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Molecular and Cellular Biology, December 2005, p. 10907-10915, Vol. 25, No. 24
0270-7306/05/$08.00+0     doi:10.1128/MCB.25.24.10907-10915.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.

Human TopBP1 Ensures Genome Integrity during Normal S Phase

Ja-Eun Kim,1 Sarah A. McAvoy,2 David I. Smith,2 and Junjie Chen1*

Department of Oncology,1 Division of Experimental Pathology, Mayo Clinic College of Medicine, Rochester, Minnesota 559052

Received 3 March 2005/ Returned for modification 31 March 2005/ Accepted 21 September 2005

Cell cycle checkpoints are essential for maintaining genomic integrity. Human topoisomerase II binding protein 1 (TopBP1) shares sequence similarity with budding yeast Dpb11, fission yeast Rad4/Cut5, and Xenopus Cut5, all of which are required for DNA replication and cell cycle checkpoints. Indeed, we have shown that human TopBP1 participates in the activation of replication checkpoint and DNA damage checkpoints, following hydroxyurea treatment and ionizing radiation. In this study, we address the physiological function of TopBP1 in S phase by using small interfering RNA. In the absence of exogenous DNA damage, TopBP1 is recruited to replicating chromatin. However, TopBP1 does not appear to be essential for DNA replication. TopBP1-deficient cells have increased H2AX phosphorylation and ATM-Chk 2 activation, suggesting the accumulation of DNA double-strand breaks in the absence of TopBP1. This leads to formation of gaps and breaks at fragile sites, 4N accumulation, and aberrant cell division. We propose that the cellular function of TopBP1 is to monitor ongoing DNA replication. By ensuring proper DNA replication, TopBP1 plays a critical role in the maintenance of genomic stability during normal S phase as well as following genotoxic stress.

* Corresponding author. Mailing address: Department of Oncology, Mayo Clinic College of Medicine, Rochester, MN 55905. Phone: (507) 538-1545. Fax: (507) 284-3906. E-mail: Chen.Junjie{at}mayo.edu.

Molecular and Cellular Biology, December 2005, p. 10907-10915, Vol. 25, No. 24
0022-538X/05/$08.00+0     doi:10.1128/MCB.25.24.10907-10915.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.



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