DIVERSITY ASSOCIATED WITH THE 2ND EXPRESSED HLA-DRB LOCUS IN THE HUMAN-POPULATION

Although diversity within the HLA-DRB region is predominantly focused in the DRB1 gene, the second expressed DRB loci, DRB3, DRB4, and DRB5, also exhibit variation. Within DRB115 or DRB1*16 haplotypes, four ne w variants were identified: 1) two new DRB5 alleles, DRB50104 and DRB 50204, 2) a haplotype carrying a DRB1*15 or *16 allele without the us ual accompanying DRB5 allele, and 3) a haplotype carrying a DRB50101 allele without a DRB115 or *16 allele. The evolutionary origins of th ese haplotypes were postulated based on their associations with the DR B6 pseudogene. Within HLA haplotypes which carry DRB3, a new DRB30205 allele and one unusual DRB3 association were identified. Finally, two new null DRB4 alleles are described: DRB40201N, which exhibits a del etion in the second exon, and a second allele, DRB4null, which lacks the second exon completely. Gene conversion-like events and variation in the number of functional genes through reciprocal recombination and inactivation contribute to the diversity observed in the second expre ssed HLA-DRB loci.