Clinicopathologic Significance of Nuclear HER4 and Phospho-YAP(S) in Human Breast Cancers and Matching Brain Metastases

Background: Human epidermal growth factor receptor-4 (HER4) and yes-associated protein-1 (YAP) are candidate therapeutic targets in oncology. YAP's transcriptional coactivation function is modulated by the HER4 intracellular domain (HER4-ICD) , but the clinical relevance of this has not been established. This study investigated the potential for targeting the HER4-YAP pathway in brain metastatic breast cancer.

Methods: We performed immuno-phenotypic profiling of pathway markers in a consecutive breast cancer series with 25 years of clinical follow up ( = 371), and patient-matched breast and metastatic brain tumours ( = 91; 30 pairs).

Results: Membrane localisation of phospho-HER4 [pHER4(Y)] was infrequent in primary breast cancer, but very frequent in brain metastases (5.9% 75% positive), where it was usually co-expressed with pHER3(Y) ( < 0.05). The presence of YAP in tumour cell nuclei was associated directly with nuclear pERK5(T/Y) ( = 0.003). However, relationships with disease-specific survival depended on oestrogen receptor (ER) status. Nuclear pYAP(S) was associated with smaller, good prognostic ER+ breast tumours (log-rank hazard-ratio 0.53;  = 9.6E), but larger, poor prognostic triple-negative cancers (log-rank hazard-ratio 2.78;  = 1.7E), particularly when co-expressed with nuclear HER4-ICD ( = 0.02). This phenotype was associated with stemness and mitotic instability markers (vimentin, SOX9, ID1, SPAG5, TTK, geminin;  < 0.05). YAP expression in brain metastases was higher than matched primary tumours; specifically, nuclear pYAP(S) in ER-negative cases ( < 0.05). Nuclear YAP was detected in ~70% of ER-negative, HER4-activated brain metastases.

Discussion: Our findings suggest that the canonical-mechanism where Hippo pathway-mediated phosphorylation of YAP ostensibly excludes it from the nucleus is dysfunctional in breast cancer. The data are consistent with pYAP(S) having independent transcriptional functions, which may include transducing neuregulin signals in brain metastases. Consistent with mechanistic studies implicating it as an ER co-factor, nuclear pYAP(S) associations with breast cancer clinical outcomes were dependent on ER status.

Conclusion: Preclinical studies investigating HER4 and nuclear YAP combination therapy strategies are warranted.