Neurogenetics Flashcards
intro to genetics
- Chromosomes are present in every cell of the human body (every living organism has a unique genetic make up)
- Chromosomes are really long strands of DNA - double helix
- Humans have 23 pairs of chromosomes
DNA structure function
- 1953: Watson, Crick, Wilkins and Franklin
- Double helix structure made from 2 chains of phosphate and deoxyribose (type of sugar)
- Deoxyribonucleic acid - that’s where the DNA come from
- 4 different bases - base pairs:
- Adenine
- Thymine
- Cytosine
- guanine
- Each chromosomes has over 100 million base pairs of DNA
human genetics
- We share ~99.9% of our DNA sequence with each other
- Natural variations in our DNA are known as Single Nucleotide Polymorphisms (SNPs) - sequence along the DNA is identical except with one base pair
- 3,300,000 SNPs identified through human genome project sequencing
- The unique combination of SNPs that we inherit from our parents are responsible for the genetic component that makes us different to one another.
cell division - mitosis and meiosis
- Mitosis - somatic cells (daughter cells identical to parent) - every cell is identical (genetic makeup)
- Meiosis - gametes (daughter cells contain half the number of chromosomes)
genetic inheritance - meiosis
- Homologous recombination or “crossing over”
- Allows genetic diversity
- Natural selection and evolution
- Offspring all share 50% of each parents genes, but a different 50% - different combination
genes to protein
- Approx. 23,000 genes on human chromosomes - within our cells
- Genes are long sequences of base pairs in the DNA, which have particular properties that can be read and encoded into proteins
- Genes are turned on by transcription factors
- Transcription factors are activated during development or by intracellular signalling cascades from other parts of the cell
- E.g., CREB
gene expression
- The DNA partially unravels, allowing a transcription factor to bind to the gene
- Transcription - in the nucleus, the gene’s DNA sequence is copied into messenger RNA (mRNA)
- Translation - a ribosome attaches to the mRNA and moves along the mRNA, reading each triplet codon (3 bases) and using transfer RNAs (tRNA) to put together the amino acid chain to make a protein.
Mendel’s law (Mendelian inheritance)
- Gregor Mendel (1865) - inheritance through “Transmissible units”
- Peas:
- Inherited properties in pea plants: tall v short
- Height in peas: dichotomous trait (tall or short, no in between)
- Trait that is controlled by a single gene - either tall or short
- When cross-fertilised all of the first generation (F1) offspring are tall. But the short character reappears in the second generation (F2) in just a quarter of the offspring.
Mendel’s law
· Gene is in one of two forms (known as alleles) - either tall or short
· 2 copies of the gene in each parent pea
· 1 copy is carried to each of the offspring
· Height: Tall (T) is dominant, Short (s) is recessive
· If the genes are identical (TT or ss) - homozygous - two identical parts of the gene
· If genes are not identical (Ts) - heterozygous - two different parts of the gene
· Dominant and recessive inheritance … not just peas.
genotype ve phenotype
· Genotype - genetic information, whether the offspring is homozygous, or heterozygous
· Phenotype - how it displays (interaction of genotype with environment)
- E.g., PKU - the genotype will show a mutation in the genetic code but as long as dietary intervention then the phenotype will appear normal
· Alleles - variants of a gene e.g., tall vs short alleles of height gene in peas
genetic variations affecting brain and behaviour
· Single gene disorders:
- Dominant
- Recessive
· Gene variations/mutations:
- Affect function (coding sequence) e.g., PKU/Huntington’s
- Affect expression (non-coding, regulatory sequences)
· Chromosomal abnormalities
· X - linked chromosomes
huntington’s chorea - dominant inheritance
· Degeneration of the brain (striatum) leading to progressive deterioration of movement, temperament and cognition.
· See the onset of this disease in their 40s and 50s, they start to see movement changes, then can lead to changes in temperament and cognition - late onset
· Autosomal dominant inheritance: single copy will be dominant and lead to the disease (if 1 parent has Huntington’s, 50% of the offspring will develop Huntington’s).
· Single gene disorder on Chromosome 4 (Gusella et al., 1983)
· Due to excessive repeat of CAG bases - in the Huntington gene (normal chromosome has 11 to 34 copies of this base repeat, Huntington’s gene has excess of 40 copies of this base repeat)
· Disease onset (age 35-55), number of repeats (average 44)
· Early onset (is when there is 60+ base repeats)
· Unstable triplet and can therefore increase in subsequent generations
Phenylketonuria - recessive inheritance
· Mutation in the PAH gene (phenylalanine hydroxlase) - if you have a normally functioning PAH gene, you will be normal
· Enzyme that breaks down dietary phenylalanine
· Carrier: 1 in 50
· Disease: 1 in 10,000
· If both parents are carriers:
- 25% of offspring will have the disease,
- 50% will be carriers
· Build up of phenylalanine toxic to developing brain:
- learning disabilities
- behavioural difficulties
- epilepsy
· PKU screening at birth in UK, as symptoms can be prevented by diet (same genotype, different phenotype (interplay of genes and environment))
chromosomal abnormalities
· Monosomy: single copy of a chromosome.
