Abstract 4049: The gain-of-function properties of mutant p53 rescues cells from telomere dysfunction-induced crisis and promotes tumorigenesis.
Authors: Chang Li-Ju , Wu Fengxia , Guo Xiaolan , Deng Yibin 
Published: August 2014

Abstract
Telomeres are nucleoprotein complexes that cap the ends of eukaryotic chromosomes and maintain chromosomal stability. Telomeres are protected by shelterin, a complex of six core proteins (TRF1, TRF2, RAP1, TIN2, TPP1 and POT1) that prevents the chromosome ends from being mistakenly recognized as damaged DNA. Telomeres that can no longer exert end-protective functions are said to be dysfunctional. Telomere dysfunction can be caused by progressive telomere attrition (telomere shortening) or through disruption of the shelterin complex (telomere uncapping). To study whether chromosomal instability caused by telomere uncapping leads to tumorigenesis, we used the Cre-lox system to delete the telomere binding protein TRF2 in mouse embryonic fibroblasts (MEFs). In the presence of tumor suppressor protein p53, loss of TRF2 inhibited cell proliferation via induction of cellular senescence. In contrast, the absence of p53 allowed the TRF2-deficient cells to bypass senescence as these cells were negative for SA-β-galactosidase staining. However, these p53-deficient cells stopped proliferating ten days after TRF2 depletion and eventually entered telomere uncapping-induced crisis (indicated by the significant appearance of chromosomal end to end fusions, mitotic failure and induction of apoptosis). Our mechanistic studies revealed that these p53-deficient cells underwent a novel ATM-caspase 2-dependent apoptosis upon loss of TRF2. Accordingly, knocking out caspase 2 rescued these cells from apoptosis, and thus increased long-term survival of the cell colonies as well as promoted cell transformation. Mutations of p53 are predominantly missense mutations that result in accumulation of mutant p53 proteins with loss of wild-type p53 tumor-suppressive functions and gain of oncogenic activities. To study whether the gain-of-function properties of mutant p53 can rescue cells undergoing telomere crisis from cell death, we generated MEFs expressing mutant p53R172H that corresponds to the p53R175H “hot spot” mutation in human cancers. We observed that mutant p53R172H did not prevent TRF2 depletion-induced chromosomal instability as indicated by extensive chromosome fusions. However, expression of mutant p53R172H significantly rescued cells from TRF2 loss-initiated cell apoptosis and promoted cell transformation as determined by anchorage-independent soft agar assay. More importantly, mutant p53R172H promoted tumor development of TRF2-deficient MEFs in immunodeficient mice. Collectively, our findings provide compelling genetic evidence that telomere uncapping-driven chromosomal instability activates ATM-caspase 2-dependent apoptosis to suppress tumorigenesis. However, the gain-of-function properties of mutant p53 can rescue the cells undergoing telomere crisis from cell death and thus promote tumorigenesis.
Citation Format: Li-Ju Chang, Fengxia Wu, Xiaolan Guo, Yibin Deng. The gain-of-function properties of mutant p53 rescues cells from telomere dysfunction-induced crisis and promotes tumorigenesis. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 4049. doi:10.1158/1538-7445.AM2013-4049