Premed · Premed · Genetics
Lecture 14: Epigenetics and Chromatin Remodeling
Genetics
Learning Objectives
By the end of this lecture, students will be able to:
- Define epigenetics and distinguish epigenetic changes from genetic mutations
- Explain the mechanisms of DNA methylation and its role in gene silencing
- Describe the histone code and how histone modifications regulate chromatin state
- Explain genomic imprinting and its clinical significance
- Describe X-chromosome inactivation as an epigenetic phenomenon
- Discuss the role of epigenetic dysregulation in human disease, including cancer
Lecture Content
I. What is Epigenetics?
Epigenetics refers to heritable changes in gene expression that occur without alterations to the underlying DNA sequence. Epigenetic modifications are reversible (unlike DNA mutations), heritable through cell division (mitotic inheritance) and sometimes across generations (transgenerational epigenetic inheritance), and influenced by environment, development, aging, and disease. Three major epigenetic mechanisms operate in mammalian cells: DNA methylation, histone modifications, and non-coding RNA-mediated regulation.
Epigenetic marks establish and maintain cell identity. Although all cells in an organism share the same genome, each cell type expresses a different set of genes, and it is the epigenome -- the complete set of epigenetic modifications across the genome in a given cell type -- that determines which genes are active and which are silent.
II. DNA Methylation in Depth
5-methylcytosine (5mC), sometimes called the "fifth base," is the principal DNA methylation mark in mammals and occurs primarily at CpG dinucleotides. Approximately 70-80% of CpG sites are methylated genome-wide, but CpG islands at gene promoters are typically unmethylated when those genes are expressed.
The enzymes responsible for DNA methylation include DNMT1, the maintenance methyltransferase, which recognizes hemimethylated DNA after replication and methylates the new strand to preserve the methylation pattern through cell division. DNMT1 associates with PCNA and UHRF1 at the replication fork. DNMT3A and DNMT3B are de novo methyltransferases that establish new methylation patterns during development, and DNMT3L, though catalytically inactive, stimulates DNMT3A/3B activity.
Demethylation can occur passively through failure to maintain methylation during DNA replication, or actively through the action of TET enzymes (TET1, TET2, TET3), which oxidize 5mC through a series of intermediates: 5-hydroxymethylcytosine (5hmC), then 5-formylcytosine (5fC), then 5-carboxylcytosine (5caC). The final oxidized products are then removed by TDG and the base excision repair pathway. Notably, 5hmC is not merely an intermediate but appears to have its own regulatory functions.
Two major waves of epigenetic reprogramming occur during development. The first occurs after fertilization, when the zygote undergoes genome-wide demethylation (except at imprinted genes). The second occurs in primordial germ cells, where imprints are erased and re-established. Re-methylation occurs during implantation, carried out by DNMT3A and DNMT3B.
III. Histone Modifications and the Histone Code
Post-translational modifications of histone tails (the N-terminal tails extending from the nucleosome core) regulate chromatin state through a "writer-reader-eraser" paradigm. Acetylation of lysine residues promotes gene activation. Writers include HATs such as p300/CBP, GCN5, and PCAF; erasers include HDACs of Class I, II, III (Sirtuins), and IV; and readers are bromodomain-containing proteins. Methylation of lysine and arginine residues can either activate or repress genes depending on context. Writers include SET domain proteins such as MLL (for H3K4), EZH2 (for H3K27), and SUV39H1 (for H3K9). Erasers include demethylases such as LSD1 and the JMJD family. Readers include chromodomain proteins (HP1 reads H3K9me3), Tudor domains, and PHD fingers. Phosphorylation of serine and threonine residues is involved in chromosome condensation (H3S10ph) and the DNA damage response (gamma-H2AX). Ubiquitination of H2B at K120 promotes methylation of H3K4 and H3K79, both marks of active transcription.
Combinatorial histone marks define distinct chromatin states. Active promoters carry H3K4me3 and H3K27ac. Active enhancers carry H3K4me1 and H3K27ac. Poised enhancers carry H3K4me1 alone without H3K27ac. Polycomb-repressed regions are marked by H3K27me3. Constitutive heterochromatin carries H3K9me3. Bivalent domains, seen in embryonic stem cells at developmental genes, carry both H3K4me3 and H3K27me3 simultaneously, keeping these genes poised for either activation or repression.
<image>Panel A: Nucleosome diagram with histone tails extending outward, labeled with specific modification sites: H3K4 (methylation = active), H3K9 (methylation = repressive), H3K27 (methylation = Polycomb repression, acetylation = active enhancer/promoter), H3K36 (methylation = active gene body), H4K16 (acetylation = active). Writers, readers, and erasers are shown for each mark. Panel B: Chromatin state map showing a genomic region with color-coded segments: active promoter (H3K4me3 + H3K27ac), active enhancer (H3K4me1 + H3K27ac), transcribed gene body (H3K36me3), Polycomb-repressed region (H3K27me3), and heterochromatin (H3K9me3). Panel C: Diagram of DNA methylation maintenance by DNMT1 at the replication fork: hemimethylated CpG sites on the parent strand recognized by UHRF1, which recruits DNMT1 to methylate the corresponding CpG on the daughter strand.</image>
IV. Chromatin Remodeling Complexes
ATP-dependent chromatin remodelers alter nucleosome position and composition. The SWI/SNF complex (BAF/PBAF in mammals) slides or ejects nucleosomes to expose regulatory elements and functions as a tumor suppressor: mutations in SWI/SNF subunits (SMARCB1, SMARCA4, ARID1A) are found in over 20% of human cancers. The ISWI complex (SNF2H/SNF2L) promotes regular nucleosome spacing and can either promote or restrict access. The NuRD complex (Nucleosome Remodeling and Deacetylase) combines chromatin remodeling activity (via CHD3/CHD4) with histone deacetylation (via HDAC1/2) to function as a transcriptional repression complex.
