Activation of immunoglobulin mu gene expression involves stepwise demethylation

Area of Science:

• Immunology
• Molecular Biology
• Genetics

Background:

• Understanding B cell development requires identifying stages of mu gene activation.
• Pre-B cell lines with rearranged JH loci but no mu polypeptides were studied to elucidate these stages.

Purpose of the Study:

• To identify stages of mu gene activation following VHDHJH assembly.
• To investigate the role of DNA demethylation in mu gene regulation.

Main Methods:

• Analysis of two pre-B cell lines (A1 and A8) with rearranged JH loci.
• Assessment of mu gene transcription and DNA methylation status in VHDHJH and C mu regions.
• Stimulation with lipopolysaccharide and fusion to a plasmacytoma.

Main Results:

• A1 cells showed incorrect VHDHJH assembly on both alleles, blocking mu production.
• A8 cells exhibited restricted transcription to a single mu allele, with an out-of-phase rearrangement.
• Low-level mu transcription correlated with demethylation of VHDHJH and enhancer regions, but not C mu.
• LPS stimulation or fusion led to elevated mu transcription, with fusion inducing C mu demethylation.

Conclusions:

• Stepwise demethylation is implicated in the regulation of mu gene expression.
• Enhanced mu gene expression can occur independently of, or precede, C mu demethylation.