Inhibition of the transcriptional kinase CDK7 overcomes therapeutic resistance in HER2-positive breast cancers

Abstract

Resistance of breast cancer to human epidermal growth factor receptor 2 (HER2) inhibitors involves reprogramming of the kinome through HER2/HER3 signaling via the activation of multiple tyrosine kinases and transcriptional upregulation. The heterogeneity of induced kinases prevents kinase targeting by a single kinase inhibitor and presents a major challenge to the treatment of therapeutically recalcitrant HER2-positive breast cancers (HER2+ BCs). As a result, there is a critical need for effective treatment that attacks the aberrant kinome activation associated with resistance to HER2-targeted therapy. Here, we describe a novel treatment strategy that targets cyclin-dependent kinase 7 (CDK7) in HER2 inhibitor-resistant (HER2iR) breast cancer. We show that both HER2 inhibitor-sensitive (HER2iS) and HER2iR breast cancer cell lines exhibit high sensitivity to THZ1, a newly identified covalent inhibitor of the transcription regulatory kinase CDK7. CDK7 promotes cell cycle progression through inhibition of transcription, rather than via direct phosphorylation of classical CDK targets. The transcriptional kinase activity of CDK7 is regulated by HER2, and by the receptor tyrosine kinases activated in response to HER2 inhibition, as well as by the downstream SHP2 and PI3K/AKT pathways. A low dose of THZ1 displayed potent synergy with the HER2 inhibitor lapatinib in HER2iR BC cells in vitro. Dual HER2 and CDK7 inhibition induced tumor regression in two HER2iR BC xenograft models in vivo. Our data support the utilization of CDK7 inhibition as an additional therapeutic avenue that blocks the activation of genes engaged by multiple HER2iR kinases.

CDK7 Inhibition Is Effective in all the Subtypes of Breast Cancer: Determinants of Response and Synergy with EGFR Inhibition

“…These results suggest that combining CDK7 and EGFR inhibitors may be a novel therapeutic approach for the treatment of breast cancer. In agreement with previous reports, THZ1 significantly inhibited the phosphorylation of RNA Pol II C-terminal domain at S2, S5 and S7 in a dose-dependent manner in all cell lines [2,9,17,18,26], confirming functional CDK7 inhibition. However, the magnitude of THZ1-induced decreases in Pol II phosphorylation did not correlate with THZ1 sensitivity.…”

“…Consistent with previous reports [18], we found a strong positive correlation between high CDK7 expression and worse relapse free survival in TNBC. However, we also found strong positive correlations between CDK7 RNA expression and survival in both luminal B (rapidly growing ER+) and HER2+ breast cancer patients, the HER2+ findings consistent with a recent report [17]. In contrast to negative prognostic correlations of CDK7 RNA levels, a study that used immunohistochemistry- Having found that many of the genes inhibited by THZ1 are regulated in an EGF-dependent manner, we hypothesized that inhibition of CDK7-mediated transcription via THZ1 could potentiate the effects of EGFR inhibition in breast cancer.…”

“…We treated a panel of 13 breast cancer cell lines with THZ1 for 2 or 7 days and found that almost all of the tested cell lines were inhibited at nanomolar concentrations of THZ1, but there was still a wide range of variability of IC 50 values within each subtype. Other studies have reported that MCF7, a widely used ER+ breast cancer cell line, was sensitive to THZ1 [26] and that HER2-expressing cancers also showed a high level of THZ1 sensitivity [17]. Hence, other factors not specifically related to breast cancer molecular subtype influence sensitivity to CDK7 inhibition.…”

“…CDK7 plays a key role in regulating the phosphorylation of RNA Pol II and THZ1 has previously been shown to inhibit the phosphorylation of RNA Pol II at S2, S5 and S7 positions of the heptapeptide repeat comprising the C-terminal domain (CTD) in HER2+ and TNBC cells [2,17]. We tested the effects of THZ1 on the phosphorylation of RNA Pol II at S2, S5 and S7 across multiple breast cancer cell lines and found that regardless of subtype, THZ1 inhibited the phosphorylation of Previous reports have indicated that THZ1 acts as an inducer of apoptosis rather than cell cycle arrest in the TNBC cell line MDA-MB-468 [2].…”

Blockade of CDK7 Reverses Endocrine Therapy Resistance in Breast Cancer

“…CDK7 inhibitors combined with tamoxifen reduced the expression level of cyclin D1 and E1 which were reported to be associated with tamoxifen resistance [35,36]. Also, CDK7's role in overcoming chemotherapeutic resistance in breast cancer was mentioned before, where CDK7 inhibition by TZH1 was found to reverse the resistant in HER2+ breast cancer patients [37]. Moreover, CDK7 inhibition increased tamoxifen-induced apoptosis in vitro and In vivo models compared to inhibitor or tamoxifen individually.…”