Clinical and pathological characterization of HER2 mutations in human breast cancer: a systematic review of the literature

HER2 gene is a member of the epidermal growth factor receptor (EGFR) family. Across different malignancies, aberrations of HER2 gene commonly correspond to gain-of-function alterations leading to increased receptor signaling. We have reviewed the literature currently available on HER2 mutations in human breast cancer (BC) evaluating type and frequency of such mutations. The primary objective was to determine the frequency and the number of patients with HER2-mut in the series analyzed. The secondary objectives were to assess characteristics of mutated cases (ER and HER2 status and stage of disease, type of mutations, and finally the clinical outcome if reported). We retrieved 31 published papers, and the pooled rate of HER2 mutations across 12,905 BC patients was calculated. Overall, the frequency of HER2 mutations was 2.7% with most involving the intracellular domain. About 4% of patients were finally mutated. The predictive role was not described. Only 30% of these patients were simultaneously HER2 positive and 63% were ER positive. We have found that the prevalence of HER2 mutations is about 3%. These genic alterations are independently associated with HER2 amplification status, occurring in both ER-positive/HER2-negative diseases or HER2-enriched cancers. Ongoing trials are investigating small molecules tyrosine kinase inhibitors in patients harboring these mutations.