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BackgroundThe histone H3 variant CENP-A is normally tightly regulated to ensure only one centromere exists per chromosome. Native CENP-A is often found overexpressed in human cancer cells and a range of human tumors. Consequently, CENP-A misregulation is thought to contribute to genome instability in human cancers. However, the consequences of such overexpression have not been directly elucidated in human cancer cells.ResultsTo investigate native CENP-A overexpression, we sought to uncover CENP-A-associated defects in human cells. We confirm that CENP-A is innately overexpressed in several colorectal cancer cell lines. In such cells, we report that a subset of structurally distinct CENP-A-containing nucleosomes associate with canonical histone H3, and with the transcription-coupled chaperones ATRX and DAXX. Furthermore, such hybrid CENP-A nucleosomes localize to DNase I hypersensitive and transcription factor binding sites, including at promoters of genes across the human genome. A distinct class of CENP-A hotspots also accumulates at subtelomeric chromosomal locations, including at the 8q24/Myc region long-associated with genomic instability. We show this 8q24 accumulation of CENP-A can also be seen in early stage primary colorectal tumors.ConclusionsOur data demonstrate that excess CENP-A accumulates at noncentromeric locations in the human cancer genome. These findings suggest that ectopic CENP-A nucleosomes could alter the state of the chromatin fiber, potentially impacting gene regulation and chromosome fragility.Electronic supplementary materialThe online version of this article (doi:10.1186/1756-8935-8-2) contains supplementary material, which is available to authorized users.

The innate immune system guards against virus infection through a variety of mechanisms including mobilization of the host interferon system, which attacks viral products mainly at a posttranscriptional level. The influenza virus NS1 protein is a multifunctional facilitator of virus replication, one of whose actions is to antagonize the interferon response. Since NS1 is required for efficient virus replication, it was reasoned that chemical inhibitors of this protein could be used to further understand virus-host interactions and also serve as potential new antiviral agents. A yeast-based assay was developed to identify compounds that phenotypically suppress NS1 function. Several such compounds exhibited significant activity specifically against influenza A virus in cell culture but had no effect on the replication of another RNA virus, respiratory syncytial virus. Interestingly, cells lacking an interferon response were drug resistant, suggesting that the compounds block interactions between NS1 and the interferon system. Accordingly, the compounds reversed the inhibition of beta interferon mRNA induction during infection, which is known to be caused by NS1. In addition, the compounds blocked the ability of NS1 protein to inhibit double-stranded RNA-dependent activation of a transfected beta interferon promoter construct. The effects of the compounds were specific to NS1, because they had no effect on the ability of the severe acute respiratory syndrome coronavirus papainlike protease protein to block beta interferon promoter activation. These data demonstrate that the function of NS1 can be modulated by chemical inhibitors and that such inhibitors will be useful as probes of biological function and as starting points for clinical drug development.

Influenza virus non-structural protein 1 (NS1) is the centrepiece of the viral response to the host interferon (IFN) system. NS1 has been demonstrated previously to be a potential therapeutic target for antiviral therapy by identification of specific small-molecule inhibitors. This study demonstrated the biological mechanism for a potent new NS1 antagonist. Compound JJ3297 inhibited virus replication by more than three orders of magnitude without affecting cell viability. Importantly, it efficiently reversed NS1-induced inhibition of IFN mRNA production. The hypothesis was tested that JJ3297 facilitates IFN production in infected cells, leading to protection of the surrounding uninfected cells. Accordingly, the compound efficiently prevented virus spread through a cell population during a 48 h multi-cycle infection initiated at a very low m.o.i. Consistent with the hypothesis, the compound had no detectable influence on a 6 h single-cycle infection initiated at a high m.o.i. The effect of JJ3297 on virus replication was not caused by inhibition of NS1 expression or its mislocalization in the cell. JJ3297 facilitated the induction of an IFN-like antiviral state, resulting in increased resistance to subsequent challenge with vesicular stomatitis virus. The activity of JJ3297 absolutely required the function of cellular RNase L, indicating that an intact IFN system is required for function of the compound. These results support a model in which inhibition of NS1 function results in restoration of the IFN-induced antiviral state and inhibition of virus replication and spread. This represents a new direction for anti-influenza virus drug development that exploits the IFN pathway to challenge virus replication.

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Histone variants such as H3.3, macroH2A, H2A.Z, and CENP-A are important epigenetic modifiers of the chromatin state in eukaryotic genomes. The centromeric histone H3 variant CENP-A/CENH3 epigenetically marks centromeres and is required for assembly of the kinetochore complex, a region of the chromosome that is responsible for proper genome segregation during mitosis. Several diverse techniques using biochemical, cell biology, and biophysical approaches have been utilized to study the nature of the CENP-A nucleosome across the cell cycle. In this chapter, we describe methods for CENP-A nucleosome purification and separation of CENP-A from other core histones using traditional SDS-PAGE and more resolving techniques such as Triton acid urea (TAU) and two-dimensional gels. We also discuss methods for observation of CENP-A on chromatin fibers using immunofluorescence. Finally, we provide a detailed description of analysis of chromatin structures using atomic force microscopy.

The structure of the centromere-specific histone centromeric protein A (CENP-A) nucleosome has been a hot topic of debate for the past five years. Structures proposed include octamers, hexamers, homotypic and heterotypic tetramers. This controversy has led to the proposal that CENP-A nucleosomes undergo cell-cycle dependent transitions between the multiple states previously documented to exist in vivo and in vitro. In recent work from our laboratory, we sought to test this hypothesis. We discovered that CENP-A nucleosomes undergo unique oscillations in human cells, a finding mirrored in a parallel study performed in budding yeast. This review provides additional insights into the potential mechanisms for the interconversion of CENP-A nucleosomal species, and speculates on a biological role for oscillations in vivo.

SummaryProtein VII is an abundant component of adenovirus particles and is tightly associated with the viral DNA. It enters the nucleus along with the infecting viral genome and remains bound throughout early phase. Protein VII can be visualized by immunofluorescent staining as discrete dots in the infected cell nucleus. Comparison between protein VII staining and expression of the 72 kDa DNA binding protein revealed a one-to-one correspondence between protein VII dots and infectious viral genomes. A similar relationship was observed for a helper-dependent adenovirus vector expressing green fluorescent protein. This relationship allowed accurate titration of adenovirus preparations, including wild-type and helper-dependent vectors, using a one-day immunofluorescence method. The method can be applied to any adenovirus vector and gives results equivalent to the standard plaque assay.