UC San Diego

Cancer arises from genetic alterations consisting of gene mutations, gene over-activation, or gene loss of function. Chromosomal translocations encoding functional oncogenic proteins have been identified as mutations and drivers of multiple cancer types. Specifically, fibroblast growth factor receptors (FGFRs), members of the receptor tyrosine kinase (RTK) family, have been found as recurrent translocation partners in both solid and hematologic malignancies. Constitutively activated FGFR1 fusion proteins give rise to 8p11 myeloproliferative syndrome (EMS), also known as stem cell leukemia/lymphoma (SCLL). This work focuses on the t(8;22)(p11;q11) chromosomal translocation, resultant in the Breakpoint Cluster Region (BCR)-FGFR1 fusion protein. Patients diagnosed with SCLL have a poor prognosis and there are few therapies for patients positive for this cancer subtype. As such, this work performs an in depth biochemical and biological characterization of the BCR-FGFR1 fusion protein along with the analysis of novel therapeutic options to treat BCR-FGFR1 driven SCLL. BCR-FGFR1 retains the coiled-coil dimerization domain, serine/threonine kinase domain, and partial RhoGEF domain contributed by BCR, and a tyrosine kinase domain contributed by FGFR1. It is demonstrated that BCR-FGFR1 relies on the tyrosine kinase activity of FGFR1 for cell transformation, cell proliferation and downstream pathway activation. Furthermore, the coiled-coil dimerization domain contributed by BCR is suggested to be stabilized by three electrostatic salt-bridge interactions. Disruption of these three salt-bridges interactions is sufficient to abrogate BCR-FGFR1 driven cell transformation. Hsp90 and PLCγ1 are established as potential therapeutic targets to treat SCLL. BCR-FGFR1 is a client of the Hsp90 chaperone protein and is sensitive to Ganetespib (STA-9090), a potent Hsp90 inhibitor, suggesting that BCR-FGFR1 relies on the Hsp90 complex to evade proteasomal degradation. Furthermore, the importance of the small adapter protein Grb2, and the enzyme PLCγ1 for BCR-FGFR1 mediated oncogenic cell growth, is demonstrated through cell-based assays and quantitative mass spectrometry-based proteomics analyses. PLCγ1 was identified as a potential therapeutic target to treat BCR-FGFR1 mediated SCLL, as the PLCγ1 inhibitor U73122 was able to suppress BCR-FGFR1 mediated cell growth, even in the presence of tyrosine kinase activating mutations. Together, this data unravels the essential roles of Hsp90, Grb2, and PLCγ1 in BCR-FGFR1 mediated oncogenic growth, suggests that Hsp90 and PLCγ1 are novel therapeutic targets for SCLL.