Early attempts to create animal models for BRCA1 associated breast cancer were not successful, as BRCA1 deficiency invariably results in embryonic lethality, primarily due to elevated cell death and growth retardation (28
). An animal model carrying mammary specific disruption of BRCA1 (BRCA1Co/Co;MMTV-Cre
) also exhibited increased apoptosis and abnormal mammary branch morphogenesis before tumor formation at low frequency with long latency (25
). Why do BRCA1 deficient cells die? Mounting experimental evidence indicates that organisms have acquired anti-proliferative and cell death–inducing mechanisms that prevent clonal expansion of mutant cells (104
). A key mechanism is the DDR, which can be activated not only by DNA damage but also by some other conditions, such as oncogene expression, loss of tumor suppressors (105
). Recently, it was shown that during the early cancerous development of lung or bladder tumors, oncogene induced aberrations in DNA replication activates DDR, as evidenced by the posphorylation of H2AX (γH2AX) followed by the activation of ATM-Chk2-p53 signaling in the tumor tissues (106
). The progression to malignant transformation requires the inactivation of DDR, which would in turn create genetic instability and accelerate cancer evolution.
Does BRCA1 deficiency activate DDR and thereby cause growth arrest and/or apoptosis? Several lines of evidence indicate that it may be the case. It was shown that the death of BRCA1 mutant cells is directly linked to the activation of p53, as deletion of p53 and/or its downstream mediator p21 partially rescues BRCA1-null embryos (28
). Moreover, it was shown that elimination of either one or both wild-type p53
alleles completely overcame embryonic lethality caused by the targeted deletion of full-length Brca1 and allowed Brca1Δ11/Δ11
mutant mice to survive to adulthood (29
). Further analysis indicated that haploid loss or complete loss of p53 resulted in attenuated apoptosis and G1
-S checkpoint control, allowing Brca1Δ11/Δ11
cells to proliferate (29
). Interestingly, most Brca1Δ11/Δ11;p53+/−
male mice suffered premature aging, exhibiting higher expression levels of p53 compared with controls (108
). This suggests that remaining wild-type p53 is activated, which may be a cause for premature aging.
It has been reported that over 90% of human BRCA1 deficient breast cancers also bear p53 mutations, while p53 alterations are only found in about 40% sporadic breast cancers (109
). Furthermore, most of mammary tumors developed in the BRCA1Co/Co;MMTV-Cre
mouse model also spontaneously mutated their p53 (25
), suggesting that the loss of p53 may be responsible for tumorigenesis. To directly test the role of p53 in BRCA1 associated tumorigenesis, a p53-null mutation allele was introduced into BRCA1 conditional mutant model, and the data indicated that introduction of a p53-null mutation into these mice can significantly accelerate mammary tumor formation (25
). This data indicated that inactivation of p53 and Brca1 deficiency synergistically induce mammary tumor formation. However the factors responsible for p53 activation in the absence of Brca1 are poorly understood. To investigate this, we employed a genetic test by crossing Brca1Δ11/+
mice with mutant mice carrying targeted mutations of genes in the DDR pathway, including ATM, Chk1, Chk2, p19, Pten, Parp-1, p21 and Gadd45. Our data indicated that ATM or Chk2 inactivation is equivalent to p53 inactivation in that it allows Brca1Δ11/Δ11
embryos to survive to adulthood (L. Cao and C. X. Deng unpublished data). An earlier investigation also revealed that Brca1 deficiency resulted in Chk2 phosphorylation and the Chk2-dependent activation of p53 (110
). These observations support a model indicating that BRCA1 deficiency results in genetic instability, leading to the activation of ATM-Chk2-p53 DDR signaling, which, in turn, serves as a natural barrier against malignant transformation of BRCA1 mutant cell ().
A model illustrating connections among cell cycle checkpoints, centrosome duplication, DNA damage repair, genetic instability, DNA damage response, developmental abnormalities and tumorigenesis caused by BRCA1 deficiency.
Experimental data also indicate that the inactivation of DDR is not sufficient for Brca1 deficient cells to undergo malignant transformation. Brca1 mutant mice in either a p53 or Chk2 mutant genetic background developed tumors in a stochastic fashion, suggesting additional factor(s) is needed to tumorigenesis. Consistent with this notion, it has been demonstrated that BRCA1 associated tumors are frequently associated with increased expression of oncogenes, such as cyclin D1, c-Myc and ErbB2, and loss of heterozygosity of tumor suppressor genes (23