Mismatch Repair Deficiency Testing Research Articles

Real-world genetic testing outcomes of pan-cancer testing for mismatch repair deficiency.

In 2017, the Food and Drug Administration approved pembrolizumab for treatment of any mismatch repair-deficient (dMMR) tumor making MMR immunohistochemistry (IHC) testing beneficial for all tumor types. For the first time, MMR IHC was not performed exclusively to screen for Lynch syndrome (LS). In this study, all MMR IHC reports issued between 2017 and 2021 at an academic hospital were reviewed and completion of genetic testing was determined through chart review. Colorectal cancers (CRCs), endometrial cancers (ECs), and noncancerous lesions were excluded. Between 2017 and 2021, MMR IHC was completed in 1939 patients with a malignancy other than CRC or EC. Absent or weak staining for at least one MMR protein was detected in 115 (5.9%) patients and 59 (51%) of those completed germline genetic testing. Overall, the identification rate of LS in this cohort was 0.72%, which is similar to the rate in our previously reported CRC and EC universal screening cohort. A diagnosis of LS was most commonly made in patients with dMMR brain (18.75%) and small intestinal cancers (10.20%). Five additional patients were found to carry a pathogenic variant in a non-LS gene. Pan-cancer MMR testing for pembrolizumab consideration can identify LS cases at a rate similar to universal CRC and EC screening programs. A persistent challenge is subsequent uptake of genetic testing. MMR testing should be prioritized in brain and small intestinal tumors, and multigene panel testing is recommended in patients with dMMR, as unexpected pathogenic variants in non-LS genes were found as frequently as LS gene variants.

Current Management Practices for Endometrial Cancer (EC) in the UK: A National Healthcare Professional Survey (KNOW-EC)

The clinical landscape for endometrial cancer in the UK is evolving to include new management guidelines and targeted treatment options. An understanding of current treatment and management practices in the UK will help services plan and adapt to upcoming changes. AimThe purpose of this survey was to understand current and anticipated real-world practices for endometrial cancer care in the UK and potential areas for optimisation. Materials and MethodsTelephone interviews were conducted in November/December 2021 with UK-based healthcare professionals involved in endometrial cancer management. Questions were aligned with the British Gynaecological Cancer Society/European Society for Medical Oncology recommendations, covering the pathway from diagnosis and treatment to follow-up. ResultsA total of 63 healthcare professionals (HCPs) involved in the management of patients with endometrial cancer participated in telephone interviews. The results highlighted variations in management and treatment practices for endometrial cancer and suggest that current UK practice appears to diverge from national and international guidance in some instances. While somatic mismatch repair deficiency testing was used by 89.7% of respondents as mainstream testing, the survey highlighted a lack of access to other key molecular biomarker tests, such as polymerase epsilon (POLE) sequencing (used by only 9.8% of HCPs at the time of the survey). ConclusionThe results highlighted several perceived practical barriers to the swift adoption of new therapeutic options, including funding access, limited staff, treatment-related resources, staff education, and support. Our findings support the need for better access to biomarkers that could enable more effective and targeted treatments.

Real-world study on microsatellite instability and mismatch repair deficiency testing patterns among patients with metastatic colorectal cancer in Spain

PurposeClinical practice guidelines recommend that all patients with metastatic colorectal cancer (mCRC) should be tested for mismatch repair deficiency (dMMR) or microsatellite instability-high (MSI-H). We aimed to describe the dMMR/MSI-H testing practice in patients with mCRC in Spanish centers.MethodsMulticenter, observational retrospective study that included patients newly diagnosed with mCRC or who progressed to a metastatic stage from early/localized stages.ResultsThree hundred patients were included in the study from May 2020 through May 2021, with a median age of 68 years, and two hundred twenty-five (75%) had stage IV disease at initial diagnosis; two hundred eighty-four patients received first-line treatment, and dMMR/MSI-H testing was performed in two hundred fifty-one (84%) patients. The results of the dMMR/MSI-H tests were available in 61 (24%) of 251 patients before the diagnosis of metastatic disease and in 191 (81%) of 236 evaluable patients for this outcome before the initiation of first-line treatment. Among the 244 patients who were tested for dMMR/MSI-H with IHC or PCR, 14 (6%) were MMR deficient. The most frequent type of first-line treatment was the combination of chemotherapy and biological agent, that was received by 71% and 50% of patients with MMR proficient and deficient tumors, respectively, followed by chemotherapy alone, received in over 20% of patients in each subgroup. Only 29% of dMMR/MSI-H tumors received first-line immunotherapy.ConclusionOur study suggests that a high proportion of patients with mCRC are currently tested for dMMR/MSI-H in tertiary hospitals across Spain. However, there is still room for improvement until universal testing is achieved.Trial registration: Not applicable.

