Liposarcoma is diagnosed by a characteristic amplification of locus 12q14, the site of two key oncogenes, MDM2 and CDK4. While CDK4 amplification can disrupt proper regulation of the Rb pathway, it has not been formally proven that such a role is sufficient to explain this amplification event. Additionally, these tumors are characterized by having cytosolic accumulation of the cdk inhibitor p27, a phenomena that other groups have suggested to reflect some novel pro-oncogenic function for the protein. Consequently, we felt that liposarcoma might provide an interesting new venue in which to ask if there were novel roles for cdk4 and what the role of cytoplasmic p27 is in disease.
A critical limitation to understanding the molecular biology of liposarcoma is the lack of a suitable mouse model system. Whereas, the critical pathways altered in adenocarcinoma are well-characterized, the critical pathways in mesenchymal-derived soft tissue sarcomas are not clear. Because there are few treatments available for patients with these diseases, it is essential that we characterize the molecular changes important in sarcoma transformation and metastasis. Our collaborator, Dr. Samuel Singer, has been isolating cell lines from human tumors and has interogated these at the molecular level to define differences in gene expression correlating to tumor type, clinical response, and other parameters. Thus, we have set out to use network and pathway analysis to begin to understand sarcomagenesis and attempt to develop a mouse model which will allow us to assess the significance of such changes in gene expression. Additionally, such a mouse model will allow us to leverage a direct examination of the significance of cytosolic-p27 in a mouse model.
Since cyclins are the limiting partners of the cyclin-CDK holoenzyme complex, amplification of cdk4 may serve to titrate Ink4 type inhibitors. However, biochemical fractionation indicated that there was sufficiently more cdk4 than was necessary to accomplish this, and we were able to purify a host of novel cdk4 complexes and interacting proteins. Thus, we have begun cellular experiments to assess the significance of such cdk4 containing complexes in the behavior of sarcoma cell lines.