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近年クロマチン免疫沈降（ChIP）を基として、さまざまなクロマチン解析技術が産み出されています。ヒストンなどの DNA 結合タンパク質（またはその修飾部位）に対する抗体を用い、それらタンパク質が結合する DNA の領域とその配列を明らかにするのが ChIP の目的ですが、抗体のバリエーションを増やすことにより、また qPCR、マイクロアレイ、シークエンシングなど他の新しい技術と組み合わせることにより、ChIP は今や高い柔軟性を持ったアプリケーションとなり、さまざまな研究現場で活用されるようになりました。特に、ヒストン以外の DNA 結合タンパク質、例えば転写因子、DNA 修復タンパク質などの解析における有用な手段となっています（Collas, 2010）。
ChIP とマイクロアレイを組み合わせた ChIP on Chip 法は現在、ヒトゲノム解析の主要な技術となっています（Kim et al., 2005）。また、次世代シークエンサーと組み合わせて一度に膨大な量の解析を効率よく行うことのできる ChIP–Seq 法は、国際エピゲノムプロジェクト ENCODE（エンコード）の進展に貢献しました(Landt et al., 2012)。さらに近年のデータ解析技術の発展により、過去に得られたデータの再検討や、複数データの統合などが可能となり、データの信頼性の向上や未知の情報の発見に繋がっています（Klein et al., 2014）。
制限酵素で断片化されたクロマチン複合体をサンプルとし、目的とするタンパク質の結合に関与する DNA 配列の情報を集め、これを基にゲノムの三次元解析を行うという試みもされています。バイサルファイト処理法などの手法と組み合わせることで、修飾ヒストンとメチル化 DNA の相互作用の解析も行われています。
ChIP-loop detects DNA-DNA interactions mediated by a specific protein of interest. This is achieved by adding an immunoprecipitation (IP) step to a relatively standard Chromatin Conformation Capture (3C) protocol:
The result is that predicted DNA-DNA interactions that are mediated by a protein of interest are confirmed.
The primary advantage of ChIP-loop over ChIP or 3C alone is specificity. For one, a reduction in background noise over a simple 3C assay is achieved by removing a large amount of genomic DNA by IP. Further, by targeting analysis to a specific protein of interest only specific, biologically relevant interactions are detected.
As a drawback, some argue that ChIP-loop may not be fully informative on its own, and that ChIP-loop should be cross-referenced against ChIP data. For example, if ChIP-loop interacting DNAs are not both enriched in a ChIP experiment, then the interaction should not be considered valid (Simonis et al., 2007).
ChIP and 3C were first combined in 2005 to study a mouse model of Rhett Syndrome (Horike et al., 2005). These researchers found that formation of a silent-chromatin loop is important for the function of the Dlx50Dlx6 locus. Further, they found that this loop was mediated by MECP2 and that this interaction is lost in individuals with Rhett Syndrome.
ChIP-loop is ideally suited to examine DNA loops mediated by transcription factor is a master regulator of chromatin-loop structure at the Kcnq5 locus.
ChIP-loop tips and tricks
Minimize ligation bias by having sufficient starting material. By performing the ligation when the DNA is concentrated in a small volume, undesirable cross-ligation can occur between chromatin fragments.
Validate with ChIP alone if possible. As stated above, cross-referencing ChIP-loop with ChIP data greatly reduces false-positives in the ChIP-loop assay.
Use appropriate controls.
Chromatin Interaction Analysis by Paired-End Tag sequencing (ChIA-PET) is a hybrid of ChIP and 3C techniques. ChIA-PEET is a high-throughput version of ChIP-loop that is capable of detecting long-range chromatin interactions via a protein of interest across the genome. The first steps in the protocol are very similar to a standard ChIP experiment.
ChIA-PET provides researchers with genome-wide interactions involving a protein of interest.
ChIA-PET has many advantages over similar techniques. The use of ChIP reduces the amount of DNA to be sequenced, reducing complexity and increasing the specificity of the sequencing reaction. ChIA-PET works with all the tag-based next generation platforms (Roche 454 pyrosequencing, SOLiD, Helicos etc.). ChIA-PET can be used to interrogate the interactions of any protein, given a suitable antibody.
Like any ChIP technique, ChIA-PET is limited by the specificity of the antibody. It is also limited to known genomes, since the calls must be mapped back to a reference genome. Finally, while ChIA-PET can identify if a protein is present in an interacting complex, it cannot determine if that protein actually mediates the interaction. For example, the protein may bind two other proteins that themselves bind the DNA.
Adapted from: Zhang et al., Nature (Nov 2013).
ChIA-PET was introduced in a 2009 paper by (Fullwood et al., 2009). These researchers developed ChIA-PET to map the binding sites of oestrogen receptor alpha (ER-alpha) in human cells. They found that ER-alpha mediates the formation of long-range looping at gene promoters, bringing genes together for coordinated transcription.
ChIA-PET has also been used to characterize the binding dynamics of other transcription factors such as CTCF (Li et al., 2013). Li et al. found that CTCF may have a role in organizing recently identified topological domains of chromatin. Chen et al. (2013) used ChIA-PET to examine miRNA regulation, and found that miRNA expression is coordinated in large chromatin domains.
ChIA-PET tips and tricks
Use sufficient starting material. The ChIP step is critical for generation of the ChIA-PET library. At least 100 ng of ChIP DNA is needed to generate a sufficiently complex ChIA-PET library. If using cells from culture, use at least 108 cells to obtain 100-300 ng of ChIP DNA (IP will need to be optimized for cell type and protein of interest).
