SRSF2-P95H decreases JAK/STAT signaling in hematopoietic cells and delays myelofibrosis development in mice

Heterozygous mutation targeting proline 95 in Serine/Arginine-rich Splicing Factor 2 (SRSF2) is associated with V617F mutation in Janus Activated Kinase 2 (JAK2) in some myeloproliferative neoplasms (MPNs), most commonly primary myelofibrosis. To explore the interaction of Srsf2^P95H with Jak2^V617F, we generated Cre-inducible knock-in mice expressing these mutants under control of the stem cell leukemia (Scl) gene promoter. In transplantation experiments, Srsf2^P95H unexpectedly delayed myelofibrosis induced by Jak2^V617F and decreased TGFβ1 serum level. Srsf2^P95H reduced the competitiveness of transplanted Jak2^V617F hematopoietic stem cells while preventing their exhaustion. RNA sequencing of sorted megakaryocytes identified an increased number of splicing events when the two mutations were combined. Focusing on JAK/STAT pathway, Jak2 exon 14 skipping was promoted by Srsf2^P95H, an event detected in patients with JAK2^V617F and SRSF2^P95 co-mutation. The skipping event generates a truncated inactive JAK2 protein. Accordingly, Srsf2^P95H delays myelofibrosis induced by the thrombopoietin receptor agonist Romiplostim in Jak2 wild-type animals. These results unveil JAK2 exon 14 skipping promotion as a strategy to reduce JAK/STAT signaling in pathological conditions.

Stepwise GATA1 and SMC3 mutations alter megakaryocyte differentiation in a Down syndrome leukemia model

Acute megakaryoblastic leukemia of Down syndrome (DS-AMKL) is a model of clonal evolution from a preleukemic transient myeloproliferative disorder requiring both a trisomy 21 (T21) and a GATA1s mutation to a leukemia driven by additional driver mutations. We modeled the megakaryocyte differentiation defect through stepwise gene editing of GATA1s, SMC3^+/–, and MPL^W515K, providing 20 different T21 or disomy 21 (D21) induced pluripotent stem cell (iPSC) clones. GATA1s profoundly reshaped iPSC-derived hematopoietic architecture with gradual myeloid-to-megakaryocyte shift and megakaryocyte differentiation alteration upon addition of SMC3 and MPL mutations. Transcriptional, chromatin accessibility, and GATA1-binding data showed alteration of essential megakaryocyte differentiation genes, including NFE2 downregulation that was associated with loss of GATA1s binding and functionally involved in megakaryocyte differentiation blockage. T21 enhanced the proliferative phenotype, reproducing the cellular and molecular abnormalities of DS-AMKL. Our study provides an array of human cell–based models revealing individual contributions of different mutations to DS-AMKL differentiation blockage, a major determinant of leukemic progression.

Automated Pulmonary Embolism Risk Assessment Using the Wells Criteria: Validation Study

Background

Computed tomography pulmonary angiography (CTPA) is frequently used in the emergency department (ED) for the diagnosis of pulmonary embolism (PE), while posing risk for contrast-induced nephropathy and radiation-induced malignancy.

Objective

We aimed to create an automated process to calculate the Wells score for pulmonary embolism for patients in the ED, which could potentially reduce unnecessary CTPA testing.

Methods

We designed an automated process using electronic health records data elements, including using a combinatorial keyword search method to query free-text fields, and calculated automated Wells scores for a sample of all adult ED encounters that resulted in a CTPA study for PE at 2 tertiary care hospitals in New York, over a 2-month period. To validate the automated process, the scores were compared to those derived from a 2-clinician chart review.

Results

A total of 202 ED encounters resulted in a completed CTPA to form the retrospective study cohort. Patients classified as “PE likely” by the automated process (126/202, 62%) had a PE prevalence of 15.9%, whereas those classified as “PE unlikely” (76/202, 38%; Wells score >4) had a PE prevalence of 7.9%. With respect to classification of the patient as “PE likely,” the automated process achieved an accuracy of 92.1% when compared with the chart review, with sensitivity, specificity, positive predictive value, and negative predictive value of 93%, 90.5%, 94.4%, and 88.2%, respectively.

Conclusions

This was a successful development and validation of an automated process using electronic health records data elements, including free-text fields, to classify risk for PE in ED visits.

Calreticulin del52 and ins5 knock-in mice recapitulate different myeloproliferative phenotypes observed in patients with MPN

Somatic mutations in the calreticulin (CALR) gene are associated with approximately 30% of essential thrombocythemia (ET) and primary myelofibrosis (PMF). CALR mutations, including the two most frequent 52 bp deletion (del52) and 5 bp insertion (ins5), induce a frameshift to the same alternative reading frame generating new C-terminal tails. In patients, del52 and ins5 induce two phenotypically distinct myeloproliferative neoplasms (MPNs). They are equally found in ET, but del52 is more frequent in PMF. We generated heterozygous and homozygous conditional inducible knock-in (KI) mice expressing a chimeric murine CALR del52 or ins5 with the human mutated C-terminal tail to investigate their pathogenic effects on hematopoiesis. Del52 induces greater phenotypic changes than ins5 including thrombocytosis, leukocytosis, splenomegaly, bone marrow hypocellularity, megakaryocytic lineage amplification, expansion and competitive advantage of the hematopoietic stem cell compartment. Homozygosity amplifies these features, suggesting a distinct contribution of homozygous clones to human MPNs. Moreover, homozygous del52 KI mice display features of a penetrant myelofibrosis-like disorder with extramedullary hematopoiesis linked to splenomegaly, megakaryocyte hyperplasia and the presence of reticulin fibers. Overall, modeling del52 and ins5 mutations in mice successfully recapitulates the differences in phenotypes observed in patients.

Calreticulin del52 and ins5 mutations induce two phenotypically distinct myeloproliferative neoplasms in patients. Here the authors show that modeling these mutations in knock-in mice recapitulate the two diseases and highlight how they impact the different hematopoietic compartments.

