Knock-in Allele Research Articles

Abstract 837: Different Disease States in Heart Have Distinct Cardiac Interstitial Cells Contributing to Fibrosis

Cardiac fibrosis is a grim consequence for almost all myocardial injuries. In myocardial infarction (MI), what starts as a protective scarring process to prevent ventricular wall rupture becomes a pathological remodeling of the tissue with the accumulation of excess extracellular matrix (ECM) proteins. Eventually, this adaptation impedes the mechanical and electrical properties of the myocardium resulting in heart failure. Recently, we showed that periostin (Postn) expressing resident cardiac fibroblasts (CFs) are a potential therapeutic target since they differentiate into the scar associated, matrix-producing myofibroblasts (MFs) after injury. In fact, deletion of these cells after an acute injury eliminates interstitial fibrosis but results in ventricular rupture which is a hallmark outcome of impaired ECM deposition during the acute phase of MI. On the other hand, ablation of these cells during a chronic injury such as pressure overload-induced cardiac fibrosis model, we observe sustained perivascular fibrosis. Previous studies report heterogeneity of origin and function for ECM producing cells associated with different cardiac diseases. Here we utilized several novel mouse models that permit lineage tracing of all activated MFs as well as perivascular mural cells in the heart to elucidate the role and fate of these distinct cardiac interstitial cells during fibrogenesis. Cells were lineage traced with a tamoxifen-inducible cre recombinase cDNA knock-in alleles (PostnMCM and Gli1CreER) in combination with a Rosa26-eGFP cre-dependent reporter. Hearts subjected to MI, TAC or Angiotensin injury were processed for extensive histological and RNAseq analyses. Results show that interstitial fibrosis in acute MI injury is a result of Postn+ MFs activity, whereas a subset of Gli1+ mural cells are responsible for the perivascular fibrosis observed after pressure overload models. Therefore, we concluded that pathological ECM deposition resulting in fibrosis comes from disease-specific specialized sub-populations of interstitial cells of the heart with distinct gene expressions and require manipulation of alternative cell- and state-specific therapeutic targets.

Expression of Lhx6 in the adult and developing mouse retina

PurposeTo investigate the expression patterns of LIM Homeobox 6 (Lhx6) in the adult and developing mouse retina. MethodsThe Lhx6-GFP knock-in allele was used to activate constitutive expression of a GFP reporter in Lhx6 expressing cells. Double labeling with GFP and retinal markers in the mouse retina at postnatal day 56 (P56) was performed to identify the cell types expressing Lhx6. To determine the neuronal cell types that express Lhx6, double labeling with GFP and various retinal markers was employed in the differentiating retina at P7 and P15. ResultsGFP + Lhx6 lineage cells were determined in Brn3a + retinal ganglion cells (RGCs), ChAT + amacrine cells (ACs), and Islet-class LIM-homeodomain 1 (Isl1+) ACs in the mouse retina at P56. In the ganglion cell layer (GCL), Lhx6 was expressed in Brn3a + RGCs but not Brn3b + RGCs at P15. Moreover, in the inner nuclear layer (INL), Lhx6 was not expressed in Bhlhb5+ ACs at P15. However, Lhx6 was weakly expressed in Glyt1+ ACs and Pax6+ ACs, and strongly expressed in Isl1+ and ChAT + ACs at P15. ConclusionLhx6 was expressed in RGCs and ACs in both the adult and developing mouse retina.

Generation and validation of novel conditional flox and inducible Cre alleles targeting fibroblast growth factor 18 (Fgf18).

Fibroblast growth factor 18 (FGF18) functions in the development of several tissues, including the lung, limb bud, palate, skeleton, central nervous system, and hair follicle. Mice containing a germline knockout of Fgf18 (Fgf18 -/- ) die shortly after birth. Postnatally, FGF18 is being evaluated for pathogenic roles in fibrosis and several types of cancer. The specific cell types that express FGF18 have been difficult to identify, and the function of FGF18 in postnatal development and tissue homeostasis has been hampered by the perinatal lethality of Fgf18 null mice. We engineered a floxed allele of Fgf18 (Fgf18 flox ) that allows conditional gene inactivation and a CreERT2 knockin allele (Fgf18 CreERT2 ) that allows the precise identification of cells that express Fgf18 and their lineage. We validated the Fgf18 flox allele by targeting it in mesenchymal tissue and primary mesoderm during embryonic development, resulting in similar phenotypes to those observed in Fgf18 null mice. We also use the Fgf18 CreERT2 allele, in combination with a conditional fluorescent reporter to confirm known and identify new sites of Fgf18 expression. These alleles will be useful to investigate FGF18 function during organogenesis and tissue homeostasis, and to target specific cell lineages at embryonic and postnatal time points.