Mismatch repair deficiency testing in Lynch syndrome-associated urothelial tumors.

Lynch syndrome-associated cancer develops due to germline pathogenic variants in one of the mismatch repair (MMR) genes, MLH1, MSH2, MSH6 or PMS2. Somatic second hits in tumors cause MMR deficiency, testing for which is used to screen for Lynch syndrome in colorectal cancer and to guide selection for immunotherapy. Both MMR protein immunohistochemistry and microsatellite instability (MSI) analysis can be used. However, concordance between methods can vary for different tumor types. Therefore, we aimed to compare methods of MMR deficiency testing in Lynch syndrome-associated urothelial cancers. Ninety-seven urothelial (61 upper tract and 28 bladder) tumors diagnosed from 1980 to 2017 in carriers of Lynch syndrome-associated pathogenic MMR variants and their first-degree relatives (FDR) were analyzed by MMR protein immunohistochemistry, the MSI Analysis System v1.2 (Promega), and an amplicon sequencing-based MSI assay. Two sets of MSI markers were used in sequencing-based MSI analysis: a panel of 24 and 54 markers developed for colorectal cancer and blood MSI analysis, respectively. Among the 97 urothelial tumors, 86 (88.7%) showed immunohistochemical MMR loss and 68 were successfully analyzed by the Promega MSI assay, of which 48 (70.6%) were MSI-high and 20 (29.4%) were MSI-low/microsatellite stable. Seventy-two samples had sufficient DNA for the sequencing-based MSI assay, of which 55 (76.4%) and 61 (84.7%) scored as MSI-high using the 24-marker and 54-marker panels, respectively. The concordance between the MSI assays and immunohistochemistry was 70.6% (p = 0.003), 87.5% (p = 0.039), and 90.3% (p = 1.00) for the Promega assay, the 24-marker assay, and the 54-marker assay, respectively. Of the 11 tumors with retained MMR protein expression, four were MSI-low/MSI-high or MSI-high by the Promega assay or one of the sequencing-based assays. Our results show that Lynch syndrome-associated urothelial cancers frequently had loss of MMR protein expression. The Promega MSI assay was significantly less sensitive, but the 54-marker sequencing-based MSI analysis showed no significant difference compared to immunohistochemistry. Data from this study alongside previous studies, suggest that universal MMR deficiency testing of newly diagnosed urothelial cancers, using immunohistochemistry and/or sequencing-based MSI analysis of sensitive markers, offer a potentially useful approach to identification of Lynch syndrome cases.

The clinical impact of testing for biomarkers in gastric cancer patients: a real-world cohort.

In gastric cancer (GC), HER2 was the first biomarker for guided therapy registered for clinical use. Considering the recent approvals of immune check-point blockade (ICB) in gastro-oesophageal cancers, testing for mismatch repair deficiency (dMMR), Epstein-Barr virus (EBV) and PD-L1 combined positive score (CPS) is becoming increasingly important. Here we describe a real-world cohort on biomarker assessment in GC patients. Patients diagnosed with GC between 2017 and 2021 were included. Biomarker results were retrieved from electronic patient files. PD-L1 CPS was determined retrospectively on dMMR and EBV-positive (EBV+) tumours. Data on genomic sequencing were analysed separately. Of 363 patients identified, 45% had metastatic disease. In 335 patients (92%) at least one biomarker was tested. The prevalence of HER2+, dMMR and EBV+ tumours was 10% (32 of 319), 7% (20 of 294) and 1% (three of 235), respectively. Of the dMMR and EBV+ tumours, 95% had a PD-L1 CPS ≥ 5. Therapeutic strategy was adjusted in 31 of 55 patients and consisted of anti-HER2 therapies as well as ICB in clinical trials. Genomic alterations were found in 44 of 60 tested patients. TP53 (73%) and PIK3CA (20%) mutations were most common, followed by KRAS mutations (11%) and amplifications (11%). In this real-world cohort, testing for HER2, dMMR and EBV status affected treatment decisions in 56% of the patients. Although most dMMR and EBV+ tumours had a PD-L1 CPS ≥ 5, not all patients with a high probability of treatment response are identified. Based on these results, a stepwise diagnostic strategy is proposed.

