Imprinting Disorders

Question 1

Name clinical features of PWS

Answer 1

Neonatal: Severe hypotonia. feeding difficulties. failure to thrive.
Childhood: hyperphagia. Absence of satiety. Obesity.
Mild- moderate MR. Milestone delays. Behavioural problems (tamper tantrums, stubbornness, obsessive compulsive). Hypogonadism. Short stature/small hands & feet. Almond shaped eyes/ triangular mouth.

Question 2

What’s the cause of Angelmans syndrome

Answer 2

Paternal UPD (5%).
Maternal deletion (70-75%) (UBE3A).
Imprinting error (2-3%) (10-15% of these are deletions).
Point mutations UBE3A (10%).
No abn detected (10%).

Question 3

Name clinical features of Angelmans

Answer 3

Severe MR, dev del by 6 months, ID. Gait ataxia. Hand flapping.
Absent speech. Microcephaly. Seizures. Characteristic EEG. Unique behaviour (inappropriate laughter, happy demeanour, fascination with water & mirrors)

Question 4

What genes are expressed on the paternal allele

Answer 4

SNURF-SNRPN, MKRN3, MAGEL2, IPW.
Unmethylated on pat allele.
Methylated in mat allele

Question 5

Where is the mat allele of UBE3A expressed

Answer 5

Mat UBE3A is expressed in the brain (neurons).
(Mat: unmethylated:/pat: methylated)
UBE3A is biallelically expressed in all other tissues.

Question 6

How is imprinting controlled in PWS and AS

Answer 6

By a bipartite imprinting centre:
It executes its function amidst several allele-specific covalent Histone modifications and other chromatin features.
PWS-IC includes major promotor and exon 1 of SNURF-SNRPN.

Question 7

What does SNURF-SNRPN encode

Answer 7

Bicistronic transcript that produces two proteins (SNURF and SNRPN), and antisense transcript, lncRNA, snoRNA and a novel RNA species called sno-lncRNA.

Question 8

What does UBE3A encode

Answer 8

E3 ubiquitin ligase. The mRNA has 7 isoforms. The antisense transcript to UBE3A most likely regulates its imprinted expression leading to repression of the pat allele.
Disruption could affect crucial neuronal processes of protein degradation/ replacement which is crucial for development of normal synapses.

Question 9

What are the common breakpoints in PWS AS

Answer 9

15q11-q13.

bp1-bp3 (37%) or bp2- bp3 (60%)

Question 10

What’s prader Willi syndrome caused by

Answer 10

Maternal UPD (25%).
Paternal deletion (75%) SNURF-SNRPN.
Imprinting error (~1%) (10-15% of these are IC deletions).

Question 11

What’s the incidence of Beckwith Wiedemann

Answer 11

1/13,700 livebirths.
85% sporadic; 15% familial.
Autosomal,dominant

Question 12

What are the clinical features of Beckwith Wiedemann

Answer 12

Paediatric overgrowth disorder with variable phenotype.

Major features: pre/post natal macrosomia, macroglossia, omphalocele, visceromegaly, hemihyperplasia, development of embryonal tumours, anterior lobe creases.

Minor: neonatal hypoglycaemia, advanced bone age, mid face hypoplasia.

Question 13

Describe prenatal Beckwith Wiedemann features

Answer 13

Polyhydromnios, premature birth, fetal macrosomia, long umbilical cord, enlarged placenta, placental mesenchymal dysplasia.

Question 14

Describe the monitoring of Beckwith Wiedemann

Answer 14

Annual renal screen 8yrs to mid adolescence to ID nephrocalcinosis or medullary sponge kidney disease.

Abdominal ultrasound every 3 months until 8yrs old: screen for embryonal tumours.

AFP (alpha fetoprotein) concentration monitoring every 2-3 months until 4yrs old: screen heptoblastoma.

Question 15

Describe Beckwith Wiedemann imprinted region

Answer 15

Consists of 2 independent imprinting domains separated by a non-imprinted region. iC1 (H19DMR) and IC2 (KvMDR1).

IC1: paternal: METHYLATED: IGF2 expressed.
Maternal: UNMETHLATED: H19 expressed.

IC2: paternal: UNMETHYLATED: KCNQ1OT1 expressed.
Maternal: METHYLATED: CDKN1C, KCNQ1 expressed.

Question 16

Describe the causes of Beckwith Wiedemann

Answer 16

1) paternal UPD (20%).
2) maternal IC1: GAIN of methylation: loss H19 expression (~5%).
3) maternal IC2: LOSS of methylation: loss CDKN1C, KCNQ1 expression (~50%).
4) maternal CDKN1C LoF mutation (~5% dn, ~40% familial).
5) chromosome abnormality: paternal duplication (increase of IGF2 expression); maternal inv/t (disruption of KCNQ1). (

Question 17

What are the clinical features of Russell Silver

Answer 17

1/100,000. IUGR, postnatal growth retardation. Normal head circumference. Limb, body, facial asymmetry (hemihypoplasia).

5th finger clinodactyly, triangular fancies (frontal bossing/ prominent forehead, small chin). Cafe au lait spots, motor, speech, cognitive delays. Feeding problems. Hypoglycaemia.

Question 18

What’s are the causes of Russell Silver

Answer 18

1) maternal UPD11 (single cases).
2) paternal IC1: LOSS methylation: H19 expressed, IGF2 silenced (~40%).
3) maternal segmental duplications (1-2%).
4) paternal IC1 mutations (microdel/dup) (1%).
5) maternal UPD7 (possibly 7q31- MEST, CPA4, COPG2) (~10%).
6) no result (~48%).

