Detection of Microsatellite Instability in Colonoscopic Biopsies and Postal Urine Samples from Lynch Syndrome Cancer Patients Using a Multiplex PCR Assay

Simple Summary Lynch syndrome is caused by inherited defects in genes which repair DNA, and predisposes affected individuals to cancers of the bowel, endometrium (uterus lining), and urinary tract, among others. Most of these cancers exhibit a characteristic pattern of DNA instability in Lynch syndrome patients. Here, we adapt an existing DNA instability test for use with small clinical samples, and show that it works with tumour biopsies taken during colonoscopic investigation just as effectively as with larger samples taken during surgery. We also analyse preoperative postal urine samples where DNA recovery is limited, identifying instability in samples from a Lynch syndrome patient with a urinary tract cancer, and a patient with a suspected endometrial cancer. The assay could enable earlier detection of these tumours, benefiting these patients. Identification of mismatch repair (MMR)-deficient colorectal cancers (CRCs) is recommended for Lynch syndrome (LS) screening, and supports targeting of immune checkpoint inhibitors. Microsatellite instability (MSI) analysis is commonly used to test for MMR deficiency. Testing biopsies prior to tumour resection can inform surgical and therapeutic decisions, but can be limited by DNA quantity. MSI analysis of voided urine could also provide much needed surveillance for genitourinary tract cancers in LS. Here, we reconfigure an existing molecular inversion probe-based MSI and BRAF c.1799T > A assay to a multiplex PCR (mPCR) format, and demonstrate that it can sample >140 unique molecules per marker from <1 ng of DNA and classify CRCs with 96-100% sensitivity and specificity. We also show that it can detect increased MSI within individual and composite CRC biopsies from LS patients, and within preoperative urine cell free DNA (cfDNA) from two LS patients, one with an upper tract urothelial cancer, the other an undiagnosed endometrial cancer. Approximately 60-70% of the urine cfDNAs were tumour-derived. Our results suggest that mPCR sequence-based analysis of MSI and mutation hotspots in CRC biopsies could facilitate presurgery decision making, and could enable postal-based screening for urinary tract and endometrial tumours in LS patients.