A Durable Response to Pembrolizumab in a Patient with Uterine Serous Carcinoma and Lynch Syndrome due to the MSH6 Germline Mutation

Abstract

Pembrolizumab, a programmed death 1 ligand (PD‐1) checkpoint inhibitor, has elicited responses in mismatch repair (MMR)–deficient advanced solid tumors, leading to its agnostic approval by the US Food and Drug Administration in 2017 when no other therapeutic options are available. However, there are still insufficient data on the response to checkpoint inhibitors in advanced endometrial cancer related to Lynch syndrome (LS) and, specifically, in uterine serous carcinoma, which is uncommon in LS. Here we report a case of metastatic uterine serous carcinoma due to a germline MSH6 mutation (Lynch syndrome) that was discovered because of a patient's tumor MMR deficiency. The patient was started on first‐line pembrolizumab in 2018 and sustained a partial response. She remains asymptomatic and progression free for more than 2 years. Tumor sequencing showed a high mutational burden and an upstream somatic mutation in the same gene, p.F1088fs. Immunohistochemical staining was negative for PD‐L1 expression. We discuss clinical characteristics of the patient, molecular features of her tumor, and the mechanism of her tumor response. We also discuss the duration of immunotherapy in her case.

Our case demonstrated a partial response and a long‐term remission from the frontline single‐agent pembrolizumab in a woman with metastatic uterine serous carcinoma and Lynch syndrome due to a germline MSH6 gene mutation. Our experience suggests a potential significant clinical benefit of checkpoint inhibitors used as single agents early on in the treatment of MMR‐deficient/high microsatellite instability/hypermutated uterine cancers in women with Lynch syndrome.

Key Points

• Even though checkpoint inhibitors are effective in mismatch repair‐deficient endometrial cancer, it is unknown whether the response to them differs between women with endometrial cancer due to germline mutations in a mismatch repair gene (Lynch syndrome) and women with sporadic endometrial cancer.

• In our case, a patient with Lynch syndrome and recurrent mismatch repair‐deficient serous endometrial cancer achieved a durable remission on the first‐line therapy with the checkpoint inhibitor pembrolizumab and remains progression free after more than 2 years.

• Based on our observation and the data, suggesting the stronger immune activation in women with Lynch syndrome–associated endometrial cancer, we propose to use checkpoint inhibitor monotherapy early in the course of their treatment and stratify patients for the presence of Lynch syndrome in clinical trials.

Short abstract

This case report describes a partial response and a long‐term remission from the front‐line single‐agent pembrolizumab in a patient with metastatic uterine serous carcinoma and Lynch syndrome due to a germline MSH6 gene mutation, suggesting a potential significant clinical benefit of checkpoint inhibitors used as single agents early on in the treatment of uterine cancers in women with Lynch syndrome.

Introduction

Endometrial carcinoma (EC) is the most frequent gynecologic cancer in the U.S. with an estimated incidence of 65,620 in 2020 and estimated mortality of more than 12,590 women in 2020 [1]. Prognosis of advanced‐stage disease is poor (5‐year survival for Federation of Gynecology and Obstetrics stage III: 51%–68%; for stage IV: 21% [2]).

Approximately 2%–5% of ECs are caused by Lynch syndrome (LS), an autosomal dominant cancer genetic predisposition syndrome due to a germline mutation in one of the mismatch repair (MMR) genes MLH1, MSH2, MSH6, and PMS2. The risk of EC varies with the gene mutated [3] with a cumulative incidence of 34%, 51%, 49%, and 24%, respectively [4]. In addition to high risk of EC, individuals with LS also have up to 61% lifetime risk of colorectal cancer [3]. It is recommended that all colorectal and endometrial cancers are screened for possible LS with immunohistochemistry for MMR proteins and/or polymerase chain reaction–based microsatellite instability (MSI) testing [3]. High MSI (MSI‐H) in the tumor is a phenotypic manifestation of MMR deficiency and is characteristic for LS [3]. Sporadic EC may also demonstrate microsatellite instability. In fact, 31% of all ECs are MSI‐H [5]. However, in sporadic EC, microsatellite instability is caused by somatic DNA alterations in the tumor, commonly, epigenetic silencing (inactivation) of MLH‐1 gene promoter through methylation or two somatic mutations in the MMR gene. Typically, EC due to LS occurs at a younger age than sporadic EC.

