Pathogenesis, morphology and molecular correlation of colorectal carcinoma / HNPCC/ Lynch syndrome. Give diagrams or flow chart directly from Robbins too

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

Sources: Robbins & Kumar Basic Pathology (9780323790185) and Robbins, Cotran & Kumar Pathologic Basis of Disease (9780443264528)

Overview and Epidemiology

Colorectal adenocarcinoma is the most common malignancy of the GI tract and responsible for nearly 10% of all cancer deaths worldwide. Approximately 1.2 million new cases occur annually. Incidence peaks at 60-70 years of age. The United States accounts for ~10% of worldwide cases, representing ~15% of all US cancer deaths - second only to lung cancer.

Risk factors:

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

Robbins, Cotran & Kumar Pathologic Basis of Disease (Table 17.10) describes three pathways:

Etiology	Molecular Defect	Target Genes	Transmission	Predominant Site	Histology
Familial adenomatous polyposis	APC/Wnt pathway	APC	Autosomal dominant	None (pancolonic)	Tubular, villous; typical adenocarcinoma
HNPCC (Lynch syndrome)	DNA mismatch repair	MSH2, MLH1	Autosomal dominant	Right colon	Sessile serrated polyp; mucinous adenocarcinoma
Sporadic (70-80%)	APC/Wnt pathway	APC	None	Left colon	Tubular, villous; typical adenocarcinoma
Sporadic (10%)	DNA mismatch repair	MSH2, MLH1	None	Right colon	Sessile serrated polyp; mucinous adenocarcinoma
Sporadic (5-10%)	Hypermethylation (CIMP)	MLH1, BRAF	None	Right colon	Sessile serrated polyp; mucinous adenocarcinoma

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

Accounts for up to 80% of sporadic colon cancers.

The Adenoma-Carcinoma Sequence - Robbins Diagram

Adenoma-Carcinoma Sequence (Robbins, Cotran & Kumar Pathologic Basis of Disease, Fig. 17.52)

Fig. 17.52 - Robbins, Cotran & Kumar: Morphologic and molecular changes in the adenoma-carcinoma sequence. Loss of one normal copy of the tumor suppressor gene APC occurs early (first hit per Knudson hypothesis), followed by loss of the second copy (second hit). Additional mutations in KRAS, losses at 18q21 (SMAD2 and SMAD4), and inactivation of TP53 lead to carcinoma.

Adenoma-Carcinoma Sequence (Robbins & Kumar Basic Pathology, Fig. 13.36)

Fig. 13.36 - Robbins & Kumar Basic Pathology: The APC at 5q21 → APC/β-catenin → KRAS at 12p12 → TP53 (17p13), LOH at 18q21 (SMAD2 and 4) → Telomerase activation sequence. COX-2 overexpression occurs at the adenoma stage.

Molecular Steps in APC/β-catenin Pathway

Step 1 - APC loss (chromosome 5q21):

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

Step 2 - KRAS mutation (chromosome 12p12):

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

Step 3 - SMAD2 and SMAD4 loss (18q21):

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

Step 4 - TP53 inactivation (17p13):

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)

Step 5 - Telomerase activation:

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

Hereditary nonpolyposis colorectal cancer (HNPCC), also known as Lynch syndrome, is an autosomal dominant condition caused by inherited mutations in DNA mismatch repair (MMR) genes - proteins responsible for detection, excision, and repair of errors occurring during DNA replication.

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

Cancer risk extends well beyond the colon:

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

Molecular Mechanism: Microsatellite Instability

The human genome contains approximately 50,000-100,000 microsatellites - short repeating sequences prone to expansion during DNA replication. These are the most frequent mutation sites in HNPCC.

When MMR is deficient:

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

Distinguishing Features of HNPCC-Associated Cancers vs. Sporadic

Feature	HNPCC/Lynch	Sporadic (APC pathway)
Age at onset	Younger	Older (peak 60-70)
Location	Right colon (proximal)	Left colon (distal)
Polyp precursor	Sessile serrated adenoma	Tubular/villous adenoma
Histology	Mucinous, often with signet ring cells	Typical glandular adenocarcinoma
Molecular defect	MMR deficiency, MSI	CIN, APC/β-catenin mutations
Number of precursor polyps	Few	Many (in FAP) or moderate

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

Right-sided (cecum/ascending colon) tumors:

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

Left-sided (descending colon/sigmoid/rectum) tumors:

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

Robbins Gross Photos:

Colorectal carcinoma gross appearance - Robbins (Fig. 17.54)

Fig. 17.54 - Robbins, Cotran & Kumar: (A) Circumferential, ulcerated rectal cancer - note the anal mucosa at the bottom. (B) Cancer of the sigmoid colon that has invaded through the muscularis propria into subserosal adipose tissue; areas of chalky necrosis (arrow) within the colon wall.

