Mary Ellen Moynahan
Department of Medicine & Program in Cell Biology
Memorial Sloan-Kettering Cancer Center, New York, NY 10021
(Tel: 212 639-2168 FAX: 212 717-3821 Email: moynaham@mskcc.org)
Homology-directed repair of DNA double strand breaks (DSBs) is important for maintaining genetic stability. A defective pathway may potentially promote tumorigenesis through an increase in the accumulation of genetic damage. In homology-directed repair, a DNA sequence with identity or similarity to a damaged chromosome directs the repair of the damage. In organisms such as bacteria and yeast, homologous recombination is the primary pathway for DNA repair. Rad51 is central to homologous repair pathways. Like its yeast and E. coli counterparts, the mammalian Rad51 protein polymerizes onto single-stranded DNA forming a nucleoprotein filament that promotes pairing and strand transfer with a homologous DNA molecule [1]. Multiple protein interactions have been demonstrated for the mammalian Rad51 protein and include the products of the hereditary breast cancer genes BRCAI and BRCA2 [2, 3]. When chromosome breaks occur, either through exogenous or endogenous processes, it can result in cell death. If the cell survives and attempts cell division with an unrepaired chromosome aneuploidy may result through chromosome mis-segregation and degradation. The DNA DSB repair pathways that can prevent these deleterious outcomes include homologous recombination and non-homologous end joining. The most direct evidence for a major role for homologous recombination in mammalian DNA repair came from analysis of the repair of an endonuclease-generated DSB in the chromosome [4]. In this study approximately half of the repair events were homology-directed and half were nonhomologous.
The evidence that the hereditary breast cancer genes, BRCA1 and BRCA2, are involved in DNA repair, in particular homologous recombination, is substantial. Both proteins co-localize with Rad51 to nuclear foci following DNA damage. In addition to changes in nuclear localization upon DNA damage, BRCA1 also undergoes changes in phosphorylation. Further evidence that BRCA1 and BRCA2 are involved in DNA repair comes from the DNA damage response of cell lines deficient in these proteins (for review, [5]). DSB repair has been examined in a Brcal-deficient mouse ES cell line using a rare-cutting endonuclease to introduce a DSB into the genome in vivo [6]. Whereas NHEJ repair of DSBS is normal, homology-directed repair is significantly impaired. In addition to defective HDR of a chromosome break, the Brca1-deficient cells are exquisitely sensitive to the interstrand DNA cross-linking agent mitomycin-C and are observed to have an increase in spontaneously occuring chromosome breaks by karotype analysis. Since these ES cells contain the exon 11 deletion that is found in mice that conditionally developed mammary gland tumors [7], the homologous repair defect is thus far found to correlate with tumorigenesis. Cells mutant for BRCA2 also have significantly impaired homology-directed repair of Induced chromosome breaks [8].
References:
1. Baumann, P., F.E. Benson, and S.C. West, Human Rad5 1 protein promotes ATP-dependent homologous pairing and strand transfer reactions in vitro. Cell, 1996. 87(4): p.757-66.
2. Scully, R., et al., Association of BRCAI with Rad5 1 in mitotic and meiotic cells. Cell, 1997. 88(2): p. 265-75.
3. Chen, J., et al., Stable interaction between the products of the BRCAI and BRCA2 tumor suppressor genes in mitotic and meiotic cells. Mol Cell, 1998. 2(3): p. 317-28.
4. Liang, F., et al., Homology-directed repair is a major double-strand break repair pathway in mammalian cells. Proc Natl Acad Sci U S A, 1998. 95(9): p. 5172-7.
5. Chen, J.J., et al., BRCAI, BRCA2, and Rad5 1 operate in a common DNA damage response pathway. Cancer Res, 1999. 59(7 Suppl): p. 1752s-1756s.
6. Moynahan, M.E., et al., Brca1 controls homology-directed DNA repair. Mol Cell, 1999. 4(4): p. 511-8.
7. Xu, X., et al., Conditional mutation of Brca1 in mammary epithelial cells results in blunted ductal morphogenesis and tumour formation. Nat Genet, 1999. 22(1): p. 37-43.
8. Moynahan, M.E., A.J. Pierce, and M. Jasin, BRCA2 is required for homology-directed repair of chromosomal breaks. Mol Cell, 2001. 7: p. 263-272.
Mary Ellen Moynahan, M.D. 1980 B.A. Boston University, Boston, MA
1986 M.D. Boston University School of Medicine, Boston, MA
1993 Clinical Assistant, Memorial Sloan-Kettering Cancer Center, New York, NY
1998 Assistant Professor of Medicine, Weill Medical College of Cornell University, New York, NY
1998 Assistant Attending Physician, Department of Medicine, Division of Solid Tumor Oncology, Memorial Hospital, New York, NY