The carboxy terminus of pp60c-src is a regulatory domain and is involved in complex formation with the middle-T antigen of polyomavirus

A large number of mutations were introduced into the carboxy-terminal domain of pp60c-src. The level of phosphorylation on Tyr-416 and Tyr-527, the transforming activity (as measured by focus formation on NIH 3T3 cells), kinase activity, and the ability of the mutant pp60c-src to associate with the middle-T antigen of polyomavirus were examined. The results indicate that Tyr-527 is a major carboxy-terminal element responsible for regulating pp60c-src in vivo. A good but not perfect correlation exists between lack of phosphorylation at Tyr-527 and increased phosphorylation at Tyr-416, between elevated phosphorylation on Tyr-416 and activated kinase activity, and between activated kinase activity and transforming activity. Phosphorylation of Tyr-527 was insensitive to the mutation of adjacent residues, indicating that the primary sequence only has a minor role in recognition by kinases or phosphatases which regulate it in vivo. Three mutants which have in common a modified Glu-524 residue were phosphorylated on Tyr-416 and Tyr-527 and were weakly transforming. This suggests that other mechanisms besides complete dephosphorylation of Tyr-527 can lead to increased phosphorylation of Tyr-416 and activation of the transforming activity of pp60c-src. Furthermore, the residues between Asp-518 and Pro-525 were required to form a stable complex with middle-T antigen. The proximity of these sequences to Tyr-527 suggests a model in which middle-T activates pp60c-src by binding directly to this region of the molecular and thereby preventing phosphorylation of Tyr-527. Alternatively, middle-T binding may mediate a conformational change in this region, which in turn induces an alteration in the level of phosphorylation at Tyr-527 and Tyr-416.

Sequential synthesis of small capped RNA transcripts in vitro by vesicular stomatitis virus

Using purified viral or intracellular transcriptive complexes (RNP cores) of vesicular stomatitis virus (VSV), we have identified several small RNA species, ranging in size from 12 to 47 nucleotides in length that are synthesized in vitro by the genomic RNA. One group of small RNA transcripts is composed of three species that are capped at their 5' termini. Two of the capped species are from the start of the N gene and one is from the start of the NS gene. Unlike the previously described 5' triphosphated small RNAs, the templates encoding these small capped RNAs had uv target sizes greater than their respective lengths. In addition, these RNAs appeared sequentially during synchronized in vitro transcription reactions. Thus, these results provide evidence that sequences representing the 5'-capped termini of N and NS mRNAs are synthesized concomitantly with their respective mRNAs rather than simultaneously at the onset of transcription as proposed for the multiple entry, start-stop model (D. Testa, P. K. Chanda, and A. K. Banerjee, 1980, Cell 21, pp. 267-275). Together with the inability of the internally initiated 5'-triphosphated RNAs to be chased into mRNA (R. A. Lazzarini, I. Chien, F. Yang, and J. D. Keene, 1982, J. Gen. Virol. 58, 429-441), these results support a single entry model of VSV mRNA transcription.

Retrovirus shuttle vector for study of kinase activities of pp60c-src synthesized in vitro and overproduced in vivo

We have constructed a recombinant murine retrovirus which efficiently transduces avian pp60c-src into murine cells and which is easily rescued from infected cells in plasmid form. To characterize the virus, several randomly selected NIH 3T3 lines were isolated after infection with recombinant retroviral stocks. All lines overproduced avian pp60c-src and appeared morphologically normal. Immunoprecipitates made from these lines with antisera specific for pp60c-src were tested for their kinase activities in vitro. We find that both autokinase and enolase kinase activities increase proportionately with the level of pp60c-src in the immunoprecipitates. To further test the authenticity of the pp60c-src encoded by the retroviral vector, these analyses were repeated in the presence of polyomavirus middle T antigen. Avian pp60c-src was activated as a protein kinase, indicating that the virally encoded pp60c-src interacts normally with middle T antigen. Interestingly, by increasing the intracellular levels of pp60c-src 15-fold over normal endogenous levels, we were unable to obtain a proportionate increase in the amount of middle-T-antigen-pp60c-src complex. Finally, using the shuttle features designed into the vector, we have isolated the first fully processed cDNA encoding functional avian pp60c-src X pp60c-src synthesized in vitro with this cDNA had intrinsic protein kinase activity and no detectable phosphatidylinositol kinase activity.

