Abstract PD3-05: Co-occurring gain-of-function mutations in HER2 and HER3 cooperate to enhance HER2/HER3 binding, HER-dependent signaling, and breast cancer growth

ERBB2, the gene encoding HER2, is mutated in 2-4% of breast cancers. The HER2 tyrosine kinase inhibitor neratinib has shown clinical activity against breast cancers harboring HER2 activating mutations, suggesting these tumors depend on HER2 signaling. Co-occurring HER2 and HER3 (ERBB3) mutations have been reported in patients who respond to neratinib (Hanker et al., Cancer Discov. 2017) suggesting the possibility of cooperativity of both oncogenes. Co-expression of the mutant intracellular domains of HER2 and HER3 in HEK293 cells enhanced phosphorylation of HER3 and ERK compared to expression of either mutant alone, which was blocked by 100 nM neratinib. Interrogation of TCGA, METABRIC, Project GENIE, and Foundation Medicine datasets revealed that gain-of-function mutations in ERBB2 and ERBB3 co-occur with a statistically significant frequency. For example, in GENIE, ERBB2 mutations co-occur with mutations in ERBB3 (8.3% of ERBB2-mutant vs 2.3% of ERBB2 WT; q=1.37x10-10).

We hypothesized that co-occurring mutations in HER2 and HER3 cooperate to enhance HER2 signaling and dependence and breast cancer progression.

Thirty-four unique breast cancers were found to harbor co-occurring mutations in HER2 and HER3, the most common of which were ERBB2L755S/ERBB3E928G (n=10), ERBB2V777L/ERBB3E928G(n=6), and ERBB2L869R/Q/ERBB3E928G (n=4). Using co-immunoprecipitation assays with HER2 and HER3 antibodies in transfected HEK293 cells, we found that co-expression of HER3E928G with wild type (WT) HER2, or co-expression of HER2L755S or HER2L869R with HER3WT, slightly increased HER2-HER3 dimerization. However, binding was strongest between double mutants. This was accompanied by the highest levels of Y1289 p-HER3 in cells expressing both HER3E928G and each HER2L755S, HER2V777L, or HER2L869R compared to cells expressing each HER2 or HER3 mutant with a respective WT heterodimer partner. Structural modeling of the HER2L869R/HER3E928G double-mutant predicted that the HER3 mutation, located at the dimer interface, may enhance heterodimerization of the kinase domains through decreased bulk and electrostatic repulsion. We also noted that the HER2L755S mutation is predicted to be in close proximity to HER3E928G (<4 Å) and may impact binding affinity. Investigation of the structural basis for the enhanced binding of other double mutants is in progress.

MCF7 “knock-in” cells incorporating HER2L755S, HER2V777L, or HER2L869R (or HER2WT) were stably transduced with HER3E928G or HER3WT. Co-expression of double mutants strongly enhanced estrogen-independent growth in 3D Matrigel over cells expressing either mutant alone. We are currently testing inhibitors of HER2/HER3 signaling, including neratinib ± trastuzumab, trastuzumab + pertuzumab, and the ERBB1-3 antibody mixture Sym013, to determine therapeutic strategies to block the cooperative growth induced by co-occurring HER2 and HER2 mutations.

Conclusions: Co-expression of mutant HER2 and mutant HER3 promotes HER2/HER binding, HER3 phosphorylation, and breast tumor cell proliferation. We aim to identify therapeutic vulnerabilities for patients with co-occurring HER2 and HER3 mutations.

Citation Format: Hanker AB, Koch JP, Ye D, Sliwoski G, Sheehan J, Kinch LN, Red Brewer M, He J, Miller VA, Lalani AS, Cutler, Jr. RE, Croessmann S, Zabransky DJ, Meiler J, Arteaga CL. Co-occurring gain-of-function mutations in HER2 and HER3 cooperate to enhance HER2/HER3 binding, HER-dependent signaling, and breast cancer growth [abstract]. In: Proceedings of the 2018 San Antonio Breast Cancer Symposium; 2018 Dec 4-8; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2019;79(4 Suppl):Abstract nr PD3-05.