Besides being a hallmark of LS, MMR deficiency as determined by immunohistochemistry or MSI was found to be a biomarker of cancer response to immunotherapy with checkpoint inhibitors. Pembrolizumab was the first checkpoint inhibitor that demonstrated the overall response rate of 53% in MMR‐deficient tumors [6]. It was Food and Drug Administration (FDA) approved for their treatment in 2017, regardless of tumor type, when no other standard therapy options are available [7]. Even though patients with MMR‐deficient tumors due to LS are ideal candidates for pembrolizumab, there are limited data on their treatment outcomes and whether they differ from the outcomes of patients with sporadic MMR‐deficient tumors.

Herein we report a case of a patient with metastatic uterine serous carcinoma (USC) and LS due to MSH6 germline mutation who has sustained a durable response after first‐line treatment with pembrolizumab. We discuss her clinical course and the molecular mechanisms of the response.

Patient Story

A 68‐year‐old woman of Lithuanian descent sought a second opinion on the treatment of metastatic USC in June 2018. She initially presented in March 2017 at the age of 67 after developing vaginal bleeding. She underwent a total abdominal hysterectomy, bilateral salpingo‐oophorectomy, bilateral pelvic lymph node dissection, and periaortic lymph node dissection in April. She was diagnosed with FIGO stage IB USC (60% serous and 40% endometrioid; Fig. 1) according to the World Health Organization Classification of Female Genital Tumors. The Classification calls mixed histology EC “serous” or “clear cell” based on the presence of these higher‐grade components and irrespective of their relative percentages in the tumor because these components behave more aggressively and thus are more likely to recur [8, 9]. Lymphovascular invasion was noted. Tumor screening by immunohistochemistry for LS showed loss of MSH6 expression (Fig. 2). MMR deficiency was confirmed by detecting MSI‐H. These findings prompted genetic testing, which confirmed a germline mutation in the MSH6 gene, c.3848_3851del (p.I1283fs), and a diagnosis of LS. She received concurrent adjuvant whole‐pelvic radiation with cisplatin, followed by four cycles of adjuvant carboplatin and paclitaxel completed in October 2017. In June 2018, a restaging computed tomography scan showed new pulmonary and peritoneal metastases. A lung biopsy showed metastatic high‐grade USC. The patient was recommended carboplatin/paclitaxel/bevacizumab for disease control. But she sought a second opinion, wanting to avoid chemotherapy, and was interested in immunotherapy. Because of the known tumor MMR deficiency, she was started on pembrolizumab.

Figure 1.

(A): The entire photomicrograph shows serous carcinoma (low magnification ×4) with cells arranged in glands surrounded by cells with high‐grade cytologic features (arrow): nuclear pleomorphism, high nuclear–cytoplasmic ratio, large nuclei with prominent nucleoli, necrosis. (B): Serous carcinoma (×40 magnification) with large nuclei and prominent nucleoli (arrow)

Figure 2.

Immunostaining of the patient's endometrial cancer for mismatch repair proteins (MLH1, PMS2, MSH2, MSH6) with the absent staining for MSH6 protein.

Molecular Tumor Board

To identify additional therapeutic targets, somatic tumor testing on the primary EC and lung metastasis were performed. A blood sample was also sent for testing to aid in determining if variants identified were of somatic or germline origin. Germline mutations may be reported incidentally in this setting; however, this analysis is not a comprehensive germline test (Table 1). As expected, the same p.I1283fs germline MSH6 mutation was detected in both tissue samples, along with an upstream p.F1088fs somatic mutation in the same MSH6 gene. The tumor mutational burden (TMB) was 30.4 mutations/Mb in the primary tumor specimen and 29.2 m/MB in the lung metastasis. Immunohistochemical staining was negative for programmed death 1 ligand (PD‐L1) expression.

Table 1.

