DNA damage in blood cells in relation to chemotherapy and nutritional status in colorectal cancer patients.

Colorectal cancer (CRC) is the third most common cancer worldwide, with Norway ranking among the highest in incidence rate. Advancement in early detection and treatment strategies have resulted in a growing population of CRC survivors. In addition to the risk of cancer recurrence, CRC survivors also face an elevated risk of various chronic diseases compared to the general population. Accumulation of DNA damage caused by oxidative stress plays an essential role in the pathogenesis of CRC and several other life-style related chronic diseases affecting CRC survivors. DNA bases are particularly sensitive to oxidation mediated by oxidative stress. Furthermore, inflammation is closely related to oxidative stress as an underlying disease mechanism. Emerging evidence suggests that the diet has a strong impact on the integrity of DNA. A large number of bioactive compounds have been identified in plant foods which protecting DNA from oxidative damage via different modes of action. The Norwegian Dietary Guidelines and Colorectal Cancer Survival (CRC-NORDIET) study is a randomized controlled trial (RCT) designed to investigate the effect of a dietary intervention on disease outcome and survival in CRC patients. While the dietary intervention emphasized foods and drinks that have been suggested to dampen oxidative stress and inflammation, the complete intervention is fully in accordance with the prudent diet recommended by the Norwegian food-based dietary guidelines. In human biomonitoring, the comet assay (single-cell gel electrophoresis) is a widely used method to assess DNA damage and DNA repair in peripheral lymphocytes. The overall aim of this project (sub-project within the CRC-NORDIET) was to investigate the long-term host response and the effect of a dietary intervention in post-surgery CRC patients, utilizing the comet assay. The findings from this project revealed that patients diagnosed with regional disease (stage III) had a higher level of DNA base oxidation compared with those with localized disease (stage I-II) at the baseline visit (2-9 months post-surgery). Interestingly, the initial elevated levels of DNA base oxidation observed in CRC patients diagnosed with regional disease, subsequently decreased to levels comparable to patients with localized disease after 6 and 12 months. Further, at the baseline visit, in patients exposed to chemotherapy, DNA base oxidation was negatively associated with number of days since last dosage. Additionally, both high absolute and relative amounts of fat tissue, as well sedentary behaviour, were positively associated with DNA base oxidation in the chemotherapy group. Our findings demonstrates that a one-year personalized intensive dietary intervention led to reduction in DNA base oxidation. However, where we investigated whether dietary factors could affect DNA repair, no effect was observed after the first 6 months of the dietary intervention. In summary, this project suggests that DNA base oxidation varies according to tumour and lifestyle related factors in CRC patients treated for non-metastatic disease. Further, adherence to a prudent diet, which includes foods and drinks known to dampen oxidative stress and inflammation, may serve as a potential moderator for DNA protection against oxidation damage.

The work presented in this thesis has advanced the field by applying the comet assay to measure DNA damage and repair within a population of CRC patients treated for non-metastatic cancer. Additionally, our exploration on the effects of a dietary intervention has provided valuable insight into the role of a post-diagnostic diet. The findings from this project revealed that patients diagnosed with regional disease (stage III) had a higher level of DNA base oxidation compared with those with localized disease (stage I-II) at the baseline visit (2-9 months post-surgery). Interestingly, the initial elevated levels of DNA base oxidation observed in CRC patients diagnosed with regional disease, subsequently decreased to levels comparable to patients with localized disease after 6 and 12 months. Further, at the baseline visit, in patients exposed to chemotherapy, DNA base oxidation was negatively associated with number of days since last dosage. Additionally, both high absolute and relative amounts of fat tissue, as well sedentary behaviour, were positively associated with DNA base oxidation in the chemotherapy group. Our findings demonstrate that a one-year personalized intensive dietary intervention led to reduction in DNA base oxidation. However, where we investigated whether dietary factors could affect DNA repair, no effect was observed after the first 6 months of the dietary intervention. Finally, intervention studies should be designed to investigate whether shifting to a more antioxidant-rich and anti-inflammatory diet changes the levels of DNA base oxidation in CRC patients, resulting in better disease outcomes. These intervention studies should investigate the levels of DNA base oxidation, response to treatment, recurrence rate, and CRC-specific mortality. If confirmed in intervention studies, these results could be translated into dietary guidelines for CRC survivors. In future studies, we will assess the primary outcomes, as well as DNA damage and repair, at later follow-ups of the CRC-NORDIET study. These future studies may reveal any possible clinical relevance of the observed reduction in DNA base oxidation by a prudent antioxidant-rich and anti-inflammatory diet.

The randomized controlled trial ‘CRC-NORDIET’ is the first study to test whether dietary intervention in colorectal cancer (CRC) patients after surgery can improve disease-free survival. CRC patients are randomly assigned either to a group receiving intensive encouragement to adopt a diet in accordance with the World Cancer Research Fund (WCRF) recommendations, or to a control group receiving standard care. CRC patients are clinically assessed at baseline, 6 months, 1 year and 3 years, and will be monitored for up to 15 years after diagnosis. Peripheral blood mononuclear cells, buffy coats and plasma are stored frozen at -80°C. They will form the material basis for this PhD project. We will test whether postdiagnostic adherence to WCRF recommendations by CRC patients from the CRC-NORDIET trial improves disease-free survival. The comet assay (single cell gel electrophoresis) will be optimized for an automated high throughput sample processing: for each patient, we will have measures of basal DNA breakage, antioxidant resistance and endogenous oxidative damage. In addition, we will obtain data on individual capacity for DNA repair. These biomarkers taken together are reliable indicators of genome (in)stability. Comparing data from samples taken at different time-points will allow us to assess changes at the individual level, while we will also be able to compare data for the intervention group as a whole with data from the control group. This approach will test the hypothesis that post-diagnostic adherence to the modified diet will decrease endogenous DNA damage, increase resistance to exogenous oxidative damage, and boost DNA repair capacity. In addition, it will be possible to examine links between these biomarkers and the various clinical and biochemical markers that will emerge from the regular clinical assessments.