Prognostic and biological divergence of upper gastrointestinal adenocarcinomas.

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350 Background: Esophageal (EAC), gastroesophageal junction (GEJ) or gastric (GAC) adenocarcinoma share similar etiologies and treatment approaches. However, it is not clear if they possess similar or different prognostic and biological characteristics. We aimed to examine overall survival (OS) in associations of the common genomic alterations with these three anatomically closely linked adenocarcinomas. Methods: Eligible patients had recurrent or de novo metastatic GAC, GEJAC or EAC whose tumors had undergone next generation sequencing (NGS) performed from November 2017 to December 2023. We used Cox regression modeling to examine the association between GAC, GEJAC, EAC and OS, adjusting for demographics, performance status, Charlson comorbidity index, receipt of chemotherapy, and HER2 overexpression or amplification (HER2 positive), p53 (mutp53 ) , KRAS (mutKRAS), CDKN2A , PIK3CA co-mutations and MYC amplification. Results: Of 875 total eligible patients, 173 had EAC, 276 had GEJAC and 426 had GAC. GEJAC had substantially better OS than EAC (HR = 0.68, [95% CI, 0.54-0.86]), and modestly better OS than GAC (HR = 0.85, [95% CI, 0.67-1.09]). HER2 positivity was associated with substantially better OS among EAC (HR = 0.62; [95% CI, 0.40-0.96]) and GEJAC (HR = 0.59; [95% CI, 0.38-0.87]) but not among GAC (HR = 0.98; [95% CI, 0.78-1.23]) patients. In addition, p53 gain-of-function versus non-gain-of-function mutations were associated with substantially worse OS among GAC (HR = 1.41; [95% CI, 0.98-2.01]) but not among EAC or GEJAC. MutKRAS was associated with substantially worse OS among EAC (HR = 1.81; [95% CI, 1.10-2.99]) but not among GEJAC or GAC. Surprisingly, MYC amplification was associated with dramatically better OS among EAC (HR = 0.19; [95% CI, 0.08-0.42]) but substantially worse OS among GEJAC (HR = 1.98, 95% CI, 1.14-3.40]) and GAC (HR = 1.75; [95% CI, 1.10-2.80]). Conclusions: GEJAC, EAC and GAC possess substantially different OS that appears differentially associated with HER2 status, mutp53, mutKRAS and MYC amplification. These results suggest distinct prognostic and biological characteristics of these three anatomically closely linked adenocarcinomas that could have important implications in clinical practice and on further investigations on their biological and topological mechanisms.