Abstract GS5-05: Resistance to neoadjuvant chemotherapy in triple negative breast cancer mediated by a reversible drug-tolerant state

Approximately 50% of patients with localized triple negative breast cancer (TNBC) have substantial residual cancer burden following treatment with neoadjuvant chemotherapy (NACT), resulting in distant metastasis and death for most of these patients. While genomic and phenotypic intra-tumor heterogeneity are pervasive features of TNBCs at the time of diagnosis, the functional contributions of heterogeneous tumor cell populations to chemoresistance have not been elucidated.

To investigate tumor evolution accompanying NACT, we employed orthotopic patient-derived xenograft (PDX) models of treatment-naïve TNBC, which retain intra-tumor heterogeneity characteristic of human TNBC. We discovered that some PDX models initially exhibited partial sensitivity to standard front-line NACT (Adriamycin plus Cytoxan, AC). Following AC, residual tumors were resistant to chemotherapy but repopulated tumors with chemo-sensitive cells if left untreated, indicating that tumor cells possessed inherent plasticity. To identify the tumor cell subpopulation(s) conferring chemoresistance, we conducted barcode-mediated clonal tracking in three independent PDX models by introducing a high-complexity pooled lentiviral barcode library into PDX tumor cells which were then orthotopically engrafted into recipient mice. Strikingly, residual tumors maintained the same heterogeneous clonal architecture as naïve tumors. Concordantly, whole-exome sequencing revealed conservation of genomic subclonal architecture throughout treatment. These results were corroborated by genomic sequencing of serial biopsies pre- and post-AC obtained directly from TNBC patients enrolled on an ongoing clinical trial at MD Anderson (ARTEMIS; NCT02276443). Together, these studies revealed that genomically distinct pre-treatment subclones were equally capable of surviving AC to reconstitute tumors after treatment.

To identify functional addictions of residual tumor cells, we conducted histologic and transcriptomic profiling. Residual tumors following AC-treatment exhibited extensive fibrotic desmoplasia and tumor cell pleomorphism in both PDX models and in serial biopsies obtained from TNBC patients enrolled on the ARTEMIS trial. Strikingly, these AC-induced features were reverted upon regrowth of residual tumors in PDXs and in patients' tumors. Similarly, residual tumors exhibited unique transcriptomic features, many of which are also de-regulated in cohorts of human TNBCs undergoing chemotherapy treatment. These features were nearly completely reverted after tumors regrew, suggesting that the residual tumor state may be a unique and transient therapeutic window. Gene set enrichment analyses revealed that residual tumors had increased activation of oxidative phosphorylation and decreased glycolytic signaling. Pharmacologic targeting of oxidative phosphorylation with a small-molecule inhibitor of mitochondrial electron transport chain complex I (IACS-010759) significantly delayed the regrowth of AC-treated residual tumors in three independent PDX models. Collectively, these studies reveal that a reversible phenotypic state can confer chemoresistance in the absence of genomic selection and that the residual tumor state is a novel therapeutic window for chemo-refractory TNBC.

Citation Format: Echeverria GV, Ge Z, Seth S, Jeter-Jones SL, Zhang X, Zhou X, Cai S, Tu Y, McCoy A, Peoples M, Lau R, Shao J, Sun Y, Bristow C, Carugo A, Ma X, Harris A, Wu Y, Moulder S, Symmans WF, Marszalek JR, Heffernan TP, Chang JT, Piwnica-Worms H. Resistance to neoadjuvant chemotherapy in triple negative breast cancer mediated by a reversible drug-tolerant state [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 GS5-05.

