SUMMARY A partial cDNA for Arsenic resistance protein 2 (Ars2) was originally identified in a screen for genes that conferred arsenic resistance. Here we show that Ars2 is a component of the nuclear RNA cap binding complex (CBC) and is critical for proliferation. Unlike other components of the CBC, Ars2 expression is linked to the proliferative state of the cell. Deletion of Ars2 causes developmental lethality. In adult mice, deletion of Ars2 led to bone marrow failure, while parenchymal organs composed of non-proliferating cells were unaffected. Depletion of Ars2 or CBP80 from proliferating cells impairs miRNA-mediated repression. Ars2 functions in miRNA biogenesis at the level of nuclear miRNA processing. Depletion of Ars2 protein led to alterations in primary miRNA processing and reduced levels of several miRNAs implicated in cellular transformation, including miR-21, let-7, and miR-155. These findings provide evidence for a role for Ars2 in RNA interference regulation during cell proliferation.

Purpose To evaluate the clinical activity of sequential therapy with sorafenib and sunitinib in FLT3-ITD-positive AML and monitor the emergence of secondary FLT3 tyrosine kinase domain (TKD) mutations during treatment. Experimental Design Six children with relapsed/refractory AML were treated with sorafenib in combination with clofarabine and cytarabine, followed by single-agent sorafenib if not a candidate for transplantation. Sunitinib was initiated after sorafenib relapse. Bone marrow samples were obtained for assessment of FLT3 TKD mutations by deep amplicon sequencing. The phase of secondary mutations with ITD alleles was assessed by cloning and sequencing of FLT3 exons 14 through 20. Identified mutations were modeled in Ba/F3 cells and the effect of kinase inhibitors on FLT3 signaling and cell viability was assessed. Results Four patients achieved complete remission, but 3 receiving maintenance therapy with sorafenib relapsed after 14–37 weeks. Sunitinib reduced circulating blasts in 2 patients and marrow blasts in 1. Two patients did not respond to sorafenib combination therapy or sunitinib. FLT3 mutations at residues D835 and F691 were observed in sorafenib resistance samples on both ITD-positive and –negative alleles. Deep sequencing revealed low-level mutations and their evolution during sorafenib treatment. Sunitinib suppressed leukemic clones with D835H and F691L mutations, but not D835Y. Cells expressing sorafenib-resistant FLT3 mutations were sensitive to sunitinib in vitro. Conclusions Sunitinib has activity in patients that are resistant to sorafenib and harbor secondary FLT3 TKD mutations. The use of sensitive methods to monitor FLT3 mutations during therapy may allow individualized treatment with the currently available kinase inhibitors.

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