Colorectal Carcinoma: Pathogenesis, Morphology & Molecular Correlation (with HNPCC/Lynch Syndrome)

Overview and Epidemiology

• Low dietary fiber, high refined carbohydrate and fat intake
• Obesity, tobacco smoking, alcohol consumption
• Protective: aspirin and NSAIDs (via COX-2 inhibition - COX-2 is overexpressed in 90% of colorectal carcinomas and 40-90% of adenomas)

Three Major Molecular Pathways to Colorectal Carcinoma

PATHWAY 1: APC/β-catenin Pathway (Classic Adenoma-Carcinoma Sequence)

The Adenoma-Carcinoma Sequence - Robbins Diagram

Molecular Steps in APC/β-catenin Pathway

• APC is a key negative regulator of β-catenin in the WNT signaling pathway
• APC normally binds to and promotes degradation of β-catenin
• With APC loss: β-catenin accumulates → translocates to nucleus → forms complex with DNA-binding factor TCF → activates transcription of MYC and cyclin D1 → promotes proliferation
• Both APC alleles must be lost (mutation or epigenetic silencing) for adenoma formation
• In FAP: patients are born with one mutant allele; somatic loss of the second allele triggers adenoma development
• Key note: some colon cancers without APC mutations harbor β-catenin mutations that prevent APC-dependent degradation - same net effect

• Activating mutations promote growth and prevent apoptosis
• Present in <10% of adenomas <1 cm, but 50% of adenomas >1 cm and 50% of invasive adenocarcinomas
• This temporal pattern confirms KRAS mutation is a late event

• These are effectors of TGF-β signaling
• TGF-β normally inhibits the cell cycle; loss allows unrestrained cell growth

• Mutated in 70-80% of colon cancers but uncommonly in adenomas
• Confirms this is a very late event in tumor progression
• Often caused by chromosomal deletions (chromosomal instability, CIN)

• Allows indefinite replication without telomere shortening
• Marks transition to frank carcinoma

PATHWAY 2: Microsatellite Instability (MSI) Pathway - The HNPCC/Lynch Syndrome Pathway

HNPCC / Lynch Syndrome

Definition and Genetics

• HNPCC accounts for 2-4% of all colorectal cancers - the most common syndromic form
• Most patients have mutations in MSH2 or MLH1; at least 5 MMR genes are known
• Patients inherit one mutant gene and one normal allele
• When the second copy is lost (mutation or epigenetic silencing): defects in mismatch repair → mutation rates up to 1000 times higher than normal

Cancer Spectrum in Lynch Syndrome

• Colorectum (most common - right colon predominance)
• Endometrium (second most common)
• Stomach, ovary, ureters, brain, small bowel, hepatobiliary tract, pancreas, skin

Molecular Mechanism: Microsatellite Instability

1. Errors at microsatellite repeats are not corrected
2. Accumulation of mutations at rates 1000x higher than normal
3. This phenomenon is called microsatellite instability (MSI)
4. Key tumor suppressor genes within microsatellite-containing regions are silenced - including TGF-β receptor II and BAX (pro-apoptotic protein)
5. Cancer develops through a sessile serrated adenoma → mucinous adenocarcinoma sequence

Distinguishing Features of HNPCC-Associated Cancers vs. Sporadic

PATHWAY 3: CpG Island Methylator Phenotype (CIMP)

• Marked by silencing of genes via promoter hypermethylation - most commonly MLH1
• MLH1 silencing → microsatellite instability without a germline MMR mutation
• Often also involves BRAF mutations
• This explains the ~5-10% of sporadic MSI-high colon cancers that occur via epigenetic rather than mutational MMR gene loss
• Predominantly right-sided, mucinous adenocarcinomas

MORPHOLOGY

Gross Appearances

• Tend to grow as polypoid, exophytic masses that extend along one wall
• Rarely cause obstruction (wide luminal diameter)
• Clinical presentation: occult blood loss → iron deficiency anemia

• Tend to be annular, encircling lesions producing "napkin-ring" constriction
• Produce early obstruction
• Clinical features: change in bowel habits, obstipation, cramping

Histologic Appearances

1. Well-differentiated adenocarcinoma - recognizable glandular structures, elongated hyperchromatic nuclei, luminal necrotic debris ("dirty necrosis")
2. Poorly differentiated adenocarcinoma - few glands, predominantly infiltrating solid nests
3. Mucinous (colloid) adenocarcinoma - extracellular mucin pools with floating signet-ring cells - characteristic of MSI/HNPCC pathway, right colon, worse prognosis

