Dynamic expression pattern of Hoxc8 during mouse early embryogenesis

Identification of a Chondrocyte-Specific Enhancer in the Hoxc8 Gene

Hox genes encode transcription factors whose roles in patterning animal body plans during embryonic development are well-documented. Multiple studies demonstrate that Hox genes continue to act in adult cells, in normal differentiation, in regenerative processes, and, with abnormal expression, in diverse types of cancers. However, surprisingly little is known about the regulatory mechanisms that govern Hox gene expression in specific cell types, as they differentiate during late embryonic development, and in the adult organism. The murine Hoxc8 gene determines the identity of multiple skeletal elements in the lower thoracic and lumbar region and continues to play a role in the proliferation and differentiation of cells in cartilage as the skeleton matures. This study was undertaken to identify regulatory elements in the Hoxc8 gene that control transcriptional activity, specifically in cartilage-producing chondrocytes. We report that an enhancer comprising two 416 and 224 bps long interacting DNA elements produces reporter gene activity when assayed on a heterologous transcriptional promoter in transgenic mice. This enhancer is distinct in spatial, temporal, and molecular regulation from previously identified regulatory sequences in the Hoxc8 gene that control its expression in early development. The identification of a tissue-specific Hox gene regulatory element now allows mechanistic investigations into Hox transcription factor expression and function in differentiating cell types and adult tissues and to specifically target these cells during repair processes and regeneration.

Altered Expression of Candidate Genes in Mayer-Rokitansky-Küster-Hauser Syndrome May Influence Vaginal Keratinocytes Biology: A Focus on Protein Kinase X

Mayer-Rokitansky-Küster-Hauser (MRKH) syndrome is a rare and complex disease defined by congenital aplasia of the vagina and uterus in 46,XX women, often associated with kidney and urinary tract anomalies. The aetiopathogenesis of MRKH syndrome is still largely unknown. Herein, we investigated the role of selected candidate genes in the aetiopathogenesis of MRKH syndrome, with a focus on PRKX, which encodes for protein kinase X. Through RT-qPCR analyses performed on vaginal dimple samples from patients, and principal component analysis (PCA), we highlighted a phenotype-related expression pattern of PRKX, MUC1, HOXC8 and GREB1L in MRKH patients. By using an in vitro approach, we proved that PRKX ectopic overexpression in a cell model of vaginal keratinocytes promotes cell motility through epithelial-to-mesenchymal transition (EMT) activation, a fundamental process in urogenital tract morphogenesis. Moreover, our findings showed that PRKX upregulation in vaginal keratinocytes is able to affect transcriptional levels of HOX genes, implicated in urinary and genital tract development. Our study identified the dysregulation of PRKX expression as a possible molecular cause for MRKH syndrome. Moreover, we propose the specific role of PRKX in vaginal keratinocyte biology as one of the possible mechanisms underlying this complex disease.

Genes Frequently Coexpressed with Hoxc8 Provide Insight into the Discovery of Target Genes

Identifying Hoxc8 target genes is at the crux of understanding the Hoxc8-mediated regulatory networks underlying its roles during development. However, identification of these genes remains difficult due to intrinsic factors of Hoxc8, such as low DNA binding specificity, context-dependent regulation, and unknown cofactors. Therefore, as an alternative, the present study attempted to test whether the roles of Hoxc8 could be inferred by simply analyzing genes frequently coexpressed with Hoxc8, and whether these genes include putative target genes. Using archived gene expression datasets in which Hoxc8 was differentially expressed, we identified a total of 567 genes that were positively coexpressed with Hoxc8 in at least four out of eight datasets. Among these, 23 genes were coexpressed in six datasets. Gene sets associated with extracellular matrix and cell adhesion were most significantly enriched, followed by gene sets for skeletal system development, morphogenesis, cell motility, and transcriptional regulation. In particular, transcriptional regulators, including paralogs of Hoxc8, known Hox co-factors, and transcriptional remodeling factors were enriched. We randomly selected Adam19, Ptpn13, Prkd1, Tgfbi, and Aldh1a3, and validated their coexpression in mouse embryonic tissues and cell lines following TGF-β2 treatment or ectopic Hoxc8 expression. Except for Aldh1a3, all genes showed concordant expression with that of Hoxc8, suggesting that the coexpressed genes might include direct or indirect target genes. Collectively, we suggest that the coexpressed genes provide a resource for constructing Hoxc8-mediated regulatory networks.