Overview of the baculovirus expression system

Baculoviruses have emerged as a popular system for overproducing recombinant proteins in eukaryotic cells. This unit gives an overview of the baculovirus expression system, including discussion of the baculovirus life cycle, and post-translational modifications that occur in insect cells. In addition, the steps for overproducing proteins in the baculovirus systems are described along with recommendations for choosing an appropriate baculovirus vector and DNA, and reagents and equipment necessary for implementing the whole overexpression system.

Abstract P4-06-03: An annotated collection of pre- and post-therapy breast cancer patient-derived xenograft models built from fine needle aspiration samples aligned with ongoing clinical trials documenting response to treatment

BACKGROUND: Patient-derived xenograft (PDX) models of breast cancer replicate the diverse histologic and molecular features of patient tumors and provide a renewable source of human tumor tissue; however collection of tissue by core needle biopsy is problematic due to patient discomfort, bleeding risk and the limited number of passes a patient can tolerate. In addition, FDA guidelines caution that multiple core needle biopsies could lead to an overestimation of the true pCR rate in neoadjuvant trials.

METHODS: To support the neoadjuvant molecular diagnostic and drug development program in TNBC, a pilot study was conducted to determine if fine needle aspiration (FNA) could be used for building PDX models. Prior to engraftment, FNA samples were analysed for cell number and viability.

RESULTS: Six PDX models were successfully generated from eight individual tumor samples. These models retain histologic and molecular features of the original patient tumors as determined by immunohistochemistry, RNA expression profiling, and deep whole-exome and targeted gene sequencing. In addition, the tested PDX models recapitulate the responses to therapies across multiple chemotherapeutic agents.

Based on this success, we have standardized the use of FNAs to generate PDX models both pre- and post-therapy in two ongoing neoadjuvant clinical trials:

1. MDACC 2014-0185 (PI Stacy Moulder, 360 patients), 'Improving outcomes in TNBC using molecular triaging and diagnostic imaging to guide neoadjuvant therapy'

2. MDACC 2014-0045 (PI Jennifer Litton, 20+ patients), 'A pilot study of BMN673 as a neoadjuvant study in patients with a diagnosis of invasive breast cancer and a deleterious BRCA mutation'

 FNA cells (x10^4)Cell viability (%)Total viable cells (x10^4)Study entry biopsy (n=67)144.5050.6544.14Post treatment biopsy (n=16)47.0732.5428.38

To date, treatment-naïve primary tumor samples from 67 patients enrolled onto these neoadjuvant trials, and 16 matched non-responsive post treatment tumor samples have been analysed for cell count and viability (table below) prior to being engrafted into the humanized mammary fat pads of NOD/SCID mice.

CONCLUSION: We have demonstrated success in using FNAs to build PDX models that recapitulate the biology and clinical course of the original tumor. In our pilot study, we successfully generated six PDX models using FNA for TNBC, including some harboring deleterious BRCA1/2 mutations. Because of the high concordance in histologic, genomic, and clinical attributes, we are now using this approach to develop a rich resource of pre- and post-treatment PDX models for the investigation of therapeutic resistance.

Citation Format: Echeverria GV, Chang JT, Cai S, Tu Y, McCoy A, Lau R, Redwood A, Kaffiabasabadi S, Rauch GM, Adrada BE, Jennifer L, Moulder SL, Symmans WF, Piwnica-Worms H. An annotated collection of pre- and post-therapy breast cancer patient-derived xenograft models built from fine needle aspiration samples aligned with ongoing clinical trials documenting response to treatment [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 P4-06-03.

Replication of the vesicular stomatitis virus genome in permissive and nonpermissive host cells

Permissive infections of BHK cells and nonpermissive infections of Raji cells were probed for the accumulation of vesicular stomatitis virus intracellular RNAs. In Raji cells, the onset of vesicular stomatitis virus transcription and replication was delayed when compared to BHK cells, and the accumulation of plus and minus sense leader RNAs was significantly reduced. In contrast, full length plus and minus strand replicative RNAs accumulated in Raji cells to levels approximately equivalent to those in BHK cells. In both cell types, approximately four times as many minus strands as plus strands were detected late in the infections. At 16 h postinfection, 12% of the total genomic RNA synthesized in BHK cells was packaged and released whereas only 0.8% was released from Raji cells. In addition, of those particles released by Raji cells, only 1% were infectious whereas 77% of those released by BHK cells were infectious. The virions released from both cell types contained similar amounts of the five viral proteins, however. Analysis of virions from Raji cells revealed a faster electrophoretic mobility of the glycoprotein than the glycoprotein in virions released from BHK cells. These results suggest that Raji cells may be restricted in their ability to support a complete infection at the level of virus assembly rather than at the level of RNA replication.

