Neural induction promotes large-scale chromatin reorganisation of the Mash1 locus

Determining how genes are epigenetically regulated to ensure their correct spatial and temporal expression during development is key to our understanding of cell lineage commitment. Here we examined epigenetic changes at an important proneural regulator gene Mash1 (Ascl1), as embryonic stem (ES) cells commit to the neural lineage. In ES cells where the Mash1 gene is transcriptionally repressed, the locus replicated late in S phase and was preferentially positioned at the nuclear periphery with other late-replicating genes (Neurod, Sprr2a). This peripheral location was coupled with low levels of histone H3K9 acetylation at the Mash1 promoter and enhanced H3K27 methylation but surprisingly location was not affected by removal of the Ezh2/Eed HMTase complex or several other chromatin-silencing candidates (G9a, SuV39h-1, Dnmt-1, Dnmt-3a and Dnmt-3b). Upon neural induction however, Mash1 transcription was upregulated (>100-fold), switched its time of replication from late to early in S phase and relocated towards the interior of the nucleus. This spatial repositioning was selective for neural commitment because Mash1 was peripheral in ES-derived mesoderm and other non-neural cell types. A bidirectional analysis of replication timing across a 2 Mb region flanking the Mash1 locus showed that chromatin changes were focused at Mash1. These results suggest that Mash1 is regulated by changes in chromatin structure and location and implicate the nuclear periphery as an important environment for maintaining the undifferentiated state of ES cells.

Heritable gene silencing in lymphocytes delays chromatid resolution without affecting the timing of DNA replication

Temporal control of DNA replication has been implicated in epigenetic regulation of gene expression on the basis of observations that certain tissue-specific genes replicate earlier in expressing than non-expressing cells. Here, we show evidence that several leukocyte-specific genes replicate early in lymphocytes regardless of their transcription and also in fibroblasts, where these genes are never normally expressed. Instead, the heritable silencing of some genes (Rag-1, TdT, CD8alpha and lambda5) and their spatial recruitment to heterochromatin domains within the nucleus of lymphocytes resulted in a markedly delayed resolution of sister chromatids into doublet signals discernable by 3D fluorescence in situ hybridization (FISH). Integration of transgenes within heterochromatin (in cis) did, however, confer late replication and this was reversed after variegated transgene expression. These findings emphasise that chromosomal location is important for defining the replication timing of genes and show that retarded sister-chromatid resolution is a novel feature of inactive chromatin.

Transcription and the territory: the ins and outs of gene positioning

When cells exit mitosis, the neat rod-like chromosomes decondense into their interphase state. However, the chromatin threads are not randomly dispersed throughout the nucleoplasm. Rather, individual chromosomes appear to be organized into discrete, non-overlapping "territories". Current studies attempt to unravel how gene loci are organized within these territories, whether their subterritorial positions are dependent on transcription, and the extent to which the loci can move.

Antihuman epidermal growth factor receptor 2 antibody herceptin inhibits autocrine motility factor (AMF) expression and potentiates antitumor effects of AMF inhibitors

Overexpression of the human epidermal growth factor receptor (HER) 2 has been linked to the development and maintenance of malignant phenotypes in breast tumors. In addition, the growth and dissemination of human cancers are regulated in part by the autocrine motility factor (AMF)/phosphoglucose isomerase shown to be up-regulated by heregulin (HRG) in breast cancer cells. This study was undertaken to explore the effect of anti-HER2 monoclonal antibody 4D5 [Herceptin (HCT)] on AMF expression and the potential of its augmentation by specific simple sugar AMF inhibitors. Here we show that HCT treatment of high HER2-expressing breast cancer SK-BR3, BT-474, and ZR-75R cells resulted in down-regulation of AMF mRNA and protein. HCT inhibited the ability of HRG to induce AMF expression in cells with a normal HER2 level, and HCT-mediated down-regulation could be reversed by HRG treatment in breast cancer cells with a high HER2 level. HCT also inhibited transcription from a chimeric pGL3-Luc vector-based reporter system containing the 1.8-kb promoter region of human AMF. Treatment of breast cancer cells with the combination of HCT and specific AMF inhibitors, erythrose 4-phosphate or D-mannose 6-phosphate, resulted in an additive inhibitory effect on both the growth rate and invasiveness of cells as compared with treatment with each agent alone. Results presented here suggest that HCT can effectively block both ligand-induced and constitutive expression of AMF associated with high HER2 overexpression, implying a role of the AMF pathway in the action of HCT. Accordingly, the combination of AMF inhibitor with HCT can potentiate the growth-inhibitory and anti-invasive action of HCT in breast cancer cells.

Subchromosomal positioning of the epidermal differentiation complex (EDC) in keratinocyte and lymphoblast interphase nuclei

The epidermal differentiation complex (EDC) at 1q21 is host to many structurally and functionally related genes coding for proteins involved in the differentiation process of keratinocytes. The grouping together of these genes which share spatial and temporal expression and interrelated functions is a remarkable genomic feature which has led to suggestions that the region may have a coordinated transcription control mechanism. With the growing awareness that the organization of the genome within the interphase nucleus is relevant to transcriptional activity, we have investigated the spatial organization of the EDC in the nuclei of keratinocytes, where the EDC genes are highly expressed, and lymphoblasts, where they are silent. Using 2D and 3D FISH we find that in keratinocyte nuclei the EDC is frequently positioned external to the chromosome 1 territory compared to lymphoblasts where the EDC more often adopts a peripheral or internal location. It has been previously shown that the MHC region can extend from the chromosome 6 territory in relation to transcriptional activity. This study of the EDC thus provides a further example of a gene-dense complex capable of assuming extraterritorial positioning in relation to cell type/transcription status.