Presentation on theme: "Epigenetics Heritable alterations in chromatin structure can govern gene expression without altering the DNA sequence. Viterbo Università degli Studi della."— Presentation transcript:
1 EpigeneticsHeritable alterations in chromatin structure can govern geneexpression without altering the DNA sequence.ViterboUniversità degli Studi della Tuscia
2 Epigenetics denotes all those hereditary phenomena in which the phenotype is not onlydetermined by the genotype (the DNAsequence itself) but also by theestablishment over the genotype(in greek “epi” means “over”) of animprint that modulates itsfunctional behavior
3 Invertebrates and Plants Polycomb group proteins Epigenetic phenomenaVertebrates,Invertebrates and PlantsGenic, chromosome and genomic imprintingHeterochromatin formationCentromere functionRNA interference (PTGS)ParamutationRIP e MIP (Quelling)Polycomb group proteinsTransvectionEukaryotesMammalsDrosophilaEukaryotesDrosophilaPlantsFungi
5 x A x x x A x A zygote Differential behavior of homologous chromosomes maternal genomepaternal genomezygotexASciara coprofilaxxDifferentialbehavior of homologous chromosomesembryoxAembryoxAThe chromosome which passes through the male germ line aquires an imprint that results in behaviour exactly opposite to the imprintconferred on the same chromosome by thefemale germ line(H. Crouse, 1960)
6 Nuclear transplantation in mammals zygotePMMPandrogenetic embryos(two male pronuclei)gynogenetic embryos(two female pronuclei)Poor development of the embryo properPoor development of extraembryoniccomponents
7 Angelman, Prader-Willi syndromes Usually caused by large (megabase+) deletions of 15q11-q13Delete maternal chromosome = ASDelete paternal chromosome = PWS
8 Prader-Willi Syndrome - obesity, mental retardation, short stature.Angelman Syndrome - uncontrollable laughter, jerkymovements, and other motor andmental symptoms.
11 Imprinting cycle establishment, maintenance and erasure
12 What Mendel (fortunately) didn’t find in his experiments with peas 1:1
13 NO Does the genomic imprinting falsifies the Mendel’s rules? Neither the segregation of single gene alleles, nor the indipendentbehavior of different genes are affected by the existenceof imprintingWhat the imprinting may mask are the dominance relationsbetween alleles, and hence only the phenotypic output of a cross
14 NUCLEATION AND MAINTENANCE HETEROCHROMATINNUCLEATION AND MAINTENANCE
15 In 1928, Heitz defined the heterochromatin as regions of chromosomes In 1928, Heitz defined the heterochromatin as regions of chromosomes that do not undergo cyclical changes in condensation during cell cycle as the other chromosome regions (euchromatin) do.Heterochromatin is not only allocyclic but also very poor of active genes, leading to define it as genetically inert (junk DNA).Heterochromatin can be subdivided into two classes: constitutive heterochromatin and facultative heterochromatin.Constitutive heterochromatin indicates those chromatin regions that are permanently heterochromatic. These regions occupy fixed sites on the chromosomes of a given species, are present in both homologous chromosomes, throughout the life cycle of the individual.Facultative heterochromatization is a phenomenon leading to the developmentally or tissue-specific co-ordinate reversible inactivation of discrete chromosome regions, entire chromosomes or whole haploid chromosome sets.
16 Position Effect Variegation (PEV) White+pericentricheterochromatinDrosophila melanogaster X chromosomeW+W-YinversionWhite+Wm4Wm4W-Y
17 In all cases an inversion or translocation changed the position of the gene from a euchromatic to heterochromatic positionthis results in variegationSome rearrangements gave large patches of red facets adjacent tolarge patches of whiteConclusion: Decision on expression of white is made early duringtissue development and maintained through multiple cell divisionsGene is not mutated – movement of the rearranged allele awayfrom heterochromatin can restore expressionPEV is not limited to Drosophila: see telomeric silencing in yeast
18 X chromosome inactivation The Barr bodyXYXXXXXXXXXYXXXXX
19 Genotype is Xyellow/Xblack Yellow patches: black allele is inactive In mammals the dosage compensation of the X chromosome products, between XX females and XY males is achieved by inactivating one of the two Xs in each cell of a female (Mary Lyon, 1961)Genotype is Xyellow/XblackYellow patches: black allele is inactiveBlack patches: yellow allele is inactiveXyellow/Xblack
28 HP1 and modified histone tails interactions during heterochromatin formation non histone chromatinproteins: HP1euchromatinheterochromatin9KMe
29 Histone Code and Transcriptional Silencing Epigenetic modifications leading to gene silencing.(A) Gene repression through histone methylation.Histone deacetylase deacetylates lysine 9 in H3,which can then be methylated by HMTs. Methylatedlysine 9 in H3 is recognised by HP1, resulting inmaintenance of gene silencing.B) Gene repression involving DNA methylation. DNAmethyltransferases methylate DNA by converting SAMto SAH, a mechanism that can be inhibited by DNMTinhibitors (DNMTi). MBPs recognise methylated DNAand recruit HDACs, which deacetylate lysines in thehistone tails, leading to a repressive state.(C) Interplay between DNMTs and HMTs results inmethylation of DNA and lysine 9 in H3, and consequentlocal heterochromatin formation. The exact mechanismof this cooperation is still poorly understood.
30 Histone Code and Transcriptional Activation Epigenetic modifications leading to gene activation.(A) Setting 'ON' marks in histone H3 to activate genetranscription. Lysine 4 in H3 is methylated by HMT (forexample MLL) and lysine 9 is acetylated by HAT, allowinggenes to be transcribed. It is not known, if HMTs and HATshave a direct connection to each other.(B) In the postulated 'switch' hypothesis, phosphorylationof serines or threonines adjacent to lysines displaceshistone methyl-binding proteins, accomplishing a bindingplatform for other proteins with different enzymaticactivities. For example, phosphorylation of serine 10 inH3 may prevent HP1 from binding to the methyl mark onlysine 9. Other lysines in H3 may be acetylated by HATs,therefore overwriting the repressive lysine 9 methyl markand allowing activation.(C) Although there is no HDM identified to date, one canspeculate that, if this enzyme exists, serine 10 phosphorylationin H3, for example, by Aurora kinases, can lead to recruitmentof HDMs that in turn demethylate lysine 9 in H3. Histoneacetyltransferases might then acetylate lysine 9 and HMTsmethylate lysine 4, resulting in the loosening of the chromatinstructure and allowing gene transcription.
31 Histone Modification Cassettes Methylation of Lys-9 by DIM-5 (SUVAR39H1) recruits HP1 via its chromodomain.In turn, HP1 can recruit additional SUVAR39H1 and other silencing proteins toestablish heterochromatin.Phosphorylation of Ser-10 abolishes methylation of Lys9 by DIM-5 (SUVAR39H1) andbinding of the HP1, thereby blocking heterochromatin formation.Phosphorylation of Ser-10 can modestly stimulate acetylation of Lys14 by GCN5,thus promoting transcription.Lys-9 and Ser-10 have been referred to as a methyl/phos switch:Fischle W, Wang Y, Allis CD. Nature. 2003;425:475-9.