Genomic alterations identified in primary EC tumor and lung metastasis (collected on April 26, 2017, and June 29, 2018, respectively) with corresponding targeted agents

Endometrial, predominantly papillary‐serous carcinoma (primary tumor)			Lung metastasis
Gene	Variant	Variant allele fraction (%)	Gene	Variant	Variant allele fraction (%)	Therapeutic indications
Somatic mutations
PTEN	p.233*	43.4	PTEN	p.R233*	37.2	‐mTOR inhibitora ‐PARP inhibitora
TP53	p.R273H	38.9	TP53	p.R273H	25.8	‐WEE1 inhibitorb
PIK3R1	p.Q572*	38.8	PIK3R1	p.Q572*	31.7	‐MEK inhibitorb ‐JNK inhibitorb
ERBB2 (HER2)	p.R678Q	30.9	ERBB2 (HER2)	p.R678Q	29.2	‐EGFR inhibitora ‐Pan‐HER TKIa ‐Anti‐HER2 Maba
TP53	c.376‐2A>G	29.3	TP53	c.376‐2A>G	26.9	‐WEE1 inhibitorb
FBXW7	p.R465H	23.6	FBXW7	p.R465H	14.1
PTEN	p.R130Q	22.6	PTEN	p.R130Q	11.2	‐mTOR inhibitora ‐PARP inhibitora
KRAS	p.G13C	16.4	KRAS	p.G13C	15.8	‐MEK inhibitora ‐Pan‐RAF inhibitorb
FAT1	Copy number loss	—	—	—	—	—
MSH6	p.F1088fs	46.3	MSH6	p.F1088fs	34.1	—
EP300	p.H2324fs	39.6	EP300	p.H2324fs	31.1	—
KMTC2 (MLL3)	p.K2792fs	32.1	—	—	—	—
PIK3R2	p.G373R	23.8	PIK3R2	p.G373R	12.3	—
—	—	—	ARID1A	p.G406*	17.1	—
—	—	—	HNF1A	p.291fs	13.6	—
Germline mutation
MSH6	p.I1283fs	—	MSH6	p.I1283fs	—	Anti‐PD‐1 Mab

^* Protein‐truncating mutation. fs ‐ Frameshift mutation (germline and second somatic “hit” frameshift mutations in MSH6 gene are in bold).

^a FDA‐approved therapy.

^b Investigational therapy.

Abbreviations: EC, endometrial carcinoma; EGFR, epidermal growth factor receptor; FDA, Food and Drug Administration; Mab, monoclonal antibody; PD‐1, programmed death 1.

Discussion

Women with LS due to germline MSH6 mutations are usually older at the time of cancer diagnosis and commonly present with endometrial rather than colorectal cancer [10]. Her tumor screening was characteristic for LS, with loss of MSH6 protein expression and MSI‐H.

Tumor DNA analysis improved our understanding of EC molecular subtypes. In endometrioid (type I EC), somatic mutations frequently occur in the PTEN, ARID1A, and ARID2 genes, whereas TP53 mutations are characteristic for uterine serous carcinomas (USC, type II, or high‐grade EC) [11]. LS‐associated ECs commonly have a “second‐hit” somatic mutation either in the same germline‐mutated MMR gene or in MSH2 gene (regardless of which MMR gene has a germline mutation), or in the SMARCA4 gene [12].

The patient's tumor has a mutational signature consistent with an LS‐associated EC: a high TMB, somatic PTEN mutations, and a “second‐hit” mutation in the MSH6 gene. It also contained two somatic TP53 mutations consistent with the molecular profile of USC [13]. The tumor sample, however, had neither somatic MSH2 nor SMARCA4, which may be related to its serous histology as opposed to mostly endometrioid EC in the previous study [12]. Interestingly, 86% of EC in LS is of endometrioid histology and those of a different histology are commonly associated with MSH2 germline mutations [14]. USC is usually characterized by mismatch repair proficiency rather than deficiency and is less likely to respond to checkpoint inhibitors as single agents [15, 16]. We found an insignificant difference in molecular signature of the primary tumor and a metastasis in our patient: FAT1 and KMTC2 mutations were lost, whereas ARID1A and HNF1A mutations were gained in the lung metastasis.

Pembrolizumab is an anti–programmed death 1 (PD‐1) monoclonal antibody that interacts with PD‐1 on T lymphocytes [17] and restores the ability of immune system to attack cancer cells [17]. It has been FDA approved for seven cancer types based on the positive PD‐L1 expression on stromal or cancer cells [17]. However, PD‐L1 expression varies in EC [18, 19]. Moreover, it has not been shown to be a predictive biomarker for response to checkpoint inhibitors in EC, with a modest response of 13% in a phase II trial [18]. As mentioned, our patient's tumor was negative for PD‐L1 expression, consistent with only 8.3% of ECs demonstrating MSI‐H and expressing PD‐L1 [19].

