Lynch syndrome (LS), also known as hereditary nonpolyposis colorectal cancer (HNPCC), is an autosomal dominant genetic disorder associated with an increased lifetime risk of developing colorectal cancer (CRC) (30–73%), endometrial carcinoma (EC) (30–51%) and, less frequently, other malignancies such as gastric, ovarian, urinary tract, pancreatic, small bowel, biliary tract, brain, and skin sebaceous cancers.
1. Introduction
Lynch syndrome (LS), also known as hereditary nonpolyposis colorectal cancer (HNPCC), is an autosomal dominant genetic disorder associated with an increased lifetime risk of developing colorectal cancer (CRC) (30–73%), endometrial carcinoma (EC) (30–51%) and, less frequently, other malignancies such as gastric, ovarian, urinary tract, pancreatic, small bowel, biliary tract, brain, and skin sebaceous cancers
[1][2][3][4]. LS is due to germline PVs in the DNA mismatch repair (MMR) genes (
MLH1,
MSH2,
MSH6 or
PMS2) or epithelial cell adhesion molecule (
EPCAM, which causes epigenetic silencing of
MSH2), which are crucial to correct DNA mismatches during DNA replication. The deficient MMR (dMMR) mechanism leads to the accumulation of frequent somatic mutational events in cancer-related genes containing tandemly repeated DNA motifs called microsatellites (
Figure 1). The condition of genetic hypermutation due to impaired DNA MMR is called microsatellite instability (MSI).
Figure 1. Overview of microsatellite stability and instability mechanism and response to immunotherapy. When the DNA mismatch repair protein complex is intact, DNA replication errors are repaired. MSS tumors have a low neoantigen burden, therefore blockade of the PD-1-PD-L1 interaction through immune checkpoint inhibitors results in low T-cell activation against tumor cells. In MMR deficiency, failure in the elimination of the DNA replication error leads to a high number of somatic mutations and neoantigens. This results in a stronger anti-tumor immune response and sensitivity to the immune checkpoint blockade.
2. dMMR/MSI-H Colorectal Cancer Associated to Hereditary Predisposition
Due to its implications for prevention and treatments, tumor testing with immunohistochemistry for MMR proteins and/or MSI is recommended for every new CRC diagnosis
[4][5]. It is important to underline that approximately 10% of CRCs display MLH1 loss of expression because of somatic hypermethylation of the
MLH1 promoter, which is often associated with
BRAF V600E PV in sporadic CRC. For this reason, methylation analysis of the
MLH1 promoter in the tumor and/or analysis for somatic
BRAF V600E PV should be carried out first to exclude LS
[6]. Based on these findings, full germline genetic testing should be offered to every new MSI-H/dMMR cancer diagnosis—excluding those who show
BRAF V600E somatic PV and/or hypermethylation of the
MLH1 promoter—and to families who display high clinical risk
[4].
It is noteworthy that in a proportion of patients with MSI/dMMR tumors, the somatic alteration is not associated with a germline PVs in MMR genes. This might be due to the presence of germline PVs in genes other than MMR that can mimic the LS phenotype. Particularly, germline PVs in the
POLE/POLD1 proofreading domain cause polymerase proofreading-associated polyposis (PPAP), a dominant genetic disorder associated with an increased risk of CRC, EC and other malignancies with an ultra-mutated phenotype
[7].
3. Chemotherapy
Historically, the role of dMMR is known as a negative predictor of response to treatment with fluoropyrimidines in selected subgroups of patients. Sargent et al., reported a detrimental role of the use of adjuvant 5-fluorouracil (5-FU) in stage II dMMR CRC patients
[8]. On the contrary, patients with stage III dMMR CRC showed a statistically significant benefit from 5-FU adjuvant treatment
[9]. As a consequence, MMR protein status assessment is recommended by the National Comprehensive Cancer Network (NCCN) and the European Society of Medical Oncology (ESMO) guidelines for patients with resected stage II CRC, since adjuvant chemotherapy may be avoided in dMMR individuals
[4][10]. Furthermore, in the neoadjuvant setting, subgroup analysis of the FOxTROT trial suggested less benefit from neoadjuvant FOLFOX-based chemotherapy in dMMR patients
[11].
4. Immune-Checkpoint Inhibitors
The dysfunctional MMR system causes 10 to 100 times as many somatic PVs compared to MMR proficient (pMMR) malignancies, which leads to the accumulation of many neoantigens (
Figure 1)
[12]. Moreover, dMMR cancers have a highly immunogenic tumor microenvironment with prominent lymphocyte infiltrates
[13][14]. Based on this evidence, Le et al., hypothesized and demonstrated that dMMR tumors were more responsive to immune checkpoint blockade with pembrolizumab than those with pMMR
[12]. The study was later expanded across 12 different dMMR tumor types, with 53% of objective radiographic responses (ORR) and 21% of complete responses (CR)
[15]. Based on these findings, in 2017, the FDA granted accelerated approval for the use of the anti-PD-1 pembrolizumab for adult and pediatric patients with unresectable or metastatic dMMR/MSI-H solid tumors that have progressed after previous treatment, irrespective of tumor type, representing the first FDA approval agnostic of a cancer site. Subsequently, the clinical trials KEYNOTE-164
[16] and KEYNOTE-177
[17] confirmed the efficacy of pembrolizumab in advanced MSI-H/dMMR CRC in the second- and first-line setting, respectively. Moreover, based on the data from the CHECKMATE 142 study
[18], the anti-PD-1 nivolumab gained FDA approval for the treatment of dMMR/MSI-H CRC that has progressed with regard to dMMR/MSI-H EC, in the phase 2 KEYNOTE-158 trial, pembrolizumab showed clinical benefit in patients with previously treated unresectable or metastatic non-colorectal MSI-H/dMMR cancer. Following these results, the FDA approved pembrolizumab for the treatment of patients with advanced MSI-H/dMMR EC who progressed following prior systemic therapy and are not candidates for curative surgery or radiation
[19]. Based on the KEYNOTE-158 results, pembrolizumab was also approved for the treatment of gastric, small intestine and biliary tract cancers that have progressed on or following at least one prior therapy
[19]. Other immune checkpoint inhibitors that have proven to be effective in dMMR EC include durvalumab
[20], avelumab
[21] and dostarlimab
[22].
