Chromosomal instability pathway
APC loss, aneuploidy and copy-number change, KRAS during progression, and late TP53 loss in a conventional adenoma-carcinoma sequence.
GI
Colorectal Cancer Molecular Pathways
APC starts one road, mismatch repair failure destabilizes another, and serrated methylation opens a third.
- Order the APC-driven adenoma-carcinoma sequence and its major cooperating alterations.
- Distinguish chromosomal instability from mismatch repair deficiency and microsatellite instability.
- Connect BRAF, CIMP, MLH1 methylation, and sessile serrated lesions.
- Separate inherited Lynch syndrome from common sporadic mismatch repair deficiency.
- Translate universal tumor testing and metastatic biomarkers into treatment implications.
Progression ladder
Follow progression from driver to treatment
The figure separates tumor initiation, progression, staging, and treatment decisions.
Quick check
A 46-year-old woman has a right-sided colon adenocarcinoma. Tumor testing shows loss of MLH1 and PMS2 with high microsatellite instability. Her mother had endometrial cancer at age 49.
Which additional finding would most strongly support Lynch syndrome rather than sporadic MLH1 silencing?
Reason it through
- Which repair complex is implicated by paired MLH1 and PMS2 loss?MLH1 is the obligate partner whose loss also destabilizes PMS2.
- What common sporadic mechanism can mimic Lynch syndrome?MLH1 promoter hypermethylation can produce dMMR and MSI-H.
- What establishes inherited risk?A pathogenic germline mismatch repair variant.
Recall what universal tumor testing reveals
New colorectal cancers undergo mismatch-repair or microsatellite testing to guide hereditary evaluation and treatment.
Immunohistochemistry shows which mismatch repair proteins are absent; MSI testing measures the genomic consequence of failed repair.
Loss of MLH1 and PMS2 should trigger evaluation for sporadic MLH1 methylation, often using tumor BRAF V600E and/or direct MLH1 promoter methylation testing before germline interpretation.
Isolated loss patterns can direct germline evaluation, but genetic counseling integrates tumor results with age and personal and family history.
Select the correct test interpretation.
Abnormal tumor screening opens the germline workup; it does not finish it.
Walk the conventional adenoma-carcinoma sequence
The APC-driven pathway accumulates mutations as normal mucosa progresses through adenoma to invasion.
Biallelic APC inactivation activates WNT signaling and beta-catenin-mediated proliferation, initiating many conventional adenomas.
KRAS activation can promote growth and progression of an established adenoma; additional chromosomal losses accumulate.
Late TP53 loss is associated with transition toward invasive carcinoma, although individual tumors do not follow an identical fixed order.
Order the high-yield progression.
- 1Normal epithelium loses APC function
WNT pathway activation allows inappropriate crypt-cell proliferation.
- 2Early conventional adenoma forms
Clonal dysplastic epithelium becomes a visible conventional adenomatous polyp.
- 3KRAS and additional CIN changes accumulate
Growth signaling and copy-number alterations favor enlargement and advanced dysplasia.
- 4Late TP53 pathway loss removes a checkpoint
Genomically abnormal cells survive and progress toward invasion.
- 5Invasive adenocarcinoma emerges
Malignant glands breach the muscularis mucosae and gain access to lymphovascular routes.
APC opens, KRAS grows, TP53 guards the late gate.
Separate the three molecular roads
Most colorectal cancers can be understood through overlapping patterns of chromosomal instability, mismatch repair failure, and serrated-pathway methylation.
The conventional adenoma-carcinoma pathway typically begins with APC/WNT dysregulation, accumulates copy-number changes, and often acquires KRAS activation and late TP53 loss.
Mismatch repair deficiency permits insertion-deletion errors to accumulate at microsatellites, producing MSI-H tumors with abundant frameshift neoantigens.
The serrated pathway commonly links sessile serrated lesions, BRAF activation, widespread CpG island methylation, and sometimes MLH1 promoter methylation with secondary MSI-H.
Contrast the dominant molecular signatures.
Lynch-associated MSI pathway
A germline mismatch repair defect plus somatic loss of the remaining functional allele produces dMMR and MSI-H.
Sporadic MSI pathway
BRAF-associated CIMP can methylate MLH1, producing acquired dMMR and MSI-H without a germline mismatch repair variant.
Serrated precursor
A sessile serrated lesion, often proximal and subtle, can progress through BRAF-driven methylation biology.
MSI describes the scar; dMMR describes the broken repair machinery.
Locate the clue in the pathway
Precursor morphology, tumor side, methylation, and protein-loss pattern help localize a cancer within the molecular map.
Sessile serrated lesions are often flat or subtle, occur disproportionately in the proximal colon, and can be missed when bowel preparation or inspection is poor.
Lynch-associated cancers also frequently arise proximally but originate from inherited mismatch repair susceptibility rather than the typical sporadic BRAF-CIMP-MLH1 methylation chain.
Classic familial adenomatous polyposis produces numerous conventional adenomas through inherited APC susceptibility, making prophylactic and family-based management distinct.
Place each finding where it best fits.