Validity of a two-antibody testing algorithm for mismatch repair deficiency testing in cancer; a systematic literature review and meta-analysis

Reflex mismatch repair immunohistochemistry (MMR IHC) testing for MLH1, PMS2, MSH2 and MSH6 is used to screen for Lynch syndrome. Recently MMR-deficiency (MMRd) has been approved as a pan-cancer predictive biomarker for checkpoint inhibitor therapy, leading to a vast increase in the use of MMR IHC in clinical practice. We explored whether immunohistochemical staining with PMS2 and MSH6 can be used as a reliable substitute. This two-antibody testing algorithm has the benefit of saving tissue, cutting costs and saving time. PubMed, Embase and Cochrane library were systematically searched for articles reporting on MMR IHC. The weighed percentage of cases with isolated MLH1 or MSH2 loss or combined MLH1/MSH2 loss alone was analyzed using a random effects model meta-analysis in R. The search yielded 1704 unique citations, of which 131 studies were included, describing 9014 patients. A weighed percentage of 1.1% (95% CI 0.53–18.87, I = 87%) of cases with isolated MLH1 or MSH2 loss or combined MLH1/MSH2 loss alone was observed. In the six articles with the main aim of investigating the two-antibody testing algorithm all MMRd cases were detected with the two-antibody testing algorithm, there were no cases with isolated MLH1 or MSH2 loss or combined MLH1/MSH2 loss alone. This high detection rate of MMRd of the two-antibody testing algorithm supports its use in clinical practice by specialized pathologists. Staining of all four antibodies should remain the standard in cases with equivocal results of the two-antibody testing algorithm. Finally, educational sessions in which staining pattern pitfalls are discussed will continue to be important.

The clinical features and management of Lynch syndrome-associated ovarian cancer.

Lynch syndrome (LS) is one of the most common hereditary cancer syndromes, characterized by mutations in mismatch repair genes and autosomal dominant inheritance. Women with LS have an additional increased risk of gynecologic malignancies, including endometrial cancer (EC) and ovarian cancer (OC). Compared with EC, OC is relatively under investigation. This review thoroughly summarizes the current clinical evidence of surveillance, screening, and prevention strategies, and describes the molecular and clinical characteristics of LS-associated OC. An electronic search from databases of PubMed and Google Scholar was carried out using key words pertaining to Lynch syndrome and ovarian cancer. A review of the literatures including review articles, experimental, and observational studies published between 2000 and 2021 was conducted. The lifetime risk of OC in women with LS of MLH1, MSH2, and MSH6 mutations is approximately 7%, with the median age at onset being 46 years, 10-15 years earlier than that in sporadic cases. Histologically, LS-associated OCs are primarily endometrioid (40%), high-grade (25%), and low-grade (11%) serous, or clear cell (6%) in nature. Eighty-five percent of patients are diagnosed at an early stage, presenting with a good prognosis at 84% 5-year survival. Optimal screening strategies for OC in LS are controversial; combined screening of patients' clinical and family history, immunohistochemical analysis, and microsatellite instability testing for mismatch repair deficiency have been proven efficient. The clinical features were different between ovarian cancer in Lynch syndrome and sporadic cases. More research are needed for a greater understanding of the prevention and medical treatment of LS-associated OC.

How Should We Test for Lynch Syndrome? A Review of Current Guidelines and Future Strategies.

Simple SummaryCarriers of Lynch syndrome (LS) have an increased risk for several types of tumour, in particular bowel and uterine cancers. LS is caused by inheritance of a faulty mismatch repair (MMR) gene, and, in accordance with clinical guidelines, bowel cancers are tested for MMR deficiency to screen for LS. We review the significant barriers to following these guidelines in routine practice, and discuss guideline limitations. We also look at the advances in our knowledge and technology that may address some of these barriers and limitations. We further discuss additional LS screening strategies, in particular MMR deficiency testing of other tumour types and normal tissues to identify LS gene carriers.International guidelines for the diagnosis of Lynch syndrome (LS) recommend molecular screening of colorectal cancers (CRCs) to identify patients for germline mismatch repair (MMR) gene testing. As our understanding of the LS phenotype and diagnostic technologies have advanced, there is a need to review these guidelines and new screening opportunities. We discuss the barriers to implementation of current guidelines, as well as guideline limitations, and highlight new technologies and knowledge that may address these. We also discuss alternative screening strategies to increase the rate of LS diagnoses. In particular, the focus of current guidance on CRCs means that approximately half of Lynch-spectrum tumours occurring in unknown male LS carriers, and only one-third in female LS carriers, will trigger testing for LS. There is increasing pressure to expand guidelines to include molecular screening of endometrial cancers, the most frequent cancer in female LS carriers. Furthermore, we collate the evidence to support MMR deficiency testing of other Lynch-spectrum tumours to screen for LS. However, a reliance on tumour tissue limits preoperative testing and, therefore, diagnosis prior to malignancy. The recent successes of functional assays to detect microsatellite instability or MMR deficiency in non-neoplastic tissues suggest that future diagnostic pipelines could become independent of tumour tissue.

