PURPOSE: Functional studies have demonstrated that some mutations of ERBB3, which encodes for human epidermal growth factor receptor (HER) 3, are oncogenic via activation of the ErbB family signaling pathway. Significant clinical activity of anti-HER2 therapies (trastuzumab plus lapatinib combination or afatinib) has been reported in patients with ERBB3-mutated cancers. This study was designed to report the rate of activating ERBB3 mutations in small bowel adenocarcinoma (SBA), a rare tumor type in which we previously reported a high rate (12%) of ERBB2-activating mutations. MATERIALS AND METHODS: DNA from 74 SBAs, previously characterized for ERBB2 mutations and mismatch repair status, was submitted for sequencing of ERBB3 exons 3, 6, 7, 8, and 23. Orthogonal validation by targeted next-generation sequencing was performed. RESULTS: Four of 74 SBAs (5.4%) displayed ERBB3-activating mutations, including three p.V104M mutations (c.310 G>A) in exon 3 and one p.E928G mutation (c.2783 A>G) in exon 23. No mutations were detected in exons 6, 7, and 8. ERBB3-activating mutations were associated with microsatellite instability (P = .002) and the presence of ERBB2-activating mutations (P = .002). Two SBAs with co-occurrence of ERBB2 and ERBB3 mutations were further analyzed by targeted next-generation sequencing. Mutant allelic frequencies suggested that both mutations were shared by the same clone rather than being harbored by mutually exclusive tumor subclones. CONCLUSION: SBAs display a high rate of ERBB3-activating mutations, which have been shown to be targetable by anti-HER2 therapies. Strikingly, ERBB3 was frequently comutated with ERBB2, suggesting a strong oncogenic addiction of these SBAs to the HER2 pathway.
ERBB3-Activating Mutations in Small Bowel Adenocarcinomas