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Genetics > Genomic Imprinting > Flashcards

Genomic Imprinting Flashcards

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1
Q

What is genomic imprinting?

A

For most genes, both copies are expressed

For some genes >>> either maternally or paternally derived copy is preferably used >>> this phenomenon is known as >>> Genomic imprinting

(imprinted = inactivated)

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2
Q

Examples of genomic imprinting

A

The best known:

Prader- Willi syndrome

Angelman syndrome

Others:

Albright’s hereditary osteodystrophy

Beckwith-Wiedeman syndrome

Russell Silver symdrome

Familial paraganglionoma

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3
Q

Which parental gene is deleted in Prader Willi syndrome?

A

Gene deleted from father

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4
Q

Which parental gene is deleted in Angelman syndrome?

A

Gene is deleted from mother

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5
Q

Cause of genomic imprinting in Prader Willi and Angelman syndrome

A

Either:

Cytogenetic microdeletions of the same region of chromosome 15 (long arm, q) OR

Uniparental disomy of chromosome 15

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6
Q

What is uniparental disomy?

A

Both copies of chromosome are derived from one parent

(NO copy from another parent)

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7
Q

Chromosomal and genetic abnormality in Prader Willi syndrome

A

70% cases: Microdeletion on Paternal (father) chromosome 15 OR

30% cases: Maternal uniparental disomy (= Both copies of chromosome 15 is derived from mother, none from father)

Many of remainder due to mutation in UBE3A gene chromosome 15 (q, long arm)

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8
Q

Chromosomal and genetic abnormality in Angelman syndrome

A

80% cases: deletion om maternal chromosome 15

2-3% paternal uniparental disomy ( = no maternal contribution)

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9
Q

Incidence of Prader Willi syndrome

A

1.2-1.3 per 10,000

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10
Q

Lifestyle of a Prader Willi patient

A

they can achieve high level of functioning to hold down simple part time job

But impossible for them to control appetite, to limit weight gain, or to limit food access

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11
Q

Body structure of Prader Willi syndrome

A

Short stature

Dysmorphic features

Hypotonia + obesity in late childhood

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12
Q

Limb features of Prader Willi syndrome

A

Small hand

Small feet

Incurred feet

Club foot

Congenital hip dislocation

Joint anomaly

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13
Q

Facial features of Prader-Willi syndrome

A

Squint

Almond shaped ears

Deafness

Cleft palate

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14
Q

Trunk features in Prader Willi syndrome

A

Asthma

Heart disease

Cor pulmonale

Gall stone

Duodenal ulcer

Bowel obstruction

Renal stone

Rectal prolapse

Scoliosis

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15
Q

Behavioural Changes in Prader Willi syndrome

A

Hyperphagia (starts 1 to 4years)

Moderate Learning difficulties

Behavioural problems in adolescent

Mental retardation

Self-injury

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16
Q

Reproductive features in Prader Willi syndrome

A

Small genitalia in male, female

Micropenis

Cryptorchidism

Hypogonadism

Infertility

Amenorrhoea in female

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17
Q

Skin problems in Prader Willi syndrome

A

Acanthosis nigricans

Boils

Hypothermia

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18
Q

Enodocrine disease in Prader Willi syndrome

A

Diabetes

19
Q

Neonatal and childhood problems in Prader Willi syndrome

A

Neonatal: hypotonia + poor feeding

Childhood: Hyperphagia + obesity

20
Q

Angelman syndrome: Behaviour

A

Happy puppet

Unprovoked laughter

Unprovoked clapping

21
Q

Angelman syndrome: features in brain

A

Microcephaly

Severe learning disability

Seizures

Characteristic EEG

22
Q

Angelman syndrome: Gait

A

Ataxia

Borad-based gait

23
Q

Learning ability in Prader Willi syndrome and Angelman syndrome

A

Prader Willi syndrome: Moderate learning disability

Angelman syndrome: Severe learning disability

24
Q

Albright’s hereditary osteodystrophy

A

Chromosome 20

Gene: GNAS

Encodes: alpha sub-unit of adenyl cyclase stimulating G-protein, Gs

Mutation deactivates GNAS gene

25
Q

Genomic imprinting of Albright’s hereditary osteodystrophy

A

Mutation in maternally derived GNAS allele

26
Q

Associated disease of Albright’s hereditary osteodystrophy

A

Pseudohypoparathyroidism

because it has the same gene mutation GNAS on chromosome 20

27
Q

Stature and weight in Albright’s hereditary osteodystrophy

A

Short adult stature + tendency to obesity

28
Q

Facial feature in Albright’s hereditary osteodystrophy

A

Round facies

29
Q

Learning ability in Albright’s hereditary osteodystrophy

A

Mild-to-moderate learning disabilities

30
Q

Bony feature in Albright’s hereditary osteodystrophy

A
  • Brachydactyly:

​Short metacarpals (particularly 4th and 5th)

Short distal phalanges (particularly thumb)

  • ​Ectopic ossifications

Associated disease: pseudohypoparathyroidism also has short 4th and 5th metacarpals, as both are caused by GNAS gene mutation on chromosome 20

31
Q

Beckwith-Wiedemann syndrome: Chromosome and gene (Genomic imprinting)

A

Chomosome 11

Gene: IGF2/ H19/ P57^kip/ KvLqQT1 gene cluster

32
Q

Weight & size in Beckwith-Wiedemann syndrome

A
  • Large birthweight
  • Hemihypertrophy

​

33
Q

Facial feature in Beckwith-Wiedemann syndrome

A

Facial nevus flammeus

34
Q

Endocrine problem in Beckwith-Wiedemann syndrome

A

Neonatal hyperinsulinism >>> hypoglycaemia

35
Q

Abdominal features in Beckwith-Wiedemann syndrome

A
  • Omphalocele (Exomphalos)
  • High risk of childhood abdominal tumours (particularly Wilm’s tumour and hepatoblastoma)
36
Q

Russell-Silver syndrome: Genomic imprinting

A

In around 10% cases, maternal uniparental disomy for chromosome 7 + abnormalities of other imprinted regions

37
Q

Onset of Russell-Silver syndrome

A

Prenatal onset

38
Q

Prenatal features of Russell-Silver syndrome

A

Small stature + relative macrocephaly

39
Q

Feeding in Russell-Silver syndrome

A

Very poor feeding

40
Q

Bony features of Russell-Silver syndrome

A

Triangular face

Asymmetry

fifth finger clinodactyly

41
Q

Main feature of Familial paraganglioma

A

Paragangliomas, including pheochromocytomas and glomus jugulare tumours occur throughout the body ( >> most common in the head and neck)

42
Q

Familial paraganglioma: Genes

A

SDHB, SDHC, SDHD

43
Q

Familial paraganglioms: Genomic imprinting

A

If a paerson carries mutated gene from mother >>> gets imprinted (=inactivated) >>> so, will not cause the disease

The mutation causes disease only when it is inherited from father

44
Q

Familial paraganglioma: inheritence

A

Autosomal dominant

Each child has 50% has to be affected

But whether the mutant allele is expressed >> depends upon parental origin of the allele (whether mother/father)

Genetics (28 decks)

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