The DNA methyltransferase inhibitor 5-aza-4'-thio-2'-deoxycytidine induces C>G transversions and acute lymphoid leukemia development.

DNA methyltransferase inhibitors (DNMTi), most commonly cytidine analogs, are compounds that decrease 5'-cytosine methylation. DNMTi are used clinically based on the hypothesis that cytosine demethylation will lead to re-expression of tumor suppressor genes. 5-Aza-4'-thio-2'-deoxycytidine (Aza TdCyd or ATC) is a recently described thiol substituted DNMTi that has been shown to have anti-tumor activity in solid tumor models. Here, we investigated the therapeutic potential of ATC in a murine transplantation model of myelodysplastic syndrome. ATC treatment led to transformation of transplanted wild-type bone marrow nucleated cells into lymphoid leukemia, and healthy mice treated with ATC also developed lymphoid leukemia. Whole exome sequencing revealed thousands of acquired mutations, almost all of which were C>G transversions in a specific 5'-NCG-3' context. These mutations involved dozens of genes involved in human lymphoid leukemia, such as Notch1, Pten, Pax5, Trp53, and Nf1. Human cells treated in vitro with ATC showed thousands of acquired C>G transversions in a similar context. Deletion of Dck, the rate-limiting enzyme for the cytidine salvage pathway, eliminated C>G transversions. Taken together, these findings demonstrate a highly penetrant mutagenic and leukemogenic phenotype associated with ATC.

Mcm2 hypomorph leads to acute leukemia or hematopoietic stem cell failure, dependent on genetic context.

Minichromosome maintenance proteins (Mcm2-7) form a hexameric complex that unwinds DNA ahead of a replicative fork. The deficiency of Mcm proteins leads to replicative stress and consequent genomic instability. Mice with a germline insertion of a Cre cassette into the 3'UTR of the Mcm2 gene (designated Mcm2Cre ) have decreased Mcm2 expression and invariably develop precursor T-cell lymphoblastic leukemia/lymphoma (pre-T LBL), due to 100-1000 kb deletions involving important tumor suppressor genes. To determine whether mice that were protected from pre-T LBL would develop non-T-cell malignancies, we used two approaches. Mice engrafted with Mcm2Cre/Cre Lin- Sca-1+ Kit+ hematopoietic stem/progenitor cells did not develop hematologic malignancy; however, these mice died of hematopoietic stem cell failure by 6 months of age. Placing the Mcm2Cre allele onto an athymic nu/nu background completely prevented pre-T LBL and extended survival of these mice three-fold (median 296.5 vs. 80.5 days). Ultimately, most Mcm2Cre/Cre ;nu/nu mice developed B-cell precursor acute lymphoblastic leukemia (BCP-ALL). We identified recurrent deletions of 100-1000 kb that involved genes known or suspected to be involved in BCP-ALL, including Pax5, Nf1, Ikzf3, and Bcor. Moreover, whole-exome sequencing identified recurrent mutations of genes known to be involved in BCP-ALL progression, such as Jak1/Jak3, Ptpn11, and Kras. These findings demonstrate that an Mcm2Cre/Cre hypomorph can induce hematopoietic dysfunction via hematopoietic stem cell failure as well as a "deletor" phenotype affecting known or suspected tumor suppressor genes.

Immuno-transcriptomic profiling of extracranial pediatric solid malignancies.

We perform an immunogenomics analysis utilizing whole-transcriptome sequencing of 657 pediatric extracranial solid cancer samples representing 14 diagnoses, and additionally utilize transcriptomes of 131 pediatric cancer cell lines and 147 normal tissue samples for comparison. We describe patterns of infiltrating immune cells, T cell receptor (TCR) clonal expansion, and translationally relevant immune checkpoints. We find that tumor-infiltrating lymphocytes and TCR counts vary widely across cancer types and within each diagnosis, and notably are significantly predictive of survival in osteosarcoma patients. We identify potential cancer-specific immunotherapeutic targets for adoptive cell therapies including cell-surface proteins, tumor germline antigens, and lineage-specific transcription factors. Using an orthogonal immunopeptidomics approach, we find several potential immunotherapeutic targets in osteosarcoma and Ewing sarcoma and validated PRAME as a bona fide multi-pediatric cancer target. Importantly, this work provides a critical framework for immune targeting of extracranial solid tumors using parallel immuno-transcriptomic and -peptidomic approaches.

Mutant Idh2 Cooperates with a NUP98-HOXD13 Fusion to Induce Early Immature Thymocyte Precursor ALL.

