What is MMR (Mismatch Repair) or MSI (Microsatellite Instability) testing and is it a tumor marker?

MMR and MSI Testing: Definition and Classification

MMR (Mismatch Repair) and MSI (Microsatellite Instability) testing are tumor biomarkers that detect defects in the DNA repair system, serving as both predictive markers for immunotherapy response and screening tools for Lynch syndrome. 1

What These Tests Measure

DNA Mismatch Repair (MMR)

• MMR is a highly conserved cellular mechanism that corrects DNA replication errors, including single base mismatches and short insertions/deletions 1
• Four critical genes encode the MMR proteins: MLH1, MSH2, MSH6, and PMS2 1
• These proteins function as heterodimers: MLH1-PMS2 and MSH2-MSH6 1
• Inactivation through germline/somatic mutations or epigenetic silencing (particularly MLH1 promoter methylation) results in defective MMR (dMMR) 1

Microsatellite Instability (MSI)

• MSI represents the phenotypic consequence of defective MMR—a condition of genetic hypermutability characterized by mutations clustering in repetitive DNA sequences (microsatellites) 1
• Microsatellites are short tandem repeats distributed throughout the genome that are particularly vulnerable to replication errors when MMR is defective 1
• MSI is therefore a marker of dMMR status, and tumors with MSI accumulate thousands of mutations, creating a hypermutable cellular state 1

Clinical Significance as Tumor Markers

Yes, They Function as Tumor Markers in Multiple Contexts:

1. Predictive Biomarker for Immunotherapy

• MSI/dMMR status predicts response to immune checkpoint inhibitors (anti-PD-1/PD-L1 therapy) across multiple cancer types 1, 2
• This has become the primary driver for expanded testing beyond colorectal cancer 1

2. Screening Tool for Lynch Syndrome

• MSI/dMMR testing identifies patients who may have hereditary nonpolyposis colorectal cancer (HNPCC/Lynch syndrome) 1, 3
• Found in >90% of Lynch syndrome-associated cancers 3, 4
• Also occurs in 8-18% of sporadic colorectal cancers without family history 1

3. Prognostic Marker

• MSI-H tumors demonstrate improved prognosis with lower lymph node metastases and better overall survival in certain cancer types 1, 3, 4

Testing Methodology

ESMO Consensus Recommendations (2019):

Primary Method: Immunohistochemistry (IHC)

• IHC testing for all four MMR proteins (MLH1, MSH2, MSH6, PMS2) represents the first-line approach for assessing dMMR status (strong consensus 8.7/10) 1
• Loss of nuclear staining indicates defective MMR 1
• Widely available and applicable to formalin-fixed paraffin-embedded tissue 1, 2

Secondary Method: PCR-Based Molecular Testing

• Testing uses panels of five microsatellite markers, including at least BAT-25 and BAT-26 (strong agreement) 1
• Two validated panels exist: one with two mononucleotide and three dinucleotide repeats, another with five poly-A mononucleotide repeats 1
• MSI-H is defined as instability in ≥2 of 5 markers (or ≥30% if >5 markers tested) 1
• MSI-L (low) shows instability in 1 of 5 markers; MSS (stable) shows no instability 1

Emerging Method: Next-Generation Sequencing

• NGS coupling MSI and tumor mutational burden (TMB) analysis may represent a decisive tool for immunotherapy patient selection (very strong agreement) 1
• Particularly valuable for rare cancers not in the Lynch syndrome spectrum 1

Cancer Types Where Testing Applies

Lynch Syndrome Spectrum Cancers (where both IHC and PCR methods are validated):

• Colorectal cancer 1
• Endometrial cancer 1
• Gastric cancer 1
• Small intestine cancer 1
• Urothelial cancer 1
• Central nervous system tumors (gliomas/glioblastomas) 1
• Sebaceous gland tumors 1

Important Clinical Caveats

Discordance Between IHC and PCR

• Concordance between MSI and MMR IHC is generally high (98.3% overall), but discordance occurs more frequently in pancreaticobiliary and endometrial cancers 5
• When Lynch syndrome is suspected in these tumor types, both MSI and MMR IHC should be performed 5
• 3.5% of MSI-H cases may show proficient MMR by IHC; 1.2% of MSS cases may show dMMR by IHC 5

Technical Considerations

• IHC can produce false results due to pre-analytical issues (tissue fixation problems) or aberrant staining patterns (cytoplasmic, dot-like, or perinuclear) 1
• Internal positive controls (normal tissue) are essential for interpretation 1
• MLH1 and MSH2 mutations typically cause loss of both the primary protein and its partner (PMS2 or MSH6, respectively), while isolated PMS2 or MSH6 loss may not affect the primary partner 1

Relationship to Other Biomarkers

• The relationship between MSI, TMB, and PD-1/PD-L1 expression is complex and varies by tumor type 1
• These biomarkers do not always overlap, and their combined use versus single marker testing remains under investigation 1