Identification of Rab5 as a Gene Involved in Chronic Myeloid Leukemia (CML) Progression

The role of Bcr-Abl in the pathogenesis of Chronic Myeloid Leukemia (CML) is well established, however, the mechanisms leading to CML progression remain poorly understood. By using our model of transgenic Drosophila Melanogaster (Dm) for human Bcr-Abl driven CML we have identified Rab5 as a gene involved in the regulation of CML progression. The Rab5 is a member of gene family small GTPases which are involved in the regulation of vesicular transport. Lately several important reports have linked some members of the Rab family to invesivness and migration of cancer cells. Rab5 is associate with alpha-integrin subunits and modulates their endosomal traffic and subcellular localization. We have observed that a loss of function of Rab5 gene have induced a worsening of the CML phenotype generated by hBcr-Abl expression. In contrast, Rab gain of function rescued Bcr-Abl phenotype. The aim of the study was to evaluate the expression of Rab5 in CML cells to better understand if a potential correlation with progression, which has been observed in the model, could be confirmed in patients. Methods: Rab5 gene expression was measured by Real Time PCR in 90 samples from 80 CML patients (32 PB and 58 BM). Among those, 53 are collected at diagnosis (19 of 53 patients have been enrolled in TOPS study). In addition, 9 samples from in CP patients have been collected at the time of imatinib resistance, 7 in accelerated phase and 11 in BC. In 14 patients, genes expression was analyzed during remission as, well. In parallel, 21 healthy donors (10 PB and 11 BM) have been evaluated. Rab5 protein expression was investigated by Western Blot and Immunofluorescence. We have also utilized K562 transfected with Rab5 plasmid, which we have generated to gain insight about the effects of Rab5 on cell proliferation and apoptosis. Results: Rab5 transfection and overexpression in K562 significantly reduced proliferation and affected apoptosis. We found that in CML patients Rab5 expression levels were significantly decreased in either BM or PB (p<0.001 and p<0.0001) as compared to healthy subjects. Furthermore, in blast crisis samples we have found Rab5 transcripts levels to be further decreased. In contrast, at the time of remission, the transcript levels were comparable to normal values. Our preliminary analysis of samples from TOPS trial have shown a trend that Rab5 levels are lower among those patients achieving MMR by 12 months, when compared to the group of patients non achieving MMR on 400 mg, but that difference was not statistically significant (p=0.2). Among those randomized to receive imatinib 800 mg the difference was statistically significant with a median value among those achieving MMR of 1.27 vs 2.14 in the group without MMR (p=0.04). The protein levels have been analyzed by Western Blot and immunofluorescence and allow us to show detectable levels of Rab5 in samples collected at remission, but undetectable levels in course of active CML disease. Although preliminary, our results show a significant decrease of Rab5 expression in blast crisis samples, when compared to CP CML and healthy volunteers, which suggest a role of Rab5 in slowing down or suppressing a progression. Surprisingly, among CP CML patients the responders to TKI therapy have been detected to express a lower level of Rab5 than non responders. We are conducting further studies to better explain these data, which we find intriguing and suggesting that molecular factors involved in the regulation of CML progression could be uncoupled from the mechanisms regulating response to TKI therapy. Supported by Novartis Oncology, Clinical Development, TOPS Clinical Correlative Studies Network