Specific 5' and 3' regions of the mu-chain gene are undermethylated at distinct stages of B-cell differentiation.

The mu-chain gene is expressed differently in successive stages of B-lymphocyte development. The heavy chain product appears as a cytoplasmic constituent in pre-B-cells, as part of the IgM receptor in maturing B cells, and as a component in the pentamer IgM antibody synthesized and secreted by the antigen-stimulated cell. We have used the methylation of CpG sequences as an assay system to define the chromatin changes associated with different expression of the mu-chain. The methylation status of eight index sites was followed by restriction enzyme analysis of murine cell lines representing the major stages in the developmental pathway. The analyses showed that a single Msp I/Hpa II site 5' to the immunoglobulin enhancer becomes undermethylated with the onset of mu-chain gene transcription. Four midgene Msp I/Hpa II sites exhibit a progressive loss of methyl groups unrelated to changes in mu-chain gene expression, whereas a Msp I/Hpa II site and two Hha I sites surrounding the exon encoding the carboxyl terminus of the secreted form of mu chain (mus) become undermethylated during the transition to IgM secretion. These results indicate that structural changes in local regions of the mu-chain gene correlate with specific developmental events.