Elevated Calprotectin and Abnormal Myeloid Cell Subsets Discriminate Severe from Mild COVID-19

Blood myeloid cells are known to be dysregulated in coronavirus disease 2019 (COVID-19), caused by SARS-CoV-2. It is unknown whether the innate myeloid response differs with disease severity and whether markers of innate immunity discriminate high-risk patients. Thus, we performed high-dimensional flow cytometry and single-cell RNA sequencing of COVID-19 patient peripheral blood cells and detected disappearance of non-classical CD14^LowCD16^High monocytes, accumulation of HLA-DR^Low classical monocytes (Human Leukocyte Antigen - DR isotype), and release of massive amounts of calprotectin (S100A8/S100A9) in severe cases. Immature CD10^LowCD101⁻CXCR4^+/− neutrophils with an immunosuppressive profile accumulated in the blood and lungs, suggesting emergency myelopoiesis. Finally, we show that calprotectin plasma level and a routine flow cytometry assay detecting decreased frequencies of non-classical monocytes could discriminate patients who develop a severe form of COVID-19, suggesting a predictive value that deserves prospective evaluation.

Multilayer intraclonal heterogeneity in chronic myelomonocytic leukemia

The functional diversity of cells that compose myeloid malignancies, i.e., the respective roles of genetic and epigenetic heterogeneity in this diversity, remains poorly understood. This question is addressed in chronic myelomonocytic leukemia, a myeloid neoplasm in which clinical diversity contrasts with limited genetic heterogeneity. To generate induced pluripotent stem cell clones, we reprogrammed CD34⁺ cells collected from a patient with a chronic myelomonocytic leukemia in which whole exome sequencing of peripheral blood monocyte DNA had identified 12 gene mutations, including a mutation in KDM6A and two heterozygous mutations in TET2 in the founding clone and a secondary KRAS(G12D) mutation. CD34⁺ cells from an age-matched healthy donor were also reprogrammed. We captured a part of the genetic heterogeneity observed in the patient, i.e. we analyzed five clones with two genetic backgrounds, without and with the KRAS(G12D) mutation. Hematopoietic differentiation of these clones recapitulated the main features of the patient's disease, including overproduction of granulomonocytes and dysmegakaryopoiesis. These analyses also disclosed significant discrepancies in the behavior of hematopoietic cells derived from induced pluripotent stem cell clones with similar genetic background, correlating with limited epigenetic changes. These analyses suggest that, beyond the coding mutations, several levels of intraclonal heterogeneity may participate in the yet unexplained clinical heterogeneity of the disease.

A Recurrent Activating Missense Mutation in Waldenström Macroglobulinemia Affects the DNA Binding of the ETS Transcription Factor SPI1 and Enhances Proliferation

The ETS-domain transcription factors divide into subfamilies based on protein similarities, DNA-binding sequences, and interaction with cofactors. They are regulated by extracellular clues and contribute to cellular processes, including proliferation and transformation. ETS genes are targeted through genomic rearrangements in oncogenesis. The PU.1/SPI1 gene is inactivated by point mutations in human myeloid malignancies. We identified a recurrent somatic mutation (Q226E) in PU.1/SPI1 in Waldenström macroglobulinemia, a B-cell lymphoproliferative disorder. It affects the DNA-binding affinity of the protein and allows the mutant protein to more frequently bind and activate promoter regions with respect to wild-type protein. Mutant SPI1 binding at promoters activates gene sets typically promoted by other ETS factors, resulting in enhanced proliferation and decreased terminal B-cell differentiation in model cell lines and primary samples. In summary, we describe oncogenic subversion of transcription factor function through subtle alteration of DNA binding leading to cellular proliferation and differentiation arrest. SIGNIFICANCE: The demonstration that a somatic point mutation tips the balance of genome-binding pattern provides a mechanistic paradigm for how missense mutations in transcription factor genes may be oncogenic in human tumors.This article is highlighted in the In This Issue feature, p. 681.

Biology and prognostic impact of clonal plasmacytoid dendritic cells in chronic myelomonocytic leukemia

Islands of CD123^high cells have been commonly described in the bone marrow of patients with chronic myelomonocytic leukemia (CMML). Using a multiparameter flow cytometry assay, we detected an excess of CD123⁺ mononucleated cells that are lineage-negative, CD45⁺, CD11c^-, CD33^-, HLA-DR⁺, BDCA-2⁺, BDCA-4⁺ in the bone marrow of 32/159 (20%) patients. Conventional and electron microscopy, flow cytometry detection of cell surface markers, gene expression analyses, and the ability to synthesize interferon alpha in response to Toll-like receptor agonists identified these cells as bona fide plasmacytoid dendritic cells (pDCs). Whole-exome sequencing of sorted monocytes and pDCs identified somatic mutations in genes of the oncogenic RAS pathway in the two cell types of every patient. CD34⁺ cells could generate high amount of pDCs in the absence of FMS-like tyrosine kinase 3-ligand (FLT3L). Finally, an excess of pDCs correlates with regulatory T cell accumulation and an increased risk of acute leukemia transformation. These results demonstrate the FLT3L-independent accumulation of clonal pDCs in the bone marrow of CMML patients with mutations affecting the RAS pathway, which is associated with a higher risk of disease progression.

Mutations of the integrin αIIb/β3 intracytoplasmic salt bridge cause macrothrombocytopenia and enlarged platelet α-granules

Rare gain-of-function mutations within the ITGA2B or ITGB3 genes have been recognized to cause macrothrombocytopenia (MTP). Here we report three new families with autosomal dominant (AD) MTP, two harboring the same mutation of ITGA2B, αIIbR995W, and a third family with an ITGB3 mutation, β3D723H. In silico analysis shows how the two mutated amino acids directly modify the salt bridge linking the intra-cytoplasmic part of αIIb to β3 of the integrin αIIbβ3. For all affected patients, the bleeding syndrome and MTP was mild to moderate. Platelet aggregation tended to be reduced but not absent. Electron microscopy associated with a morphometric analysis revealed large round platelets; a feature being the presence of abnormal large α-granules with some giant forms showing signs of fusion. Analysis of the maturation and development of megakaryocytes reveal no defect in their early maturation but abnormal proplatelet formation was observed with increased size of the tips. Interestingly, this study revealed that in addition to the classical phenotype of patients with αIIbβ3 intracytoplasmic mutations there is an abnormal maturation of α-granules. It is now necessary to determine if this feature is a characteristic of all mutations disturbing the αIIb R995/β3 D723 salt bridge.

