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Title: Colorectal cancer in those with a genetic susceptibility
Author: Newton, Katherine
Awarding Body: University of Manchester
Current Institution: University of Manchester
Date of Award: 2013
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In the UK colorectal cancer is the second most common cancer in women and the third most common in men. Colorectal cancer is a heterogeneous disease. Improved knowledge of the molecular pathways involved in colorectal carcinogenesis has yet to translate into improved clinical outcomes. This thesis investigates three different aspects of colorectal cancer caused by a genetic susceptibility. Through these three separate studies this thesis aims to understand the relationship between molecular genetics and the screening for, clinical behaviour of, and outcomes following, colorectal cancer. Study 1: Background: Lynch syndrome patients have DNA mismatch repair deficiency and up to an 80% life-time risk of colorectal cancer. Screening of mutation carriers reduces colorectal cancer incidence and mortality. Selection for germline mutation testing relies on family history (Amsterdam and Bethesda Criteria) and tumour derived biomarkers. Initial tumour biomarker analysis uses mismatch repair protein immunohistochemistry and microsatellite instability. Abnormalities in either identify mismatch repair deficiency but do not differentiate sporadic epigenetic defects, due to MLH1 promoter region methylation (13% of all CRCs), and Lynch Syndrome (4% of all CRCs). A diagnostic biomarker capable of making this distinction would be valuable. This study compared two biomarkers in tumours with known mismatch repair; quantitative methylation of the MLH1 promoter region using a novel assay and BRAF V600E mutation in identification of germline mutations. Methods: Tumour DNA was extracted from preserved (FFPE) tumour tissue and pyrosequencing used to test for MLH1 promoter methylation and BRAF p.V600E mutation in 71 CRCs from individuals with pathogenic MLH1 mutations and 73 CRCs with sporadic MLH1 loss. Specificity and sensitivity were compared. Results: Unmethylated MLH1 promoter: sensitivity 94.4% (95% CI 86.2-98.4%), specificity 87.7% (95% CI 77.9-94.2%), Wild-type BRAF p.V600E: sensitivity 65.8% (95% CI 53.7-76.5%), specificity 98.6% (95% CI 92.4-100.0%) for the identification of those with pathogenic germline MLH1 mutations. Conclusions: Quantitative MLH1 promoter region methylation using pyrosequencing is superior to BRAF mutation in identifying germline mutations in mismatch repair deficient tumours. Study 2: Background: Familial adenomatous polyposis (FAP) is a colorectal cancer predisposition syndrome caused by autosomal dominantly inherited mutations in the Adenomatous Polyposis Coli (APC) gene. It has been divided into three clinical subtypes; mild, classical and severe FAP. MutYH associated polyposis is a recently described autosomal recessively inherited condition which causes mild polyposis and an increased risk of colorectal cancer. Prior to the discovery of the MutYH gene these patients were thought to have mild FAP. The aim of this study was to investigate for a correlation between genotype (site of the APC mutation, or MutYH mutation) and phenotype. Method: A retrospective longitudinal study of 492 patients on the Manchester Polyposis Registry was conducted. Patients were grouped according to genotype: 0; unknown mutation, 1; APC 0-178 (and 312-412 of exon 9), 2; APC>1550, 3; APC 179-1249, 4; APC 1250-1549, 5; MutYH. Date of onset of polyposis, incidence of colorectal cancer (CRC), actuarial survival and actuarial time to surgery were calculated. Results: Median age of onset of polyposis: genotype 0; 20.3 years, genotype 1; 35.6 years, genotype 2; 32.2, genotype 3; 15.9 years, genotype 4; 14.8 years (p<0.0001). Age and onset of CRC was similar between genotypes. Median survival: genotype 0; 56.6, genotype 1; 74.9 years, genotype 2; 61.0, genotype 3; 63.0, genotype 4; 48.1, genotype 5; 69.7 (p=0.003).This survival difference was also seen when patients who underwent screening and those who did not were analysed separately. Patients with genotype 4 (APC 1249-1549) have a significantly worse survival despite screening and early prophylactic surgery. Conclusions: This analysis supports a genotype-phenotype correlation. Patients with a mutation APC 1249-1549 develop polyposis younger and have a worse survival. Patients with a mutation APC 0-178 or 312-412 develop polyposis later and have an improved survival. This is the first study to demonstrate this survival difference.Study 3: Background: Life-time risk of metachronous colorectal cancer is 0.6%-3% following sporadic colorectal cancer and 15-26% in Lynch syndrome. The life-time incidence of incident colorectal cancer in individuals with moderate familial risk is 8-17%. Risk of metachronous colorectal cancer is unknown. Method: A retrospective longitudinal study of the Regional Familial colorectal cancer Registry was performed. Patients who had at least one colorectal cancer were categorised as follows: moderate risk (n=383), LS (n=528) and population risk (n=409). Kaplan-Meier estimate (1-KM) and cumulative incidence function (CI) were used to calculate the risk of metachronous colorectal cancer. 1-KM gives the risk for individuals remaining at risk (alive) at a given time point thus is useful for counselling. CI gives the risk for the whole population. Results: 1-KM and CI demonstrated that the risk of metachronous colorectal cancer was significantly higher in moderate risk patients compared with population risk (1-KM p= 0.008, CI p= <0.005). Both were lower than Lynch Syndrome. Moderate risk 1-KM was 2.7%, 6.3% and 23.5% at five, ten and 20 years. Population risk 1-KM was 1.3%, 3.1% and 7.0% at 5, 10 and 20 years and CI was 0.3%, 0.6% and 2.4%. Conclusion: These data indicate that the risk of metachronous colorectal cancer is significantly higher in patients with a moderate family history than in those at population risk. This justifies pro-active life-long surveillance.
Supervisor: Evans, Gareth Sponsor: Not available
Qualification Name: Thesis (M.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available