MLH1 Promoter Hypermethylation Does Not Impact Survival Outcomes in dMMR Endometrial Cancer Treated with Immune Checkpoint Inhibitors

Highlight

– MLH1 promoter hypermethylation accounted for 74.4% of mismatch repair deficiency (dMMR) cases in a large real-world endometrial cancer cohort.
– Response rates to pembrolizumab or dostarlimab immune checkpoint inhibitors were similar between patients with epigenetic MLH1 silencing and those with germline or somatic Lynch syndrome mutations.
– Two-year progression-free and overall survival did not significantly differ between the MLH1 hypermethylation group and Lynch syndrome mutation carriers.
– These findings suggest that the underlying mechanism of dMMR does not impact clinical outcomes in advanced and recurrent endometrial adenocarcinoma treated with immunotherapy.

Study Background

Endometrial cancer is a prevalent gynecologic malignancy with an increasing global incidence. A significant subset of endometrial tumors exhibit mismatch repair deficiency (dMMR), resulting from loss of function in DNA mismatch repair genes such as MLH1, MSH2, MSH6, and PMS2. dMMR tumors are characterized by high microsatellite instability (MSI-H) and a high mutational burden, features that sensitize them to immune checkpoint inhibitors such as pembrolizumab and dostarlimab.

Among dMMR cases, MLH1 promoter hypermethylation (hMLH1) leads to epigenetic silencing and consequent loss of MLH1 protein expression, whereas Lynch Syndrome arises from germline mutations in MMR genes. Somatic MMR mutations also contribute to dMMR. Understanding whether the etiology of dMMR, epigenetic versus genetic, influences immunotherapy response and survival outcomes is clinically important in tailoring treatment strategies and prognostication.

Study Design

This retrospective cohort study utilized the Endometrial Cancer Molecularly Targeted Therapy Consortium (ECMT2) database to identify 133 patients with advanced or recurrent dMMR endometrial adenocarcinoma treated with immune checkpoint inhibitors, either pembrolizumab or dostarlimab, from 2016 to early 2023. Patients were stratified into two groups based on the underlying cause of dMMR: (1) germline or somatic Lynch Syndrome mutations (gLS/sLS, n=34, 25.6%) and (2) MLH1 promoter hypermethylation (hMLH1, n=99, 74.4%).

Demographic, clinical, and pathological data were collected, including age, tumor stage and histology, treatment regimens, response rates to immunotherapy, recurrence, progression-free survival (PFS), and overall survival (OS). The primary endpoints were response rate and survival outcomes comparing gLS/sLS and hMLH1 groups.

Key Findings

Patients in both groups showed similar baseline characteristics regarding age, tumor stage at diagnosis, histologic subtype, and other clinical features, supporting valid group comparisons. The overall immune checkpoint inhibitor response rate for the entire cohort was 57.1%. Response rates were comparable between the groups: 55.9% in the gLS/sLS group and 57.6% in the hMLH1 group (p=0.90), indicating no statistically significant difference.

Survival analyses revealed no significant difference in 2-year progression-free survival between groups: 68.0% in the gLS/sLS group versus 59.0% in the hMLH1 group (p=0.77). Similarly, 2-year overall survival was 70.9% for gLS/sLS patients compared to 62.2% for hMLH1 patients (p=0.55). These results demonstrate that the underlying dMMR mechanism — whether genetic Lynch Syndrome mutations or epigenetic MLH1 promoter hypermethylation — does not significantly affect response to checkpoint blockade nor survival outcomes in this patient population.

Expert Commentary

The results align with the growing understanding that high tumor mutational burden and microsatellite instability, regardless of the cause, elicit robust anti-tumor immune responses. Immune checkpoint inhibitor efficacy appears largely agnostic to the dMMR etiology. This real-world evidence supports the use of immune checkpoint inhibitors broadly among dMMR endometrial cancer patients, including those with MLH1 promoter hypermethylation, who represent the majority.

However, limitations include the retrospective design and possible selection bias. Further studies with larger cohorts and prospective designs could validate these findings and investigate whether additional biomarkers may refine immunotherapy predictions. Moreover, integrating molecular subtyping with immune microenvironment profiling might improve therapeutic stratification.

Conclusion

This large multicenter cohort study reveals that MLH1 promoter hypermethylation and Lynch Syndrome mutations define molecularly distinct but clinically equivalent subsets of mismatch repair deficient endometrial cancers in terms of response and survival following immune checkpoint inhibition. These findings support current clinical practice guidelines advocating dMMR/MSI testing to guide immunotherapy, without necessity to distinguish between genetic and epigenetic causes for treatment decision-making.

Future research should examine optimal combination approaches and resistance mechanisms to further improve outcomes in this heterogeneous disease group.

Funding and ClinicalTrials.gov

The study was conducted through the ECMT2 consortium. Specific grant funding details and clinical trial registrations were not provided in the publication.

References

1. Borden LE, et al. MLH1 promoter hypermethylation and associations with survival outcomes: A real-world endometrial cancer molecularly targeted therapy consortium cohort study. Gynecologic Oncology. 2026;211:231-237.

2. Le DT, Durham JN, Smith KN, et al. Mismatch repair deficiency predicts response of solid tumors to PD-1 blockade. Science. 2017;357(6349):409-413.

3. Marabelle A, et al. Efficacy of pembrolizumab in patients with non-colorectal high microsatellite instability/mismatch repair–deficient cancer: Results from the phase II KEYNOTE-158 study. J Clin Oncol. 2020;38(1):1-10.