Rett’s

Question 1

State link between Rett’s and autism

Answer 1

Used to be classed as autism disorder, DSM-V (Diagnostic and Statistical Manual of Mental Disorders) changed that.

There is co-morbidity with autism and autsism like symptoms such as avoidal of eye contact.

Rett’s can be seen as an issue with differentiation that impacts cortical layers (autism differential cortical patterns are seen).

Question 2

State main symptoms of Rett’s syndrome

Answer 2

Hand skill impairment (repetitive movement), poor verbal and non-verbal communication, progressive motor and mental retardation.

Question 3

Why and how does Rett’s syndrome impact males and females differently?

Answer 3

X-linked: more females will acquire the disease but male patients will often have more severe cases that cause loss of life in infancy.

Question 4

How is Rett’s inherited?

Answer 4

Mostly via de novo mutations in paternal germ line but can rarely be passed down from mothers that have mild cognitive impairment or are asymptomatic due to skewed X-inactivation.

Question 5

State genetic profile of males with classical Rett’s

Answer 5

XXY karyotype or somatic mosaicisms.

Question 6

State genetic profile of males with severe neonatal encephalopathy and infantile death

Answer 6

XY. MeCP2 mutation inherited from mildly sympomatic or asymptomatic mother.

Question 7

How is Rett’s diagnosed?

Answer 7

Detectable from 6-18 months: normal early regression, marked regression and seizures.

symptoms in marked regression: microcephaly, hypotonia, stereotypic hand movements, breathing irregularities, loss of eye contact, loss of speech skills.

Question 8

Outline different life stages of Rett’s patients.

Answer 8

Normal development.
Stagnation: ~1yrs, postural delays, microcephaly.
Rapid regression: 1+yrs, loss of motor skills and communication, breathing irreg, seizures.
Pseudo-stationary: 2+yrs, more interest in surroundings, increased alertness, hand apraxia.
Motor deterioration: 10+yrs, not all will get to this stage, severe physical disability, scoliosis, bradykinesia.

Question 9

Mutation in what gene is liked to most prominent form of Rett’s?

Answer 9

MeCP2: methyl CpG binding protein 2 that leads to loss of MeCP2 function

Question 10

What is the role of MeCP2

Answer 10

Intrinsically disordered protein that binds methylated DNA via methyl-CpG-binding Domain (MBD). Widely regarded as repressor of transcription, specifically for long genes over 100kb long.

Found in mecp2 mutant that these gene expression upregulated. Can also activate more transcription but has low affinity for this mechanism.

More recent evidence suggests that secondary effects of MeCP2 most important: post-transcriptional regulation of gene expression via microRNA.

Review by Ip et al., 2018.

Question 11

What mutations of MeCP2 cause Rett’s?

Answer 11

Rett’s: methyl binding domain of protein is changed (this is the only structurally ordered part of the protein) most common. Review, Good et al., 2021.

Mostly arise from mutations in germ cells from father. 8 ‘hotspot’ mutations constitute 60% of documented cases.

Question 12

How do Rett’s patients brains differ to norm?

Answer 12

Gross observation of reduced brain volume in RTT individuals: explained by abnormally small and densely packed neurons with reduced dendritic complexity and spine density.

In females: parietal and temporal lobe volume decreased but occipital not.

Review by Ip et al., 2018.

Question 13

How does loss of MeCP2 function cause imbalance of excitatory and inhibitory circuits (no astrocytes)?

Answer 13

Impaired neurogenesis and differentiate seen by decreased dendritic spine density and smaller somas.

Transcription factor Dlx5 is direct target of of MECP2 = regulates maturation and differentiation of forebrain GABAergic interneurons.

NMDAR and pre glutamate transporter 1 (VGLUT1) reduced upon removal of mecp2 which regulates post-transcriptionally as seen in levels of mRNA coding remaining the same.

Microcircuits (pyramidal and inhibitory) that ensure excitatory and inhbitory balance aren’t set up properly due to disruption of neuronal differentiation.

Decreased miniature excitatory postsynaptic current response and frequency (absence of AP).

Question 14

How does loss of MeCP2 function cause imbalance of excitatory and inhibitory circuits (astrocytes)?

Answer 14

Astrocytes buffer extracellular potassium via inwardly rectifying potassium channels. in MeCP2 deficient mice low expression of these channels and therefore increased extracellular potassium.

This leads to…
1) longer action potential due to increased rate of rapid depolarisation.
2) Depolarisation of resting potential so smaller stimulus can trigger AP.

Review from Kadam et al., 2019

Question 15

What is the link between the circadian cycle and Rett’s?

Answer 15

Levels of MeCP2 change and have subtle influence of gene expression at diff points of circadian cycle.

Question 16

How is MeCP2 activated?

