A family with an apparent HLA-DR triplet: evidence for exchange of functional HLA-DR beta-genes between different haplotypes.

Serological and protein analysis showed that HLA-DR1 and DR2 short (s) segregated together as one haplotype, resulting in a HLA-DR triplet as found in a family study. Both DR1 and DR2 molecules were coordinately expressed and were shown to function as restriction elements in antigen presentation assays. This unique HLA-DR1, 2s haplotype was further studied by Southern blot analysis. Based upon well-known restriction fragment length polymorphisms for the involved gene sets, i.e. DR1 and DR2 along with the DR type from the other haplotype, the genes as identified by restriction fragment length polymorphism of the triplet could be established. Pseudogenes, which are included in the previously described gene sets of HLA-DR1 and DR2 are apparently lacking in the triplet. We therefore postulate that during an unequal crossing-over event the DR beta-pseudogene of DR2 could be exchanged by a functional DR1 beta-gene.