Identification of core DNA elements that target somatic hypermutation

Area of Science:

• Immunology
• Molecular Biology
• Genetics

Background:

• Somatic hypermutation (SHM) diversifies immunoglobulin (Ig) genes, crucial for antibody affinity maturation.
• Activation-induced deaminase (AID) mediates SHM by deaminating deoxycytosine residues in activated B cells.
• SHM and AID preferentially target Ig loci, but the underlying mechanisms remain unclear.

Purpose of the Study:

• To identify and characterize elements responsible for the preferential targeting of SHM and AID activity to Ig loci.
• To understand the molecular mechanisms governing SHM targeting within Ig genes.

Main Methods:

• Utilized an SHM reporter cassette assay in chicken DT40 B cells, inserted into the Ig L chain locus (IgL).
• Performed systematic deletion analysis of a 1.9-kb chicken IgL-derived element (DIVAC).
• Employed chromatin immunoprecipitation (ChIP) to assess epigenetic marks and RNA polymerase II phosphorylation.

Main Results:

• Identified a 1.9-kb DIVAC element from chicken IgL that significantly enhances AID-dependent mutation activity.
• Deletion analysis revealed that targeting activity is dispersed, with two critical regions: an IgL enhancer region and a 3' element.
• DIVAC increased serine-5 phosphorylated RNA polymerase II, suggesting an impact on transcriptional elongation/pausing, without altering other epigenetic marks.

Conclusions:

• Multiple, dispersed DNA elements, exemplified by DIVAC, collaborate to recruit and activate the SHM machinery.
• These elements play a critical role in ensuring the preferential targeting of SHM to Ig gene variable regions.
• Understanding these targeting mechanisms is key to comprehending antibody diversification and B cell maturation.