MicroRNAs as the Target of Deletions on der(9) in Chronic Myeloid Leukemia.

Deletions on der(9) are associated with chronic myeloid leukemia(CML) in 15–18% of cases. To date, the biological significance of this genomic loss in the pathogenesis of CML is unknown. The most plausible hypothesis is that the loss of a tumor suppressor gene may confer a proliferative advantage to the Philadelphia-positive clone. On the other hand, it has now become evident that microRNAs (miRNAs) play an important regulatory role in some hematological malignancies. To investigate the presence of miRNAs within the genomic regions lost on der(9) we analyzed 60 CML patients with der(9) deletions. Methods. Genomic characterization of the deleted sequences was performed by fluorescence in situ hybridization (FISH) using a contig of DNA clones; the miRBase (http://microrna.sanger.ac.uk/) was queried to assess the presence of miRNAs in the der(9) deleted genomic regions. FISH experiments showed that the genomic loss on der(9) of the 9 (centromeric to ABL) and 22 (telomeric to BCR) chromosome sequences ranged from 260 Kb to 54 Mb and from 230 Kb to 12.9 Mb, respectively. Consultation of the miRBase revealed that in 16 (27%) patients there was loss of miRNAs mapping on chromosome 9 whereas no known miRNAs were mapped on the deleted genomic sequences belonging to chromosome 22. Moreover, 4 cases with a complex t(9;22) rearrangement and der(9) deletions showed loss of the miRNAs sequence also on the third derivative chromosome (4p16, 7p14, 13q14, and 11q13, respectively); among them, only in one case the loss of miRNAs on the third derivative was not associated with the miRNAs deletion mapped on chromosome 9. The most recurrent miRNAs deleted on der(9) were mir-219–2 (deleted in 100% of cases) and mir-199-b (lost in 67% of cases). It is noteworthy that mir-219–2 neighbors and overlaps CpG-islands, suggesting a potential role of this miRNA in CpG-island methylation. Experimental studies indicate that miRNAs can function as tumor suppressor genes or as oncogenes. In fact, in chronic lymphocytic leukemia associated with del(13)(q14) it has been demonstrated that the miRNAs loss can induce downregulation of the antiapoptotic BCL-2 protein. The novel evidence that deletions on der(9) in CML are associated with miRNAs loss may shed new light on the significance of genomic sequences loss. Further studies are needed since it is known that some microRNAs may have as many as a few thousand targets, so prediction algorithms and strategies allowing large-scale screening of multiple target genes are required.