Epigenetics, Nucleosomes, Histone Modifications, and DNA Methylation

Fundamentals of Epigenetics

• Definition of Epigenetics:

  • Heritable changes in gene expression that do not involve changes in underlying DNA sequences.

  • The study of mitotically and/or meiotically heritable changes in gene function that do not entail a change in DNA sequence (Wu & Morris 2001; Bird 2007; Morgan et al. 1999).

• Phenotypic Examples of Epigenetic Variation:

  • Toadflax (Linaria vulgaris): Epigenetic variants exhibit distinct floral symmetry phenotypes, transitioning between wild-type (zygomorphic) and peloric (radially symmetric) forms without underlying genetic alterations.

*   **Isogenic Agouti ($A^{vy}/a$) Mice:** Genetically identical mice express a continuous spectrum of coat colors ranging from yellow to pseudo-agouti/brown due to variable epigenetic regulation at the viable yellow agouti locus.

• Molecular Features of Epigenetic Regulation:

  • Small interfering RNAs (siRNAs).

  • Histone Post-Translational Modifications (PTMs).

  • Cytosine DNA Methylation ($5\text{-methylcytosine}$).

  • All these molecular mechanisms act concertedly to alter gene expression in a non-Mendelian manner.

• Primary Downstream Effect:

  • Epigenetic processes primarily alter gene expression by directly modulating chromatin conformation (open versus closed states), which dictates transcriptional accessibility.

Nucleosome Structure and Chromatin Conformation

• The Nucleosome as the Basic Unit:

  • The nucleosome represents the fundamental repeating structural unit of chromatin responsible for packaging genomic DNA inside the nucleus.

  • In human cells, approximately $2\text{ meters}$ of linear genomic DNA is compacted into a nucleus measuring only $6\text{ to }10\,\mu\text{m}$ in diameter.

• Structural Composition of a Nucleosome:

  • Consists of a core of $147\text{ bp}$ of double-stranded DNA wrapped in $1.65$ superhelical turns around a central histone octamer protein complex.

  • Histone Octamer Core Composition:

    • $2 \times \text{Histone 2A (H2A)}$

    • $2 \times \text{Histone 2B (H2B)}$

    • $2 \times \text{Histone 3 (H3)}$

    • $2 \times \text{Histone 4 (H4)}$

  • Linker Histone H1: Binds to the linker DNA region entering and exiting the nucleosome particle.

  • Linker DNA: DNA segment located between adjacent nucleosomes, typically spanning $\approx 10\text{ to }80\text{ bp}$ in length.

• Chromatin Conformation States:

  • Open Chromatin Conformation (Euchromatin):

    • Decondensed, accessible state.

    • Allows transcription factors, co-activator complexes, and RNA Polymerase II to physically access DNA binding sites and initiate transcription.

    • Associated with active gene transcription ("switched on").

  • Closed Chromatin Conformation (Heterochromatin):

    • Highly condensed, inaccessible state.

    • Sterically hinders and prevents transcription factors and RNA Polymerase II from accessing promoter and enhancer regions.

    • Associated with transcriptional repression and gene silencing ("switched off").

• Factors Controlling Chromatin Conformation:

  • Histone post-translational modifications (PTMs) and histone variants (e.g., $H2AZ$, $H2AW$).

  • Cytosine DNA methylation patterns.

  • ATP-dependent nucleosome remodeling complexes (e.g., $SWI/SNF$ complex).

  • Transcription factors and associated co-activator or co-repressor complexes.

Histone Post-Translational Modifications (PTMs)

• Biochemical Characteristics of Histone Marks:

  • Histone tails are flexible, unstructured amino-terminal ends extending outward from the nucleosome core particle.

  • Amino acid residues within these N-terminal tails undergo covalent biochemical modifications catalyzed by specific chromatin-modifying enzymes.

• Major Types of Covalent Modifications:

  • Acetylation ($-COCH_3$): Neutralizes positive charges on lysine residues; added by Histone Acetyltransferases (HATs) and removed by Histone Deacetylases (HDACs).

  • Methylation ($-CH_3$): Mono-, di-, or tri-methylation on lysine ($K$) or arginine ($R$) residues; added by Histone Methyltransferases (HMTs) and removed by Histone Demethylases.

  • Ubiquitination ($Ub$): Covalent attachment of a $76\text{-amino acid}$ ubiquitin protein moiety to lysine residues; removed by Deubiquitinating enzymes (DUBs).

  • Phosphorylation ($-PO_3^{2-}$): Addition of phosphate groups to serine ($S$), threonine ($T$), or tyrosine ($Y$) residues.

