E2F6 initiates stable epigenetic silencing of germline genes during embryonic development.

In mouse development, long-term silencing by CpG island DNA methylation is specifically targeted to germline genes; however, the molecular mechanisms of this specificity remain unclear. Here, we demonstrate that the transcription factor E2F6, a member of the polycomb repressive complex 1.6 (PRC1.6), is critical to target and initiate epigenetic silencing at germline genes in early embryogenesis. Genome-wide, E2F6 binds preferentially to CpG islands in embryonic cells. E2F6 cooperates with MGA to silence a subgroup of germline genes in mouse embryonic stem cells and in embryos, a function that critically depends on the E2F6 marked box domain. Inactivation of E2f6 leads to a failure to deposit CpG island DNA methylation at these genes during implantation. Furthermore, E2F6 is required to initiate epigenetic silencing in early embryonic cells but becomes dispensable for the maintenance in differentiated cells. Our findings elucidate the mechanisms of epigenetic targeting of germline genes and provide a paradigm for how transient repression signals by DNA-binding factors in early embryonic cells are translated into long-term epigenetic silencing during mouse development.

A rapid screening system evaluates novel inhibitors of DNA methylation and suggests F-box proteins as potential therapeutic targets for high-risk neuroblastoma.

After extensive research on radiochemotherapy, 5-year survival rates of children with high risk neuroblastoma still do not exceed 50%, owing to adverse side-effects exemplified by doxorubicin-induced cardiomyopathy. A promising new approach is the combination of conventional therapies with specific modulation of cell signaling pathways promoting therapeutic resistance, such as inhibition of aberrant kinase activity or re-expression of silenced tumor suppressor genes by means of chromatin remodeling. In this regard, we established a system that allows to identify potential drug targets as well as to validate respective candidate inhibitors in high-risk neuroblastoma model cell lines. Cell culture, drug exposure, shRNA-mediated knockdown and phenotype analysis are integrated into an efficient and versatile single well-based protocol. By utilizing this system, we assessed RG108, SGI-1027 and nanaomycin A, three novel DNA methyltransferase inhibitors that have not been tested in neuroblastoma cell lines so far, for their potential of synergistic anti-tumor activity in combination with doxorubicin. We found that, similarly to azacytidine, SGI-1027 and nanaomycin A mediate synergistic growth inhibition with doxorubicin independently of N-Myc status. However, they display high cytotoxicity but lack global DNA demethylation activity. Secondly, we conducted a lentiviral shRNA screen of F-box proteins, key regulators of protein stability, and identified Fbxw11/ß-TrCP2 as well as Fbxo5/Emi1 as potential therapeutic targets in neuroblastoma. These results complement existing studies and underline the reliability and versatility of our single well-based protocol.

A role for E2F6 in the restriction of male-germ-cell-specific gene expression.

E2F transcription factors play a pivotal role in the regulation of cellular proliferation and can be subdivided into activating and repressing family members [1]. Like other E2Fs, E2F6 binds to E2F consensus sites, but in contrast to E2F1-5, it lacks an Rb binding domain and functions as an Rb-independent transcriptional repressor [2, 3, 4 and 5]. Instead, E2F6 has been shown to complex with Polycomb (PcG) group proteins [6 and 7], which have a well-established role in gene silencing. Here, we show that E2F6 plays an unexpected and essential role in the tissue specificity of gene expression. E2F6-deficient mice ubiquitously express the alpha-tubulin 3 and 7 genes, which are expressed strictly testis-specifically in control mice. Like an additional E2F6 target gene, Tex12, that we identified, tubulin 3 and 7 are normally expressed in male germ cells only. The promoters of the alpha-tubulin and Tex12 genes share a perfectly conserved E2F site, which E2F6 binds to. Mechanistically, E2F6-mediated repression involves CpG hypermethylation locking target promoters in an inactive state. Thus, E2F6 is essential for the long-term somatic silencing of certain male-germ-cell-specific genes, but it is dispensable for cell-cycle regulation.

The prothrombin 3'end formation signal reveals a unique architecture that is sensitive to thrombophilic gain-of-function mutations.

The functional analysis of the common prothrombin 20210 G>A(F2 20210(*)A) mutation has recently revealed gain of function of 3'end processing as a novel genetic mechanism predisposing to human disease. We now show that the physiologic G at the cleavage site at position 20210 is the functionally least efficient nucleotide to support 3'end processing but has evolved to be physiologically optimal. Furthermore, the F2 3'end processing signal is characterized by a weak downstream cleavage stimulating factor (CstF) binding site with a low uridine density, and the functional efficiency of F2 3'end processing can be enhanced by the introduction of additional uridine residues. The recently identified thrombosis-related mutation (F2 20221(*)T) within the CstF binding site up-regulates F2 3'end processing and prothrombin biosynthesis in vivo. F2 20221(*)T thus represents the first example of a likely pathologically relevant mutation of the putative CstF binding site in the 3'flanking sequence of a human gene. Finally, we show that the low-efficiency F2 cleavage and CstF binding sites are balanced by a stimulatory upstream uridine-rich element in the 3'UTR. The architecture of the F2 3'end processing signal is thus characterized by a delicate balance of positive and negative signals. This balance appears to be highly susceptible to being disturbed by clinically relevant gain-of-function mutations.

Abnormally spliced beta-globin mRNAs: a single point mutation generates transcripts sensitive and insensitive to nonsense-mediated mRNA decay.

Nonsense-mediated mRNA decay (NMD) represents a phylogenetically widely conserved splicing- and translation-dependent mechanism that eliminates transcripts with premature translation stop codons and suppresses the accumulation of C-terminally truncated peptides. Elimination of frameshifted transcripts that result from faulty splicing may be an important function of NMD. To test this hypothesis directly, this study used the IVS1 + 5 G>A thalassemia mutation of the human beta-globin gene as a model system. We generated beta-globin gene constructs with this mutation and an iron-responsive element in the 5' untranslated region, which allowed specific experimental activation and inactivation of translation and, hence, NMD of this transcript. Premessenger RNAs with IVS1 + 5 G>A were spliced at normal sites and cryptic sites, enabling a direct comparison of the effect of NMD on the accumulation of normal and frameshifted messenger RNAs. In transfected HeLa cells, the predominant frameshifted transcript was degraded under conditions of active NMD, whereas accumulation to high levels occurred under conditions of specifically disabled NMD, thereby indicating an important physiologic function of NMD in the control of the splicing process. An unexpected finding was that accumulation of a second aberrant transcript remained unaffected by NMD. The IVS1 + 5 G>A mutation thus revealed the presence of an unknown cis-acting determinant that influences the NMD sensitivity of a putative NMD substrate. It can therefore serve as a useful tool for defining the mechanisms that permit specific transcripts to circumvent the NMD pathway.