20.03.11 PWS/AS

Question 1

Clinical features of PWS

Answer 1

• Severe hypotonia - delayed cognitive development and behavioural complications - Hyperphagia leading to obesity - OCD - Temper tantrums - Hypogonadism (incomplete puberty, infertility) - Short stature, small hands and feet

Question 2

Clinical features of AS

Answer 2

• Severe DD, LD - Severe speech impairment (normally no words) - Gait ataxia, movement/balance problems - Epilepsy/seizures - Sleep disorder - Microcephaly - Hyperactivity - Happy demeanour, laughing/smiling - Hand flapping - Scoliosis

Question 3

Genomic region and genes involved in PWS/AS?

Answer 3

• 15q11-q13 - Includes imprinted genes - need to be expressed on one parental allele - LoF or over expression of imprinted genes causes PWS, AS and 15q11-q13 dup syndrome - Recurrent deletions occur due to unequal crossing over between PCRs - 5 common breakpoints (BP1-5) - Most common are BP1/2 and BP3 deletions

Question 4

Paternally expressed genes in region

Answer 4

• SNURF/SNRPN, MRKN3, MAGEL2 and NDN - Introns of SNRPN include paternally expressed snoRNAs (including SNORD115 and SNORD116) - SNORD116 - loss of this region causes PWS (other genes contribute to phenotype)

Question 5

Maternally expressed genes in region

Answer 5

• UBE3A - produces a protein involved in protein degradation, so disruption causes degradation of crucial brain synapses - ATP10C - may modulate AS phenotype

Question 6

Genetic defects leading to PWS

Answer 6

• 70-75% = de novo deletion of pat allele - 25-30% = mat UPD(15) - 1% = Imprinting defect (no IC del) (10-15% of imp defect have a IC deletion) - all have low (<1%) recurrence risk apart from IC del that has a 50% risk

Question 7

Genetic defects leading to AS

Answer 7

• 75% = de novo deletion of mat allele
• 1-2% = pat UPD(15)
• 3% = Imprinting defect (no IC del) (10-15% of imp defect have a IC deletion)
• 5-10% = UBE3A mutation
• 10-15% = Unknown cause
• all have low (<1%) recurrence risk apart from IC del and UBE3A mutations that have a 50% risk

Question 8

What is the testing strategy for PWS and AS referrals?

Answer 8

Question 9

What methods are routinely used to test for PWS/AS?

Answer 9

1) MS-PCR (kubota assay) - to detect PWS or AS
2) MS-MLPA - to investigate molecular mechanism (important for recurrence risk)
3) Array - can pick up des/dups but can’t say if on mat or pat allele (sometimes picked up this way in LD cases)
4) Microsatellite analysis - to see if aberrant methylation is due to UPD (needs parental samples)
5) Seq of IC to look for microdeletionif not UPD

Question 10

What are the differential diagnoses for PWS?

Answer 10

• if methylation is normal in a PWS referral then unlikely to have PWS (as 99% have abnormal methylation)
  1) DM1 or SM1 fro hypotonia in newborn
  2) Craniopharyngioma and its treatment
  3) Hyperphagic short stature (get over eating, growth hormone deficiency and mild LD)
  4) mat UPD(14) - neonatal hypotonia and later onset obesity
  5) Cohen syndrome or Bardet0biedl syndrome as both present with obesity and DD

Question 11

What are the differential diagnoses for AS?

Answer 11

• Normal methylation does NOT rule out AS (only 80% have abnormal methylation)
• Up to 40% of AS caused by imprinting defects can be mosaic and these can be missed by MS=PCR
• Still may get two bands but may be of different intensities
• Doesn’t pick up 10% of cases with UBE3A point mutations and also end up with 10% of cases with unknown genetc cause
• Unknown cases may be due to mutations in UBE3A regulatory regions, other genes not yet identified or incorrect clinical diagnosis
  1) Rett syndrome in girls
  2) Mowat-Wilson sundrome (ZEB2)
  3) Pitt-Hopkins syndrome
  4) ATR-X

Question 12

Current treatment for PWS/AS

Answer 12

• Only treat clinical symptoms
• drugs for seizures (AS)
• medical intervention for feeding issues (PWS)
• strict food supervision (PWS)
• growth hormone therapy
• hyperphagia treatments

Question 13

Future therapies for AS

Answer 13

• Genomic therapies are being developed that restore UBE3A expression in neurons
• Plan is to activate the expression of the paternally derived UBE3A gene
• This could be done by inhibiting expression of UBE3A antisense strand on paternal chromosome
• Two methods for this:
  1) Artifical transcription factors (e.g. CRISPR) could target SNRPN gene to inhibit UBE3A-AS
  2) Could truncate UBE3A-AS by inserting poly(A) cassette