HLA class II polymorphism in the Ticuna of Brazil: evolutionary implications of the DRB1*0807 allele.

The alleles at the HLA class II loci HLA-DRB1, DQA1, DQB1 and DPB1 were determined for 49 individuals of the Ticuna, a Native South American population living in Brazil, using PCR/SSO probe hybridization and DNA sequencing. A newly described DRB108 variant, DRB10807, which has previously been reported only in native Colombians and contemporary Brazilians of African and Caucasian descent, was identified in the Ticuna at a high frequency (f=0.225). Because *0807 has been observed only in South American populations, we propose that it was generated from a parental 0802 allele recently, after the isolation of various Native South American populations, and infer that the DRB10807 allele was generated by a C to T change at codon 57 (Asp-->Val, GAT-->GTT) from the ancestral *0802. This inference is supported by the sequence of a complex VNTR in the second intron of the DRB1 gene. The DPB1 alleles *0401, *0402 and 1401 constituted 76% of the observed Ticuna DPB1 alleles (f=0.166, 0.427 and 0.166 respectively). In addition, the DPB1 allele 3501, which has been observed in a few other Native South American groups, was observed at a frequency of 0.053 and may have been generated from the putative ancestral allele 1401 allele in South America. The DRB1 and DPB1 allele frequencies for the Ticuna deviate from expected Hardy-Weinberg equilibrium proportions, while DQA1 and DQB1 allele frequencies do not. When this deviation, which involves an observed excess of DRB10807 heterozygotes, is considered with the high frequency of the DRB10807 and DPB11401 alleles, we infer that native South American populations may have been under selection pressure for increased allele diversity.