What are the molecular subtypes of gastric cancer and how do they influence treatment outcomes?

Molecular Subtypes of Gastric Cancer

Gastric cancer is classified into four distinct molecular subtypes according to The Cancer Genome Atlas (TCGA): EBV-positive, microsatellite instability-high (MSI-H), chromosomal instability (CIN), and genomically stable (GS) tumors, each with unique molecular characteristics that directly influence treatment selection and outcomes. 1

The Four TCGA Molecular Subtypes

1. EBV-Positive Gastric Cancer

• Characterized by recurrent PIK3CA mutations, extreme DNA hypermethylation, and amplification of JAK2, CD274 (PD-L1), and PDCD1LG2 (PD-L2) 2
• Demonstrates enhanced immunogenicity and superior response to PD-1/PD-L1 checkpoint inhibitors 3
• Associates strongly with male sex and intestinal histological type 4
• Shows better 5-year survival in intestinal-type tumors compared to genomically stable tumors (HR = 0.57) 4
• Can be diagnosed using in-situ hybridization for EBV-encoded small RNA 5

2. Microsatellite Instability-High (MSI-H) Gastric Cancer

• Displays elevated mutation rates with defects in DNA mismatch repair genes (MLH1, MSH2, MSH6, PMS2) 1, 2
• Exhibits high mutational load making these tumors highly sensitive to immunotherapy 1, 3
• Prevalence is approximately 8% in Japanese populations versus 21% in Western populations 1
• Associates with intestinal histological type 4
• Critical caveat: MSI-H/dMMR patients who receive preoperative chemotherapy have worse outcomes than those undergoing surgery alone, making MSI testing essential to identify patients who should avoid neoadjuvant treatment 6
• Can be tested by immunohistochemistry (MLH1, PMS2, MSH2, MSH6) or molecular-biological analysis 5

3. Chromosomal Instability (CIN) Gastric Cancer

• The most common subtype, characterized by marked aneuploidy and focal amplification of receptor tyrosine kinases including HER2, EGFR, FGFR2, and MET 1, 2
• Enriched for copy number changes in key oncogenes 1
• Associates with intestinal histological type and positive lymph node status 4
• In diffuse-type tumors specifically, CIN subtype shows significantly worse 5-year survival compared to genomically stable tumors (HR = 1.57) 4
• HER2 overexpression occurs in 10-20% of cases, with higher prevalence in proximal/gastroesophageal junction cancers and intestinal subtype 1
• ncRNAs such as miR-22 and TERRA exacerbate chromosomal instability and correlate with poor prognosis 3

4. Genomically Stable (GS) Gastric Cancer

• Enriched for diffuse histological variant with mutations in RHOA or fusions involving RHO-family GTPase-activating proteins, plus CDH1 and CTNNA1 mutations 2, 3
• Comprises approximately 30% of Japanese gastric cancers versus 20% in Western populations 1
• Associates with diffuse histological type and negative lymph node status 4
• Features epithelial-to-mesenchymal transition (EMT) characteristics 1, 3
• ncRNAs including HOX antisense intergenic RNA, ZFAS1, and Linc00152 regulate invasion and metastasis through EMT pathways 3

Clinical Actionability and Treatment Implications

Mandatory Biomarker Testing

HER2 status and PD-L1 combined positive score (CPS) must be evaluated in all patients with metastatic gastric cancer to tailor first-line treatment 1

• HER2-positive patients (IHC 3+ or IHC 2+ with FISH positive) benefit from trastuzumab plus platinum-fluoropyrimidine chemotherapy 1
• PD-L1 CPS ≥1 indicates positive expression and predicts response to PD-1 inhibitors 1
• Universal MSI testing by PCR/NGS or MMR testing by IHC is recommended in all newly diagnosed patients 1
• CLDN18.2 testing should be performed if advanced/metastatic disease is documented or suspected 1
• Next-generation sequencing should be considered for comprehensive molecular profiling 1

Subtype-Specific Treatment Considerations

• EBV-positive and MSI-H subtypes demonstrate high sensitivity to immunotherapy and should be prioritized for PD-1/PD-L1 checkpoint inhibitors 1, 3
• CIN subtype with HER2 amplification requires HER2-targeted therapy with trastuzumab 3
• Genomically stable tumors with EMT features may not respond to single-agent anti-PD-1 therapy 1

Important Caveats and Pitfalls

Tumor Heterogeneity

Intratumoural and intertumoural heterogeneity is a defining feature of gastric carcinoma that creates diagnostic and therapeutic challenges 1

• HER2 expression shows significant intratumoural heterogeneity, hampering efficacy of HER2-targeted treatment 1
• Quantitative reporting of the proportion of tumor cells staining positive for HER2 by IHC and gene amplification ratio is essential 1

Classification Discrepancies

• The Asian Cancer Research Group (ACRG) classification shows some overlap with TCGA but differs in non-MSI subtypes 1
• ACRG subtypes have demonstrated prognostic value while TCGA subtypes do not consistently predict prognosis across all studies 1
• When combining histological and molecular information, the prognostic effects differ between intestinal- and diffuse-type cancers, requiring separate analysis 4

Practical Implementation

• Simplified approaches using IHC for p53, MLH1, E-cadherin, plus ISH for EBV, combined with NGS can achieve molecular classification in routine practice 7
• This approach defines five practical subtypes: MSI, EBV-associated, EMT-like, p53 aberrant (surrogate for CIN), and p53 proficient (surrogate for GS) 7
• Integrating lymph node status with p53 aberrant tumors achieves better prognostic stratification, separating patients into poor versus good-to-intermediate prognosis groups 7