• Standard Nomenclature for Histone PTMs:

  • Written as: [Histone Name][Amino Acid Single-Letter Code][Residue Position][Modification Type].

  • Example: $H3K27me3$ denotes trimethylation of lysine ($K$) at position $27$ on the N-terminal tail of Histone 3 ($H3$).

• Biological Processes Regulated by Histone PTMs:

  • Gene expression and transcriptional activation.

  • Gene silencing and heterochromatin formation.

  • Transcriptional elongation.

  • Histone deposition and nucleosome assembly.

  • Chromosome condensation during mitosis/meiosis.

• Histone PTM Patterns and Chromatin States:

*   **Mechanisms Linked to Transcriptional Activation (Open Chromatin):**
    *   Histone acetylation via HATs (e.g., $H3K9ac$, $H3K14ac$, $H3K27ac$, $H3K18ac$, $H3K23ac$, $H4K16ac$).
    *   Lysine methylation at specific sites: $H3K4me1/2/3$, $H3K6$, $H3K79me1/2$.
    *   Lysine demethylation at repressive sites: Demethylation of $H3K9$, $H3K27$, $H4K20$.
    *   Mono-ubiquitination (e.g., $H2B\text{-ub}$).
*   **Mechanisms Linked to Transcriptional Repression (Closed Chromatin):**
    *   Histone deacetylation via HDACs.
    *   Lysine methylation at repressive sites: $H3K9me2/3$, $H3K27me3$, $H4K20me$ (catalyzed by HMTs).
    *   Lysine demethylation at activating sites: Demethylation of $H3K4$, $H3K6$, $H3K79$.
    *   De-ubiquitination via DUBs.

Profiling Histone Marks: ChIP and the modENCODE Project

• Chromatin Immunoprecipitation (ChIP) Technique:

  • Method used to profile protein-DNA interactions and specific histone PTM locations in vivo across genome regions.

  • Step-by-Step Protocol:

    1. Crosslinking: Treat cells/tissues with formaldehyde to covalently crosslink DNA to histones and associated chromatin proteins in vivo.

    2. Shearing: Sonicate or enzymatically digest chromatin into short DNA fragments measuring $150\text{ to }300\text{ bp}$ in length.

    3. Immunoprecipitation: Incubate sheared chromatin with a highly specific antibody targeting the specific modified histone residue (or chromatin protein).

    4. Reverse Crosslinking: Wash immune complexes, unbind non-specific background, and heat to reverse covalent protein-DNA crosslinks.

    5. Purification & Sequencing: Purify isolated DNA and subject to high-throughput sequencing (ChIP-seq) or microarray hybridization (ChIP-chip).

• The modENCODE Project (Drosophila melanogaster):

  • The Model Organism Encyclopedia of DNA Elements (modENCODE) project mapped genome-wide chromatin landscapes.

  • Conducted systematic ChIP experiments using antibodies against $18$ distinct histone PTMs in Drosophila cell lines.

  • Data integration revealed $9$ major combinatorial patterns of histone PTMs, defining distinct functional chromatin states ($1$ through $9$):

    • State 1: Active promoters / transcription start site (TSS)-proximal regions (enriched for $H3K4me3$, $H3K4me2$, $H3K9ac$, $H3K27ac$, $H3K23ac$, RNA Pol II).

    • State 2 & 3: Active gene bodies and exons/introns (enriched for $H3K36me3$, $H3K79me$).

    • State 5: Heterochromatin-like regions (enriched for $H4K16ac$).

    • State 6: Polycomb-mediated repressive domains (highly enriched for $H3K27me3$ and Polycomb proteins like $PC$, $E(Z)$, $dRING$).

    • State 7 & 8: Pericentromeric heterochromatin / constitutive silencing (enriched for $H3K9me2$, $H3K9me3$, $HP1a$, $SU(VAR)3-9$).

    • State 9: Silent intergenic background regions.

• Interpreting Data with Heatmaps:

  • Heatmaps are graphical data visualization tools where matrix values are represented as colors.

  • Rows represent specific variables (e.g., individual genes or functional chromatin states).

  • Columns represent samples, specific histone modifications, or genomic regions.

  • Color intensity gradients correspond to relative quantitative levels (e.g., read depth or $\log_2$ fold enrichment), guided by a color legend/scale bar.

Whole-Genome Analysis of H3K27me3 in Arabidopsis

• Experimental Design (Zhang et al. 2007):

  • Research Question: Map the global genomic distribution of $H3K27me3$ (a classic Polycomb-mediated gene silencing mark) across the Arabidopsis thaliana genome.

  • Method: Performed ChIP on Arabidopsis seedlings using anti-$H3K27me3$ antibodies and hybridized purified DNA to high-density, whole-genome tiling microarrays.

