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Assessing Somatic Hypermutation in Ramos B Cells after Overexpression or Knockdown of Specific Genes

Assessing Somatic Hypermutation in Ramos B Cells after Overexpression or Knockdown of Specific Genes

Published on: November 1, 2011

Decrease in topoisomerase I is responsible for activation-induced cytidine deaminase (AID)-dependent somatic

Maki Kobayashi¹, Zahra Sabouri, Somayeh Sabouri

¹Department of Immunology and Genomic Medicine, Graduate School of Medicine, Kyoto University, Kyoto 606-8501, Japan.

Proceedings of the National Academy of Sciences of the United States of America

|November 15, 2011

Summary

Activation-induced cytidine deaminase (AID) decreases topoisomerase I (Top1) to promote somatic hypermutation (SHM). Transcription is essential for this AID-induced DNA instability, crucial for immune diversity.

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Area of Science:

Immunology
Molecular Biology
Genetics

Background:

Somatic hypermutation (SHM) and class-switch recombination (CSR) are essential for adaptive immunity.
Both processes require transcription and activation-induced cytidine deaminase (AID).
AID influences topoisomerase I (Top1) levels during CSR, impacting DNA structure.

Purpose of the Study:

To investigate the role of Top1 in SHM.
To elucidate the interplay between AID, Top1, and transcription in generating immune diversity.

Main Methods:

Top1 knockdown and haploinsufficiency experiments.
Top1 overexpression studies.
Inhibition of Top1 activity using camptothecin.
Assessment of SHM levels under varying transcription conditions.

Main Results:

Top1 reduction or haploinsufficiency enhanced SHM.
Top1 overexpression suppressed SHM.
Top1 inhibition by camptothecin reduced SHM.
Abolition of SHM upon transcription suppression, even with Top1 knockdown.
Evidence supporting a model where reduced Top1 enhances transcription-induced DNA cleavage.

Conclusions:

AID-induced Top1 decrease is critical for SHM.
Transcription is indispensable for SHM, acting in concert with Top1 reduction.
The findings suggest a conserved mechanism for transcription-coupled genome instability adopted for immune diversification.