Presentation on theme: "The Emerging Role of Epigenetics in Human Diseases"— Presentation transcript:
1The Emerging Role of Epigenetics in Human Diseases David P. Gardner, Ph.D.Professor of Biochemistry
2ObjectivesProvide a working definition of epigenetics and contrast an epigenetic change with a mutation.Contrast the normal process of genomic imprinting with abnormal changes in epigenetic tags seen in several diseases described.Describe evidence that nutritional status can influence the epigenetic profile of later generations.Illustrate and describe epigenetic tags involving cytosine methylation and histone acetylation.Describe the mechanism of action of Vorinostat.Interpret the divergence of epigenomes of identical twins with respect to potential difference in disease presentation between twins.
3Epigenetics Epigenetics literally means ‘above’ the genetics. Has had multiple definitions over time.2008 Cold Spring Harbor Epigenetics meeting:“An epigenetic trait is a stably heritable phenotype resulting from changes in a chromosome without alterations in the DNA sequence.”Alterations in the DNA sequence = mutations
4Epigenetic ResearchThe number of publications in the field has increased dramatically in the last 10 years.Genetic Engineering and Biotechnology News Feb 1, 2013 (Vol. 33, No. 3)
5Epigenetic EffectsThe effects of epigenetics have been known for many years.Lyon Hypothesis from 1961.Renamed the Lyon Law in 2011.Inactive X chromosome is heavily epigenetically modified.
6Another Familiar Epigenetic Case Chromosome 15 imprintingPrader-Willi SyndromeAngelman Syndrome
7Angelman/Prader-Willi Commonly referred to as genomic imprinting.Imprinting is not the cause of these syndromes but is responsible for the unique presentation of these diseases.
8Genomic ImprintingWith genomic imprinting, it is thought that the maternal or paternal imprint is erased with each succeeding generation (meiotic division).A male receives a maternally imprinted and paternally imprinted chromosome 15 but will always transmit a paternally imprinted chromosome 15.Note that the maternal/paternal imprinting is heritable through mitosis.Somatic Cells***Germ Cells****
9Genomic ImprintingImportantly, X-inactivation and genomic imprinting are normal processes.Much of the recent research has analyzed when the process of epigenetics is altered from normal.This has involved the study of changes within somatic cells in disease.It has also involved the study of changes within the germ cells (heritable epigenetics).
10Heritable Epigenetics Evidence suggests that environmental information could be propagated through meiosis.Studies of Dutch famine of 1944.Famine during last two trimesters of pregnancy:8-9% decrease in child’s birth weight (SGA).Offspring of these SGA children tended to be normal size.Famine early in pregnancy but not late:Normal size infants were born.Offspring of these non-SGA children exhibited high rate of SGA.
11Överkalix Study Retrospective study conducted in Överkalix, Sweden. Divided population into three cohorts:Born1890Born1905Born1920Assessed each cohort for access to food during slow growth period (SGP) of adolescence (8-10 girls, 9-12 boys).Cardiovascular and diabetes mortality determined by nutrition during parents' and grandparents' slow growth period. Kaati G, Bygren LO, Edvinsson S. Eur J Hum Genet. 2002, 10:682-8.
12Överkalix Study Results When the father (P=0.05) was exposed to a famine during his SGP, his offspring exhibited protection against cardiovascular causes of death.Paternal grandmother exposure to famine also showed a trend (P=0.11) towards similar protection in grandchildren.If the paternal grandfather lived through a famine during his SGP it tended to protect grandchildren from diabetes (P=0.09).
13Överkalix Study Results If the paternal grandfather had an abundance of food during their SGP, their grandchildren had a four-fold increased risk for death of diabetes mellitus.One mechanism to explain these results is transmission of epigenetic markers that were influenced by the environment of the parent.Effect on grandchildren suggests the markers are maintained through multiple generations.
14Lamarkism? Jean Batiste Lamark (1744-1829) Inheritance of acquired characteristics.Largely discounted with Darwin’s publication of Origin of Species and the rediscovery of work of Mendel.Recent work in epigenetics suggest Larmark may have been correct to some degree.
15Molecular Basis of Epigenetics Two primary mechanisms identified.Methylation of cytosine nucleotides in DNAPosttranslational modification to histone proteins.Includes acetylation, methylation and phosphorylationA third proposed mechanism involves expression of small interfering RNAs (siRNA).
16Cytosine MethylationMethylation of cytosine occurs at CpG dinucleotides.Often located just upstream of genes (promoter regions).Associated with attenuation of expression of nearby genes.
17Histone ModificationHistones are the proteins that organize the genetic material.Have a high percentage of basic amino acids, which gives histones an overall positive charge.Positively charged amino acids associate with the overall negative charge of the DNA.
18Histone ModificationMost histone modification occurs on the extended tails of histone proteins.Modifications influence the association of histones with the DNA and patterns of gene expression.Best studied modification is histone acetylation.
