[Relevant mutations in predictive breast cancer pathology]

Whereas predictive immunohistochemistry has represented a core element of breast cancer classification for decades, predictive molecular pathology, with the exception of in situ hybridization for assessment of HER2 amplification, has only recently gained importance because novel drugs have been approved for treatment of metastatic disease. For the use of PARP inhibitors, proof of BRCA1 or BRCA2 mutation is mandatory. When mutation of the catalytic subunit α of the phosphatidylinositol‑4.5‑bisphosphate 3‑kinase gene (PIK3CA) is present, which can be encountered in up to 40% of luminal breast cancers, the option for treatment with the specific inhibitor alpelisib arises. The HER2 -encoded growth factor receptor contributes to neoplastic transformation not only by amplification and overexpression but also by activating the mutation of the kinase domain, which is responsive to tyrosine kinase inhibitors of the tucatinib/neratinib type. Up to 30% of metastatic and endocrine treated luminal breast cancers acquire an activating mutation of the estrogen receptor gene ESR1, resulting in an autocrine and ligand-independent growth stimulation resistant to aromatase inhibitors. Larotrectinib-sensitive mutation of tropomyosinreceptor kinase is present in up to 50% of secretory breast cancers, whereas the other histologic subtypes display an incidence of below 1%. In conclusion, predictive molecular pathology has gained importance in metastatic breast cancer.