The distribution of HLA class II haplotypes reveals that the Sardinian population is genetically differentiated from the other Caucasian populations

In this study we have established the frequencies of the DRB1-DQA1-DQB1 haplotypes in a large cohort of Sardinian new-borns and found that the most frequent haplotypes were detected at frequencies unique to the Sardinians. Other haplotypes, common in other Caucasian populations, are rare or absent across the island. Next, the DRB1-DQA1-DQB1 haplotype frequencies obtained in Sardinians and those reported in other human populations were used to compute genetic distances and construct phylogenetic trees. A clear-cut pattern appeared with a split between the three major human groups: Caucasians, Asians and Blacks. Among the Caucasians there were three major clusters: a group representing the North-Africans, a group including most of the European-derived populations and a group encompassing Bulgaria, Greece and Sardinia. When we increased the resolution of the tree using the genetic distances calculated from both DRB1-DQA1-DQB1 haplotypes and class I HLA A, B, C allelic frequencies, the Sardinians clearly emerged as the major outlier among the various European populations considered in this study. These results indicate that the genetic structure of the present Sardinian population is the result of a fixation of haplotypes, which are very rare elsewhere, and are most likely to have originated from a relatively large group of founders.