The Polycomb and Trithorax systems maintain stable gene expression states throughout cell divisions. Polycomb Repressive Complex 2 (PRC2) contains the methyltransferase EZH2, which writes the repressive H3K27me3 mark. PRC1 recognizes H3K27me3 through its chromodomain and ubiquitinates H2A at K119 to compact chromatin. The opposing Trithorax/MLL complexes write the activating H3K4me3 mark. Together, Polycomb and Trithorax maintain the stable on/off states of developmental genes through successive cell divisions.
V. Genomic Imprinting
Genomic imprinting is parent-of-origin-specific gene expression, in which certain genes are expressed only from the maternal allele or only from the paternal allele. The silent allele is marked by DNA methylation at an imprinting control region (ICR). Approximately 150-200 genes are imprinted in humans, and many cluster together in imprinted domains. Imprinting marks are established in the germline (in the oocyte or sperm), maintained through somatic cell divisions, and erased and re-established each generation in primordial germ cells.
The IGF2/H19 locus on chromosome 11p15 provides a well-characterized example. IGF2 (insulin-like growth factor 2) is expressed from the paternal allele, while H19 (a non-coding RNA) is expressed from the maternal allele. A differentially methylated ICR between the two genes controls this pattern. On the maternal allele, the ICR is unmethylated, allowing the CTCF insulator protein to bind and block enhancer access to IGF2, so the enhancers activate H19 instead. On the paternal allele, the ICR is methylated, preventing CTCF binding and allowing the enhancers to activate IGF2 while H19 is silenced.
The clinical significance of imprinting is illustrated by several disorders. Prader-Willi syndrome results from loss of paternally expressed alleles at 15q11-13 (through deletion, uniparental disomy, or imprinting defect) and features hypotonia, obesity, intellectual disability, and hypogonadism. Angelman syndrome results from loss of maternal UBE3A expression at the same locus and features severe intellectual disability, seizures, ataxia, and a characteristically happy demeanor. Beckwith-Wiedemann syndrome arises from an imprinting defect at 11p15 leading to IGF2 overexpression and presents with macrosomia, macroglossia, omphalocele, and increased cancer risk (particularly Wilms tumor).
<image>Panel A: Diagram of the IGF2/H19 imprinted locus showing the maternal allele (ICR unmethylated, CTCF bound as insulator, enhancers activate H19) and the paternal allele (ICR methylated, no CTCF binding, enhancers activate IGF2), with DNA methylation marks shown as filled circles and unmethylated CpGs as open circles. Panel B: Imprinting life cycle diagram showing: erasure of imprints in primordial germ cells, sex-specific re-establishment during gametogenesis (different marks in oocyte vs. sperm), maintenance after fertilization (surviving the global demethylation wave), and somatic maintenance through development. Panel C: Chromosome 15q11-13 map showing the Prader-Willi region (paternally expressed genes: SNRPN, MKRN3, MAGEL2, NDN) and Angelman region (maternally expressed gene: UBE3A), with the imprinting center indicated, and the clinical consequences of paternal vs. maternal deletions.</image>
VI. Epigenetics and Disease
Cancer epigenetics represents one of the most clinically significant areas. Global DNA hypomethylation promotes genomic instability, reactivation of transposons, and oncogene activation. Local hypermethylation silences tumor suppressor gene promoters including RB1, p16/CDKN2A, BRCA1, and MLH1. Mutations in epigenetic regulators are common in cancer: DNMT3A mutations in acute myeloid leukemia, EZH2 mutations (both gain-of-function and loss-of-function) in lymphoma, IDH1/IDH2 mutations that produce 2-hydroxyglutarate and inhibit TET enzymes and histone demethylases creating a hypermethylation phenotype, and SWI/SNF subunit mutations in over 20% of all cancers. Epigenetic therapies have entered clinical practice: DNMT inhibitors (azacitidine and decitabine) for MDS and AML, HDAC inhibitors (vorinostat and romidepsin) for cutaneous T-cell lymphoma, and EZH2 inhibitors (tazemetostat) for epithelioid sarcoma and follicular lymphoma.
Epigenetic dysregulation also contributes to neurological disorders such as Rett syndrome (caused by MECP2 mutations affecting the methyl-CpG binding protein), Fragile X syndrome (caused by FMR1 promoter methylation), and ICF syndrome (caused by DNMT3B mutations). Imprinting disorders including Prader-Willi, Angelman, Beckwith-Wiedemann, and Silver-Russell syndromes further highlight the clinical importance of epigenetic regulation.
VII. Transgenerational Epigenetic Inheritance
There is growing evidence that epigenetic marks can be transmitted across generations beyond the directly exposed generation. This phenomenon is well documented in plants and C. elegans, but evidence in mammals remains more limited and controversial. The Dutch Hunger Winter studies showed that children of mothers who experienced famine during pregnancy had altered metabolic profiles. In the agouti mouse model, maternal diet rich in methyl donors affects coat color and obesity in offspring through DNA methylation at the agouti viable yellow locus.
Significant challenges remain in this field, including the difficulty of distinguishing true transgenerational inheritance from direct exposure effects, genetic confounders, and behavioral transmission. Potential mechanisms include incomplete reprogramming of epigenetic marks in germ cells and the transmission of small RNAs in sperm.