Metastatic Disease

Prognosis and Staging

1. Depth of invasion (T stage):
   • Confined to mucosa (no crossing of muscularis mucosae): ~100% 5-year survival
   • Into submucosa or muscularis propria: 95% and 70-90% respectively
   • Through visceral serosa or into adjacent organs: further reduction
2. Lymph node metastases (N stage): significantly reduces survival; most LN+ cases receive adjuvant chemotherapy ± radiation (neoadjuvant approach common for rectal cancer)
3. Distant metastasis (M stage): only 15% or fewer alive at 5 years

Summary: Molecular Correlation Table

PATHWAY          GENE DEFECT       MECHANISM                  CANCER TYPE
─────────────────────────────────────────────────────────────────────────────
APC/Wnt         APC (5q21)        β-catenin accumulation      Left colon, tubular/villous
(80% sporadic)  KRAS (12p12)      Proliferation/survival      adenocarcinoma
                SMAD2/4 (18q21)   Loss TGF-β brake            (Chromosomal instability)
                TP53 (17p13)      Loss apoptosis

MSI Pathway     MSH2 / MLH1       MMR deficiency → MSI        Right colon, mucinous
(HNPCC, 2-4%;  (germline mut.)    1000x mutation rate         adenocarcinoma
sporadic 10%)                     TGFβRII, BAX silenced       (MSI-high)

CIMP Pathway    MLH1 (epigenetic) Promoter hypermethylation    Right colon, mucinous
(sporadic 5-10%) BRAF mutation     → MLH1 silencing → MSI      adenocarcinoma

Clinical Pearls for HNPCC/Lynch Syndrome

• Amsterdam criteria (clinical diagnosis): 3 relatives with Lynch syndrome-associated cancers, across 2 generations, one diagnosed <50 years, one being first-degree relative of the other two
• Testing: MMR protein immunohistochemistry (loss of MLH1, MSH2, MSH6, PMS2) + MSI PCR testing
• MSI-high tumors (whether sporadic or Lynch-associated) tend to respond better to immune checkpoint inhibitors (pembrolizumab) - this is a major clinical implication
• Colonoscopic surveillance every 1-2 years from age 20-25 for Lynch syndrome carriers

Colorectal Carcinoma — Pathogenesis, Morphology & Molecular Correlation

Three Molecular Pathways (Summary Table)

Pathway 1 — APC/β-catenin (Adenoma-Carcinoma Sequence)

1. APC loss (5q21) - "first hit" → β-catenin accumulates → nuclear translocation → MYC + cyclin D1 activation → proliferation
2. KRAS mutation (12p12) - promotes growth, prevents apoptosis (late event; present in 50% of adenomas >1 cm)
3. SMAD2/4 loss (18q21) - loss of TGF-β cell cycle brake
4. TP53 loss (17p13) - mutated in 70-80% of cancers, rarely in adenomas (very late)
5. Telomerase activation - immortalization → frank carcinoma

Pathway 2 — MSI Pathway / HNPCC / Lynch Syndrome

What is HNPCC?

• Autosomal dominant condition; germline mutations in MMR genes (MSH2 or MLH1 in most cases)
• Accounts for 2-4% of all colorectal cancers
• Cancers also at: endometrium, stomach, ovary, ureters, brain, biliary tract, pancreas, skin

Mechanism

• Patient inherits one mutant MMR allele; when the second copy is lost → MMR deficiency
• Mutations accumulate at 1000× normal rate in microsatellite repeat regions (50,000-100,000 in the genome)
• Key targets silenced: TGF-β receptor II, BAX (pro-apoptotic) → unchecked growth

HNPCC vs. Sporadic Colon Cancer

Morphology

Gross

• Right-sided: Polypoid, exophytic mass → occult bleeding → iron-deficiency anemia
• Left-sided: Annular "napkin-ring" constriction → obstruction, change in bowel habits

Histology

Metastasis & Prognosis

1. Depth of invasion - mucosa-limited: ~100% survival; through muscularis propria: 70-90%
2. Lymph node metastases - significantly worsens prognosis; guides adjuvant therapy

Clinical pearl: MSI-high tumors (both Lynch and sporadic CIMP) respond well to immune checkpoint inhibitors (pembrolizumab). Rectal cancers bypass the liver via systemic veins → lung metastases first.

