Somatic mutation of immunoglobulin light-chain variable-region genes

Area of Science:

• Immunology
• Molecular Biology
• Genetics

Background:

• Immunoglobulin variable region genes undergo rearrangement and somatic hypermutation during B-cell development.
• The VK167 germline gene is expressed in MOPC167 and MOPC511 myeloma cells.

Purpose of the Study:

• To investigate the genetic alterations in the VK167 gene during B-cell differentiation.
• To determine the mechanism of nucleotide sequence changes in expressed immunoglobulin genes.

Main Methods:

• Nucleotide sequencing of germline and expressed VK167 genes.
• Southern blot analysis to detect restriction-site alterations.
• Comparative analysis of germline and somatic sequences.

Main Results:

• Expressed VK167 genes in MOPC167 and MOPC511 cells show single-base-pair mutations compared to the germline sequence.
• These mutations alter a restriction-enzyme-recognition site, confirming sequence divergence.
• Southern blot analysis confirmed that the mutated VK167 sequence is not present in the germline genome.
• Mutations were found in both framework and hypervariable regions, indicating a broader mechanism than previously thought.
• Evidence suggests mutations arose from single-base-pair substitutions, not recombination.

Conclusions:

• Somatic mutation, not recombination, is responsible for the observed nucleotide alterations in the VK167 gene.
• Somatic mutations occur throughout the variable region, not exclusively in hypervariable segments.
• A model proposing a link between somatic mutation introduction and DNA replication during V-J joining is suggested.