The BRCA1 E3 Ubiquitin Ligase Controls Centrosome Dynamics

[Cell Cycle 5:17, 1946-1950, 1 September 2006]; ©2006 Landes Bioscience

The BRCA1 E3 Ubiquitin Ligase Controls Centrosome Dynamics Review

ABSTRACT

Original manuscript submitted: 07/03/06 Manuscript accepted: 07/10/06

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ACKNOWLEDGEMENTS

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GFP RHA BIF HU Aur-A

breast cancer tumor suppressor protein 1 BRCA1 associated RING domain protein 1 green fluorescent protein RNA helicase A BRCA1 inhibitory fragment hydroxyurea aurora kinase A

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ABBREVIATIONS

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BRCA1, BARD1, centrosome, ubiquitin, microtubules, γ-tubulin

The BRCA1 locus linked a gene to familial breast cancer.1 Mutations in the BRCA1 gene are found in about half of cases of familial breast cancer,2 but in sporadic cases mutation of BRCA1 is almost never found. In these nonfamilial cases, BRCA1 is lost at the protein or mRNA level rather than the genetic level.3,4 In about 80% of invasive ductal carcinomas there is a loss of BRCA1 detectable by immunohistochemistry4 and many of these likely have hypermethylated promoters,5 evidence of epigenetic down regulation. Thus, loss of BRCA1 protein, either by mutation or by repression of gene expression, is a prevalent component of breast cancer. From many experiments designed to generate null alleles of BRCA1 in mice, it has been found that all cells require BRCA1.6,7 Mice harboring hypomorphic alleles of BRCA1 are viable but prone to cancers in a variety of tissues.8 If these truncations or internal deletions are conditionally deleted in mammary epithelia, then the mice survive and are prone to cancer of the mammary glands.9 The major phenotypes associated with these BRCA1 deficient cells is a loss of genomic stability8,10 and multiple defects in DNA damage repair.11-14 If loss of the entire BRCA1 protein is fatal to the cell, then why is loss of BRCA1 tumor growth promoting to a subset of cell types? One hypothesis that explains this conundrum is that breast and ovarian cells contain a protein not present in other cell types that suppresses the lethality of loss of the gene.15 While this model may prove true, such a suppressor protein has not yet been identified. An alternative explanation is that BRCA1 has a special function in breast cells that is executed by other proteins in nonbreast cell types. This latter hypothesis is the subject of this review. When the BRCA1 gene was cloned, sequence analysis suggested that it is a transcription factor.16 Eventually, it was realized that the amino-terminal RING domain endowed the BRCA1 protein with E3 ubiquitin ligase activity, particularly when BRCA1 is a heterodimer with the BARD1 protein.17,18 Data suggest that BRCA1 is normally bound to BARD1,19 and in this review we will simply refer to the heterodimer as “BRCA1”. Given that BRCA1 has an enzymatic activity, how does this activity contribute to the healthy growth of breast cells? BRCA1 is primarily a nuclear protein, and many proteins with which BRCA1 interacts and also ubiquitinates are found in the nucleus of the cell.20,21 Recent studies have revealed that several centrosomal proteins are bound and modified by BRCA1 and that the BRCA1 ubiquitin ligase activity is important for the appropriate regulation of centrosomes.

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INTRODUCTION

Previously published online as a Cell Cycle E-publication: http://www.landesbioscience.com/journals/cc/abstract.php?id=3208

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*Correspondence to: Jeffrey D. Parvin; Department of Pathology; Brigham and Women's Hospital; NRB 630; 77 Avenue Louis Pasteur; Boston, Massachusetts 02115 USA; Tel.: 617.525.4406; Fax: 617.525.4422; Email: JParvin@rics.bwh. harvard.edu

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Brigham and Women’s Hospital and Harvard Medical School; Boston, Massachusetts USA

The breast cancer specific tumor suppressor protein 1, BRCA1, mediates functions for all cells to grow. The puzzle of BRCA1 is that its loss is only associated with tumors in breast and ovarian epithelial cells. In this focused review, we highlight the data linking BRCA1 to the centrosome function, and we suggest that the specificity for breast tumors is due to a loss in restraint on centrosome function. Amplification of centrosome numbers secondary to loss of BRCA1 can drive the cell into the aneuploid state, thus, by this perspective loss of BRCA1 is a mutator phenotype.

