Selective activation of p53-mediated tumour suppression in high-grade tumours

Non-small cell lung carcinoma (NSCLC) is the leading cause of cancer-related death worldwide, with an overall 5-year survival rate of only 10–15% 1. Deregulation of the Ras pathway is a frequent hallmark of NSCLC, often through mutations that directly activate Kras 2. p53 is also frequently inactivated in NSCLC and, since oncogenic Ras can be a potent trigger of p53 3, it seems likely that oncogenic Ras signalling plays a major and persistent part in driving the selection against p53. Hence, pharmacological restoration of p53 is an appealing therapeutic strategy for treating this disease 4. Here, we model the likely therapeutic impact of p53 restoration in a spontaneously evolving mouse model of NSCLC initiated by sporadic oncogenic activation of endogenous Kras 5. Surprisingly, p53 restoration failed to induce significant regression of established tumours although it did result in a significant decrease in the relative proportion of tumours classed as high grade. This is due to selective activation of p53 only in the more aggressive tumour cells within each tumour. Such selective activation of p53 correlates with marked up regulation in Ras signal intensity and induction of the oncogenic signalling sensor p19(ARF) 6. Our data indicate that p53-mediated tumour suppression is triggered only when oncogenic Ras signal flux exceeds a critical threshold. Importantly, the failure of low-level oncogenic Kras to engage p53 reveals inherent limits in the capacity of p53 to restrain early tumour evolution and to the efficacy of therapeutic p53 restoration to eradicate cancers.