Targeted deep-intronic sequencing in a cohort of unexplained cases of suspected Lynch syndrome

Whole genome sequencing completes the molecular genetic testing workflow of patients with Lynch syndrome

Multigene panel tests (MGPTs) revolutionized the diagnosis of Lynch syndrome (LS), however noncoding pathogenic variants (PVs) can only be detected by complementary methods including whole genome sequencing (WGS). Here we present a DNA-, RNA- and tumor tissue-based WGS prioritization workflow for patients with a suspicion of LS where MGPT detected no LS-related PV. Among the 100 enrolled patients, MGPT detected 28 simple PVs and an additional 3 complex PVs. Among the 69 MGPT-negative patients, the lack of somatic MLH1 promoter methylation in a patient with a distinguished MLH1 allelic imbalance selected this sample for WGS. This returned a germline deep intronic MLH1 variant, with further functional studies confirming its' pathogenicity. Interestingly, all three complex PVs and the MLH1 deep intronic PV were found to be recurrent at our center. Our straightforward and cost-effective prioritization workflow can optimally include WGS in the genetic diagnosis of LS.

Spectrum and genotyping strategies of "dark" genetic matter in germline susceptibility genes of tumor syndromes

PURPOSE

Despite the widespread use of high-throughput genotyping strategies, certain mutation types remain understudied. We provide an overview of these often overlooked mutation types, with representative examples from common hereditary cancer syndromes.

METHODS

We conducted a comprehensive review of the literature and locus-specific variant databases to summarize the germline pathogenic variants discovered through non-routine genotyping methods. We evaluated appropriate detection and analysis methods tailored for these specific genetic aberrations. Additionally, we performed in silico splice predictions on deep intronic variants registered in the ClinVar database.

RESULTS

Our study suggests that, aside from founder mutations, most cases are sporadic. However, we anticipate a relatively high likelihood of splice effects for deep intronic variants. The findings underscore the significant clinical utility of genome sequencing techniques and the importance of applying relevant analysis methods.

Transcript capture and ultradeep long-read RNA sequencing (CAPLRseq) to diagnose HNPCC/Lynch syndrome

PURPOSE

Whereas most human genes encode multiple mRNA isoforms with distinct function, clinical workflows for assessing this heterogeneity are not readily available. This is a substantial shortcoming, considering that up to 25% of disease-causing gene variants are suspected of disrupting mRNA splicing or mRNA abundance. Long-read sequencing can readily portray mRNA isoform diversity, but its sensitivity is relatively low due to insufficient transcriptome penetration.

METHODS

We developed and applied capture-based target enrichment from patient RNA samples combined with Oxford Nanopore long-read sequencing for the analysis of 123 hereditary cancer transcripts (capture and ultradeep long-read RNA sequencing (CAPLRseq)).

RESULTS

Validating CAPLRseq, we confirmed 17 cases of hereditary non-polyposis colorectal cancer/Lynch syndrome based on the demonstration of splicing defects and loss of allele expression of mismatch repair genes MLH1, PMS2, MSH2 and MSH6. Using CAPLRseq, we reclassified two variants of uncertain significance in MSH6 and PMS2 as either likely pathogenic or benign.

CONCLUSION

Our data show that CAPLRseq is an automatable and adaptable workflow for effective transcriptome-based identification of disease variants in a clinical diagnostic setting.

A tumor focused approach to resolving the etiology of DNA mismatch repair deficient tumors classified as suspected Lynch syndrome