Mutations in the isocitrate dehydrogenase 1 (IDH1) and IDH2 genes are frequently observed in a wide variety of hematologic malignancies, including myeloid and T-cell leukemias. In this study, we generated Idh2R140Q transgenic mice to examine the role of the Idh2R140Q mutation in leukemia. No leukemia developed in Idh2R140Q transgenic mice, suggesting a need for additional genetic events for leukemia development. Because myeloid cells from NUP98-HOXD13 fusion (NHD13) transgenic mice frequently acquire somatic Idh mutations when they transform to acute myeloid leukemia, we generated Idh2R140Q/NHD13 double transgenic mice. Idh2R140Q/NHD13 transgenic mice developed an immature T-cell leukemia with an immunophenotype similar to double-negative 1 (DN1) or DN2 thymocytes. Idh2R140Q/NHD13 leukemic cells were enriched for an early thymic precursor transcriptional signature, and the gene expression profile for Idh2R140Q/NHD13 DN1/DN2 T-ALL closely matched that of human early/immature T-cell precursor (EITP) acute lymphoblastic leukemia (ALL). Moreover, recurrent mutations found in patients with EITP ALL, including KRAS, PTPN11, JAK3, SH2B3, and EZH2 were also found in Idh2R140Q/NHD13 DN1/DN2 T-ALL. In vitro treatment of Idh2R140Q/NHD13 thymocytes with enasidenib, a selective inhibitor of mutant IDH2, led to a marked decrease in leukemic cell proliferation. These findings demonstrate that Idh2R140Q/NHD13 mice can serve as a useful in vivo model for the study of early/immature thymocyte precursor acute lymphoblastic leukemia development and therapy. SIGNIFICANCE: T-cell leukemia induced in Idh2R140Q/NUP98-HOXD13 mice is immunophenotypically, transcriptionally, and genetically similar to human EITP ALL, providing a model for studying disease development and treatment.

The S-phase-induced lncRNA SUNO1 promotes cell proliferation by controlling YAP1/Hippo signaling pathway.

Cell cycle is a cellular process that is subject to stringent control. In contrast to the wealth of knowledge of proteins controlling the cell cycle, very little is known about the molecular role of lncRNAs (long noncoding RNAs) in cell-cycle progression. By performing genome-wide transcriptome analyses in cell-cycle-synchronized cells, we observed cell-cycle phase-specific induction of >2000 lncRNAs. Further, we demonstrate that an S-phase-upregulated lncRNA, SUNO1, facilitates cell-cycle progression by promoting YAP1-mediated gene expression. SUNO1 facilitates the cell-cycle-specific transcription of WTIP, a positive regulator of YAP1, by promoting the co-activator, DDX5-mediated stabilization of RNA polymerase II on chromatin. Finally, elevated SUNO1 levels are associated with poor cancer prognosis and tumorigenicity, implying its pro-survival role. Thus, we demonstrate the role of a S-phase up-regulated lncRNA in cell-cycle progression via modulating the expression of genes controlling cell proliferation.

1q21.1 deletion and a rare functional polymorphism in siblings with thrombocytopenia-absent radius-like phenotypes.

Thrombocytopenia-absent radii (TAR) syndrome, characterized by neonatal thrombocytopenia and bilateral radial aplasia with thumbs present, is typically caused by the inheritance of a 1q21.1 deletion and a single-nucelotide polymorphism in RBM8A on the nondeleted allele. We evaluated two siblings with TAR-like dysmorphology but lacking thrombocytopenia in infancy. Family NCI-107 participated in an IRB-approved cohort study and underwent comprehensive clinical and genomic evaluations, including aCGH, whole-exome, whole-genome, and targeted sequencing. Gene expression assays and electromobility shift assays (EMSAs) were performed to evaluate the variant of interest. The previously identified TAR-associated 1q21.1 deletion was present in the affected siblings and one healthy parent. Multiple sequencing approaches did not identify previously described TAR-associated SNPs or mutations in relevant genes. We discovered rs61746197 A > G heterozygosity in the parent without the deletion and apparent hemizygosity in both siblings. rs61746197 A > G overlaps a RelA-p65 binding motif, and EMSAs indicate the A allele has higher transcription factor binding efficiency than the G allele. Stimulation of K562 cells to induce megakaryocyte differentiation abrogated the shift of both reference and alternative probes. The 1q21.1 TAR-associated deletion in combination with the G variant of rs61746197 on the nondeleted allele is associated with a TAR-like phenotype. rs61746197 G could be a functional enhancer/repressor element, but more studies are required to identify the specific factor(s) responsible. Overall, our findings suggest a role of rs61746197 A > G and human disease in the setting of a 1q21.1 deletion on the other chromosome.

