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Improved Xenotransplant Model for Human Breast Cancer
Before new breast cancer drugs are allowed into clinical trials, FDA requires promising effects on human breast cancer xenografted into immunodeficient mice. Many drugs show great promise in mice, but the vast majority fail in clinical tests in patients. The hormones estrogen, progesterone, and prolactin are the three cardinal hormones controlling growth of normal and malignant breast epithelia. Since the majority of human breast cancers express prolactin receptors, a major overlooked problem of modeling human breast cancer in mice is that mouse prolactin does not activate human prolactin receptors. Therefore, all human breast tumor cell lines currently being used for drug testing in mouse xenograft models have been artificially selected for prolactin-independent growth. Thus, current mouse models fail to mimic the hormonal environment of patients with breast cancer and may not be suitable for drug testing.
Our solution is to generate an immunodeficient mouse model that expresses physiological levels of circulating human prolactin. First, using a gene knock-in strategy, I have replaced the mouse prolactin gene with human prolactin cDNA. Five positive mouse embryonic stem cell clones have been established and blastocyst injections are expected to result in chimeras carrying the human allele. Second, the human prolactin-expressing mice will be crossed with immunodeficient mice to generate appropriate hosts for xenotransplantation. Third, I will compare growth characteristics and drug sensitivity of human breast cancer lines in these modified recipients and in immunodeficient mice currently being used in preclinical trials.
Since human prolactin activates both mouse and human prolactin receptors equally well, we expect that mice expressing physiological levels of human prolactin will reproduce and have a normal phenotype. Furthermore, I expect to see major differences in drug responsiveness, proliferation and differentiation of human breast cancer cells in mice expressing human prolactin. The concept of a humanized prolactin mouse represents a paradigm shift in experimental modeling of human breast cancer. The new model will allow scientists to a) test more reliably breast cancer drugs in the preclinic, b) reevaluate progression, invasion and metastasis of existing human breast cancer models, c) establish new transplantable lines of human breast cancer that are not selected under human prolactin-free conditions, and d) develop improved models for human breast carcinogenesis and progression using untransformed, immortalized human epithelia and stroma.
Extensive efforts have been made to develop drugs to cure breast cancer. Before a new breast cancer drug is allowed into clinical testing in patients, the drug needs to show promising effect on human breast cancer implanted into mice. Unfortunately, the vast majority of promising drugs that work on human breast tumors in mice do not work in patients. I propose that an important reason for this frequent failure is that the hormonal environment in mice does not mimic the hormonal environment in patients. Specifically, I have discovered that human breast tumors are not responsive to mouse prolactin, which is one of main hormones that regulate human breast cancer growth. I therefore seek postdoctoral funding for a project that aims to correct this major hormonal shortcoming of current mouse models.
The interaction of three different hormones - estrogen, progesterone and prolactin - is central for growth regulation of normal and cancerous breast cells. As I will show in this application, mouse prolactin fails to functionally activate human prolactin receptors on human breast tumor cells. In other words, human breast tumors grown in mice do not “see” or “sense” mouse prolactin, and therefore need to grow in absence of this growth factor. I therefore believe that only more aggressive breast tumors will grow in mice, and that their responsiveness to drugs does not reflect the responsiveness of the same tumors grown in the presence of human prolactin. Consequently, current mouse models fail to mimic the hormonal environment of patients.
During my first four months as a postdoctoral fellow at the Lombardi Comprehensive Cancer Center at Georgetown University, I have 1) proven that mouse prolactin does not activate human prolactin receptors, 2) verified that human prolactin, in contrast, works well on mouse prolactin receptors, and 3) generated and successfully injected a targeting DNA vector that carries the human prolactin gene into mouse embryonic stem cells. We expect the first mice to be ready for further testing and analyses by October, 2004. These mice will be mated with immunodeficient mice to generate appropriate hosts for growing human breast tumors. I will compare tumor drug responsiveness in the new mice and in immunodeficient mice currently used in preclinical trials. I expect to observe extensive differences in drug responsiveness and growth of human breast cancer grown in mice expressing human prolactin. These modified mice have the potential to revolutionize experimental human breast cancer testing.