The effects of E(μ) 3'α (hs 1,2) and 3'κ enhancers on mutation of an Ig-VDJ-Cγ2a Ig heavy gene in cultured B cells

The Ig κ intronic and 3'κ light chain enhancers have been shown to be necessary for V region hypermutation in κ light chain transgenes. To investigate the role of the E(μ) intronic enhancer in V region hypermutation of heavy chain genes, E(μ) and its associated matrix attachment regions (MAR) were replaced with the SV40 or the cytomegalovirus (CMV) enhancer in a γ2a construct that hypermutates its rearranged VDJ region in the NSO plasmacytoma and 18.81 pre-B cell lines. In this model in vitro system, mutation rates of stable transfectants were determined by reversion analysis using a V region stop codon that, when mutated, allowed the detection of cellular revertants by ELISA spot assay. The γ2a constructs with the E(μ), SV40 and CMV enhancers mutated at comparably high rates in the B cell lines, but not in L cells, indicating that the E(μ) enhancer and its associated MAR were not specifically required for IgH hypermutation in this system. In parallel experiments, the addition of the 3'α heavy chain enhancer (hs 1,2) or the 3'κ light chain enhancer did not increase the mutation rate of a related μ reporter construct in which the associated VDJ mutates at a moderately low rate in NSO cells or in cell hybrids made between 18.81 and NSO. These results imply that cis-acting IgH elements that promote hypermutation may not be restricted to Ig-specific transcriptional enhancers.