Bcl-xL deamidation in oncogenic tyrosine kinase signalling

I have been interested in the molecular mechanisms of Haematopoietic malignant diseases such as leukaemia and lymphoma, especially those involving oncogenic tyrosine kinases. About 30 of the 90 tyrosine kinases in the human genome have been implicated in cancer (Blume-Jensen P, 2001). The oncogenic tyrosine kinases (OTKs), such as Bcr-Abl (product of chromosomal translocations of two genes bcr and abl) in Chronic Myelogenous Leukaemia, and Erythroblastic leukaemia viral oncogene homolog 2(Erb-B2) in mammary and other cancers, mediate their transforming effects via a diverse array of signalling pathways involved in DNA damage, cell survival and cell cycle regulation (Deutsch E, 2001; Skorski T, 2002; Kumar R, 1996). My work has been centred around the analysis of a mouse cancer model that is driven by an oncogenic tyrosine kinase – p56 Lck-F505 expressed on CD45 knock- out background (Baker M, 2000). The investigation of this mouse model has revealed that oncogenic inhibition of deamidation of the Bcl-xL survival protein plays a critical role in protecting thymocytes from DNA-damage induced apoptosis. Cells that would normally be eliminated due to accumulating DNA damage are instead preserved with an increasing load of double-stranded breaks, leading to genomic instability, chromosomal abnormalities and transformation. This work was published in Cancer Cell (An oncogenic tyrosine kinase inhibits DNA repair and DNA-damage-induced BclxL deamidation in T cell transformation. Zhao R, 2004). Following that I have tried to elucidate the different roles of the two deamidated species of Bcl-xL in apoptosis, and also the molecular mechanisms of DNA damage- induced Bcl-xL deamidation in order to understand the inhibition of Bcl-xL deamidation by oncogenic tyrosine kinases. Recently I have shown that Bcl-xL deamidation, whereby two critical Asn residues are converted to iso-Asp, cripples the ability of the protein to sequester pro-apoptotic BH3-only proteins such as Bim and p53- upregulated modulator of apoptosis (PUMA), thereby explaining its loss of pro-survival functionality. In vivo, DNA damage causes intracellular alkalinisation that is both necessary and sufficient to deamidate Bcl-xL, promoting apoptosis: no enzyme is necessary for this process. In pre-tumourigenic thymocytes alkalinisation is blocked, so preserving Bcl-xL in its pro-survival mode. Furthermore murine tumours are protected from genotoxic attack by native Bcl-xL, but enforced alkalinisation and consequent Bcl-xL deamidation promotes apoptosis. This part of work was published in Plos Biology (DNA damage-induced Bcl-xL deamidation is mediated by NHE-1 antiport regulated intracellular pH. Zhao R, 2007). Through collaboration with Prof AR Green’s research group at the Department of Haematology of the University of Cambridge, I have also analysed the Bcl-xL deamidation pathway in human myeloproliferative disorders, e.g. Polycythemia vera(PV) and Chronic Myelogenous Leukaemia (CML). We found that the oncogenic tyrosine kinases involved in these disorders, i.e. Jak2V617F and Bcr-Abl also inhibit the Bcl-xL deamidation pathway in DNA damage responses. These findings shed light on potential therapeutic application of the Bcl-xL deamidation pathway in human malignancies. This piece of work was recently published in the New England Journal of Medicine (Inhibition of the Bcl-xL deamidation pathway in myeloproliferative disorders. Zhao R, 2008). Overall the cited work has led to several important new insights into the molecular mechanisms involved in oncogenesis: first, that Bcl-xL deamidation is important in the cascade of events leading from DNA damage to apoptosis; second, that oncogenic tyrosine kinases inhibit these events in both the murine and human context; third, that up-regulation of the NHE-1 antiport and consequent intracellular alkalinisation are critical events in this DNA damage-induced cascade leading to apoptosis. In the process I have demonstrated the first in vivo mechanism for the deamidation of an internal protein Asn. Essentially, a completely new and unexpected signalling pathway has been uncovered that seems to pertain to all murine and human haematopoietic cell lineages that have been investigated so far.