Abstract P5-06-06: Comprehensive analysis of the DNA damage repair and maintenance pathways that regulate TNBC sensitivity to replication stress

This abstract was not presented at the symposium.

Abstract P5-05-01: A molecularly annotated collection of breast cancer patient-derived xenograft models aligned with ongoing clinical trials built from fine needle aspiration samples throughout neoadjuvant treatment

BACKGROUND: Patient-derived xenograft (PDX) models of breast cancer replicate the diverse histologic and molecular features of patient tumors and provide a renewable source of human tumor tissue. However, collection of tissue by core needle biopsy is problematic due to patient discomfort, bleeding risk and the limited number of passes a patient can tolerate. Several studies have catalogued the maintenance of molecular features of patient tumors in PDX models of breast cancer.

METHODS: To support the neoadjuvant molecular diagnostic and drug development program in triple negative breast cancer (TNBC), a pilot study was conducted to determine if fine needle aspiration (FNA) could be used for building PDX models. Subsequently, PDX models are being established in alignment with ongoing clinical trials at MDACC. The molecular evolution of patient's tumors, matched with PDXs engrafted from their tumors, is under study throughout the neoadjuvant treatment of TNBC using RNA sequencing, whole-exome sequencing, deep sequencing of cancer genes, and histologic analyses.

RESULTS: To date, 20 established PDX models have been developed and stable PDX models continue to be generated at a rate of 2-3 per month. Several of these models are derived from serial FNAs derived from patients throughout neoadjuvant treatment. These models retain histologic and molecular features of the original patient tumors. Serial patient biopsies, matched with PDX models, have enabled measurement of the mutational and transcriptomic evolution in vivo of TNBC undergoing neoadjuvant treatment.

We have standardized the use of FNAs to generate PDX models both pre- and post-neoadjuvant therapy in the following ongoing neoadjuvant clinical trials:

1. MDACC 2014-0185 (PI Stacy Moulder, 360 patients), 'ARTEMIS: A Randomized TNBC-Enrolling trial to confirm Molecular profiling Improves Survival'

2. MDACC 2014-0045 (PI Jennifer Litton, 20+ patients), 'A pilot study of BMN673 as a neoadjuvant study in patients with a diagnosis of invasive breast cancer and a deleterious BRCA mutation'

CONCLUSION: We demonstrated that PDX models from tissue collected by FNA recapitulate the biology and clinical course of the patient's tumor. Sequencing analyses revealed that neoadjuvant chemotherapy and PDX engraftment enrich for cancer gene mutations. We observe association of the rate of successful PDX engraftment with clinical parameters such as the patient's residual cancer burden (RCB) status at the time of surgery (upon completion of neoadjuvant treatment). In addition, we observe that PDX models derived from serial patient biopsies throughout treatment are more resistant to chemotherapy treatment. These models recapitulate the variety of chemotherapy responses observed in patients with TNBC and serve as powerful tools for preclinical biomarker and discovery studies.

Citation Format: Echeverria GV, Cai S, Tu Y, McCoy A, Lau R, Redwood A, Rauch G, Adrada B, Candelaria R, Santiago L, Thompson A, Litton J, Moulder S, Symmans F, Chang JT, Piwnica-Worms H. A molecularly annotated collection of breast cancer patient-derived xenograft models aligned with ongoing clinical trials built from fine needle aspiration samples throughout neoadjuvant treatment [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 P5-05-01.