P53 activation inhibits all types of hematopoietic progenitors and all stages of megakaryopoiesis

TP53 also known as p53 is a tumor suppressor gene mutated in a variety of cancers. P53 is involved in cell cycle, apoptosis and DNA repair mechanisms and is thus tightly controlled by many regulators. Recently, strategies to treat cancer have focused on the development of MDM2 antagonists to induce p53 stabilization and restore cell death in p53 non-mutated cancers. However, some of these molecules display adverse effects in patients including induction of thrombocytopenia. In the present study, we have explored the effect of SAR405838 not only on human megakaryopoiesis but also more generally on hematopoiesis. We compared its effect to MI-219 and Nutlin, which are less potent MDM2 antagonists than SAR405838. We found that all these compounds induce a deleterious effect on all types of hematopoietic progenitors, as well as on erythroid and megakaryocytic differentiation. Moreover, they inhibit both early and late stages of megakaryopoiesis including ploidization and proplatelet formation. In conclusion, MDM2 antagonists induced a major hematopoietic defect in vitro as well as an inhibition of all stages of megakaryopoiesis that may account for in vivo thrombocytopenia observed in treated patients.

Critical role of the HDAC6-cortactin axis in human megakaryocyte maturation leading to a proplatelet-formation defect

Thrombocytopenia is a major side effect of a new class of anticancer agents that target histone deacetylase (HDAC). Their mechanism is poorly understood. Here, we show that HDAC6 inhibition and genetic knockdown lead to a strong decrease in human proplatelet formation (PPF). Unexpectedly, HDAC6 inhibition-induced tubulin hyperacetylation has no effect on PPF. The PPF decrease induced by HDAC6 inhibition is related to cortactin (CTTN) hyperacetylation associated with actin disorganization inducing important changes in the distribution of megakaryocyte (MK) organelles. CTTN silencing in human MKs phenocopies HDAC6 inactivation and knockdown leads to a strong PPF defect. This is rescued by forced expression of a deacetylated CTTN mimetic. Unexpectedly, unlike human-derived MKs, HDAC6 and CTTN are shown to be dispensable for mouse PPF in vitro and platelet production in vivo. Our results highlight an unexpected function of HDAC6–CTTN axis as a positive regulator of human but not mouse MK maturation.

Histone deacetylase (HDAC) inhibitors, a class of cancer therapeutics, cause thrombocytopenia via an unknown mechanism. Here, the authors show that HDAC6 inhibition impairs proplatelet formation in human megakaryocytes, and show that this is linked to hyperacetylation of the actin-binding protein cortactin.

ETO2-GLIS2 Hijacks Transcriptional Complexes to Drive Cellular Identity and Self-Renewal in Pediatric Acute Megakaryoblastic Leukemia

Chimeric transcription factors are a hallmark of human leukemia, but the molecular mechanisms by which they block differentiation and promote aberrant self-renewal remain unclear. Here, we demonstrate that the ETO2-GLIS2 fusion oncoprotein, which is found in aggressive acute megakaryoblastic leukemia, confers megakaryocytic identity via the GLIS2 moiety while both ETO2 and GLIS2 domains are required to drive increased self-renewal properties. ETO2-GLIS2 directly binds DNA to control transcription of associated genes by upregulation of expression and interaction with the ETS-related ERG protein at enhancer elements. Importantly, specific interference with ETO2-GLIS2 oligomerization reverses the transcriptional activation at enhancers and promotes megakaryocytic differentiation, providing a relevant interface to target in this poor-prognosis pediatric leukemia.

Whole-exome sequencing of single circulating tumor cells is a useful tool for studying the intrapatient genetic heterogeneity in metastatic prostate cancer

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Background: Molecular characterization of metastatic castration resistant prostate cancer (mCRPC) is limited by tumor tissue availability. The analysis of circulating tumor cells (CTC) offers an attractive noninvasive surrogate option to analyze molecular alterations. We report whole exome sequencing (WES) of CTCs at the single cell level in mCRPC patients. Methods: Blood samples were drawn from 11 enzalutamide or abiraterone pre-treated mCRPC patients enrolled in the clinical program MOSCATO (NCT02613962). CTC enrichment, immunofluorescent detection and single cell isolation were performed using three methods (ISET filtration, CellSearch and the VyCap puncher system and RosetteSep enrichment) to obtain pools of 1-10 CTCs with distinct epithelial or mesenchymal phenotypes. After Whole Genome Amplification (WGA), WES was performed on the Illumina HiSeq 2000 platform. GATK Haplotype Caller enabled identification of germline polymorphisms from each patient in normal DNA, metastatic sample and CTCs in order to consider WGA induced bias. The detection of sSNV in tumor biopsies and CTCs was assessed with Mutect and IndelGenotyper respectively. Results: 189 WGA of CTC pools were performed. 34 pools of phenotypically different CTCs from 7 patients were selected and sequenced. Mean coverage of 51% was obtained at a sequencing depth of 10X. Allelic drop out was lower for CTC pools containing 5-10 cells. 17/34 (50%) CTC samples had shared sSNV with the paired tumor sample (range 0.35%-68%) Epithelial CTCs had more shared sSNV with metastatic biopsies than CTCs of other phenotypes but shared sSNV were also detected in large non epithelial CTC pointing out a high level of genetic heterogeneity between CTC. Overall, 89 deleterious protein-coding mutations were found only in pools of CTC, including mutations affecting oncogenic drivers such as MAPK1, HSP90AB1 or KDM5B. Conclusions: We present single cell WES of CTCs harboring distinct phenotypes. The detection of shared sSNV between CTC pools and corresponding biopsy could validate the use of CTCs as a liquid biopsy. The finding of sSNV specific to CTCs could offer additional data on tumor heterogeneity.