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Jeffrey D. Parvin* Satish Sankaran

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This work was supported by a postdoctoral fellowship from the Susan G. Komen Breast Cancer Foundation (S.S.) and grant CA111480 from the National Cancer Institute (J.D.P.).

BRCA1 LOCALIZES TO THE CENTROSOME

The first findings that linked BRCA1 with centrosomes discovered that BRCA1 localizes to this organelle during mitosis.22,23 BRCA1 and BARD1 have both been detected at centrosomes during interphase as well.24,25 In the latter two sets of experiments, BRCA1 1946

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BRCA1 Regulation of Centrosomes

was abundantly nuclear during S and G2 phases of the cell cycle, but in those cells in which centrosomes did not overlie the nucleus, BRCA1 was clearly detected at the centrosome. As a stringency control, these cells were treated with nocodazole to ensure that this colocalization was independent of microtubule function. During G1 phase of the cell cycle, BRCA1 content in the cell was very low, and all of the detected BRCA1 was associated with the centrosome.25 Data have been published that contradict these localization results: GFP-tagged BRCA1 was expressed in cells and did not localize to the centrosomes, and data suggested that antibodies specific for BRCA1 nonspecifically labeled centrosomes.26 GFP-tagged protein fusions may not fully replicate the activities of the non-tagged protein, and this may be the cause of the discrepancy of results on BRCA1 localization. On the other hand, immunostaining of centrosomes can be problematic due to the stickiness of the organelle and due to the need to have epitopes exposed. While there are contradictory results published, the localization experiments done by Sankaran et al. (2005) used high dilution of BRCA1-specific antibody and were stringently controlled. We conclude that while BRCA1 is predominantly nuclear, it is also present at the centrosome.

BRCA1 REGULATES CENTROSOME NUMBER

The pathogenesis of breast cancer is associated with changes in centrosome number and function. Early tumor lesions, such as ductal carcinoma in situ (DCIS), have extra centrosomes.27,28 There is, thus, a loose temporal correlation of loss of BRCA1 protein with extra centrosomes. Is centrosome amplification a cause or effect of aneuploidy? Since supernumerary centrosomes appear in early breast lesions, these could cause the aneuploidy evident in tumors. One could imagine how extra centrosomes will result in unequal distribution of chromosomes among the two daughter cells following cytokinesis, or failed cell division would cause tetraploidy. Cells with tetraploid genomes are more likely to cause tumors than genetically identical diploid cells.29 These findings are consistent with the concept that centrosome amplification promotes tumorigenesis, but this concept is unproven. The first experiments linking BRCA1 to controlling centrosome number utilized murine cells expressing an internal deletion of BRCA1 in place of the wild type protein. Fibroblasts from this mutant BRCA1 mouse line were found to have centrosome amplification.30 A domain of BRCA1 (residues 504–803) was identified to bind to γ-tubulin when both proteins were overexpressed in cells. Overexpression of the γ-tubulin binding domain of BRCA1 in the COS cell line resulted in a marked centrosome amplification with abnormal mitoses.31 Experiments were done to inhibit BRCA1 transcription function by expressing in breast cell lines a fragment of RNA helicase A (RHA), a protein that binds to the carboxy-terminus of BRCA1 and coactivates BRCA1 transcription function.32 At the time it was reasoned that expression of the BRCA1 binding domain would block the transcription function and, indeed the expression of this BRCA1 inhibitory fragment of RHA (called “BIF”) did inhibit the association of BRCA1 with DNA repair foci.33 While modest changes in gene expression were noted, instead a strong phenotype of binucleate cells and centrosome amplification were observed in a non-transformed mammary epithelial cell line.33 By contrast, blocking BRCA1 in a control osteosarcoma cell line did not accumulate supernumerary centrosomes. In the BRCA1-mutant breast cancer cell line, HCC1937, centrosomes were amplified,33 supporting the concept that the effect was indeed due to loss of BRCA1 protein. www.landesbioscience.com

