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Concepts Of Disease > Epigenetics > Flashcards

Epigenetics Flashcards

1
Q

What are two types of epigenetic marks?

A
  1. DNA methylation
  2. Histone modifications
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2
Q

What is DNA methylation? Where does it occur? What is the product of DNA methylation?

A

DNA methylation is a chemical modification of DNA by the addition of methyl (CH3 group) .

Methyl group is added to number 5 carbon or cytosine pyramidine ring. Only occurs if CG.

Results in 5-methylcytosine.

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3
Q

What are the enzymes involved in methylation?
What are the types of these enzymes?

A

Methyltransferases: DNMT1, DNMT3a, DNMT 3b.

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4
Q

What happens when a methyl group is lost?

A

When DNA methylated it is less stable- so undergoes a reaction so its a thymine CG dinucleotides are 5 times less frequent that expected because mC has tendency to mutate to T.
Undergoes spontaneous deamination.

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5
Q

What are GpG islands? Are cytosines usually unmethylated or methylated?

A

A region where there are more CG dinucleotides.
0.5-2Kb region.
Cytosines are generally unmethylated in CgP islands

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6
Q

What are the main mechanisms by which the methylation of DNA can prevent the transcription of genes?

A

The methyl group changes the recognition sequence - this can prevent protein binding. Attracts a different protein.- blocks protein binding.
Attracts methyl- binding domain proteins (MBDs) which recruit protei complexes that modify histones.

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7
Q

What is the role of DNMT1?

A

Recognises hemi-methylated DNA- So adds methyl group to other strand.

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8
Q

Where does DNMT1 accumulate?

A

Replication sites by binding to proliferating cell nuclear antigen (PCNA) and an E3 ubiquitin ligase (UHRF1- scan genome looking for hemi-methylated).

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9
Q

In the each generation what is the percentage methylation in what percentage of cells?
G0
G1
G2

A

G0- 100% methylation in 100% cells
G1- 50% methylation in 100% cells
G2- 50% methylation in 50% cells, 0% in 50% cells

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10
Q

What are the two ways of DNA demethylation?

A

Passive demethylation
Rapid demethylation

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11
Q

What is histone modification?

A

Tails that protrude from nucleosomes- these tails are modified - through methylation, phosphorylation, Ubiquitinalated.

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12
Q

What is euchromatin?

A

Looser- more active

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13
Q

What is heterochromatin?

A

Tighter- silent

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14
Q

What is acetylation? What enzyme is involved?

A

Acetylation of lysine on histone H3 Tails- by histone acetyl transferases (HATs) - NEGATIVE So repel- so become loose

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15
Q

How are acetyl group removed?

A

Histone decetylases (HDACs) - Associated with gene silencing- DNA crunch back up as charge is removed

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16
Q

What are the two suggest models of how histone modifications can be inherited through the cell cycle?

A

Model 1- After DNA replication, the methylated nucleosomes are distributed equally to the two daughter strands
New MTCs re-establish the methylated domain.
Model 2- After DNA replication, the modified histones are lost but the MTC complex is retained at the replication fork
The MTC complex then methylated the new histones

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17
Q

What is epigenetics?

A

study of mitotically/meiotically heritable changes in gene function that do not entail a change in DNA sequence

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18
Q

What does the Dolly the sheep expriment prove regarding cloned cells?

A

that cells can retain the ability to make DNA even when they are differentiated - shows that all cells have the same genetic information, they just express different genes

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19
Q

What are epigenetic modifications?

A

marks that can be added/removed from DNA that control when genes are transcribed. they are inherited when cells divide and some are passed on to the next generation

20
Q

Which chromosome is bigger

A

X chromosome - 150 million bc compared to less than 60 on the Y

21
Q

what structure does the inactive chromosome form

A

the ‘Barr body’ - a heterochromatic structure

22
Q

What is the XIC

A

the X inactivation centre, cis-acting master switch locus that controls X inactivation

23
Q

What is Xist

A

X inactive specific transcription
a 15-75 kb spliced and polyadenylated non-coding transcript
transcribed and not translated as the rna is the functional part
rna coats the inactive X chromosome to inactivate them

24
Q

What is the relationship between Xist and Tsix

A

at the beginning, both X chromosomes express both until one chromosome starts expressing more Xist and becomes the inactive X

25
Q

What are features of the active X chromosome

A

histone tails acetylation, histone H3 lysine 4 tri-methylation on promoters, variant histones H3.3 and H2Abb enriched, poly comb histone modifications at silent loci, gene promoters depleted of DNA methylation, DNA methylation in gene bodies

26
Q

what are features of the inactive X chromosome

A

expresses Xist, histone tails hypo-acetyl ate, histone H3 lysine 4 methylation depleted, poly comb histone modifications enriched, variant histone macroH2A enriched, DNA methylation of gene promoters, DNA hypomethylation in gene bodies

27
Q

How is X inactivation maintained

A

DNA methylation

28
Q

What are the chances of inheriting an X-linked recessive disease if the mother or father have the mutant?