Abstract 1884: Development of a novel VIM-RFP reporter line for colorectal cancer EMT study and drug discovery

Abstract Epithelial-to-Mesenchymal Transition (EMT) describes a dynamic and reversible process by which cells lose their epithelial characteristics and acquire mesenchymal properties. Accumulating evidence indicates that EMT displays an array of intermediate states, a phenotype referred to as “partial MET”. EMT is executed in response to signaling pathway molecules and microRNAs (miRNAs) that induce the expression of specific EMT associated transcription factors (EMT-TFs), including Zeb1/2, Snail1/2, and Twist. There is an ever-growing body of research and clinical evidence indicating that EMT plays an important role in cancer cell dissemination and distal metastasis. Therefore, targeting EMT is considered a novel opportunity in anti-cancer treatment and drug development. Vimentin, a hallmark of mesenchymal cells, is of increasing interest as a novel anti-cancer therapeutic drug target. In colorectal cancer (CRC) patients, increased expression of the vimentin protein predicts a poor prognosis. Here, we developed a novel CRC HCT-116 VIM-RFP reporter line using CRISPR/Cas9 technology. In this cell line, the RFP reporter was incorporated into the endogenous vimentin gene just before the stop codon at the last exon, enabling real-time monitoring of EMT states in live cells. The VIM-RFP knock-in allele was confirmed at genomic, transcriptional, and translational levels. Bio-functional evaluation data revealed that miRNA-200 inhibitor treatment induced the increased expression of VIM-RFP, and decreased expression of E-cadherin, a hallmark of epithelial cells. The expression of EMT-TFs ZEB-1 and ZEB-2 was also upregulated upon induction. In addition, we showed that induced VIM-RFP cells displayed increased migration capacity. These data suggested that miRNA-200 inhibitor induced VIM-RFP cells have undergone EMT. Azacitidine, a clinically approved demethylating agent, has been extensively evaluated in a number of clinical trials as a treatment for CRC patients. Studies reported that azacitidine can induce MET in a number of cancer cell lines. We showed that azacitidine treatment of VIM-RFP cells can effectively induce RFP expression, suggesting a potential application of this VIM-RFP reporter line as a platform for drug evaluation and compound screening. Taken together, the HCT-116 Vim-RFP EMT reporter line could be a valuable tool for dissecting the molecular mechanisms underlying EMT and for evaluating or screening compounds targeting EMT in CRC. Citation Format: Weiguo Shu, Diana Douglas, Sangeeta Kumari, Luis Romero, Luis G. Rodriguez, Chaozhong Zou, Robert Newman. Development of a novel VIM-RFP reporter line for colorectal cancer EMT study and drug discovery [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 1884.

Chondrocyte-Specific RUNX2 Overexpression Accelerates Post-traumatic Osteoarthritis Progression in Adult Mice.

RUNX2 is a transcription factor critical for chondrocyte maturation and normal endochondral bone formation. It promotes the expression of factors catabolic to the cartilage extracellular matrix and is upregulated in human osteoarthritic cartilage and in murine articular cartilage following joint injury. To date, in vivo studies of RUNX2 overexpression in cartilage have been limited to forced expression in osteochondroprogenitor cells preventing investigation into the effects of chondrocyte-specific RUNX2 overexpression in postnatal articular cartilage. Here, we used the Rosa26Runx2 allele in combination with the inducible Col2a1CreERT2 transgene or the inducible AcanCreERT2 knock-in allele to achieve chondrocyte-specific RUNX2 overexpression (OE) during embryonic development or in the articular cartilage of adult mice, respectively. RUNX2 OE was induced at embryonic day 13.5 (E13.5) for all developmental studies. Histology and in situ hybridization analyses suggest an early onset of chondrocyte hypertrophy and accelerated terminal maturation in the limbs of the RUNX2 OE embryos compared to control embryos. For all postnatal studies, RUNX2 OE was induced at 2 months of age. Surprisingly, no histopathological signs of cartilage degeneration were observed even 6 months following induction of RUNX2 OE. Using the meniscal/ligamentous injury (MLI), a surgical model of knee joint destabilization and meniscal injury, however, we found that RUNX2 OE accelerates the progression of cartilage degeneration following joint trauma. One month following MLI, the numbers of MMP13-positive and TUNEL-positive chondrocytes were significantly greater in the articular cartilage of the RUNX2 OE joints compared to control joints and 2 months following MLI, histomorphometry and Osteoarthritis Research Society International (OARSI) scoring revealed decreased cartilage area in the RUNX2 OE joints. Collectively, these results suggest that although RUNX2 overexpression alone may not be sufficient to initiate the OA degenerative process, it may predetermine the rate of OA onset and/or progression following traumatic joint injury. © 2019 American Society for Bone and Mineral Research.

