MSI-H/dMMR and cancer immunotherapy: current state and future implications

Immunotherapy has revolutionized the treatment landscape of several hematological and solid tumors by producing unprecedented algorithm shifts in a relatively short period of time [1]. However, immune checkpoint inhibitors (ICIs) have been suggested to be effective in approximately onethird of all cancer patients, with the antitumor activity of immunotherapy varying among different malignancies [2]. Thus, the identification of potential responders has recently become one of the key challenges in medical oncology, since there is an urgent need to develop reliable biomarkers that could guide clinicians in patient selection [3,4]. In fact, several predictors of response to ICIs have been tested and evaluated, three of whom have been approved by the United States Food and Drug Administration (FDA): programmed death ligand 1 (PD-L1), tumor mutational burden (TMB), and microsatellite instability/defective mismatch repair (MSI/dMMR) [5]. Notably enough, all these predictors present notable differences in terms of methodology and specificity as well as strengths and weaknesses. Other potentially useful elements as predictors and/or for prognostic stratification are under evaluation, including tumorinfiltrating lymphocytes (TILs) [6]. Two years after the landmark approval of PD-L1 as predictive biomarker in non-small cell lung cancer (NSCLC), pembrolizumab was approved by the FDA for the treatment of patients with MSI-high (MSI-H)/dMMR advanced solid tumors in 2017 [7]. In particular, this approval was based on the results observed in MSI-H/dMMR malignancies across five clinical trials [7]. In fact, in these studies, the PD-1 inhibitor pembrolizumab reported an overall response rate (ORR) and a complete response (CR) rate of 39.6% and 7%, respectively, in MSI-H/dMMR solid tumors; in addition, the 78% of responders presented duration of response of 6 months or longer. Notably enough, the approval of MSI-H had some historical significance, being the first ‘orphan’ approval of a biomarker, regardless of histology and tumor type. From a molecular point of view, dMMR malignancies accumulate mutations across the genome, leading to the formation of neoantigens as well as the activation of antitumor responses [8]. Mismatch errors are particularly frequent in short tandem repeats, and thus, mutations are more commonly observed in microsatellite regions, a condition termed as MSI. Three different testing methods are available for detecting MSI-H/dMMR status: polymerase chain reaction (PCR) and next-generation sequencing (NGS) for MSI-H while dMMR is commonly determined through immunohistochemistry (IHC) [9]. Two PCR panels are more frequently used in clinical practice to determine MSI-H, the first of which is known as the Bathesda panel, including two mononucleotide (BAT-25 and BAT-26) and three dinucleotide (D5S346, D2S123, and D17S250) repeats [10]. Of note, both cancer and paired normal tissue are necessary for the evaluation of MSI-H using this panel. Conversely, the second panel is based on the assessment of five poly-A mononucleotide repeats (BAT-25, BAT-26, NR-21, NR-24, and NR-27), and has shown higher specificity and sensitivity compared with the previous one [11]. In addition, this panel does not require normal tissue, and if at least two out of five repeats lose stability, the malignancy is determined as MSI-H. However, the assessment of MSI-H by PCR is currently based on the analysis of a selected number of microsatellites, and thus, false negative results are observed in a proportion ranging from 0.5% to 10% of cases [12]. In addition, the prevalence of MSI-H varies greatly according to the tumor type, with no data available in several malignancies, including renal cell carcinoma and melanoma; conversely, other malignancies present a prevalence of approximately 1– 2% [13]. NGS approaches have been evaluated to overcome the limitations associated to the assessment of MSI-H/dMMR through PCR [14]. In fact, this novel assay based on tumor gene panels or whole exome sequencing has the potential to evaluate several different types of microsatellites; in addition, NGS may be used for all malignancies and is able to evaluate TMB. Thus, the evaluation of MSI-H/dMMR by NGS could also integrate TMB [15]; however, NGS presents some disadvantages, including its high cost and the lack of wide availability. IHC is another method commonly used to determine dMMR through the evaluation of MLH1, MSH2, MSH6, and PMS2, where the loss of expression of at least one MMR protein is defined as dMMR [16]. The strengths of IHC include its simplicity and cost; in addition, IHC is greatly available and may be easily done in all centers. However, IHC has some important weaknesses, including the low