BRCA1, DNA damage response and aging

我们早期的研究表明，与BRCA 1缺陷相关的遗传不稳定性激活DNA损伤反应（DDR），从而导致生长停滞和/或凋亡。分析Brca 1（delta 11/delta 11）p53+/-小鼠，我们发现，全长BRCA 1的缺失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的单倍体丢失克服了胚胎衰老，但未能防止成年突变小鼠过早衰老，其特征在于寿命缩短，体脂沉积减少，骨质疏松症，皮肤萎缩和伤口愈合减少。我们进一步证明了逃避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/分子改变，包括p53的丢失。这些观察结果提供了第一个体内证据，表明细胞衰老与BRCA 1的肿瘤抑制功能受损和p53激活有关。然而，在缺乏Brca 1的情况下负责p53激活的因素知之甚少。 为了研究这一点，在过去的一年中，我们采用了一种遗传测试，将Brca 1（delta 11/+）小鼠与携带DDR途径中靶向基因突变的突变小鼠杂交，包括53 BP 1，ATM，Chk 1，Chk 2，p19，Pten，Parp-1，p21和Gadd 45。我们的数据表明，53 BP 1，ATM或Chk 2失活与p53失活相当，因为它废除了DDR，并允许Brca 1（delta 11/delta 11）胚胎存活至成年。这些观察结果支持了一个模型，该模型表明BRCA 1缺陷导致遗传不稳定性，导致53 BP 1-ATM-Chk 2-p53 DDR信号转导的激活，这反过来又充当了抵抗BRCA 1突变细胞恶性转化的天然屏障。 我们表明，减少表达和/或Chk 2的情况下，允许Brca 1（delta 11/delta 11）小鼠从胚胎致死逃脱。与Brca 1（delta 11/delta 11）p53+/-小鼠相比，Brca 1（delta 11/delta 11）Chk 2-/-小鼠的寿命显着延长。对Brca 1（delta 11/delta 11）Chk 2-/-小鼠的分析显示，在快速增殖的器官中，由Brca 1缺乏引起的p53依赖性凋亡和生长缺陷显著减弱。然而，在以后的生活中，Brca 1（delta 11/delta 11）Chk 2-/-雌性小鼠发生了多个肿瘤。此外，ATM的单倍体丢失也挽救了Brca 1缺陷相关的胚胎致死性和过早衰老。因此，在响应Brca 1缺陷，ATM-Chk 2-p53信号通路的激活有助于抑制肿瘤转化，同时导致受损的生物体内平衡。我们的数据强调了基因组完整性的准确维护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.