Uncoupling of the Hippo and Rho pathways allows megakaryocytes to escape the tetraploid checkpoint

Megakaryocytes are naturally polyploid cells that increase their ploidy by endomitosis. However, very little is known regarding the mechanism by which they escape the tetraploid checkpoint to become polyploid. Recently, it has been shown that the tetraploid checkpoint was regulated by the Hippo-p53 pathway in response to a downregulation of Rho activity. We therefore analyzed the role of Hippo-p53 pathway in the regulation of human megakaryocyte polyploidy. Our results revealed that Hippo-p53 signaling pathway proteins are present and are functional in megakaryocytes. Although this pathway responds to the genotoxic stress agent etoposide, it is not activated in tetraploid or polyploid megakaryocytes. Furthermore, Hippo pathway was observed to be uncoupled from Rho activity. Additionally, polyploid megakaryocytes showed increased expression of YAP target genes when compared to diploid and tetraploid megakaryocytes. Although p53 knockdown increased both modal ploidy and proplatelet formation in megakaryocytes, YAP knockdown caused no significant change in ploidy while moderately affecting proplatelet formation. Interestingly, YAP knockdown reduced the mitochondrial mass in polyploid megakaryocytes and decreased expression of PGC1α, an important mitochondrial biogenesis regulator. Thus, the Hippo pathway is functional in megakaryocytes, but is not induced by tetraploidy. Additionally, YAP regulates the mitochondrial mass in polyploid megakaryocytes.

Abstract 4953: Whole-exome sequencing of single circulating tumor cells according to epithelial-mesenchymal marker expression in metastatic prostate cancer

Molecular characterization of metastatic castration-resistant prostate cancer (mCRPC) is limited by tumor tissue availability. The analysis of circulating tumor cells (CTCs) offers an attractive non invasive surrogate option to analyze molecular alterations. We report whole exome sequencing (WES) of CTCs at the single cell level in 11 mCRPC patients. We examined single somatic nucleotide variant (sSNV) shared between matched metastatic tumor sample and CTCs and sSNV specific to CTCs.

Blood samples were drawn from 11 patients enrolled in the clinical program MOSCATO (2011-A00841-40). CTC enrichment, detection and single cell isolation were performed using three methods to obtain pools of 1-10 CTCs. The first method used ISET filtration, immunofluorescent staining (CD45, pan-cytokeratin, EpCAM, Vimentin and Hoechst 33342) on filters and laser microdissection of single CTCs; the second combined CellSearch and the VyCap puncher system; the third used RosetteSep enrichment, immunofluorescent staining and isolation by cell sorting. Whole Genome Amplification (WGA) was performed using the Ampli1 kit. WGA quality was assessed by qPCR of 7 genes located on different regions of the genome. WES was performed by preparation of a genomic DNA bank, Agilent capture and sequencing on the Illumina HiSeq 2000 platform. Data were aligned to the human genome reference hg19. GATK Haplotype Caller enabled identification of germline polymorphisms from each patient in normal DNA, metastatic sample and CTCs in order to consider WGA induced bias. The detection of sSNV in tumor biopsies and CTCs was assessed with Mutect and IndelGenotyper respectively.

189 WGAs of CTC pools were performed. A first round of WES showed that at least 3 well amplified genes were required to obtain a coverage of at least 50% at 10X depth sequencing. 34 pools of phenotypically different CTCs from 7 patients were selected and sequenced. Mean coverage of 51% was obtained at a sequencing depth of 10X. Allelic drop out was lower for CTC pools containing 5 to 10 cells. 17/34 (50%) CTC samples (4 patients) had shared sSNV with the paired tumor sample (range 0.35%-68%). Epithelial CTCs had more shared sSNV with metastatic biopsies than CTCs of other phenotypes but shared sSNV were also detected in large Cytokeratin-Vimentin- CTC. Shared sSNV in cancer genes between epithelial CTC pools, but not in the paired biopsy, were present in 2 patients.

We report WES of CTC pools harboring distinct EMT marker phenotypes is possible with the use of 3 different approaches to enrich, detect and isolate CTCs. The detection of shared sSNV between CTC pools and corresponding biopsy could validate the use of CTCs as a liquid biopsy. The finding of sSNV specific to CTCs could offer additional data on tumor heterogeneity. Ongoing work examining if sSNV detected in phenotypically different CTCs converge to similar signaling pathways will be presented.

Citation Format: Vincent Faugeroux, Céline Lefebvre, Emma Pailler, Valérie Pierron, Fanny Billiot, Charles Marcaillou, Philippe Vielh, Semih Dogan, Philippe Rameau, Maud Ngocamus, Jean Charles Soria, Karim Fizazi, Yohann Loriot, Sylvia Julien, Françoise Farace. Whole-exome sequencing of single circulating tumor cells according to epithelial-mesenchymal marker expression in metastatic prostate cancer. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 4953.

Developmental changes in human megakaryopoiesis

BACKGROUND

The molecular bases of the cellular changes that occur during human megakaryocyte (MK) ontogeny remain unknown, and may be important for understanding the significance of MK differentiation from human embryonic stem cells (hESCs)

METHODS

We optimized the differentiation of MKs from hESCs, and compared these with MKs obtained from primary human hematopoietic tissues at different stages of development.

RESULTS

Transcriptome analyses revealed a close relationship between hESC-derived and fetal liver-derived MKs, and between neonate-derived and adult-derived MKs. Major changes in the expression profiles of cell cycle and transcription factors (TFs), including MYC and LIN28b, and MK-specific regulators indicated that MK maturation progresses during ontogeny towards an increase in MK ploidy and a platelet-forming function. Important genes, including CXCR4, were regulated by an on-off mechanism during development.

DISCUSSION

Our analysis of the pattern of TF network and signaling pathways was consistent with a growing specialization of MKs towards hemostasis during ontogeny, and support the idea that MKs derived from hESCs reflect primitive hematopoiesis.

