SLC45A2 variations in Indian oculocutaneous albinism patients

Oculocutaneous albinism (OCA) is an autosomal recessive disorder of melanin biosynthesis that results in congenital hypopigmentation of ocular and cutaneous tissues. It is also associated with common developmental abnormalities of the eye. Mutations in the solute carrier family 45, member 2 gene (SLC45A2, also called MATP) cause oculocutaneous albinism type 4 (OCA4), which is the second most prevalent type of OCA in Japan. So far, 24 pathological mutations have been reported in SLC45A2, but there is no report from India. Interestingly, in almost 31% of the cases, the second mutation has never been found. The purpose of this study was to investigate the molecular basis of OCA among Indians using SLC45A2 as the candidate gene.

From our patient pool, consisting of 50 unrelated OCA pedigrees covering 17 ethnic groups of eastern and southern India, 20 patients (from 19 affected families) lacking any mutation in the tyrosinase gene (TYR) were screened further for nucleotide variants in SLC45A2. All seven exons and splice-site junctions of SLC45A2 were amplified and sequenced from the OCA patients and from 50 ethnically matched healthy controls. Nucleotide changes were detected by identifying 'double peaks' in the chromatogram due to heterozygosity as well as by pairwise BLAST analysis of the sequence output data with a normal copy of SLC45A2. Haplotype analysis was done among the affected sibs using three newly identified microsatellite markers placed within and in flanking regions of the SLC45A2 locus.

Four novel mutations (c.126G>A [Met42Ile], c.190G>A [Gly64Ser], c.904A>T [Thr302Ser], and c.1042C>T [Arg348Cys]) and one reported mutation (c.469G>A [Asp157Asn]) were identified in SLC45A2. All the novel changes cosegregated with the disease and none were present in control samples. Consistent with previous reports, we did not find the second mutant allele in three unrelated patients. Haplotype analysis using microsatellite markers in the family of one such proband suggested that the affected sibs inherited the mutant allele (Arg348Cys) from their father but different SLC45A2 alleles from the mother. In addition, five single nucleotide variants were identified which included E272K and L374F polymorphisms that have been reported to be associated with human ethnicities.

Our study reveals that 10% of the total OCA cases from eastern and southern Indian ethnic groups carry mutations in SLC45A2. Among 10 variants found in the gene, five are pathogenic changes. Our data, based on haplotype analysis on a single family, suggest that the disease is caused in the affected sibs either by a single mutation in SLC45A2 and a defect in another locus, or SLC45A2 is not responsible for the disorder in the family, but the pathogenesis is caused by a mutation in another gene not yet characterized in these patients.