Structure and chromosomal location of the human CD6 gene: detection of five human CD6 isoforms

The CD6 protein has been shown to play important roles in T cell costimulation and adhesion. Recently, variably spliced isoforms of CD6 mRNA have been identified in both human and murine T cells. Here we report on the genomic organization of the human CD6 gene, its chromosomal localization, and the characterization of novel isoforms. Human CD6 is encoded by at least 13 exons. The amino terminal signal sequence, extracellular region, and transmembrane domain are encoded by seven exons, while the cytoplasmic domain of CD6 is encoded by six exons. Each of the three extracellular scavenger receptor cysteine-rich domains is encoded by a separate exon. Fluorescence in situ hybridization studies and screening of a chromosome-specific YAC (yeast artificial chromosome) library revealed that the gene encoding CD6 is located on chromosome 11 at 11q13 in close proximity to the gene encoding the related molecule CD5 and within 600 kb of CD20. Analysis of mRNA transcripts encoding CD6 isolated from mitogen-activated PBMC and from B cells obtained from patients with chronic lymphocytic leukemia revealed the presence of at least five different CD6 transcripts. These transcripts arise via variable splicing of exons encoding the cytoplasmic domain of CD6. The existence of these isoforms suggests that signaling through CD6 could be regulated via alternative splicing of cytoplasmic encoding exons.