Rett syndrome Flashcards
Main characteristics of Rett
- Normal early development 6-18 months
- Loss of acquired fine and gross motor skills and communication
- Stereotypic hand movements
Genes associated with Rett
MECP2: methyl CpG binding protein (90-95%)
Clinically overlapping phenotype:
CDKL5
Netrin G1
MECP2 gene
2 isoforms created by alternative splicing of exon 2 and the use of 2 alternative start codons:
- e1 (contains exon 1 but alternatively splices out exon 2, predominant in brain)
- e2 (exon 1 absent, predominant in fibroblasts/lymphocytes)
MECP2 protein
Two functional domains:
- methyl binding domain (binds specifically to DNA at methylated CpGs)
- transcription repression domain (recruits corepressor proteins that repress transcription by deacetylation and condensation of chromatin)
E1 form most abundant
MECP2 mutations
- Almost all de novo
- Parents can be germline mosaic
- Majority arise on paternal Chr.
- 200 known nucleotide changes
- 8 most common missense and nonsense =70% of mutations
- ~80% mutations in classic Rett in exons 3-4
- Exon4 and intron 2 contain highly repetitive regions, recombination between these can lead to del exons 3-4
- Deletion hotspot in c-terminal 9% of mutations
- Whole exon deletions described
- Mutations identfied in 80-95% females with classic Rett
Complication of MECP2 mutations
Mutations in MECP2 do not constitute a diagnosis of Rett
Mutations also seen in individuals with Angelman like phenotype, non-syndromic X-linked MR, autism, neonatal encephalopathy
Rett syndrome
Severe neurodevelopmental disorder mostly affecting females
Rett in males
Classical Rett is infrequently observed
Neonatal onset encephalopathy with breathing abnormalities and early lethality more common
Deleterious mutations in 1 copy of the X result in severe neonatal encephalopathy and early lethality
May be mosaic or have a 47XXY karyotype (more classical symptoms)
Other Rett phenotypes
Abnormal breathing patterns
Seizures
Autonomic nervous system dysfunction
Factors influencing severity
Skewed X inactivation
Genotype-phenotype correlation (truncation X more severe)
MECP2 duplications
Also results in neurodevelopmental impairment
- early onset infantile hypotonia
- delayed cognitive development
- ID
- epilepsy
- spasticity
Mutation spectrum
Missense mutations predominantly found in methyl biding domain (MBD)
Nonsense mutations found in MBD and transcription repression domain (TRD)
Deletions mostly found in hotspot at c-terminal
Male MECP2 mutations
Previously thought to be lethal
Three types of mutations observed in males:
- Classic Rett mutations
- Mutations inherited from mother not found in females with Rett
- Males with deletion of the whole gene and neighbouring genes resulting in a severe phenotype
- MECP2 duplication syndrome (most common mutation in males)
MECP2 genotype/phenotype correlation
Studies have given conflicting results
Milder mutations:
- missense milder than truncating
- 3’ to TRD region
- Females with missense mutations milder than nonsense/fs
- deletions, if small and towards 3’ end
Skewed X-inactivation may modulate clinical severity
Testing strategy
Bidirectional Sanger sequencing detects 85-90% mutations in classic Rett and 30-40% in atypical Rett
MLPA detects deletions/duplications
Possible testing problems
reports of false negative results due to primer site polymorphisms
SNPcheck used regularly to look for new SNPs (assess freq info)
PCR primers tagged for use of universal sequencing primers
Analyse sequencing data using Mutation Surveyor
Diagnostic referrals (male and female)
- accepted from GPs, paediatrics, clinical genetics
- parental samples requested if variant identified in child
- offer testing to other female relatives of proband
- test male relatives with possible Rett phenotype
Referral types
Diagnostic
Parental testing following testing in proband
Prenatal
Parental referrals
- only accepted from clinical genetics
- following identification of a variant pathgenic of VUS in proband
- determine recurrence risk
- risk of germline mosaicism
Prenatal diagnosis
- if mother carries mutation risk is 50%
- test for MCC
X inactivation detection
Females can show skewed X inactivation
If mother is found to have a mutation, analysis to determine degree of skewing may help in counselling
PCR of polymorphic CAG repeat in exon1 of the Androgen receptor
DNA subject to restriction digest HpaII and CfoI
Can detect ration of 75:25 or unilateral inactivation
may aid in determining the phenotype
Alternative testing
CDKL5 Angelman NTNG1 FOXG1 SLC9A6
CDKL5
atypical Rett
group characterised by infantile spasms and jerks, early onset seizures
Angelman syndrome
Present in early childhood and presents with global dev delay, speech and communication problems, stereotypical hand movement, autistic
NTNG1
NetrinG1- dev of CNS
atypical Rett
FOXG1
Congenital form presenting within first month
SLC9A6
X-linked ID microcephaly epilepsy ataxia phenotype mimics Angelman
Pathophysiology of Rett
Regression arises from altered neuronal function rather than neurodegeneration
Maintenance of an appropriate level of MECP2 essential for neuronal function (enhanced as well as reduced function results in impairment)
Brains have smaller and more densely packed neurons
Impaired epigenetic transcriptional regulation
- MECP2 binds 2 alternatively methylated forms of DNA (methylated cytosine adjacent non G base and hydroxymethylcytosine)
- These methylation patterns accumulate postnatally during neuronal maturation
- This coincides with surge in MECP2 expression
- Binding of MECP2 may regulate transcription
- timing coincides with increased synaptogenesis
- mechanism may contribute to delayed onset of symptoms
Altered chromatin architecture
MECP2 is able to compact chromatin and influence the higher-order structure of DNA
Impacts the ability of other nuclear proteins to interact with chromatin
e.g. chromatin remodelling protein ATRX, shown to lose normal localisation at heterochromatic foci in null mice (this correlates with disease onset)
Neuronal circuit dysfunction
Third aspect in timing of Rett onset
Progressive molecular and nuclear changes in neurons impair neuronal function and culminate in altered circuit connectivity (may explain delay)