Gene expression profile of patients with Mayer-Rokitansky-Küster-Hauser syndrome: new insights into the potential role of developmental pathways

Mayer-Rokitansky-Küster-Hauser syndrome (MRKHS) is a rare disease characterized by congenital aplasia of uterus and vagina. Although many studies have investigated several candidate genes, up to now none of them seem to be responsible for the aetiology of the syndrome. In our study, we identified differences in gene expression profile of in vitro cultured vaginal tissue of MRHKS patients using whole-genome microarray analysis. A group of eight out of sixteen MRKHS patients that underwent reconstruction of neovagina with an autologous in vitro cultured vaginal tissue were subjected to microarray analysis and compared with five healthy controls. Results obtained by array were confirmed by qRT-PCR and further extended to other eight MRKHS patients. Gene profiling of MRKHS patients delineated 275 differentially expressed genes, of which 133 downregulated and 142 upregulated. We selected six deregulated genes (MUC1, HOXC8, HOXB2, HOXB5, JAG1 and DLL1) on the basis of their fold change, their differential expression in most patients and their relevant role in embryological development. All patients showed upregulation of MUC1, while HOXB2 and HOXB5 were downregulated, as well as Notch ligands JAG1 and DLL1 in the majority of them. Interestingly, HOXC8 was significantly upregulated in 47% of patients, with a differential expression only in MRKHS type I patients. Taken together, our results highlighted the dysregulation of developmental genes, thus suggesting a potential alteration of networks involved in the formation of the female reproductive tract and providing a useful clue for understanding the pathophysiology of MRKHS.

Akt1 as a putative regulator of Hox genes

In mammals, precise spatiotemporal expressions of Hox genes control the main body axis during embryogenesis. However, the mechanism by which Hox genes are regulated is poorly understood. To discover the putative regulator of Hox genes, in silico analyses were performed using GEO profiles, and Akt1 emerged as a candidate regulator of Hox genes in E13.5 MEFs. The results of the RT-PCR showed that 5' Hoxc genes, including ncRNA were upregulated in Akt1 null MEF. Combined bisulfite restriction analysis (COBRA) and bisulfite sequencing showed that the CpG island of a 5' Hoxc gene was hypomethylated in Akt1 null cells. These results indicate that Hox expression could be controlled by the function of Akt1 through epigenetic modification such as DNA methylation.

Protein networks as logic functions in development and cancer

Many biological and clinical outcomes are based not on single proteins, but on modules of proteins embedded in protein networks. A fundamental question is how the proteins within each module contribute to the overall module activity. Here, we study the modules underlying three representative biological programs related to tissue development, breast cancer metastasis, or progression of brain cancer, respectively. For each case we apply a new method, called Network-Guided Forests, to identify predictive modules together with logic functions which tie the activity of each module to the activity of its component genes. The resulting modules implement a diverse repertoire of decision logic which cannot be captured using the simple approximations suggested in previous work such as gene summation or subtraction. We show that in cancer, certain combinations of oncogenes and tumor suppressors exert competing forces on the system, suggesting that medical genetics should move beyond cataloguing individual cancer genes to cataloguing their combinatorial logic.