A

if the mother has it: 50% of male offspring will have the disease, 50% of female will be carriers
if the father has it: none of the male have it, all female will have it

29
Q

Give two examples of x-linked recessive diseases and their symptoms

A

haemophilia A - deficiency of blood clotting, joint and muscle bruising and haemorrhaging
hypohidrotic ectodermal dysplasia - mutation in ectodysplasinA, defect in sweat glands hair and teeth

30
Q

What are the chances of both a mother and father passing on a mutant in X-linked dominant

A

if the mutant is from the mother - 50% chance all children have the disease
if the mutant is from the father - none of the sons will, all of the daughters will

31
Q

what is rett syndrome

A

x-linked dominant, mutation in MCGB binding protein 2. causes a regression and autism, dementia, loss of speech, mental retardation

32
Q

what are intermediate phenotypes

A

phenotype depends on circulating products such as in haemophilia, has an averaging effect between normal and abnormal cells

33
Q

what is skewed X inactivation

A

in heterozygous females, x linked disorders:
dominant mutations - if mutant X is inactivated = normal phenotype
recessive mutations - if normal X is inactivated, disease phenotype

34
Q

What is a begnin ovarian teratoma?

A

A germ cell tumour
Rare
Consists of tissues derived from all three germ layers
Can have hair, sebaceous glands, skin and teeth
Most ovarian teratomas are benign
Most have two maternal genomes

35
Q

What is a Hydatidiform mole?

A

A gestational trophoblastic disorder
• Rare
• About one in every 1,200 pregnant women
• Disorganised mass of placental tissue without a fetus
• Most molar pregnancies are benign
• Most have two paternal genomes.

36
Q

What is genomic imprinting?

A

The process by which only one copy of a gene in an individual (either maternal or paternal) is expresssed, while other copy is suppressed

37
Q

In a teratoma what genes have a loss of expression and what genes have a gain in expression?

A

Loss of expression of normally paternally expressed genes
Gain in expression of normally maternally expressed gene

38
Q

In the Hydatidiform mole what genes have a loss of expression and what genes have a gain in expression?

A

Gain in expression of normally paternally expressed genes
Loss of expression of normally maternally expressed genes

39
Q

What is the role of IGF2?

A

A growth factor that promotes cell division and growth. It is a paternally expressed gene- which encodes insulin-like growth factor 2

40
Q

What is CDKN1C?

A

Is a cell cycle inhibitor which inhibits cell division and growth. A maternally expressed gene- encoded cyclin- dependent kinase inhibitor 1c

41
Q

What are the characteristics of beckwith weidemann syndrome?

A

• Affects 1 in 13,700
• Babies in the 95th weight percentile at birth
• Macroglossia (large tongue)
• Omphalocoele (abdominal wall defects)
• Predisposition to Wilms’ tumour
• Cleft palate
• Placentomegaly (large placenta)
• Neonatal hypoglycaemia (low blood sugar)

42
Q

What causes beckwith weidemann syndrome?

A

Two paternal copies of IGF2 and no CDKN1C

43
Q

What are the characteristics of silver Russel syndrome?

A

• Affects 1 in 100,000
• Low birth weight – below 5th percentile
• Poor postnatal growth
• Classic facial phenotype
• Asymmetry
• Lack of subcutaneous fat
• Night sweats

44
Q

What causes Silver Russel syndrome?

A

Two maternal copies- so too much CDKN1C and no IGF2

45
Q

What can cause diseases to be more prevalent in some people?

A

Those that are exposed to adversity in early life- i.e diet stress, obesity, smoking - cans lead to low brith weight or an increase in near development disorder, mental health disorders, metabolic syndrome, cardiovascular disease.

46
Q

What are epidrugs? What types are there?

A

Chemical agents that modify the structure of DNA and disruption. Histone deactylases removes acetyl groups from chromatin
DNA methylation inhibitors remove dna Methylation

Concepts Of Disease (5 decks)

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