Roger Abounader, MD, PhD

Roger Abounader, MD, PhD

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Roles and targeting of c-Met in brain tumors; c-Met/PTEN interactions in gliomas; PTEN/mutant-p53 interactions in gliomas; microRNAs in brain tumors

Our laboratory conducts basic and translational brain tumor research. The research focuses on understanding how key oncogenic pathways and tumor suppressors interact in brain tumor malignancy and on translating discoveries into experimental therapies.  The following projects are currently ongoing in the laboratory:

Interactions between PTEN and c-Met dependent pathways in gliomas
Activation of oncogenes or inactivation of tumor suppressors can lead to irreversible activation of growth regulatory pathways and tumor formation and growth.  A number of oncogenes are tyrosine kinases which, when activated, trigger downstream phosphorylation cascades that mediate their functions.  Phosphorylation of signal transduction molecules is regulated by phosphatases which can function as tumor suppressors.  The proto-oncogene product and tyrosine kinase receptor c-Met and the tumor suppressor phosphatase PTEN are frequently simultaneously altered in glioblastoma.  This project focuses on studying the interactions and interdependencies of PTEN and c-Met-dependent pathways in these tumors.  We are studying the effects of PTEN on c-Met-dependent cell signaling, gene expression regulation and malignancy.  We are testing the effects of combining PTEN reconstitution and c-Met inhibition on in vivo tumor growth and sensitization to conventional radio- and chemotherapy.  We are also examining how the PTEN status affects emerging clinically applicable anti-c-Met therapies.  Thereby, we will use glioma xenogfrafts established from primary cells as well as from glioma stem cells.  The findings could serve as a paradigm for receptor tyrosine kinase/PTEN interactions in gliomas and provide important information on multi-targeting strategies for glioma therapy.

PTEN and p53 in human gliomas
We found, for the first time, that the well characterized tumor suppressor PTEN has tumor promoting properties in mutant-p53 gliomas.  We hypothesized that PTEN stabilizes gain-of-function p53 mutants and thereby enhances their tumor promoting effects.  Ongoing research investigates the interactions between PTEN, p53 and their mutants in brain tumor malignancy.  We are studying potential correlations between the expression levels of PTEN, p53 and mutants in human brain tumor surgical specimens.  We are analyzing the molecular mechanisms with which PTEN regulates mutant p53 levels and the involvement of Mdm2 and direct physical binding in this process.  Using gain of function/restoration and loss of function approaches, we plan to manipulate co-expression levels of both tumor suppressors and their mutants and study the effects of these combinations on various malignancy endpoints including tumor growth and survival in animal models.  The findings will provide new insights into the interactions of the two most commonly mutated tumor suppressors in gliomas and human cancer in general.  The results could give prognostic value to the combined PTEN/p53 status in gliomas.  Importantly, understanding the combined effects of PTEN and p53 in gliomas could have novel and critical implications on therapies that aim at restoring wild-type tumor suppressor gene expression to gliomas and other human tumors.

Role of c-Met in embryonal CNS tumors
Embryonal CNS tumors which comprise medulloblastoma are the most common brain tumors in children.  Although the implication of c-Met and its ligand HGF in human malignancy is well established, the role of this pathway in embryonal CNS tumors is not known.   This project aims at a systematic characterization of the c-Met pathway in embryonal CNS tumors and its assessment as a potential target for therapy.  We are studying the expression levels of c-Met and its ligand hepatocyte growth factor (HGF) in human tumor specimens and their potential correlation to tumor subtype and clinical outcome.  We are using gain of function and loss of function approaches to study the effects of activation or inhibition of the pathway on various malignancy endpoints including apoptosis, cell cycle regulation, tumor cell migration and invasion and angiogenesis in cell culture as well as in animal models. We plan to dissect the signal transduction pathways and transcriptional events that mediate the malignant effects of c-Met in these tumors.  We will target c-Met expression in vivo using gene knock-down approaches, small molecule inhibitors and monoclonal antibodies also in combination with chemo/radiotherapy and thereby assess the experimental therapeutic value of targeting c-Met pathway in embryonal CNS tumors.  The findings will lead to the first characterization of the c-Met pathway in embryonal CNS tumors and could lead to a timely validation of this pathway as a target for the promising small molecular inhibitors of c-Met and anti-HGF monoclonal antibodies that are expected to enter clinical trials soon. 

microRNAs in brain tumors
microRNAs are small noncoding regulatory RNA molecules, with profound impact on a wide array of biological processes.  microRNAs modulate protein expression by binding to the 3' untranslated region (3'UTR) of mRNA and promoting RNA degradation, inhibiting mRNA translation, and also affecting transcription.  microRNAs are thought to play important roles in cancer by regulating the expression of various oncogenes and tumor suppressors.   Expression profiling identified microRNA-34a (miR-34a) as one of several microRNAs that are downregulated in some cancer cells.   miR-34a expression was shown to be regulated by p53.  We recently found that miR-34a is strongly downregulated in some glioma cells and that its restoration to glioma cells leads to inhibition of tumor cell proliferation, cell survival and cell invasion.  We also found that miR-34a binds to the 3'UTR of the receptor tyrosine kinase c-Met and downregulates its expression in glioma cells.  Therefore, we formulated the hypothesis that miR-34a is a new tumor suppressor in gliomas that is downregulated in mutant p53 tumors and that functions in part through downregulation of the c-Met oncogene.  To test this hypothesis we are measuring the levels of miR-34a in human surgical glioma specimens and primary cells and analyzing their correlation with c-Met protein and p53 mutations in the same samples.  We plan to determine if c-Met mediates the tumor suppressive effects of miR-34a using rescue type experiments.  We will assess the in vivo tumor suppressive effects of miR-34a. The proposed experiments could establish miR-34a as a new tumor suppressor in gliomas.