Retroactive assessment for DNA mismatch repair deficiency in women with a history of endometrial cancer

Objectives. Beginning in 2014, the Society of Gynecologic Oncology and the American College of Obstetricians and Gynecologists have recommended universal tumor testing for mismatch repair deficiency in endometrial cancer. Mismatch repair testing can triage patients who may benefit from genetic testing for Lynch syndrome. Many women previously diagnosed with endometrial cancer have not undergone mismatch repair tumor testing. We sought to determine the feasibility of retroactive assessment for mismatch repair deficiency among women with diagnosed with endometrial cancer prior to 2014. Methods. Between 2016 and 2018, we identified 36 patients presenting for gynecologic oncology follow-up visits who were previously diagnosed with endometrial cancer. The endometrial pathology underwent tumor assessment for loss of expression of mismatch repair proteins by immunohistochemistry. Patients with abnormal mismatch repair testing were referred to genetic counseling and, if indicated, for germline genetic testing. Results. Thirty-six patients underwent retroactive tumor immunohistochemistry, yielding 10 (28%) abnormal results, including nine (25%) with loss of one or more mismatch repair proteins and one with inconclusive staining (2.8%). All ten patients with abnormal immunohistochemistry were referred to genetic counseling; 9 (90%) accepted the referral and proceeded with genetic testing. One pathogenic mutation was identified in CHEK2 (11%). Five patients (56%) were found to have a variant of unknown significance. Conclusions. Implementation of universal retroactive tumor testing for mismatch repair deficiency in patients previously diagnosed with endometrial cancer is feasible. With the growing use of new molecular classification protocols for endometrial tumors, identification of mismatch repair deficiency may have significant clinicopathologic implications.

Mismatch repair deficiency testing in colorectal cancer and factors associated with nonadherence to guidelines.

e16069 Background: Mismatch repair (MMR) deficiency is characteristic of Lynch syndrome, which typically affects patients < 50 years old with colorectal cancer (CRC) but can also occur in sporadic CRC. MMR deficiency causes microsatellite instability (MSI) and affects therapy response and prognosis. Guidelines now recommend universal testing of MMR/MSI in all CRC patients, but universal testing has not been achieved. Our study sought to evaluate factors associated with MMR/MSI testing in CRC patients. Methods: The National Cancer Database (NCDB) identified 433,968 patients diagnosed with stages I-IV CRC from 2011-2016. Variables included gender, race, stage, insurance status, education, income, and treatment facility type. Baseline characteristics were compared using Pearson's chi‐square test between patients not tested versus tested for MMR/MSI status. Multivariate logistic regression analysis determined the factors associated with increased probability of MMR/MSI testing. Results: Only 34.7% patients were tested for MMR/MSI and those £ 40 years old were more likely to be tested as were Caucasian patients. Patients with private insurance were more likely to be tested compared to uninsured (OR 1.21; 95% CI 1.1601.27, p < 0.001). Testing also correlated with increasing education, reported as the percentage of adults over 25 years old who did not graduate high school within a zip code. Diagnosis at an academic/research program was also a positive predictor of MMR/MSI testing compared to a community program (OR 1.74; 95% 1.71-1.77, p < 0.001). Testing was dependent on stage with the greatest likelihood of testing in stage II (OR 1.44, 95% CI 1.41-1.46, p < 0.001) and the least in stage IV (OR 0.72; 95% CI 0.71-0.74, p < 0.001) compared to stage I. Still, the trend for testing increased over the years (trend p < 0.001), with that in 2016 being 4 times more likely than in 2011 (OR 4.66; 95% CI 1.53-4.80, p < 0.001). Conclusions: Our study identified established risk factors affecting care such as African American race, treatment at a non-academic facility and being uninsured. It also showed age and stage were factors associated with noncompliance to new guidelines regarding testing for MMR/MSI with testing being less frequent in older patients and those with later stages. An explanation and study limitation is that NCDB only collected diagnoses until 2016 while immunotherapy in MSI-H or dMMR CRC was approved in 2017. With more treatment options available now to these patients, testing rates will hopefully improve with more research needed to capture this.

