MiR-125b Cooperates with BCR-ABL and Enhances Therapy Resistance in Childhood Ph+ ALL

Abstract 78

The treatment improvements in the past two decades led childhood acute lymphoblastic leukemia (ALL) cure rates reach over 80%, but children who carry the Philadelphia (Ph+) chromosome still have high risk of relapse due to therapy resistance. Different results with large series of patients have shown that an earlier remission after induction with glucocorticoids and intrathecal methotrexate is correlated to a better outcome. Besides, the minimal residual disease (MRD) risk measured after induction is also correlated to the therapy response, indicating that an early response can predict Ph+ ALL overall outcome. Several studies have shown that Ph+ ALL is heterogeneous in terms of clinical parameters even if all patients carry BCR-ABL and most of them carry the oncogenic isoform of IKAROS. They respond differently to the treatment, which suggests the presence of additional mechanisms involved in leukemogenesis. In an attempt to find secondary genetic abnormalities that may be responsible for the Ph+ ALL chemo resistance and heterogeneity, we studied the miRNA expression profile in two patient groups discriminated by the initial therapy response.

We included samples from 78 consecutive Ph+ ALL children diagnosed in Italy between 2000 and 2010. The miRNA signature was analyzed by miRNA array using Applied Biosystems Array-Cards comparing two patient groups differentiated according to MRD and prednisone response. A particular miRNA profile was found in the poor responder group and it was confirmed using specific miRNA single assays from AB. Specially the miR-125b was up to ten-fold overexpressed compared to the good responder group (p =.006). To investigate the functional role of miR-125b in Ph+ ALL we used BCR-ABL positive cell lines (3 ALL and one CML in blast-crisis) to create a xenograft leukemia model and induced miRNA upregulation by direct inoculation of synthetic miR-125b (premiR). Tumors injected with premiR-125b (n=14) were significantly bigger than the scrambled oligonucleotide (n=10) and mock controls (n=4) (p=.04). After two weeks the premiR-125b tumors grew six-fold more than controls and average tumor weights for the scrambled oligonucleotides and the premiR-125b inoculated mice were 0.63 g and 1.56 g, respectively (p =.01). Further, we determined whether miR-125b protects the tumor cells from apoptosis after treatment with Dexamethasone. While tumors injected with scrambled molecules were 70% reduced after 10 days of corticoid treatment, tumors injected with premiR-125b were only 30% reduced (p=.05).

Together, these results suggest that miR-125b acts as an oncogene in Ph+ ALL and its overexpression accelerates the role of BCR-ABL, increasing therapy resistance and leukemia aggressiveness. Additional studies will be necessary to understand why this miRNA is overexpressed in some patients and how it acts in cooperation with BCR-ABL to induce leukemia. In the future, novel therapies using miRNAs as targets can emerge as strategies to be added to the anti-tyrosine kinase drugs in order to improve treatment response and survivor in Ph+ ALL.