• Reading Genome Browser Tracks:

  • Horizontal Axis: Chromosomal positions and coordinates.

  • Vertical Read Peaks (Blue/Cyan): Signals representing $H3K27me3$ ChIP read density at specific genomic loci.

  • Green Boxes: Positions and exon structures of individual genes located on the top (sense) versus bottom (antisense) strands.

  • Red Box Highlights: Specific target genes analyzed (e.g., FUSCA3, AGAMOUS, MEDEA, PHERES1, SHOOT MERISTEMLESS, AGAMOUS-LIKE 19).

• Key Experimental Findings:

  • $H3K27me3$ modifications cover extensive regions covering full transcribed gene bodies of target loci.

  • Approximately $4{,}400$ genes in the Arabidopsis genome are marked by $H3K27me3$.

  • Transcriptional Correlation: Integration of ChIP-chip profile data with RNA expression datasets demonstrated that genes marked by $H3K27me3$ exhibit significantly lower overall average expression levels compared to non-marked genes.

*   **Tissue Specificity:** Genes marked by $H3K27me3$ show a dramatically higher degree of tissue-specific gene expression than non-target genes, demonstrating that $H3K27me3$ acts as a key mark regulating gene expression during developmental processes.

Principles of DNA Methylation

• Chemical Structure:

  • DNA methylation is a covalent modification where a methyl group ($-CH_3$) is added to the 5th carbon atom of a cytosine pyrimidine ring, forming 5-methylcytosine (5mC).

• Biological Roles of DNA Methylation:

  • Silences and inactivates transposable elements (TEs) and foreign viral DNA inserts to maintain genome stability.

  • Regulates normal, cell-type-specific endogenous gene expression.

  • Essential for embryonic development, cell lineage specification, genomic imprinting, and dosage compensation (X-chromosome inactivation).

  • Implicated in human pathologies including cancer, neurological disorders, and cardiovascular diseases.

  • Aging Dynamics: Global methylation patterns become progressively more randomized and disorganized over an organism's lifespan.

• Sequence Contexts of Cytosine Methylation:

  • Symmetric Methylation:

    • $CG$ (or $CpG$): Methylated on both strands at symmetric dinucleotides. Dominant form in mammals and plants. In Arabidopsis, $\approx 24\%$ of all $CG$ dinucleotides are methylated.

    • $CHG$ (where $H = A, T, \text{or } C$): Symmetric trinucleotide methylation found predominantly in plants. In Arabidopsis, $\approx 6.7\%$ of $CHG$ sites are methylated.

  • Asymmetric Methylation:

    • $CHH$ (where $H = A, T, \text{or } C$): Non-symmetric context methylated on only a single DNA strand. Highly prominent in plants ($\approx 1.7\%$ of $CHH$ sites in Arabidopsis); also present in animal embryonic stem cells and cancer tissues.

Maintenance and De Novo DNA Methylation Mechanisms

• Enzymatic Machinery for Methylation:

*   **Establishment (*De Novo* Methylation):**
    *   *Plants:* Catalyzed by DRM2 (Domains Rearranged Methyltransferase 2, an ortholog of animal Dnmt3), targeting unmethylated $CG$, $CHG$, and $CHH$ loci.
    *   *Animals:* Catalyzed by DNMT3A and DNMT3B.
*   **Maintenance Methylation:**
    *   *$CG$ Context:* Maintained during DNA replication by MET1 (Methyltransferase 1 in plants; orthologous to animal DNMT1). MET1 recognizes hemi-methylated $CG$ sites on newly replicated daughter strands and restores full symmetric methylation.
    *   *$CHG$ Context:* Maintained by CMT3 (Chromomethylase 3) in plants through a self-reinforcing loop with $H3K9me2$ marks.
    *   *$CHH$ Context:* Cannot be maintained by passive replication machinery because it is asymmetric; requires continuous targeting by DRM2 *de novo* methylation enzymes.

• RNA-Directed DNA Methylation (RdDM) Pathway:

  • Mechanism by which small interfering RNAs (siRNAs) guide de novo DNA methylation to specific target genomic sites.

