Abstract 1310: ATR checkpoint kinase suppression in combination with oncogenic Ras expression synergistically increases genomic instability and potentiates tumorigenesis in mice
Authors: Gilad Oren , Nabet Barzin , Durham Amy C. , Brown Eric J. 
Published: February 2011

Abstract
Oncogenic stress, such as that induced by expression of constitutively active forms of Ras, leads to the activation of the ATR-Chk1 pathway. The ATR-Chk1 signaling network is responsible for maintaining genome stability during S phase by preventing replication fork collapse into DNA double strand breaks (DSB). We asked whether oncogenic Ras expression increases the reliance on the ATR pathway for genome maintenance. Here, we report that activation of the ATR-Chk1 pathway functions prominently in suppressing genomic instability following oncogenic Ras expression and limits progression to malignancy in vivo. Independently derived isogenic mouse fibroblasts cultures expressing multiple copies of H-RasG12V or a single copy K-RasG12D exhibited significantly elevated levels of ATR-dependent Chk1 phosphorylation in comparison to untransformed controls. Consistent with a critical function for the ATR-Chk1 pathway in maintaining genome stability under oncogenic stress, H2AX phosphorylation and the frequency of sister chromatid exchange were increased to significantly higher levels by partial ATR suppression or Chk1 inhibition in H-RasG12V-transformed cells than in untransformed counterparts. The effect of combining ATR-Chk1 pathway suppression with H-RasG12V expression on genomic instability was greater than that observed following ATR-Chk1 pathway suppression or H-RasG12V expression alone and was not due to increased S phase representation. These results indicate that ATR limits genomic instability following oncogenic Ras expression and putatively provides an important barrier to further genetic alterations that contribute to malignant progression in vivo. To test this model, endogenous levels of K-RasG12D were conditionally induced via mosaic cre/lox activation of the K-RasG12D-LSL allele in ATR wild-type and haploinsufficient mice. We demonstrate that ATR haploinsufficiency synergizes with K-RasG12D expression to cause significantly increased incidences of lung adenocarcinoma, thymic lymphoma and spindle cell sarcoma. Notably, increased tumor incidence in ATR haploinsufficient mice was associated with elevated p53 LOH, consistent with a greater reliance on ATR for maintaining genomic stability under conditions of oncogenic stress. These data indicate that single copy loss of ATR or, potentially, other members of the ATR-Chk1 pathway (&gt;20) may accelerate the malignant progression cancers induced by oncogenic forms of Ras in humans. Genetic alterations such as these may provide selectivity for therapeutic strategies that involve further suppression of ATR-Chk1 pathway activity to levels that are incompatible with continued cellular proliferation.
Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 1310.