Investigating disease and radiotherapy response associations with rectal tumour expression of the DNA damage response proteins, ATM, MRE11, NBS1 and RAD50

Abstract

Rectal cancers are thought to contribute approximately one third of all colorectal cancers worldwide, and are associated with considerable morbidity and mortality. Worryingly, the incidence of rectal cancer is increasing in developed economies, as genetics and environment converge to cause pathology in increasingly older populations. Despite current detailed knowledge of various molecular mechanisms responsible for oncogenesis, in general, the precise sequence of events causing disease, or influencing prognosis, in a particular patient is not completely understood. This is unsurprising given the multifactorial nature of disease and treatment responses in human populations with highly variable clinical histories. To overcome this knowledge gap, this thesis sought to further refine the understanding of the molecular mechanisms at work during rectal cancer development, and their effect on treatment responses and patient outcomes. Furthermore, by defining the molecular mechanisms of disease, biomarkers (single or multiple molecules whose expression levels serve to identify disease processes common to many patients with the same disease) can be developed and applied to clinical situations "" helping to diagnose, prognosticate and determine treatment modalities, depending on the application in question. To this end, and given the large heritage literature concerning DNA damage response proteins and cancer pathophysiology, the expression of four central DNA repair proteins (ATM, MRE11, NBS1 and RAD50) in rectal cancers has been quantified by immunohistochemistry. This will enable correlation of expression levels in different regions of the tumour with available clinicohistopathological variables, such as overall and disease-free survival. Furthermore, although radiotherapy represents a first-line treatment for rectal cancer, highly variable treatment responses have been documented amongst patients. As not all patients stand to benefit from such treatments, the expression of the candidate proteins "" central to repairing damaged DNA generated by radiotherapy "" and the association with radiotherapy responses are investigated in rectal tumours. Firstly, in the case of ATM, it was found that reduced expression in the growing edge of the tumour (tumour periphery) was associated with better responses to radiotherapy and improved disease-free survival. ATM expression in the tumour centre was also associated with disease-free survival by uni- and multi-variate analyses. Secondly, MRE11 expression was found to be predictive of patient outcomes, when patients were also scored positive for high-grade disease, metastasis positive, and showed perineural invasion. In contrast, NBS1 expression levels in rectal tumours were only found to have a marginal association with patient overall survival, necessitating additional studies of NBS1 in rectal cancer. Low RAD50 expression was associated with worse disease-free survival. RAD50 levels also proved to be useful to delineate low- and high-grade disease subgroups. Together, the analysis of these four markers individually, led to several novel associations with regards to rectal cancer "" highlighting their 'biomarker' potential in this clinical context. Furthermore, by combining expression of these proteins into combinatorial panels "" made up of either ATM and MRE11, or MRE11, NBS1 and RAD50 "" a greater predictive power of their expression levels with respect to patient outcomes was demonstrated, and support the use of multiple markers to better understand disease in different patient groups. Therefore, the utility of examining DDR proteins in the context of rectal cancer is demonstrated in this thesis, and the results provide evidence to support future studies investigating the roles of these proteins in larger rectal cancer patient cohorts and other cancers. Further studies and validation of the results of this thesis will help determine whether such proteins can serve as clinically-useful biomarkers for disease intervention.

Date of Award	2018
Original language	English