Routine screening of tumors for DNA mismatch repair (MMR) deficiency (dMMR) in colorectal (CRC), endometrial (EC) and sebaceous skin (SST) tumors leads to a significant proportion of unresolved cases classified as suspected Lynch syndrome (SLS). SLS cases (n = 135) were recruited from Family Cancer Clinics across Australia and New Zealand. Targeted panel sequencing was performed on tumor (n = 137; 80×CRCs, 33×ECs and 24xSSTs) and matched blood-derived DNA to assess for microsatellite instability status, tumor mutation burden, COSMIC tumor mutational signatures and to identify germline and somatic MMR gene variants. MMR immunohistochemistry (IHC) and MLH1 promoter methylation were repeated. In total, 86.9% of the 137 SLS tumors could be resolved into established subtypes. For 22.6% of these resolved SLS cases, primary MLH1 epimutations (2.2%) as well as previously undetected germline MMR pathogenic variants (1.5%), tumor MLH1 methylation (13.1%) or false positive dMMR IHC (5.8%) results were identified. Double somatic MMR gene mutations were the major cause of dMMR identified across each tumor type (73.9% of resolved cases, 64.2% overall, 70% of CRC, 45.5% of ECs and 70.8% of SSTs). The unresolved SLS tumors (13.1%) comprised tumors with only a single somatic (7.3%) or no somatic (5.8%) MMR gene mutations. A tumor-focused testing approach reclassified 86.9% of SLS into Lynch syndrome, sporadic dMMR or MMR-proficient cases. These findings support the incorporation of tumor sequencing and alternate MLH1 methylation assays into clinical diagnostics to reduce the number of SLS patients and provide more appropriate surveillance and screening recommendations.

A tumor focused approach to resolving the etiology of DNA mismatch repair deficient tumors classified as suspected Lynch syndrome

Routine screening of tumors for DNA mismatch repair (MMR) deficiency (dMMR) in colorectal (CRC), endometrial (EC) and sebaceous skin (SST) tumors leads to a significant proportion of unresolved cases classified as suspected Lynch syndrome (SLS). SLS cases (n=135) were recruited from Family Cancer Clinics across Australia and New Zealand. Targeted panel sequencing was performed on tumor (n=137; 80xCRCs, 33xECs and 24xSSTs) and matched blood-derived DNA to assess for microsatellite instability status, tumor mutation burden, COSMIC tumor mutational signatures and to identify germline and somatic MMR gene variants. MMR immunohistochemistry (IHC) and MLH1 promoter methylation were repeated. In total, 86.9% of the 137 SLS tumors could be resolved into established subtypes. For 22.6% of these resolved SLS cases, primary MLH1 epimutations (2.2%) as well as previously undetected germline MMR pathogenic variants (1.5%), tumor MLH1 methylation (13.1%) or false positive dMMR IHC (5.8%) results were identified. Double somatic MMR gene mutations were the major cause of dMMR identified across each tumor type (73.9% of resolved cases, 64.2% overall, 70% of CRC, 45.5% of ECs and 70.8% of SSTs). The unresolved SLS tumors (13.1%) comprised tumors with only a single somatic (7.3%) or no somatic (5.8%) MMR gene mutations. A tumor-focused testing approach reclassified 86.9% of SLS into Lynch syndrome, sporadic dMMR or MMR-proficient cases. These findings support the incorporation of tumor sequencing and alternate MLH1 methylation assays into clinical diagnostics to reduce the number of SLS patients and provide more appropriate surveillance and screening recommendations.

Unexplained mismatch repair deficiency: Case closed

To identify Lynch syndrome (LS) carriers, DNA mismatch repair (MMR) immunohistochemistry (IHC) is performed on colorectal cancers (CRCs). Upon subsequent LS diagnostics, MMR deficiency (MMRd) sometimes remains unexplained (UMMRd). Recently, the importance of complete LS diagnostics to explain UMMRd, involving MMR methylation, germline, and somatic analyses, was stressed. To explore why some MMRd CRCs remain unsolved, we performed a systematic review of the literature and mapped patients with UMMRd diagnosed in our center. A systematic literature search was performed in Ovid Medline, Embase, Web of Science, Cochrane CENTRAL, and Google Scholar for articles on UMMRd CRCs after complete LS diagnostics published until December 15, 2021. Additionally, UMMRd CRCs diagnosed in our center since 1993 were mapped. Of 754 identified articles, 17 were included, covering 74 patients with UMMRd. Five CRCs were microsatellite stable. Upon complete diagnostics, 39 patients had single somatic MMR hits, and six an MMR germline variant of unknown significance (VUS). Ten had somatic pathogenic variants (PVs) in POLD1, MLH3, MSH3, and APC. The remaining 14 patients were the only identifiable cases in the literature without a plausible identified cause of the UMMRd. Of those, nine were suspected to have LS. In our center, complete LS diagnostics in approximately 5,000 CRCs left seven MMRd CRCs unexplained. All had a somatic MMR hit or MMR germline VUS, indicative of a missed second MMR hit. In vitually all patients with UMMRd, complete LS diagnostics suggest MMR gene involvement. Optimizing detection of currently undetectable PVs and VUS interpretation might explain all UMMRd CRCs, considering UMMRd a case closed.

