Down syndrome and Fragile X

• Down Syndrome (Trisomy 21)


  Caused by the triplication of chromosome 21 (3 copies of chromosome 21)

  
• Individuals with Down Syndrome

  • Mild-moderate intellectual disability but degree of impairment can vary a lot
  • Delayed language and behavioural problems like attention and impulsivity
  • Might have trouble expressing thoughts and feelings so can be misunderstood, difficulty understanding social cues and interpreting emotions (isolation)
  • Motor skill development delayed so everyday tasks affected and may need extra support
  • Distinct facial appearance and birth defects
• Health issues associated with Down Syndrome

  • Higher risk in having hearing defects
  • Digestive system defects
  • Developing Alzheimer's
  • Congenital heart defects (e.g. Heart septal defects)
  • Sterility in males
  • Blood cell abnormalities (e.g. leukaemia)
  • Immune system may not function properly making them susceptible to infections and cancers
  • Obstructions in GI tract where digestive organs are abnormally formed, can contribute to developing celiac disease
  • Sleep apnoea
  • Higher risk of developing type 1 diabetes
• Brain changes in Down Syndrome

  • Reduced brain volume
  • Reduced frontal and temporal lobe volume
  • Reduced hippocampal volume – linked to dementia risk
• Down Syndrome was only identified just over 60 years ago, and since research life expectancy for DS individuals has increased from rarely surviving beyond 11-13years to now 60+
• This longer life expectancy has raised discussion with the link with Alzheimer's Disease
• Physical features of Down Syndrome

  • Simian crease in hands
  • Gap between big toe (sandal gap)
  • Flat face
  • Flat nose bridge – because of flat face and nose leads to protruding tongue due to less space I the mouth to contain the tongue
  • Small ears set lower
  • Short neck with excess skin folds
  • Up-slanting palpebral fissures (upper and lower eyelids) e.g. almond shaped eyes
  • Epicanthal folds in eye
  • Short height
  • Hypothyroidism – hormonal problems, problems in producing thyroid hormone impacting growth and metabolism
  • Hypotonia – poor muscle tone, appears floppy
  • Super flexible joints can lead to skeletal problems even dislocation of spine – paralysis
  • Vision problems, hearing loss
  • 1/14 autism
• Maternal age


  A major risk factor for Down Syndrome, e.g. less than 20yrs 1/1500 whereas 40+ 1/25 risk
• Nondisjunction (95% of DS cases)

  1. 3 complete copies of chr21, chromosomes fail to separate from their centre during meiosis

  2. Normally, sperm and egg carries one copy of every chromosome

  3. Chromosomes don't split apart, one cell ends up with both chromosomes during meiosis and the other has none. This results in 2 cells with an extra chromosome and 2 cells missing a chromosome. But this can also happen with sperm cells that have too many or too few copies.

  
• Unbalanced Robertsonian translocation (4% of DS cases)

  1. Bulk of the chromosomes attach to each other and the smaller parts of each chromosome attach to each other, but it is possible to have a balanced translocation (normal phenotype) because of balanced genetic material

  2. Occurs in acrosomal chromosomes where the centromere is shifted towards the end of the chromosome not the middle, leading to a long end (q arm) and a short end (p arm)

  3. Breakage and re-joining - During miotic cell division, part of chromosome 21 breaks off and merges onto chromosomal 14, leading to a long chromosome and short chromosome, so there is this extra material leading to 3 overall

  4. 2 normal copies of chromosome 21 plus an extra material from chromosome 21 attached to chromosome 14 which is the Robertsonian translocation

  5. Translocation occurs with chromosome 21 and chromosome 14 or 22 or other chromosomes. The cross over chromosome contains more genetic material than a normal one (like 2 for 1)

  
• Mosaic Down Syndrome (1% of cases)

  1. Normal: affected cell ratio, some cells will be normal

  2. Occurs during mitosis

  3. Cells are mixed, some have 46 chromosomes and some have 47 chromosomes

  4. Zygote divides over and over again to produce cells in the body (mitosis) so nondisjunction of chromosome 21 can also happen during mitosis so you might get one cell that has an extra 21 and one without one. Therefore One cell ends up with 47 chromosomes and one with 45 (which ends up dying because of the mutation) so 47 continues to carry out mitosis, if this happens during the early stages, a lot of cells will end up having 47 chromosomes.