Isocitrate dehydrogenase 1-mutated cancers are sensitive to the green tea polyphenol epigallocatechin-3-gallate

BackgroundMutations in isocitrate dehydrogenase 1 (IDH1) occur in various types of cancer and induce metabolic alterations resulting from the neomorphic activity that causes production of D-2-hydroxyglutarate (D-2-HG) at the expense of α-ketoglutarate (α-KG) and NADPH. To overcome metabolic stress induced by these alterations, IDH-mutated (IDHmut) cancers utilize rescue mechanisms comprising pathways in which glutaminase and glutamate dehydrogenase (GLUD) are involved. We hypothesized that inhibition of glutamate processing with the pleiotropic GLUD-inhibitor epigallocatechin-3-gallate (EGCG) would not only hamper D-2-HG production, but also decrease NAD(P)H and α-KG synthesis in IDHmut cancers, resulting in increased metabolic stress and increased sensitivity to radiotherapy.MethodsWe performed 13C-tracing studies to show that HCT116 colorectal cancer cells with an IDH1R132H knock-in allele depend more on glutaminolysis than on glycolysis for the production of D-2-HG. We treated HCT116 cells, HCT116-IDH1R132H cells, and HT1080 cells (carrying an IDH1R132C mutation) with EGCG and evaluated D-2-HG production, cell proliferation rates, and sensitivity to radiotherapy.ResultsSignificant amounts of 13C from glutamate accumulate in D-2-HG in HCT116-IDH1wt/R132H but not in HCT116-IDH1wt/wt. Preventing glutamate processing in HCT116-IDH1wt/R132H cells with EGCG resulted in reduction of D-2-HG production. In addition, EGCG treatment decreased proliferation rates of IDH1mut cells and at high doses sensitized cancer cells to ionizing radiation. Effects of EGCG in IDH-mutated cell lines were diminished by treatment with the IDH1mut inhibitor AGI-5198.ConclusionsThis work shows that glutamate can be directly processed into D-2-HG and that reduction of glutamatolysis may be an effective and promising new treatment option for IDHmut cancers.

A Publicly Guided Effort to Rapidly Produce and Disseminate 100 Intersectional Genetic Alleles for the Study of Nervous System Circuit Development and Function in the Mouse

The goal of this project is to build and distribute a community library of more than 100 mouse intersectional genetic alleles to facilitate the developmental, anatomical, molecular, and functional characterization of neural circuit organization in behavior and physiology based on public input. Even within narrowly defined cell types, significant diversity is found at multiple levels including genetic and molecular signatures, activity patterns, and synaptic connectivity. Current challenges now center on tools to identify, access, and study cell populations with increasing precision. Intersectional genetics offers exceptionally high resolution to consistently delineate distinct cell types in the embryo and adult mouse for functional, molecular and anatomical studies. Intersectional genetics utilizes a ubiquitously expressed conditional allele that is activated by Cre and Flp site specific recombinases. Upon activation, the intersectional genetic alleles may express any number of Genetically Encodable Effector Molecules (GEEMS), such as channelrhodopsins for neural activity perturbations or an L10‐GFP ribotrap fusion to affinity purify translating mRNAs. The intersectional allele is activated by overlapping expression of both Flp and Cre recombinases in the same cell. Traditionally, these recombinases have been deployed as gene knock‐in or transgenic alleles that are designed to express in a cell or gene specific fashion. The use of two selectors (Cre and Flp) to define a cellular population enables high specificity and modularity in combining any set of Cre, Flp and intersectional alleles to fit an experiment. The number of Cre and Flp recombinase mouse lines are constantly growing, giving greater access to increasing numbers of cell types. Additionally, these recombinases are the focus of multiple efforts to deploy them in ways that select cells based on other unique properties such as neuronal activity and synaptic connectivity. To make this powerful genetic approach more accessible to as many research laboratories as possible, we are building a production pipeline to create a suite of resources for anyone to easily make their own intersectional mouse line, to produce over 100 targeted ES cell lines that can be developed into mouse lines, as well as 10–15 high demand mouse lines all for public distribution. Priority for the GEEMS to be intersectionally deployed will be based on a public input collected at mouseintersectionalgenetics.org. All plasmids, cell lines and mouse lines developed under this effort will be distributed without restriction to non‐profit research organizations.Support or Funding InformationMcNair Medical InstituteR01HL130249R01HL130249‐02S1This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Expanding the CRISPR Toolbox in Zebrafish for Studying Development and Disease.

