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Somatic hypermutation introduces insertions and deletions into immunoglobulin V genes
P C Wilson1, O de Bouteiller, Y J Liu
1Molecular Immunology Center, Department of Microbiology, University of Texas Southwestern Medical Center at Dallas, Texas 75235-9140, USA.
The Journal of Experimental Medicine
|January 31, 1998
Summary
This study reveals that DNA insertions and deletions, though rare, occur during the somatic hypermutation process in immunoglobulin genes. These events, often in CDR regions, suggest a link to DNA repair mechanisms and polymerase activity during antibody maturation.
Area of Science:
- Immunology
- Molecular Biology
- Genetics
Background:
- Somatic hypermutation introduces mutations into immunoglobulin V genes during germinal center reactions to enhance antibody affinity and B cell diversity.
- Antigen-driven selection of B cells with high-affinity surface Ig leads to antibody affinity maturation.
- While typically base-pair substitutions, DNA insertions and deletions occur at a low frequency in Ig genes.
Purpose of the Study:
- To investigate and characterize insertion and deletion events within somatically mutated VH gene cDNA molecules.
- To determine if these events are associated with the somatic hypermutation process and not experimental artifacts.
Main Methods:
- Sequencing of 395 cDNA clones from human tonsil B cell populations (germinal center and memory B cells).
- Extensive characterization of the genomic VH4 repertoire to identify germline origins.
- Statistical analysis comparing event frequency in mutated vs. unmutated IgD+CD38- follicular mantle cDNA clones.
Main Results:
- Identified five insertion and four deletion events in VH gene cDNA, with six originating from germinal center and memory B cells.
- No germline genes encoding these events were found in the donor's genomic VH4 repertoire.
- Eight of nine events occurred in CDR1 or CDR2, and all occurred in triplets or multiples, maintaining the reading frame.
- These events were statistically associated with somatic hypermutation (P = 0.014) and not found in unmutated follicular mantle cells.
Conclusions:
- Insertion and deletion events are part of the somatic hypermutation process, likely occurring in triplets or multiples to maintain reading frame.
- These events are concentrated in CDR1/CDR2, suggesting selection, and are linked to DNA repair alterations and polymerase activity.
- Secondary DNA structures and sequence motifs resembling mutation hotspots may predispose these events, consistent with models of error-prone DNA repair during hypermutation.