Biological outcomes are often determined by modules of proteins working in combination. In classic biological studies, these modules have been shown to encode a diverse repertoire of logic functions which provide the means to express complex regulatory programs using a limited number of proteins. Here, we integrate gene expression profiles and physical protein interaction maps to provide a systematic and global view of combinatorial network modules underlying representative developmental and cancer programs. We develop a new method that associates decision trees with concise network regions to identify network decision modules predictive of biological or clinical outcome. The resulting network signatures prove robust across different sample cohorts and capture causal mechanisms of development or disease. Furthermore, we find that the most predictive network decision functions rely on both coherent and opposing gene activities. Notably, in cancer progression the predictive gene associations often map to physical interactions between known oncogenes and tumor suppressors, where the combined activity of these genes determines disease outcome.

Hoxc8 downregulates Mgl1 tumor suppressor gene expression and reduces its concomitant function on cell adhesion

Hoxc8 is a homeobox gene family member, which is essential for growth and differentiation. Mgl1, a mouse homologue of the Drosophila tumor suppressor gene lgl, was previously identified as a possible target of Hoxc8. However, the biological effects and underlying molecular mechanism of Hoxc8 regulation on Mgl1 has not been fully established. The endogenous expression patterns of Hoxc8 were inversely correlated with those of Mgl1 in different types of cells and tissues. Here we showed that Hoxc8 overexpression downregulated the Mgl1 mRNA expression. Characterization of the ~2 kb Mgl1 promoter region revealed that the upstream sequence contains several putative Hox core binding sites and chromatin immunoprecipitation assay confirmed that Hoxc8 directly binds to the 5' upstream region of Mgl1. The promoter activity of this region was diminished by Hoxc8 expression but resumed by knockdown of Hoxc8 using siRNA against Hoxc8. Functional study of Mgl1 in C3H10T1/2 cells revealed a significant reduction in cell adhesion upon expression of Hoxc8. Taken together, our data suggest that Hoxc8 downregulates Mgl1 expression via direct binding to the promoter region, which in turn reduces cell adhesion and concomitant cell migration.

Mouse Homologue of the Schizophrenia Susceptibility Gene ZNF804A as a Target of Hoxc8

Using a ChIP-cloning technique, we identified a Zinc finger protein 804a (Zfp804a) as one of the putative Hoxc8 downstream target genes. We confirmed binding of Hoxc8 to an intronic region of Zfp804a by ChIP-PCR in F9 cells as well as in mouse embryos. Hoxc8 upregulated Zfp804a mRNA levels and augmented minimal promoter activity in vitro. In E11.5 mouse embryos, Zfp804a and Hoxc8 were coexpressed. Recent genome-wide studies identified Zfp804a (or ZNF804A in humans) as a plausible marker for schizophrenia, leading us to hypothesize that this embryogenic regulatory control might also exert influence in development of complex traits such as psychosis.

Proliferating cell nuclear antigen (Pcna) as a direct downstream target gene of Hoxc8

Hoxc8 is a member of Hox family transcription factors that play crucial roles in spatiotemporal body patterning during embryogenesis. Hox proteins contain a conserved 61 amino acid homeodomain, which is responsible for recognition and binding of the proteins onto Hox-specific DNA binding motifs and regulates expression of their target genes. Previously, using proteome analysis, we identified Proliferating cell nuclear antigen (Pcna) as one of the putative target genes of Hoxc8. Here, we asked whether Hoxc8 regulates Pcna expression by directly binding to the regulatory sequence of Pcna. In mouse embryos at embryonic day 11.5, the expression pattern of Pcna was similar to that of Hoxc8 along the anteroposterior body axis. Moreover, Pcna transcript levels as well as cell proliferation rate were increased by overexpression of Hoxc8 in C3H10T1/2 mouse embryonic fibroblast cells. Characterization of 2.3kb genomic sequence upstream of Pcna coding region revealed that the upstream sequence contains several Hox core binding sequences and one Hox-Pbx binding sequence. Direct binding of Hoxc8 proteins to the Pcna regulatory sequence was verified by chromatin immunoprecipitation assay. Taken together, our data suggest that Pcna is a direct downstream target of Hoxc8.