The study of model organisms has revolutionized our understanding of the mechanisms underlying normal development, adult homeostasis, and human disease. Much of what we know about gene function in model organisms (and its application to humans) has come from gene knockouts: the ability to show analogous phenotypes upon gene inactivation in animal models. The zebrafish (Danio rerio) has become a popular model organism for many reasons, including the fact that it is amenable to various forms of genetic manipulation. The RNA-guided CRISPR/Cas9-mediated targeted mutagenesis approaches have provided powerful tools to manipulate the genome toward developing new disease models and understanding the pathophysiology of human diseases. CRISPR-based approaches are being used for the generation of both knockout and knock-in alleles, and also for applications including transcriptional modulation, epigenome editing, live imaging of the genome, and lineage tracing. Currently, substantial effort is being made to improve the specificity of Cas9, and to expand the target coverage of the Cas9 enzymes. Novel types of naturally occurring CRISPR systems [Cas12a (Cpf1); engineered variants of Cas9, such as xCas9 and SpCas9-NG], are being studied and applied to genome editing. Since the majority of pathogenic mutations are single point mutations, development of base editors to convert C:G to T:A or A:T to G:C has further strengthened the CRISPR toolbox. In this review, we provide an overview of the increasing number of novel CRISPR-based tools and approaches, including lineage tracing and base editing.

Identification of Enteroendocrine Regulators by Real-Time Single-Cell Differentiation Mapping

Homeostatic regulation of the intestinal enteroendocrine lineage hierarchy is a poorly understood process. We resolved transcriptional changes during enteroendocrine differentiation in real time at single-cell level using a novel knockin allele of Neurog3, the master regulator gene briefly expressed at the onset of enteroendocrine specification. A bi-fluorescent reporter, Neurog3Chrono, measures time from the onset of enteroendocrine differentiation and enables precise positioning of single-cell transcriptomes along an absolute time axis. This approach yielded a definitive description of the enteroendocrine hierarchy and its sub-lineages, uncovered differential kinetics between sub-lineages, and revealed time-dependent hormonal plasticity in enterochromaffin and L cells. The time-resolved map of transcriptional changes predicted multiple novel molecular regulators. Nine of these were validated by conditional knockout in mice or CRISPR modification in intestinal organoids. Six novel candidate regulators (Sox4, Rfx6, Tox3, Myt1, Runx1t1, and Zcchc12) yielded specific enteroendocrine phenotypes. Our time-resolved single-cell transcriptional map presents a rich resource to unravel enteroendocrine differentiation.

Mice carrying a humanized Foxp2 knock-in allele show region-specific shifts of striatal Foxp2 expression levels

Genetic and clinical studies of speech and language disorders are providing starting points to unravel underlying neurobiological mechanisms. The gene encoding the transcription factor FOXP2 has been the first example of a gene involved in the development and evolution of this human-specific trait. A number of autosomal-dominant FOXP2 mutations are associated with developmental speech and language deficits indicating that gene dosage plays an important role in the disorder. Comparative genomics studies suggest that two human-specific amino acid substitutions in FOXP2 might have been positively selected during human evolution. A knock-in mouse model carrying these two amino acid changes in the endogenous mouse Foxp2 gene (Foxp2hum/hum) shows profound changes in striatum-dependent behaviour and neurophysiology, supporting a functional role for these changes. However, how this affects Foxp2 expression patterns in different striatal regions and compartments has not been assessed. Here, we characterized Foxp2 protein expression patterns in adult striatal tissue in Foxp2hum/hum mice. Consistent with prior reports in wildtype mice, we find that striatal neurons in Foxp2hum/hum mice and wildtype littermates express Foxp2 in a range from low to high levels. However, we observe a shift towards more cells with higher Foxp2 expression levels in Foxp2hum/hum mice, significantly depending on the striatal region and the compartment. As potential behavioural readout of these shifts in Foxp2 levels across striatal neurons, we employed a morphine sensitization assay. While we did not detect differences in morphine-induced hyperlocomotion during acute treatment, there was an attenuated hyperlocomotion plateau during sensitization in Foxp2hum/hum mice. Taken together, these results suggest that the humanized Foxp2 allele in a mouse background is associated with a shift in striatal Foxp2 protein expression pattern.