Thymic precursor cells generate acute myeloid leukemia in NUP98-PHF23/NUP98-HOXD13 double transgenic mice.

Transgenic mice that express either a NUP98-PHF23 (NP23) or NUP98-HOXD13 (NHD13) fusion in the hematopoietic compartment develop a wide spectrum of leukemias, including myeloid, erythroid, megakaryocytic and lymphoid, at age 9-14 months. NP23-NHD13 double transgenic mice were generated by interbreeding NP23 and NHD13 mice. Remarkably, 100% of the NP23-NHD13 double transgenic mice developed acute myeloid leukemia (AML) within three months, characterized by replacement of the thymus with leukemic myeloblasts. The marked infiltration of thymus led to the intriguing hypothesis that AML generated in NP23-NHD13 mice arose in the thymus, as opposed to the bone marrow (BM). Transplantation of CD4-CD8- double negative (DN) thymocytes (which were also negative for Mac1 and Gr1) from leukemic NHD13/NP23 mice demonstrated that DN thymocytes could transmit AML, and limiting dilution studies showed that leukemia initiating cells were increased 14-fold in the thymus compared to BM. Further thymocyte fractionation demonstrated that DN1 and DN2, but not DN3 or DN4 fractions transmitted AML, and a marked expansion (100-fold) of Lineage-Sca1 + Kit + (LSK) cells in the thymus of the NP23-NHD13 mice. Taken together, these results show that the thymus of NP23-NHD13 mice acts as a reservoir for AML initiating cells and that thymic progenitors can transmit AML.

A unique mutator phenotype reveals complementary oncogenic lesions leading to acute leukemia.

Mice homozygous for a hypomorphic allele of DNA replication factor minichromosome maintenance protein 2 (designated Mcm2cre/cre) develop precursor T cell lymphoblastic leukemia/lymphoma (pre-T LBL) with 4-32 small interstitial deletions per tumor. Mice that express a NUP98-HOXD13 (NHD13) transgene develop multiple types of leukemia, including myeloid and T and B lymphocyte. All Mcm2cre/cre NHD13+ mice develop pre-T LBL, and 26% develop an unrelated, concurrent B cell precursor acute lymphoblastic leukemia (BCP-ALL). Copy number alteration (CNA) analysis demonstrated that pre-T LBLs were characterized by homozygous deletions of Pten and Tcf3 and partial deletions of Notch1 leading to Notch1 activation. In contrast, BCP-ALLs were characterized by recurrent deletions involving Pax5 and Ptpn1 and copy number gain of Abl1 and Nup214 resulting in a Nup214-Abl1 fusion. We present a model in which Mcm2 deficiency leads to replicative stress, DNA double strand breaks (DSBs), and resultant CNAs due to errors in DNA DSB repair. CNAs that involve critical oncogenic pathways are then selected in vivo as malignant lymphoblasts because of a fitness advantage. Some CNAs, such as those involving Abl1 and Notch1, represent attractive targets for therapy.

Engineered Bcor mutations lead to acute leukemia of progenitor B-1 lymphocyte origin in a sensitized background.

Approximately 10% of NUP98-PHF23 (NP23) mice develop an aggressive acute lymphoblastic leukemia of B-1 lymphocyte progenitor origin (pro-B1 ALL), accompanied by somatic frameshift mutations of the BCL6 interacting corepressor (Bcor) gene, most commonly within a 9-bp "hotspot" in Bcor exon 8. To determine whether experimentally engineered Bcor mutations would lead to pro-B1 ALL, we used clustered, regularly interspaced, short palindromic repeats-associated protein 9 to introduce a Bcor frameshift mutation into NP23 hematopoietic stem and progenitor cells through the use of Bcor small guide RNAs (Bcor sgRNAs). Recipient mice transplanted with NP23 bone marrow or fetal liver cells that had been transduced with a Bcor sgRNA developed pro-B1 ALL, characterized by a B-1 progenitor immunophenotype, clonal Igh gene rearrangement, and Bcor indel mutation, whereas control recipients did not. Similar to a subset of human B-cell precursor ALL, the murine pro-B1 ALL had acquired somatic mutations in Jak kinase genes. JAK inhibitors (ruxolitinib and tofacitinib) inhibited the growth of pro-B1 ALL cell lines established from Bcor sgRNA/NP23 recipients at clinically achievable concentrations (100 nM). Our results demonstrate that Bcor mutations collaborate with NP23 to induce pro-B1 ALL, and that JAK inhibitors are potential therapies for pro-B1 ALL.

