Developmental disorders of the synapse

Question 1

what is a synapse

Answer 1

connection between neurons

Question 2

overview of ASD

Answer 2

2 core features: persistent deficits in communication(language&eye contact), social interaction & repetitive behaviours
symptoms (intellectual disability/irritability/immune dysfunction/GI disorders) occur in early childhood (heterogenous)
affects 1-2%, more boys (1:4), no treatment for core symptoms

Question 3

overview of schizophrenia

Answer 3

positive symptoms: delusions/hallucinations/disorganised speech negative symptoms: catonia/avolition/asociality
diagnosis requires: social/occupational dysfunction, >6months of symptoms

Question 4

Intellectual disability

Answer 4

deficits in mental abilities approx 2SD below IQ

Question 5

ASD cause

Answer 5

period of overgrowth/arrested growth/possible decline/degeneration

Question 6

structural changes in ASD brains

Answer 6

ASD babies have a large brain
MRIs show that the frontal regions have large cortical thickness and brain size (Bedford et al., 2020)

Question 7

ASD pathology

Answer 7

more dendritic spines (failure of synaptic pruning)
difficult to obtain controls since ASD is heterogenous

Question 8

twin concordance rate in ASD and schizophrenia

Answer 8

ASD - 90%
Schizophrenia - 50%

2 swedish families (NLGN3/4 - postsynaptic transmembrane protein) neurexin is presynaptic

Question 9

Copy number Variations (CNV)

Answer 9

deletions/duplications in DNA (only unbalanced chromosomal changes)
uses microarray-based comparative genomic hybridization (CGH)
above 0 - duplication
below 0 - deletion

Question 10

CNVs and de novo mutations

Levy at el., 2011

Answer 10

de novo = mother and father do not have mutation, child is affected
more females have de novo CNVs in ASD
females have larger CNVs
females are resistant to the phenotypic effects of ASD causing mutations

Question 11

network analysis

Answer 11

node size = importance of gene to the cluster score (how many times it appears)
length width = proportional to the likelihood of sharing the phenotype

Question 12

percentages of CNVs in ASD

Answer 12

5-10% non syndromic
10-25% syndromic (multiple features occur together)
de novo CNVs contribute to simplex ASD
majority of causes not related to CNVs (multiple genes instead)

Question 13

ASD exome sequencing

Answer 13

synaptic genes identified are GRIN2B (NMDAR subunit)/SCN1A (sodium channel subunit)
>100 genes involved in synapse formation/transcription regulation/chromatin remodelling

Question 14

common disease common variant hypothesis

Answer 14

majority of risk comes from several common alleles (>5%) each conferring modest risk
no evidence

Question 15

GWAS

Answer 15

looks for SNPs by comparing case and controls , SNPs point to an area of the genome which contributes to the disease
3 alleles: 5p (between cadherin 9/10), 80kB from Sema5A gene, MACROD2

limitations: studies are underpowered, each study fails to replicate findings, decreased confidence for all risk alleles, no common variants confer even modest (>1.2 fold) statistical increase in the risk of AD

Question 16

simplex vs multiplex

Answer 16

simplex = one affected child disorder
multiplex = multiple families

Question 17

mouse assays to understand behaviour

Answer 17

social interaction assay: 3 chamber task - analyse time spent in object/mouse chamber. If more time spent with object then ASD symptom. Mouse tried to kill other mouse (measure aggression)

repetitive behaviour assay: grooming/bar biting/marble burying. Tests cognitive flexibility, MWM-water must be cold. ultrasonic vocalisations = sqeaks

Question 18

what does functional imaging show

Answer 18

local hypoconnectivity with loss of long-range connectivity
increased brain volume with rapid overgrowth and decreased white matter imaging

Question 19

SHANK3 mutant

Answer 19

post synaptic scaffolding protein, binds to Homer and PSD-95, forms a scaffold, binds to glutamate.
Shank3 deletions cause Phelan Mcdermid Syndrome (22q13.3-haploinsufficency)/PDZ domain is deleted.
behavioural phenotypes similar to ASD(excessive grooming and social interaction deficits)

Question 20

Feng Lab observations of the Shank3 mutants

Peca et al., 2011

Answer 20

observed cortico-striatal synapses only gamma subunits - no trafficking and enlarged striatum (overgrowth), dendrites had more complex dendritic aborisations-longer length of spines but reduced density

Question 21

Buxbaum lab observations of the Shank3 mutant

Bodzagi et al., 2010

Answer 21

deleted ankyrin repeat domain:
reduced AMPA responses/mEPSPc amplitude, loss of LTP and LTD, decreased GluR1 puncta in hippocampus and impaired social interaction

Question 22

Rett syndrome

Answer 22

mainly affects girls since males die (heterozygous X)
at 1st year then 12-18 months: regression of speech and motor skills, cognitive/motor-hand/respiratory abnormalities
stereotypic motor behaviour (hand wringing), seizures, ataxia
caused by MeCP2 mutant on the X chromosome

Question 23

mouse models of rett syndrome

Answer 23

MeCP-/y MeCP308/y (truncated, hypomorph) MeCPTG (duplication) - Dani et al., 2005
in pyramidal neuron cortical slices recordings: reduced spontaneous firing in mutants, reduced excitation (via mEPSPs), increased inhibition (mIPSCs), less dendritic spines in the hippocampus
duplications increased synapses

Question 24

KO of MeCP2 from GABAergic inhibitory interneurons

Answer 24

(same phenotype as null mice) - stereotyped paw movements/compulsive grooming/impaired coordination
reduced somatic GABA/GAD1/GAD2 levels in neurons - risk of early death
decreased mIPSCs, no change on frequencies

Question 25

gene therapy and interneurons

Answer 25

MeCP2 null - adult mouse has no movement MeCP2+gene rescue at 3-4wks - movement (Guy et al., 2007)
SST/PV+ altered inhibition
PYR - reduced excitation

Question 26

rett syndrome treatment

Answer 26

neurons from skin neuroblasts from 4 patients with rett syndrome
add IGF1 - rescues glutamatergic synapse number/promotes excitatory synapse maturation and neuronal survival/crosses BBB

Question 27

what is (1-3)IGF

Answer 27

synthetic tripeptide
causes extended lifespan and rescues spine density
rhIGF (recombinant IGF1) increases survival and breathing rate

Question 28

IGF1 trials

Answer 28

2016 Tropea trial (Pini et al., 2016) Rx mecasermin improved disease severity
2018 Sahin group - double blind crossover had no benefit

Question 29

Trofinetide

Answer 29

novel synthetic analog of N-terminus peptide of IGF-1 phase II had promising results - approved in mar 2023

Question 30

why does the same pathway/mutations cause differing human phenotypes

Answer 30

2-hit model
genetics - background effect in animal models/epistatic interactions
environmental factors - febrile seizures/head injuries/psychosocial stressors
intrinsic stochastic developmental variation