Homologous recombination-mediated targeted integration in monkey embryos using TALE nucleases

BackgroundNon-human primate (NHP) models can closely mimic human physiological functions and are therefore highly valuable in biomedical research. Genome editing is now developing rapidly due to the precision and efficiency offered by engineered site-specific endonuclease-based systems, such as transcription activator-like effector nucleases (TALENs) and the clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein-9 nuclease (Cas9) system. It has been demonstrated that these programmable nucleases can introduce genetic changes in embryos from many species including NHPs. In 2014, we reported the first genetic editing of macaques using TALENs and CRISPR/Cas9. Subsequently, we characterized the phenotype of a methyl CpG binding protein 2 (MECP2)-mutant cynomolgus monkey model of Rett syndrome generated using the TALEN approach. These efforts not only accelerated the advance of modeling genetic diseases in NHPs, but also encouraged us to develop specific gene knock-in monkeys. In this study, we assess the possibility of homologous recombination (HR)-mediated gene replacement using TALENs in monkeys, and generate preimplantation embryos carrying an EmGFP fluorescent reporter constructed in the OCT4 gene.ResultWe assembled a pair of TALENs specific to the first exon of the OCT4 gene and constructed a donor vector consisting of the homology arms cloned from the monkey genome DNA, flanking an EmGFP cassette. Next, we co-injected the TALENs-coding plasmid and donor plasmid into the cytoplasm of 122 zygotes 6–8 h after fertilization. Sequencing and immunofluorescence revealed that the OCT4-EmGFP knock-in allele had been successfully generated by TALENs-mediated HR at an efficiency of 11.3% (7 out of 62) or 11.1% (1 out of 9), respectively, in monkey embryos.ConclusionWe have successfully, for the first time, obtained OCT4-EmGFP knock-in monkey embryos via HR mediated by TALENs. Our results suggest that gene targeting through TALEN-assisted HR is a useful approach to introduce precise genetic modification in NHPs.

The Y227N mutation affects bestrophin-1 protein stability and impairs sperm function in a mouse model of Best vitelliform macular dystrophy.

ABSTRACTHuman bestrophin-1 (BEST1) is an integral membrane protein known to function as a Ca2+-activated and volume-regulated chloride channel. The majority of disease-associated mutations in BEST1 constitute missense mutations and were shown in vitro to lead to a reduction in mutant protein half-life causing Best disease (BD), a rare autosomal dominant macular dystrophy. To further delineate BEST1-associated pathology in vivo and to provide an animal model useful to explore experimental treatment efficacies, we have generated a knock-in mouse line (Best1Y227N). Heterozygous and homozygous mutants revealed no significant ocular abnormalities up to 2 years of age. In contrast, knock-in animals demonstrated a severe phenotype in the male reproductive tract. In heterozygous Best1Y227N males, Best1 protein was significantly reduced in testis and almost absent in homozygous mutant mice, although mRNA transcription of wild-type and knock-in allele is present and similar in quantity. Degradation of mutant Best1 protein in testis was associated with adverse effects on sperm motility and the capability to fertilize eggs. Based on these results, we conclude that mice carrying the Best1 Y227N mutation reveal a reproducible pathologic phenotype and thus provide a valuable in vivo tool to evaluate efficacy of drug therapies aimed at restoring Best1 protein stability and function.

Integrated Genomic and Proteomic Analysis of Murine CLL-like Cells Reveals SF3B1 Mutation to Impact DNA Damage Response and BCR Signaling