Rapid and reversible suppression of ALT by DAXX in osteosarcoma cells.

Many tumors maintain chromosome-ends through a telomerase-independent, DNA-templated mechanism called alternative lengthening of telomeres (ALT). While ALT occurs in only a subset of tumors, it is strongly associated with mutations in the genes ATRX and DAXX, which encode components of an H3.3 histone chaperone complex. The role of ATRX and DAXX mutations in potentiating the mechanism of ALT remains incompletely understood. Here we characterize an osteosarcoma cell line, G292, with wild-type ATRX but a unique chromosome translocation resulting in loss of DAXX function. While ATRX and DAXX form a complex in G292, this complex fails to localize to nuclear PML bodies. We demonstrate that introduction of wild type DAXX suppresses the ALT phenotype and restores the localization of ATRX/DAXX to PML bodies. Using an inducible system, we show that ALT-associated PML bodies are disrupted rapidly following DAXX induction and that ALT is again restored following withdrawal of DAXX.

Dynamics of Genome Alterations in Crohn's Disease-Associated Colorectal Carcinogenesis.

Purpose: Patients with inflammatory bowel diseases, that is, ulcerative colitis and Crohn's disease (CD), face an increased risk of developing colorectal cancer (CRC). Evidence, mainly from ulcerative colitis, suggests that TP53 mutations represent an initial step in the progression from inflamed colonic epithelium to CRC. However, the pathways involved in the evolution of CRC in patients with CD are poorly characterized.Experimental Design: Here, we analyzed 73 tissue samples from 28 patients with CD-CRC, including precursor lesions, by targeted next-generation sequencing of 563 cancer-related genes and array-based comparative genomic hybridization. The results were compared with 24 sporadic CRCs with similar histomorphology (i.e., mucinous adenocarcinomas), and to The Cancer Genome Atlas data (TCGA).Results: CD-CRCs showed somatic copy-number alterations (SCNAs) similar to sporadic CRCs with one notable exception: the gain of 5p was significantly more prevalent in CD-CRCs. CD-CRCs had a distinct mutation signature: TP53 (76% in CD-CRCs vs. 33% in sporadic mucinous CRCs), KRAS (24% vs. 50%), APC (17% vs. 75%), and SMAD3 (3% vs. 29%). TP53 mutations and SCNAs were early and frequent events in CD progression, while APC, KRAS, and SMAD2/4 mutations occurred later. In four patients with CD-CRC, at least one mutation and/or SCNAs were already present in non-dysplastic colonic mucosa, indicating occult tumor evolution.Conclusions: Molecular profiling of CD-CRCs and precursor lesions revealed an inflammation-associated landscape of genome alterations: 5p gains and TP53 mutations occurred early in tumor development. Detection of these aberrations in precursor lesions may help predicting disease progression and distinguishes CD-associated from sporadic colorectal neoplasia. Clin Cancer Res; 24(20); 4997-5011. ©2018 AACR.

PSIP1/p75 promotes tumorigenicity in breast cancer cells by promoting the transcription of cell cycle genes.

Breast cancer (BC) is a highly heterogeneous disease, both at the pathological and molecular level, and several chromatin-associated proteins play crucial roles in BC initiation and progression. Here, we demonstrate the role of PSIP1 (PC4 and SF2 interacting protein)/p75 (LEDGF) in BC progression. PSIP1/p75, previously identified as a chromatin-adaptor protein, is found to be upregulated in basal-like/triple negative breast cancer (TNBC) patient samples and cell lines. Immunohistochemistry in tissue arrays showed elevated levels of PSIP1 in metastatic invasive ductal carcinoma. Survival data analyses revealed that the levels of PSIP1 showed a negative association with TNBC patient survival. Depletion of PSIP1/p75 significantly reduced the tumorigenicity and metastatic properties of TNBC cell lines while its over-expression promoted tumorigenicity. Further, gene expression studies revealed that PSIP1 regulates the expression of genes controlling cell-cycle progression, cell migration and invasion. Finally, by interacting with RNA polymerase II, PSIP1/p75 facilitates the association of RNA pol II to the promoter of cell cycle genes and thereby regulates their transcription. Our findings demonstrate an important role of PSIP1/p75 in TNBC tumorigenicity by promoting the expression of genes that control the cell cycle and tumor metastasis.