  5. On the other hand, if the zygote starts with having 47 chromosomes, when mitosis occurs the cell will have 4 chromosome 21s so that cell ends up dying, the other cell with end up with 46 (loses one 21) which is the normal amount that will continue mitosis – opposite happens, if it happens early in development more cells will be normal, but late development cells will have 47

  
• Nondisjunction is different to Robertsonian translocation because during breakage and re-joining, chromosome can be spliced in the middle of a gene or between genes, introducing a new DNA molecule within a gene can cause that gene to be non-functioning

  
• Prenatal diagnostic tests for Down Syndrome

  • Chorionic villus sampling - Biopsy from placenta, analysing foetal cells karyotype (chromosomal analysis), highly accurate
  • Amniocentesis - Amniotic fluid sample, low AFP ratio between amniotic fluid and maternal serum indicates high chance of DS
  • PUBS - Umbilical cord examining the blood
  • Quad screen – least invasive taking maternal blood sample looking for alpha-fetoprotein (AFP), estriol, increase in human chorionic gonadotropin (HCG) and increase in inhibin A
• Chromosome 21 is the smallest human chromosome with 48 million base pairs representing 1.5%-2% of total DNA, 200-300 genes, highest percentage of long non-coding RNAs

  
• Mouse genomic sequencing in 2002 stimulated the development of DS mouse models
• Genome sequencing of the human chromosome 21 and the full mouse genome allowed a full mapping of the corresponding syntenic regions between human chromosome 21 and chromosomes MMU16, 17 & 10 of the mouse genome

  
• Ts65DN mouse model


  Plays a key role in understanding the neurological features of DS and to develop drugs to potentially help cognitive deficits we see in DS which can play along term role in preventing Alzheimer's Disease

  
• Brain regions most affected in Down Syndrome

  • Prefrontal cortex – contributes to executive function
  • Hippocampus – learning and memory
  • Cerebellum – dramatic reduction in size, cerebellum is complex in it's role of planning, adjusting and executing movements and a major role in motor learning
• In Down Syndrome individuals


  Un-coordination in motor movements, is this due to trisomy 21 or due to a secondary affect of the physical and neurological limitations of DS?
• Trisomy 21 impacts brain regions which causes the behavioural difficulties that we see in DS, or is it that the physical features of DS that prevents messages from sending strong or efficient enough to these brain regions – what is the primary or secondary factor or is it both?
• Neuronal function in Down Syndrome

  • In DS older children and adults there is perturbed neuronal function
  • In genetic mouse models studies have shown APs in the dorsal root ganglia are shortened due to the increase in speed of depolarisation and repolarisation, this acceleration of action potential polarisation is specific to DS
  • Mouse hippocampal neurons are much slower (deceleration of speed). It is suggested this is due to reduced density of sodium channels in hippocampus and so a smaller inward sodium current. The combination of these 2 effects memory and progressive decline in DS
• Imbalance in glutamate and GABA


  Excitatory and inhibitory inputs into the hippocampus. In Ts65DN mice, downregulation of GABAergic neurons seems to restore the glutamate and GABA inputs and it normalised performance in some hippocampal based tasks relating to learning and memory
• Kcnj6 gene


  Found on Chr21 that codes for GIRK2 (g protein inwardly rectifying channel). Selective or K+ ions causes hyperpolarisation and reduced neuronal excitability when opened, GIRK2 are found in the heart to control spike frequency, found in atrial pacemaker cells in the nervous system to control inhibitory potentials in neurons
• Activation of GIRk2 channels