Reviewing the occurrence of large genomic rearrangements in patients with inherited cancer predisposing syndromes: importance of a comprehensive molecular diagnosis

INTRODUCTION

Hereditary cancer predisposition syndromes are caused by germline pathogenic or likely pathogenic variants in cancer predisposition genes (CPG). The majority of pathogenic variants in CPGs are point mutations, but large gene rearrangements (LGRs) are present in several CPGs. LGRs can be much more difficult to characterize and perhaps they may have been neglected in molecular diagnoses.

AREAS COVERED

We aimed to evaluate the frequencies of germline LGRs in studies conducted in different populations worldwide through a qualitative systematic review based on an online literature research in PubMed. Two reviewers independently extracted data from published studies between 2009 and 2020. In total, 126 studies from 37 countries and 5 continents were included in the analysis. The number of studies in different continents ranged from 3 to 48 and for several countries there was an absolute lack of information. Asia and Europe represented most of the studies, and LGR frequencies varied from 3.04 to 15.06% in different continents. MLPA was one of the methods of choice in most studies (93%).

EXPERT OPINION

The LGR frequencies found in this review reinforce the need for comprehensive molecular testing regardless of the population of origin and should be considered by genetic counseling providers.

Prevalence of CNV-neutral structural genomic rearrangements in MLH1, MSH2, and PMS2 not detectable in routine NGS diagnostics

Routine diagnostics for colorectal cancer patients suspected of having Lynch-Syndrome (LS) currently uses Next-Generation-Sequencing (NGS) of targeted regions within the DNA mismatch repair (MMR) genes. This analysis can reliably detect nucleotide alterations and copy-number variations (CNVs); however, CNV-neutral rearrangements comprising gene inversions or large intronic insertions remain undetected because their breakpoints are usually not covered. As several founder mutations exist for LS, we established PCR-based screening methods for five known rearrangements in MLH1, MSH2, or PMS2, and investigated their prevalence in 98 German patients with suspicion of LS without a causative germline variant or CNV detectable in the four MMR genes. We found no recurrence of CNV-neutral structural rearrangements previously described: Neither for two inversions in MLH1 (exon 1 and exon 16-19) within 33 MLH1-deficient patients, nor for two inversions in MSH2 (exon 1-7 and exon 2-6) within 48 MSH2-deficient patients. The PMS2 insertion in intron 7 was detected in one of 17 PMS2-deficient patients. None of the four genomic inversions constitutes a founder event within the German population, but we advise to test the rare cases with unsolved PMS2-deficiency upon the known insertion. As a next diagnostic step, tumour tissue of the unsolved patients should be sequenced for somatic variants, and germline analysis of additional genes with an overlapping clinical phenotype should be considered. Alternatively, full-length cDNA analyses may detect concealed MMR-defects in cases with family history.

Diagnosis of Lynch Syndrome and Strategies to Distinguish Lynch-Related Tumors from Sporadic MSI/dMMR Tumors

Simple Summary

Microsatellite instability (MSI) is a hallmark of Lynch syndrome (LS)-related tumors but is not specific, as most of MSI/mismatch repair-deficient (dMMR) tumors are sporadic. Therefore, the identification of MSI/dMMR requires additional diagnostic tools to identify LS. In this review, we address the hallmarks of LS and present recent advances in diagnostic and screening strategies to identify LS patients. We also discuss the pitfalls associated with current strategies, which should be taken into account in order to improve the diagnosis of LS.

Abstract

Microsatellite instability (MSI) is a hallmark of Lynch syndrome (LS)-related tumors but is not specific to it, as approximately 80% of MSI/mismatch repair-deficient (dMMR) tumors are sporadic. Methods leading to the diagnosis of LS have considerably evolved in recent years and so have tumoral tests for LS screening and for the discrimination of LS-related to MSI-sporadic tumors. In this review, we address the hallmarks of LS, including the clinical, histopathological, and molecular features. We present recent advances in diagnostic and screening strategies to identify LS patients. We also discuss the pitfalls associated with the current strategies, which should be taken into account to improve the diagnosis of LS and avoid inappropriate clinical management.