Further experiments using the transient expression of the BRCA1inhibiting BIF peptide in nine different cell lines revealed that inhibition of BRCA1 causes centrosome amplification in breast cells and not in nonbreast cells.34 Of the nine different cell lines studied, four of the five breast cell lines amplified centrosomes after BRCA1 inhibition. The four nonbreast cell lines did not accumulate extra centrosomes. Since the publication of that paper, two more breast cell lines were analyzed and inhibition of BRCA1 using specific siRNAs also revealed centrosome amplification (Starita LM, Parvin JD, unpublished). The non-breast cell lines studied may amplify centrosomes under other conditions, but not due to inhibition of BRCA1 function. Since the phenotype observed with overexpression of the BIF peptide was identical to that observed when BRCA1 expression was inhibited by transfection of specific siRNA oligonucleotides,34 it was likely that BRCA1 function controlled centrosome number. Is BRCA1 regulation of centrosome number due to direct action on the centrosome, or is it indirect via, for example, transcription? BRCA1 and BARD1 are abundantly nuclear during S phase of the cell cycle, but at all stages of the cell cycle the two proteins also colocalize to the centrosome, even during G1 when there is very little of either protein in the cell.24,25 Thus, the possibility exists for direct activity of BRCA1 on the centrosomes resulting in control of centrosome amplification in breast cells.

BRCA1 E3 UBIQUITIN LIGASE ACTIVITY CONTROLS CENTROSOME NUMBER

BRCA1 is a multifunctional protein, and any of several mechanisms might control centrosome number. It was found that the E3 ubiquitin ligase activity is critical for controlling centrosome number. In one instance, the expression in the mammary cell line of a BRCA1 mutant lacking the amino terminal 300 amino acids, spanning the RING domain, resulted in centrosome amplification.35 The definitive experiment in this regard was the expression in a breast cancer cell line of a point mutant (Ile-26 to Ala) of full length BRCA1 that is stable, binds to BARD1, but unable to bind to the E2 protein.36 This mutant BRCA1 is enzymatically inert with regard to its ubiquitin ligase activity. Just as the expression of kinases with mutations in the active site produce a dominant-negative phenotype, the expression of this point mutant BRCA1(I26A) had caused centrosome amplification.24 This dominant-negative mutant of BRCA1 demonstrated that the E3 ubiquitin ligase enzymatic activity of the tumor suppressor controlled centrosome number and function. Purified BRCA1 was found to ubiquitinate in vitro multiple proteins from purified centrosomes. One of these was identified as γ-tubulin,34 an important centrosomal protein that has been implicated as an initiator of microtubule nucleation by centrosomes. Data from in vitro experiments using Xenopus egg extracts and sperm nuclei suggest that γ-tubulin also controls the formation of centrosomes in sperm.37 The lysines on γ-tubulin that serve as the acceptor for ubiquitin were identified and expression in cells of a mutant γ-tubulin with the key lysine mutated to arginine had profound effects on the centrosomes. Expression of this mutant γ-tubulin in a breast cancer cell line resulted in centrosome amplification. This result suggests an enzyme-substrate relationship between BRCA1 and γ-tubulin. Inhibition of the enzyme (BRCA1) or expression of a mutant substrate that can no longer be ubiquitinated by the enzyme had the same phenotype.34 In that study, the ubiquitinated γ-tubulin was not detected in cells, and it was hypothesized that a potent

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REGULATION OF CENTROSOME FUNCTION BY BRCA1 UBIQUITINATION ACTIVITY

Figure 1. Model for the role of the BRCA1 E3 ubiquitin ligase in regulating centrosome number. The BRCA1 ubiquitin ligase modifies several centrosomal proteins, including γ-tubulin. We propose that the net effect of these modifications is to prevent reduplication of the centrosomes. One possibility from the in vitro data suggests that the ubiquitination destabilizes γ-tubulin association with the centrosome, decreasing functional activity, which in turn diminishes the potential for synthesizing the microtubule bundles that comprise the centriole. Data show that breast cell lines tend to depend on BRCA1 for this centrosome ubiquitination function. Other cell types likely have ubiquitin ligases that can replace this function of BRCA1. Loss of BRCA1 in breast cells could then result in the loss of this signal restraining centrosome amplification, resulting in too many centrosomes, driving the genome into aneuploidy.

