Epigenetics: Fundamentals

Clinical Applications – Epigenetic Diseases

Cancer. Cancer was the first human disease to be linked to epigenetics. Studies performed by Feinberg and Vogelstein in 1983, using primary human tumor tissues, found that genes of colorectal cancer cells were substantially hypomethylated compared with normal tissues.1 DNA hypomethylation can activate oncogenes and initiate chromosome instability, whereas DNA hypermethylation initiates silencing of tumor suppressor genes. An accumulation of genetic and epigenetic errors can transform a normal cell into an invasive or metastatic tumor cell. Additionally, DNA methylation patterns may cause abnormal expression of cancer-associated genes. Global histone modification patterns are also found to correlate with cancers such as prostate, breast, and pancreatic cancer. Subsequently, epigenetic changes can be used as biomarkers for the molecular diagnosis of early cancer.

Mental Retardation Disorders. Epigenetic changes are also linked to several disorders that result in intellectual disabilities such as ATR-X, Fragile X, Rett, Beckwith-Weidman (BWS), Prader-Willi and Angelman syndromes..2 For example, the imprint disorders Prader-Willi syndrome and Angelman syndrome, display an abnormal phenotype as a result of the absence of the paternal or maternal copy of a gene, respectively. In these imprint disorders, there is a genetic deletion in chromosome 15 in a majority of patients. The same gene on the corresponding chromosome cannot compensate for the deletion because it has been turned off by methylation, an epigenetic modification.  Genetic deletions inherited from the father result in Prader-Willi syndrome, and those inherited from the mother, Angelman syndrome.

Immunity & Related Disorders. There are several pieces of evidence showing that loss of epigenetic control over complex immune processes contributes to autoimmune disease. Abnormal DNA methylation has been observed in patients with lupus whose T cells exhibit decreased DNA methyltransferase activity and hypomethylated DNA. Disregulation of this pathway apparently leads to overexpression of methylation-sensitive genes such as the leukocyte function-associated factor (LFA1), which causes lupus-like autoimmunity. Interestingly, LFA1 expression is also required for the development of arthritis, which raises the possibility that altered DNA methylation patterns may contribute to other diseases displaying idiopathic autoimmunity.

Neuropsychiatric Disorders. Epigenetic errors also play a role in the causation of complex adult psychiatric, autistic, and neurodegenerative disorders. Several reports have associated schizophrenia and mood disorders with DNA rearrangements that include the DNMT genes. DNMT1 is selectively overexpressed in gamma-aminobutyric acid (GABA)-ergic interneurons of schizophrenic brains, whereas hypermethylation has been shown to repress expression of Reelin (a protein required for normal neurotransmission, memory formation and synaptic plasticity) in brain tissue from patients with schizophrenia and patients with bipolar illness and psychosis. A role for aberrant methylation mediated by folate levels has been suggested as a factor in Alzheimer’s disease; also some preliminary evidence supports a model that incorporates both genetic and epigenetic contributions in the causation of autism. Autism has been linked to the region on chromosome 15 that is responsible for Prader-Willi syndrome and Angelman syndrome. Findings at autopsy of brain tissue from patients with autism have revealed a deficiency in MECP2 expression that appears to account for reduced expression of several relevant genes.

Pediatric Syndromes. In addition to epigenetic alterations, specific mutations affecting components of the epigenetic pathway have been identified that are responsible for several syndromes: DNMT3B in ICF (immunodeficiency, centromeric instability and facial anomalies) syndrome, MECP2 in Rett syndrome, ATRX in ATR-X syndrome (a-thalassemia/mental retardation syndrome, X-linked), and DNA repeats in facioscapulohumeral muscular dystrophy. In Rett syndrome, for example, MECP2 encodes a protein that binds to methylated DNA; mutations in this protein cause abnormal gene expression patterns within the first year of life. Girls with Rett syndrome display reduced brain growth, loss of developmental milestones and profound mental disabilities. Similarly, the ATR-X syndrome also includes severe developmental deficiencies due to loss of ATRX, a protein involved in maintaining the condensed, inactive state of DNA. Together, this constellation of clinical pediatric syndromes is associated with alterations in genes and chromosomal regions necessary for proper neurologic and physical development.

The increased knowledge and technologies in epigenetics over the last ten years allow us to better understand the interplay between epigenetic change, gene regulation, and human diseases, and will lead to the development of new approaches for molecular diagnosis and targeted treatments across the clinical spectrum.

Ready to learn about the first epigenetic mechanism? Read on: DNA Methylation

Show 2 footnotes

  1. Feinberg, AP and Vogelstein, B. Hypomethylation distinguishes genes of some human cancers from their normal counterparts. Nature 301, 89-92 (1983).
  2. Egger G. et al. Epigenetics in human disease and prospects for epigenetic therapy. Nature 429, 457-463 (2004).