"Beyond HER2 overexpression: somatic alterations in HER2 and PI3K genes in HER2-high and HER2-low/TNBC breast cancer.

HER2-overexpressing (3+) breast cancer exhibits distinct clinicopathological characteristics compared to HER2 1+ and 2+ tumors; however, differences in their molecular features remain poorly defined. This study aimed to investigate clinicopathologic and somatic alterations in breast tumors stratified by HER2 status. Ninety breast cancer patients were stratified by HER2 expression (1+, 2+, 3+) using immunohistochemistry and confirmed by FISH. For each patient, paired diagnostic biopsies and post-neoadjuvant chemotherapy (NACT) residual tumors were analyzed. Targeted next-generation sequencing (NGS) was performed on 34 paired samples, and shortlisted variants were validated by digital droplet PCR (ddPCR) in 90 cases. Among the 90 paired samples, 40 were HER2-high, and 50 were HER2-low/TNBC. HER2-high tumors presented with a larger mean size (3.34 cm vs. 2.29 cm) but lower lymph-node metastasis (40% vs. 60%) compared to HER2-low/TNBC. NGS identified frequent mutations in PIK3CA (24%), TP53 (32%), and ERBB2 (9%). PIK3CA variants His1047Arg and Glu542Lys exhibited significantly higher variant allele frequencies in post-NACT tumors (p < 0.05). HER2-high tumors demonstrated a greater prevalence of ERBB2 mutations (13.5%), whereas in HER2-low patients, such mutations appeared only after neoadjuvant chemotherapy (NACT). Importantly, PIK3CA mutations were correlated with increased lymph-node metastasis (p = 0.04) and poorer survival outcomes, underscoring their prognostic impact. HER2-high and HER2-low/TNBC breast cancers demonstrate distinct clinicopathologic and molecular profiles. Therapy-associated enrichment of PIK3CA mutations and their association with aggressive behavior underscore their prognostic and therapeutic significance in HER2-positive disease. Further studies are needed to clarify their role in guiding personalized treatment strategies.