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.

Single-turnover kinetic analysis of Trypanosoma cruzi S-adenosylmethionine decarboxylase

Trypanosoma cruzi S-adenosylmethionine decarboxylase (AdoMetDC) catalyzes the pyruvoyl-dependent decarboxylation of S-adenosylmethionine (AdoMet), which is an important step in the biosynthesis of polyamines. The time course of the AdoMetDC reaction under single-turnover conditions was measured to determine the rate of the slowest catalytic step up to and including decarboxylation. Analysis of this single-turnover data yields an apparent second-order rate constant for this reaction of 3300 M(-1) s(-1) in the presence of putrescine, which corresponds to a catalytic rate of >6 s(-1). This rate is minimally 100-fold faster than the steady-state rate suggesting that product release, which includes Schiff base hydrolysis, limits the overall reaction. AdoMetDC exhibits an inverse solvent isotope effect on the single-turnover kinetics, and the pH profile predicts a pK(a) of 8.9 for the basic limb. These results are consistent with a Cys residue functioning as a general acid in the rate-determining step of the single-turnover reaction. Mutation of Cys-82 to Ala reduces the rate of the single turnover reaction to 11 M(-1) s(-1) in the presence of putrescine. Further, a solvent isotope effect is not observed for the mutant enzyme. Reduction of the wild-type enzyme with cyanoborohydride traps the Schiff base between the enzyme and decarboxylated substrate, while little Schiff base species of either substrate or product was trapped with the C82A mutant. These data suggest that Cys-82 functions as a general acid/base to catalyze Schiff base formation and hydrolysis. The solvent isotope and pH effects are mirrored in single-turnover analysis of reactions without the putrescine activator, yielding an apparent second-order rate constant of 150 M(-1) s(-1). The presence of putrescine increases the single-turnover rate by 20-fold, while it has relatively little effect on the affinity of the enzyme for product. Therefore, putrescine likely activates the T. cruzi AdoMetDC enzyme by accelerating the rate of Schiff base exchange.

Cloning and kinetic characterization of the Trypanosoma cruzi S-adenosylmethionine decarboxylase

The gene for S-adenosylmethionine decarboxylase (AdoMetDC), a rate-limiting enzyme in the biosynthesis of polyamines, has been cloned from a Trypanosoma cruz cDNA library. The cDNA clone contains a 1.1 kb open reading frame predicted to encode a 42 kDa protein that shares 31% sequence identity to the human proenzyme. T. cruzi AdoMetDC expressed and purified from E. coli is auto-catalytically processed into two subunits of 32 kDa (alpha) and 10 kDa (beta). The catalytic activity of the purified recombinant enzyme is activated by the addition of putrescine to the reaction. To determine the effect of putrescine on the kinetics of the reaction, the velocity data collected at various substrate and putrescine concentrations were fit to the rate equation describing a non-essential activator. In the presence of fully saturating putrescine, k(cat) increases by 9-fold over the unactivated rate to 0.06 s(-1). The model derived Km for AdoMet is 0.05 mM in the absence of putrescine and the model-derived Kd for putrescine binding to free enzyme is 2.5 mM. The Km for AdoMet increases by alpha 2-fold when the enzyme is fully saturated with putrescine. Unlike human AdoMetDC, cadaverine activates the T. cruzi enzyme to a similar extent as putrescine.

High-dose chemotherapy combined with escalating doses of cyclosporin A and an autologous bone marrow transplant for the treatment of drug-resistant solid tumors: a phase I clinical trial

High response rates are seen in patients undergoing dose-intensive chemotherapy and autologous marrow transplantation due to the ability of the therapy to overcome inherent or acquired drug resistance. However, relapse rates are also high because this drug resistance reversal is incomplete. Because both P-glycoprotein- and platinum-induced resistance appear to be clinically important and can be reversed in vitro with a short exposure of cyclosporin A (CSA) at 2000 and 5000 ng/ml, respectively, we undertook a trial of high-dose chemotherapy with carboplatin (1500mg/m2), mitoxantrone (75 mg/m2), and cyclophosphamide (120 mg/kg) over a 5-day period combined with escalating doses of CSA. Thirty-seven patients with primarily breast cancer (61% doxorubicin resistant) and ovarian cancer (85% platinum resistant) were treated with CSA given as a bolus 18 h prior to chemotherapy, followed by a 5-day infusion at doses of 5.0-28.2 mg/kg/day and the chemotherapy. The maximum tolerated dose of CSA was a bolus of 5.5 mg/kg and an infusion of 15. 9 mg/kg/day, which gave a mean serum CSA level of 1544 ng/ml. The dose-limiting toxicity was severe mucositis and enteritis, leading to infectious complications. Nephrotoxicity was seen in 42% and, while usually mild and reversible, was fatal in two patients with pretreatment creatinine clearances h80 ml/min. Grade III-IV isolated hyperbilirubinemia was seen in 39%, but appeared to be of no clinical significance. The overall response rate for the 26 patients with measurable/evaluable disease was 73% and 63% for those with doxorubicin- or platinum-resistant disease. The median overall survival and progression-free survival for the group were 18.1 and 8. 0 months. The overall survival for the nine patients with doxorubicin-resistant breast cancer was 19.3 months. Although we did not achieve CSA levels needed to reverse platinum resistance in vivo, levels approaching those needed to reverse P-glycoprotein resistance were reached at the maximum tolerated dose. The strategy of combining dose intensity with drug resistance reversal deserves further study, especially with the advent of potentially less toxic agents available to reverse P-glycoprotein-mediated resistance.

