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Session Length
~17 min
Adaptive Checks
15 questions
Transfer Probes
8
Epigenetics is the study of heritable changes in gene expression that occur without alterations to the underlying DNA sequence. The term, coined by Conrad Waddington in 1942, literally means 'above genetics' and refers to the molecular mechanisms that determine which genes are turned on or off in a given cell. These mechanisms include DNA methylation, histone modification, and non-coding RNA regulation, all of which work together to control how cells read and interpret genetic information. Unlike mutations, epigenetic changes do not alter the genetic code itself but instead modify the accessibility and activity of genes, providing a crucial layer of regulation that allows a single genome to produce the hundreds of distinct cell types found in a complex organism.
Epigenetic modifications play essential roles in normal development, cellular differentiation, and the maintenance of tissue-specific gene expression patterns. During embryonic development, epigenetic programming guides stem cells to become specialized cell types such as neurons, muscle cells, or blood cells, even though all these cells share the same DNA. Disruptions to the epigenome have been implicated in a wide range of diseases, including cancer, autoimmune disorders, neurological conditions, and metabolic syndromes. In cancer, for example, aberrant DNA methylation can silence tumor suppressor genes or activate oncogenes, driving uncontrolled cell growth. The reversible nature of epigenetic marks makes them attractive targets for therapeutic intervention, and several epigenetic drugs have already been approved for clinical use.
One of the most fascinating aspects of epigenetics is the discovery that certain epigenetic changes can be influenced by environmental factors such as diet, stress, toxin exposure, and physical activity, and that some of these changes may be transmitted across generations. This concept of transgenerational epigenetic inheritance challenges the traditional view that only DNA sequence changes can be inherited and has profound implications for our understanding of evolution, public health, and disease risk. Research in model organisms and human epidemiological studies, such as the Dutch Hunger Winter cohort, have provided evidence that environmental exposures in one generation can affect the health of subsequent generations through epigenetic mechanisms.
One step at a time.
The addition of a methyl group to the cytosine base in DNA, typically at CpG dinucleotides. Methylation of gene promoter regions generally suppresses gene transcription by blocking transcription factor binding or recruiting repressive protein complexes.
Example: In cancer cells, the promoter of the tumor suppressor gene BRCA1 can become hypermethylated, silencing its expression and removing an important brake on cell proliferation.
Chemical modifications to histone proteins, including acetylation, methylation, phosphorylation, and ubiquitination, that alter how tightly DNA is wound around histones and thus control gene accessibility.
Example: Acetylation of histone H3 at lysine 27 (H3K27ac) loosens chromatin structure and is a hallmark of active gene enhancers, while trimethylation at the same position (H3K27me3) marks repressed genes.
The dynamic restructuring of chromatin architecture by ATP-dependent remodeling complexes that slide, eject, or restructure nucleosomes, thereby regulating access to DNA for transcription, replication, and repair.
Example: The SWI/SNF chromatin remodeling complex repositions nucleosomes at gene promoters to allow RNA polymerase access, and mutations in SWI/SNF subunits are found in roughly 20% of all human cancers.
An epigenetic phenomenon in which certain genes are expressed in a parent-of-origin-specific manner. One allele is silenced through DNA methylation depending on whether it was inherited from the mother or father.
Example: The IGF2 gene is normally expressed only from the paternal allele. Loss of imprinting at IGF2 leads to biallelic expression and is associated with Beckwith-Wiedemann syndrome and increased cancer risk.
The process in female mammals by which one of the two X chromosomes is transcriptionally silenced to achieve dosage compensation with males. The non-coding RNA XIST coats the inactive X chromosome and recruits silencing complexes.
Example: Calico cats display patches of orange and black fur because different X chromosomes are randomly inactivated in different skin cell populations during early embryonic development.
Regions of DNA with a high frequency of cytosine-guanine dinucleotides, often located near gene promoters. CpG islands are typically unmethylated in normal cells, allowing gene expression, and their aberrant methylation can lead to gene silencing.
Example: Approximately 70% of human gene promoters are associated with CpG islands. In colorectal cancer, the CpG island methylator phenotype (CIMP) involves widespread hypermethylation of these islands, silencing multiple tumor suppressor genes simultaneously.
Regulatory RNA molecules, including microRNAs (miRNAs), long non-coding RNAs (lncRNAs), and small interfering RNAs (siRNAs), that influence gene expression through epigenetic mechanisms without being translated into protein.
Example: The lncRNA HOTAIR recruits the PRC2 polycomb complex to specific genomic loci, directing histone methylation and gene silencing. Overexpression of HOTAIR is associated with metastasis in breast cancer.
The transmission of epigenetic marks from one generation to the next without changes in DNA sequence, potentially allowing environmental exposures in parents to influence gene expression and phenotype in offspring.
Example: Studies of the Dutch Hunger Winter (1944-1945) found that children conceived during the famine had higher rates of obesity and cardiovascular disease decades later, and some effects were observed in their grandchildren.
More terms are available in the glossary.
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