Differential Expression of BIRC Family Genes In The Course Of Chronic Myeloid Leukemia – BIRC3 and BIRC8 As Potential New Candidates To Identify Disease Progression

Although majority of chronic myeloid leukemia (CML) patients benefit from targeted therapy, there is an unmet need to identify as early as possible patients who develop resistance to tyrosine kinase inhibitors (TKI) and/or patients who progress to blastic phase (CML-BP). Searching for potential candidates of disease progression we have focused on BIRC (baculoviral IAP repeat-containing) genes expression in various stages of CML. This family includes eight functionally- and structurally-related proteins, most of them are believed to serve as endogenous inhibitors of apoptosis. Overexpression of various BIRC genes has been associated with cancer progression, multidrug resistance, poor prognosis and short survival in several types of neoplasms including hematological malignancies. In CML so far expression of BIRC5 (encoding survivin) and BIRC4 (encoding XIAP) has been associated with progression of the disease. However, there is no data on the role of other BIRC members in myeloproliferative diseases.

To study the expression of BIRC genes in CML we employed RT-qPCR following MIQE guidelines. We analyzed sequential samples of peripheral blood leukocytes obtained from CML patients at various stages of the disease. Initially we looked at the samples from patients in chronic phase (CML-CP): at the diagnosis and after development of TKI resistance (confirmed as a loss of cytogenetic response, n=5). Then we analyzed samples from patients who progressed either to accelerated phase (CML-AccPh) or to blastic phase (CML-BP) (n=6). Among eight BIRC genes we observed significant decrease in BIRC3 (encoding cIAP2) and BIRC8 (encoding ILP2) expression. This was associated both with TKI resistance and with progression of CML to accelerated or blastic phase. Simultaneously, we observed marked increase in BIRC5 expression in samples from CML-AccPh and BP as compared to chronic phase (as was previously shown by others) but we observed no significant difference in BIRC5 expression between CML-CP diagnostic and TKI-resistant samples. Expression of BIRC1 (NAIP), BIRC2 (cIAP1), BIRC4 (XIAP), BIRC6 (BRUCE) and BIRC7 (LIVIN) was comparable in sequential samples from CML-CP and CML-BC and was not related to TKI-resistance.

We verified these results by comparing larger group of patients in either CML-CP at the diagnosis prior to any treatment (n=15) or CML-BP (n=11). To compare the expression of BIRC family in CML to normal hematopoietic cells, we included also cDNA from healthy blood donors (n=10). In accordance with paired samples analysis, we observed downregulation of BIRC3 and BIRC8 expression in CML-BP (as compared to CML-CP and healthy blood donors), while BIRC5 was upregulated in CML-BP patients (as compared to CML-CP and healthy blood donors). There was no difference in expression of other BIRC family members.

In conclusion, this is the first comprehensive analysis of the expression of all eight BIRC genes in the course of CML. In addition to the previously described upregulation of BIRC5, we observed significant downregulation of BIRC3 and BIRC8 associated with TKI-resistance and also with progression to accelerated or blastic phase. Recently Rossi D. et al. (Blood 2012;119: 2854-62) described potential role of disruption of BIRC3 (through mutation or gene deletion) in resistance to fludarabine in chronic lymphocytic leukemia. Together with our results this shows that current view linking BIRC genes upregulation with tumor progression and drug resistance may not be true for all of BIRC genes. Our results suggest novel and unexpected role of BIRC3 and BIRC8 in the clonal evolution of CML and open a new area for further exploration of the role of BIRC in CML progression.