High frequency of the Ala481Thr mutation of the P gene in the Japanese population

P-gene-related oculocutaneous albinism (OCA2) (MIM#203200) is an autosomal recessive disorder that results from mutations of the P gene [Rinchik et al., 1993; Rosemblat et al., 1994], the human homologue to the mouse pink-eyed dilution gene. The function of the P gene product has not been clarified yet, although several potential functions have been proposed, e.g., a membrane transporter of a substrate [Rosemblat et al., 1998], a stabilizer of a melanosomal protein complex [Manga and Orlow, 2001], a regulator of melanosomal pH [Brilliant, 2001], or a sorter of tyrosinase to melanosome [Puri et al., 2000]. OCA2 is the most common type of albinism and accounts for approximately 50% of OCA worldwide. More than fifty different mutations of the P gene have been reported [Oetting and King, 1999; Passmore et al., 1999]. One of them is a missense substitution at codon 481 in which threonine (ACC) is substituted for alanine (GCC) (A481T) in the predicted luminal (or extracellular) loop of the P protein [Rinchik et al., 1993]. The A481T substitution is a relatively commonmutant allele of thePgene in theCaucasianpopulation (1/50, q1⁄4 0.01) and is associated with a very mild phenotype with no distinctive skin manifestations, clinically classified as ‘‘autosomal recessive ocular albinism,’’ rather than as OCA2 [Lee et al., 1994]. Transfection experiments of p null mouse melanocytes with the A481T mutant P cDNA showed that the A481T allele had approximately 70% function in melanogenesis compared with that of thewild-type humanP cDNA [Sviderskaya et al., 1997], which is consistent with this being a relatively mild OCA2 mutant allele. We report here that the A481T mutant allele is much more frequently found in Japanese than inCaucasians, i.e., approximately 10%of OCA patients and 12% of normally-pigmented people (Table I). One hundred and four unrelated normally-pigmented Japanese volunteers and40JapaneseOCApatientswho had no mutation detected in their tyrosinase gene were enrolled in this study for analysis of the P gene in this report. The twenty-four coding exons of the human P gene plus adjacent noncoding sequences were amplified from genomic DNA by PCR using the primers described previously [Lee et al., 1995]. The amplified segments were then screened for mutations by SSCP and heteroduplex method [Spritz et al., 1992]. Exon segments that yielded aberrant patterns were independently reamplified from genomic DNA and sequenced directly. This study was approved by the Ethical Committee of Nagoya University School of Medicine. We found that sixOCApatientswereheterozygous for the A481T allele and one was homozygous (Table I). Then,wedetermined the frequency of themutatedallele in 104 unrelated normally-pigmented Japanese. As shown in Table I, twenty of these 104 normal subjects were heterozygous and two were homozygous for the mutation allele, and the allele frequency of A481T in normally-pigmented Japanese was 0.12. Two homozygotes appeared entirely normal in terms of the biosynthesis ofmelanin pigment of the skin, eyes, and hair. These findings are totally consistent with the fact that the A481T allele had approximately 70% function in melanogenesis compared with that of the wild-type human P cDNA [Sviderskaya et al., 1997]. Because, generally speaking, less than 50% loss of function doesn’t cause symptoms in case of an autosomal recessive disorder. The A481T mutant allele does not cause OCA2 by itself even in homozygotes, but does cause a mild type of OCA2 when in combination with a null or almost null mutant allele. Only one Japanese OCA2 patient has been reported, and the patient was hemizygous for A481T [Saitoh et al., 2000], supporting the