Abstract OT2-01-22: NCT02456857: A phase II trial of liposomal doxorubicin, bevacizumab and everolimus (DAE) in patients (pts) with localized triple-negative breast cancer (TNBC) with tumors predicted insensitive to standard neoadjuvant chemotherapy (NACT)

BACKGROUND: Approximately 50% of TNBC pts treated with standard taxane/anthracycline-based NACT will have chemo-insensitive disease (CID) manifested as extensive residual disease (RCB-II or III) at the time of surgery. 40-80% of these pts will develop recurrence within 3 years of initial diagnosis. Recent advances in molecular profiling have identified subsets of TNBC with distinct, targetable molecular features. We developed a clinical trial to identify and characterize CID (ARTEMIS: A Randomized, TNBC Enrolling trial to confirm Molecular profiling Improves Survival). In the ARTEMIS trial, treatment naïve pts with localized TNBC undergo a pretreatment biopsy and then immediately start their initial phase of anthracycline-based chemotherapy so that the results of the molecular characterization are used in combination with response assessment (clinical exam/diagnostic imaging) to identify CID and inform the second phase of NACT, thus using a 'second hit' strategy in the middle of NACT to overcome drug resistance. The mesenchymal subtypes of TNBC have a high incidence of PI3K pathway activation. Preclinical models demonstrated response to PI3K inhibitors in this subtype. Metaplastic breast cancers make up ∼30% of tumors characterized as 'claudin-low/mesenchymal' by gene signature and are also associated with a high rate of PI3K activating molecular aberrations. A combination regimen of liposomal doxorubicin, bevacizumab and the mTOR inhibitors temsirolimus or everolimus (DAT or DAE) demonstrated response (including durable complete responses) in metastatic metaplastic breast cancer.

PRIMARY OBJECTIVE: Determine the rate of pathologic complete response (pCR/RCB-0) or minimal residual disease (RCB-I) after 4 cycles of DAE for treatment of mesenchymal TNBC deemed to be CID through the ARTEMIS trial

TRIAL DESIGN AND STATISTICAL METHODS: Only pts deemed to have mesenchymal CID on the ARTEMIS trial can enter this non-randomized phase II study. Realizing that pts without response to their initial cycles of chemotherapy have very low chance (5%) of achieving pCR with additional cycles of chemotherapy, it would be clinically meaningful to see pCR in this pt population improved to 20%. Counting pCR (RCB-0) or RCB-I as response, a two-stage Gehan-type design will be employed with 14 pts in the first stage. If at least one pt responds, 23 more pts will be added for a total of 37 pts. This design has a 49% chance of terminating after the first stage if the true response rate is 0.05, 23% chance if the true rate is 0.10, 10% if the true rate is 0.15 and 4% if the true rate is 0.20. If accrual continues to the second stage and a total of 37 pts are enrolled, the 95% confidence interval for a 0.20 response rate will extend from 0.10 to 0.35.

BRIEF ELIGIBILITY CRITERIA: Inclusion: localized TNBC enrolled onto ARTEMIS trial, adequate organ, bone marrow and cardiac parameters Exclusion: metastatic disease, pregnant or lactating pts, medical illness that increases chance of moderate to severe toxicity

CORRELATIVE SCIENCE: Correlate vimentin expression by IHC, mesenchymal signatures and PI3K pathway aberrations with response.

Citation Format: Moulder S, Hess K, Rauch M, Astrada B, Litton J, Mittendorf E, Ueno N, Tripathy D, Lim B, Piwnica-Worms H, Thompson A, Symmans WF. NCT02456857: A phase II trial of liposomal doxorubicin, bevacizumab and everolimus (DAE) in patients (pts) with localized triple-negative breast cancer (TNBC) with tumors predicted insensitive to standard neoadjuvant chemotherapy (NACT) [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 OT2-01-22.

Abstract P3-07-01: Towards a therapeutically relevant subtyping scheme for triple-negative breast cancer (TNBC), profiling results from A Randomized, TNBC Enrolling trial to confirm Molecular profiling Improves Survival (ARTEMIS)

Triple-negative breast cancer is a highly diverse group of cancers, with poor prognosis, and currently, there are no targeted drugs available in the clinic. In TNBC around 50% percent of the patients respond to chemotherapy, while, the other 50% percent relapse with poor prognosis. There is a need to understand better the targetable mechanisms driving TNBC via integrative analysis of gene-expression, copy-number, and mutational data.