The response to checkpoint inhibitors in MMR‐deficient tumors is linked to the large number of mutant‐associated neoantigens (MANAs) [6, 20]. MANAs result in increased CD3+ and CD8+ tumor‐infiltrating lymphocytes and a compensatory upregulation of immune checkpoints [21]. Another measurement of MANAs and a predictive biomarker is high TMB (≥10 mutations/Mb = million bases; a new FDA‐approved indication for pembrolizumab [17]). Typically, an MSI‐H EC is hypermutated [12, 13]: 31% of ECs are MSI‐H and have high TMB [19]. Our patient's high TMB (30.4; 98th percentile) is another molecular underpinning of her exceptional response to pembrolizumab.

The tumor microenvironment of inherited MMR‐deficient EC is distinct from that of sporadic MMR‐deficient EC. Its immune landscape is characterized by the higher TMB [22] and stronger PD‐1 expression [23] as well as the significantly higher T‐cell infiltration and elevated cytotoxic CD8+, memory CD45RO+, regulatory FOXP3+, and PD‐1+ T cells at the invasive margin [24]. These findings suggest that the mechanisms underlying microsatellite instability alter the immune response [25] and thus may alter the effect of checkpoint inhibitors in these subtypes of MMR‐deficient EC.

Because MMR deficiency/MSI‐H/high TMB are new indications for pembrolizumab, our experience with it and other checkpoint inhibitors in MMR‐deficient tumors is still limited, even more so in MMR‐deficient tumors caused by LS [6, 16]. In the original study [6], there were 23 patients with LS (21 patients with colorectal cancer and 2 patients with EC among the total of 14 patients with EC; J. Durham, personal communication, April 20, 2020). Their outcomes were similar to the outcomes of patients without LS, but the numbers are too small to make a conclusion [6]. The number of published trials on checkpoint inhibitors in EC is also still small and data on the presence of LS are not necessarily reported [26]. The majority of these trials as well as ongoing clinical trials explore the efficacy and safety of checkpoint inhibitors in combination with molecularly targeted agents, chemotherapy, or radiation as opposed to monotherapy. For example, pembrolizumab in combination with the vascular endothelial growth factor receptor inhibitor lenvatinib has been shown to elicit a response rate of 36% in patients with advanced microsatellite‐stable EC, including USC, who had previously received chemotherapy [27]. In this study, the response rate in MSI‐H EC was 64% but the publication does not report on the presence of LS in the study participants. The combination of pembrolizumab and lenvatinib received the FDA approval for not MSI‐H or mismatch repair‐deficient (dMMR) advanced EC progressing on prior chemotherapy regimen in 9/19. However, this regimen resulted in grade ≥ 3 treatment‐related adverse events in 70% of patients [27] as compared with 17% of patients treated with single‐agent pembrolizumab [28]. The ongoing frontline clinical trial (LEAP‐001) investigates this regimen in patients with stage III and IV EC. Other ongoing frontline phase III clinical trials study the combinations of pembrolizumab with standard chemotherapy (carboplatin and paclitaxel) and pembrolizumab with radiation (NRG‐GY018 and NRG‐GY020, respectively). Similarly, a phase III trial of a PD‐1 inhibitor, dostarlimab, with standard chemotherapy (carboplatin and paclitaxel) explores this combination in the frontline treatment of advanced or recurrent EC (RUBY trial). These trials either specifically include only women with dMMR EC (NRG‐GY020) or stratify them according to the tumor MMR status (NRG‐GY018 and RUBY). Clearly, these trials will expand our knowledge on the outcomes of checkpoint inhibitor therapy in MMR‐deficient and MMR‐proficient EC.

In summary, based on tumor molecular characteristics, our patient was a reasonable candidate for pembrolizumab despite available chemotherapy options. As described above, she developed a durable partial response and continues to do well. Her tumor profile showed multiple potential molecular targets consistent with the hypermutated phenotype of LS‐associated EC. Therefore, she would have many targeted agents available in the event of cancer progression.

Our case also raises a question about the duration of immunotherapy when a complete or partial response is achieved. In the aforementioned trial [6], pembrolizumab was discontinued after 2 years with a higher chance of progression‐free survival if a complete response was achieved. Similar results have been seen in other tumor types (e.g., non‐small cell lung cancer and melanoma) [29, 30] after 2 years of checkpoint blockade where only ~30% of patients experienced tumor progression off immunotherapy, and among them, ~40% were successfully salvaged with a repeat course of the checkpoint inhibitor.

