DNA Adductomics for Colorectal Cancer Investigation

Colorectal cancer (CRC) develops as a result of multiple genetic mutations causing normal intestinal epithelium to transform into a colorectal carcinoma. Genetic mutations may be caused by many different genetic events including chemical damage to the DNA nucleosides. These chemical modifications are due to both exogenous compounds coming from diet, environment and gut microbiota, or endogenous compounds produced by our own metabolic processes like inflammation and oxidative stress. Such genetic alteration is thought to be the starting event leading to development of sporadic CRC. However, there is little understanding on which DNA nucleoside modifications are associated with increased risk of CRC and their mechanism of action. The study of these DNA nucleoside modifications has been addressed in the recent years by a new research field, called DNA adductomics. DNA adductomics uses the new advanced high resolution mass spectrometry (HRMS) instrumentations for identifying the complexes that are formed between toxic compounds and DNA, namely DNA adducts.

Some studies have been previously identified DNA adducts in CRC with older technology. However, there is not a real evidence on which DNA adducts are related to sporadic CRC, hereditary non polyposis colorectal cancer (HNPCC) and other diseases such as familial adenomatous polyposis (FAP) which turns into CRC with a 95% risk before the age of 35. The lack of more recent human studies in DNA adductomics is mainly due to the lack of appropriate analytical methods. Developing such methods requires sufficient sample material and the amount of sample in a colon biopsy is too low to be used for method development. In this study, colon epithelial tissue obtained by resection of colon during surgery will be used for developing a more sensitive method, possibly allowing DNA adduct analysis from biopsies in future studies. In order to ascertain that the developed method can differentiate the level of DNA-adducts between inherited CRC, sporadic CRC and non-CRC subjects, also materials from other groups coming to the hospital for colon resections will be obtained. By analyzing the materials obtained in a case-control manner, we might also be able to resolve whether some of the DNA adducts differ between the different CRC cases or in comparison with cancer-free subjects. This knowledge should provide a preliminary basis for suggesting prevention and intervention approaches to reduce morbidity and mortality from CRC.

However, in case-control studies, a proper selection of the subjects should be carried out by assuring gender and age balance between the control group and the CRC group. This will be difficult in the first part of the current study since 1) there is limited possibility of obtaining resected colon from healthy subjects 2) CRC incidence rates are markedly higher in men than in women, and 3) different types of CRC develop at different ages. It is also obvious that a method relying on analyses of colon resections would have a limited application in preventive medicine. A solution to these issues may be the use of appropriate surrogate samples like blood, faeces and urine. Indeed, since DNA lesions may be removed from the genome by the DNA repair system, they are often excreted in urine, in faeces, or in blood. In order to know whether we may substitute tissues with surrogate samples, we will explore whether there is a correlation between DNA-repair product level in surrogate samples and DNA adducts in colon tissues. Substituting colon tissues with surrogates, or developing a sensitive method for DNA analysis from biopsies, would allow an easier collection of the samples, giving the possibility, in the future, of performing large and controlled clinical studies as well as less invasive sampling from patients. This could allow to confirm a causal relationship between specific DNA-adducts and CRC, providing real advances in prevention and intervention approaches.

Finally, after the identification of the DNA adducts and DNA-repair products possibly associated with CRC, it will be important to identify the real cause of DNA adducts formation. Our final purpose is therefore identifying which life-style, dietary or environmental factors are possibly associated with the DNA adducts and DNA-repair products identified in colon and surrogate samples, respectively. For this purpose, we will perform a metabolic profiling of serum, urine and faeces, a microbial profiling of faeces, and we will correlate it with basic information on patient life styles about smoking, alcohol consumption and intake of red meat, e.g. factors suspected to influence risk of colonic diseases.

Establishing a causal relationship between specific DNA-adducts and CRC or other colonic diseases, and understanding the causes for DNA adducts formation, will not only yield much richer insights into the molecular defects but will also result in advances in prevention and intervention approaches.