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MD1 - Reproduction Block > 2. Gene dosage & imprinting > Flashcards

2. Gene dosage & imprinting Flashcards

1
Q

What is biallelic gene expression?

A

Gene expression from both gene copies giving 100% protein levels

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2
Q

What is aneuploidy?

A

Unbalanced sets of chromosomes due to excess or deficiency of individual chromosomes - Polyploidy/trisomy/monosomy

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3
Q

What is the cause of aneuploidy?

A

Non-disjunction of two homologous chromosomes or sister chromatids during cell division, either in meiosis or mitosis

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4
Q

What are some exampled of trisomy that reach full term?

A

Trisomy 13, 18 and 21 XXY, XXXY, XYY

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5
Q

What are some monosomies that reach full term?

A

XO (turner syndrome) but no autosomal monosomies Most monosomies are lethal

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6
Q

What is down syndrome?

A

Trisomy 21 A cause of intellectual disability and congenital heart disease - Characteristic set of facial and physical features - Associated with congenital anomalies of the GIT, increased risk of leukaemia, immune system defects and an Alzheimer-like dementia (premature aging)

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7
Q

What is Edwards syndrome?

A

Trisomy 18 1 in 3000 births Often fatal before birth - Overlapping fingers - Club foot - Heart defects - Developmental disability

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8
Q

Describe autosomal recessive inheritance

A
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9
Q

Describe autosomal dominant inheritance

A
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10
Q

Describe haploinsufficiency

A

Occurs when a diploid organism has only a single functional copy of a gene (with the other copy inactivated by mutation) and the single functional copy does not produce enough of a gene product (typically a protein) to bring about a wild-type condition, leading to an abnormal or diseased state.

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11
Q

What is monoallelic gene expression?

A

Certain genes MUST be expressed from ONLY ONE copy

Mechanisms to operate to INACTIVE one of the two copies

  • X chromosomes inactivation in females = Epigenetic
  • Genomic imprinting = Epigenetic and genetic
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12
Q

Describe epigenetics

A

EPigentics is a change in gene expression without a change in DNA sequence passed on through cell division

Cellular ‘memory’ encoded in chromatin

Mechanisms

  • DNA Methylation of promoter regons of genes
  • Histone modifications - chromatin remodelling
  • Gene silencing through RNA mediated pathway induced by small RNAs
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13
Q

What is X-chromosome inactivation

A

One X chromosome in inactivated in female to ensure equal levels of X-linked genes is equal to males

Mechanism

Random inactivation in cells of early embryo (<100 cells)

Involved epigenetic mechanisms: DNA methylation, hyperchromatin structure and non-coding RNA (Xist) acting in cis

Females are mosaic for cells that have one or the other inactivate X

Permanent and heritable

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14
Q

What is Monosomy X?

A

Turner syndrome (XO)

Characteristics

Short stature

Infertility due to absent or immature gonadal development

Absence of secondary sexual development

Impaired neurocognitive function (visuospatial, perceptual)

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15
Q

What is genomic imprinting?

A

Maternal and paternal genetic material is equivalent in DNA sequence but NOT in function.

Their expression of certain genes depends on whether they are inherited from the maternal of paternal gamete.

Genomic imprinting describes the process whereby the parental origin of a particular gene is marked by reversible epigenetic mechanism

  • DNA methylation
  • Chromatin condensation
  • ncRNAs
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16
Q

Describe the changes in DMA methylation during development

A
  • Erasure of parental epigentic settings during gonadal differentiation
  • de novo methylation and establishment of imprinting marks during germ cells development
  • Erasure again during early blastocyst
17
Q

How many genes are imprinted?

A

100 identified

1-2% of human protein coding?

Chromosome 15 and 11 are where they are clustered

18
Q

What happens during abnormal embryogenesis

A

Parthenogenetic = All 46 chromosomes from mother

Typically non-viable embryos; ovarian teratomas

Androgenetic = All 46 chromosomes from father, form hydatiform moles –> Malignant choriocarcinoma

19
Q

What is significant in chromosome 11p15.5?

A

Chromosome 11p15.5 contain maternally and paternally imprinted genes which are differentially methylated (DMRs)

  • IGF2 is imprinted in many tissues but biallelic expression in brain, adult liver and chondrocytes
  • there are 6 genes clustered around imprinting centres, role in fetal growth and development
20
Q

What is the “parental conflict hypothesis”?

A

Maternally expressed genes tend to limit fetal growth - involved in resource conservation and less flow to the fetus

Paternally expressed genes tend to promote fetal growth - involved in resource extraction to give more energy to the fetus

21
Q

What is Uniparental disomy (UPD)?

A

Deficiency of maternally active genes +

Duplication of paternally active genes

22
Q

What is the origin of UPD?

A
23
Q

What is epimutation and what are its consequences ?

A

Epimutation is the loss of imprinting often associated with tumorigenesis (particularly IGF2)

24
Q

What does a mutation in 11p.15.5 cause? Where is the mutation?

A

Beckwith-Wiedemann Syndrome (BWS)

60% epimutation on meternal allele

20% patUDP11 (mosaic, segmental)

10% mutation in meternal allele of CDKN1C

1-2% cytogenetically visible deletion or duplication

Characteristics

  • 1 in 15000
  • Macroglossia
  • pre/post natal overgrowth
  • anterior abdominal wall defects
  • neonatal hypoglycaemia
  • ear pits/creases
  • hemihypertrophy
  • facial naevus flammeus
  • increased risk of abdominal tumours
25
Q

What conditions occur due to microdeletions (causing LOH) on chromosome 15

A

Prader-willi syndrome

Angelman syndrome

26
Q

Describe Prader-Willi syndrome

A

Deficiency of paternally expressed genes (15q11-13) due to deletion (70%) or maternal uniperental disomy (UPD) 25%

Characteristics

  • Infantile hypotonia
  • Hyperphagia, obesity
  • Hypogonadism
  • Small hands and feet
  • Moderate intellectual disabolity
27
Q

Describe angelman syndrome

A

Deficiency of maternally expressed genes 15q11-13 due to deletion (70%) or UPD (2%

Characteristics

  • Severe intellectual disability
  • Absent speech
  • Inappropriate laughter
  • Repetitive ataxic movement (hand flapping)
  • Large mouth, protruding tongue
  • Unusual seizures
28
Q

What is the pattern of inheritance of mutation in imprinted genes?

A
29
Q

Describe the inheritance of mutations in imprinted genes if mutation occurs in active maternal or inactive paternal

A
30
Q

What re the implications of genetic and epigenetic alteration in imprinted genes?

A

MD1 - Reproduction Block (15 decks)

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