Gilteritinib is a potent and selective FLT3 kinase inhibitor with single-agent clinical effi cacy in relapsed/refractory FLT3 -mutated acute myeloid leukemia (AML). In this context, however, gilteritinib is not curative, and response duration is limited by the development of secondary resistance. To evaluate resistance mechanisms, we analyzed baseline and progression samples from patients treated on clinical trials of gilteritinib. Targeted next-generation sequencing at the time of AML progression on gilteritinib identifi ed treatment-emergent mutations that activate RAS/ MAPK pathway signaling, most commonly in NRAS or KRAS. Less frequently, secondary FLT3 -F691L gatekeeper mutations or BCR-ABL1 fusions were identifi ed at progression. Single-cell targeted DNA sequencing revealed diverse patterns of clonal selection and evolution in response to FLT3 inhibition, including the emergence of RAS mutations in FLT3 -mutated subclones, the expansion of alternative wild-type FLT3 subclones, or both patterns simultaneously. These data illustrate dynamic and complex changes in clonal architecture underlying response and resistance to mutation-selective tyrosine kinase inhibitor therapy in AML. SIGNIFICANCE:Comprehensive serial genotyping of AML specimens from patients treated with the selective FLT3 inhibitor gilteritinib demonstrates that complex, heterogeneous patterns of clonal selection and evolution mediate clinical resistance to tyrosine kinase inhibition in FLT3 -mutated AML. Our data support the development of combinatorial targeted therapeutic approaches for advanced AML.

SUMMARY The formation of epithelial tissues containing lumens requires not only the apical-basolateral polarization of cells, but also the coordinated orientation of this polarity such that the apical surfaces of neighboring cells all point toward the central lumen. Defects in extracellular matrix (ECM) signaling lead to inverted polarity so that the apical surfaces face the surrounding ECM. We report a molecular switch mechanism controlling polarity orientation. ECM signals through a β1-integrin/FAK/p190RhoGAP complex to down-regulate a RhoA/ROCK/Ezrin pathway at the ECM interface. PKCβII phosphorylates the apical identity-promoting Podocalyxin/NHERF1/Ezrin complex, removing Podocalyxin from the ECM-abutting cell surface and initiating its transcytosis to an apical membrane initiation site for lumen formation. Inhibition of this switch mechanism results in the retention of Podocalyxin at the ECM interface and the development instead of collective front-rear polarization and motility. Thus, ECM-derived signals control the morphogenesis of epithelial tissues by controlling the collective orientation of epithelial polarization.

Inositol phospholipids have been implicated in almost all aspects of cellular physiology including spatiotemporal regulation of cellular signaling, acquisition of cellular polarity, specification of membrane identity, cytoskeletal dynamics, and regulation of cellular adhesion, motility, and cytokinesis. In this review, we examine the critical role phosphoinositides play in these processes to execute the establishment and maintenance of cellular architecture. Epithelial tissues perform essential barrier and transport functions in almost all major organs. Key to their development and function is the establishment of epithelial cell polarity. We place a special emphasis on highlighting recent studies demonstrating phosphoinositide regulation of epithelial cell polarity and how individual cells use phosphoinositides to further organize into epithelial tissues.P hosphoinositides (PIs) are essential components of cellular membranes in eukaryotes. Though these specialized lipids comprise less than 1% of the cellular lipid cohort, they play key roles in many fundamental biological processes (Di Paolo and De Camilli 2006;Saarikangas et al. 2010). PIs possess such far ranging roles by serving as specialized membrane docking sites for effectors of numerous cellular signal transduction cascades. PIs also serve as precursors of lipid second messengers. They are concentrated on the cytosolic face of cellular membranes (Fig. 1A) and rapidly diffuse within the plane of the membrane. Reversible phosphorylation of the myo-inositol head group of phosphatidylinositol (PtdIns) at positions 3, 4, and 5 (Fig. 1B) gives rise to the seven PI isoforms identified in eukaryotic cells. PtdIns(4)P and PtdIns(4,5)P 2 are constitutively present in membranes and comprise the largest pool of cellular PIs, whereas PtdIns(3,4,5)P 3 is essentially undetectable in most types of unstimulated cells (Lemmon and Ferguson 2000;Saarikangas et al. 2010).The spatiotemporally regulated production and turnover of phosphoinositides is crucial for localized PI signaling and function. Numerous phosphatidylinositol kinases and phosphatases are involved in regulating the metabolism of the various PI isoforms (Fig. 1)

