The Piwnica-Worms Laboratory research portfolio includes:
- Understanding the function and regulation of proteins that are involved in mammalian cell cycle and checkpoint control.
- Identifying mutations that are functionally significant in the development and progression of premalignant and invasive breast cancer.
- Developing and exploiting models that enable cell cycle regulatory proteins, breast cancer drivers, and therapeutic targets to be studied in vertebrate animals.
- Translating the results of basic, genomic, and preclinical studies into improved clinical interventions for cancer patients.
We perform mechanistic studies to understand how the mammalian cell cycle is regulated and how checkpoints interface with the cell cycle machinery to bring about cell cycle delays. Major efforts are devoted to studying the CHK1 and CHK2 protein kinases and the CDC25 family of protein phosphatases. Checkpoints induce cell cycle arrest or apoptosis in response to genotoxic or replication stress, while defects in checkpoints result in genomic instability and cancer predisposition. The ATM and ATR protein kinases are critical components of checkpoints that phosphorylate a plethora of cellular proteins to regulate DNA repair, cell death, and cell cycle progression. Downstream of ATM and ATR are two structurally unrelated protein kinases, CHK1 and CHK2. CHK2 is activated by ATM in cells with DNA double-strand breaks, and CHK1 is activated by ATR in cells that experience replication stress or those that contain DNA single-strand breaks. ATM and CHK2 are mutated in certain hereditary cancer predisposition syndromes. We are investigating how ATM, ATR, CHK1, and CHK2 are regulated and are studying the downstream effectors of these protein kinases.
Molecular Imaging Studies
Research in the area of cell cycle control is now sufficiently mature that many of the key players have been discovered and the pathways to which they belong have been delineated. However, there is still much to be learned regarding how pathway components communicate with each other and are regulated, especially in the context of vertebrate animals. Much of our mechanistic understanding of mammalian cell cycle control has come from studying cells that have been cultured in vitro. It is unclear whether the regulatory pathways discovered using cultured cells are operational in the context of a living, breathing vertebrate animal. Therefore, we have developed mouse models that enable regulatory pathways to be studied non-invasively and repetitively in living mice using molecular imaging strategies. These models enable the interrogation of pathways in vertebrate animals under steady-state conditions and after exposure to various forms of genotoxic stress.
Generation of Patient-Derived Xenograft Models of Breast Cancer
Breast cancer is the leading cause of cancer-related death in women worldwide, and metastasis is responsible for the vast majority of these deaths. Triple-negative (negative for estrogen receptor, progesterone receptor, and HER2 gene amplification) breast cancer (TNBC) is an aggressively metastatic subtype with a disproportionate degree of TP53 mutation compared to other breast cancer subtypes. TNBC comprises 15% to 20% of breast cancers in western countries and disproportionately affects African American, Hispanic, and young women. A major focus of our research program is the generation of patient-derived xenograft (PDX) models that enable TNBC to be studied in mice using the human-in-mouse (HIM) model (PNAS 2004, 101:4966-4971). In this model, tumors are obtained directly from patients with breast cancer and are immediately engrafted into the humanized mammary fat pads of NOD/SCID female mice. These models are being used to study breast cancer metastasis and determine the mechanisms of drug resistance. We are particularly interested in understanding the mechanism of resistance to PARP inhibitors.
Breast Cancer Metastasis
The preferential site of metastasis varies by breast cancer subtype, with TNBC preferring visceral organs, including the lungs. A major goal of our research is to identify the proteins that drive metastasis in patients with TNBC and determine their mechanism of action. The goals of our study are to (i) identify the transcriptional changes that drive breast cancer metastasis to the lungs and other organs; (ii) functionally characterize how these changes contribute to breast cancer metastasis; and (iii) validate our findings in human breast cancer samples. We are using preclinical HIM models of TNBC in our proposed studies.
Using our HIM models of TNBC, we demonstrated that tumor cells metastasize from mammary glands and take up residence in the lymph nodes, lungs, brain, bones, and liver. TNBC cells metastasize most robustly to the lungs in these models. We are able to isolate metastatic cells, re-passage them in recipient mice, and generate mammosphere cultures for isolating tumor-initiating cells (TICs). Using HIM models of TNBC, we will identify genes that are differentially expressed in lung metastases compared with in the mammary tumors from which they were derived. We will also identify genes that are differentially expressed in TICs from lung metastasis compared with those from mammary tumor. Once potential metastasis drivers have been identified through comprehensive gain-of-function and loss-of-function screens in vivo, we will validate their clinical relevance by examining expression levels in human breast tumor samples and corresponding (matched) metastases. Prioritized hits will be functionally assayed to determine how they function in the metastatic cascade.
Clinical Translational Research
Our laboratory has been actively involved in phase I/II clinical trials aimed at translating our fundamental knowledge of cell cycle and checkpoint control into improved targeted therapies for cancer patients. We completed phase I/II studies that tested the combination of irinotecan (topoisomerase I inhibitor) and UCN-01 (non-selective CHK1 inhibitor) in patients with resistant solid tumor malignancies, including breast cancer. The responses observed in a subset of breast cancer patients paved the way for our current efforts, which are focused on targeting p53-deficient breast cancers. We will continue to devote considerable effort over the next decade to evaluating tumor responses in both preclinical and clinical settings. Our immediate goal is to combine selective CHK1 inhibitors with various DNA-damaging agents or anti-metabolites in preclinical models of breast cancer, with the ultimate goal of using our findings to develop interventions for breast cancer patients.
Another major goal is to functionally test the mutations identified in premalignant and malignant breast cancers in our preclinical models to prioritize the vast body of somatic mutations for further study. Our long-term goal is to design and activate early-phase clinical trials of patients with somatic mutations, in collaboration with breast medical oncologists at MD Anderson.