19.03.12 Beckwith-Wiedemann and Russell-Silver syndrome

Question 1

What region is important for BWS and SRS?

Answer 1

-11p15 - Aberrant genomic imprinting of this region is important for both syndromes

Question 2

What controls the normal imprinting cluster?

Answer 2

• Two domains each controlled by an imprinting centre (IC1 and IC2) - these control the expression of the cluster of imprinted genes - Can regulate expression of gene sin cis over large distances and show differential methylation of the parental alleles

Question 3

Domain 1 - what are its 4 main units?

Answer 3

1) IGF2 - fetal growth factor 2) H19 - non-translated mRNA 3) IC1 (also called H19DMR) - regulates IGF2 and H19. It is methylated on paternal allele, causing H19 to be expressed from maternal allele and IGF2 to be expressed from paternal allele 4) CTCF - binds to unmethylated IC1 causing H19 to be expressed. H19 then interacts with IGF2 enhancers, blocking IGF2 expression.

Question 4

Domain 2 - what are its 4 main units?

Answer 4

1) KCNQ1 - K+ channel 2) KCNQ1OT1 - non-coding RNA with antisense transcription to KCNQ1 (spans introns 10-11 of KCNQ1) 3) IC2 (also called KvDMR) - methylated on maternal alleles - causing CDKN1C and KCNQ1 to be expressed on maternal allele, and KCNQ10T1 to be expressed on paternal allele 4) CDKN1C - cyclic dependent kinase inhibitor

Question 5

What is the phenotype of BWS?

Answer 5

Pediatric overgrowth disorder Variable phenotype (over 30 features) Major features are: Macrosomia (excessive birth weight) Anterior ear lobe creases Macroglossia (large tongue) Omphalocele (abdom wall does not fuse) Visceromegaly (enlarged organs) Hemihyperplasia (abnormal growth on one side)

Question 6

What are the 5 main molecular causes of BWS?

Answer 6

• 85% are sporadic, 15% are familial (AD) - of the sporadic cases: 1) 50-60% IC2 hypomethylation (mosaic) - get complete or partial loss of maternal methylation at IC2 2) 20% paternal UPD - causes hypermethylation of IC2 and hypomethylation of IC2 3) 5-10% CDKN1C point mutations 4) 2-7% IC1 hypermethylation (mosaic) - gain of maternal IC1 methylation which causes H19 bialleleic expression 5) <1% - translocations or inversions - duplications tend to be paternally inherited, inversions and translocations tend to be on maternal allele

Question 7

What is the current BWS testing strategy?

Answer 7

1) Copy number analysis of 11p15 and methylation analysis of 11p15 (array and MS-MLPA) - If methylation/CNV found: - Gain of H19 meth - if associated CNV found could indicate heritable so test parents - Loss of KvDMR meth - if not associated CNV, likely to be sporadic - Both of above - supports UPD (confirm by microsatellite analysis) 2) Karyotype - If chr 11p15 abnormality found, indicates cause is a heritable dup, trans or inv

Question 8

What can MS-MLPA detect?

Answer 8

• Detects majority of epigenetic and genetic changes associated with BWS Microdeletions Microduplication Changes in gene dosage DNA methylation (including UPD)

Question 9

What are the differential diagnoses for BWS?

Answer 9

• Over over growth syndromes (Sotos, Weaver, Costello)

Question 10

What is the phenotype of SRS?

Answer 10

Intrauterine and postnatal growth restriction Dysmorphic facial features (triangular face, small face) Normal head cirumference Limb, body and or facial asymmetry

Question 11

What are the 2 chromosomal regions associated with SRS?

Answer 11

Either 11p15.5 related or chromosome 7 related

Question 12

What are the molecular mechanisms at 11p15.5 that cause SRS?

Answer 12

1) 30-50% hypomethylation of IC1 - most common cause - get loss of methylation on paternal allele 2) 1-2% maternal dup of 11p15 3) Maternal UPD11 - rare and normally mosaic - All associated with hypomethylation of IC1 which causes bialleleic H19 expression and biallelic silencing of IGF2 - resulting in growth restriction - Hypomethylation at paternal IC1 is normally a postzygotic event - therefore most SRS cases are mosaic

Question 13

What are the molecular mechanisms at chromosome 7 that cause SRS?

Answer 13

• 7-10% mat UPD7 - both isodisomy and heterodisomy observed. Can also be mosaic, or segmental UPD - Rare but can get 7q deletions

Question 14

What are the differential diagnoses for SRS?

Answer 14

• DNA repair disorder - FA, Bloom syndrome as you get IUGR - 3-M syndrome, temple syndrome, SHORT syndrome - as you get pre- and postnatal growth retardation

Question 15

What is the current SRS testing strategy?

Answer 15

1) Test for 11p15 ICRs (methylation testing) - Normal meth = UPD(7)mat testing - Abnormal meth = IC1 hypometh (40%) then do more MS testing at additional loci (found in 7% of cases), if UPD11 then confirm by micro satellite testing, if 11p15 duplication (1-2%) then do array to confirm 2) UPD(7)mat - If positive - confirm by micro satellite testing - If negative - do array - 1% of cases have small dels and dups 3) At the end 48% of patients with a clinical diagnosis of SRS have a negative result

Question 16

What are Multi-Locus Methylation Defects (MLMD)?

Answer 16

• Significant number of SRS and BWS cases have aberrant methylation at lots of imprinted loci not just at 11p15.5 - 9.5% of SRS and 22% of BWS have abnormal meth at other sites - No difference in clinical phenotypes was seen