  Require simultaneous action of beta-gamma subunits of G-proteins which are produced by the activation of GPCRs and PIP2. Opening of these channels by K+ shifts the membrane potential so that the neurons become hyperpolarised (-94mV) reducing neuronal excitability (important in E/I balance and controlling resting membrane potential and neuronal excitability)
• Kcnj6 in Ts65N mouse model

  • Resides on the mouse model Ts65N chromosome MMU16, exhibiting 50% overexpression of GIRK2 in hippocampus, frontal cortex, SubNigra and cerebellum. The mice showed increased GABA-induced Girk2 activation – therefore increased neuronal inhibition. Distribution of GIRK2 is altered with disproportional increases in specific brain regions. Membrane targeting of GIRK2 is enhanced in DS
  • Reduced LTP and insignificantly enhanced LTD (key players in synaptic strength and neuroplasticity affecting memory and learning) GIRK2 trisomy with impaired hippocampal-dependent spatial memory and associative learning
• When evaluating evidence from animal studies and comparing them to human conditions, an animal model has many different chromosomes, CNS structural differences and behaviour. These findings can only INFORM research into human disorders. It is still amazing and facilitates huge research and clinical advances in understanding the limitations of DS.
• Reduction in kcnj6 gene


  Restored normal hippocampal GIRK2 levels, long term memory improved
• Increased GABAb/GIRK2 signalling


  Can reduce synaptic plasticity and therefore lead to cognitive deficits in DS seen in models using GABAbR antagonists or GIRK2 blockers or genetically reducing kcnj6 gene dose
• LTP restored in DS mice with 2 functional copies of kcnj6 gene
• Treatments targeting GIRK2


  • Fluoxetine (serotonin reuptake inhibitor) directly blocks GIRK2 and restores LTP – this suppression of signalling needs to be within certain limits for effectiveness
  • Ethosuximide partial blocker of GIRK2 not effective due to inhibiting low-threshold ca2+ channels which may negatively affect synaptic plasticity and memory
• But not all abnormal behavioural parameters were restored with these treatments, so only selective improvements so maybe timing of interventions may be critical
• GIRK2


  G protein-coupled inwardly rectifying potassium channel 2, a type of ion channel, and its location varies across different tissues and organs including the heart. GIRK2 are present in the central nervous system, including hippocampus and neocortex. They are involved in regulating neuronal excitability and neurotransmission. They regulate flow of K+ ions across cell membranes mediating inhibitory effects of neurotransmitters GABA and serotonin. When activated they lead to hyperpolarisation and decreased neuronal excitability.
• In Down Syndrome, we see increased expression of kcnj6 gene which encodes GIRK2, associated with abnormal neural circuit formation, synaptic plasticity and cognitive impairments and increased risk of congenital heart abnormalities in DS.
• Dysregulation of neuronal excitability and synaptic function can impact learning, memory and overall cognitive function.
• Congenital heart disease (CHD)

  Affects 40-60% DS newborns. Increased dosage of genes on chromosome 21 can contribute to CHD, some genes playing a role in cardiac development and this additional copy can disrupt the normal balance of gene expression.
• Genes in DSCR (DS-CHD critical region) on chromosome 21

  • DSCAM – immunoglobin superfamily of cell adhesion molecules, increased dosage due to trisomy may lead to atrioventricular septal defects AVSD, it is involved in nervous system development.
  • COL6A1 and COL6A2 – collagen VI composed of 3 chains by genes including these two which are located on chr21, over expression linked to AVSD and can be co-expressed with DSCAM
  • KCNJ6 – overexpression linked to congenital heart defects and cognitive impairments in DS
• GIRK2


  Associated with abnormal neural circuit formation, synaptic plasticity and cognitive impairments and increased risk of congenital heart abnormalities in DS
• Dysregulation of neuronal excitability and synaptic function can impact learning, memory and overall cognitive function
• Congenital heart disease CHD affects 40-60% DS newborns
• Increased dosage of genes on chromosome 21


  Can contribute to CHD, some genes playing a role in cardiac development and this additional copy can disrupt the normal balance of gene expression