A phase II trial of high-dose mitoxantrone, carboplatin, and cyclophosphamide with autologous bone marrow rescue for recurrent epithelial ovarian carcinoma: analysis of risk factors for clinical outcome

Despite high initial response rates to platinum-based chemotherapy, most patients with advanced ovarian carcinoma die of drug-resistant disease. Drug resistance can be overcome in the hematologic malignancies and lymphomas with high-dose therapy and bone marrow transplantation (BMT) when used early, suggesting that this therapy may also be of value in ovarian carcinoma. As a prelude to the use of high-dose chemotherapy with BMT early in the management of advanced ovarian carcinoma, we evaluated a new high-dose regimen in patients with relapsed/refractory ovarian carcinoma to define toxicities and responses. Thirty patients were treated, of whom 20 were platinum resistant and 22 had > 1 cm maximum diameter disease. They received mitoxantrone (75 mg/m2), carboplatin (1500 mg/m2), and cyclophosphamide (120 mg/kg), followed by an autologous BMT. Overall, 89% responded, with clinical complete responses seen in 88 vs 47% (P = 0.06) of platinum-sensitive vs -resistant disease. There was only one early death (3.3%) due to Aspergillus pneumonia. Median survival for all 30 patients was 29 months, and at 3 years 23% are alive without disease. There was a 10.1- vs 5.1-month progression-free survival for patients with platinum-sensitive versus -resistant disease, and at a median follow-up of 12 months, 80% of the platinum-sensitive patients are alive. This regimen is safe, and for platinum-sensitive disease appears superior to other salvage therapies. Its use should be explored earlier in the management of advanced ovarian carcinoma.

Acidic residues important for substrate binding and cofactor reactivity in eukaryotic ornithine decarboxylase identified by alanine scanning mutagenesis

Ornithine decarboxylases from Trypanosoma brucei, mouse, and Leishmania donovani share strict specificity for three basic amino acids, ornithine, lysine, and arginine. To identify residues involved in this substrate specificity and/or in the reaction chemistry, six conserved acidic resides (Asp-88, Glu-94, Asp-233, Glu-274, Asp-361, and Asp-364) were mutated to alanine in the T. brucei enzyme. Each mutation causes a substantial loss in enzyme efficiency. Most notably, mutation of Asp-361 increases the Km for ornithine by 2000-fold, with little effect on kcat, suggesting that this residue is an important substrate binding determinant. Mutation of the only strictly conserved acidic residue, Glu-274, decreases kcat 50-fold; however, substitution of N-methylpyridoxal-5'-phosphate for pyridoxal-5'-phosphate as the cofactor in the reaction restores the kcat of E274A to wild-type levels. These data demonstrate that Glu-274 interacts with the protonated pyridine nitrogen of the cofactor to enhance the electron withdrawing capability of the ring, analogous to Asp-222 in aspartate aminotransferase (Onuffer, J. J., and Kirsch, J. F. (1994) Protein Eng. 7, 413-424). Eukaryotic ornithine decarboxylase is a homodimer with two shared active sites. Residues 88, 94, 233, and 274 are contributed to each active site from the same subunit as Lys-69, while residues 361 and 364 are part of the Cys-360 subunit.

Formation of functional cross-species heterodimers of ornithine decarboxylase

The two active sites in ornithine decarboxylase (ODC) are formed at the dimer interface with Lys-69 and Cys-360 contributing to each active site from opposite monomers [Tobias, K. E., & Kahana, C. (1993) Biochemistry 32, 5842-5847]. To gain insight into the organization of the substrate binding site and the nature of the dimer interface, analysis of ornithine decarboxylase from two parasitic protozoa, Trypanosoma brucei and Leishmania donovani, and from mouse was undertaken. Though T. brucei and mouse ornithine decarboxylase share only 60% sequence identity, the cross-species heterodimers form spontaneously, as measured by the restoration of enzyme activity upon mixing inactive K69A and C360A mutant enzymes. Thus, the amino acid composition of the dimer interface is apparently highly conserved between the T. brucei and mouse enzymes. Cross-species heterodimers were not formed between either T. brucei or mouse ODC and L. donovani ODC. Unlike the mouse and T. brucei ODC, the subunits of L. donovani ODC are not in rapid equilibrium, and incubation with a denaturant is required to induce reassociation. Kinetic analysis of the wild-type mouse and parasite ODCs revealed differences in the substrate binding sites between the three enzymes. The substrate binding properties of the restored active site in the T. brucei:mouse cross-species heterodimer mimic the characteristics of the wild-type enzyme from the species which contributes the subunit with a functional Lys-69.