de-ubiquitinase reverses the ubiquitin modification in extracts. Modification by phosphorylation of γ-tubulin has been detected in the amoeba.38 Though the specific modification was different, the control of the centrosome number was similar since phosphorylation of γ-tubulin was correlated with preventing centrosome reduplication.38 BRCA1 control of the centrosome number appears to be due to prevention of reduplication. Blocking cells in early S phase, by incubating cells in hydroxyurea (HU), resulted in halting centrosome duplication at the two centrosome stage regardless of BRCA1 status.34 In other words, the centrosomes had duplicated, but amplification of centrosome number due to loss of BRCA1 required a later stage of the cell cycle. Experiments inhibiting BRCA1 in cells that had been blocked in early S phase using HU and subsequently released revealed that loss of BRCA1 caused a second round of centrosome duplication prior to mitosis.39 In a different setting, it had been shown that centrosomes from cells in the G2 phase of the cell cycle are intrinsically marked to prevent reduplication.40 It is thus suggested that BRCA1 ubiquitination of centrosomes, via γ-tubulin or other target molecules, results in covalently marking a centrosome as already-duplicated. This model, which needs to be confirmed, is diagrammed in Figure 1. It follows from this model that loss of BRCA1 in breast cells would result in a loss of this covalent tag, permitting centrosomes to reduplicate. We hypothesize that in nonbreast cell lines this function of BRCA1 may be executed by redundant ubiquitin ligases that are not present in breast cells.

The centrosome is the microtubule organizing center of the cell. During interphase it regulates cell shape and polarity, and during mitosis the centrosome organizes the bipolar mitotic spindle. Thus, a major component of centrosome function is the nucleation of microtubules. This centrosome function can be assayed in situ in tumor samples,27 in vivo in tissue culture cells41,42 and in vitro with purified centrosomes.43 Using breast cancer pathology samples, the centrosomes in breast tumors were found to be hyperactive.27 In tissue culture cells, blocking BRCA1 by siRNA transfection results in hyperactive microtubule nucleation.25 Just as centrosome duplication control required the E3 ubiquitin ligase enzymatic activity of BRCA1, transfection of the BRCA1(I26A) ligase-negative mutant resulted in an increase of cells with hyperactive centrosomes.24 These results suggest that this enzymatic activity of BRCA1 inhibits centrosome microtubule nucleation. The effect on centrosome function of the BRCA1 ubiquitin ligase was tested directly using in vitro reactions. In these experiments, purified centrosomes were preincubated with BRCA1 plus ubiquitination factors and then assayed for microtubule nucleation potential using a Xenopus extract.25 It was found that low concentrations of BRCA1 (15 nM) significantly inhibited microtubule nucleation. Deletion mutants of BRCA1 and BARD1 were analyzed in this assay, and it was found that the highest level of inhibition was observed with full-length BRCA1 and full-length BARD1.24 Strikingly, deletion of the carboxy-terminal 336 amino acids of BRCA1 severely reduced its activity. Since most disease-associated mutations of BRCA1 are frame shift mutants and result in a loss of the carboxy-terminus, this requirement for the carboxy-terminus may reflect the functionality lost in tumors harboring BRCA1 mutations. An internal fragment of the BRCA1 protein (from amino acid residues 770–1290) was dispensable for high level inhibition of microtubule nucleation. Deletions of other domains of either BRCA1 or BARD1 caused a decrease in BRCA1 activity in this functional assay24 suggesting that full length BRCA1 and BARD1 are required to inhibit centrosome function. In this in vitro assay, it was observed that BRCA1 ubiquitination activity results in the loss of γ-tubulin content of the centrosome. In cells in which BRCA1 was knocked down using BRCA1-specific siRNA, the γ-tubulin content of centrosomes was increased relative to pericentrin, centrin, and α-tubulin,25 consistent with the in vitro observations that BRCA1 controls γ-tubulin content of centrosomes. This loss of γ-tubulin may be direct via ubiquitination of the γ-tubulin, or it may be indirect by the ubiquitination of another as yet unknown protein that anchors the γ-tubulin to the centrosome. Since γ-tubulin is the key component of the γ-TuRC complex that nucleates microtubule polymerization, these results provided a mechanistic explanation for the inhibition of centrosomal function by BRCA1: the key initiation factor is removed from the centrosome.

