Mismatch repair and immunology: Targeted therapy for targeted patients
McMeekin DS, Tritchler DL, Cohn DE, et al. Clinicopathologic significance of mismatch repair defects in endometrial cancer: an NRG oncology/gynecologic oncology group study. J Clin Oncol. 2016;34(25):3062-3068.
Le DT, Uram JN, Wang H, et al. PD-1 blockade in tumors with mismatch-repair deficiency. N Engl J Med. 2015;372(26):2509-2520.
The most frequent genetic mutation in endometrial cancer is mismatch repair (MMR) deficiency. Loss of this pathway leads to a failure of repairing replication errors and gives rise to small repeated sequences of DNA, known as MSI. Germline mutations in MMR (Lynch syndrome) occur in only 3% to 5% of endometrial cancers. Somatic mutations in MMR give rise to 10% to 20% of colorectal cancers and upwards of 20% to 40% of endometrial cancers.
Given this high frequency, universal screening utilizing immunohistochemistry of proteins MLH1, MSH2, MSH6, and PMS2 has become the standard of care in tumors to identify MMR deficiency. MMR-deficient endometrial tumors are associated with higher grade and lymphovascular space invasion. The actual clinical prognosis of these tumors, however, has not been well described.7 McMeekin and colleagues set out to examine prognosis.
Details of the study by McMeekin and colleagues
In the collaborative study, researchers assessed 1,024 tumors for MMR and categorized them into 1 of 4 groups: normal(62.4%), epigenetic MMR-defective (25.78%),MMR-probable mutation (9.67%), or MSI-low (2.15%). The researchers found that the pathologic features were associatedwith MMR status. For instance, MMR-defective tumors were more likely thanMMR-normal tumors to be Grade 2 (50% vs 40.7%, respectively). Lymphovascular space invasion also occurred more frequently in MMR-defective than in MMR-normal tumors (32.7% vs 17.13%, respectively). Approximately 22% of patients with MMR-defective tumors had stage III or IV disease, while only 13% to 14% of the other groups presented with such advanced stage.
On univariate analysis, an MMR-defective tumor was associated with worsened progression-free survival (hazard ratio [HR], 1.37). On subsequent multivariate analysis, no difference in survival in MMR-defective vs MMR-normal tumors was found. The authors concluded that MMR status is predictive of response to adjuvant therapy.
An intriguing biologic explanation of how MMR status affects response to adjuvant therapy is that MMR-defective tumors contain lymphocytic infiltrates, consistent with an increased immunologic response.8 Similar to the previously discussed POLE mutations, MMR-defective tumors have a tremendous increase in somatic mutations that are on the order of 10 to 100 times that of MMR-proficient tumors. These MMR-defective tumors likely give rise to increased antigen exposure to the immune system.
These immune infiltrates will show signs of exhaustion and upregulate negative feedback systems, which is the point at which the PD-1 pathway becomes critically important. The PD-1 receptor is expressed predominately on T-cells and its ligands regulate the immune system by inhibition of self-reactive T-cells.9
MMR deficiency and anti-programmed death receptor 1
The study by McMeekin and colleagues shows MMR-defective tumors have poor prognostic features but the same survival as those with MMR proficiency or good prognostic features. Why is this the case? A recent study by Le and colleagues analyzed this question.
Details of the study by Le and colleagues
The investigators performed a phase 2 trial evaluating pembrolizumab (10 mg/kg IV every 14 days), an anti-PD 1 immune checkpoint inhibitor in patients with tumors demonstrating MMR-deficiency. The 3 cohorts included: MMR-defective colorectal cancer (n = 10), MMR-proficient colorectal cancer (n = 18), and MMR-defective noncolorectal cancer (n = 7, including 2 endometrial cancers). Objective response rates were 40%, 0%, and 71% for each group, respectively.
MMR-defective tumors had a striking HR of disease progression or death of 0.04 (95% confidence interval, 0.01-0.21; P<.001). Genomic analysis was performed and identified 578 potential mutation- associated neoantigens in the MMR-defective groups (compared with only 21 in the MMR-proficient tumors). These findings promote the concept of a mutation-associated antigen component to the endogenous immune response.10
WHAT THIS EVIDENCE MEANS FOR PRACTICEWe are on the verge of being able to manipulate the immune system to help us kill cancer. MMR-deficient cells have increased somatic mutations and antigen exposure, with a potential immune response making them excellent candidates for targeted therapy with immune checkpoint inhibition.
These studies support the growing evidence that molecular events have a powerful clinical impact that has the potential to supplant traditional histopathologic staging.
Conclusion
The above-stated mutations of mismatch repair and POLE are changing our perspective of endometrial cancer and shedding light on the complexities of tumor biology. As future research increasingly incorporates genomic profiling, we anticipate clinical trials may build evidence that adjuvant therapy will be directed by molecular staging, as opposed to traditional surgical or even histologic staging, as these mutations are the root cause of the tumor phenotype.
Key for readers to take away from this Update is that genomic profiling and enrollment in clinical trials is critical to understanding the implications of these mutations and how to best treat our patients. In addition, we should encourage our patients with endometrial cancer to see genetic counselors and have appropriate screening of MMR-deficiency. This will continue to advance our understanding as well as to provide patients with valuable information regarding their diagnosis.
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