DUX4 and DUX4 downstream target genes are expressed in fetal FSHD muscles

Facioscapulohumeral muscular dystrophy (FSHD) is one of the most prevalent adult muscular dystrophies. The common clinical signs usually appear during the second decade of life but when the first molecular dysregulations occur is still unknown. Our aim was to determine whether molecular dysregulations can be identified during FSHD fetal muscle development. We compared muscle biopsies derived from FSHD1 fetuses and the cells derived from some of these biopsies with biopsies and cells derived from control fetuses. We mainly focus on DUX4 isoform expression because the expression of DUX4 has been confirmed in both FSHD cells and biopsies by several laboratories. We measured DUX4 isoform expression by using qRT-PCR in fetal FSHD1 myotubes treated or not with an shRNA directed against DUX4 mRNA. We also analyzed DUX4 downstream target gene expression in myotubes and fetal or adult FSHD1 and control quadriceps biopsies. We show that both DUX4-FL isoforms are already expressed in FSHD1 myotubes. Interestingly, DUX4-FL expression level is much lower in trapezius than in quadriceps myotubes, which is confirmed by the level of expression of DUX4 downstream genes. We observed that TRIM43 and MBD3L2 are already overexpressed in FSHD1 fetal quadriceps biopsies, at similar levels to those observed in adult FSHD1 quadriceps biopsies. These results indicate that molecular markers of the disease are already expressed during fetal life, thus opening a new field of investigation for mechanisms leading to FSHD.

Thrombocytopenia induced by the histone deacetylase inhibitor abexinostat involves p53-dependent and -independent mechanisms

Abexinostat is a pan histone deacetylase inhibitor (HDACi) that demonstrates efficacy in malignancy treatment. Like other HDACi, this drug induces a profound thrombocytopenia whose mechanism is only partially understood. We have analyzed its effect at doses reached in patient plasma on in vitro megakaryopoiesis derived from human CD34⁺ cells. When added at day 0 in culture, abexinostat inhibited CFU-MK growth, megakaryocyte (MK) proliferation and differentiation. These effects required only a short incubation period. Decreased proliferation was due to induction of apoptosis and was not related to a defect in TPO/MPL/JAK2/STAT signaling. When added later (day 8), the compound induced a dose-dependent decrease (up to 10-fold) in proplatelet (PPT) formation. Gene profiling from MK revealed a silencing in the expression of DNA repair genes with a marked RAD51 decrease at protein level. DNA double-strand breaks were increased as attested by elevated γH2AX phosphorylation level. Moreover, ATM was phosphorylated leading to p53 stabilization and increased BAX and p21 expression. The use of a p53 shRNA rescued apoptosis, and only partially the defect in PPT formation. These results suggest that HDACi induces a thrombocytopenia by a p53-dependent mechanism along MK differentiation and a p53-dependent and -independent mechanism for PPT formation.

Presence of a defect in karyokinesis during megakaryocyte endomitosis

Megakaryocyte is the naturally polyploid cell that gives rise to platelets. Polyploidization occurs by endomitosis, a process corresponding to a late failure of cytokinesis with a backward movement of the daughter cells. Generally, a pure defect in cytokinesis produces a multinucleated cell, but megakaryocytes are characterized by a single polylobulated nucleus with a 2 (N) ploidy. Here, we show the existence of a defect in karyokinesis during the endomitotic process. From late telophase until the reversal of cytokinesis, some dipolar mitosis/endomitosis and most multipolar endomitosis present a thin DNA link between the segregated chromosomes surrounded by an incomplete nuclear membrane formation, which implies that sister chromatid separation is not complete. This observation may explain why polyploid megakaryocytes display a single polylobulated nucleus along with an increase in ploidy.

Abstract 2022: P53-dependent thrombocytopenia induced by the histone deacetylase inhibitor S78454

S78454 (also called PCI-24781) is an orally bioavailable, hydroxamate-based pan-HDACi currently being tested in clinical trials in the US and EU. Like many other HDACi, this drug induces a reversible thrombocytopenia. This thrombocytopenia, which was associated with a decrease in either GATA1 transcriptional activity or the expression of proteins of the Rho GTPase family (RhoA, Cdc42 or Rac), remains poorly understood. Given that the S78454 plasma level is above 100nM in treated patients, we performed a dose-response analysis (from 10 to 100 nM) of the drug effects on CFU-MK growth and cell proliferation. CFU-MKs were generated by ex vivo culture of CD34-positive cells and their differentiation was dose-dependently decreased after either a 24-hour treatment or a continuous exposure to S78454. This effect was associated with a dose-dependent decrease in MK proliferation. When added at day 8 of the culture, at a time-point when MK have nearly finished their endomitotic process and are entering in the cytoplasmic maturation step, S78454 induced a dose-dependent decrease (ten- to- two fold at 100nM and 20nM, respectively) in proplatelet formation, even when secondarily removed from the culture medium. The decreased proliferation was associated with an increased apoptosis, as demonstrated by the presence of a sub-G1 peak after propidium iodide staining, Annexin V binding, and caspase-3 proteolysis. We did not observed any blockade in the TPO/MPL/JAK2 signaling since ERK, Stat3 and Stat5 phosphorylation remained unaffected. Interestingly, MK exposed to S78454 displayed an increase in γH2AX foci formation and a decrease in Rad51 expression, a major mediator of homologous recombination, indicating a defective repair of DNA double-strand breaks. Consequently, P53 became phosphorylated in MKs, and the expression of its target genes BAX and P21 was increased. Altogether, these results suggest that S78454-induced thrombocytopenia may be mediated by induction of apoptosis in MK due to DNA-double-strand breaks and p53 activation.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 2022. doi:1538-7445.AM2012-2022

Antigen-driven interactions with dendritic cells and expansion of Foxp3+ regulatory T cells occur in the absence of inflammatory signals

Foxp3+ regulatory T cells (Tregs) play a pivotal role in the maintenance of peripheral T cell tolerance and are thought to interact with dendritic cells (DC) in secondary lymphoid organs. We analyzed here the in vivo requirements for selective expansion of Ag-specific Treg vs CD4+CD25- effector T cells and engagement of Ag-specific Treg-DC interactions in secondary lymphoid organs. Using i.v. Ag delivery in the absence of inflammation, we found that CD4+CD25+Foxp3+ Tregs undergo vigorous expansion and accumulate whereas naive CD4+CD25-Foxp3- T cells undergo abortive activation. Quantifying directly the interactions between Tregs and CD11c+ DC, we found that Tregs establish cognate contacts with endogenous CD11c+ DC in spleen and lymph nodes at an early time point preceding their expansion. Importantly, we observed that as few as 10(3) Tregs selectively expanded by i.v. Ag injection are able to suppress B and T cell immune responses in mouse recipients challenged with the Ag. Our results demonstrate that Tregs are selectively mobilized by Ag recognition in the absence of inflammatory signals, and can induce thereafter potent tolerance to defined Ag targets.