The phase I single-arm GARNET trial investigated the role of the anti-PD-1 monoclonal antibody dostarlimab in advanced solid tumors that have limited available treatment options. Interim data reported that dostarlimab is associated with durable antitumor activity and an acceptable safety profile for MSI-H/dMMR EC patients after prior platinum-based chemotherapy
[22]. Based on these data, in 2021, dostarlimab gained FDA and EMA approval as a monotherapy for the treatment of recurrent or advanced MSI-H/dMMR ECs that have progressed after a platinum-containing regimen. Furthermore, dostarlimab recently met its primary endpoint of PFS in a planned interim analysis of the phase III RUBY trial of dostarlimab plus carboplatin-paclitaxel versus chemotherapy only in the first line setting in advanced or recurrent EC, both in the dMMR/MSI-H subgroup and in the overall population (NCT03981796,
[23]).
Immune checkpoint inhibitors have also recently been tested in the resectable and locally advanced disease setting. A phase 2 study
[24] investigated the activity of the anti-PD-1 monoclonal antibody dostarlimab in a cohort of 12 dMMR patients affected by stage II or III rectal adenocarcinoma. A clinical CR was shown in 100% of evaluable patients, resulting in the omission of the standard approach with chemoradiotherapy (CRT) and surgery. Furthermore, at the ESMO Congress 2022, Chalabi presented the preliminary results of the non-randomized phase 2 NICHE-2 study
[25], in which 112 non-metastatic dMMR colon cancer patients received neoadjuvant treatment with nivolumab plus ipilimumab, showing a pathologic response rate of 99%, a major pathologic response rate of 95% and a pathologic CR rate of 67%. Considering the possible biological differences between Lynch- and non-Lynch-associated dMMR tumors, the pCR rate was assessed between these two groups. Among the 97 patients in the per protocol population for whom Lynch status was available, 32 had LS and experienced 78% of pCR compared to 58% in sporadic dMMR tumors. This result is consistent with evidence that LS-associated dMMR CRCs tend to exhibit stronger immunological reactions compared to sporadic dMMR tumors, due to the greater burden of somatic PVs, neoantigens and tumor infiltrating lymphocytes
[26][27]. The 3-year disease-free survival data of the NICHE-2 study are expected in 2023. Of note, these unprecedented findings open the possibility of organ-preserving strategies in selected dMMR locally advanced CRCs.
The EMA-approved immune checkpoint inhibitors in dMMR/MSI-H tumors are summarized in Table 1.
Table 1. EMA-approved immune checkpoint inhibitors in dMMR/MSI-H malignancies (ICI: immune checkpoint inhibitor; CRC: colorectal cancer; EC: endometrial cancer; LS: Lynch Syndrome).
5. Future Perspectives
Despite the impressive results obtained for neoadjuvant immune checkpoint inhibitors in dMMR CRC, validation in larger cohorts is awaited, and many questions still need to be answered. Future research on this topic should focus on identifying the best treatment regimen (monotherapy or combination), whether CRT and surgery should be omitted and if adjuvant treatment might be necessary. A randomized phase II/III clinical trial is currently assessing the efficacy of the anti-PD-1 sintilimab with or without CRT in patients with locally advanced rectal cancer in both dMMR and pMMR patients (NCT04304209). Similarly, a phase II single-arm study is investigating the efficacy of induction PD-1 blockade with dostarlimab in dMMR early stage rectal cancer patients (NCT04165772). In this trial, participants who exhibit clinical CR will proceed with a “watch and wait” strategy, whereas those who do not show clinical CR will receive standard CRT and surgery. Furthermore, the ongoing phase III MK-347 (NCT05173987) and DOMENICA (NCT05201547) trials are, respectively assessing the safety and efficacy of pembrolizumab and dostarlimab compared to carboplatin and paclitaxel in previously untreated dMMR EC. In addition, the phase III double-blind randomized placebo-controlled AtTEnd trial is evaluating the activity of atezolizumab in combination with carboplatin and paclitaxel in women with advanced/recurrent EC, including dMMR patients (NCT03603184).
Current research is also focusing on chemoprevention in LS patients. To date, no drug has been approved for this indication, and aspirin represents the only pharmacological therapy suggested for LS patients by the NCCN guidelines
[10]. Current research aims to identify the optimal aspirin dose (CAPP-3 trial) and other potentially chemopreventive agents, such as progestins for EC prevention
[28] and atorvastatin (NCT04379999), omega-3-acid ethyl esters (NCT03831698) and mesalamine (NCT04920149) for CRC prevention. The role of the anti-PD-1 tripleitriumab in preventing adenomatous polyps and second primary tumors in patients with LS is under evaluation in a randomized phase III trial (NCT04711434). Moreover, preclinical
[29] and clinical
[30] findings support the preventive role of neoantigen-based vaccines for LS-related cancers and have led to the development of two ongoing clinical trials (NCT05078866, NCT05419011).