Patient Update

The patient had a partial response (Fig. 3) and remains progression free as of February 2021 (31 months). She had no side effects from pembrolizumab and agreed to discontinue it in March 2021 after 32 months of therapy owing to sustained remission. All of her three children tested negative for her germline mutation. Her brother with recurrent prostate cancer tested positive for the familial mutation.

Figure 3.

(A): At the beginning of pembrolizumab therapy. Computed tomography (CT) scan of the abdomen and pelvis, September 11, 2018. Peritoneal cystic mass: 3.8 × 4.2 cm (arrow). (B): After 22 months of pembrolizumab therapy. CT scan of the abdomen and pelvis, February 3, 2021. Peritoneal cystic mass: 1.6 × 1.7 cm (arrow).

Conclusion

Our case demonstrates a significant clinical benefit from first‐line immunotherapy with pembrolizumab in an older woman with metastatic uterine serous carcinoma and LS due to a germline MSH6 gene mutation. We hope that ongoing clinical trials with checkpoint inhibitors in EC will eventually generate more data on treatment outcomes in LS‐associated EC. Meanwhile, we believe that our observation is informative for clinicians taking care of patients with EC including a small percentage of these patients who had developed EC because of their genetic predisposition to cancer (LS). Based on this case, we also believe that studies of immunotherapy in MMR‐deficient/MSI‐H/hypermutated tumors should stratify patients for the presence of LS and investigate the potential benefit of monotherapy with checkpoint inhibitors in the first‐line and adjuvant settings in this condition.

Glossary of Genomic Terms and Nomenclature

Germline mutation: a gene change in a body's reproductive cell (egg or sperm) that becomes incorporated into the DNA of every cell in the body of the offspring.

Somatic mutation: an alteration in DNA that occurs after conception.

Mismatch repair deficiency: cells that have mutations in certain genes that are involved in correcting mistakes made when DNA is copied in a cell.

Lynch syndrome: an inherited disorder in which affected individuals have a higher‐than‐normal chance of developing colorectal cancer and certain other types of cancer, often before the age of 50.

MSI (microsatellite instability): a change that occurs in certain cells (such as cancer cells) in which the number of repeated DNA bases in a microsatellite (a short, repeated sequence of DNA) is different from what it was when the microsatellite was inherited.

MSI‐High (MSI‐H): cancer cells that have a high number of mutations (changes) within microsatellites.

Tumor mutational burden (TMB): the total number of mutations found in the DNA of cancer cells.

PD‐1: a protein found on T cells (a type of immune cell) that helps keep the body's immune responses in check. When PD‐1 is bound to another protein called PD‐L1, it helps keep T cells from killing other cells, including cancer cells. Immune checkpoint inhibitors are anticancer drugs which block PD‐1. When this protein is blocked, the “brakes” on the immune system are released and the ability of T cells to kill cancer cells is increased.

PD‐L1: a protein that acts as a “brake” to keep the immune responses under control. PD‐L1 may be found on some normal cells and in higher‐than‐normal amounts on some types of cancer cells.

Epigenetic gene silencing: non‐mutational gene inactivation that can be faithfully propagated from precursor cells to clones of daughter cells. The addition of methyl groups to cytosine residues in CpG dinucleotides in DNA is a biochemical modification that meets this requirement.

NOTE: The primary source of definitions is the NCI Dictionary of Cancer Terms, which can be found at https://www.cancer.gov/publications/dictionaries/cancer-terms.

Author Contributions

Conception/design: Kelsey T Danley, Karen Schmitz, Lydia Usha

Provision of study material or patients: Lydia Usha

Collection and/or assembly of data: Kelsey T Danley, Karen Schmitz, Ritu Ghai, Joy S. Sclamberg, Lela E. Buckingham, Kelly Burgess, Lydia Usha

Data analysis and interpretation: Karen Schmitz, Ritu Ghai, Joy S. Sclamberg, Lela E. Buckingham, Kelly Burgess, Lydia Usha

Manuscript writing: Kelsey T Danley, Karen Schmitz, Lydia Usha

Final approval of manuscript: Kelsey T Danley, Karen Schmitz, Ritu Ghai, Joy S. Sclamberg, Lela E. Buckingham, Kelly Burgess, Timothy M. Kuzel, Lydia Usha

Disclosures

The authors indicated no financial relationships.