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Patients with HER2-positive gastric cancer (GC) can benefit from the addition of trastuzumab. However, not all patients with HER2-positive GC respond to trastuzumab. Biomarkers affecting its efficacy in patients with advanced gastric cancer (AGC) are largely unknown. Therefore, classifying GC into molecularly distinct subtypes to accurately distinguish between GC patients who would and would not benefit from trastuzumab is worthwhile. Tumor mutation burden (TMB) is a notable feature in GC and whether TMB influences trastuzumab efficacy is still unknown. Herein, we report the case of a 61-year-old man who was diagnosed with metastatic HER2-positive gastric adenocarcinoma that had spread to the liver (T4aN0M1, stage IV). Esophagogastroduodenoscopy revealed a circular ulcer in the posterior wall of the stomach. A computed tomography (CT) scan revealed a 2-cm diameter liver metastasis. Immunohistochemical analysis of the endoscopic biopsy tumor revealed 3+ positive expression for HER2. Whole-exome sequencing (WES) of the tumor tissue revealed 3,736 somatic mutations in 2,423 genes and a very high TMB of 50.3 mutations/Mb. Immunohistochemistry revealed that the patient had mismatch repair-proficient (pMMR) GC. The patient received first-line trastuzumab-containing chemotherapy, and after 2 courses of sequential metronomic trastuzumab-containing chemotherapy, restaging CT showed that the liver metastasis had disappeared. Following resection, the patient had no recurrence and no new tumor metastasis after a follow-up of period nearly 7 years. This study is the first to report that pMMR GC with a high TMB has a favorable response to trastuzumab. The combination of HER2 positivity and a high TMB may be sufficiently predictive of sensitivity to trastuzumab in AGC.

High tumor mutation burden in a patient with metastatic gastric cancer sensitive to trastuzumab: a case report