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[At left: p53 staining in the small intestine of codon 72 polymorphic mice (P/P v. R/R)]
We have found that a common polymorphism at codon 72 in p53 significantly influences the ability of this protein to induce programmed cell death. Specifically, cells containing the proline 72 variant of p53, which is much more common in African Americans than Caucasian Americans, demonstrate decreased apoptosis by p53. Along with our colleague Monica Hollstein, Ph.D. (University of Leeds, UK), we have created the first mouse model for the codon 72 polymorphism of p53. This knock-in mouse contains a humanized version of p53 (Humanized p53 knock-in, or Hupki), encoding either the P72 or R72 variants.
We show in this mouse model that mice containing the P72 and R72 variants have significantly altered levels of apoptosis in different tissues. We also show that these polymorphic variants have distinct transcriptional potential, as assessed by Quantitative RT-PCR. Our mouse model of the codon 72 polymorphism will be important in our efforts to understand inter-individual differences in cancer risk, and to tailor chemotherapy to different individuals based upon their genetics.
[At right: Increased apoptosis in the thymus of P/P mice]
In addition to a common polymorphism at codon 72, p53 also has a rare polymorphism at codon 47.
Wild type p53 encodes proline at this residue, but in small percentage of African Americans it is serine (S47). We found that the S47 variant has reduced phosphorylation on serine 46, and up to three-fold decreased apoptotic function. Therefore, in terms of apoptotic ability, R72 > P72 > S47. We are in the process of creating an S47 mouse, in order to test the hypothesis that the altered apoptotic function of this variant impacts its ability to suppress tumor development.
p53 is a central signaling molecule in tumorigenesis and the response to therapy. We previously reported that three coding region variants in p53 have markedly different apoptotic potential in cell lines, and we now report similar findings in mice. Because the lesser-functioning S47 and P72 variants are significantly more common in African Americans, the proposed research will have direct impact on our understanding of disparities in cancer risk and efficacy of therapy in this population.
We have obtained data that the pathway of autophagy is critical for tumor survival. We find that inhibiting this pathway using chemical or genetic inhibitors greatly impedes tumor progression, suggesting that this pathway is an Achilles heel for cancer. Recently we discovered a novel inhibitor of the chaperone protein HSP70 that is a potent and effective anti-cancer agent. This inhibitor, which we call PES, can successfully eradicate multiple tumor types, including melanoma and acute myeloid leukemia, while protecting normal cells from toxicity. Notably, our data indicate that PES also targets rare cells in the tumor called cancer stem cells. These rare cells are believed to be responsible relapse after cancer therapy, so our finding that PES can target and eliminate these cells is very exciting. The goal of research on HSP70, autophagy and PES is to better understand the role of HSP70 and autophagy in cancer survival, and to perform research that will push PES into clinical applications for cancer therapy.
The microscope in the image belonged to William E. Horner, M.D., a collaborator with Caspar Wistar, M.D., in the early 1800s.
Dr. Horner, a lecturer at the University of Pennsylvania, was a pioneer of the use of microscopes in anatomical and medical research. He authored Special Anatomy and Histology, a seminal text on the subject.