PATHCHAT Edition No. 53
April 2019
Please contact your local Ampath pathologist for more information.
Authors:
- Dr. Robert M. Maritz, FC Path (SA) Anatomy
- Dr. Jaysen Knezovich, PhD Genetics, Principal Medical Scientist
Introduction
✅ Colorectal carcinoma (CRC) is the third-most common cancer worldwide.
- A leading cause of cancer-related death.
- Five-year survival rate remains poor (~55%).
- Advances in molecular testing have improved understanding of CRC genetics and treatment options.
- Molecular testing now guides targeted therapy selection.
📌 This document outlines key molecular pathways and genetic markers used in CRC diagnosis and management.
Molecular Pathways in Colorectal Carcinogenesis
✅ CRC arises from colorectal epithelium via an adenoma-carcinoma sequence.
- Progression from normal epithelium to adenomas with dysplasia occurs with increasing genetic mutations.
- High-grade dysplastic adenomas evolve into invasive CRC.
🔹 Three Major Molecular Pathways in CRC:
✔ 1. Chromosomal Instability (CIN) Pathway (Most Common, ~85% of CRCs)
- Sporadic and hereditary CRCs (except Lynch syndrome) develop via this pathway.
- Key Mutated Genes:
- APC (>90% of CIN-related CRCs).
- KRAS, TP53, PIK3CA.
✔ 2. Microsatellite Instability (MSI) Pathway (~15% of CRCs)
- Microsatellites are repetitive DNA sequences prone to replication errors.
- Mismatch repair (MMR) genes (MLH1, MSH2, MSH6, PMS2) normally correct these errors.
- Deficient MMR leads to MSI and increased cancer risk.
✔ 3. CpG Island Methylator Phenotype (CIMP) Pathway
- Hypermethylation of CpG islands silences tumour suppressor genes.
- Commonly mutated genes: KRAS, BRAF.
📌 Understanding these pathways helps identify therapeutic targets and hereditary cancer syndromes.
Key Molecular Markers in CRC
1. KRAS Mutations
✅ KRAS is an oncogene that promotes tumour growth.
- Mutations occur early in CRC development.
- Present in both CIN and CIMP pathways.
🔹 Clinical Significance:
- KRAS mutations predict resistance to anti-EGFR monoclonal antibodies (cetuximab, panitumumab).
- Mutations in KRAS codons 12, 13, 59, 61, 117, and 146 are associated with poor response.
- NRAS mutations (codons 12, 13, 59, 61) also indicate resistance to anti-EGFR therapy.
📌 KRAS mutation testing is essential before starting anti-EGFR therapy.
2. BRAF Mutations
✅ BRAF encodes a serine/threonine kinase that acts downstream of KRAS.
- BRAF V600E is the most common mutation in CRC.
- Mutually exclusive with KRAS mutations.
🔹 Clinical Significance:
- BRAF-mutant CRCs have poor prognosis and are often resistant to anti-EGFR therapy.
- Limited response to BRAF inhibitors (vemurafenib, dabrafenib) due to EGFR reactivation.
- Commonly associated with peritoneal metastases.
📌 BRAF mutation testing helps guide targeted therapy and prognosis assessment.
3. Microsatellite Instability (MSI) & Mismatch Repair (MMR) Deficiency
✅ Loss of MMR function leads to MSI, increasing mutation rates in oncogenes and tumour suppressors.
- Up to 15% of CRCs exhibit MSI.
🔹 Clinical & Therapeutic Implications:
- MSI-high (MSI-H) tumours respond poorly to standard chemotherapy (5-FU-based regimens).
- Predicts improved response to immune checkpoint inhibitors (pembrolizumab, nivolumab).
- Associated with Lynch syndrome (hereditary non-polyposis colorectal cancer, HNPCC).
🔹 Testing for MSI/MMR Deficiency:
- Immunohistochemistry (IHC) for MLH1, MSH2, MSH6, PMS2.
- Polymerase chain reaction (PCR) for microsatellite markers.
- Germline genetic testing for Lynch syndrome if MSI-H with intact BRAF.
📌 MSI testing helps identify Lynch syndrome and select patients for immunotherapy.
Molecular Testing Algorithm for CRC
✅ Stepwise Approach to Molecular Testing:
✔ Step 1: Determine MSI/MMR Status
- MSI-H (Instability Present): Check BRAF mutation to differentiate sporadic vs. Lynch syndrome.
- MSI-Stable (MSS): Proceed to KRAS/NRAS testing.
✔ Step 2: KRAS/NRAS Mutation Testing
- If mutated: Anti-EGFR therapy is NOT recommended.
- If wild-type: Proceed to BRAF testing.
✔ Step 3: BRAF Mutation Testing
- If mutated: Poor prognosis; limited response to anti-EGFR therapy.
- If wild-type: Anti-EGFR therapy may be considered.
📌 A structured testing approach optimizes treatment selection and patient outcomes.
Available CRC Genetic Tests at Ampath
✅ Somatic (Acquired) Mutation Screening:
- KRAS, NRAS, BRAF, PIK3CA, TP53.
✅ Inherited (Germline) Mutation Screening for Lynch Syndrome:
- MLH1, MSH2, MSH6, PMS2, EPCAM.
- Performed via Next-Generation Sequencing (NGS).
📌 For CRC genetic testing, contact Ampath Genetics at 012 678 1361 or email ngs@ampath.co.za.
Conclusion
✅ Molecular testing is essential for personalized CRC management.
✅ KRAS and NRAS mutations predict anti-EGFR resistance.
✅ BRAF mutations indicate poor prognosis and guide targeted therapy.
✅ MSI-H tumours respond well to immunotherapy.
✅ Comprehensive testing improves treatment selection and survival outcomes.
📌 Genomic advancements in CRC are transforming treatment strategies and precision medicine approaches.