MBNL1 As a New Therapeutic Target in MLL-Fusion Gene Leukemia
Authors: Itskovich Svetlana S , Clark Jason , Mulloy James C. , Disney Matthew D , Kumar Ashish R 
Published: October 2019

Abstract
Translocations of the Mixed Lineage Leukemia (MLL) gene located on chromosome 11 are commonly found in infants with AML or ALL and in secondary leukemia at all ages. A majority of patients with these translocations have a poor prognosis. Gene expression profiling studies demonstrate that one of the most consistently overexpressed genes in these leukemias (compared to all other leukemias) is muscleblind-like 1 (MBNL1). Further, MBNL1 was also identified as a direct transcriptional target of MLL-fusion proteins. An RNA-binding protein, MBNL1 is known to be a key factor in the pathophysiology of Myotonic Dystrophy Type I (DM), where sequestration of MBNL1 leads to splicing defects in muscle and neuronal cells. However, the role of MBNL1 in hematopoiesis and leukemogenesis is unknown. To determine the role of MBNL1 in normal hematopoiesis we studied MBNL1-/- mice. Compared to littermate controls, MBNL1-/- mice showed no differences in peripheral blood counts or bone marrow cellularity. When challenged with 5-FU, both MBNL1-/- and wild type mice displayed similar kinetics of peripheral blood cytopenia and recovery. Next we examined the role of MBNL1 in hematopoietic stem cell function using a competitive transplantation assay. Lethally irradiated mice were transplanted with a 1:1 mix of CD45.1 and CD45.2 bone marrow, with the latter being wild-type or MBNL1-/-. Flow cytometry analysis of peripheral blood at 4 weeks post-transplant showed donor chimerism being 53±4.14% in recipients of wild type marrow and 25±5.41 % in the MBNL1-/- recipients. Successive analyses every 4 weeks showed the chimerism to be stable over the next 16 weeks. To determine the role of MBNL1 in leukemia, we transformed MBNL1-/- or wild type bone marrow cells with various oncogenes delivered via retroviral transduction and compared them in methylcellulose colony replating assays. Absence of MBNL1 significantly reduced colony formation in MLL-AF9 and E2A-HLF transformed cells by 59.5% (± 27.1) and 50.7% (± 23) respectively, compared to controls. To assess the role of MBNL1 in leukemia in vivo, we transplanted MLL-AF9-transformed wild type or MBNL1-/- cells into irradiated mice. All recipients injected with wild-type MLL-AF9-transformed cells succumbed to leukemia with a median time of 106 days. In contrast, the majority of recipients of MBNL1-/- cells survived leukemia-free for at least 140 days post-transplantation (p=0.0017, log rank test). We next assessed the role of MBNL1 in human leukemia cells. Lentiviral-shRNA knockdown of MBNL1 in leukemia cell lines (MV4;11, THP-1) significantly inhibited cell growth, both in liquid culture and methylcellulose colony forming assays. To determine the requirement of MBNL1 for leukemia propagation in vivo, we used cord blood-derived leukemia cells bearing the MLL-AF9 fusion gene and mutant NRAS (MA9NRAS). MA9NRAS cells transduced with MBNL1-specific or control (non-targeting, NT) shRNA were transplanted into immunodeficient mice. Six weeks after transplant, bone marrow aspirates showed persistence of lentiviral-transduced cells in 85% of the NT-group. On the other hand, MBNL1-shRNA transduced cells were not detected in any of the recipient mice. These results suggest that MBNL1 is essential for leukemia cell propagation in vivo. Finally, we tested therapeutic targeting of MBNL1 in MLL-fusion gene leukemia. A lead inhibitor that prevents binding of MBNL1 to its targets was recently identified. Treatment of MA9NRAS cells with the inhibitor for 48 hours led to significant apoptosis whereas normal cord blood CD34+ cells were relatively less sensitive. Blockade of MBNL1 in leukemia cells either by shRNA-knockdown or by the inhibitor showed identical changes in splicing patterns of known MBNL1 target genes. Collectively, our data suggest that MBNL1 is required for the initiation and propagation of MLL-fusion gene leukemia while it appears relatively dispensable for normal hematopoiesis. Further, we have identified a promising lead inhibitor that could be developed for novel treatments for therapy-resistant leukemias.

Disclosures
No relevant conflicts of interest to declare.