Abstract P1-13-08: Extended adjuvant neratinib/fulvestrant blocks ER/HER2 crosstalk and maintains complete responses of ER+/HER2+ tumors following treatment with chemotherapy and anti-HER2 therapy

Background: Neratinib is a potent, irreversible pan-HER tyrosine kinase inhibitor. The phase III trial ExteNET showed improved disease-free survival in patients (pts) with HER2+ breast cancer treated with neratinib vs placebo after trastuzumab-based adjuvant therapy. The benefit from neratinib appeared to be greater in pts with ER+ tumors. Thus, we sought to elucidate mechanisms that may explain the benefit from extended adjuvant therapy with neratinib in pts with ER+/HER2+ breast cancer using a human-in-mouse model that simulates the clinical outcomes seen in ExteNET.

Results: Mice with established ER+/HER2 amplified MDA-361 tumors were treated with trastuzumab (tz) + paclitaxel (pac) for 4 weeks, and then randomized to fulvestrant (fulv) ± neratinib for 4 weeks. All MDA-361 tumors exhibited a prompt and marked reduction in volume after tz/pac treatment; 10 mice achieved a complete response (CR) before receiving 'extended adjuvant' therapy with fulv (n=5) or neratinib/fulv (n=5). A CR was maintained with neratinib/fulv following tz/pac. However, mice treated with fulv alone, relapsed rapidly (p<0.05 at week 8) despite of a complete downregulation of tumor ER levels. In a second experiment, nude mice with established MDA-361 xenografts were treated with pertuzumab/tz/pac for 4 weeks. Following a CR, mice were randomized to neratinib/fulv vs. fulv. Again, mice treated with neratinib/fulv maintained a CR, while mice in the fulv alone arm exhibited tumor progressions within a week. In three ER+/HER2+ cell lines (MDA-361, BT474 and UACC893) but not in ER+/HER– MCF7 cells, treatment with neratinib induced ER reporter transcriptional activity whereas treatment with fulv resulted in an increase in HER2 phosphorylation, suggesting compensatory crosstalk between the ER and HER2 pathways. To further understand the molecular basis of this crosstalk, MDA-361 tumor-bearing mice were treated with either fulv, neratinib or the combination for 7 days, after which tumors were harvested and analyzed using a Nanostring breast cancer ER panel consisting of 196 ER-regulated genes. Compared to vehicle or fulv-treated tumors, tumors treated with neratinib alone and neratinib/fulv showed marked downregulation of cyclin D1 mRNA expression. Similarly, in MDA-361, BT474 and UACC893 cells but not in MCF7 cells, only neratinib/fulv downregulated cyclin D1, P-AKT and P-ERK. Finally, treatment with neratinib/fulv but not fulv alone reduced cyclin D1 transcriptional reporter activity and cyclin D1 protein levels, and induced cell cycle arrest, suggesting double blockade is required to overcome compensatory crosstalk between ER and amplified HER2.

Conclusions: Neratinib/fulv but not fulv alone maintained complete responses of ER+/HER+ tumors following treatment with tz/pac or pertuzumab/tz/pac, reminiscent of the results in ExteNET. Neratinib treatment promoted ER transcriptional activity whereas ER downregulation with fulv was associated with increased HER2 signaling. In ER+/HER2+ breast cancer cells and tumors, neratinib/fulv synergistically inhibited growth, cyclin D1 expression, and AKT and MAPK activation, thus providing a plausible mechanism to explain the results in the ExteNET trial.

