Lecture 19: Epigenetics Flashcards
(41 cards)
What provides evidence for the importance of epigenetics?
genotype =/= phenotype
- monozygotic twins are genotypical identical but not always phenotypically identical, and these differences can’t always be explained by environment
- mice with genotype A^vy/a genotype are also genetically identical, but some have brown fur and some have yellow fur
define epigenetic
the study of mitotically and/or meiotically heritable changes in gene function that cannot be explained by changes in DNA sequence
state the 4 molecular mechanisms underlying epigenetics
- DNA methylation on CpG islands
- covalent modification of histone tails
- noncovalent modification of histones
- non-coding RNAs
DNA methylation on CpG islands
CpG = Cytosine-phosphate-guanine nucleotides
- some regions have many CpG islands, in which cytosine may be either methylated or unmethylated
- typically, unmethylated promoters enable gene expression
- typically, methylated promoters result in heterochromatin, repressing gene expression
- the effects of methylation depend on the region; sometimes DNAm leads to silencing, sometimes it leads to increased expression
covalent modification of histone tails
- acetylation: loosens chromatin by neutralizing lysine charges, promoting gene expression
- methylation: can activate or repress gene expression, depending on the site and number of methyl groups.
- ubiquitination: tags histones for degradation or signals chromatin remodeling, affecting transcriptional activity
non-covalent modification of histones
reposition nucleosomes to make DNA more or less accessible.
non-coding RNAs
Transcriptional Silencing:
Small RNAs (like siRNAs) guide chromatin modifiers to DNA, causing heterochromatin formation and blocking transcription.
how does chromatin structure affect transcription?
- promoters can be hidden when wrapped in nucleosomes, leading to lowered gene expression
- chromatin remodelling complexes can expose gene promoters, allowing RNA polymerase to bind
- nucleosomes in heterochromatin can be tightly packed, generating silenced heterochromatin
epigenetic changes causing coat colour differences can be influenced by
the environment
regulation of gene expression in A^vy/a mice
- the agouti gene is normally expressed in a regulated pattern, giving banded hairs (black-yellow-black) and brown/gray fur.
- the Aᵛʸ allele contains an IAP (Intracisternal A Particle) retrotransposon upstream of the Agouti gene.
- the IAP has a promoter that can drive ectopic (abnormal) Agouti expression.
- Unmethylated IAP → promoter is active → overexpression of Agouti → yellow coat.
- Methylated IAP → promoter silenced → Agouti expressed normally (or not at all) → brown or mottled coat.
define a metastable epiallele
an allele whose expression is not fixed but can vary in a stable way between cells or individuals due to epigenetic states established early in development.
how is A^vy an example of a metastable epiallele?
its expression is controlled by DNA methylation at a retrotransposon promoter, leading to variable, heritable gene expression without DNA sequence changes.
what two main points were derived from experiments involving agouti mice?
- Maternal diet can directly affect the epigenetic regulation of genes in offspring.
- These changes can be stable enough to affect not just the immediate offspring, but also subsequent generations.
How was the idea that maternal diet can directly affect the epigenetic regulation of genes in offspring understood?
- When pregnant mothers were fed a diet rich in methyl donors, their offspring showed a higher proportion of the pseudoagouti phenotype (brown fur).
- This was because methylation silenced the Agouti gene, preventing ectopic expression and resulting in darker coat color.
- When mothers were fed a normal diet, their offspring showed more of the yellow or mottled phenotypes.
- This was due to lower methylation of the Agouti gene, allowing it to be expressed and leading to lighter coat color.
How was the idea of intergenerational epigenetic inheritance understood?
- F0 mothers were supplemented with methyl donors during pregnancy.
- Her F1 offspring showed increased methylation at the Agouti locus.
- The F2 generation also exhibited more pseudoagouti phenotypes, despite not being directly exposed to the diet.
- This suggested that the epigenetic marks were inherited through the germline.
- F0 mothers were supplemented with no methyl supplementation.
- the F1 and F2 offspring showed more yellow or mottled phenotypes, indicating low methylation.
how is epigenetic required for normal development?
specific functions of different cell types are generated through differential gene regulation
explain the randomness of X-chromosome inactivation and its consequences
- random X-chromosome inactivation in females occurs early during development for dosage compensation
- these X-chromosomes are reactivated in germ cells
- inactivation of the paternal/maternal X chromosome is random but persists in the subsequent cells produced
- this means that females express X^m in some cells and X^p in others, leading to clonal patches
give an example of X-chromosome inactivation and its phenotypic consequences in animals
in the calico cat, X-inactivation leads to a mosaic of fur colours
mechanism of X-chromosome inactivation
- process begins at the X-inactivation centre (XIC), which activates the gene Xist.
- Xist makes a long non-coding RNA (lncRNA) that spreads across the X chromosome, coating it and starting the silencing process.
- Hypoacetylation of a Lys of histones (H3/H4), methylation of histone H3 and underlying DNA shut down gene expression
- the inactive X becomes tightly packed into a structure called a Barr body.
- most genes on this X are turned off, but a few, called escapees, remain active.
how does histone acetylation lead to more loosely packed DNA?
- histone tail usually positively charged, DNA negatively charged
- when the tail is acetylated, the charge is neutralised and DNA becomes more loosely packed
Hypoacetylation vs Hyperacetylation
Hypoacetylation = regions that are silenced
Hyperacetylation = transcriptionally active regions
gene therapy approach for Down syndrome
autosome + one copy of Xist -> autosome becomes a heterochromatic Barr body
thus, we could add Xist to one of the 3 copies of chromosome 21 in children with Down syndrome
what is a synonym for parental imprinting?
genomic imprinting
how does parental imprinting come about?
it results from transcriptional silencing
