Comprehensive Paired Tumor/Germline Testing for Lynch Syndrome: Bringing Resolution to the Diagnostic Process

Abstract

PURPOSE The current diagnostic testing algorithm for Lynch syndrome (LS) is complex and often involves multiple follow-up germline and somatic tests. We aimed to describe the results of paired tumor/germline testing performed on a large cohort of patients with colorectal cancer (CRC) and endometrial cancer (EC) to better determine the utility of this novel testing methodology. MATERIALS AND METHODS We retrospectively reviewed a consecutive series of patients with CRC and EC undergoing paired tumor/germline analysis of the LS genes at a clinical diagnostic laboratory (N = 702). Microsatellite instability, MLH1 promoter hypermethylation, and germline testing of additional genes were performed if ordered. Patients were assigned to one of five groups on the basis of prior tumor screening and germline testing outcomes. Results for each group are described. RESULTS Overall results were informative regarding an LS diagnosis for 76.1% and 60.8% of patients with mismatch-repair–deficient (MMRd) CRC and EC without and with prior germline testing, respectively. LS germline mutations were identified in 24.8% of patients in the group without prior germline testing, and interestingly, in 9.5% of patients with previous germline testing; four of these were discordant with prior tumor screening. Upon excluding patients with MLH1 promoter hypermethylation and germline mutations, biallelic somatic inactivation was seen in approximately 50% of patients with MMRd tumors across groups. CONCLUSION Paired testing identified a cause for MMRd tumors in 76% and 61% of patients without and with prior LS germline testing, respectively. Findings support inclusion of tumor sequencing as well as comprehensive LS germline testing in the LS testing algorithm. Paired testing offers a complete, convenient evaluation for LS with high diagnostic resolution.

Immune Checkpoint Inhibition in Colorectal Cancer: Microsatellite Instability and Beyond

“…In the analysis of ICI clinical trials, MMR deficiency origin was determined by investigators based on past medical history collected from clinical records without a systematic molecular approach that might skew the results [1,11,28]. Moreover, it has been recently proven that Lynch-like tumors (MSI tumors without MLH1 promotor hypermethylation, BRAF mutation, or germline MMR gene mutation) arise from biallelic somatic MMR gene inactivation in approximately 50% of patients [29]. Therefore, further studies are warranted to confirm these findings.…”

Tumor-Based Genetic Testing and Familial Cancer Risk

“…Approximately 15% of colorectal tumors will show high MSI, with about one-third of those having Lynch syndrome (de la Chapelle and Hampel 2010). Not all MSI-high tumors will have Lynch syndrome as other events, particularly somatic hypermethylation of the MLH1 promoter region and biallelic somatic inactivation of one of the genes associated with Lynch syndrome, can also lead to MSI-high findings (Salvador et al 2019).…”

“…As part of the tumorigenesis process, the wild-type allele will characteristically become nonfunctional (through an acquired somatic variant or loss of heterozygosity) leading to a lack of protein expression in tumor tissue, whereas normal tissue will retain detectable protein levels because of one functional wild-type allele. Abnormal IHC results can also be acquired, similar to high microsatellite instability, through acquired biallelic somatic variants and hypermethylation of the MLH1 promoter region (Salvador et al 2019).…”

Immunohistochemistry in screening for heritable colorectal cancer: what to do with an abnormal result

“…NGS allows simultaneous evaluation of MSI and multiple genes including MLH1 , MSH2 , PMS2 , MSH6 and EPCAM , effectively collapsing screening into a single‐step process . Several studies have verified its superiority compared to current multi‐step LS screening algorithms and its practicality has already been demonstrated in the investigation of other tumours . With the increasing use and availability of NGS, we can quite conceivably achieve a more straightforward screening and diagnostic process for LS in the near future.…”