Colorectal carcinoma gross - endoscopic and resection views (Robbins Basic Pathology, Fig. 13.38)

Fig. 13.38 - Robbins & Kumar Basic Pathology: (A) Endoscopic view of ulcerated ascending colon adenocarcinoma. (B) Resected rectum showing circumferential adenocarcinoma with anal mucosa at bottom. (C) Sigmoid colon cancer invading through muscularis propria into subserosal fat (left), with chalky necrosis (arrows).

Histologic Appearances

Histologic appearance of colorectal carcinoma - Robbins Pathologic Basis of Disease (Fig. 17.55)

Fig. 17.55 - Robbins, Cotran & Kumar: (A) Well-differentiated adenocarcinoma - elongated hyperchromatic nuclei; necrotic debris in gland lumen is typical. (B) Poorly differentiated adenocarcinoma - forms few glands, largely infiltrating nests. (C) Mucinous adenocarcinoma with signet-ring cells and extracellular mucin pools (characteristic of HNPCC-associated tumors).

Histologic appearance - Robbins Basic Pathology (Fig. 13.39)

Fig. 13.39 - Robbins & Kumar Basic Pathology: Same panel - (A) Well-differentiated with elongated hyperchromatic nuclei and luminal necrotic debris. (B) Poorly differentiated, infiltrating nests. (C) Mucinous adenocarcinoma with signet-ring cells and extracellular mucin pools.

Key histologic subtypes:

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

Metastatic Disease

Because of portal drainage of the colon, the liver is the most common site of distant metastasis.

The rectum does not drain via portal circulation - rectal and anorectal carcinomas can bypass the liver and metastasize to the lung first.

Metastatic colorectal carcinoma - Robbins Basic Pathology (Fig. 13.40)

Fig. 13.40 - Robbins & Kumar Basic Pathology: (A) Lymph node metastasis - glandular structures in subcapsular sinus. (B) Solitary subpleural lung metastasis. (C) Liver with multiple metastatic deposits - note central necrosis.

Prognosis and Staging

The two most important prognostic factors are:

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
Lymph node metastases (N stage): significantly reduces survival; most LN+ cases receive adjuvant chemotherapy ± radiation (neoadjuvant approach common for rectal cancer)
Distant metastasis (M stage): only 15% or fewer alive at 5 years

This staging correlates with the historic Dukes classification (A-D) and is now standardized as the TNM system.

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

All content and images sourced directly from: Robbins & Kumar Basic Pathology (Basic Pathology, 9780323790185) and Robbins, Cotran & Kumar Pathologic Basis of Disease (9780443264528)

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Colorectal Carcinoma — Pathogenesis, Morphology & Molecular Correlation

Robbins & Kumar Basic Pathology + Robbins, Cotran & Kumar Pathologic Basis of Disease

Three Molecular Pathways (Summary Table)

Etiology	Defect	Key Genes	Site	Histology
Sporadic (80%) / FAP	APC/Wnt pathway	APC, KRAS, SMAD2/4, TP53	Left colon	Tubular/villous adenocarcinoma
HNPCC / Lynch (2-4%)	DNA mismatch repair	MSH2, MLH1	Right colon	Mucinous adenocarcinoma
Sporadic CIMP (5-10%)	Epigenetic (hypermethylation)	MLH1, BRAF	Right colon	Mucinous adenocarcinoma

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

Adenoma-Carcinoma Sequence — Robbins, Cotran & Kumar Fig. 17.52

Fig. 17.52 — Robbins Pathologic Basis of Disease: Step-wise molecular changes from healthy mucosa → adenoma → carcinoma

Sequential mutations:

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

Feature	HNPCC/Lynch	Sporadic (APC)
Age	Younger	60-70 years
Location	Right colon	Left colon
Precursor	Sessile serrated adenoma	Tubular/villous adenoma
Histology	Mucinous, signet-ring	Typical glandular
Mechanism	MSI (MMR defect)	CIN (chromosomal instability)

Morphology

Gross

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

Fig. 13.38 — (A) Endoscopic: ulcerated ascending colon adenocarcinoma. (B) Circumferential rectal adenocarcinoma. (C) Sigmoid cancer invading muscularis propria with chalky necrosis (arrows)

Histology

Histologic types — Robbins Pathologic Basis of Disease Fig. 17.55

Fig. 17.55 — (A) Well-differentiated: glandular structures, elongated hyperchromatic nuclei, "dirty" luminal necrosis. (B) Poorly differentiated: infiltrating nests, few glands. (C) Mucinous: extracellular mucin pools + signet-ring cells (HNPCC-type)

Metastasis & Prognosis

Fig. 13.40 — (A) Lymph node metastasis. (B) Lung nodule. (C) Liver with multiple metastases (portal drainage route)

Two key prognostic factors:

Depth of invasion - mucosa-limited: ~100% survival; through muscularis propria: 70-90%
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.

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