L296 Gene Dosage and Genomic Imprinting Flashcards

1
Q

Bi-allelic expression vs mono-allelic expression of genes

A
  • Biallelic expression = genes expressed from both gene copies
    Monoallelic expression = genes expressed from only one gene copy
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2
Q

Aneuploidy vs euploidy

A
  • Aneuploidy = abnormal # chromosomes

- Euploidy = abnormal sets of chromosomes

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3
Q

Most aneuploidy embryos survive: T/F?

A

False - most aneuploidies are incompatible with life

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4
Q

Trisomy vs monosomy

A
  • Trisomy = > 2 copies of a chromosome

- Monosomy = 1 copy of a chromosome

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5
Q

Full-term trisomies

A

trisomy 13, 18, 21 and XXY, XXXY, XYY etc

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6
Q

Full-term monosomies

A

XO, no autosomal monosomies

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7
Q

When do we see aneuploidies affecting normal function?

A

When they affect dose-sensitive genes - that must have a particular range of protein expressed

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8
Q

Trisomy 18 =

A

Edwards syndrome (47,XX+18 or 47,XY+18)

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9
Q

If a baby with trisomy 18 survives, what does this mean?

A

not every cell in the body is trisomic: they are mosaics

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10
Q

Trisomy 21 =

A

Down syndrome (47,XX+21 or 47,XY+21)

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11
Q

Which genes contribute to the development of the down syndrome phenotype?

A

• Gene loci at many regions of chromosome 21 play a role in the development of the overall phenotype of Down syndrome

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12
Q

Autosomal recessive inheritance: how many altered copies of a gene are required to cause a phenotype?

A
  • Two altered/non-functional copies cause phenotype

- One normal copy is sufficient for cell function

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13
Q

Autosomal dominant inheritance: how many altered copies of a gene are required to cause a phenotype?

A
  • Alteration/loss of a single gene copy causes a specific phenotype
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14
Q

Why might having one altered gene cause a new phenotype (in autosomal dominant inheritance)?

A
  • One altered copy of a gene might have a novel ‘gain-of-function’ to cause phenotype OR
  • Having only one normal copy of a gene is not sufficient to support normal cell function (haploinsufficiency)
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15
Q

How might monoallelic gene expression come about?

A

Inactivation of one of the gene copies

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16
Q

Examples of mechanisms that operate to inactivate a genetic copy for monoallelic gene expression

A
  1. X chromosome inactivation in females (epigenetic)

2. Genomic imprinting (epigenetic and genetic)

17
Q

Epigenetics =

A

= change in gene expression/repression with no change in DNA sequence, passed on through cell division

18
Q

X-inactivation: at which cell stage does it occur at, and what does it leave behind?

A
  • Occurs at 8-100 cell stage of embryo

- Barr body = condensed, inactive X chromosome remnant

19
Q

Comment on the heritability of X-inactivation

A

Heritable: when cell divides, same chromosome is inactivated i.e. every cell from that cell in chromosomally similar

20
Q

X-inactivation: does it mean that females only need one X chromosome? Why/why not?

A

No - some genes must be expressed from the inactivated X e.g. Turner syndrome (45, X) is a monosomy X

21
Q

What is genomic imprinting?

A

Describes the process whereby the parental origin of a particular gene is “marked” by a reversible epigenetic mechanism

22
Q

What do we say expression of genes in genomic imprinting is regulated by?

A

Parent-of-origin effects: expression of certain genes depends on whether they are inherited from maternal/paternal gamete

23
Q

When does imprinting occur?

A

• Imprints are erased during gametogenesis in haploid cell, re-established in a parental-specific pattern in mature gametes and maintained during embryogenesis

24
Q

Which chromosomes in particular have clusters of imprinted genes?

A

Chr 11 and 15

25
Abnormal embryogenesis that leads to all 46 chromosomes being from father/mother - what is this called, and what can this lead to?
- Parthogenesis: all from mother, androgenesis: all from father - typically lead to non-viable embryos and risk of cancer
26
Which hypothesis explains the need for parent-of-origin effects, and explain this hypothesis.
“Parental conflict hypothesis” - Maternally expressed genes tend to limit foetal growth – involved in resource conservation and less flow to the fetus - Paternally expressed genes tend to promote foetal growth - involved in resource extraction to give more energy to the fetus
27
Name 4 changes that underlie recognised imprinting disorders
1. Loss of heterozygosity (LOH) 2. Uniparental disomy (UPD) 3. Epimutation 4. DNA mutations in genes that are usually imprinted or in imprinting control centres
28
LOH results from...?
Large deletions or duplications of chromosome regions that contain imprinted genes → loss of heterozygosity
29
What is UPD?
Embryo ends up with two copies of one-parental chromosome
30
List 4 ways UDP might occur
1. Meiotic non-disjunction in both gametes 2. Meitoic non-disjunction in one gamete with duplication in zygote 3. Loss of chromosome in zygote and duplication of other chromosome during mitosis 4. Meiotic non-disjunction in one gamete followed by loss of chromosome in zygote
31
Epimutation =
= alteration of epigenetic changes at imprinted loci without changes in DNA sequence
32
Describe two types of epimutation that might result in imprinting disorders
- Primary epimutation: = change in epigenetics without any change to DNA sequence e.g. loss of co-factors in environment - Secondary epimutation: mutation in cis/trans-acting regulator of epigenetic change
33
Inheritance of mutations in imprinted genes: mutation in active paternal/inactive maternal allele - what are the inheritance rules?
- Carrier males and affected males can have affected children but not carrier children - Carrier females and affected females cannot have affected children but can have carrier children
34
Inheritance of mutations in imprinted genes: mutation occurs in active maternal or inactive paternal allele - what are the inheritance rules?
- Carrier females and affected females (with paternal inactive allele that is mutated) can have affected children but not carrier children - Carrier males and affected males cannot have affected children but can have carrier children
35
Three examples of imprinting disorders, and which chromosome they occur on
1. Beckwith-Wiedemann syndrome (BWS) - Chr 11 2. Prader-Willi syndrome (PWS) - Chr 15 3. Angelman syndrome (AS) - Chr 15
36
BWS, PWS and AS - what kind of changes result in these disorders?
- BWS: epimutation | - PWS, AS: LOH
37
BWS, PWS and AS - which allele is most commonly affected, maternal or paternal?
- BWS: maternal epimutation - PWS: paternal deficiency (25% from maternal UDP) - AS: maternal deficiency