A 58-year-old male from rural India presents with progressive dysphagia and early satiety for 6 months. Upper endoscopy reveals a large ulcerated lesion in the gastric antrum with surrounding erythema. Biopsy confirms adenocarcinoma. Regarding the molecular and genetic alterations in gastric carcinoma, all of the following are recognized alterations EXCEPT:

Explanation

## Molecular Alterations in Gastric Adenocarcinoma ### Correct Answer Analysis **KRAS mutations are NOT the most frequent oncogenic driver in gastric cancer.** While KRAS mutations do occur, they are found in only ~20–30% of gastric adenocarcinomas, not >70%. TP53 is far more commonly mutated (>50%), making it the most frequent driver. ### Molecular Landscape of Gastric Cancer | Gene/Alteration | Frequency | Significance | Cancer Type | |-----------------|-----------|--------------|-------------| | **TP53** | >50% | Tumor suppressor loss, poor prognosis | Both types | | **KRAS** | 20–30% | Oncogenic driver (NOT most common) | Intestinal-type | | **CDH1** | ~10% (somatic); Hereditary | E-cadherin loss, diffuse type, HDGC | Diffuse-type | | **ARID1A** | 15–20% | Chromatin remodeling, poor prognosis | Both types | | **TP63** | ~15% | Squamous differentiation | Intestinal-type | | **PIK3CA** | 15–25% | PI3K/AKT pathway activation | Both types | | **Mismatch Repair (MLH1, MSH2, MSH6, PMS2)** | 20–30% | Microsatellite instability (MSI) | Both types | ### Key Molecular Concepts **Key Point:** TP53 is the **most frequently mutated gene** in gastric adenocarcinoma (>50% of cases), not KRAS. TP53 mutations correlate with advanced stage, poor differentiation, and worse overall survival [cite:Robbins 10e Ch 17]. **High-Yield:** KRAS mutations occur in only ~20–30% of gastric cancers and are more common in intestinal-type adenocarcinoma. KRAS is NOT the dominant driver in gastric cancer (unlike in pancreatic or colorectal cancer, where KRAS is very frequent). **Key Point:** **CDH1 (E-cadherin) mutations** are the hallmark of **hereditary diffuse gastric cancer (HDGC)**. Germline CDH1 mutations confer ~40% lifetime risk of diffuse gastric cancer and warrant prophylactic total gastrectomy in carriers. **Key Point:** **Microsatellite instability (MSI)** due to mismatch repair gene defects (MLH1, MSH2, MSH6, PMS2) occurs in 20–30% of gastric cancers. MSI-high tumors have better response to immunotherapy (PD-L1 inhibitors) and are associated with better prognosis in some contexts. **Mnemonic:** **STAMP** — TP53, ARID1A, KRAS, PIK3CA, Mismatch repair (MSI) — common drivers in gastric cancer (though not all equally frequent). **Clinical Pearl:** The patient in this vignette likely has intestinal-type adenocarcinoma (antral location, chronic H. pylori exposure in rural India). Such tumors typically harbor TP53 and KRAS mutations, but TP53 is far more prevalent. ### Why the Distractors Are Correct - **Option 0 (TP53 mutations >50%):** TRUE — TP53 is the most frequently mutated gene in gastric cancer and is associated with poor prognosis and advanced stage. - **Option 2 (CDH1 loss in HDGC):** TRUE — CDH1 germline mutations are pathognomonic for hereditary diffuse gastric cancer and lead to loss of E-cadherin-mediated cell adhesion. - **Option 3 (MSI from mismatch repair defects):** TRUE — MLH1, MSH2, MSH6, and PMS2 defects cause MSI in a subset of gastric cancers and have prognostic and therapeutic implications.