Hoxc8 represses BMP-induced expression of Smad6

Proper regulation of bone morphogenetic protein (BMP) signaling is critical for correct patterning and morphogenesis of various tissues and organs. A well known feedback mechanism is a BMP-mediated induction of Smad6, an inhibitor of BMP signaling. Hoxc8, one of the Hox family transcription factors, has also been shown to negatively regulate BMP-mediated gene expression. Here we add another level of Hoxc8 regulation on BMP signaling. Our results show that Hoxc8, when over-expressed in C3H10T1/2 or C2C12 cells, suppressed basal Smad6 promoter activity and its mRNA expression. Activation of Smad6 transcription either by BMP2 treatment or Smad1 over-expression was also abolished by Hoxc8. When chromatin was precipitated from mouse embryos with anti-Smad1 or anti-Hoxc8 antibody, Smad6 promoter sequence was enriched, suggesting that Hoxc8 proteins make complexes with Smad1 in the Smad6 promoter region. Yet, a lack of Hox binding motifs in the Smad6 promoter sequence suggests that instead of directly binding to the DNA, Hoxc8 may regulate Smad6 expression via an indirect mechanism. Our results suggest that the Smad6-mediated negative feedback mechanism on BMP signaling may be balanced by the repression of Smad6 expression by Hoxc8.

Additional sex combs-like 1 belongs to the enhancer of trithorax and polycomb group and genetically interacts with Cbx2 in mice

The Additional sex combs (Asx) gene of Drosophila behaves genetically as an enhancer of trithorax and polycomb (ETP) in displaying bidirectional homeotic phenotypes, suggesting that is required for maintenance of both activation and silencing of Hox genes. There are three murine homologs of Asx called Additional sex combs-like1, 2, and 3. Asxl1 is required for normal adult hematopoiesis; however, its embryonic function is unknown. We used a targeted mouse mutant line Asxl1(tm1Bc) to determine if Asxl1 is required to silence and activate Hox genes in mice during axial patterning. The mutant embryos exhibit simultaneous anterior and posterior transformations of the axial skeleton, consistent with a role for Asxl1 in activation and silencing of Hox genes. Transformations of the axial skeleton are enhanced in compound mutant embryos for the polycomb group gene M33/Cbx2. Hoxa4, Hoxa7, and Hoxc8 are derepressed in Asxl1(tm1Bc) mutants in the antero-posterior axis, but Hoxc8 expression is reduced in the brain of mutants, consistent with Asxl1 being required both for activation and repression of Hox genes. We discuss the genetic and molecular definition of ETPs, and suggest that the function of Asxl1 depends on its cellular context.

Effect of excessive iodine exposure on the placental deiodinase activities and Hoxc8 expression during mouse embryogenesis

Excessive iodine induces thyroid dysfunction. However, the effect of excessive iodine exposure on maternal-fetal thyroid hormone metabolism and on the expression of genes involved in differentiation, growth and development is poorly understood. Since a thyroid hormone receptor response element was found in the Hoxc8 promoter region, Hoxc8 expression possibly regulated by excessive iodine exposure was firstly investigated. In the present study, Balb/C mice were given different doses of iodine in the form of potassium iodate (KIO3) at the levels of 0 (sterile water), 1.5, 3.0, 6.0, 12.0 and 24.0 microg/ml in drinking water for 4 months, then were mated. On 12.5 d postcoitum, placental type 2 and type 3 deiodinase activities and fetal Hoxc8 expression were determined. The results showed that excessive iodine exposure above 1.5 microg/ml resulted in an increase of total thyroxine and a decrease of total triiodothyronine in the serum of maternal mice, which was mainly associated with the inhibition of type 1 deiodinase activity in liver and kidney. Placental type 2 deiodinase activity decreased, showing an inverse relationship with maternal thyroxine level. Hoxc8 mRNA and protein expression at 12.5 d postcoitum embryos were down regulated. Because Hoxc8 plays an important role in normal skeletal development, this finding provides a possible explanation for the skeletal malformation induced by excessive iodine exposure and also provides a new clue to study the relationship between iodine or thyroid hormones and Hox gene expression pattern.