12. BWS/RSS Flashcards
What is the result of DNA methylation?
Repressing/silencing transcription
Which residues can be methylated? What is the mechanism?
Cytosine residues with a downstream neighbouring guanine residue (CpG islands)
Methly group added to cytosine residue to produce 5-methylcytosine
What is the definition of imprinting?
Genes that are differentially methylated depending on their parent of origin
Leads to monallelic, parent-specific gene expression
When are imprinting patterns erased?
During the development of the parental gametes for transmission to offspring
What are the clinical features of BWS?
Overgrowth, macroglossia, exomphalos, hemihyperplasia, Wilms tumour (nephroblastoma), macrosomia
What are the clinical features of RSS?
Severe intrauterine and postnatal growth retardation, short stature, characteristic triangular facies, body asymmetry
What is the BWS/RSS critical region?
11p15
What are the 2 ICs at 11p15 and what genes do the regulate expression of?
IC1/H19DMR - IGF2 & H19
IC2/KvDMR - CDKN1C, KCNQ1, KCNQ1OT1
Describe the methylation pattern of IC1/H19DMR
IC1 methylated on paternal chromosome, unmethylated on maternal chromosome
Paternal chr - H19 suppressed, IGF2 expressed
Maternal chr - H19 expressed, IGF2 suppressed
What are the roles of IFG2 and H19?
IGF2 - fetal growth factor (regulates cell proliferation, growth, migration, differentiation and survival)
H19 - non-coding RNA thought to act as tumour supressor
Describe the methylation pattern of IC2/KvDMR
IC2 unmethylated on paternal chromosome, methylated on maternal chromosome
Paternal chr - KCNQ1OT1 expressed, KCNQ1 & CDKN1C suppressed
Maternal chr - KCNQOT1 suppressed, KCNQ1 & CDKN1C expressed
What proportion of BWS cases are sporadic vs inherited?
85% sporadic, 15% inherited (AD)
What is the most common cause of sporadic BWS?
Loss of methylation (hypomethylation) at maternal IC2/kvDMR
Approx 50% of cases
Causes loss of CDKN1C (and KCNQ1) expression –> overgrowth, increased expression of KNCQ1OT1
What is the second most common cause of sporadic BWS?
What does it result in?
Mosaic paternal UPD11p15 - segmental isodisomy
Approx. 20% of cases
Hypermethylation of IC1 AND hypomethylation of IC2
Increased IGF2 expression, decreased CDKN1C expression
What causes paternal UPD11p15 in BWS and why is it always mosaic?
Post-zygotically due to mitotic exchange between non-sister chromatids in early embryogenesis
Means that UPD should be present in each daughter cell - maternal UPID in one and paternal UPID in the other one (mosaicism)
Why is mosaicism for mat and pat segmental isodisomy not seen in BWS?
Because maternal isodisomy is lethal
What is the third most common cause of sporadic BWS?
Gain of methylation (hypermethylation) at maternal IC1/H19DMR
Approx. 10% of cases
Causes loss of H19 expression + increased expression of IGF2
What is the role of CTCF?
Regulates expression of IGF2 and H19 by creating chromatin loops
Unmethylated maternal allele, CTCF binds IC1 to HIDAD - H19 under control of enhancer & expressed, acts as an insulator and prevents IGF2 from being expressed
Methylation on paternal allele prevents CTCF from binding IC1 to HIDAD –> forms alternative loop so that IGF2 can be activated by enhancers
Why do mutations in CTCF binding sites result in BWS?
Cause paternal pattern of DNA looping on both chromosomes - IGF2 activated by enhancers and so is expressed biallelically, H19 not expressed from maternal allele
What is the recurrence risk associated with CTCF binding site mutations?
Up to 50%
What are the fourth and fifth most common causes of BWS?
- Maternal CDKN1C point mutations (5% of sporadic, 40% of familial)
- Duplications and deletions at 11p15 - must have an accompanying methylation change
What is the role of CDKN1C?
Negative regulator of cell proliferation - inhibits cyclin/cyclin-dependent complexes of the G1 phase
Expressed from maternal allele - loss results in overgrowth seen in BWS
Mutations in maternally inherited allele cause BWS
In a patient with ?BWS, what does a normal MS-MLPA result indicate?
CDKN1C testing
Pathogenic variants have high recurrence risk
When is SNP array required?
- Deletion & methylation change on MS-MLPA - define breakpoints
- GOM at IC1 and LOM at IC2 - suggestive of pat isodisomy UPD11, array to confirm
- Borderline methylation result for IC1 and IC2 - ?pat UPD11