Epigenetic gene expression noise and phenotypic diversification of clonal cell populations

Spontaneous emergence of phenotypic heterogeneity in cultures of genetically identical cells is a frequently observed phenomenon that provides a simple in vitro experimental system to model the problems of in vivo differentiation. In the present study, we have investigated whether stochastic variation of gene expression levels could contribute to phenotypic change in human cells. We have applied the two fluorescence-coding gene method and the expression variability of the two reporter genes to human cells in culture. We have quantified the portion of gene expression variation determined by global, promoter-specific, or by epigenetic sources. These two types of variation appear to contribute, in different ways, to the phenotypic diversification of clonal cell populations. Global, or promoter-specific, gene expression noise increases with cellular stress and contributes to the emergence of cellular diversity by diversifying the gene-expression levels. Epigenetic mechanisms act to increase the robustness of the cellular state by stabilizing gene transcription levels or by reinforcing the silenced state.

Expression of mdr1 is required for efficient long term regeneration of dystrophic muscle

The mouse mdr1a and mdr1b genes are expressed in skeletal muscle, though their precise role in muscle is unknown. Dystrophic muscle is characterized by repeated cycles of degeneration and regeneration. To explore the role of the mdr1 genes during muscle regeneration, we have created a triple knockout mouse lacking the mdr1a, mdr1b, and the dystrophin genes. The resulting ReX mice developed normally and were fertile. However, as adults, ReX had a higher proportion of degenerating muscle fibers and greater long-term loss of muscle mass than mdx. ReX muscles were also characterized by a reduced proportion of muscle side population (mSP) cells, of myogenic cells, and a reduced capacity for muscle regeneration. We found too that mSP cells derived from dystrophic muscle are more myogenic than those from normal muscle. Thus, in dystrophic muscle, the mdr1 gene plays an important role in the preservation of the mSP and of the myogenic regenerative potential. Moreover, our results suggest a hitherto unappreciated role of mdr1 in precursor cells of regenerating tissue; they therefore provide an important clue to the physiological significance of mdr1 expression in stem cells.

The origin of phenotypic heterogeneity in a clonal cell population in vitro

Background

The spontaneous emergence of phenotypic heterogeneity in clonal populations of mammalian cells in vitro is a rule rather than an exception. We consider two simple, mutually non-exclusive models that explain the generation of diverse cell types in a homogeneous population. In the first model, the phenotypic switch is the consequence of extrinsic factors. Initially identical cells may become different because they encounter different local environments that induce adaptive responses. According to the second model, the phenotypic switch is intrinsic to the cells that may occur even in homogeneous environments.

Principal Findings

We have investigated the “extrinsic” and the “intrinsic” mechanisms using computer simulations and experimentation. First, we simulated in silico the emergence of two cell types in a clonal cell population using a multiagent model. Both mechanisms produced stable phenotypic heterogeneity, but the distribution of the cell types was different. The “intrinsic” model predicted an even distribution of the rare phenotype cells, while in the “extrinsic” model these cells formed small clusters. The key predictions of the two models were confronted with the results obtained experimentally using a myogenic cell line.

Conclusions

The observations emphasize the importance of the “ecological” context and suggest that, consistently with the “extrinsic” model, local stochastic interactions between phenotypically identical cells play a key role in the initiation of phenotypic switch. Nevertheless, the “intrinsic” model also shows some other aspects of reality: The phenotypic switch is not triggered exclusively by the local environmental variations, but also depends to some extent on the phenotypic intrinsic robustness of the cells.

Combination of quantification and observation methods for study of “Side Population” cells in their “in vitro” microenvironment

BACKGROUND

Qualitative and quantitative analyses of the rare phenotypic variants in in vitro culture systems is necessary for the understanding of cell differentiation in cell culture of primary cells or cell lines. Slide-based cytometry combines image acquisition and data treatment, and associates the power of flow cytometry (FCM) and the resolution of the microscopic studies making it suitable for the analysis of cells with rare phenotype. In this paper we develop a method that applies these principles to a particularly hot problem in cell biology, the study of stem cell like cells in cultures of primary cells, cancer cells, and various cell lines.

METHODS

The adherent cells were labeled by the fluorescent dye Hoechst 33342. The images of cell populations were collected by a two-photon microscope and processed by a software developed by us. The software allows the automated segmentation of the nuclei in a very dense cell environment, the measurement of the fluorescence intensity of each nucleus and the recording of their position in the plate. The cells with a given fluorescence intensity can then be located easily on the recorded image of the culture plate for further analysis.

RESULTS

The potential of our method is illustrated by the identification and localization of SP cells in the cultures of the C2C12 cell line. Although these cells represent only about 1% of the total population as calculated by flow cytometry, they can be identified in the culture plate with high precision by microscopy.

CONCLUSION

Cells with the rare stem-cell like phenotype can be efficiently identified in the undisturbed cultures. Since the fluorescence intensity of rare events and the position of thousands of surrounding cells are recorded at the same time, the method associates the advantage of the FCM analysis and the microscopic observation.

Efficient control of gene expression in the hematopoietic system using a single Tet-on inducible lentiviral vector

This work addresses the problem of efficient control of gene expression in the context of viral vectors, which still represents a difficult challenge. A number of lentiviral vectors incorporating the different elements of regulatable transcriptional systems have been described, but they fail to perform satisfactorily either because of a poor dynamic range of transcription levels or because they display high background activities in the uninduced state and mediocre inducer response. We report here on the systematic comparison of vector designs containing the elements of the doxycycline-inducible Tet-on system in their most advanced versions (rtTA2S-M2 transactivator and tTS(Kid) repressor). We show that a simple "all-in-one" vector can be obtained and used for efficient control of transgene expression in long-term tissue culture and in the hematopoietic system of mice following bone marrow transplantation. Using this vector, the uninduced state can be kept at background levels and induction factors of 100-fold are repeatedly obtained over months both in tissue culture and in vivo. Interestingly, the low background activity of the all-in-one vector renders the use of the tTS repressor dispensable, avoiding the problem of progressive loss of inducibility over time associated with irreversible modifications of the chromatin surrounding proviral sequences.

