ALBERT

Description: Background: Acute Lymphoblastic Leukemia (ALL) in infants is characterized by a high incidence (~80%) of chromosomal rearrangements of the Mixed Lineage Leukemia (MLL) gene, fusing the N-terminal portion of MLL to the C-terminal region of one of its translocation partner genes. MLL-rearranged infant ALL patients are challenged by a very poor prognosis (i.e. 30-40% 5-year EFS), hence the need for better risk stratification and improved therapeutic solutions is evident. We recently screened a relatively large cohort (n=109) infant ALL patients (all enrolled in INTERFANT treatment protocols) for the presence of KRAS and NRAS mutations and found that the incidence of such mutations ranges between 14-24%, depending on the type of MLL translocation. Moreover, these mutations were found to represent independent predictors of exceedingly poor prognosis; patients carrying RAS mutations essentially stand no chance in surviving their malignancy, as all RAS mutation-positive MLL-rearranged infant ALL patients deceased within 3 years from diagnosis. Aims: Here we aimed to identify a therapeutic strategy to improve the prognosis of MLL-rearranged infant ALL patients carrying RAS mutations. Methods: For this, 8 small molecule inhibitors against different RAS-pathway components were selected and initially tested for anti-leukemic activity against the MLL-rearranged ALL cell lines SEM and RS4;11 (RASwt) and KOPN8 (RASmut) using MTS cell viability assays. Next, primary MLL-rearranged infant ALL samples (n=20) all carrying MLL translocation t(4;11) (giving rise to the MLL-AF4 fusion protein) either with (n=6) or without (n=14) RAS mutations were exposed to these inhibitors in MTT cytotoxicity assays. In addition, we assessed the RAS activity in RAS mutated and wild-type MLL-rearranged infant ALL cells, and performed immunoblotting analysis of downstream MEK and ERK both in the absence and presence of the MEK inhibitors. Results: We found that the MEK inhibitors MEK162, Selumetinib and Trametinib effectively reduced the viability of KOPN8 cells (RASmut), whereas SEM and RS4;11 cells (RASwt) largely remained unaffected. In line with this, MLL-AF4+ infant ALL patient samples carrying RAS mutations were significantly more sensitive to these MEK inhibitors when compared with patients carrying wild-type RAS genes: LC50 values for MEK162 were 0.04 vs. 26.9 µM (p

Description: BACKGROUND: MLL-rearranged acute lymphoblastic leukemia (MLLr-ALL) in infants (

Description: BACKGROUND: Infant acute lymphoblastic leukaemia (ALL) is a rare but aggressive malignancy, mainly presenting with chromosomal rearrangements of the MLL (Mixed Lineage Leukaemia) gene locus on 11q23. The majority of these MLL rearrangements involve the translocation partners AF4, AF9 or ENL within the translocation events t(4;11)(q21;q23), t(9;11)(p22;q23) and t(11;19)(q23;p13.3), respectively. The resulting fusion genes, MLL-AF4, MLL-AF9 and MLL-ENL, code for chimeric transcription regulators acting as strong oncogenic drivers, rewriting the epigenetic landscape of the cell and profoundly altering gene expression. Consequently, these cytogenetic lesions define an ALL subtype both biologically and clinically distinct from other subtypes, strongly associated with drug resistance to first-line chemotherapeutics, high relapse rates and a dismal prognosis. Hence, novel treatment strategies which specifically target the underlying molecular pathobiology of this disease are urgently needed. AIMS: Previously, our group performed extensive patient cohort profiling on both transcript and epigenetic level in order to understand the molecular events underlying the disease, and identified histone deacetylase inhibitors (HDACi) as effective therapeutic drugs both in silico and in vitro. The aim of the current study was to elucidate potential molecular mechanisms by which the candidate HDACi Panobinostat is able to target MLL-rearranged ALL (MLLr-ALL) cells, and to confirm its efficacy in vivo using pre-clinical MLLr-ALL xenograft mouse models able to recapitulate the disease phenotype observed in humans. METHODS: Immunodeficient NOD.Cg-PrkdcscidIl2rgtm1Wjl/SzJ (NSG) mice were injected intrafemurally with a MLL-AF4+ B-ALL cell line (SEM) genetically modified to express a luciferase reporter. These mice were subsequently either treated with low-dose (1mg/kg) or high-dose (5mg/kg) Panobinostat using a continuous 5-day-on-2-day-off regimen for a period of up to 12 weeks, or they were assigned to a control group and left untreated. Disease onset and progression was monitored using in vivo bioluminescence imaging, and systemic human ALL cell infiltration was determined by multi-colour flow cytometry and histochemistry. In addition, molecular changes induced by Panobinostat exposure in MLLr-ALL and non-MLLr-ALL cell lines were assessed in vitro using immunoblotting and cell death assays. RESULTS: High-dose Panobinostat resulted in a significantly and substantially delayed MLLr-ALL disease onset and progression in NSG mice when compared to controls; this was accompanied by a reduction of the systemic disease burden, as evidenced by significantly lower whole-body luminescence signals and substantially decreased splenomegaly. Furthermore, immunohistochemical and flow cytometric data showed hypocellularity and increased cell death in the BM of xenografted NSG mice treated with Panobinostat when compared to untreated control xenografts. This finding correlated well with in vitro results, where exposure with 5 nM Panobinostat induced cell death in MLLr-ALL cells, but not in non-MLLr ALL cells, as determined by both ANNEXINV/7AAD flow cytometry assays and immunoblotting. In addition, on a molecular level, in vitro exposure with Panobinostat induced histone H3 hyperacetylation in all leukaemic cell lines, but did not affect other histone modification marks investigated such as, i.e., histone H3K4 methylation or histone H3K79 methylation. A notable exception was observed in MLLr-ALL cell lines, where Panobinostat exposure correlated with a reduction in histone H2B ubiquitination, a histone modification recently reported to be pivotal for MLLr leukaemogenesis. Concomitantly, Panobinostat - or more generally - HDACi-mediated loss of H2B ubiquitination might play a role in the observed sensitivity of MLLr-ALL cell towards this drug class. CONCLUSIONS: Both the in vivo and the molecular in vitro results show the HDACi Panobinostat to have promising therapeutic potential against MLLr-ALL. Currently, we are investigating Panobinostat in combination with other epigenetic drugs in xenograft models with primary MLLr-ALL patient material in order to consolidate these observations, and to confirm HDACi as a novel powerful treatment strategy in MLLr-ALL. Disclosures No relevant conflicts of interest to declare.