Analysis of HLA class II haplotypes in the Cayapa Indians of Ecuador: a novel DRB1 allele reveals evidence for convergent evolution and balancing selection at position 86.

PCR amplification, oligonucleotide probe typing, and sequencing were used to analyze the HLA class II loci (DRB1, DQA1, DQB1, and DPB1) of an isolated South Amerindian tribe. Here we report HLA class II variation, including the identification of a new DRB1 allele, several novel DR/DQ haplotypes, and an unusual distribution of DPB1 alleles, among the Cayapa Indians (N = 100) of Ecuador. A general reduction of HLA class II allelic variation in the Cayapa is consistent with a population bottle-neck during the colonization of the Americas. The new Cayapa DRB1 allele, DRB108042, which arose by a G-->T point mutation in the parental DRB10802, contains a novel Val codon (GTT) at position 86. The generation of DRB108042 (Val-86) from DRB10802 (Gly-86) in the Cayapa, by a different mechanism than the (GT-->TG) change in the creation of DRB108041 (Val-86) from DRB10802 in Africa, implicates selection in the convergent evolution of position 86 DR beta variants. The DRB108042 allele has not been found in > 1,800 Amerindian haplotypes and thus presumably arose after the Cayapa separated from other South American Amerindians. Selection pressure for increased haplotype diversity can be inferred in the generation and maintenance of three new DRB108042 haplotypes and several novel DR/DQ haplotypes in this population. The DPB1 allelic distribution in the Cayapa is also extraordinary, with two alleles, DPB11401, a very rare allele in North American Amerindian populations, and DPB10402, the most common Amerindian DPB1 allele, constituting 89% of the Cayapa DPB1.(ABSTRACT TRUNCATED AT 250 WORDS)