Exon duplication and triplication in the human T-cell receptor gamma constant region genes and RFLP in French, Lebanese, Tunisian, and black African populations.

The human T-cell receptor gamma (TCRG) locus comprises 14 variable genes (TRGV), five joining segments (TRGJ), and two constant region genes (TRGC). The constant gamma 1 gene, TRGC1, consists of three exons, whereas the TRGC2 gene contains four or five exons due to the duplication or triplication of exon 2 and spans 9.5 kb or 12 kb, respectively. In this paper, we define the alleles of the T-cell receptor gamma J2 and C2 genes, and we show that two Hind III allelic fragments, 5.4 kb and 8 kb, characterize unambiguously the C2 gene with duplication or triplication of exon 2. We show also that the cDNA of the HPB-MLT cell line results from the transcription of an allelic TRGC2 gene with duplicated exon 2. We propose a model involving unequal crossing-overs to explain the organization and the evolution of the TRGC locus. Moreover, we analyze the TCRG haplotypes in four different populations (French, Lebanese, Tunisian, and Black African) to underline their interest for population genetics.