Common Molecular Subtypes Among Asian Hepatocellular Carcinoma and Cholangiocarcinoma.

Intrahepatic cholangiocarcinoma (ICC) and hepatocellular carcinoma (HCC) are clinically disparate primary liver cancers with etiological and biological heterogeneity. We identified common molecular subtypes linked to similar prognosis among 199 Thai ICC and HCC patients through systems integration of genomics, transcriptomics, and metabolomics. While ICC and HCC share recurrently mutated genes, including TP53, ARID1A, and ARID2, mitotic checkpoint anomalies distinguish the C1 subtype with key drivers PLK1 and ECT2, whereas the C2 subtype is linked to obesity, T cell infiltration, and bile acid metabolism. These molecular subtypes are found in 582 Asian, but less so in 265 Caucasian patients. Thus, Asian ICC and HCC, while clinically treated as separate entities, share common molecular subtypes with similar actionable drivers to improve precision therapy.

Targeting MYC as a Therapeutic Intervention for Anaplastic Thyroid Cancer.

Context: Recent studies showed that transcription of the MYC gene is driven by the interaction of bromodomain and extraterminal domain (BET) proteins with acetylated histones on chromatin. JQ1, a potent inhibitor that effectively disrupts the interaction of BET proteins with acetylated histones, preferentially suppresses transcription of the MYC gene. We recently reported that JQ1 decreased thyroid tumor growth and improved survival in a mouse model of anaplastic thyroid cancer (ATC) by targeting MYC transcription. The role of MYC in human ATC and whether JQ1 can effectively target MYC as a treatment modality have not been elucidated. Objective: To understand the underlying molecular mechanisms of JQ1, we evaluated its efficacy in human ATC cell lines and xenograft models. Design: We determined the effects of JQ1 on proliferation and invasion in cell lines and xenograft tumors. We identified key regulators critical for JQ1-affected proliferation and invasion of tumor cells. Results: JQ1 markedly inhibited proliferation of four ATC cell lines by suppression of MYC and elevation of p21and p27 to decrease phosphorylated Rb and delay cell cycle progression from the G0/G1 phase to the S phase. JQ1 blocked cell invasion by attenuating epithelial-mesenchymal transition signals. These cell-based studies were further confirmed in xenograft studies in which the size and rate of tumor growth were inhibited by JQ1 via inhibition of p21-cyclin/cyclin-dependent kinase-Rb-E2F signaling. Conclusions: These results suggest targeting of the MYC protein could be a potential treatment modality for human ATC for which effective treatment options are limited.

Somatic mutations in murine models of leukemia and lymphoma: Disease specificity and clinical relevance.

Malignant transformation is a multistep process that is dictated by the acquisition of multiple genomic aberrations that provide growth and survival advantage. During the post genomic era, high throughput genomic sequencing has advanced exponentially, leading to identification of countless cancer associated mutations with potential for targeted therapy. Mouse models of cancer serve as excellent tools to examine the functionality of gene mutations and their contribution to the malignant process. However, it remains unclear whether the genetic events that occur during transformation are similar in mice and humans. To address that, we chose several transgenic mouse models of hematopoietic malignancies and identified acquired mutations in these mice by means of targeted re-sequencing of known cancer-associated genes as well as whole exome sequencing. We found that mutations that are typically found in acute myeloid leukemia or T cell acute lymphoblastic leukemia patients are also common in mouse models of the respective disease. Moreover, we found that the most frequent mutations found in a mouse model of lymphoma occur in a set of epigenetic modifier genes, implicating this pathway in the generation of lymphoma. These results demonstrate that genetically engineered mouse models (GEMM) mimic the genetic evolution of human cancer and serve as excellent platforms for target discovery and validation.

Progenitor B-1 B-cell acute lymphoblastic leukemia is associated with collaborative mutations in 3 critical pathways.