Citation Format: Sudhan DR, Schwarz LJ, Guerrero-Zotano AL, Nixon M, Formisano L, Croessmann S, Gonzalez Ericsson PI, Sanders ME, Balko JM, Avogadri-Connors F, Cutler RE, Lalani AS, Bryce R, Auerbach A, Arteaga CL. Extended adjuvant neratinib/fulvestrant blocks ER/HER2 crosstalk and maintains complete responses of ER+/HER2+ tumors following treatment with chemotherapy and anti-HER2 therapy [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr P1-13-08.

Abstract PD2-05: Inhibition of mutant HER2 results in synthetic lethality when combined with ER antagonists in ER+/HER2 mutant human breast cancer cells

Background: Human epidermal growth factor receptor 2 (ERBB2 or commonly known as HER2) missense mutations have been reported in 2-4% of breast cancers and occur primarily in the absence of HER2 gene amplification. Based on TCGA, approximately 60% of these tumors are hormone-dependent and express estrogen receptor (ER) a. Among ER+ breast cancers with HER2 missense mutations, more than 80% are in the HER2 kinase domain. We examined herein whether ER+/HER2 mutant breast cancer cells are resistant to anti-estrogen therapies and, thus, whether they should be treated with combined ER and HER2 inhibitors.

Methods: Three common HER2 activating mutations (G309A, L755S, V777L) and wild type (WT) HER2 were incorporated into ER+ MCF7 cells using AAV-mediated homologous recombination. The isogenic incorporation of a heterozygous mutation more accurately represents primary human tumors as compared to transfection and overexpression of exogenous vectors. We examined cell viability and ER transcriptional activity, using an ERE-luciferase reporter, in response to estrogen deprivation and treatment with fulvestrant (a selective ER downregulator) and neratinib (an irreversible, pan-HER tyrosine kinase inhibitor), either alone or in combination. Signaling downstream mutant HER2 was examined by immunoblot analysis. In vivo anti-tumor efficacy of fulvestrant ± neratinib is currently being assessed in ovariectomized athymic mice bearing MCF7/HER2V777L xenografts.

Results: MCF7 cells containing HER2 kinase missense mutations (L755S and V777L), but not cells with HER2WT or an extracellular domain mutation (G309A), were able to proliferate exponentially in estrogen-free medium. MCF7/HER2L755S and MCF7/HER2V777L were also resistant to 1 mM fulvestrant, despite fulvestrant's ability to downregulate ER in these cells. Additionally, MCF7/HER2L755S and MCF7/HER2V777L showed increased levels of pERK and p70S6K. Treatment with 200 nM neratinib potently inhibited growth of MCF7/HER2L755S and MCF7/HER2V777L in estrogen-free conditions and resensitized them to fulvestrant while partially downregulating HER2 levels. Addition of 1 nM estradiol markedly rescued all three HER2 mutant cells from neratinib-induced cell death suggesting that the inhibition of both ER and mutant HER2 is required for tumor cell apoptosis. Using ERE-luc reporter assays, neratinib did not inhibit basal or estrogen-induced ER transcriptional activity or ERα Ser118 phosphorylation, thus not supporting HER2 mutation-to-ER crosstalk in these genetically engineered cells. This result also suggests that the ER and HER2 mutant pathways can operate independently and it is the dual pathway inhibition that results in synthetic lethality.

Conclusions: These data suggest that, in ER+ breast cancers and similar to HER2 gene amplification, HER2 kinase domain mutations induce resistance to antiestrogen therapies. Therefore, we propose simultaneous therapeutic targeting of both ER and HER2 signaling pathways is required for maximal inhibition of ER+ breast cancers also harboring HER2 activating mutations, as is currently being investigated in the phase II SUMMIT trial (NCT01953926).

Citation Format: Croessmann S, Zabransky DJ, Cutler, Jr. RE, Lalani AS, Park BH, Arteaga CL. Inhibition of mutant HER2 results in synthetic lethality when combined with ER antagonists in ER+/HER2 mutant human breast cancer cells [abstract]. In: Proceedings of the 2016 San Antonio Breast Cancer Symposium; 2016 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2017;77(4 Suppl):Abstract nr PD2-05.