Complementary activities of DOT1L and Menin inhibitors in MLL-rearranged leukemia

Chromosomal rearrangements of the mixed lineage leukemia (MLL/KMT2A) gene leading to oncogenic MLL-fusion proteins occur in ~10% of acute leukemias and are associated with poor clinical outcomes, emphasizing the need for new treatment modalities. Inhibition of the DOT1-like histone H3K79 methyltransferase (DOT1L) is a specific therapeutic approach for such leukemias that is currently being tested in clinical trials. However, in most MLL-rearranged leukemia models responses to DOT1L inhibitors are limited. Here, we performed deep-coverage short hairpin RNA sensitizer screens in DOT1L inhibitor-treated MLL-rearranged leukemia cell lines and discovered that targeting additional nodes of MLL complexes concomitantly with DOT1L inhibition bears great potential for superior therapeutic results. Most notably, combination of a DOT1L inhibitor with an inhibitor of the MLL-Menin interaction markedly enhanced induction of differentiation and cell killing in various MLL disease models including primary leukemia cells, while sparing normal hematopoiesis and leukemias without MLL rearrangements. Gene expression analysis on human and murine leukemic cells revealed that target genes of MLL-fusion proteins and MYC were suppressed more profoundly upon combination treatment. Our findings provide a strong rationale for a novel targeted combination therapy that is expected to improve therapeutic outcomes in patients with MLL-rearranged leukemia.

Short communication: altered synaptic clustering of GABAA receptors in mice lacking dystrophin (mdx mice)

Dystrophin is selectively localized in the postsynaptic density of neurons in cerebral cortex, hippocampus and cerebellum. Here, we show by double-immunofluorescence staining that dystrophin is extensively colocalized with GABAA receptor subunit clusters in these brain regions. To determine the relevance of this observation, we investigated in mdx mice, which provide a model of Duchenne muscular dystrophy, whether the absence of dystrophin affects the synaptic clustering of GABAA receptors. A marked reduction in the number of clusters immunoreactive for the alpha1 and alpha2 subunits was observed in, respectively, cerebellum and hippocampus of mdx mice, but not in striatum, which is normally devoid of dystrophin. Furthermore, these alterations were not accompanied by a change in gephyrin staining, although gephyrin is colocalized with the majority of GABAA receptor clusters in these regions. These results indicate that dystrophin may play an important role in the clustering or stabilization of GABAA receptors in a subset of central inhibitory synapses. These deficits may underlie the cognitive impairment seen in Duchenne patients.