A 58-year-old man from rural Maharashtra presents with a 6-month history of altered bowel habits and weight loss. Colonoscopy reveals a stenosing lesion in the sigmoid colon with biopsy-confirmed adenocarcinoma. Which of the following molecular/genetic alterations is NOT typically associated with the development of colorectal adenocarcinoma via the adenoma-carcinoma sequence?

Question

Accepted Answer

D. Amplification and overexpression of HER2 (human epidermal growth factor receptor 2), a hallmark of gastric carcinoma but not colorectal cancer. ## Molecular Pathogenesis of Colorectal Adenocarcinoma ### The Adenoma-Carcinoma Sequence (Fearon-Vogelstein Model) **Key Point:** Colorectal cancer develops through a stepwise accumulation of genetic alterations in the adenoma-carcinoma sequence: ```mermaid flowchart LR A[Normal epithelium]:::outcome --> B[APC loss]:::action B --> C[Early adenoma]:::outcome C --> D[KRAS activation]:::action D --> E[Intermediate adenoma]:::outcome E --> F[TP53 loss]:::action F --> G[Late adenoma/Carcinoma]:::outcome ``` ### Key Genetic Alterations in CRC **High-Yield:** The classic sequence involves: | Gene | Type | Frequency | Role in Pathogenesis | |------|------|-----------|----------------------| | APC | Tumor suppressor | 80% of sporadic CRC | Initiating event; loss of Wnt pathway regulation | | KRAS | Oncogene | 40–50% of CRC | Promotes proliferation; occurs in adenoma stage | | TP53 | Tumor suppressor | 50–70% of CRC | Loss drives adenoma-to-carcinoma transition | | SMAD2/SMAD4 | Tumor suppressor | 30% of CRC | TGF-β pathway inactivation | | HER2 | Oncogene | <5% of CRC | NOT a typical driver in CRC | **Clinical Pearl:** HER2 amplification is a hallmark of gastric and breast cancers, not colorectal cancer. While rare HER2-positive CRCs exist (<5%), HER2 is NOT a typical molecular driver of the adenoma-carcinoma sequence. ### APC Loss — The Initiating Event **Key Point:** Loss of APC is the earliest and most frequent alteration: - Occurs in ~80% of sporadic colorectal cancers - Leads to dysregulation of the Wnt/β-catenin pathway - Causes adenoma initiation - Found in adenomas as small as 1 mm ### KRAS Activation **High-Yield:** KRAS mutations occur in 40–50% of colorectal adenocarcinomas: - Typically arise in intermediate-sized adenomas - Promote cellular proliferation and survival - Associated with worse prognosis in some studies - Predict resistance to EGFR inhibitors (anti-EGFR therapy not effective in KRAS-mutant tumors) ### TP53 Inactivation **Key Point:** TP53 loss is a late event: - Occurs in 50–70% of colorectal cancers - Marks the transition from adenoma to carcinoma - Loss of p53 function removes apoptotic checkpoints - Associated with progression to invasive disease ### HER2 in Colorectal Cancer **Warning:** Do NOT confuse HER2 status between cancer types: - **Gastric cancer:** HER2 amplification in 15–20%; trastuzumab approved - **Breast cancer:** HER2 amplification in 15–20%; trastuzumab standard - **Colorectal cancer:** HER2 amplification in <5%; NOT a typical driver; HER2-targeted therapy not standard **Clinical Pearl:** The primary targetable pathways in CRC are EGFR (in KRAS wild-type tumors) and BRAF (in BRAF-mutant tumors), not HER2.

Answer

A. Activation of KRAS oncogene, occurring in approximately 40–50% of colorectal adenocarcinomas

Answer

B. Loss of APC (adenomatous polyposis coli) tumor suppressor gene, typically the initiating event in sporadic colorectal cancer

Answer

C. Inactivation of TP53 tumor suppressor gene, typically occurring in the transition from adenoma to carcinoma

Explanation

Molecular Pathogenesis of Colorectal Adenocarcinoma

The Adenoma-Carcinoma Sequence (Fearon-Vogelstein Model)

Key Genetic Alterations in CRC

APC Loss — The Initiating Event

KRAS Activation

TP53 Inactivation

HER2 in Colorectal Cancer

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Gene	Type	Frequency	Role in Pathogenesis
APC	Tumor suppressor	80% of sporadic CRC	Initiating event; loss of Wnt pathway regulation
KRAS	Oncogene	40–50% of CRC	Promotes proliferation; occurs in adenoma stage
TP53	Tumor suppressor	50–70% of CRC	Loss drives adenoma-to-carcinoma transition
SMAD2/SMAD4	Tumor suppressor	30% of CRC	TGF-β pathway inactivation
HER2	Oncogene	<5% of CRC	NOT a typical driver in CRC