Transcription analysis, physical mapping, and molecular characterization of a nonclassical human leukocyte antigen class I gene.

The human major histocompatibility complex contains approximately 20 class I genes, pseudogenes, and gene fragments. These include the genes for the three major transplantation antigens, HLA-A, HLA-B, and HLA-C, as well as a number of other genes or pseudogenes of unknown biological significance. Most of the latter have C + G-rich sequences in their 5' ends that are unmethylated in the B-lymphoblastoid cell line 3.1.0. We investigated one of these genes, HLA-H, in more detail. The gene is, overall, strongly homologous in sequence to HLA-A but differs in several potentially significant ways, including changes in conserved promoter sequences, a single-base deletion producing a translation termination codon in exon 4, and a region of sequence divergence downstream of the transcribed portion of the gene. Nevertheless, mouse L cells transfected with the gene accumulated small amounts of apparently full-length polyadenylated RNA. A portion of this RNA begins at the transcription site predicted by analogy to certain class I cDNA clones, while another portion appears to begin shortly upstream. L cells transfected with a hybrid gene containing the first three exons of HLA-H and the last five exons of HLA-B27 accumulated full-length HLA transcripts at the same level as cells transfected with an HLA-B27 gene; both levels are at least 15- to 20-fold higher than that directed by HLA-H alone. In addition, we isolated a cDNA clone for HLA-H that contains a portion of intron 3 attached to a normally spliced sequence comprising exons 4 through 8. These results suggest that low levels of translatable mRNA for the truncated class I heavy chain encoded by HLA-H are produced under physiologic circumstances and that sequences 3' of intron 3 decrease the levels of stable transcripts.