title: Absence of BLM leads to accumulation of chromosomal DNA breaks during both unperturbed and disrupted S phases.
abstract: Bloom's syndrome (BS), a disorder associated with genomic instability and cancer predisposition, results from defects in the Bloom's helicase (BLM) protein. In BS cells, chromosomal abnormalities such as sister chromatid exchanges occur at highly elevated rates. Using Xenopus egg extracts, we have studied Xenopus BLM (Xblm) during both unperturbed and disrupted DNA replication cycles. Xblm binds to replicating chromatin and becomes highly phosphorylated in the presence of DNA replication blocks. This phosphorylation depends on Xenopus ATR (Xatr) and Xenopus Rad17 (Xrad17), but not Claspin. Xblm and Xenopus topoisomerase IIIalpha (Xtop3alpha) interact in a regulated manner and associate with replicating chromatin interdependently. Immunodepletion of Xblm from egg extracts results in accumulation of chromosomal DNA breaks during both normal and perturbed DNA replication cycles. Disruption of the interaction between Xblm and Xtop3alpha has similar effects. The occurrence of DNA damage in the absence of Xblm, even without any exogenous insult to the DNA, may help to explain the genesis of chromosomal defects in BS cells.
authors.affiliation: Division of Biology 216-76, California Institute of Technology, 1200 E. California Blvd., Pasadena, CA 91125, USA.
authors.forename: Wenhui
authors.initials: W
authors.lastname: Li
authors.forename: Soo-Mi
authors.initials: SM
authors.lastname: Kim
authors.forename: Joon
authors.initials: J
authors.lastname: Lee
authors.forename: William G
authors.initials: WG
authors.lastname: Dunphy
mesh_terms: D000251:Adenosine Triphosphatases / Q000172:deficiency* / Q000235:genetics*; D000818:Animals; D001816:Bloom Syndrome / Q000235:genetics; D002467:Cell Nucleus / Q000502:physiology / Q000648:ultrastructure; D002843:Chromatin / Q000502:physiology / Q000648:ultrastructure; D004249:DNA Damage; D004265:DNA Helicases / Q000172:deficiency* / Q000235:genetics*; D005260:Female; D006801:Humans; D008297:Male; D008715:Methionine / Q000378:metabolism; D010063:Ovum / Q000166:cytology; D053484:RecQ Helicases; D016196:S Phase / Q000235:genetics / Q000502:physiology; D013094:Spermatozoa / Q000166:cytology; D014981:Xenopus
chemical_list: D002843:Chromatin; D008715:Methionine; D000251:Adenosine Triphosphatases; C097151:Bloom syndrome protein; D004265:DNA Helicases; D053484:RecQ Helicases