What are the clinical and pathological features of proficient mismatch repair (pMMR) and deficient mismatch repair (dMMR) colorectal cancer?

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Last updated: October 13, 2026 • View editorial policy

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Clinical and Pathological Features of dMMR vs pMMR Colorectal Cancer

Deficient mismatch repair (dMMR) colorectal cancers exhibit a distinct clinicopathological profile characterized by right-sided location, poor differentiation, mucinous histology, larger tumor size, and paradoxically lower rates of lymph node metastasis compared to proficient mismatch repair (pMMR) tumors.

Key Distinguishing Features

Tumor Location

  • dMMR tumors: Predominantly right-sided (proximal colon), with studies showing 61.1% vs 22.6% in pMMR 1
  • pMMR tumors: More evenly distributed throughout the colon, with higher representation in distal colon and rectum

Histological Characteristics

Differentiation Grade:

  • dMMR tumors show significantly higher rates of poor differentiation (50.0% vs 19.4% in pMMR) 1
  • This poor differentiation paradoxically coexists with better prognosis in early-stage disease 2

Histologic Type:

  • Mucinous adenocarcinoma is markedly more common in dMMR (27.8% vs 6.5% in pMMR) 1
  • Standard adenocarcinoma predominates in pMMR tumors (~90% well/moderately differentiated) 2

Tumor Size and Stage Distribution

  • dMMR tumors tend to be larger (≥5 cm in 55.6% vs 30.6% in pMMR) 1
  • dMMR is more frequently diagnosed at Stage II rather than advanced stages 3, 4
  • Stage-dependent histological shift occurs in dMMR, with poorly differentiated histology increasing from 11.5% in Stage I to 60.0% in Stage IV 2

Lymph Node Metastasis

  • Critical distinction: dMMR tumors show significantly lower rates of lymph node metastasis (22.2% vs 46.8% in pMMR, P=0.052) 1
  • This finding holds across multiple studies and contributes to better prognosis in Stage II disease 4

Molecular Patterns of MMR Protein Loss

Frequency of dMMR

  • Overall prevalence: 7.6-25.5% of all colorectal cancers, with variation by population 1, 5, 4
  • Lower incidence reported in Asian populations compared to Western countries 5

Common Loss Patterns (in order of frequency):

  1. MLH1/PMS2 co-deletion: 41-55.7% of dMMR cases 3, 5, 4
  2. MSH2/MSH6 co-deletion: 20% of dMMR cases 4
  3. Isolated PMS2 loss: 50% of dMMR cases in some cohorts 1
  4. Isolated MSH6 loss: Associated with younger age and poor differentiation 1

Clinical Implications by Protein Loss Pattern

MLH1/PMS2 deficiency:

  • Strongly associated with right-sided location, T4 stage, and poor differentiation 4
  • When combined with BRAF V600E mutation (present in ~75% of cases), indicates sporadic rather than Lynch syndrome etiology 6
  • Requires MLH1 promoter methylation testing to distinguish from germline Lynch syndrome 6

MSH2/MSH6 deficiency:

  • More common in younger patients (≤65 years) 4
  • Higher incidence in transverse colon-splenic flexure region 4
  • More likely to represent Lynch syndrome (germline mutation) 6

Prognostic Implications

Overall Survival and Disease-Free Survival

  • dMMR confers superior prognosis compared to pMMR, particularly in Stage II disease 7
  • Five-year disease-free survival: 82.2% (dMMR) vs 68.9% (pMMR), P=0.02 7
  • dMMR is an independent favorable prognostic factor in multivariate analysis 7

Stage-Specific Considerations

Stage II:

  • dMMR shows most pronounced survival advantage 7
  • Lower lymph node metastasis rates contribute to better outcomes 1

Stage III:

  • Prognostic advantage of dMMR is attenuated 2
  • Critical caveat: Poorly differentiated histology within dMMR Stage III tumors associates with early recurrence and worse outcomes, negating the typical dMMR survival benefit 2
  • Well/moderately differentiated dMMR Stage III tumors maintain excellent prognosis with no recurrences observed in some cohorts 2

Testing Recommendations

Primary Testing Method

Immunohistochemistry (IHC) for all four MMR proteins (MLH1, MSH2, MSH6, PMS2) represents the first-line approach 8. This guideline recommendation is based on:

  • High sensitivity (100%) and specificity (87.5%) 9
  • Excellent concordance with PCR-based MSI testing (98.4-98.7%) 5, 9
  • Ability to identify specific protein loss patterns that guide Lynch syndrome evaluation 6

Confirmatory and Reflex Testing

When MLH1 loss is detected:

  1. BRAF V600E mutation testing should be performed to distinguish sporadic from Lynch syndrome cases 6
  2. If BRAF wild-type, proceed to MLH1 promoter methylation testing 6
  3. Absence of both BRAF mutation and MLH1 methylation warrants germline genetic testing for Lynch syndrome 6

Alternative/Complementary Methods

  • PCR-based MSI testing using five microsatellite markers (including BAT-25 and BAT-26) serves as second-line method 8
  • Next-generation sequencing coupling MSI and tumor mutational burden analysis may represent optimal approach for immunotherapy selection 8

Clinical Pitfalls to Avoid

  1. Do not assume all poorly differentiated dMMR tumors have favorable prognosis: In Stage III disease, poor differentiation within dMMR tumors associates with early relapse 2

  2. Do not skip BRAF testing in MLH1-deficient tumors: This is essential to avoid unnecessary Lynch syndrome workup in sporadic cases (75% of MLH1-deficient tumors harbor BRAF mutations) 6

  3. Do not rely solely on IHC without considering clinical context: While IHC-PCR concordance is high (98.4%), discordant cases exist and may require additional molecular testing 9

  4. Do not overlook immunotherapy implications: dMMR status predicts exceptional response to immune checkpoint inhibitors regardless of stage, fundamentally altering treatment algorithms 8, 10

References

5
Research

[Clinicopathological features and types of microsatellite instability in 1394 patients with colorectal cancer].

Nan fang yi ke da xue xue bao = Journal of Southern Medical University, 2020

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This information is intended for healthcare professionals. Any medical decision-making should rely on clinical judgment and independently verified information. The content provided herein does not replace professional discretion and should be considered supplementary to established clinical guidelines. Healthcare providers should verify all information against primary literature and current practice standards before application in patient care. Dr.Oracle assumes no liability for clinical decisions based on this content.