PhD ThesisColorectal cancer (CRC) is the third most common cancer in Western societies and
approximately 15% are mismatch repair deficient (MMRd). MMRd CRCs have a distinct
prognosis, respond to immunotherapy, and occur at a high rate in patients with Lynch
syndrome or constitutional mismatch repair deficiency (CMMRD). Detection of MMR
deficiency, therefore, guides treatment and identification of associated cancerpredisposition syndromes. However, there is a need for novel biomarkers to detect MMRd
CRC, and innovative assays to improve Lynch syndrome and CMMRD diagnosis.
I assessed autoantibodies generated against MMRd CRCs as a liquid-biopsy
biomarker for cancer detection, by analysing the sera of 464 Lynch syndrome gene carriers
using a recently published, multiplex method. Although autoantibodies correlated with a
history of CRC, a lack of signal from patients who developed CRC shortly after sampling
suggests the method has poor sensitivity. Microsatellite instability (MSI) is an established
biomarker of MMR deficiency. I used single molecule molecular inversion probes to develop
a sequencing-based MSI assay with an automated results analysis, suitable as a companion
diagnostic for immunotherapy, and for streamlined Lynch syndrome screening. The assay
achieved 100% accuracy in 197 CRCs, and was robust to sample variables, including quantity,
quality, and tumour cell content. Subsequently, I adapted the MSI assay to detect low-level
MSI in non-neoplastic tissues of CMMRD patients. The assay separated all 32 CMMRD
patients from 94 controls. For both CRC and CMMRD diagnostics, the MSI assay is cheaper
and faster than current methods, and is scalable to large cohorts.
These results suggest that the humoral immune response to MMRd CRCs cannot
readily be used as a biomarker to detect disease, and that alternatives should be sought.
However, the MSI assay could be deployed into clinical practice to meet the high demand for
MMR deficiency testing of CRCs and to improve CMMRD diagnostics.the Barbour Foundatio