Humans; D014442:Monophenol Monooxygenase / Q000235:genetics* / Q000378:metabolism; D020125:Mutation, Missense / Q000235:genetics*; D010641:Phenotype; D020641:Polymorphism, Single Nucleotide / Q000235:genetics; D014446:Tyrosine 3-Monooxygenase / Q000378:metabolism
chemical_list: D014446:Tyrosine 3-Monooxygenase; D014442:Monophenol Monooxygenase