Overlooking MMR deficiency in carriers of certain pathogenic variants by routine MSI and/or IHC testing in Lynch syndrome: Implications for a wider MMR deficiency testing.

e16107 Background: DNA mismatch repair (MMR) deficiency occurs in both inherited/sporadic colorectal cancer (CRC) and endometrial cancer, but it may also be found in some other types of cancer. At present, MMR status testing in clinical practice is recommended for all CRC patients in order to identify those who should be offered genetic testing for the Lynch syndrome (LS), inform disease prognosis, and guide therapeutic management.There are two commonly accepted methods for MMR deficiency analysis, one based on the detection of microsatellite instability (MSI) by PCR and the other based on the detection of protein expression of the MMR genes using immunohistochemistry (IHC). The objective of this study was to evaluate the concordance between IHC and MSI in tumors from 18 LS patients with known pathogenic germline variants in MMR genes (MLH1, MSH2, PMS2 and MSH6). Methods: The MSI testing was performed using the five gene Bethesda panel (BAT25, BAT26, D2S123, D5S346, D17S250) while the IHC testing was done with the use of a standard 4 antibody panel (MLH1, MSH2, PMS2 and MSH6). Results: High concordance of the two methods was observed in 13/18 (72.2%) patients, mainly with disruptive mutations in the МLH1, MSH2 and PMS2 genes. Inconsistent results were obtained in 5/18 (28.8%) patients, of whom two had a positive result only with the use of the PCR method [carriers of MLH1 c.62C > T (p.Ala21Val) and c.244A > G (p.Thr82Ala) missense variants], other two had a positive result only with IHC [carriers of MSH6 c.3514dupA (p.Arg1172LysfsTer5) and c.2384T > C (p.Ile795Thr)] and one patient had normal results using both methods (carrier of MSH6 c.457+1G > T splice site mutation that results in exon 3 skipping). A positive predictive value of either MSI or IHC used as a single methods for screening was 83.3%, which indicates that a substantial number of cases with MMR tumors can be misdiagnosed by using only either one or the other of these two methods. Conclusions: These results have a potential implication not only for LS screening in CRC patients, but also for the detection of the MMR deficiency in patients with various tumors that might benefit from the checkpoint inhibitor immunotherapy. The use of extended MSI NGS panels might provide a higher sensitivity for the detection of MMR deficiency compared to the standard MSI or ICH testing.

Low prevalence of mismatch repair deficiency in Chinese colorectal cancers: a multicenter study.

BackgroundAlthough universal testing for mismatch repair deficiency (dMMR) has been recommended to all colorectal cancer (CRC) patients, related evidence for the Chinese population is lacking. Here, we investigated the prevalence and clinicopathological features of dMMR patients in a large Chinese CRC cohort.MethodsWe included 7,373 CRC patients treated at four Chinese medical centers between August 2010 and September 2016. Patients’ baseline characteristics and pathological features were recorded. The clinicopathological features were compared between patients with MLH1/PMS2 deficiency (dMLH1/PMS2) and MSH2/MSH6 deficiency (dMSH2/MSH6).ResultsAmong the investigated patients, 654 (8.9%) were identified with dMMR CRCs and, of them, 401 (61.3%) were males, with a median age of 55 years (range, 22–87 years); 355 (54.3%) had stage II CRC based on American Joint Committee on Cancer 8th edition. The prevalence of the dMLH1/PMS2 group and the dMSH2/MSH6 group were 51.5% (337/654) and 25.1% (164/654), respectively. Compared with dMSH2/MSH6 patients, those with dMLH1/PMS2 were older (57 vs 52 years, P < 0.001), more likely to be female (45.7% vs 31.5%, P = 0.004), prone to having tumors located in the right-hand side of the colon (59.0% vs 47.6%, P = 0.015), and less likely to have a family history of tumors (29.7% vs 43.3%, P = 0.003).ConclusionsThe prevalence of dMMR in Chinese CRC patients was low, especially in the dMLH1/PMS2 group. The clinicopathological features were different between dMMR subgroups.

