Women with Lynch syndrome are known to carry an approximately 60% lifetime risk of endometrial cancer. These cancers result from inherited deleterious mutations in genes that code for mismatch repair proteins. However, mismatch repair deficiency (MMR-d) is not exclusively found in the tumors of patients with Lynch syndrome, and much is being learned about this group of endometrial cancers, their behavior, and their vulnerability to targeted therapies.
Dr. Emma C. Rossi
During the processes of DNA replication, recombination, or chemical and physical damage, mismatches in base pairs frequently occurs. Mismatch repair proteins function to identify and repair such errors, and the loss of their function causes the accumulation of the insertions or deletions of short, repetitive sequences of DNA. This phenomenon can be measured using polymerase chain reaction (PCR) screening of known microsatellites to look for the accumulation of errors, a phenotype which is called microsatellite instability (MSI). The accumulation of errors in DNA sequences is thought to lead to mutations in cancer-related genes.
The four predominant mismatch repair genes include MLH1, MSH2, MSH 6, and PMS2. These genes may possess loss of function through a germline/inherited mechanism, such as Lynch syndrome, or can be sporadically acquired. Approximately 20%-30% of endometrial cancers exhibit MMR-d with acquired, sporadic losses in function being the majority of cases and only approximately 10% a result of Lynch syndrome. Mutations in PMS2 are the dominant genotype of Lynch syndrome, whereas loss of function in MLH1 is most frequent aberration in sporadic cases of MMR-d endometrial cancer.1
Endometrial cancers can be tested for MMR-d by performing immunohistochemistry to look for loss of expression in the four most common MMR genes. If there is loss of expression of MLH1, additional triage testing can be performed to determine if this loss is caused by the epigenetic phenomenon of hypermethylation. When present, this excludes Lynch syndrome and suggests a sporadic form origin of the disease. If there is loss of expression of the MMR genes (including loss of MLH1 and subsequent negative testing for promotor methylation), the patient should receive genetic testing for the presence of a germline mutation indicating Lynch syndrome. As an adjunct or alternative to immunohistochemistry, PCR studies or next-generation sequencing can be used to measure the presence of microsatellite instability in a process that identifies the expansion or reduction in repetitive DNA sequences of the tumor, compared with normal tumor.2
It is of the highest importance to identify endometrial cancers caused by Lynch syndrome because this enables providers to offer cascade testing of relatives, and to intensify screening or preventative measures for the many other cancers (such as colon, upper gastrointestinal, breast, and urothelial) for which these patients are at risk. Therefore, routine screening for MMR-d tumors is recommended in all cases of endometrial cancer, not simply those of a young age at diagnosis or for whom a strong family history exists.3 Using family history factors, primary tumor site, and age as a trigger for screening for Lynch syndrome, such as the Bethesda Guidelines, is associated with a 82% sensitivity in identifying Lynch syndrome. In a meta-analysis including testing results from 1,159 women with endometrial cancer, 43% of patients who were diagnosed with Lynch syndrome via molecular analysis would have been missed by clinical screening using Bethesda Guidelines.2
Discovering cases of Lynch syndrome is not the only benefit of routine testing for MMR-d in endometrial cancers. There is also significant value in the characterization of sporadic mismatch repair–deficient tumors because this information provides prognostic information and guides therapy. Tumors with a microsatellite-high phenotype/MMR-d were identified as one of the four distinct molecular subgroups of endometrial cancer by the Cancer Genome Atlas.4 Patients with this molecular profile exhibited “intermediate” prognostic outcomes, performing better than the “serous-like” cancers with p53 mutations, yet worse than patients with a POLE ultramutated group who rarely experience recurrences or death, even in the setting of unfavorable histology.
Beyond prognostication, the molecular profile of endometrial cancers also influence their responsiveness to therapeutics, highlighting the importance of splitting, not lumping endometrial cancers into relevant molecular subgroups when designing research and practicing clinical medicine. The PORTEC-3 trial studied 410 women with high-risk endometrial cancer, and randomized participants to receive either adjuvant radiation alone, or radiation with chemotherapy.5 There were no differences in progression-free survival between the two therapeutic strategies when analyzed in aggregate. However, when analyzed by Cancer Genome Atlas molecular subgroup, it was noted that there was a clear benefit from chemotherapy for patients with p53 mutations. For patients with MMR-d tumors, no such benefit was observed. Patients assigned this molecular subgroup did no better with the addition of platinum and taxane chemotherapy over radiation alone. Unfortunately, for patients with MMR-d tumors, recurrence rates remained high, suggesting that we can and need to discover more effective therapies for these tumors than what is available with conventional radiation or platinum and taxane chemotherapy. Targeted therapy may be the solution to this problem. Through microsatellite instability, MMR-d tumors create somatic mutations which result in neoantigens, an immunogenic environment. This state up-regulates checkpoint inhibitor proteins, which serve as an actionable target for anti-PD-L1 antibodies, such as the drug pembrolizumab which has been shown to be highly active against MMR-d endometrial cancer. In the landmark, KEYNOTE-158 trial, patients with advanced, recurrent solid tumors that exhibited MMR-d were treated with pembrolizumab.6 This included 49 patients with endometrial cancer, among whom there was a 79% response rate. Subsequently, pembrolizumab was granted Food and Drug Administration approval for use in advanced, recurrent MMR-d/MSI-high endometrial cancer. Trials are currently enrolling patients to explore the utility of this drug in the up-front setting in both early- and late-stage disease with a hope that this targeted therapy can do what conventional cytotoxic chemotherapy has failed to do.
Therefore, given the clinical significance of mismatch repair deficiency, all patients with endometrial cancer should be investigated for loss of expression in these proteins, and if present, considered for the possibility of Lynch syndrome. While most will not have an inherited cause, this information regarding their tumor biology remains critically important in both prognostication and decision-making surrounding other therapies and their eligibility for promising clinical trials.
Dr. Rossi is assistant professor in the division of gynecologic oncology at the University of North Carolina at Chapel Hill. She has no conflicts of interest to declare. Email her at obnews@mdedge.com.
References
1. Simpkins SB et al. Hum. Mol. Genet. 1999;8:661-6.
2. Kahn R et al. Cancer. 2019 Sep 15;125(18):2172-3183.
3. SGO Clinical Practice Statement: Screening for Lynch Syndrome in Endometrial Cancer. https://www.sgo.org/clinical-practice/guidelines/screening-for-lynch-syndrome-in-endometrial-cancer/
4. Kandoth et al. Nature. 2013;497(7447):67-73.
5. Leon-Castillo A et al. J Clin Oncol. 2020 Oct 10;38(29):3388-97.
6. Marabelle A et al. J Clin Oncol. 2020 Jan 1;38(1):1-10.