BRCA1 INVOLVEMENT IN THE CHECKPOINTS CONTROLLING MITOSIS

There are other pathways by which loss of BRCA1 may drive centrosome amplification and aneuploidy. BRCA1 has been shown to regulate the G2/M checkpoint that halts the cell cycle before mitosis if the DNA is damaged.30 How BRCA1 exercises this function is unknown, but it may occur at the centrosome. Like 1948

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BRCA1, the kinase Aurora-A regulates centrosome number, and the Aur-A locus is amplified in breast cancer cases.44,45 It was found that Aur-A and BRCA1 functionally interact and that Aur-A phosphorylates BRCA1 on Ser-308 in order to arrest cells in G2. Intriguingly, mutation of Ser-308 so that it could not be phosphorylated resulted in a G2 arrest independent of DNA damage, suggesting that phosphorylation of BRCA1 by Aur-A is a necessary event for progression into mitosis.46 While we have focused on the BRCA1 E3 ubiquitin ligase activity, many experiments indicate that BRCA1 transcriptional function may regulate checkpoints. As an example, BRCA1 stimulates the expression of MAD2, which is important for controlling the spindle checkpoint. Loss of BRCA1 thus causes a loss of MAD2 and an inappropriate entry into anaphase.47 Thus, BRCA1 function as a transcription factor is consistent with BRCA1 function as an E3 ubiquitin ligase since both activities combine to maintain the integrity of the genome. BRCA1 is also a factor required for the repair of DNA damage by a variety of pathways. Other DNA damage repair factors similarly affect centrosome function, including: Rad51 and its paralogs48-52 and Rad6.53 Failure to repair DNA damage can cause centrosomal fragmentation if the checkpoints are additionally compromised,54,55 suggesting that DNA damage repair, mitotic checkpoint control, and centrosome regulation are all linked by the BRCA1 acting as a transcription factor and as an E3 ubiquitin ligase.

CONCLUDING REMARKS

The mechanics of how loss of BRCA1 results in breast or ovarian cancer are unknown, but emerging data suggest that the BRCA1 enzymatic activity inhibits centrosome function and blocks reduplication of centrosomes in cells derived from mammary epithelium. These findings may provide an explanation of the tumor types that result from loss of BRCA1: nonbreast cells have additional activities that modify γ-tubulin in order to control centrosome number. We suggest that BRCA1-dependent ubiquitination of centrosomes marks post-duplicated centrosomes, preventing reduplication. In the genesis of breast tumors, centrosome amplification occurs in early lesions, and it is possible that loss of BRCA1 in these lesions is responsible. References 1. Hall JM, Lee MK, Newman B, Morrow JE, Anderson LA, Huey B, King MC. Linkage of early-onset familial breast cancer to chromosome 17q21. Science 1990; 250:1684-9. 2. Couch FJ. Genetic epidemiology of BRCA1. Cancer Biol Ther 2004; 3:509-14. 3. Magdinier F, Ribieras S, Lenoir GM, Frappart L, Dante R. Downregulation of BRCA1 in human sporadic breast cancer; analysis of DNA methylation patterns of the putative promoter region. Oncogene 1998; 17:3169-76. 4. Wilson CA, Ramos L, Villasenor MR, Anders KH, Press MF, Clarke K, Karlan B, Chen JJ, Scully R, Livingston D, Zuch RH, Kanter MH, Cohen S, Calzone FJ, Slamon DJ. Localization of human BRCA1 and its loss in high-grade, noninherited breast carcinomas. Nat Genet 1999; 21:236-40. 5. Esteller M, Herman JG. Cancer as an epigenetic disease: DNA methylation and chromatin alterations in human tumours. J Pathol 2002; 196:1-7. 6. Hakem R, de la Pompa JL, Sirard C, Mo R, Woo M, Hakem A, Wakeham A, Potter J, Reitmair A, Billia F, Firpo E, Hui CC, Roberts J, Rossant J, Mak TW. The tumor suppressor gene Brca1 is required for embryonic cellular proliferation in the mouse. Cell 1996; 85:1009-23. 7. Ludwig T, Chapman DL, Papaioannou VE, Efstratiadis A. Targeted mutations of breast cancer susceptibility gene homologs in mice: Lethal phenotypes of Brca1, Brca2, Brca1/Brca2, Brca1/p53, and Brca2/p53 nullizygous embryos. Genes Dev 1997; 11:1226-41. 8. Shen SX, Weaver Z, Xu X, Li C, Weinstein M, Chen L, Guan XY, Ried T, Deng CX. A targeted disruption of the murine Brca1 gene causes gamma-irradiation hypersensitivity and genetic instability. Oncogene 1998; 17:3115-24.