*   **Step-by-Step Pathway:**
    1.  Single-stranded RNA (ssRNA) transcribed by RNA Polymerase IV (RNA Pol IV) is copied into double-stranded RNA (dsRNA) by RDR2 (RNA-Dependent RNA Polymerase 2).
    2.  DCL3 (Dicer-like 3) cleaves dsRNA into $24\text{ nt}$ small interfering RNAs (siRNAs).
    3.  The $24\text{ nt}$ siRNA is exported to the cytoplasm and loaded into Argonaute 4 (AGO4) assisted by HSP90.
    4.  The AGO4-siRNA complex re-enters the nucleus and binds to complementary non-coding scaffold transcripts being actively transcribed by RNA Polymerase V (RNA Pol V), assisted by KTF1 and RDM1.
    5.  Recruitment of the complex targets DRM2 (and DRM3) to catalyze *de novo* cytosine methylation ($5\text{mC}$) at the local genomic locus, promoting histone deacetylase recruitment, $H3K9$ methylation, and transcriptional gene silencing.

Genome-Wide Profiling Methods for DNA Methylation

• Method 1: Methylated DNA Immunoprecipitation (MeDIP / mCIP):

  • Procedure: Genomic DNA is sonicated into short fragments and immunoprecipitated using monoclonal antibodies raised against $5\text{-methylcytosine}$ ($5\text{mC}$). Purified methylated DNA undergoes Whole Genome Amplification (WGA) followed by quantitative Real-time PCR, microarrays, or deep sequencing.

  • Alternative Affinity Purification: Uses recombinant Methyl-CpG Binding Domain (MBD) proteins (e.g., WT MBD assay).

  • Capabilities: Detects $5\text{mC}$ across $CG$, $CHG$, and $CHH$ contexts.

  • Limitations: Sensitivity depends strictly on local $5\text{mC}$ density; resolution is limited to the length of the fragmented DNA ($\approx 100\text{ to }300\text{ bp}$) and cannot pinpoint exact individual methylated cytosine bases.

• Method 2: Sodium Bisulfite Sequencing (Gold Standard):

  • Provides single-nucleotide resolution mapping of methylated cytosines across the entire genome.

  • Chemical Reactions:

    1.  **Step 1 (Denaturation):** Thermal denaturation at $95^\circ\text{C}$ yields single-stranded DNA samples.
    2.  **Step 2 (Conversion):** Incubation with sodium bisulfite ($\text{NaHSO}_3$) at $65^\circ\text{C}$ and low pH ($5.0\text{ to }6.0$) selectively deaminates unmethylated cytosine residues to form cytidylate-bisulfite adducts.
    3.  **Step 3 (Desulphonation):** Treatment with an alkaline solution (high pH) at room temperature for $15\text{ min}$ removes the sulfonate group, yielding **Uracil (U)**.
    4.  **Resilience of 5mC:** $5\text{-Methylcytosine (5mC)}$ (and $5\text{-hydroxymethylcytosine}$) residues are resistant to bisulfite-mediated deamination and remain completely intact as **Cytosine (C)**.
*   **Sequencing Read Interpretation:**
    *   During subsequent PCR amplification, DNA polymerase reads Uracil ($U$) as Thymine ($T$) and synthesizes Adenine ($A$) on the opposite strand.
    *   Cytosines that read as **T** in sequencing data correspond to original **unmethylated cytosines**.
    *   Cytosines that read as **C** in sequencing data correspond to original **5-methylcytosines (5mC)**.

Functional Genomic Analysis of DNA Methylation in Arabidopsis

• Genome-Wide Methylation Profiling (Zhang et al. 2006):

  • Mapped $5\text{mC}$ across Arabidopsis thaliana Chromosome IV using tiling microarrays combined with mCIP, MBD affinity, and transcriptome analysis in wild-type (WT) plants, ddc mutants (drm1 drm2 cmt3 triple mutants lacking de novo and $CHG$ methylation), and met1 mutants (lacking $CG$ maintenance methylation).

• Genomic Distribution Features:

  • DNA methylation is heavily concentrated in heterochromatic regions rich in repetitive DNA sequences, transposable elements, centromeres, pericentromeres, and heterochromatic knobs.

  • These repeat-rich heterochromatic regions coincide with high densities of endogenous $24\text{ nt}$ siRNAs.

• Mutant Expression Dynamics:

  • In met1 mutants, $CG$ methylation is abolished genome-wide across both pericentromeric and euchromatic regions.

  • Loss of methylation in met1 mutants triggers dramatic transcriptional reactivation/induction of silent transposons and specific endogenous genes.

  • Classic Locus Example: The FLOWERING WAGENINGEN (FWA) gene is normally silenced by promoter repeat methylation; in met1 mutants, loss of promoter methylation leads to ectopically induced FWA expression, causing a late-flowering phenotype.

• General Rules of DNA Methylation and Gene Regulation:

  • Genes containing DNA methylation within their promoter regions are consistently expressed at very low levels or silenced.

  • Promoter-methylated genes display significantly higher degrees of tissue-specific gene expression than non-methylated genes across development.