19Histone Acetylation Two enzyme types involved in histone acetylation HAT: histone acetyltransferaseHDAC: histone deacetylaseNote that acetylation eliminates the positive charge from the amino acid.It is thought that this changes the chromatin conformation to a form more open to transcription.⬆ acetylation = ⬆gene expression.
20HAT/HDAC and Hydrophobic Hormones It is thought that hydrophobic hormones like thyroid hormone and glucocorticoid influence gene expression by binding to either HDAC or HAT proteins.⬆ acetylation = ⬆gene expression.
21Epigenetic Errors Unaffected individuals have 6-50 CGG repeats. Fragile X syndrome is most commonly caused by a CGG trinucleotide repeat expansion in the 5’ region of the FMR1 gene.Unaffected individuals have 6-50 CGG repeats.>200 CGG repeats is seen in individuals with fragile X.>200 CGG repeats is correlated with hypermethylation at CpG dinucleotides and silencing of the FMR1 gene.
22Epigenetics and Cancer DNA repair is a critical process to maintain genomic fidelity.Loss of DNA repair is thought to be a major contributor to the development of cancer.Epigenetic changes involving DNA repair genes are thought to be a major early step in cancer progression.~13% of sporadic breast cancers and 5-30% of ovarian cancers present with hypermethylation of the BRCA1 gene.40-90% of sporadic colorectal cancer has hypermethylation of the MGMT gene (O6-methylguanine methyltransferase).
23Therapies Targeting Epigenetic Errors In contrast to mutations, epigenetic changes can be reversed.Are there therapies that influence epigenetic patterns?YesVorinostat (trade name Zolinza) approved by FDA for cutaneous T cell lymphoma in 2006.Vorinostat is a histone deacetylase inhibitor.⬆ acetylation = ⬆gene expression.X
24Combination Therapy Phase III Clinical Trial Vorinostat plus cytarabine and idarubicin.85% remission rate after initial treatment.
25Our EpigenomeIf epigenetic markers are dynamic and respond to environmental influences, do they change over time?Evidence suggests the answer is yes.Twin studies have been highly informative for this question.
26Author Statement“We found that, although twins are epigenetically indistinguishable during the early years of life, older monozygous twins exhibited remarkable differences in their overall content and genomic distribution of 5-methylcytosine DNA and histone acetylation, affecting their gene-expression portrait.”
27Chromosomal Level Comparative genomic hybridization for methylated DNA Yellow = similar chromosome methylation pattern between twins.Red = regions of hypomethylation in one twin compared to the other.Green = regions of hypermethylation in one twin compared to the other.
28Epigenomic Alterations If the epigenome changes as we age, what kinds of things can induce these changes?Very active area of current research.Some interesting findings:Fear conditioning induces changes in DNA methylation in the brain derived neurotrophic factor (BDNF) gene promoter region in hippocampal neurons of rat brains.
29Epigenomic Alterations In rats, social deprivation during the 1st postnatal week triggers changes in DNA methylation across the BDNF gene.This was later associated with decreased BDNF gene expression in the prefrontal cortex of adult experimental animals.A schizophrenic-type state can be induced in mice when they are chronically given l-methionine (substrate for methyltransferase enzymes).Studies with cocaine exposure suggest that the drug induces acetylation of the BDNF gene histones that is transmittable to future male offspring.
30Epigenetics and Osteopathic Manipulation Is it plausible that osteopathic manipulation could influence gene expression through modulation of epigenetic tags on treated tissue?
31SummaryEpigenetic traits are heritable phenotypes resulting from changes in chromosomes that do not involve changes in DNA sequence.Scientific and medical interest in epigenetics has increased dramatically in recent years.Two prominent epigentic mechanisms involve DNA methylation (gene silencing) and histone acetylation (gene activation).Errors in epigenetic patterns can influence the presentation of human diseases including cancer.The epigenome changes as we age and can be influenced by the environment.Drugs that influence the epigenome represent a major area of current research.
32ReferencesBerger, S.L. et. al An operational definition of epigenetics. Genes Dev. 23,Kaati, G. et. al Cardiovascular and diabetes mortality determined by nutrition during parents' and grandparents' slow growth period. Eur. J. Hum. GenetEsteller, M. et. al Promoter hypermethylation and BRCA1 inactivation in sporadic breast and ovarian tumors. J Natl Cancer Inst. 92,Shen, L. et. al MGMT promoter methylation and field defect in sporadic colorectal cancer. J Natl Cancer Inst. 97,Garcia-Manero, G Can we improve outcomes in patients with acute myelogenous leukemia? Incorporating HDAC inhibitors into front-line therapy. Best Pract Res Clin Haematol. 25,Fraga, M.F. et al Epigenetic differences arise during the lifetime of monozygotic twins. Proc Natl Acad Sci U S A. 102,Roth, T.L. et. al Lasting epigenetic influence of early-life adversity on the BDNF gene. Biol PsychiatryVassoler, F.M Epigenetic inheritance of a cocaine-resistance phenotype. Nat. Neurosci. 16,