Conventional adenoma
APC/WNT-driven dysplasia within the chromosomal instability adenoma-carcinoma sequence.
Sessile serrated lesion
A subtle, often proximal precursor linked to BRAF activation and CIMP.
MLH1 promoter
Hypermethylation can silence MLH1, cause paired MLH1/PMS2 loss, and create sporadic MSI-H cancer.
Germline mismatch repair gene
Inherited pathogenic variation creates Lynch syndrome and risk across colorectal and extracolonic organs.
Metastatic tumor panel
RAS, BRAF, mismatch repair or MSI, and other actionable markers refine systemic treatment selection.
Scale molecular data into treatment
Biomarkers matter most when they change inherited-risk counseling, adjuvant discussion, or systemic drug selection.
All newly diagnosed colorectal cancers should undergo mismatch repair or MSI assessment; abnormal results can trigger Lynch evaluation and influence treatment.
Stage II dMMR or MSI-H colon cancers have a favorable prognostic pattern and generally do not benefit from fluoropyrimidine monotherapy alone, so high-risk features and regimen choice require individualized discussion.
Advanced MSI-H or dMMR colorectal cancer is highly responsive to immune checkpoint blockade; current first-line options include single-agent or combined checkpoint strategies selected to fit the patient and disease.
In metastatic disease, RAS mutation predicts lack of benefit from anti-EGFR antibodies. BRAF V600E now identifies a first-line targeted combination with encorafenib, cetuximab, and fluorouracil-based chemotherapy, while resectability and patient fitness remain central.
Induction is the initial intensive treatment phase; maintenance is planned de-escalation after disease control, not a synonym for second-line therapy after progression. The molecular profile helps choose both phases.
Place each result from risk definition to direct drug selection.
Directly predictive for systemic therapy
Directly predictive for systemic therapy
Directly predictive for systemic therapy
Directly predictive for systemic therapy
Directly predictive for systemic therapy
Test every tumor for repair failure; test metastatic tumors for the levers that choose drugs.
Unmask inherited and sporadic look-alikes
Both Lynch syndrome and sporadic MLH1 methylation can produce a proximal MSI-H cancer, so the discriminating evidence matters.
Lynch syndrome follows autosomal dominant inheritance of a pathogenic mismatch repair gene variant and often presents with younger cancers or a family pattern involving colorectal, endometrial, and other Lynch-associated tumors.
Sporadic MSI-H cancer is commonly driven by acquired MLH1 promoter methylation in a BRAF-mutated, CIMP-high serrated pathway and does not imply the same inherited family risk.
Familial adenomatous polyposis is a different inherited pathway: germline APC susceptibility causes extensive conventional adenomas and near-certain cancer risk without prevention.
Reveal the discriminator behind each pattern.
- Lynch syndrome
Germline MLH1, MSH2, MSH6, PMS2, or EPCAM-related mismatch repair disruption.
Tumor dMMR supports evaluation; a pathogenic germline result establishes inherited susceptibility.
- Sporadic MLH1 loss
Somatic MLH1 promoter hypermethylation, frequently in a BRAF V600E and CIMP-associated cancer.
BRAF V600E or direct MLH1 methylation argues against classic Lynch syndrome.
- Familial adenomatous polyposis
Germline APC susceptibility with carpeting by conventional adenomas.
The pathway is chromosomal instability, not mismatch repair deficiency.
- Apparently sporadic pMMR cancer
No mismatch repair loss on tumor screening and no obvious syndrome pattern.
A striking phenotype or pedigree can still justify genetics evaluation for non-Lynch syndromes.
Same MSI phenotype, different family consequence.
Stage 1 of 3: Overview
Overview
Colorectal Cancer Molecular Pathways
The APC-driven pathway accumulates mutations as normal mucosa progresses through adenoma to invasion.
Work through the oncology pathway
Five tumor boards force the molecular map to answer a hereditary, prognostic, or treatment question.
Cross out stage mismatches and highlight the treatment-changing clue. Each case separates biology, stage, and intent.
A colectomy specimen contains carcinoma beside a large conventional tubulovillous adenoma. Sequencing shows APC inactivation, KRAS activation, broad copy-number alterations, and TP53 loss.
Which model best fits this tumor?
Reason it through
- Which alteration is the classic initiating gatekeeper?APC loss activates WNT signaling.
- Which driver commonly expands the adenoma?KRAS activation promotes progression.
- Which late loss supports malignant transition?TP53 loss removes a major damage checkpoint.
Which alteration is the classic initiating gatekeeper?Which driver commonly expands the adenoma?
- Which alteration is the classic initiating gatekeeper?APC loss activates WNT signaling.
- Which driver commonly expands the adenoma?KRAS activation promotes progression.
- Which late loss supports malignant transition?TP53 loss removes a major damage checkpoint.
A 39-year-old man has a right-sided colon cancer. Immunohistochemistry shows paired loss of MSH2 and MSH6 with retained MLH1 and PMS2. His sister had endometrial cancer at age 42.
Which next step best addresses the underlying concern?
