The implication of the tyrosinase assay results in light of the phenotype and the possible location of the pigment block in these forms of OCA are discussed.
Here, we describe the analysis of the HPS1 gene in 24 Japanese OCA patients who lacked mutations in the four genes known to cause OCA (TYR/OCA1, P/OCA2, TYRP1/OCA3, and MATP/OCA4), and the identification of eight different HPS1 mutations in ten of these patients, four of which were novel (W583X, L668P, 532insC, 1691delA).
This mutation, which results in a proline----leucine substitution at codon 81 of the tyrosinase polypeptide (EC 1.14.18.1), was observed in 20% (6 of 30) of oculocutaneous albinism alleles from independent probands, but it was not observed in any normal individuals.
Three PUAs (p.P152H and p.W272X of TYR, p.A486T of SLC24A5) identified in the OCA probands did not co-transmit with known pathological alleles and thus gave rise to unaffected fetuses.
Oculocutaneous albinism (OCA) is a group of autosomal recessive disorders characterized by reduced melanin that are caused by mutations in the gene encoding tyrosinase (TYR), which is the rate-limiting enzyme in the production of the pigment melanin.
Genetic analysis was subsequently conducted, and the results revealed the p. (Arg778Leu) mutation in 1 allele and the p. (Asn1270Ser) mutation in the other allele of the ATP7B gene, confirming the diagnosis of WD; the p. (D456fs) mutation in 1 allele and the p. (R299H) mutation in the other allele of the TYR gene, confirming the diagnosis of OCA.
We identified five different mutations in the TYR gene in 4 patients with severe OCA and in 2 patients with mild OCA, but found no mutations in the 6 patients with mild OCA phenotypes.
TYR gene mutations represent a relevant cause of oculocutaneous albinism in Italy, whereas mutations in P present a lower frequency than that found in other populations.
Of 75 unrelated patients that were screened, 18 individuals (24%) were identified as having OCA4; they harbored seven novel mutations, including four missense mutations (P58S, D157N, G188V, and V507L) and three frameshift mutations (S90CGGCCA-->GC, V144insAAGT, and V469delG), showing that MATP is the most frequent locus for tyrosinase-positive OCA in Japanese patients.
Compound heterozygous mutations (c.832C>T and c.929_930insC) in the TYR gene may be responsible for partial clinical manifestations of OCA, while the homozygous missense mutation c.814G>A (p.Glu272Lys) in the SLC45A2 gene may not be associated with OCA.
Whole exome or direct sequencing showed that two of the children had Hermansky-Pudlak syndrome (HPS) type-1 (HPS-1), one had HPS-3, one had HPS-4, and four had non-syndromic oculocutaneous albinism associated with TYR variants (OCA1).
Retrospective analysis in oculocutaneous albinism patients for the 2.7 kb deletion in the OCA2 gene revealed a co-segregation of the controversial variant, p.R305W.