Sonication or restriction enzyme digest: choose carefully. Sonication gets around the sequence bias and limitations of using a restriction enzyme to shorten protein-cross-linked chromatin. It also improves resolution; however, it is rather harsh, and can cause the loss of long-range interactions.
If long-range interactions are vital to your experiment, consider restriction digest. If sequence bias and resolution are issues, consider sonication.
Analysis is tricky, adapt another ChIP program, or use a previously developed software. The same group that developed ChIA-PET provide the software they used for analysis (Li et al., 2010).
Very slight modifications to traditional ChIP allow ChIP-exo to detect protein binding at single-nucleotide resolution. ChIP-exo uses restriction to digest and next-generation sequencing to determine genome-wide protein binding with high resolution.
ChIP-exo has many benefits over traditional ChIP. Most obviously, the resolution of ChIP-exo is greater than almost all other ChIP-based techniques. The single-nucleotide resolution achieved is robust across many diverse DNA-binding proteins (Rhee and Pugh, 2011; Rhee and Pugh, 2012a). Most importantly, the high degree of both experimental resolution and scale does not come with the drawbacks of comparable techniques, specifically noise associated with genome-wide approaches.
Adapted from: Rhee and Pugh, Current Protocols in Molecular Biology (Oct, 2012).
Noise is greatly reduced by the DNA digestion step: all non-protein bound DNA is digested by the lambda exonuclease. An additional single-strand specific exonuclease (such as RecJ) can also be used to digest background DNA more efficiently. The removal of this DNA greatly reduces the noise, and therefore the sequencing depth required, reducing cost.
Another major advantage is sensitivity. ChIP-exo can detect weaker DNA-protein interactions that other techniques, allowing more biologically relevant interactions to be discovered (Rhee and Pugh, 2011).
The only substantial disadvantage of ChIP-exo is related to multiple binding events by a single protein. If a protein binds several DNA loci simultaneously creating a ring or other 3D structure, such structures cannot be detected; any and all multiple-binding events are lost. ChIP-exo will simply read these events are single-bindings. This may be a problem depending on the protein, since the significance of complex ring structures in transcription is becoming apparent (Gibcus and Dekker, 2013).
ChIP-exo was presented in a 2011 Cell paper (Rhee and Pugh, 2011). These researchers examined the binding patterns of the yeast transcription factors Reb1, Gal4, Phd1, Rap1 and human CTCF. They found that each yeast transcription factor had multiple binding patterns and mechanisms. For example, they found that many low-affinity sites had high occupancy. This suggests that sequence alone cannot predict factor binding. This is explained in part by the finding that clustered sites bind more factors than lone sites, suggesting that a combination of effects including high concentrations and direct and indirect cooperation affect transcription factor binding. In a follow-up paper, these researchers also use ChIP-exo to identify TATA-box-like features in promoters that were thought to be TATA-less (Rhee and Pugh, 2012b).
ChIP-exo tips and tricks
Assess if resolution is paramount before starting. ChIP-exo is superior to ChIP-seq in almost all metrics, but it is labor and cost intensive. If the extra resolution and better signal-to-noise ratios are not vital, consider ChIP-seq for your experiment.
Consider use of magnetic beads. Serandour et al. (see additional reading) adapted the Rhee protocol to include the use of magnetic beads. Magnetic beads are believed to better pull down weak interactions and large protein complexes than agarose.
ChIP-BS-seq and BisChIP-seq are two nearly identical techniques that combine ChIP with bisulfite sequencing.
Bisulfite conversion is perhaps the most informative DNA methylation technology available. Sodium bisulfite chemically converts unmodified cytosine to uracil while leaving methylated cytosine intact (Frommer et al., 1992). Bisulfite-seq combines bisulfite treatment and next-generation sequencing platforms, providing single-nucleotide methylation resolution on a genome-wide scale.
In combination with ChIP, 5mC information can be targeted to regions bearing a specific histone modification or chromatin-binding protein of interest:
It is becoming increasingly clear that co-occurence of epigenetic marks is vital to their function. Various marks act in concert to modulate gene expression, and often co-regulate one another (Ernst et al., 2011). As such, examining two or more epigenetic marks in the same biological samples can give enhanced insight into the research problem. This is the key advantage of ChIP-BS-seq and BisChIP-seq.
Another advantage of these two techniques over traditional bisulfite-seq is the reduction in DNA used for bisulfite sequencing. This means that fewer reads are necessary in sequencing, dramatically reducing costs. However, this does mean that a good deal of input sample is needed, so ChIP-BS-seq/BisChIP-seq is not well suited to low-cell count applications.
An additional drawback of these techniques is that like all antibody-based techniques, the specificity of the experiment is determined by the quality of the antibody.
ChIP-BS-seq and BisChIP-seq were independently developed by two groups and published in the same issue of Genome Research (Brinkman et al., 2012; Statham et al., 2012). Brinkman et al. used ChIP-BS-seq to look at histone H3 lysine 27 trimethylation (H3K27me3) and DNA methylation. They found that H3K27me3 and DNA methylation are mutually exclusive at CpG islands. Statham et al. used BisChIP-seq to examine DNA methylation and H3K27me3 in normal and cancerous prostate cells. Surprisingly, they found that unmethylated and methylated DNA can be associated with H3K27me3 regions. This imples that DNA methylation status is not depended on the presence of the repressive histone mark, a widely held assumption.
ChIP-BS-seq and BisChIP-seq tips and tricks
Data analysis can be tricky. Since these techniques are relatively new no software has been specifically designed for ChIP-BS-seq/BisChIP-seq data. Statham et al. released the custom pipeline they created available here.
Adapt reference sequence to allow read mapping. Replace all the cytosines in the reference sequence with thymidines to allow the converted bisulfite sequence to be mapped.