For a patient with colon cancer, how should loss of mismatch‑repair proteins (MLH1, MSH2, MSH6, PMS2) and HER2 (ERBB2) amplification be assessed and incorporated into systemic treatment decisions?

Assessment and Treatment Implications of MMR and HER2 Status in Colon Cancer

All patients with colon cancer should undergo routine testing for mismatch repair (MMR) protein expression via immunohistochemistry for MLH1, MSH2, MSH6, and PMS2, and HER2 testing should be performed in metastatic disease to guide systemic therapy decisions.

MMR Protein Assessment

Testing Methodology

Immunohistochemistry (IHC) is the first-line method for assessing MMR status 1. The panel must include all four MMR proteins: MLH1, MSH2, MSH6, and PMS2, as testing fewer proteins results in missed diagnoses 1, 2.

• Interpretation: Loss of nuclear staining in tumor cells for any MMR protein indicates deficient MMR (dMMR), while intact expression of all four proteins indicates proficient MMR (pMMR) 3
• Pattern recognition: MLH1 loss typically occurs with PMS2 loss; MSH2 loss typically occurs with MSH6 loss, due to heterodimer instability 1, 2
• False-negative rate: Both IHC and PCR-based MSI testing have 5-10% false-negative rates 4

Alternative/Confirmatory Testing

PCR-based microsatellite instability (MSI) testing serves as a complementary method, using either:

• NCI panel: BAT-25, BAT-26, D5S346, D2S123, D17S250
• Promega panel: BAT-25, BAT-26, NR-21, NR-24, MONO-27

MSI-H is defined as instability in ≥2 of 5 markers 1. Next-generation sequencing can simultaneously assess MSI and other actionable mutations, making it the preferred approach when available 5.

When to Test

• All newly diagnosed colon cancers should undergo MMR testing, particularly:
  • All patients <50 years (Lynch syndrome screening) 6
  • Stage II disease (treatment decision-making) 6
  • All metastatic disease (immunotherapy eligibility) 7, 5

Treatment Implications of dMMR Status

Stage II Disease

dMMR status is a favorable prognostic marker but predicts LACK of benefit from fluoropyrimidine monotherapy 6. In stage II dMMR colon cancer:

• Do not routinely use adjuvant chemotherapy, as these patients have excellent prognosis without treatment and may experience harm from fluoropyrimidine monotherapy 6
• Consider observation for most stage II dMMR patients
• Reserve chemotherapy only for high-risk features (T4, perforation, inadequate lymph node sampling)

Metastatic Disease

First-line immunotherapy is superior to chemotherapy for dMMR metastatic colorectal cancer 7. The treatment algorithm is:

1. First-line: Immune checkpoint inhibitors (pembrolizumab or nivolumab-based regimens) are preferred 7
2. For immunotherapy non-responders (20-30%): Consider chemotherapy combinations, though response rates are lower than in pMMR disease (5% vs 44%) 7
3. Prognosis: dMMR mCRC has worse overall survival (16.0 vs 23.6 months in pMMR) when treated with chemotherapy 7

Critical caveat: The poor outcomes in dMMR mCRC treated with chemotherapy reflect tumor biology and reduced chemotherapy sensitivity, not intrinsic resistance to all therapies 8. The microenvironment, not tumor cell-intrinsic mechanisms, likely drives differential treatment response 8.

HER2 Assessment in Metastatic Disease

Testing Indications

HER2 testing should be performed in all patients with metastatic colorectal cancer, particularly those with RAS/BRAF wild-type tumors 5, 9.

• Testing can be performed via IHC, FISH, or NGS panels
• HER2 positivity is defined as amplification and/or overexpression
• Prevalence is low (~3-5% of mCRC), but identifies actionable targets

Treatment Implications

For HER2-positive, RAS/BRAF wild-type mCRC 9:

1. First-line: Standard chemotherapy with EGFR inhibitors or bevacizumab remains appropriate
2. Subsequent lines: HER2-targeted therapy should be incorporated
   • Preferred regimen: Trastuzumab + tucatinib (FDA-approved) 9
   • Alternatives: Trastuzumab + lapatinib, trastuzumab + pertuzumab
   • Reserve trastuzumab deruxtecan (T-DXd) for progression after other HER2-targeted therapies due to higher toxicity risk 9

For HER2-positive, RAS-mutated mCRC 9:

• Signal transduction inhibitors (trastuzumab combinations) have limited activity
• Consider T-DXd in later lines, as it shows some efficacy despite RAS mutations 9

Stability of HER2 Status

Repeat biopsy at progression is recommended to assess HER2 status changes due to tumor heterogeneity and clonal evolution 10. Preoperative treatment may alter HER2 expression, though data suggest minimal impact on overall HER2 status 11.

Key Clinical Pitfalls

1. Do not assume dMMR = Lynch syndrome: 10-15% of sporadic colon cancers show dMMR due to MLH1 promoter hypermethylation 4. Refer for genetic counseling to distinguish hereditary from sporadic cases.

2. Do not withhold immunotherapy in dMMR mCRC: These patients have dramatically better outcomes with immunotherapy than chemotherapy 7.

3. Do not use single MMR protein testing: All four proteins must be assessed to avoid false-negative results 1, 2.

4. Do not assume stable biomarker status: Repeat molecular testing at metastatic progression, especially after targeted therapy, as clonal evolution occurs 5.

5. Recognize unusual patterns: Concurrent loss of MLH1/PMS2/MSH6 (without MSH2 loss) suggests widespread DNA repair dysregulation and warrants comprehensive molecular profiling 12.