Global transcriptional characterization of SP and MP cells from the myogenic C2C12 cell line: effect of FGF6

With the use of Hoechst staining techniques, we have previously shown that the C2C12 myogenic cell line contains a side population (SP) that is largely increased in the presence of fibroblast growth factor 6 (FGF6). Here, we compared transcriptional profiles from SP and main population (MP) cells from either C2C12 or FGF6-expressing C2C12. Expression profiles of SPs show that these cells are less differentiated than MPs and display some similarities to stem cells. Moreover, principal component analysis made it possible to distinguish specific contributions of either FGF6 or differentiation effects on gene expression profiles. This demonstrated that FGF6-expanded SPs were similar to parental C2C12-derived SPs. Conversely, FGF6-treated MPs differed from parental MPs and were more related to SP cells. These results show that FGF6 pushed committed myogenic cells toward a more immature phenotype resulting in the accumulation of cells with a SP phenotype. We propose that FGF6 conditioning could provide a way to expand the pool of immature cells by myoblast dedifferentiation.

Role for the Nuclear Factor κB Pathway in Transforming Growth Factor-β1 Production in Idiopathic Myelofibrosis: Possible Relationship with FK506 Binding Protein 51 Overexpression

The release of transforming growth factor-beta1 (TGF-beta1) in the bone marrow microenvironment is one of the main mechanisms leading to myelofibrosis in murine models and probably in the human idiopathic myelofibrosis (IMF). The regulation of TGF-beta1 synthesis is poorly known but seems regulated by nuclear factor kappaB (NF-kappaB). We previously described the overexpression of an immunophilin, FK506 binding protein 51 (FKBP51), in IMF megakaryocytes. Gel shift and gene assays show that FKBP51's overexpression in a factor-dependent hematopoietic cell line, induces a sustained NF-kappaB activation after cytokine deprivation. This activation correlates with a low level of IkappaBalpha. A spontaneous activation of NF-kappaB was also detected in proliferating megakaryocytes and in circulating CD34(+) patient cells. In normal cells, NF-kappaB activation was only detected after cytokine treatment. The expression of an NF-kappaB superrepressor in FKBP51 overexpressing cells and in derived megakaryocytes from CD34(+) of IMF patients revealed that NF-kappaB activation was not involved in the resistance to apoptosis after cytokine deprivation of these cells but in TGF-beta1 secretion. These results highlight the importance of NF-kappaB's activation in the fibrosis development of this disease. They also suggest that FKBP51's overexpression in IMF cells could play an important role in the pathogenesis of this myeloproliferative disorder.

Evidence for a resident subset of cells with SP phenotype in the C2C12 myogenic line: a tool to explore muscle stem cell biology

Muscle satellite cells are heterogeneous and present functional disparities, some of them behaving as multipotent stem cells. Yet their phenotype is obscure and their isolation remains elusive. The ability to purify stem cells from a wide variety of tissues by using Hoechst 33342 staining/FACS methods has permitted access to this category of cells (side population, or SP) in a manner independent of antibodies. Here, we show that the C2C12 myogenic line comprises a minor population of cells with SP phenotype. These cells are growth-arrested and delayed in their ability to differentiate. Dye efflux in C2C12-derived SPs is likely mediated by mdr1a, whose overexpression results in increased dedifferentiation. Interestingly, growth-arrested SPs rapidly appear in purified MP populations, thus suggesting a dynamic equilibrium among different states of differentiation. Finally, transcriptional profiling of C2C12-derived SP and MP cells corroborates the many similarities of SP to stem cells.

FGF6 mediated expansion of a resident subset of cells with SP phenotype in the C2C12 myogenic line

Fibroblast growth factor 6 (FGF6) is selectively expressed during muscle development and regeneration. We examined its effect on muscle precursor cells (mpc) by forcing stable FGF6 expression in C2C12 cells in vitro. FGF6 produced in genetically engineered mpc was active, inducing strong morphological changes, altering cell adhesion and compromising their ability to differentiate into myotubes. Expression of MyoD and myogenin, but not of Myf5, was abrogated in FGF6 engineered mpc. These effects were reversed by FGF inhibitors. Ectopic expression of MyoD also restored fiber formation indicating that FGF6 interferes with the myogenic differentiation pathway upstream of MyoD. We also report that in the presence of FGF6, the minor (0.5-2%) subpopulation of cells actively excluding Hoechst 33342 in a verapamil-dependent manner (SP phenotype) was increased to 15-20% and the expression of the mdr1a gene (but not mdr1b) was upregulated by 400-fold. Our data establish a previously undescribed link between FGF6--a muscle specific growth factor--and a multidrug resistance gene expressed in stem cells, and suggest a role for FGF6 in the maintenance of a reserve pool of progenitor cells in the skeletal muscle.

Embryonic stem cell differentiation to hematopoietic cells: A model to study the function of various regions of the intracytoplasmic domain of cytokine receptors in vitro

To examine whether the in vitro model of embryonic stem (ES) cell hematopoietic differentiation is suitable to study the function of intracytoplasmic regions of cytokine receptors, we used the thrombopoietin receptor Mpl as a typical cytokine receptor.ES cells deficient in c-mpl (mpl(-/)-) were transfected with genes encoding the full-length or two mutated forms of the intracytoplasmic domain of Mpl using the pEF-BOS expression vector. The mutated forms lack box1 or box2.pEF-BOS was able to maintain protein production during ES cell differentiation. Reintroduction of full-length-c-mpl into mpl(-/)- ES cells restored the response of megakaryocyte progenitors to a truncated form of human Mpl-ligand conjugated to polyethylene glycol (PEG-rhuMGDF) and the formation of platelets, for which mpl(-/)- ES cells are defective. In addition, enforced expression of Mpl resulted in the development of all myeloid progenitors and mature cells in the presence of PEG-rhuMGDF. Blast colony-forming cells, the in vitro equivalent of the hemangioblast, also generated blast cell colonies with a hematopoietic potential equivalent to that of the wild type in the presence of PEG-rhuMGDF, although its growth is normally dependent on vascular endothelial cell growth factor (VEGF). Thus, Mpl acts as a substitute for other cytokine receptors and for a tyrosine kinase receptor, Flk-1, indicating that Mpl has no instructive role in hematopoietic cell commitment and differentiation. The Mpl mutant forms lacking box1 or box2 prevented response of ES cell-derived blast colony-forming cells or progenitors to PEG-rhuMGDF. Therefore, these two regions, essential for signaling by cytokine receptors, are required for the responses of ES cell-derived hematopoietic cells to PEG-rhuMGDF.These results show that the in vitro hematopoietic differentiation of ES cells is suitable for studying the role of various intracytoplasmic regions of cytokine receptors.