Reason it through
- Which protein pair is absent?MSH2 loss destabilizes its partner MSH6.
- What in the pedigree strengthens inheritance?Early endometrial cancer is a classic Lynch-associated clue.
- What converts suspicion into an inherited diagnosis?Identification of a pathogenic germline variant.
Which protein pair is absent?What in the pedigree strengthens inheritance?
- Which protein pair is absent?MSH2 loss destabilizes its partner MSH6.
- What in the pedigree strengthens inheritance?Early endometrial cancer is a classic Lynch-associated clue.
- What converts suspicion into an inherited diagnosis?Identification of a pathogenic germline variant.
A 72-year-old woman has a proximal colon cancer arising near a sessile serrated lesion. The tumor is MSI-H, lacks MLH1 and PMS2, contains BRAF V600E, and has MLH1 promoter methylation.
Which diagnosis is most likely?
Reason it through
- What lesion supplies the morphologic breadcrumb?The sessile serrated lesion.
- What driver points toward the serrated route?BRAF V600E.
- What event creates the MSI phenotype?MLH1 promoter methylation disables mismatch repair.
What lesion supplies the morphologic breadcrumb?What driver points toward the serrated route?
- What lesion supplies the morphologic breadcrumb?The sessile serrated lesion.
- What driver points toward the serrated route?BRAF V600E.
- What event creates the MSI phenotype?MLH1 promoter methylation disables mismatch repair.
A 63-year-old man has resected stage II colon cancer. The tumor is MSI-H with loss of mismatch repair function, margins are negative, and no obstruction, perforation, lymphovascular invasion, or inadequate nodal sampling is present.
Which adjuvant principle is most appropriate?
Reason it through
- What is the anatomic risk group?Resected stage II disease without listed high-risk features.
- What does dMMR add?A favorable prognostic pattern and lack of expected benefit from fluoropyrimidine alone.
- What remains essential?Individualized review of pathology, comorbidity, and patient preference.
What is the anatomic risk group?What does dMMR add?
- What is the anatomic risk group?Resected stage II disease without listed high-risk features.
- What does dMMR add?A favorable prognostic pattern and lack of expected benefit from fluoropyrimidine alone.
- What remains essential?Individualized review of pathology, comorbidity, and patient preference.
A 56-year-old woman has unresectable metastatic colon adenocarcinoma. Tumor profiling shows MSI-H and deficient mismatch repair; performance status is good.
Which molecularly directed treatment principle is strongest?
Reason it through
- What defect creates many frameshift neoantigens?Deficient mismatch repair with high microsatellite instability.
- Why can the immune system see this tumor better?Its high neoantigen load makes immune recognition more likely once checkpoints are blocked.
- What separate markers still matter?RAS, BRAF, and other actionable alterations remain relevant to later systemic choices.
What defect creates many frameshift neoantigens?Why can the immune system see this tumor better?
- What defect creates many frameshift neoantigens?Deficient mismatch repair with high microsatellite instability.
- Why can the immune system see this tumor better?Its high neoantigen load makes immune recognition more likely once checkpoints are blocked.
- What separate markers still matter?RAS, BRAF, and other actionable alterations remain relevant to later systemic choices.
Tumor-board pivot
Choose the finding that changes the pathway
Choose the feature that changes diagnosis, stage, or treatment intent.
Which additional finding would most strongly support Lynch syndrome rather than sporadic MLH1 silencing?
- Key finding. A 46-year-old woman has a right-sided colon adenocarcinoma. Tumor testing shows loss of MLH1 and PMS2 with high microsatellite instability. Her mother had endometrial cancer at age 49.
- Answer. A pathogenic germline MLH1 variant
- Why. A germline pathogenic mismatch repair variant establishes inherited Lynch syndrome.
- Board rule. Tumor dMMR raises the Lynch question; germline testing answers it.
Rapid review
Three questions to check
Which additional finding would most strongly support Lynch syndrome rather than sporadic MLH1 silencing?
A pathogenic germline MLH1 variant. A germline pathogenic mismatch repair variant establishes inherited Lynch syndrome.
Which alteration is the classic initiating gatekeeper?
APC loss activates WNT signaling.
Which driver commonly expands the adenoma?
KRAS activation promotes progression.
Medically reviewed
Fatima Ali, DO
PGY-1 Resident Physician in Psychiatry
University Hospitals, Columbia
DO from Kansas City University
Resident physician and founding medical reviewer at Bone Wizardry, focused on clinical accuracy, clear diagnostic reasoning, and practical board-oriented teaching across the curriculum.
Languages: English, Urdu
Primary reviewerFull physician profile
Medically reviewed
Sources
- Genetics of Colorectal Cancer (PDQ): Health Professional Version2025
- Colon Cancer Treatment (PDQ): Health Professional Version2025
- Metastatic Colorectal Cancer: ESMO Clinical Practice Guideline for Diagnosis, Treatment and Follow-up2026
- FDA Grants Traditional Approval to Encorafenib for Metastatic Colorectal Cancer with a BRAF V600E Mutation2026
Bone Wizardry is a study resource for medical students. It is not medical advice.