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DNA Topoisomerases

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Topoisomerases are enzymes that relax overwound DNA molecules during various cell processes, including DNA replication and transcription. These enzymes regulate positive and negative DNA supercoiling without changing the nucleotide sequence. DNA overwinding in a clockwise direction results in positively supercoiled DNA, whereas underwinding in a counterclockwise direction produces negatively supercoiled DNA.
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AID-induced decrease in topoisomerase 1 induces DNA structural alteration and DNA cleavage for class switch

Maki Kobayashi¹, Masatoshi Aida, Hitoshi Nagaoka

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

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

|December 19, 2009

Summary

This summary is machine-generated.

Activation-induced cytidine deaminase (AID) triggers DNA breaks during class switch recombination (CSR) by reducing Topoisomerase 1 (Top1) levels. This reduction alters DNA structure, enabling AID to cleave the S region, thus promoting CSR.

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Area of Science:

Immunology
Molecular Biology
Genetics

Background:

Class switch recombination (CSR) is crucial for antibody diversification.
Activation-induced cytidine deaminase (AID) initiates CSR by inducing DNA breaks in the S region.
Topoisomerase 1 (Top1) regulates DNA supercoiling and is implicated in genomic stability.

Purpose of the Study:

To investigate the role of Topoisomerase 1 (Top1) in AID-induced S region cleavage during CSR.
To elucidate the mechanism by which AID affects Top1 activity and its consequences on S region DNA structure.

Main Methods:

Inhibition of Top1 using camptothecin.
Overexpression and RNA-mediated knockdown of AID.
Assessment of S region cleavage and CSR.
Analysis of Top1 mRNA translation and protein levels.
Evaluation of DNA structural alterations in the S region.

Main Results:

Top1 inactivation by camptothecin blocked S region cleavage and CSR.
AID expression suppressed Top1 mRNA translation and reduced Top1 protein levels.
Reduced Top1 protein levels augmented AID-dependent S region cleavage and CSR.
Top1 reduction altered S region DNA structure, likely to a non-B form.

Conclusions:

Topoisomerase 1 (Top1) is essential for S region cleavage during CSR.
AID reduces Top1 protein levels, which facilitates S region cleavage and CSR.
AID-induced Top1 reduction alters S region DNA structure, leading to impaired Top1 religation and DNA cleavage.