Germline and Somatic Tumor Testing in Epithelial Ovarian Cancer: ASCO Guideline.

To provide recommendations on genetic and tumor testing for women diagnosed with epithelial ovarian cancer based on available evidence and expert consensus. A literature search and prospectively defined study selection criteria sought systematic reviews, meta-analyses, randomized controlled trials (RCTs), and comparative observational studies published from 2007 through 2019. Guideline recommendations were based on the review of the evidence. The systematic review identified 19 eligible studies. The evidence consisted of systematic reviews of observational data, consensus guidelines, and RCTs. All women diagnosed with epithelial ovarian cancer should have germline genetic testing for BRCA1/2 and other ovarian cancer susceptibility genes. In women who do not carry a germline pathogenic or likely pathogenic BRCA1/2 variant, somatic tumor testing for BRCA1/2 pathogenic or likely pathogenic variants should be performed. Women with identified germline or somatic pathogenic or likely pathogenic variants in BRCA1/2 genes should be offered treatments that are US Food and Drug Administration (FDA) approved in the upfront and the recurrent setting. Women diagnosed with clear cell, endometrioid, or mucinous ovarian cancer should be offered somatic tumor testing for mismatch repair deficiency (dMMR). Women with identified dMMR should be offered FDA-approved treatment based on these results. Genetic evaluations should be conducted in conjunction with health care providers familiar with the diagnosis and management of hereditary cancer. First- or second-degree blood relatives of a patient with ovarian cancer with a known germline pathogenic cancer susceptibility gene variant should be offered individualized genetic risk evaluation, counseling, and genetic testing. Clinical decision making should not be made based on a variant of uncertain significance. Women with epithelial ovarian cancer should have testing at the time of diagnosis.

Identification of Lynch syndrome by microsatellite instability and mismatch repair deficiency testing on colorectal adenomas.

Identification of Lynch syndrome by microsatellite instability and mismatch repair deficiency testing on colorectal adenomas.

Clinical implications of genetic aberrations in metastatic prostate cancer.

The utility of tumor genetic testing in metastatic prostate cancer is rapidly evolving - especially in respect to finding prognostic and predictive biomarkers. In this review, we describe genomic aberrations in clinically relevant pathways in metastatic castration-resistant prostate cancer (mCRPC) for which therapeutic targeting is possible. Recognizing the diverse array of genetic features within prostate cancer, the goal of testing in mCRPC is to match an individual patients' tumor with the best therapy. Approximately 20-25% of mCRPC patients have defects in DNA repair, which may be exploited with poly-ADP-ribose polymerase inhibitors, platinum-based chemotherapy, and/or immunotherapy. Choosing between the second-generation androgen inhibitors and chemotherapy may be informed through testing for androgen receptor splice variants or androgen receptor amplifications, but technology and outcomes are still being clarified. Genetic testing for mismatch repair deficiency (1% of mCRPC patients) is a standard of care, but may be expanded as other subpopulations that could respond to immunotherapy are found. The era of precision medicine for prostate cancer is here, but is being refined. Further studies with newer technology and standardized analytical platforms are needed and must be matched with improvement in clinical care infrastructure.

