BRCA1, DNA damage response and aging

我们早期的研究表明，与BRCA1缺乏相关的遗传不稳定性会激活DNA损伤反应(DDR)，从而导致生长停滞和/或细胞凋亡。通过对BRCA1(delta11/delta11)p53+/-小鼠的分析，我们发现BRCA1全长缺失会导致突变胚胎和培养细胞的衰老以及成年小鼠的衰老和肿瘤发生。P53单倍体缺失克服了胚胎衰老，但未能阻止成年突变小鼠过早衰老，其特征是寿命缩短，体脂沉积减少，骨质疏松，皮肤萎缩，伤口愈合减少。我们进一步证明，逃脱衰老的突变细胞经历了克隆选择，以获得更快的增殖和广泛的遗传/分子变化，包括P53的丢失。这些观察提供了第一个体内证据，将细胞衰老与由于BRCA1的肿瘤抑制功能受损和P53的激活有关的衰老联系起来。然而，在缺乏BRCA1的情况下，导致P53激活的因素还知之甚少。 为了研究这一点，在过去的一年里，我们使用了一项遗传学测试，将BRCA1(delta11/+)小鼠与携带DDR途径基因靶向突变的突变小鼠杂交，这些突变包括53BP1、ATM、Chk1、Chk2、p19、Pten、PARP-1、p21和GADD45。我们的数据表明，53BP1、ATM或Chk2的失活相当于P53的失活，因为它取消了DDR，并允许BRCA1(delta11/delta11)胚胎存活到成年。这些观察支持一个模型，该模型表明BRCA1缺乏导致遗传不稳定，导致53BP1-ATM-Chk2-P53DDR信号的激活，而这反过来又是阻止BRCA1突变细胞恶性转化的天然屏障。我们发现Chk2表达的减少和/或Chk2的缺失使得BRCA1(delta11/delta11)小鼠能够逃脱胚胎死亡。与BRCA1(delta11/delta11)P53+/-小鼠相比，BRCA1(delta11/delta11)Chk2-/-小鼠的寿命显著延长。对BRCA1(delta11/delta11)Chk2-/-小鼠的分析表明，在快速增殖的器官中，BRCA1缺乏导致的P53依赖的细胞凋亡和生长缺陷明显减弱。然而，在后来的生活中，BRCA1(delta11/delta11)Chk2/-雌性小鼠患上了多种肿瘤。此外，ATM的单倍体缺失也挽救了BRCA1缺乏相关的胚胎死亡和过早衰老。因此，作为对BRCA1缺乏的反应，ATM-Chk2-P53信号通路的激活有助于抑制肿瘤转化，同时导致机体动态平衡受损。我们的数据突出表明，准确保持基因组完整性对于抑制衰老和恶性肿瘤至关重要，并提供了衰老和癌症之间的进一步联系。

Our earlier studies indicated that genetic instability associated with BRCA1 deficiency activates the DNA damage response (DDR) and thereby causes growth arrest and/or apoptosis. Analyzing Brca1(delta11/delta11)p53+/- mice, we found that the absence of full-length BRCA1 caused senescence in mutant embryos and cultured cells as well as aging and tumorigenesis in adult mice. Haploid loss of p53 overcame embryonic senescence but failed to prevent the adult mutant mice from prematurely aging, characterized by decreased life span, reduced body fat deposition, osteoporosis, skin atrophy and decreased wound healing. We further demonstrated that mutant cells that escaped senescence had undergone clonal selection for faster proliferation and extensive genetic/ molecular alterations including the loss of p53. These observations provide the first in vivo evidence that links cell senescence to aging due to impaired tumor suppression function of BRCA1 and activation of p53. However the factors responsible for p53 activation in the absence of Brca1 are poorly understood. To investigate this, in the past year, we employed a genetic test by crossing Brca1(delta11/+) mice with mutant mice carrying targeted mutations of genes in the DDR pathway, including 53BP1, ATM, Chk1, Chk2, p19, Pten, Parp-1, p21 and Gadd45. Our data indicated that 53BP1, ATM or Chk2 inactivation is equivalent to p53 inactivation in that it abolishes DDR and allows Brca1(delta11/delta11) embryos to survive to adulthood. These observations support a model indicating that BRCA1 deficiency results in genetic instability, leading to the activation of 53BP1-ATM-Chk2-p53 DDR signaling, which, in turn, serves as a natural barrier against malignant transformation of BRCA1 mutant cells. We show that reduced expression and/or the absence of Chk2 allow Brca1(delta11/delta11) mice to escape from embryonic lethality. Compared to Brca1(delta11/delta11)p53+/- mice, lifespan of Brca1(delta11/delta11)Chk2-/- mice was remarkably extended. Analysis of Brca1(delta11/delta11)Chk2-/- mice revealed that p53-dependent apoptosis and growth defect caused by Brca1 deficiency are significantly attenuated in rapidly proliferating organs. However, in later life, Brca1(delta11/delta11)Chk2-/- female mice developed multiple tumors. Furthermore, haploid loss of ATM also rescued Brca1 deficiency-associated embryonic lethality and premature aging. Thus, in response to Brca1 deficiency, the activation of the ATM-Chk2-p53 signaling pathway contributes to the suppression of neoplastic transformation, while leading to compromised organismal homeostasis. Our data highlight how accurate maintenance of genomic integrity is critical for the suppression of both aging and malignancy, and provide a further link between aging and cancer.