Epigenetics: A historical overview Dr. Robin Holliday

Transcription

1 Epigenetics 1 Rival hypotheses Epigenisis - The embryo is initially undifferentiated. As development proceeds, increasing levels of complexity emerge giving rise to the larval stage or to the adult organism. Preformation - Complexity already exists in the fertilised egg or even gamete. 2 Thomas Hunt Morgan 3 The screen versions of these slides have full details of copyright and acknowledgements 1

2 CH Waddington 4 Ernst Hadorn1 5 Three features of development that demand explanation Differentiated cells maintain their specialised phenotypes through cell division When stem cells divide, one daughter cell is differentiated while the other daughter remains an undifferentiated stem cell X chromosome inactivation in eutherian mammals a pair of almost identical chromosomes reside in the same nucleus but one is inactivated and one is not 6 The screen versions of these slides have full details of copyright and acknowledgements 2

3 Arthur Riggs 7 DNA methylation 1975 papers by Holliday and Pugh and by Riggs Both suggested that methylation of cytosine to 5 methyl cytosine in mammalian DNA might be a mechanism of gene control. Methylation could be sequence specific The site of methylation is the C of a CpG doublet. This is a palindrome. Hemi-methylated sites would be methylated by a maintenance methylase thus providing a mechanism for the inheritance of the methylation status of any site 8 Evidence for methylation X inactiv ation gene silencing was shown to be associated with DNA methylation Cancer tumour suppressor gene promoters are methylated and these genes are therefore inactive. 9 The screen versions of these slides have full details of copyright and acknowledgements 3

4 Evidence for methylation Isoschizomers MspI and HpaII both cut GCGC but only MspI will cut this site if the first C is methyl ated. Comparing Southern blot patterns to exami ne the methylation of these sites has shown that many genes are inactivated by methyl ati on of promoter sequences. Walter Doerfler 5-aza-cytidine is a potent demethyl ati ng agent and can reactivate many inactive genes. 10 Definitions of epigenetics The study of the changes in gene expression which occur in organisms with differentiated cells and the mitotic inheritance of a given pattern of gene expression Nuclear inheritance which is not based on differences in DNA sequence All those events which lead to the unfolding of the genetic program for development 11 Dual inheritance Classical genetic inheritance is based on changes in DNA sequence and epigenetic inheritance is not based on changes in DNA sequence John Maynard Smith 12 The screen versions of these slides have full details of copyright and acknowledgements 4

5 Mechanisms other than DNA methylation Histone acetylation and methylation Alternative splicing of RNA transcripts Rearrangement of DNA sequences (as in the immunoglobulin genes) Enzyme induced mutation RNAi 13 The epigenome project Bisulphite treated DNA sequencing reveals methylated cytosines Compare different cell types Expect to see no differences in the methylation patterns of some genes Expect to see important differences in the methylation patterns of genes coding for luxury proteins The epigenotype all the genes necessary for household and luxury functions and also those specialised genes that are silent or repressed in a given cell type. 14 Summary Historical account of genetics and developmental biology Epigenetics attempted unification Insights from molecular models particularly DNA methylation New mechanisms especially those involving RNA The Human Genome project has led to the Human Epigenome Project 15 The screen versions of these slides have full details of copyright and acknowledgements 5