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M2M2 > Imprinting > Flashcards

Imprinting Flashcards

1
Q

Epigenetics

A

Mitotically and mieotically heritable variations in gene expression that are not caused by changes in DNA sequence

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2
Q

What are examples of epigenetic mechanisms that alter chromatin structure - thereby affecting gene expression?

A

Reversible, post-translational modification of histones

DNA methylation

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3
Q

Compact chromatin is characterized by?

A

DNA methylation and respressive histone marks

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4
Q

Open chromatin (active) is characterized by?

A

Less DNA methylation and

Transrcriptional Complexes

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5
Q

What does DNA methylation mediate?

A

Genetic transcription

Mainly through gene silencing

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6
Q

What kind of sequences are methylated?

A

CpG islands

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7
Q

What does having CpG islands methylated result in?

A 
Recruitment of silencing complex
Histone deacetylation
Histone methylation 
Chromatin compaction 
Transcriptional repression
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8
Q

What protein recognizes / interacts with methylation to promote gene silencing

A

MeCP2

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9
Q

Does DNA methylation ever activate gene expression?

A

In some cases - developmentally programmed 3’CpG island methylation confers tissue-and cell-type specific transcriptional activation

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10
Q

How can DNA methylation (which normally modulates gene silencing) activate genetic transcription?

A

In certain instances it can block proteins that silence gene expression

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11
Q

What is genetic imprinting?

A

Sex-dependent, epigenetic modulation of regulatory regions such as promoter sequences
i.e. - Depending if male or female you have certain genes activated or silenced because of DNA methylation

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12
Q

What mediates imprinting?

A

DNA methylation

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13
Q

Is imprinting propagated through generations?

A

Yes, through generations so that genes inherited paternally will have the same pattern of expression as dad, and genes inherited maternally will have the same pattern of expression as mom

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14
Q

What is the consequence of imprinting being propagated through generations

A

Genes maternally inherited will have same expression in progeny as in mom
Genes paternally inherited will have same expression in progeny as in dad

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15
Q

What percent of the genes in the genome are thought to be imprinted

A

10%

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16
Q

What epigenetic mark mediates genetic imprinting?

A

DNA methylation

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17
Q

When are DNA methylation marks (patterns) first established?

A

In the gamete

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18
Q

What happens with DNA methylation in somatic cells?

A

DNA methylation is stably maintained in somatic cells after fertilization

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19
Q

Is DNA methylation reversible?

A

DNA methylation is reversible so that it can be reset during gametogenesis to transmit the appropriate sex-specific imprint to progeny

20
Q

What propagates epigenetic marks in somatic cells?

A

Maintenance methyltransferase

21
Q

What is the intermediate in DNA replication somatic cells during methylation maintenance?

A

A hemi-methylated dsDNA - i.e. the newly synthesized strand (from semi-conservative replication) will not have methylation marks until maintenance methyltransferase adds them

22
Q

What occurs with epigenetic marks in germs cells?

A

Epigenetic reprogramming occurs in germ cells

  • there is erasure (demethylation) of chromosomes
  • then depending on whether sperm or oocyte, you have establishment of new epigenetic marks - sex specific methylation
23
Q

Epigenetic memory

A

System somehow knows that in Dad you have methylated region A (meaning that all sperm will have methylated region A) and in mom you have methylated region B (meaning all oocytes will have methylated region B)

24
Q

What happens with imprinted regions in somatic cells?

A

Somatic maintenance of imprinted regions occurs in somatic cells

25
Q

What would happen if there was not an epigenetic “resetting” in gametogenesis?

A

There would be a high frequency of embryos with no active / or two active copies of imprinted genes
(i.e. you could have gene A silencing in both oocyte and sperm and thus no functional gene A in embryo and double the activity of gene B)

26
Q

Prader Willi syndrome

CAUSE?

A

del(15q11-q13) on paternal chromosome
Thus you are not expressing genes that are normal unmethylated in Dad and methylated in Mom because Dad’s are absent and mom’s are methylated

27
Q

What characterizes (Clinically) PWS?

A

Excessive eating
short stature
Hypogonadism
Some degree of intellectual disability

28
Q

Angelman Syndrome

CAUSE?

A

del(15q11-q13) on maternal chromosome
Thus, you are not expressing maternal genes that should be active in this region and you are not expressing them from the father chromosome either because it has been silenced

29
Q

Angelman Syndrome

Clinical?

A

Short stature
severe intellectual disability
spasticity
seizures

30
Q

del(15q11-q13) Maternal –>

A

Angelman syndrome

31
Q

del(15q11-q13) Paternal –>

A

Prader Willi Syndrome

32
Q

What creates the DNA methylation program for sex specificity in chromosomes?

A

Imprinting Center (modulated by DNA methylation) dictates the sex specific expression of the imprinted genes

33
Q

What is responsible for 70% of Prader Willi Cases?

A

70% are caused by deletion of paternal chromasome - thus you only have expression of maternal genes (as paternal are methylated on maternal chromosome)

34
Q

What is responsible for 28% of Prader Willii Cases?

A

28% are due to two chomosomes with sex specific maternal imprint –> (Maternal uniparental disomy - this can occur when trisomic conceptus undergoes trisomy rescue

35
Q

What is responsible for 2% of Prader Willi Syndrome?

A

2% is due to mutation on imprinting center on the paternal allele so that it receives the maternal imprinting patterns

36
Q

What were the two paternally expressed genes we focused on?

A

SNRPN

IPW

37
Q

What was the maternally expressed gene we focused on?

A

UBE3A

38
Q

Absence of expression of SNRPN results in?

A

PWS

39
Q

Absence of expression of UBE3A results in?

A

AS

40
Q

What is responsible for 70% of Angelman syndrome cases?

A

Deletion of maternal 15q11-13

41
Q

What is responsible for 4% of AS

A

Paternal Uniparental disomy -
Trisomy rescue
This is interesting - I wonder why retains female copy more frequently?

42
Q

What two errors are responsible for 8% of AS, each?

A

Imprinting center mutation –> paternal pattern of imprinting (silence of madre)
or
Mutation of UBE3A gene on maternal allele

43
Q

What is responsible (usually) for the deletions in the PWS/AS region of chromosome 15?

A

Deletions result from the presence of low copy repeats in the vicinity of the common breakpoints
The repeats are derived from genomic duplications of HERC2
The repeats flanking 15q11q13 may be involved with inter and intrachromosomal misalignment and homologous recombination resulting in deletions

44
Q

At a basic level, just know that deletions are cause by…

A

low copy repeat misalignment

45
Q

PWS and AS can result from uniparental Disomy - what is this?

A

Trisomic conceptus –>

Mitotic nondisjunction event early in gestation rescues lethality (trisomy rescue)

M2M2 (27 decks)

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