Samples from 220 triple-negative breast cancer (TNBC) pts treated with NACT were prioritized for transcriptomic and genomic profiling. Non-negative matrix factorization was used on array-based profiling to identify six robust (ARTEMIS) subtypes. Comparing ARTEMIS subtypes with Vanderbilt subtypes, revealed significant overlap with 4/6 clusters while identifying two new clusters. Logistic regression on ssGSEA scores vs. subtypes revealed several pathways, selectively enriched specific subtypes. CL1/IM (Immune subtype), was enriched in INFg and INFa, while CL2 (MYC/mTOR), showed enrichment of several proliferation-related pathways. In addition, LAR and M (Mesenchymal) pts formed overlapping clusters, using either method.

Two new subtypes did not associate significantly with any of the previous subtypes. The majority of the tumors from the Vanderbilt BL2 and MSL were reclassified into a CL5 (ANGIO) cluster, which was enriched in angiogenesis geneset, including targetable genes like VEGF and FGFR. Also, an MYO (CL3) subtype was identified, with myogenesis-related genes. Of note, TIL (tumor infiltrating lymphocytes) and LAR quantification using IHC were associated with respective ARTEMIS subtypes. Finally, the IM subtype was significantly associated with higher rates of RCB 0-I and the M (CL4) subtype was associated with higher rates of RCB II-III, irrespective of the neoadjuvant treatment regimen.

ARTEMIS subtypes are a novel classification system for TNBC that is focused on therapeutic translation. Further, we show a possibility to classify previously un-classified (UNS) tumors, which will be validated using additional cohorts (TCGA/METABRIC).

Citation Format: Seth S, Huo L, Rauch G, Lau R, Gilcrease M, Adrada B, Piwnica-Worms H, Symmans WF, Draetta G, Futreal AP, Moulder S, Chang JT. Towards a therapeutically relevant subtyping scheme for triple-negative breast cancer (TNBC), profiling results from A Randomized, TNBC Enrolling trial to confirm Molecular profiling Improves Survival (ARTEMIS) [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 P3-07-01.

Threonine 68 phosphorylation by ataxia telangiectasia mutated is required for efficient activation of Chk2 in response to ionizing radiation

Eukaryotic cells activate an evolutionarily conserved set of proteins that rapidly induce cell cycle arrest to prevent replication or segregation of damaged DNA before repair is completed. In response to ionizing radiation (IR), the cell cycle checkpoint kinase, Chk2 (hCds1), is phosphorylated and activated in an ataxia telangiectasia mutated (ATM)-dependent manner. Here we show that the ATM protein kinase directly phosphorylates T68 within the SQ/TQ-rich domain of Chk2 in vitro and that T68 is phosphorylated in vivo in response to IR in an ATM-dependent manner. Furthermore, phosphorylation of T68 was required for full activation of Chk2 after IR. Together, these data are consistent with the model that ATM directly phosphorylates Chk2 in vivo and that this event contributes to the activation of Chk2 in irradiated cells.

Abstract P3-07-03: Withdrawn

This abstract was withdrawn by the authors.

Citation Format: Rinkenbaugh AL, Sinha VC, Zhang X, Shao J, Piwnica-Worms H. Withdrawn [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 P3-07-03.

Abstract OT2-03-03: Women's triple-negative, first-line treatment: Improving outcomes in triple-negative breast cancer using molecular triaging and diagnostic imaging to guide neoadjuvant therapy

BACKGROUND:

In triple negative breast cancer (TNBC), pathologic compete response/residual cancer burden-0 (pCR/RCB-0) or minimal residual disease (RCB-I) following neoadjuvant chemotherapy (NACT) is associated with a good prognosis. This is in contrast to extensive residual disease (RCB-II-III) which carries approximately a 50% chance of recurrence. These patients have a particularly poor prognosis as there are currently no targeted agents to salvage chemoresistant disease. It is important to predict pCR in order to direct responsive disease toward standard NACT and non-responsive disease (NRD) to therapy on clinical trials.

