Human Molecular Genetics Advance Access originally published online on June 19, 2009
Human Molecular Genetics 2009 18(18):3417-3428; doi:10.1093/hmg/ddp282
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Loss of Bloom syndrome protein destabilizes human gene cluster architecture
1 Department of Microbiology, Immunology and Molecular Genetics and 2 Department of Toxicology, Markey Cancer Center, University of Kentucky, Lexington, KY, USA, 3 Department of Genome System Science, Graduate School of Nanobioscience, Yokohama City University, Yokohama, Japan, 4 Department of Biochemistry and Molecular Biology, Bloomberg School of Public Health, Johns Hopkins University, Baltimore, MD, USA
* To whom correspondence should be addressed at: 207 Combs Research Building, 800 Rose Street, Lexington, KY 40536-0096, USA. Tel: +1 8593231455; Fax: +1 8592578940; Email: andrew.pierce{at}uky.edu
Received May 12, 2009; Accepted June 15, 2009
Bloom syndrome confers strong predisposition to malignancy in multiple tissue types. The Bloom syndrome patient (BLM) protein defective in the disease biochemically functions as a Holliday junction dissolvase and human cells lacking functional BLM show 10-fold elevated rates of sister chromatid exchange. Collectively, these phenomena suggest that dysregulated mitotic recombination drives the genomic instability underpinning the development of cancer in these individuals. Here we use physical analysis of the highly repeated, highly self-similar human ribosomal RNA gene clusters as sentinel biomarkers for dysregulated homologous recombination to demonstrate that loss of BLM protein function causes a striking increase in spontaneous molecular level genomic restructuring. Analysis of single-cell derived sub-clonal populations from wild-type human cell lines shows that gene cluster architecture is ordinarily very faithfully preserved under mitosis, but is so unstable in cell lines derived from BLMs as to make gene cluster architecture in different sub-clonal populations essentially unrecognizable one from another. Human cells defective in a different RecQ helicase, the WRN protein involved in the premature aging Werner syndrome, do not exhibit the gene cluster instability (GCI) phenotype, indicating that the BLM protein specifically, rather than RecQ helicases generally, holds back this recombination-mediated genomic instability. An ataxia-telangiectasia defective cell line also shows elevated rDNA GCI, although not to the extent of BLM defective cells. Genomic restructuring mediated by dysregulated recombination between the abundant low-copy repeats in the human genome may prove to be an important additional mechanism of genomic instability driving the initiation and progression of human cancer.