Answer 16

Neuronal depol, calcium influx, CamKII causes phosphorylation of MeCP2 thus activating.

BDNF calso shown to activate CamKII and activate MeCP2.

Review: Sanchez-Lafuente et al., 2022.

Question 17

How can Rett mosaicism occur?

Answer 17

Random X chromosome inactivation

Question 18

Most prominent form of Rett’s is due to loss of function of MeCP2. What happens in MeCP2 duplication syndrome?

Answer 18

100% penetrance in males with carrier mothers whereas females typically asymptomatic. Life span in 50% of sufferes severely reduced to 25 years due to reccurent respiratory infections.

also: motor dysfunction, seizures, severe mental retardation, stunted motor development and progressive spasticity. anxiety, and autism on top of this.

NB: have been cases of triplication that have been documented with even more severe symptoms.

Contrasting to loss of function: increased dendritic spine density.

Question 19

MeCP2 transgenic mouse where allele truncated similar to specific RTT mutations

Answer 19

normal for 6 weeks then forepaw rubbing, seizures, ataxia etc.

Activation of MeCP2 can reverse some neurological phenotypes in null mice.

Question 20

Outline different result of paired pulse stimulation in MeCP2 loss of funcion and duplication

Answer 20

Paired pulse stim: 2 stimulations given at varying intervals and slope of second response is compared to first.

1. Depression in second: inactivation of ca or sodium channels and indicative of increased probability of neurotransmitter release. MeCP2 loss of function
2. Facilitation of second: decreased due to residual calcium from initial stim. MeCP2 duplication.

Na et al., 2013

Question 21

Outline most commonly used animal model

Answer 21

Mecp2-null mice (males) often used more than Mecp2+- female mice as athey show early-onset severe symptoms (motor abnormalities, early death, breathing irregularities).

BDNF reduced in mouse models: synaptic maturation and plasticity.

IGF1: deficient in RTT mice, regulates pathway for excitatory synaptic maturation.

Extrasynaptic NMDAR responeses are increased. Overactivation of excitatory mechanisms tat could ‘burn out’ at older age when balance is shifted to inhibition. MeCP2 in neuronal cultures increases susceptibility to excitotoxicity.
Postnatal restoration in mice rescues several RTT phenotypes.

Question 22

Region specific MeCP2 KO

Answer 22

Brainstem and spinal: abnormal heart rate, breathing patterns and early death.

5HT: increased aggression

DA and NA: motor impairments.

All inhib: severe RTT phenotype, important of balance in brain. If excitatory only milder phenotype (tremor and increased anxiety).

• mecp2 deletion from different gabaergic = differential non-overlapping phenotypes (SOM(somatostatin)+ deletion is seizures and PV(parvalbumin)+ sensory and motor deficits)).

Question 23

Novel therapeutic treatments for Rett’s

Answer 23

DBS targeting hippo rescues memory deficits in mutant mice.

Reactivation of the silent X chromosome: issue is that RTT are heterozygous mosaic therefore reactivation may lead to increased MeCP2 in cells that are normal. This done using DNA methylation inhibitor on mice

IGF-1 (insulin growth factor): potentiates BDNF activity and analog is in phase 2 of clinical trials.

Question 24

Link between CDKL5 and Rett’s

Answer 24

cyclin-dependent kinase-like 5 mutation: accounts for atypical RTT that manifests with early refractory epilepsy (cannot be completely controlled by pharmaceutics) before the age of three months.

Seizures and sleep disturbances more common in CDKL5 but spinal curvature less.

PSD-95 (postsynatpic density protein 95) reduction and spine density reduction

Question 25

Outline link between epilepsy and Rett’s syndrome

Answer 25

Up to 90% of Rett’s patients have epilepsy co-morbidity. This is due to imbalance of inhibition and excitation. Different variants of Rett’s have differential epilepsy.

CDKL5: Hanefeld variant is characterized by early onset of seizures (infantile spasms seen on EEg as hypsarrhythmia and refractory myoclonic epilepsy).

In general the later the onset the less severe the epilepsy. There is not one type of epilepsy seen, most common are Complex partial and generalized tonic‐clonic.

NB: thought to be over-diagnosed due to other clinical manifestations being mistaken. Guerrini et al., believes actual to be 48% suffering true epilepsy.

Question 26

Outline some factors that contribute to feeding difficulties in Rett’s.

Answer 26

Appetite: effected by medication (esp epilepsy).
Chewing: bruxism - repetitive grinding of teeth that wears them down making harder to chew.
Posture: poor alignment of digestive tract causes squeezing and cramp of organs.

Helped by eating smaller, more frequent meals. Special chairs to eat in

Question 27

Define retts

Answer 27

Rare genetic neurological developmental progressive disorder that affects the way the brain develops

Question 28

Retts prevalence

Answer 28

1/10,000 females