Mapping tumour molecular mechanisms associated with common exposures: a new approach to identifying targets to prevent and treat cancer

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Every year, over 360,000 people in the UK are diagnosed with cancer and around 160,000 die as a result of the disease. Cancer costs the NHS over £5 billion annually, while the loss of human productivity due to cancer in the UK is estimated to be £18 billion a year. Above all, cancer impacts patients and their families in ways that are beyond measure. This makes cancer one of the most pressing societal challenges of this century.Cancer is a disease of the genome. Certain changes that are acquired over the course of life in the genomes of healthy cells in the human body (somatic genomic changes) dysregulate the fine balance between cell death and proliferation. These somatic genomic aberrations are the cornerstone of malignant cellular transformation. Targeting somatic genomic changes is fundamental to the practice of precision cancer medicine. We understand that common exposures and cancer risk factors such as ultraviolet light and smoking accelerate the acquisition of these changes. However, little is actually known about how everyday exogeneous and endogenous factors such as diet, obesity, and insulin resistance relate to, and likely drive, carcinogenic changes in the somatic genome. This is because it is difficult to measure lifelong trajectories of the factors retrospectively at cancer diagnosis and expensive to measure them prospectively in large numbers of individuals until some of them develop cancer. Such one-time "snapshot" measures, even where feasible, are prone to bias and confounding. Specific inherited or germline genetic variants have been found to be robustly associated with these exposures or factors. Since genetic variants are allocated at random at conception and fixed thereafter, they are less affected by bias and confounding. The factor-associated variants provide remarkable proxies for the lifetime levels of these factors even in patients in whom the factor itself has not been measured. These variants collected into polygenic scores serve as instruments in Mendelian randomisation (MR) studies that evaluate association between the germline genetically-inferred levels of the factor and a disease outcome. MR studies of cancer have so far been limited to an appraisal of the relationship between putative risk factors and cancer risk.The crucial conceptual advances being proposed here are the application of an MR-like approach to identify somatic/tumour molecular changes that operate within the cancer and are associated with factors such as obesity and the illumination of the role of the identified tumour molecular changes in driving cancer progression and response to cancer drugs. This novel shift in the conventional MR paradigm is challenging to accomplish but has dramatic potential for translational clinical impact. First, by testing for association between a comprehensive range of potentially modifiable everyday exposures and specific somatic genetic mechanisms on the pathway to cancer, the proposed research will generate a rich catalogue of precise molecular targets for further preventive intervention. The availability of a target would mean that such intervention could go beyond policies aimed at influencing behaviour and take the form of primary chemoprevention for high-risk populations. Second, these molecular targets with a clear and well-reasoned link to common exposures may serve as biomarkers for early detection and in the diagnostic or prognostic classification of cancer. Third, untangling the complex interplay between extrinsic/intrinsic exposures and the somatic genome and establishing the sequence of events from exposure to pre-malignancy to cancer may inform strategies for rational anti-tumour therapeutic development. An exhaustive set of tumour molecular changes will be evaluated but a particular focus will be on mutational signatures and anti-tumour immune cell infiltrate signatures, given that these may determine response to chemotherapy, and targeted and immuno-oncology treatments.