TRIAL DESIGN:

The use of genomic signatures (JAMA, 2011; 305:1873-81) and imaging to predict response to NACT will be validated, and the clinical impact of selecting patients with predicted NRD for targeted therapy on clinical trial will be determined. Patients will undergo primary tumor biopsy for molecular profiling and will be randomized 2:1 to know the results versus not (control). Following that, all patients will receive 4 cycles of anthracycline-based NACT, with imaging used for response assessment. Patients with molecular/imaging criteria for NRD will be offered enrollment on a clinical trial based upon molecular profiling or based upon physician/patient choice (control).

INCLUSION CRITERIA:

Tumor size ≥1.5 cm diameter; TNBC by standard assays; ≥18 years of age; LVEF ≥50%; adequate organ and bone marrow function

EXCLUSION CRITERIA:

Stage IV disease; invasive cancer within 5 years; excisional biopsy of the primary tumor; features that limit response assessment by imaging; unfit for taxane and/or antracycline regimens; prior anthracycline therapy; ≥grade II neuropathy; Zubrod performance status of ≥2; history of serious cardiac events

PRIMARY AIM:

- Prospectively determine the impact of a molecular diagnostic/imaging platform in patients with localized invasive TNBC

SECONDARY AIMS:

- Compare rates of clinical trial enrollment

- Evaluate disease free survival in the experimental arms compared to control standard NACT

- Perform integrated biomarker analyses and identify therapeutic targets for resistant disease

STATISTICAL METHODS:

A maximum of 360 patients will be randomized (2:1)using a group sequential design with one-sided O'Brien-Fleming boundaries, with two equally spaced binding interim tests for futility and superiority and one final test, having an overall Type I error .05 and power .80 to detect an improvement in pCR/RCB-I from 50% to 64%.

Citation Format: Mitri ZI, Ueno NT, Yang W, Valero V, Litton JK, Murthy RK, Ibrahim NK, Arun BK, Mittendorf EA, Hunt KK, Meric-Bernstam F, Thompson A, Piwnica-Worms H, Tripathy D, Symmans F, Moulder-Thompson S. Women's triple-negative, first-line treatment: Improving outcomes in triple-negative breast cancer using molecular triaging and diagnostic imaging to guide neoadjuvant therapy. [abstract]. In: Proceedings of the Thirty-Eighth Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2015 Dec 8-12; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2016;76(4 Suppl):Abstract nr OT2-03-03.

Abstract P6-06-01: Comprehensive analysis of the DNA damage repair and maintenance pathways that regulate TNBC sensitivity to replication stress

Agents that induce replication stress, such as inhibitors of Chk1 or ATR, are advancing in clinical development and are being tested for treatment of various solid tumors, including triple-negative breast cancer (TNBC). While the preclinical data are encouraging, additional studies are needed to predict with precision (i) which patients will most likely benefit from these inhibitors, (ii) the genetic and proteomic contexts in which these inhibitors will provide maximum therapeutic benefit as a single agent, or require additional sensitization via combination with a targeted- or chemotherapeutic agent, and (iii) exactly which targeted/chemotherapeutic agent will provide maximum therapeutic benefit for combination with replication stress inducers. To address these challenges in TNBC, we have attempted to gain a comprehensive understanding of how the DNA damage response pathways regulate TNBC cell survival in response to Chk1 inhibitors, by performing high throughput loss-of-function screens.

We have identified genes whose loss induces death of TNBC cells in the presence of (1) CHK1i alone, (2) chemotherapy alone or (3) CHK1i plus chemotherapy. In addition, given the role of TP53 as the most frequently mutated gene in TNBC, we also determined whether distinct vulnerabilities could be identified in TNBC cells that are p53-proficient versus p53-deficient. Thus, we have also identified the top synthetic lethal interactions that are either common to both p53-proficient and p53-deficient TNBC, or unique to p53-deficient TNBC; we are currently performing in vitro studies to validate the identified mechanisms. We anticipate these studies to be applicable to other agents that induce replication stress and cell cycle checkpoint bypass. Ongoing in vivo preclinical studies, which utilize patient-derived xenografts (PDXs) of TNBC to validate these findings are expected to impact patient selection for clinical trials, and also allow us to predict which chemotherapeutic agents will be most effective for combination with different cell-cycle checkpoint inhibitors.