To enable the characterization of genetic heterogeneity in tumor cell populations, we developed a novel microfluidic approach that barcodes amplified genomic DNA from thousands of individual cancer cells confined to droplets. The barcodes are then used to reassemble the genetic profiles of cells from next-generation sequencing data. By using this approach, we sequenced longitudinally collected acute myeloid leukemia (AML) tumor populations from two patients and genotyped up to 62 disease relevant loci across more than 16,000 individual cells. Targeted single-cell sequencing was able to sensitively identify cells harboring pathogenic mutations during complete remission and uncovered complex clonal evolution within AML tumors that was not observable with bulk sequencing. We anticipate that this approach will make feasible the routine analysis of AML heterogeneity, leading to improved stratification and therapy selection for the disease.

The behaviors of complex biological systems are often dictated by the properties of their heterogeneous and sometimes rare cellular constituents. Correspondingly, the analysis of individual cells from a heterogeneous population can reveal information not obtainable by ensemble measurements. Reverse-transcriptase polymerase chain reaction (RT-PCR) is a widely used method that enables transcriptional profiling and sequencing analysis on bulk populations of cells. Major barriers to successfully implementing this technique for mammalian single-cell studies are the labor, cost, and low-throughput associated with current approaches. In this report, we describe a novel droplet-based microfluidic system for performing ~50000 single-cell RT-PCR reactions in a single experiment while consuming a minimal amount of reagent. Using cell type-specific staining and TaqMan RT-PCR probes, we demonstrate the identification of specific cells from a mixed human cell population. The throughput, robust detection rate and specificity of this method makes it well-suited for characterizing large, heterogeneous populations of cells at the transcriptional level.

Although in most plant species no more than two annexin genes have been reported to date, seven annexin homologs have been identified in Arabidopsis, Annexin Arabidopsis 1–7 (AnnAt1–AnnAt7). This establishes that annexins can be a diverse, multigene protein family in a single plant species. Here we compare and analyze these seven annexin gene sequences and present the in situ RNA localization patterns of two of these genes, AnnAt1 andAnnAt2, during different stages of Arabidopsis development. Sequence analysis of AnnAt1–AnnAt7 reveals that they contain the characteristic four structural repeats including the more highly conserved 17-amino acid endonexin fold region found in vertebrate annexins. Alignment comparisons show that there are differences within the repeat regions that may have functional importance. To assess the relative level of expression in various tissues, reverse transcription-PCR was carried out using gene-specific primers for each of the Arabidopsis annexin genes. In addition, northern blot analysis using gene-specific probes indicates differences in AnnAt1 and AnnAt2expression levels in different tissues. AnnAt1 is expressed in all tissues examined and is most abundant in stems, whereas AnnAt2 is expressed mainly in root tissue and to a lesser extent in stems and flowers. In situ RNA localization demonstrates that these two annexin genes display developmentally regulated tissue-specific and cell-specific expression patterns. These patterns are both distinct and overlapping. The developmental expression patterns for both annexins provide further support for the hypothesis that annexins are involved in the Golgi-mediated secretion of polysaccharides.

Intersectin 2 localizes to centrosomes, where it regulates Cdc42 and helps to orient the apical surface correctly during cyst formation. (See also companion paper from Qin et al., in this issue.)

In multicellular epithelial tissues, the orientation of polarity of each cell must be coordinated. Previously, we reported that for Madin-Darby canine kidney cells in three-dimensional collagen gel culture, blockade of b1-integrin by the AIIB2 antibody or expression of dominant-negative Rac1N17 led to an inversion of polarity, such that the apical surfaces of the cells were misorientated towards the extracellular matrix. Here, we show that this process results from the activation of RhoA. Knockdown of RhoA by short hairpin RNA reverses the inverted orientation of polarity, resulting in normal cysts. Inhibition of RhoA downstream effectors, Rho kinase (ROCK I) and myosin II, has similar effects. We conclude that the RhoA-ROCK I-myosin II pathway controls the inversion of orientation of epithelial polarity caused by AIIB2 or Rac1N17. These results might be relevant to the hyperactivation of RhoA and disruption of normal polarity frequently observed in human epithelial cancers. Keywords: b1-integrin; epithelial polarity; myosin II; RhoA; ROCK I EMBO reports (2008) 9, 923-929.