We are interested in mechanisms that transform normal cells into cancer cells. Currently much of the lab focus is on the Huntingtin Interacting Protein 1 (HIP1) family. HIP1 is linked to neurology by virtue of its interaction with huntingtin, the protein mutated in Huntington's disease. It was originally linked to neoplasia by our discovery of the oncogenic HIP1/PDGF Receptor (HIP1/PDGFβR) fusion protein that resulted from a t(5;7) chromosomal translocation in a patient with chronic myelomonocytic leukemia. The protein structures of HIP1 and HIP1-related (HIP1r) suggest that they link the actin cytoskeleton, clathrin trafficking and phosphatidylinositol turnover. One part of the lab focuses on understanding the transforming biology of HIP1/PDGFβR and other oncogenic tyrosine kinase fusion proteins such as BCR/ABL. We have generated and analyzed a conditional HIP1/PDGFβR knock-in allele and discovered that the biology resulting from this mutation in mice is more reflective of human cancer biology than standard retroviral models of oncogene expression. This is most likely due to a more physiologic expression in the right cell, at the right time and in the right location compared to the wrong time, place and location that occurs as a result of standard retroviral models. We are also identifying and characterizing the protein substrates for the HIP1/PDGFβR tyrosine kinase that are necessary for transformation.
Peripheral blood smear showing severe myeloid leukocytosis resulting from targeted "knock-in" expression of the HIP1/PDGFβR and the t(8;21)-associated AML1-ETO oncogenic fusion proteins in mouse bone marrow. Click to enlarge.
The lab is also focusing on HIP1 and HIP1r and their roles in tumorigenesis, hematopoiesis, and clathrin trafficking. We have found that, although HIP1 is expressed in some normal epithelial tissues, its expression is restricted to the colon and prostate neoplastic epithelia and not expressed in normal colon and prostate epithelia. We have evaluated the clinical implications of this in prostate cancer patients and found that HIP1 expression is a strong marker of tumor progression. The mechanism of how HIP1 may promote tumorigenesis and progression is not known. However, we have shown that its expression is necessary in some cells to survive, that it directly transforms NIH/3T3 cells and that the transformed cells have altered levels of multiple growth factor receptors (GFRs).
Confocal immunofluorescent analysis of HIP1 and clathrin heavy chain localization in normal 3T3 cells and 3T3 over-expressing HIP1. HIP1 causes redistsribution of clathrin, decreased endocytosis, and increased growth factor receptor signaling. Click to enlarge
To test if HIP1 and HIP1r directly affect the trafficking of GFRs we have evaluated the effects of transiently altered levels of HIP1 and HIP1r on activated GFRs. These proteins, but not their dominant negative counterparts, directly stabilize endosomal pools of activated GFRs. In addition HIP1 knockout mice have a general "cell loss" phenotype as evidenced by hematopoietic defects, testicular degeneration, spinal defects and cataracts. These findings suggest that HIP1 and HIP1r stabilization of GFRs is necessary for survival or proliferation of cells from diverse tissues.
Expression of HIP1 is also found in multiple cell types that show no overt phenotypes as a result of the HIP1's deletion. We therefore suspect that some lost HIP1 functions can be compensated for by expression of HIP1r, which is also expressed in multiple tissue types. The lab has generated compound HIP1r/HIP1 deficient mice to test the latter hypothesis and found that although the HIP1r deficient mice alone are normal, loss of HIPr drastically accelerates the HIP1 deficient phenotype and the mice die by 4 months of age. Although HIP1 and HIP1r clearly function in the same cellular pathway we have found that, in contrast to HIP1, HIP1r is not elevated in primary cancers!
The putative regulation of clathrin mediated trafficking and indirectly GFR activation by HIP1 and HIP1r could explain our observations. I.e. alteration of the levels and activation of GFRs may be the reason for the altered growth characteristics during over-expression or knockout of HIP1 and HIP1r. In sum, much of the lab focus is to understand the role of the clathrin trafficking in the biology of malignancy and determination if modulation of this pathway in tumor cells is therapeutic.
The spinal defects and diminished weight of "double knockout" mice deficient in both HIP1 and HIP1-related protein are rescued by transgenic expression of the human HIP1 protein. Click to enlarge.