Question 19

What are the issues with ART assisted reproductive technologies

Answer 19

Several studies have identified high incidence of rare imprinting disorders (RSS, AS, BWS, not PWS).
Suggested the process of gamete/embryo culture could cause imprinting defect by:
1) Disrupting the process of finalising the parental imprinting which occurs later in ooctyes.
2) Disrupt maintainance of the imprint: after fertilisation there are global methylation changes in the developing embryo- culturing may expose embryos to environmental factors that disrupt the imprint.
3) medical induction of superovulation could promote the collection of immature ooctyes where maternal imprinting is incomplete.

Question 20

What’s associated with imprinting of GNAS

Answer 20

Albright Hereditary Osteodystrophy.
Mat GNAS mut/pat UPD: Pseudohypoparathyroidism type 1a/b.
Pat GNAS mut: pseudopseudohypoparathyroidism

Question 21

Discuss UPD20

Answer 21

Altered expression on maternal GNAS 20q13.11.
Paternal UPD20, mutation of maternal GNAS, loss of methylation of alternative promoter of maternal GNAS.

Clinical features: resistance to hormones including parathyroid- growth retardation, mild brachydactyly.

Question 22

When should prenatal UPD studies be offered for UPD 14 or 15

Answer 22

All balanced carriers of robertsonians.
Foetus with a familial or de novo balanced rob.
Foetus with a normal karyotype with a parental carrier of a rob.

Question 23

Discuss 14q32 imprinted region

Answer 23

Region is controlled by a paternally methylated intergenic differentially methylated region (IG-DMR).
Paternally expressed genes: PEGS: DLK1 and RTL1.
Maternally expressed genes: MEGS: MEG3 and RTLas (antisense encoding microRNA).

Question 24

Describe UPD14 mat

Answer 24

Pre/postnatal growth retardation. Hypotonia. Joint laxity. Motor/dev delay. Early onset puberty (~8yrs). Small hands and feet. Truncal obesity.

Increase in MEGS (MEG3 and RTLas), silencing of PEGS

Differential diagnosis: PWS

Question 25

Describe UPD14 pat

Answer 25

More severe phenotype to matUPD.
Polyhydromnios. Narrow bell shaped thorax. Coat hanger shaped ribs. Severe dev delay/MR. Joint contractures. Skeletal anomalies- short limbs. Dysmorphic face. scoliosis.

Increase expression of PEGS: DLK1 & RTL1 , silencing of MEGS.

Question 26

What are the features of transient neonatal diabetes mellitis

Answer 26

Hyperglycaemia. Diabetes during first 6 months of life, spontaneously resolves by 18months. IUGR. dehydration. Absence of ketoacidosis.

Question 27

What’s the cause of transient neonatal diabetes mellitis

Answer 27

1) patUPD (40%).
2) paternal duplication of ICR (32%).
3) hypomethylation of maternal ICR (28%).

PLAGL1 & HYMA1: paternally expressed genes. In TNDM they are both over-expressed.

Question 28

Discuss the 2 groups of hypomethylation of maternal ICR in TNDM

Answer 28

~50% have isolated imprinting mutations/ no other epimutations.
The rest have hypomethylation at 6q24 as part of hypomethylation at multiple imprinted loci (HIL) (autosomal recessive). The majority of these cases have a recessive LoF homozygous/compound heterozygous mutation of ZFP57 (6p22.3).

Question 29

Name the features of the subset of TNDM that have ZFP57 mutation and HIL

Answer 29

Structural brain anomalies. Dev delay. CHD.

Question 30

What are the 3 ways to test for methylation

Answer 30

MS-PCR.
Southern blotting.
MS-MLPA

Question 31

What’s the limitation of MS PCR

Answer 31

It can confirm the diagnosis but not the mechanism.
So once abn methylation pattern established- need to FISH, MLPA, aCGH, microsatellite analysis to establish deal/ UPD/IC defect

Question 32

How does MS MLPA WORK

Answer 32

DNA is denatured and probes hybridised.
Sample the. Split in two.
One half has normal MLPA: ligation and PCR of ligated products- establish copy number.
Other half: simultaneous Hha1 digestion and ligation: Hha1 digesters UNMETHYLATED DNA- gives no signal. Hha1 doesn’t disgust methylated DNA- gives a signal.
So you have two results- one that tells you copy number for each ligated product, and one give gives you a signal or not for the same ligated products. It’s a semi quantitative test.

Question 33

Differential diagnosis for PWS

Answer 33

Neonatal: SMA, myotonic dystrophy (hypotonia)
UPD14mat (hypotonia and obesity)
Older kids: obesity- Cohen syndrome, 1p36 deletion, diploid/triploidy mosaic.

Question 34

Differential diagnosis for AS

Answer 34

Girls: Retts (difficult to distinguish during infancy, both: acquired microcephaly, ataxia, frequent smiling. Later- Retts is distinguished by history of regression, hand wringing and hyperventilation).
Mowat-Wilson (ZFHX1B muspt: LD, limited speech, seizures, facial similar).
Pitt-Hopkins (TCF4) .
1p36 del, 17q21 del, 22q13 terminal del.

Question 35

In terms of infertility discuss PWS

Answer 35

Hypogonadism. Delayed and incomplete pubertal development OR precocious puberty.

Males: 80-90% cryptorchidism.

Female: primary amenorrhea. Immature breast development.