Rapid Detection of the ERV-K(C4) Retroviral Insertion Reveals Further Structural Polymorphism of the Complement C4 Genes in Old World Primates

The fourth component of complement (C4) is coded for by two tandem-duplicated genes located in the class III region of the MHC of humans as well as a number of primates. A C4 gene size polymorphism giving rise to two gene variants of 16 and 22.3 kb length can be attributed to a complete endogenous retroviral insertion of 6.3 kb termed ERV-K(C4) in intron 9 of the long C4 genes. We developed a simple PCR-based screening assay to detect the presence of this insertion, and tested a number of unrelated animals from old world primate species. The presence of the ERV insertion in the orangutan, rhesus macaque and green monkey as well as its absence in gorillas and chimpanzees could be confirmed. In addition, the insertion was also detected in the baboon and the cynomolgus macaque whereas it was not found in a single gibbon. Among rhesus and cynomolgus macaques one individual was identified in each species only carrying short C4 genes demonstrating further structural heterogeneity in these species. Based on these findings we propose that the primigenial retroviral integration occurred prior to the radiation of old world primate species, and that both the long and the short forms of the C4 gene have existed side by side since then.