Rett Syndrome

Question 1

What is rett syndrome?

Answer 1

A rare postnatal neurological disorder of the grey matter of the brain that almost exclusively affects females

Question 2

What is the main gene implicated in Rett syndrome?

Answer 2

MECP2 (4 exons) - mutations identified in 80-95% of females with classic Rett syndrome

Question 2

What can affect the degree of presentation in females?

Answer 2

The level of skewed X inactivation

Question 3

What is a synonymous mutation and how does it differ from a non-synonymous?

Answer 3

Synonymous changes do not impact on the protein - there is no change in its production. Non-synonymous changes affect the protein product

Question 4

What are the three types of non-synonymous mutation?

Answer 4

1. Missense: single base substitution results in coding for a different amino acid
2. Nonsense: substitution that results in generation of a stop codon leading to premature termination of translation
3. Frameshift: a deletion or insertion that disrupts the reading frame

Question 5

Are most cases of Rett syndrome sporadic or inherited?

Answer 5

Sporadic - ~95% caused by de novo mutation on paternal copy of MECP2

Question 6

Which exons of MECP2 are more important for disease-causing mutations?

Answer 6

Exons 3+4 account for 80% of classic Rett and 50% of the persevered speech variant of atypical Rett

Question 7

What type of nucleotides are more susceptible to mutation in the MECP2 gene?

Answer 7

CpG dinucleotides

Question 8

Are large deletions in the MECP2 gene more commonly found in classic or atypical Rett syndrome?

Answer 8

Classic

Question 9

What domains of the MECP2 gene are commonly linked with

1. Missense mutations
2. Nonsense mutations
3. Deletions

Answer 9

1. Predominantly Methyl binding domain
2. Scattered between methyl binding domain and transcription repression domain
3. Deletion prone region

Question 10

What are the three types of MECP2 mutation seen in males?

Answer 10

1. Mutations found in classic Rett
2. Mutations inherited from the mother which are not found in females with Rett. These range in severity and have good prognosis
3. Deletions of Whole gene and neighbouring genes resulting in severe phenotype

Question 11

Is there any genotype/phenotype correlation in Rett syndrome?

Answer 11

• mutations occurring 3’ to transcription repression domain often result in milder phenotype
• females with Missense mutations have a significantly milder phenotype than those with truncating mutations
• patients with deletions have milder phenotype if mutation is small and towards 3’ end of the gene

Question 12

What testing strategy for Rett syndrome?

Answer 12

• Bidirectional sequencing should detect approx. 85-90% of mutations found in classic Rett syndrome and 30-40% in those patients with atypical Rett
• all exons are sequenced and MLPA is used to detect duplications and deletions

Question 13

What can produce false negative results in Rett syndrome testing and how is it combatted?

Answer 13

• Preferential amplification of normal allele due to PCR primer polymorphism, which could be present in mutant allele
• Special software constantly used to detect presence of new SNPs and ensure primer location is SNP free

Question 14

Are the same PCR conditions used for template generation of all the MECP2 exons?

Answer 14

No. Different mastermix and PCR conditions are used for exon1

Question 15

What downstream testing is involved if variants are detected in Rett syndrome?

Answer 15

• Parental samples should be requested
• If it is a MUTATION then testing also offered to other female relatives of proband and to male relatives where there is possible Rett phenotype

Question 16

How can X inactivation impact on X linked disease?

Answer 16

Inactivation is not always complete so females can present with a mild or sometimes full expression of an Xlinked recessive disorder.

By chance the X chromosome carrying the normal genes has been inactivated in more than half the cells and this is referred to as skewed X inactivation

Question 17

Describe prenatal diagnosis in the context of Rett syndrome and MECP2 mutations

Answer 17

• risk to foetus will depend if mutation has been identified in parent. If mum carries mutation risk=50%
• if familial mutation not identified in parents then still a risk they are germ line mosaics
• some studies have shown no mutation in parents but new child has same mutation as their affected sibling
• MCC must be checked to determine if result is due to presence of maternal cells in the foetal sample

Question 18

What alternative testing is available for atypical Rett syndrome not associated with MECP2?

Answer 18

• mutation testing in CDKL5 gene at Xp22
• all atypical Rett syndrome patients with mutation in this gene have had early onset seizures (before 6 months)
• patients presenting with this atypical Rett phenotype should have screening of the CDKL5 gene considered

Question 19

Other than MECP2 and CDKL5, name four alternative testing strategies

Answer 19

1. Angelman syndrome - similar phenotype early in childhood
2. SLC9A6 gene: found in cases with similar phenotype mimicking Angelman syndrome
3. NTNG1 gene: mutations are rare cause of atypical Rett
4. FOXG1 gene: associated with congenital form of Rett syndrome that presents in first months of life