B-1 and B-2 lymphocytes are derived from distinct developmental pathways and represent layered arms of the innate and adaptive immune systems, respectively. In contrast to a majority of murine B-cell malignancies, which stain positive with the B220 antibody, we discovered a novel form of B-cell leukemia in NUP98-PHF23 (NP23) transgenic mice. The immunophenotype (Lin- B220- CD19+ AA4.1+) was identical to that of progenitor (pro) B-1 cells, and VH gene usage was skewed toward 3' V regions, similar to murine fetal liver B cells. Moreover, the gene expression profile of these leukemias was most similar to that of fetal liver pro-B fraction BC, a known source of B-1 B cells, further supporting a pro-B-1 origin of these leukemias. The NP23 pro-B-1 acute lymphoblastic leukemias (ALLs) acquired spontaneous mutations in both Bcor and Janus kinase (Jak) pathway (Jak1/2/3 and Stat5a) genes, supporting a hypothesis that mutations in 3 critical pathways (stem-cell self-renewal, B-cell differentiation, and cytokine signaling) collaborate to induce B-cell precursor (BCP) ALL. Finally, the thymic stromal lymphopoietin (Tslp) cytokine is required for murine B-1 development, and chromosomal rearrangements resulting in overexpression of the TSLP receptor (CRLF2) are present in some patients with high-risk BCP-ALL (referred to as CRLF2r ALL). Gene expression profiles of NP23 pro-B-1 ALL were more similar to that of CRLF2r ALL than non-CRLF2r ALL, and analysis of VH gene usage from patients with CRLF2r ALL demonstrated preferential usage of VH regions used by human B-1 B cells, leading to the suggestion that this subset of patients with BCP-ALL has a malignancy of B-1, rather than B-2, B-cell origin.

Bromodomain and Extraterminal Protein Inhibitor JQ1 Suppresses Thyroid Tumor Growth in a Mouse Model.

PURPOSE: New therapeutic approaches are needed for patients with thyroid cancer refractory to radioiodine treatment. An inhibitor of bromodomain and extraterminal domain (BET) proteins, JQ1, shows potent antitumor effects in hematological cancers and solid tumors. To evaluate whether JQ1 is effective against thyroid cancer, we examined antitumor efficacy of JQ1 using the ThrbPV/PVKrasG12D mouse, a model of anaplastic thyroid cancer. EXPERIMENTAL DESIGN: We treated ThrbPV/PVKrasG12D mice with vehicle or JQ1 at a dose of 50 mg/kg body weight/day starting at the age of 8 weeks for a 10-week period and monitored thyroid tumor progression. RESULTS: JQ1 markedly inhibited thyroid tumor growth and prolonged survival of these mice. Global differential gene expression analysis showed that JQ1 suppressed the cMyc (hereafter referred to as Myc) transcription program by inhibiting mRNA expression of Myc, ccnd1, and other related genes. JQ1-suppressed Myc expression was accompanied by chromatin remodeling as evidenced by increased expression of histones and hexamethylene bis-acetamide inducible 1, a suppressor of RNA polymerase II transcription elongation. Analyses showed that JQ1 decreased MYC abundance in thyroid tumors and attenuated the cyclin D1-CDK4-Rb-E2F3 signaling to decrease tumor growth. Further analysis indicated that JQ1 inhibited the recruitment of BDR4 to the promoter complex of the Myc and Ccnd1 genes in rat thyroid follicular PCCL3 cells, resulting in decreased MYC expression at the mRNA and protein levels to inhibit tumor cell proliferation. CONCLUSIONS: These preclinical findings suggest that BET inhibitors may be an effective agent to reduce thyroid tumor burden for the treatment of refractory thyroid cancer. Clin Cancer Res; 23(2); 430-40. ©2016 AACR.

miR-23a impairs bone differentiation in osteosarcoma via down-regulation of GJA1.

Osteosarcoma is the most common type of bone cancer in children and adolescents. Impaired differentiation of osteoblast cells is a distinguishing feature of this aggressive disease. As improvements in survival outcomes have largely plateaued, better understanding of the bone differentiation program may provide new treatment approaches. The miRNA cluster miR-23a~27a~24-2, particularly miR-23a, has been shown to interact with genes important for bone development. However, global changes in gene expression associated with functional gain of this cluster have not been fully explored. To better understand the relationship between miR-23a expression and bone cell differentiation, we carried out a large-scale gene expression analysis in HOS cells. Experimental results demonstrate that over-expression of miR-23a delays differentiation in this system. Downstream bioinformatic analysis identified miR-23a target gene connexin-43 (Cx43/GJA1), a mediator of intercellular signaling critical to osteoblast development, as acutely affected by miR-23a levels. Connexin-43 is up-regulated in the course of HOS cell differentiation and is down-regulated in cells transfected with miR-23a. Analysis of gene expression data, housed at Gene Expression Omnibus, reveals that Cx43 is consistently up-regulated during osteoblast differentiation. Suppression of Cx43 mRNA by miR-23a was confirmed in vitro using a luciferase reporter assay. This work demonstrates novel interactions between microRNA expression, intercellular signaling and bone differentiation in osteosarcoma.