Nucleosome stability dramatically impacts the targeting of somatic hypermutation

Area of Science:

• Immunology
• Molecular Biology
• Genetics

Background:

• Somatic hypermutation (SHM) of immunoglobulin (Ig) genes is crucial for adaptive immunity.
• The role of chromatin structure, particularly nucleosome stability, in regulating SHM is not fully understood.
• Previous studies suggested activation-induced cytidine deaminase (AID) requires transcription for DNA access within nucleosomes.

Purpose of the Study:

• To investigate the in vivo influence of nucleosome stability and positioning on SHM rates.
• To determine how engineered nucleosome structures affect AID accessibility and mutation frequency within Ig genes.

Main Methods:

• Introduction of high-affinity (MP2) and weak (M5) nucleosome positioning sequences into a variable Ig gene region in vivo.
• Assessment of SHM frequency across the nucleosome in the presence of these engineered sequences.
• Comparison of mutation patterns between stable (MP2) and less stable (M5) nucleosome configurations.

Main Results:

• A high-affinity positioning sequence (MP2) significantly reduced mutation frequency, especially near the nucleosome center.
• A weaker positioning sequence (M5) resulted in substantially higher mutation rates throughout the nucleosome.
• Mutation frequency correlated directly with nucleosome stability, indicating its critical role in SHM regulation.

Conclusions:

• Nucleosome stability and positioning are significant determinants of SHM patterns in Ig genes.
• Stable nucleosomes restrict AID access, suggesting a mechanism for regulating SHM.
• Variable Ig gene regions may have evolved for reduced nucleosome stability to enhance AID access and maximize antibody diversity.