Absence of Brca2 causes genome instability by chromosome breakage and loss associated with centrosome amplification

Abstract

Women heterozygous for mutations in the breast-cancer susceptibility genes BRCA1 and BRCA2 have a highly elevated risk of developing breast cancer [1]. BRCA1 and BRCA2 encode large proteins with no sequence similarity to one another. Although involvement in DNA repair and transcription has been suggested, it is still not understood how loss of function of these genes leads to breast cancer [2]. Embryonic fibroblasts (MEFs) derived from mice homozygous for a hypomorphic mutation (Brca2(Tr2014)) within the 3' region of exon 11 in Brca2 [3], or a similar mutation (Brca2(Tr)) [4], proliferate poorly in culture and overexpress the tumour suppressor p53 and the cyclin-dependent kinase inhibitor p21(Waf1/Cip1). These MEFs have intact p53-dependent DNA damage G(1)-S [3] [4] and G(2)-M checkpoints [4], but are impaired in DNA double-strand break repair [3] and develop chromosome aberrations [4]. Here, we report that Brca2(Tr2014/Tr2014) MEFs frequently develop micronuclei. These abnormal DNA-containing bodies were formed through both loss of acentric chromosome fragments and by chromosome missegregation, which resulted in aneuploidy. Absence of Brca2 also led to centrosome amplification, which we found associated with the formation of micronuclei. These data suggest a potential mechanism whereby loss of BRCA2 may, within subclones, drive the loss of cell-cycle regulation genes, enabling proliferation and tumourigenesis.

Targeting Tankyrase 1 as a therapeutic strategy for BRCA-associated cancer

“…HTC75 cells expressing the truncated forms of Tankyrase 1 and the isogenic control cell lines, MOCK and TANK-1, were transfected with BRCA1-or BRCA2-silencing plasmids and the number of centrosomes analysed by immunofluorescence. We observed a significant increase in the number of cells with centrosome amplification after silencing of BRCA1 or BRCA2 alone compared to controls, consistent with previous studies (Figures 4a and b; Tutt et al, 1999;Xu et al, 1999). However, this phenotype was considerably exacerbated in cells with silenced BRCA1 or BRCA2 in the presence of the ARCV Tankyrase 1 mutant (Figures 4a and b).…”

PIBF (progesterone induced blocking factor) is overexpressed in highly proliferating cells and associated with the centrosome

“…6 BRCA1 interacts with a variety of proteins regulating centrosome duplication, including BRCA2, CDK2-Cyclin A, CDK2-Cyclin E, Gadd45, p21, p53 and retinoblastoma protein (RB). 23,24 The more general tumor suppressor gene p53 has also been shown to localize to centrosomes and regulate centrosome duplication in a manner independent of its function as a transcription factor. 25 Another important group of proteins associated with the centrosomes are kinases like Aurora and Polo-like kinases (Plk) that are important for the integrity and correct separation of the mitotic spindle and therefore also for accurate segregation of chromosomes and correct cytokinesis (reviewed in references 26 -29).…”

Centrosomal abnormalities, multipolar mitoses, and chromosomal instability in head and neck tumours with dysfunctional telomeres

“…In particular, centrosomal aberrations have been correlated with a number of different genetic abnormalities in tumours, e.g. amplification of STK15 (Zhou et al, 1998), mutations in TP53 (Carroll et al, 1999), and inactivation of BRCA1 (Xu et al, 1999), BRCA2 (Tutt et al, 1999), and GADD45 (Hollander et al, 1999). In adenocarcinomas of the prostate and breast, centrosomal abnormalities are known to increase in parallel to loss of tissue differentiation and the development of aneuploidy (Pihan et al, 2001;Lingle et al, 2002).…”