Rett Syndrome

Question 1

What is the incidence of Rett Syndrome?

Answer 1

1/10,000 female births

Question 2

What are the clinical features?

Answer 2

Normal early postnatal development.
Rate of head growth abnormally low.
Developmental regression at 9-18 months
- loss of skills (hand, speech, locomotion)
- social withdrawal
- onset of hand stereotypies (hand wringing
- breathing/autonomic dysfunction, disturbed mood and sleep patterns.

After regression at about 4+ years there is a plateau
Slow progress thereafter
Social skills limited.
Need full care
Later - seizure onset, scoliosis
Can live to >50 years old, but sudden early death in 25%

Question 3

What are the clinical features in male hemizygotes?

Answer 3

Severe encephalopathy

Fatal soon after birth or earlier.

Question 4

What is the inheritance pattern for Rett Syndrome?

Answer 4

X-linked dominant.

Question 5

Which gene is affected?

Answer 5

MECP2 gene - encodes methyl CpG binding protein 2

Question 6

Is it a loss of function or gain of function mutation?

Answer 6

Loss of function.

Question 7

What is the function of MeCP2?

Answer 7

Epigenetic modification of cytosine
Associated with compact chromatin structure and reduced capacity for expression - role in suppressing transcription.
Binds methylated CpG and keeps those genes turned of.

Question 8

What explains the variance in severity?

Answer 8

Skewed X inactivation and the type of MECP2 mutation.

Question 9

What is the phenotype in the MECP2 knockout mice?

Answer 9

Brain weight and size reduced.
 - mainly frontal/temporal cortical layers affected.
 - brainstem also affected
Cortical thickness reduced
No cell loss
Neuronal cell packing density increased
 - mainly associated with reduction in 'bushiness' of dendritic arborisation
Cortical layering nearly normal.

Question 10

What happens to the neurons in KO mice?

Answer 10

All neurons reduced in size.

Question 11

What is the evidence that RTT is the result of failure of neurons to mature in their function?

Answer 11

Huge postnatal increase in MECP2 expression, coincides with maturation of neurons.

Question 12

How might research develop to treat RETT.

Answer 12

Reactivation of Mecp2 in reversible-KO mice after symptom onset could reverse and improve the condition - gene rescue.

Question 13

Have you read the review?

Answer 13

Gadalla et al. review, Biochm. J. 2011

Question 14

Could RTT be treated with gene therapy, what would be required of a vector system?

Answer 14

Must transduce non-dividing cells (neurons)
Must efficiently enter the CNS and reach and transduce enough neurons.
Must lead to stable long-term transgene expression at near WT level in transduced cells.
Must not overexpress above a certain threshold.
Must not provoke a serious immune response.

Question 15

How would the outcome of gene therapy be measured?

Answer 15

Distribution of transduced cells, cell types, MeCP2 expression level in both ‘null’ and ‘WT’.
Ability to prevent/rescue molecular phenotype - nuclear size, neuronal soma size, mature neuronal marker expression level, dendritic patterning and growth.
Ability to prevent/rescue the phsyiological phenotype - neuronal connectivity, synaptic function and regulation, network behaviour.
Ability to prevent/rescue the whole organism phenotype - body growth size, brain size, behavioural/breathing deficits.

Question 16

Have gene therapies been tested in animal models?

Answer 16

Yes, mouse model using adeno-associated virus vector.
Survival, severity and bodyweight all improved in male KO mice given AAV/Mecp2 vector.
Neuronal nuclear size became normal in cells transduced with AAV/Mecp2 vector.