Expression of Mir-196b Is Not Exclusively MLL-Driven but Especially Linked to Activation of HOXA Genes in Children with Acute Lymphoblastic Leukemia.

Abstract 580

MicroRNAs (miRNAs) play important roles in diverse biological processes including hematopoiesis. As a consequence, aberrant expression of miRNAs may contribute to hematopoietic malignancies. It has been reported that miR-196b is transcriptionally activated by MLL and MLL-fusion genes and is therefore highly expressed in MLL-rearranged leukemia. In order to investigate whether high expression levels of miR-196b are restricted to MLL-rearranged leukemia cases, we measured the expression in samples of 72 selected pediatric acute lymphoblastic leukemia (ALL) cases i.e. MLL-rearranged and non-MLL-rearranged precursor B-ALL and T-ALL patients. MiR-196b was highly expressed in 9/12 MLL-rearranged precursor B-ALL patients, but also in 14/22 T-ALL patients. In particular, 100% of T-ALL cases carrying CALM-AF10 (n=5), MLL-AF6 (n=2), SET-NUP214 (n=3) and an inversion of chromosome 7 (n=1) showed high expression levels of miR-196b comparable to the high levels found in MLL-rearranged ALL. Like MLL-rearrangements, these genetic abnormalities have been functionally linked with upregulation of HOXA cluster genes. MiR-196b expression levels in these patients were strongly correlated with the expression of HOXA family but not with HOXB and HOXC cluster genes (Rs ≥ 0.7, P≤0.003). Since miR-196b is located between HOXA9 and HOXA10 on chromosome 7, our data suggest co-activation of miR-196b and HOXA family genes in pediatric ALL. In parallel to the high expression level of miR-196b we found decreased methylation at CpG islands located 5' of miR-196b in MLL-rearranged cases compared to normal bone marrow cells, which suggests an epigenetic origin for the high expression level of miR-196b in these patients. Despite the fact that MLL-rearranged ALL patients often respond poorly to prednisolone and L-asparaginase, upregulation of miR-196b was not indicative for the resistance to these drugs in pediatric ALL. In conclusion, high-level expression of miR-196b is not only MLL-driven, but can also be found in other types of leukemia that display aberrant activation of HOXA genes.