A NEW AUSTRALIAN ABORIGINAL COMPLOTYPES CHARACTERIZED BY INCREASED C4GENE COPY NUMBER

Complement component C4 genes are located within the central region of the human MHC. The genomic arrangement of these genes is complex, wit h each chromosome usually encoding either one or two C4 genes. C4 allo typing of a group of Western Australian Aborigines demonstrated certai n discrepancies in the densitometric ratios between the C4A4 and the C 4A3 protein bands; however, the mechanism causing the increase in dens ity of the C4A4 band was unknown. Our aim was to determine whether the increase in densitometry was due to an increase in the expression of the C4A4 isotype, or whether these individuals carried a new complotyp e characterised by an increased gene copy number. Using pulsed-field g el electrophoresis and Tag I RFLP analysis we will show that the appar ent increase in C4A4 protein expression was due to the existence of ne w, previously uncharacterised Aboriginal complotypes defined by at lea st three C4 genes. Segregation analysis from an extensive family sugge sts that one of the new C4 complotypes is likely to contain the duplic ated C4A4 isotype together with a C4B2 gene (C4A4, C4A4, C4B2) and is the first such chromosomal arrangement seen in this population group.