Tandem repeats 3' of the IGHA genes in the human immunoglobulin heavy chain gene cluster.

The human IGH constant region spans 350 kb and includes nine genes and two pseudogenes. All of the constant region gene cluster has been cloned except for sequences between the IGHD and IGHG3 genes, between the IGHA1 and IGHG2 genes, and the 3' region downstream of the IGHA2 gene. The regions 3' of the IGHA genes, which are not cloned, are of interest since transcriptional control elements were found downstream of the IGHA genes in the rat and the mouse IGH loci. In addition, by pulsed-field gel electrophoresis mapping, CpG islands were identified approximately 30 kb downstream of each IGHA gene, within the uncloned portion of the human IGH. These findings indicate that the regions 3' of the IGHA genes are candidate regions for additional transcriptional elements of the human IGH genes. In an effort to characterize these regions, we screened five different libraries and determined the regions 3' of the IGHA genes to be unclonable by standard cloning methods. Therefore, we applied the Inverse-PCR technique to amplify the sequences flanking the IGHA genes. We obtained 1418 bp of new sequence 3' of the IGHA1 gene. The new sequence included tandem repeats of 20 bp, which we propose is the cause of the unclonability of this region.