We propose a new mechanism for TCL1 (T cell leukemia-1) induction of lymphoid cancers. The assumption has been that aberrant TCL1 expression promotes inappropriate AKT (protein kinase-B) activation to cause transformation. However, the extent of TCL1-augmented AKT activation is comparable to the degree of enhanced AKT activation induced by targeted deletion of the PTEN tumor suppressor in B cells. Most notably, our TCL1 transgenic mice rapidly develop mature B cell malignancies while conditional PTEN-null mice never form B cell cancers. These observations lead us to hypothesize that TCL1-mediated AKT augmentation has, at most, a minor tumorigenic effect. We cannot, however, exclude the possibility that TCL1 alters the target specificity of AKT and we address this issue briefly in this proposal. Previously, we showed that a cytoplasmic membrane factor augmented TCL1/AKT interactions. We subsequently searched for new TCL1-interacting partners in flag-tagged TCL1 co-IP reactions. Surprisingly, we identified the RNA degrading enzyme polynucleotide phosphorylase (PNPase) as the sole TCL1 binding partner in lymphoid cells. We will determine whether or not PNPase is the TCL1/AKT interaction-augmenting factor. We favor the idea that PNPase/TCL1 interactions mediate TCL1-induced tumorigenesis through TCL1 stabilization of PNPase target mRNAs rather than by effecting AKT/TCL1 interactions. Testing this hypothesis is the main aim of this proposal. We provide preliminary evidence that TCL1/PNPase interactions block the processing of specific mRNAs. As such, TCL1 may be a natural PNPase ligand that regulates developmental gene expression by altering mRNA turnover by PNPase. When dysregulated, we predict TCL1 inappropriately stabilizes specific mRNAs destined for degradation, providing a novel mechanism of aberrant gene expression resulting in transformation. The most compelling candidate target mRNA is AID, since persistent AID hypermutation could account for the broad spectrum of germinal center-derived B cell cancers in our TCL1 transgenic mouse model.
The specific aims i n this proposal form an investigation of the mechanism of TCL1-mediated transformation, focusing on functional consequences of interactions between TCL1, PNPase and to a lesser extent AKT in vitro and in vivo.
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