Currently, only patients with HER2-positive
tumors are candidates for HER2-targeted
therapies. However, recent clinical observations suggest that the survival of patients with HER2-low breast
cancers, who lack HER2 amplification, may benefit from adjuvant
therapy that targets HER2. In this study, we explored a mechanism through which these benefits may be obtained. Prompted by the hypothesis that HER2/HER3 signaling in
breast tumor-initiating cells (
TIC) promotes self-renewal and survival, we obtained evidence that
neuregulin 1 (NRG1) produced by
TICs promotes their proliferation and self-renewal in HER2-low
tumors, including in triple-negative
breast tumors. Pharmacologic inhibition of EGFR, HER2, or both receptors reduced breast
TIC survival and self-renewal in vitro and in vivo and increased
TIC sensitivity to ionizing radiation. Through a tissue microarray analysis, we found that NRG1 expression and associated HER2 activation occurred in a subset of HER2-low breast
cancers. Our results offer an explanation for why HER2 inhibition blocks the growth of HER2-low
breast tumors. Moreover, they argue that dual inhibition of EGFR and HER2 may offer a useful therapeutic strategy to target
TICs in these
tumors. In generating a mechanistic rationale to apply HER2-targeting
therapies in patients with HER2-low
tumors, this work shows why these
therapies could benefit a considerably larger number of patients with
breast cancer than they currently reach.