1. EPIGENETICS
Professor Victoria Sarafian, MD, PhD, DMSc

2. Epigenetics (Greek: επί- above, over, outside of, around;  "extra growth")
Phenotypic trait variations resulting from external or environmental factors that affect gene expression (switch genes on and off).
 Functional changes to the genome that do not involve a change in the nucleotide sequence.
 “Stably heritable phenotype resulting from changes in a chromosome without alterations in the DNA sequence"  
Mechanisms - DNA methylation & Histone modifications  

4. DNA methylation - provides stable gene silencing that plays an important role in regulating gene expression and chromatin architecture;
primarily occurs by the covalent modification of cytosine residues in CpG dinucleotides - concentrated in short CpG-rich DNA stretches called ‘CpG islands’ and regions of large repetitive sequences (e.g. centromeric repeats);
CpG islands – C separated by phosphate – G; preferentially located at the 5′ end of genes and occupy ∼60% of human gene promoters.
Most of CpG sites in the genome are methylated.
Some CpG island promoters become methylated during development, which results in long-term transcriptional silencing - X-chromosome inactivation and imprinted genes.

5. DNA methylation and histone deacetylation induce a closed chromatin configuration and transcriptional repression!!!

6. Histone modifications - histone modifying enzymes can add or remove functional groups to the histones, and these modifications influence the level of transcription of the genes wrapped around those histones and the level of DNA replication.
cancer cells - loss of histone H4 acetylation; might be battled with a histone deacetylase inhibitor (HDAC)

7. With inhibition of histone deacetylases (HDACs) by HDAC inhibitors, histones are acetylated, and the DNA that is tightly wrapped around a deacetylated histone core relaxes and leads to cell-growth arrest, differentiation and/or apoptotic cell death and, as a consequence, inhibition of tumor growth. HAT – histone acetyl transferase

8. Cancer epigenetics - epigenetic modifications to the genome of cancer cells that do not involve a change in the nucleotide sequence; as important as mutations.
normal cells – CpG preceding gene promoters are unmethylated and transcriptionally active
cancer cells - CpG preceding tumor suppressor gene promoters are often hypermethylated; CpG of oncogene promoter regions is often unmethylated.

9. Epigenetic code - set of epigenetic features that create different phenotypes in different cells. Represents the total state of the cell, with the position of each molecule accounted for in an epigenomic map, a diagrammatic representation of the gene expression, DNA methylation and histone modification status of a particular genomic region.
Specific epigenetic processes include : imprinting, gene silencing, X-chromosome inactivation, position effects, maternal effects, carcinogenesis, cloning, etc.

10. Twins - age-dependent accumulation of epigenetic differences between the two siblings of twin pairs; suggests the existence of epigenetic “drift”.

11. Addiction - disorder of the brain's reward system (centres of pleasure & desire) which arises through transcriptional  and neuroepigenetic mechanisms. They occurs over time from chronically high levels of exposure to an addictive stimulus (e.g., morphine, cocaine, sexual intercourse, gambling, etc.).
 Transgenerational
epigenetic inheritance

12. Genomic imprinting – a genetic phenomenon by which certain genes are expressed in a parent of origin-specific manner – Non-Mendelian inheritance
 For some genes, only one copy is expressed. Expression of these genes depends on which parent the gene came from -genomic imprinting. The active gene is preferentially transmitted from one parent over the other.
 Imprinting does not occur on every chromosome; only nine chromosomes are known to have imprinted genes.
 Imprinting occurs by a pattern of methylation - the copy of the gene to be inactivated is coated with methyl groups. This takes place before fertilization, in the egg and sperm cells. The methylation prevents that gene from being expressed.
 Genomic imprinting is a reversible form of gene inactivation and is not considered a mutation.

13. Different expression of an allele depending on its parental origin – the functional inequality of both alleles of a gene is due to DNA methylation
 There is no change in DNA sequences but in its structure and functional activity– epigenetic modifications!
 In gametogenesis, before fertilization, the maternal or paternal allele of a gene is inactivated (imprinted) by methylation
 Active genes are not methylated!
 Less than 1% of the genes are imprinted
There are over 80 imprinted genes in mammals – control of embryogenesis and placental development

14. Uniparental disomy (UPD) – inheritance of a chromosome or a part of it only from one of the parents, without a second copy from the other parent
Chromosomal nondisjunction during meiosis leads, after fertilization, to trisomy. Subsequent loss of one chromosome (rescue) could lead to the formation of cells with a chromosome from each parent or cells in which both chromosomes were from the disomic gamete.

15. Usually, UPD likely has no effect on health or development
Usually, UPD likely has no effect on health or development. Because most genes are not imprinted, it doesn’t matter if a person inherits both copies from one parent instead of one copy from each parent. In some cases a person with UPD may lack any active copies of essential genes that undergo genomic imprinting. This loss of gene function can lead to delayed development, intellectual disability, or other health problems.

16. Deletion of the same region in chromosome 15
The maternal chromosome is imprinted (inactivated); thus only the paternal 15 chrom. is active; in case a deletion in the paternal chromosome Prader-Willi syndrome develops (obesity, muscular hypotonia, sterility, thin upper lip, protruding nose)

17. Normally, a specific region of the long arm of the maternally inherited chromosome 15 is imprinted - turned off through methylation. An individual will not have PWS as long as he can transcribe and translate genes along that same portion of chromosome 15 from the paternally inherited chromosome. When an abnormality of the paternally inherited 15q occurs, causing loss of gene expression in that region, PWS results.

18. Deletion of the same region in chromosome 15
the paternal chromosome is imprinted (inactivated); in case a deletion in the maternal chromosome Angelman syndrome occurs - “smiling puppets”, convulsions, mental retardation
“A girl with a picture" Giovanno Caroto

19. Normally, a specific region of the long arm of the paternally inherited chromosome 15 is imprinted - turned off through methylation. An individual will not have AS as long as he can transcribe and translate genes along that same portion of chromosome 15 from the maternally inherited chromosome. When an abnormality of the maternally inherited 15q occurs, causing loss of gene expression in that region, AS results.

21. “Patience is a virtue – a hard lesson to be learned“.