Aim p63, a member of the p53 family, is a myoepithelial cell marker usually expressed in metaplastic breast carcinoma and its expression suggests a myoepithelial phenotype. However, its expression and association with clinicopathological features of human epidermal growth factor receptor 2 (HER 2)-positive breast carcinoma is poorly investigated.

Materials and methods Sixty-seven patients with oestrogen receptor-negative and progesterone receptor-negative, HER2-positive breast carcinoma who received anti-HER2-based neoadjuvant±adjuvant therapy was retrospectively analysed.

Results Twenty cases were p63-positive and 47 cases were p63-negative. The clinicopathological features and tumour responses after neoadjuvant therapy and outcomes were analysed. Among HER2-positive tumours, expression of p63 was significantly associated with younger age (42.5 vs 55.9; p=0.010). Expression of p63 was also significantly associated with histological grade 3 (11/20 (55%) vs 11/47 (23.4%); p=0.012) and negatively associated with grade 2 (9/20 (45%) vs 36/47 (76.6%); p=0.012). Intriguingly, p63-positive breast carcinomas showed significant aberrant p53 expression by immunohistochemistry (16/18 (88.9%) vs 29/47 (61.7%); p=0.03) and of TP53 mutation by Sanger sequencing (15/16 (93.8%) vs 12/22 (54.5%); p=0.009). No significant difference in tumour response after anti-HER2 neoadjuvant therapy nor in survival were found between p63-positive and p63-negative breast carcinomas.

Conclusion Expression of p63 in HER2-positive breast carcinoma is significantly associated with younger age, poor differentiation, high histological grade and aberrant expression of p53 and of TP53 mutation. HER2-positive breast carcinoma with a myoepithelial immunophenotype shows distinctive clinicopathological features representing a distinct subtype of HER2-positive breast carcinoma. Further, these findings suggest an interaction between p63 and mutant p53 in the tumorigenesis of HER2-positive breast carcinomas.