Citation Format: Redwood AB, Seth S, Cai S, Piwnica-Worms H. Comprehensive analysis of the DNA damage repair and maintenance pathways that regulate TNBC sensitivity to replication stress [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 P6-06-01.

Cdc25 regulates the phosphorylation and activity of the Xenopus cdk2 protein kinase complex

The Xenopus cdk2 gene encodes a 32-kDa protein kinase with sequence similarity to the 34-kDa product of the cdc2 gene. Previous studies have shown that the kinase activity of the protein product of the cdk2 gene oscillates in the Xenopus embryonic cell cycle with a high in M-phase and a low in interphase. In the present study cdk2 was found not to be associated with any newly synthesized proteins during the cell cycle, but the enzyme did undergo periodic changes in phosphorylation. Upon exit from metaphase, cdk2 became increasingly phosphorylated on both tyrosine and serine residues, and labeling on these residues increased progressively until entry into mitosis, when tyrosine residues were markedly dephosphorylated. Phosphopeptide mapping of cdk2 demonstrated the major sites of phosphorylation were in a phosphopeptide with a pI of 3.7 that contained both phosphoserine and phosphotyrosine. This phosphopeptide accumulated in egg extracts blocked in S-phase with aphidicolin and was not evident in cdc2 immunoprecipitated under the same conditions. Under the same conditions cdc2 was phosphorylated primarily on a phosphopeptide containing both phosphothreonine and phosphotyrosine residues, most likely threonine 14 and tyrosine 15. Affinity-purified human GST-cdc25 was able to dephosphorylate and activate cdk2 isolated from interphase cells. Phosphopeptide mapping demonstrated that the phosphate was specifically removed from the same phosphopeptide identified as the major in vivo site of phosphorylation. These results demonstrate that cdk2 is regulated in the cell cycle by phosphorylation and dephosphorylation on both serine and tyrosine residues. Moreover, the increased phosphorylation of cdk2 in aphidicolin-blocked extracts and the ability of cdc25 to mediate cdk2 dephosphorylation in vitro suggest the possibility that cdk2 is part of the mechanism ensuring mitosis is not initiated until completion of DNA replication. It also implies cdc25 may have other functions in addition to the regulation of cdc2 kinase activity.

C-TAK1 protein kinase phosphorylates human Cdc25C on serine 216 and promotes 14-3-3 protein binding

Cdc25C is a dual-specificity protein kinase that controls entry into mitosis by dephosphorylating Cdc2 on both threonine 14 and tyrosine 15. Cdc25C is phosphorylated on serine 216 throughout interphase but not during mitosis. Serine 216 phosphorylation mediates the binding of 14-3-3 protein to Cdc25C, and Cdc25C/14-3-3 complexes are present throughout interphase but not during mitosis. Here we report the cloning of a human kinase denoted C-TAK1 (for Cdc twenty-five C associated protein kinase) that phosphorylates Cdc25C on serine 216 in vitro. C-TAK1 is ubiquitously expressed in human tissues and cell lines and is distinct from the DNA damage checkpoint kinase Chk1, shown previously to phosphorylate Cdc25C on serine 216. Cotransfection of Cdc25C with C-TAK1 resulted in enhanced phosphorylation of Cdc25C on serine 216. In addition, a physical interaction between C-TAK1 and Cdc25C was observed upon transient overexpression in COS-7 cells. Finally, coproduction of Cdc25C and C-TAK1 in bacteria resulted in the stoichiometric phosphorylation of Cdc25C on serine 216 and facilitated 14-3-3 protein binding in vitro. Taken together, these results suggest that one function of C-TAK1 may be to regulate the interactions between Cdc25C and 14-3-3 in vivo by phosphorylating Cdc25C on serine 216.