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37. Stearns T, Kirschner M. In vitro reconstitution of centrosome assembly and function: The central role of gamma-tubulin. Cell 1994; 76:623-37. 38. Haren L, Remy MH, Bazin I, Callebaut I, Wright M, Merdes A. NEDD1-dependent recruitment of the gamma-tubulin ring complex to the centrosome is necessary for centriole duplication and spindle assembly. J Cell Biol 2006; 172:505-15. 39. Ko MJ, Murata K, Hwang DS, Parvin JD. Inhibition of BRCA1 in breast cell lines causes the centrosome duplication cycle to be disconnected from the cell cycle. Oncogene 2006; 25:298-303. 40. Wong C, Stearns T. Centrosome number is controlled by a centrosome-intrinsic block to reduplication. Nat Cell Biol 2003; 5:539-44. 41. Dammermann A, Merdes A. Assembly of centrosomal proteins and microtubule organization depends on PCM-1. J Cell Biol 2002; 159:255-66. 42. Nguyen HL, Gruber D, Bulinski JC. Microtubule-associated protein 4 (MAP4) regulates assembly, protomer-polymer partitioning and synthesis of tubulin in cultured cells. J Cell Sci 1999; 112(Pt 12):1813-24. 43. Mitchison T, Kirschner M. Microtubule assembly nucleated by isolated centrosomes. Nature 1984; 312:232-7. 44. Zhou H, Kuang J, Zhong L, Kuo WL, Gray JW, Sahin A, Brinkley BR, Sen S. Tumour amplified kinase STK15/BTAK induces centrosome amplification, aneuploidy and transformation. Nat Genet 1998; 20:189-93. 45. Li JJ, Weroha SJ, Lingle WL, Papa D, Salisbury JL, Li SA. Estrogen mediates Aurora-A overexpression, centrosome amplification, chromosomal instability, and breast cancer in female ACI rats. Proc Natl Acad Sci USA 2004; 101:18123-8. 46. Ouchi M, Fujiuchi N, Sasai K, Katayama H, Minamishima YA, Ongusaha PP, Deng C, Sen S, Lee SW, Ouchi T. BRCA1 phosphorylation by Aurora-A in the regulation of G2 to M transition. J Biol Chem 2004; 279:19643-8. 47. Wang RH, Yu H, Deng CX. A requirement for breast-cancer-associated gene 1 (BRCA1) in the spindle checkpoint. Proc Natl Acad Sci USA 2004; 101:17108-13. 48. Date O, Katsura M, Ishida M, Yoshihara T, Kinomura A, Sueda T, Miyagawa K. Haploinsufficiency of RAD51B causes centrosome fragmentation and aneuploidy in human cells. Cancer Res 2006; 66:6018-24. 49. Lindh AR, Rafii S, Schultz N, Cox A, Helleday T. Mitotic defects in XRCC3 variants T241M and D213N and their relation to cancer susceptibility. Hum Mol Genet 2006; 15:1217-24. 50. Lesca C, Germanier M, Raynaud-Messina B, Pichereaux C, Etievant C, Emond S, Burlet-Schiltz O, Monsarrat B, Wright M, Defais M. DNA damage induce gamma-tubulin-RAD51 nuclear complexes in mammalian cells. Oncogene 2005; 24:5165-72. 51. Daboussi F, Thacker J, Lopez BS. Genetic interactions between RAD51 and its paralogues for centrosome fragmentation and ploidy control, independently of the sensitivity to genotoxic stresses. Oncogene 2005; 24:3691-6. 52. Smiraldo PG, Gruver AM, Osborn JC, Pittman DL. Extensive chromosomal instability in Rad51d-deficient mouse cells. Cancer Res 2005; 65:2089-96. 53. Shekhar MP, Lyakhovich A, Visscher DW, Heng H, Kondrat N. Rad6 overexpression induces multinucleation, centrosome amplification, abnormal mitosis, aneuploidy, and transformation. Cancer Res 2002; 62:2115-24. 54. Dodson H, Bourke E, Jeffers LJ, Vagnarelli P, Sonoda E, Takeda S, Earnshaw WC, Merdes A, Morrison C. Centrosome amplification induced by DNA damage occurs during a prolonged G2 phase and involves ATM. Embo J 2004; 23:3864-73. 55. Hut HM, Lemstra W, Blaauw EH, Van Cappellen GW, Kampinga HH, Sibon OC. Centrosomes split in the presence of impaired DNA integrity during mitosis. Mol Biol Cell 2003; 14:1993-2004.

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