Barrett's Oesophagus

Definition

Epidemiology

• Occurs in up to 10% of patients with symptomatic GERD; up to 2% of the general population
• Peak age: 40-60 years
• Males >> Females
• More common in people of European descent
• Incidence is rising - parallels the rise of GERD and obesity in Western countries

Pathogenesis

Chronic GERD
     ↓
Repeated acid (± bile) injury to squamous mucosa
     ↓
Metaplasia: squamous → columnar (intestinal-type) epithelium
     ↓
Barrett's Oesophagus (intestinal metaplasia with goblet cells)
     ↓ (0.2-1% per year)
Low-grade dysplasia
     ↓
High-grade dysplasia
     ↓
Intramucosal carcinoma → Invasive Adenocarcinoma

• TP53 mutation - often present in early stages
• Driver mutations shared between Barrett's epithelium and adenocarcinoma (confirmed by genomic sequencing)
• Chromosomal abnormalities accumulate stepwise
• Inflammation contributes to tumour progression

Morphology

Gross / Endoscopic

• Long-segment Barrett's: ≥3 cm - higher risk of dysplasia/carcinoma
• Short-segment Barrett's: <3 cm - lower risk; may lack GERD symptoms

Microscopic - The Hallmark

Goblet cells = diagnostic of Barrett's oesophagus

• Goblet cells have distinct mucin vacuoles that stain pale blue on H&E
• The mucin vacuole pushes the cytoplasm into a "wine goblet" shape
• Non-goblet columnar cells (gastric-type foveolar cells) are also present
• The risk of cancer is greatest in those with goblet cells (true intestinal metaplasia)

Dysplasia - The Cancer Precursor

Clinical Features & Diagnosis

• Symptoms: heartburn, dysphagia, regurgitation (GERD symptoms)
• Diagnosis requires both:
  1. Endoscopic evidence of columnar mucosa >1 cm above the GOJ
  2. Histologic confirmation of intestinal metaplasia (goblet cells on biopsy)
• Advanced imaging: chromoendoscopy or narrow-band imaging increases diagnostic yield

Management Summary

Key Exam Points

1. Barrett's = intestinal metaplasia (not just any columnar metaplasia) - goblet cells are diagnostic
2. It is a complication of GERD, not a separate disease
3. Most individuals with Barrett's do NOT develop adenocarcinoma
4. Adenocarcinoma risk: 0.2-1% per year from Barrett's → dysplasia → cancer sequence
5. Located in the distal 1/3 oesophagus; may invade the gastric cardia
6. Associated adenocarcinoma produces mucin and forms glands on histology
7. TP53 mutation is an early molecular event in Barrett's → adenocarcinoma progression

Histology Comparison: Normal Oesophagus vs. Barrett's vs. Adenocarcinoma

Visual Summary of the Progression

Normal squamous   →   Barrett's          →   Low-grade Dys.   →   High-grade Dys.   →   Adenocarcinoma
──────────────────────────────────────────────────────────────────────────────────────────────────────
Flat squamous         Goblet cells            Goblet cells          Severe atypia         Invasive glands
No glands             Orderly glands          Mild nuclear          No surface            Desmoplastic
Uniform nuclei        Surface maturation      changes               maturation            stroma
                      present                 Pseudostratified      Atypical mitoses      Mucin production

Key Exam Differentiators

Molecular Events in Barrett's Oesophagus

Key Concept

The Molecular Progression Flowchart

CHRONIC GERD / ACID INJURY
         ↓
  Reactive oxygen species → DNA damage
         ↓
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
     BARRETT'S METAPLASIA
  ─ CDKN2A (p16/p19-ARF) silencing        ← EARLY
  ─ Epigenetic hypermethylation of CDKN2A
  ─ Chromosomal instability begins
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
       LOW-GRADE DYSPLASIA
  ─ TP53 mutation / allelic loss           ← EARLY-MID
  ─ Chromosomal abnormalities increase
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
       HIGH-GRADE DYSPLASIA
  ─ TP53 biallelic inactivation            ← MID-LATE
  ─ Loss of cell cycle control
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
        ADENOCARCINOMA
  ─ Oncogene amplification                 ← LATE
    (ERBB2, VEGFA, EGFR, KRAS,
     CCND1, CDK6)
  ─ Gross chromosomal instability
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

Gene-by-Gene Breakdown

Summary: Two Types of Molecular Change

1. Tumour Suppressor Loss (early)

• CDKN2A (p16/p19-ARF) - lost by mutation or epigenetic methylation
• TP53 - mutated early, one of the first detectable changes even in non-dysplastic Barrett's

2. Oncogene Amplification (late - carcinoma stage)

• ERBB2, VEGFA, EGFR, KRAS, CCND1, CDK6 - all amplified in established adenocarcinoma

Clinical Relevance of Molecular Events

Key exam point: Barrett's adenocarcinoma is driven by chromosomal instability (CIN) - NOT microsatellite instability (MSI). This distinguishes it from right-sided colon cancers (HNPCC/Lynch). The first molecular changes are TP53 mutation and CDKN2A silencing, before any dysplasia is visible histologically.