Identification of 3' alpha-hs4, a novel Ig heavy chain enhancer element regulated at multiple stages of B cell differentiation.

In addition to E mu, several elements downstream of the IgH cluster, i.e. 3' of the C alpha gene, are involved in regulating IgH gene rearrangement and expression. This entire downstream regulatory region was shown to be deleted in the mutant myeloma cell line, LP1.2. The deletion encompasses approximately 34 kb and is presumably responsible for the reduced levels of IgH expression in this cell line. An additional regulatory element, included in the LP1.2 deletion, was identified by investigation of a DNase I hypersensitivity site located approximately 33 kb downstream of the alpha gene and present in pre-B and plasma cells. This novel IgH gene enhancer element, termed 3' alpha-hs4, is capable of activity throughout B cell development. Transient transfection of 3' alpha-hs4 in a CAT reporter gene construct shows transcriptional enhancement activity approximating that of E mu in S194 plasmacytoma and M12.4.1 and A-20 B cell lines; while in a pre-B cell line, 18-81, the average activity is 25% that of E mu. Enhancer activity was localized to an 800 bp fragment. The activity of 3' alpha-hs4 is orientation independent and appears to be B cell specific. Tight regulation of 3' alpha-hs4 is inferred from its variable activity in different plasmacytoma cell lines and within the pre B cell line, 18-81.