Collective large-scale sequencing efforts have unexpectedly revealed the high frequency of mutations in the splicing factor genes (SF3B1, U2AF1, SRSF2, ZRSR2) in various solid and hematological cancers, suggesting the association of splicing dysregulation with tumorigenesis. Mutations in SF3B1 occur in 5-20% of patients with chronic lymphocytic leukemia (CLL) and are associated with poorer overall survival and chemotherapy resistance. These mutations are restricted to hotspots (>50% at K700E site) and strongly co-occur with ATM mutations (loss-of-function) and deletion of 11q (ATM minimal deleted region). Numerous studies including ours have demonstrated that somatic alterations in this gene cause RNA splicing dysregulation, however, how this splicing factor mutation alone and in combination with ATM deletion impacts cellular processes and contributes to CLL remains to be fully defined.To this end, we modeled the effects of these combined alterations by crossing mice with conditional knockout of Atm and mice with a conditional knock-in allele of SF3B1 mutation (Sf3b1-K700E). We achieved B cell-restricted expression of heterozygous Sf3b1 mutation and Atm deletion by breeding these mice with CD19-Cre homozygous transgenic mice. Conditional expression of heterozygous Sf3b1-K700E mutation in mouse B cells disrupts pre-mRNA splicing, alters B-cell development, and induces a state of cellular senescence. Combined with Atm deletion in B cells led to the overcoming of cellular senescence and the development of clonal CLL cells in elderly mice at low penetrance (6%). These malignant cells could be propagated by in vivo passaging, with detectable disease within 4 weeks following transfer, thus making this mouse line amenable to further drug discovery and biologic investigations.To fully understand the underlying mechanisms of how the combined alterations led to CLL, we performed integrated genome, transcriptome, and proteome analysis using mouse CLL (DM-CLL) cells and B cells with either Sf3b1 mutation or Atm deletion, or with double genetic lesions (DM). Whole-genome sequencing of paired DNA from B cells (or DM-CLL) and non-B cell tissue (kidney) revealed the somatic mutation rate in the CLL cells to be ~0.5 mutations/Mb. Few recurrent mutations were identified among the samples. However, copy number variation analysis of DM-CLL cells revealed recurrent amplifications of chromosomes 15 and 17. RNA-seq analysis revealed that these amplifications were associated with overexpression of 835 of 987 Chr15 and Chr17 genes detected in DM-CLL vs. DM cells. Of note, 146 genes were overexpressed in human CLLs with SF3B1 mutations (DFCI cohort), compared to normal B-cells (p<0.05).Integrated transcriptome and proteome analysis of the DM-CLL cells showed coordinated dysregulation of multiple CLL-associated cellular processes with B-cell receptor (BCR) signaling as the most dramatically downregulated compared to DM cells. Since BCR signaling is a therapeutic target in CLL and has critical roles in B cell biology, we asked how SF3B1 mutation contributes to gene expression of BCR signaling. Through RNA-seq data analysis derived from two independent patient cohorts (DFCI and ICGC), we identified downregulation of BCR gene expression in SF3B1 mutant CLL cells. In line with this, human CLLs harboring SF3B1 mutations exhibit greater sensitivity to in vitro treatment with ibrutinib, and altered response kinetics in vivo to ibrutinib, per analysis of patients with SF3B1 mutations treated with ibrutinib. These studies together highlight a role of SF3B1 mutation in BCR signaling.In summary, we have generated a genetically-engineered murine model that recapitulates human CLL genetics, and presents an informative model to functionally dissect the effects of mutant SF3B1 in a B cell context. Starting from computation-based identification of recurrent co-occurring events in CLL, our study employs murine lines that express genetic alterations in an lineage-specific fashion, utilizes integrated genomics and proteomics approaches to dissect pathways that are fundamental to CLL phenotype, and more importantly, links the dysregulated pathways back to human CLL gene expression data and clinical trials to reveal novel mechanisms underlying therapeutic response. DisclosuresWiestner:Pharmacyclics LLC, an AbbVie Company: Research Funding. Wu:Neon Therapeutics: Equity Ownership.

Utx Insufficiency Cooperates with Braf V600E in the Induction of Myeloma in Mice

Introduction:Dysfunction of epigenetic pathways has been frequently implicated in hematological malignancies. In multiple myeloma (MM), EZH2, a methyltransferase that induces histone H3 lysine 27 trimethylation (H3K27me3), acts as an oncogene as evidenced by its overexpression, which was found to be positively correlated with disease progression (Kalushkova et al. PloS One. 2010). We have shown that inhibition of both EZH2 and its homolog EZH1 is effective in eradicating MM cells in vitro and in vivo (Rizq et al. Clin Cancer Res. 2017). In addition, inactivating somatic mutations in UTX/KDM6A, an X-linked histone demethylase that removes di- and tri-methyl groups from H3K27, are found in 3 - 10% of MM patients (van Haaften et al. Nat Genet. 2009 and Pawlyn et al. Clin Cancer Res. 2016), indicating a tumor suppressive role for UTX, which has yet to be delineated. Up till now, no mouse model has been generated to test Utx insufficiency in post germinal center (GC) B cells and plasma cells. On the other hand, an activating mutation V600E in the BRAF kinase gene is closely associated with aggressive disease features such as extramedullary disease and shorter overall survival in MM patients (Andrulis et al. Cancer Discov. 2013) and could accelerate induction of myeloma in mice.Methods:To investigate whether loss of Utx cooperates with Braf V600E in myelomagenesis in mice, we generated and analyzed mice with conditional knock-out allele of Utx and/or knock-in allele of Braf V600E combined with Cγ1-Cre allele, in which Cre is activated by immunization in post GC B cells.Results:Loss of Utx and Braf V600E synergistically induced post GC B-cell lymphoma and plasma cell neoplasms in mice and significantly shortened the survival of mice compared with control mice and either allele alone. Utx-/-Braf V600E females succumbed to death earlier than Utx-/YBraf V600Emales and Utx-/+Braf V600Efemales. Of note, plasma cell neoplasms developed at a high frequency in Utx-/YBraf V600Emales and Utx-/+Braf V600Efemales and, less frequently, in Utx-/-Braf V600E females. Mice with plasma cell neoplasms showed expansion of CD138+ plasma cells in bone marrow as well as spleen and/or lymph nodes, exhibiting extramedullary disease. Loss of Utx alleles and expression of Braf V600E were confirmed by genomic PCR of plasma cells. Importantly, the clonality of plasma cells was demonstrated by genomic PCR detecting rearrangements of immunoglobulin heavy and light chain genes. In addition, M protein was detected by serum protein electrophoresis (SPEP) at a high frequency. Notably, we were able to establish murine myeloma cell lines from moribund compound mice. These cells readily engrafted in the bone marrow of NOG mice after transplantation and caused myeloma-associated phenotypes including paraplegia in recipient mice. Interestingly, Utx-/-Braf V600E cells were sensitive to dual inhibition of EZH2 and EZH1 but not to specific inhibition of EZH2 in culture. They also showed decreased susceptibility to proteasome inhibitors when compared with human MM cell lines. To gain insight into the changes in the transcriptional landscape following Utx loss, we performed RNA sequencing (RNA seq) and then gene set enrichment analysis (GSEA). We found positive enrichment of gene sets related to Myc, implying that Myc is one of the main drivers of myelomagenesis in our mouse model. In addition, gene sets related to MM were significantly enriched following Utx loss. We are now working on ChIP sequencing (ChIP seq) of UTX-related histone modifications to evaluate the epigenetic impact of Utx loss on myelomagenesis.Conclusion:Utx insufficiency cooperates with Braf V600E in the induction of myeloma in mice. Our mouse model is a promising tool for understanding the role of epigenetic dysregulation in the pathogenesis of MM and evaluating novel anti-myeloma agents. DisclosuresOkuno:Celgene: Research Funding. Tamaru:Nichirei Bioscience INC.: Research Funding; Takeda Pharmaceutical Company Limited: Speakers Bureau.