Characteristics of mismatch repair deficiency in sarcomas

Due to the efficacy of immune checkpoint inhibitor therapy in tumors with deficient mismatch repair, there has been a surge in demand for mismatch repair deficiency testing in various tumor types. Mismatch repair deficiency is not known to play a significant role in the pathogenesis of sarcomas, and the utility of testing these tumor types is not established. This study aimed to determine the frequency, pattern, and clinicopathologic correlates of mismatch repair deficiency in sarcomas. Three hundred and four sarcomas were profiled using a genomic platform that employs massively parallel sequencing to interrogate 447 cancer-associated genes. Mismatch repair status was evaluated by determining the number of small insertion/deletion events occurring in homopolymer regions per megabase of exonic sequence data across all genes. Molecular characteristics of mismatch repair-deficient sarcomas were compared to mismatch repair-deficient carcinomas (n = 70) also identified using the sequencing panel. Seven sarcomas (2.3%) were classified as mismatch repair-deficient: four unclassified sarcomas, and one each of pleomorphic rhabdomyosarcoma, epithelioid leiomyosarcoma and malignant PEComa. One patient had an established diagnosis of Lynch syndrome. In the remaining patients, the mismatch repair gene mutation was confirmed or suspected to be somatic. Mismatch repair immunohistochemistry confirmed the mismatch repair-deficiency status of all cases with alterations in the tested proteins. As expected, mismatch repair-deficient sarcomas showed a significantly elevated tumor mutation burden relative to mismatch repair-proficient sarcomas (median 16 versus 4.6, p < 0.001). However, in comparison to mismatch repair-deficient carcinomas, mismatch repair-deficient sarcomas showed a lower tumor mutation burden (median 28 versus 16, p = 0.006) and a significantly greater degree of chromosomal instability. Among mismatch repair-deficient sarcomas, PD-L1 was variably expressed on tumor-associated macrophages but not on tumor cells. Three patients received pembrolizumab: two progressed and one has stable disease with five months follow-up. Mismatch repair deficiency in histologically classifiable sarcomas is rare (1%) and is more common in unclassified sarcomas (10%). Additional study is required to determine the predictive role of mismatch repair-deficiency in sarcomas for immunotherapy.

Minimal microsatellite shift in microsatellite instability high endometrial cancer: a significant pitfall in diagnostic interpretation

Mismatch-repair deficiency testing plays a critical role in the identification of proband in Lynch Syndrome families and triaging patients with high stage or recurrent solid malignancies for check point inhibitor (Pembrolizumab) immunotherapy. We compared microsatellite shift patterns of microsatellite instability PCR analysis at 5 NCI recommended loci between microsatellite instability high endometrial carcinoma (n = 50) and microsatellite instability high colorectal cancer (n = 19). The endometrial cancer cohort included 45 endometrioid, 1 serous, and 4 clear cell carcinomas. Overall, 52% (26/50) of microsatellite instability high endometrial cancers showed minimal microsatellite shift (defined as a one to three nucleotide repeat shift at an involved locus) observed at least at one locus. Among microsatellite instability high endometrial cancers with minimal microsatellite shift, the frequencies at each involved locus were D2S123 (21/21, 100%), D17S250 (10/11, 89%), D5S346 (11/12, 92%), BAT25 (9/12, 80%), and BAT26 (8/21, 45%). Noticeably, 11 of the 26 cases (42%) showed only minimal shift. Among microsatellite instability high endometrial cancers with minimal microsatellite shift, 65% (17/26) had combined MLH1 and PMS2 loss, 8% (2/26) had combined MSH2 and MSH6 loss, 13% (3/26) had MSH6 loss and 15% (4/26) had loss of PMS2 by immunohistochemistry. In contrast, only 16% (3/19) had minimal microsatellite shift seen in colorectal cancer cohort with corresponding loss of MLH1/PMS2, MSH2/MSH6, or MSH6. Overall, 15% (7/50) of microsatellite instability high endometrial carcinomas showed isolated loss of MSH6 in contrast to 7% (1/15) seen in microsatellite instability high colorectal carcinomas. In conclusion, microsatellite instability high endometrial carcinomas have a significantly higher frequency of minimal microsatellite shift that coincides with a high percentage of combined loss of MLH1/PMS2. Microsatellite instability high endometrial cancers also have more frequent loss of MSH-6. Diagnostically, recognition of minimal microsatellite shift is crucial for accurate interpretation of microsatellite instability PCR data of endometrial carcinoma.

The Role of PD-1 Checkpoint Inhibition in Gynecologic Malignancies.

Activity of PD-1 and PD-L1 inhibitors has been demonstrated in ovarian, endometrial, and cervical cancer, with a tolerable side effect profile and the highest response rate seen in mismatch repair-deficient endometrial cancers. Other biomarkers are under active investigation. Tumor testing for mismatch repair deficiency or high microsatellite instability for treatment with pembrolizumab should be considered an option for all women with progressive gynecologic malignancy.

391P - Mismatch repair deficiency testing for immunotherapy in metastatic cancers: A single centre study from India

391P - Mismatch repair deficiency testing for immunotherapy in metastatic cancers: A single centre study from India

P-191 - Combined analysis of KRAS, NRAS, BRAF mutations and mismatch repair deficiency testing in Indian patients with metastatic colorectal carcinoma: A single centre experience

P-191 - Combined analysis of KRAS, NRAS, BRAF mutations and mismatch repair deficiency testing in Indian patients with metastatic colorectal carcinoma: A single centre experience