A cohort study of sex differences and prognostic biomarkers in colorectal cancer

Sammanfattning: Colorectal cancer (CRC) is one of the most common forms of cancer worldwide, with an annual incidence of more than 1 million cases. Despite advancements in the management of CRC, mortality remains high. Accumulating evidence indicates that CRC is a heterogeneous disease, which affects outcome beyond what can be predicted by disease stage and other conventional prognostic factors. Thus, there is a great need to identify new biomarkers to enable a more accurate prognostication, and to help select patients for adjuvant treatment. The aim of this thesis was to investigate the associations of a series of putative biomarkers with survival, treatment response and clinicopathological factors in a large cohort of incident CRC, with special attention to sex differences. The study group consisted of tumours from 557 incident cases of CRC in the Malmö Diet and Cancer Study (MDCS). The tumours, assembled in tissue microarrays, were evaluated for expression of cyclin D1, mismatch repair proteins, beta-catenin and epidermal growth factor receptor (EGFR) by immunohistochemistry, and further, EGFR gene copy number (GCN) alterations by brightfield double-in situ hybridization. In addition, KRAS and BRAF mutational status was assessed by pyrosequencing. Associations with clinicopathological and investigative factors were explored by Chi Square and Spearman’s correlation tests, and Kaplan-Meier analysis and Cox proportional hazards modelling were applied for survival analysis. We hereby found that nuclear cyclin D1 expression was associated with female sex and a favorable prognosis, although not independently, in male, but not in female, CRC. Microsatellite instability (MSI) correlated to distinctive clinicopathological features, cyclin D1 expression, and independently predicted a good prognosis in stage III-IV CRC. Moreover, beta-catenin overexpression correlated independently with a prolonged survival from stage III-IV CRC, and was associated with microsatellite stable (MSS) tumours. We also observed that KRAS codon 13 mutation predicted a poor prognosis in female CRC, but not independently of established prognostic factors. KRAS and BRAF mutations were mutually exclusive, and correlated with MSS and MSI, respectively. BRAF mutation was independently associated with a reduced survival in male patients with MSS CRC. Furthermore, both EGFR protein expression and GCN alterations were associated with a reduced survival in stage III-IV CRC, the latter, however, not independently of established prognostic factors. EGFR protein expression correlated significantly with EGFR GCN alterations, although a substantial proportion of EGFR expressing tumours displayed a normal GCN, and vice versa. Finally, EGFR alterations were significantly associated with a reduced survival in curatively treated patients with stage III-IV disease receiving adjuvant oxaliplatin. In conclusion, the results from this thesis demonstrate several relevant associations of the investigative biomarkers with prognosis and treatment response in CRC. Moreover, substantial sex differences in the clinicopathological correlates and prognostic significance of some of the biomarkers were observed.