Generation and characterization of Megf6 null and Cre knock-in alleles.

Megf6, a member of MEGF (multiple EGF-like domains) protein family, is a conserved high molecular weight protein with 30 EGF-like domains. Although many members of the MEGF protein family are essential for embryonic development and homeostasis, the role of Megf6 in development and physiology is still unknown. Here, we generated Megf6-deficient mice using CRISPR-Cas9 technique and showed that Megf6 is dispensable for embryonic development. We also constructed the Megf6Cre allele to study Megf6-expressing cell lineages. Our results showed that Megf6-expressing cells contribute to the periotic mesenchyme and its derivatives, skin epidermis, certain cells in brain and ribs. Therefore, the Megf6Cre allele can be a useful tool for conditional deletion in these tissues, in particular for periotic mesenchyme deletion.

Induction of Lrp5 HBM-causing mutations in Cathepsin-K expressing cells alters bone metabolism.

High-bone-mass (HBM)-causing missense mutations in the low density lipoprotein receptor-related protein-5 (Lrp5) are associated with increased osteoanabolic action and protection from disuse- and ovariectomy-induced osteopenia. These mutations (e.g., A214V and G171V) confer resistance to endogenous secreted Lrp5/6 inhibitors, such as sclerostin (SOST) and Dickkopf homolog-1 (DKK1). Cells in the osteoblast lineage are responsive to canonical Wnt stimulation, but recent work has indicated that osteoclasts exhibit both indirect and direct responsiveness to canonical Wnt. Whether Lrp5-HBM receptors, expressed in osteoclasts, might alter osteoclast differentiation, activity, and consequent net bone balance in the skeleton, is not known. To address this, we bred mice harboring heterozygous Lrp5 HBM-causing conditional knock-in alleles to Ctsk-Cre transgenic mice and studied the phenotype using DXA, μCT, histomorphometry, serum assays, and primary cell culture. Mice with HBM alleles induced in Ctsk-expressing cells (TG) exhibited higher bone mass and architectural properties compared to non-transgenic (NTG) counterparts. In vivo and in vitro measurements of osteoclast activity, population density, and differentiation yielded significant reductions in osteoclast-related parameters in female but not male TG mice. Droplet digital PCR performed on osteocyte enriched cortical bone tubes from TG and NTG mice revealed that ~8-17% of the osteocyte population (depending on sex) underwent recombination of the conditional Lrp5 allele in the presence of Ctsk-Cre. Further, bone formation parameters in the midshaft femur cortex show a small but significant increase in anabolic action on the endocortical but not periosteal surface. These findings suggest that Wnt/Lrp5 signaling in osteoclasts affects osteoclastogenesis and activity in female mice, but also that some of the changes in bone mass in TG mice might be due to Cre expression in the osteocyte population.