Existence of a differentiation blockage at the stage of a megakaryocyte precursor in the thrombocytopenia and absent radii (TAR) syndrome

The thrombocytopenia and absent radii (TAR) syndrome is a rare disease associating bilateral radial agenesis and congenital thrombocytopenia. Here, we investigated in vitro megakaryocyte (MK) differentiation and expression of c-mpl in 6 patients. Using blood or marrow CD34(+) cells, the colony-forming unit (CFU)-MK number was markedly reduced. CD34(+) cells were also cultured in liquid medium in the presence of a combination of 3 cytokines (stem cell factor, interleukin-3, and interleukin-6) or megakaryocyte growth and development factor (PEG-rHuMGDF) with or without SCF. In the presence of PEG-rHuMGDF, the majority of mature megakaryocytes (CD41 high, CD42 high) underwent apoptosis. This phenomenon was also observed in cultures stimulated by three cytokines. However, this last combination of cytokines allowed a more complete terminal MK differentiation. Surprisingly, a homogeneous population of CD34(-)CD41(+)CD42(-) cells accumulated during the cultures. This population was unable to differentiate along the myeloid pathways. This result suggests that a fraction of MK cells is unable to differentiate in the TAR syndrome. We subsequently investigated whether this could be related to an abnormality in c-mpl. No mutation or rearrangement in the c-mpl gene was found by Southern blots or by sequencing of the c-mpl coding region and its promoter in any of the patients. Using Western blot analysis, a decreased level of Mpl was found in patient platelets. A decreased level of c-mpl messenger RNA in TAR platelets was also detected with a lower c-mpl-P to c-mpl-K ratio in comparison to adult platelets. Altogether, these results demonstrate that the thrombocytopenia of the TAR syndrome is associated with a dysmegakaryocytopoiesis characterized by cells blocked at an early stage of differentiation. (Blood. 2000;95:1633-1641)

The thrombocytopenia of Wiskott Aldrich syndrome is not related to a defect in proplatelet formation

The Wiskott-Aldrich syndrome (WAS) is an X-linked hereditary disease characterized by thrombocytopenia with small platelet size, eczema, and increased susceptibility to infections. The gene responsible for WAS was recently cloned. Although the precise function of WAS protein (WASP) is unknown, it appears to play a critical role in the regulation of cytoskeletal organization. The platelet defect, resulting in thombocytopenia and small platelet size, is a consistent finding in patients with mutations in the WASP gene. However, its exact mechanism is unknown. Regarding WASP function in cytoskeletal organization, we investigated whether these platelet abnormalities could be due to a defect in proplatelet formation or in megakaryocyte (MK) migration. CD34(+) cells were isolated from blood and/or marrow of 14 WAS patients and five patients with hereditary X-linked thrombocytopenia (XLT) and cultured in serum-free liquid medium containing recombinant human Mpl-L (PEG-rHuMGDF) and stem-cell factor (SCF) to study in vitro megakaryocytopoiesis. In all cases, under an inverted microscope, normal MK differentiation and proplatelet formation were observed. At the ultrastructural level, there was also no abnormality in MK maturation, and normal filamentous MK were present. Moreover, the in vitro produced platelets had a normal size, while peripheral blood platelets of the same patients exhibited an abnormally small size. However, despite this normal platelet production, we observed that F-actin distribution was abnormal in MKs from WAS patients. Indeed, F-actin was regularly and linearly distributed under the cytoplasmic membrane in normal MKs, but it was found concentrated in the center of the WAS MKs. After adhesion, normal MKs extended very long filopodia in which WASP could be detected. In contrast, MKs from WAS patients showed shorter and less numerous filopodia. However, despite this abnormal filopodia formation, MKs from WAS patients normally migrated in response to stroma-derived factor-1alpha (SDF-1alpha), and actin normally polymerized after SDF-1alpha or thrombin stimulation. These results suggest that the platelet defect in WAS patients is not due to abnormal platelet production, but instead to cytoskeletal changes occuring in platelets during circulation.

[Measurement of immunologic agglutination of erythrocytes by ultrasonic interferometry]

Ultrasound interferometry is a new methodology which has been developed in our laboratories in order to measure precisely and quickly the size of particles sedimenting in liquid on horizontal surface, upon gravity. Applied to red blood cells, this method evaluates the sedimentation of erythrocytes, their aggregation induced by proteins or aggregating compounds as well as their agglutination upon immune reactions. The quantitative assessment of red cell agglutination was applied to the study of blood groups and to the search for red cell antibodies. Preliminary results show that ultrasound interferometry is 1) quantitative, measuring the size of agglutinates; 2) sensitive; 3) specific; 4) fast; 5) able to detect irregular antibodies.

Development and use of a computer system in a radiotherapy department: SISGRAD

SISGRAD, the interactive computer system of the Antoine-Lacassagne Cancer Center Radiotherapy Department, has been operational since January 1982. It completes the computerized dosimetry system installed several years earlier and is fully integrated with the institution's central network. SISGRAD is in charge of surveillance of the radiotherapy treatments given by the Center's three radiotherapy units (1400 patients per year); it is also used for administrative purposes in the Department and physically connects all of the Department's operating stations. SISGRAD consists of a series of microcomputers connected to a common mass memory; each microcomputer is used as an intelligent console. SISGRAD was developed to guarantee that the treatments comply with prescriptions, to supply extemporaneous dosimetric data, to improve administrative work, and to supply banks with data for statistical analysis and research. SISGRAD actively intervenes to guarantee treatment quality and helps to improve therapy-related security factors. The present text describes the results of clinical use over a 4-year period. The consequences of integration of the system within the Department are analyzed, with special emphasis being placed on SISGRAD's role in the prevention and detection of errors in treatment prescription and delivery.