Creating Knockin Alleles in Mouse Embryonic Stem Cells by CRISPR/Cas9-Mediated Homologous Recombination Without Drug Selection.

The rapidly evolving CRISPR/Cas9-mediated genome editing provides the convenience of genome manipulation directly in mouse zygotes for a number of genomic manipulations; but knockins of large insertions prove to be relatively inefficient at least with double-strand DNA as targeting constructs. Here, we describe an alternative approach to the direct genome editing in mouse zygotes by generating knockin alleles in mouse embryonic stem cells first with CRIPSR-mediated homologous recombination. Our results show this approach is efficient and requires no drug selection in mouse embryonic stem cells as in classic gene targeting. As the result, knockin alleles across many target loci are created in mouse embryonic stem cells and readily transmitted through germline. The knockin alleles created in ES cells can also serve as valuable tools for in vitro stem cell differentiation.

Feedback regulation of cytoneme-mediated transport shapes a tissue-specific FGF morphogen gradient.

Gradients of signaling proteins are essential for inducing tissue morphogenesis. However, mechanisms of gradient formation remain controversial. Here we characterized the distribution of fluorescently-tagged signaling proteins, FGF and FGFR, expressed at physiological levels from the genomic knock-in alleles in Drosophila. FGF produced in the larval wing imaginal-disc moves to the air-sac-primordium (ASP) through FGFR-containing cytonemes that extend from the ASP to contact the wing-disc source. The number of FGF-receiving cytonemes extended by ASP cells decreases gradually with increasing distance from the source, generating a recipient-specific FGF gradient. Acting as a morphogen in the ASP, FGF activates concentration-dependent gene expression, inducing pointed-P1 at higher and cut at lower levels. The transcription-factors Pointed-P1 and Cut antagonize each other and differentially regulate formation of FGFR-containing cytonemes, creating regions with higher-to-lower numbers of FGF-receiving cytonemes. These results reveal a robust mechanism where morphogens self-generate precise tissue-specific gradient contours through feedback regulation of cytoneme-mediated dispersion.

Impaired hematopoiesis and leukemia development in mice with a conditional knock-in allele of a mutant splicing factor gene U2af1

Mutations affecting the spliceosomal protein U2AF1 are commonly found in myelodysplastic syndromes (MDS) and secondary acute myeloid leukemia (sAML). We have generated mice that carry Cre-dependent knock-in alleles of U2af1(S34F), the murine version of the most common mutant allele of U2AF1 encountered in human cancers. Cre-mediated recombination in murine hematopoietic lineages caused changes in RNA splicing, as well as multilineage cytopenia, macrocytic anemia, decreased hematopoietic stem and progenitor cells, low-grade dysplasias, and impaired transplantability, but without lifespan shortening or leukemia development. In an attempt to identify U2af1(S34F)-cooperating changes that promote leukemogenesis, we combined U2af1(S34F) with Runx1 deficiency in mice and further treated the mice with a mutagen, N-ethyl-N-nitrosourea (ENU). Overall, 3 of 16 ENU-treated compound transgenic mice developed AML. However, AML did not arise in mice with other genotypes or without ENU treatment. Sequencing DNA from the three AMLs revealed somatic mutations homologous to those considered to be drivers of human AML, including predicted loss- or gain-of-function mutations in Tet2, Gata2, Idh1, and Ikzf1 However, the engineered U2af1(S34F) missense mutation reverted to WT in two of the three AML cases, implying that U2af1(S34F) is dispensable, or even selected against, once leukemia is established.

Laminin γ1 C-terminal Glu to Gln mutation induces early postimplantation lethality.

Laminin-integrin interactions regulate various adhesion-dependent cellular processes. γ1C-Glu, the Glu residue in the laminin γ1 chain C-terminal tail, is crucial for the binding of γ1-laminins to several integrin isoforms. Here, we investigated the impact of γ1C Glu to Gln mutation on γ1-laminin binding to all possible integrin partners in vitro, and found that the mutation specifically ablated binding to α3, α6, and α7 integrins. To examine the physiological significance of γ1C-Glu, we generated a knock-in allele, Lamc1 EQ , in which the γ1C Glu to Gln mutation was introduced. Although Lamc1 EQ/EQ homozygotes developed into blastocysts and deposited laminins in their basement membranes, they died just after implantation because of disordered extraembryonic development. Given the impact of the Lamc1 EQ allele on embryonic development, we developed a knock-in mouse strain enabling on-demand introduction of the γ1C Glu to Gln mutation by the Cre-loxP system. The present study has revealed a crucial role of γ1C-Glu-mediated integrin binding in postimplantation development and provides useful animal models for investigating the physiological roles of laminin-integrin interactions in vivo.