- Embryonic lethal
· Trisomy: three copies of a chromosome
- Very high rate of embryonic lethality
· Downs syndrome:
- trisomy in chromosome 21 (error in first meiotic division)
· Symptoms narrowed down to 20-40 genes on chromosome 21. Overexpression of these genes?
· Smaller brain size frontal lobes and cerebelum.
· Mild to moderate intellectual ability
· High risk of early onset Alzheimer’s Disease
x-linked conditions
· The wrong number of chromosomes impacts normal development reflecting importance of gene dosage e.g. Downs syndrome
· But males have XY and females have XX therefore major variation in gene dosage between sexes
· Y chromosome - very few genes, mostly governing male sexual function.
· X chromosome - many genes that play vital roles in both sexes
· Need to ensure that cells function normally with either one or two X chromosomes.
· X inactivation in females switches off one copy of X chromosome during embryogenesis
· X-linked disorders vary in their penetrance according to sex
x-inactivation in females (XX)
· X – inactivation ensures that the dosage of active genes is maintained in all individuals
· XY cells - no inactivation
· XX cells - inactivate one X
· An adult female is a mosaic of clones derived from different embryonic cells. Within a clone, all the cells inactivate the same X, but between clones the choice is random. If she happens to be a carrier of an X-linked recessive disease, this can have major implications
Rett syndrome (x linked)
· Progressive neurodevelopmental disorder almost exclusively affecting females, leading to profound disabilities.
· Rare condition: 1in 10,000
· Mutation in the gene MeCP2. This “transcriptional repressor” turns off the expression of unwanted genes during synapse formation.
· X-linked – gene the X chromosome
- (spontaneous mutation rather than inherited)
· X – inactivation means that not all cells will express mutated MePC2 gene, therefore variable penetrance, sometimes see milder symptoms.
- (Affected males do not have a “good copy” of MePC2. Much more severe phenotype, embryonic lethal or die soon after birth)
fragile X (x-linked)
· Most common inherited form of learning disability
· Relatively common: 1 in 4000 males; 1 in 6000 females (carrier of ‘premutation’: 1 in 259 females; 1 in 800 males)
· X-linked: symptoms predominantly in males
· Mutation in one end of the FMR1 gene (the 5’ untranslated region), consisting of an amplification of a CGG repeat (200+ copies; normally between 6 and 40 repeats).
· The FMR1 gene encodes the FMR protein, which is thought to shuttle select mRNAs between the cytosol and nucleus.
· (Milder penetrance in females due to X – inactivation, so not always recognised)
epigenetics
· Inherited change in phenotype
· Not due to changes in genotype (DNA sequence)
· X-inactivation (X chromosome)
· Genomic imprinting (autosomal genes)
· Environmentally activated
epigenetics and the environment
· What genes are switched on and off.
· Phenotypic differences (neuron a neuron, muscle cell a muscle cell etc)
· Epigenetic modifications can be stable through life course (e.g. cellular differentiation)
· BUT… Some affected by environment
· Early developmental influence on stress resilience / depression (in later life)
· Maternal care (pup licking) switches on serotonin, action through 5-HT7 receptor (intracellular cascade) to activate transcription factor NGFIA, which switches on gene (Nr3C1) which expressed Glucocorticoid receptor (GR)
transgenerational epigenetics
· Environmental influence on parents can affect offspring
· Disrupted histones in sperm cells:
· Showed altered RNA profile in offspring (and grand-offspring)
· (Histones can be modified by chemicals (e.g. smoking / drinking))
· Opinion piece in The Conversation explaining importance of this work:
human genetics
· We share over 99.9% of our DNA sequence with each other
· Natural variations in our DNA are known as Single Nucleotide Polymorphisms (SNPs) ~ 3,300,000 SNPs identified through human genome project sequencing
· Gene association studies:
- Gene association studies look for sorting of SNPs in candidate genes
- Genome wide association studies (GWAS) – look for what SNPs sort with disease state
- Functional SNPs vs genetic tags
genetics and Alzheimer’s disease
· Mutations in these genes cause Alzheimer’s disease
· Amyloid precursor protein (on chromosome 21 - Down’s trisomy)
· Risk genes identified in gene association or GWAS studies of late onset AD.
· One SNP difference between difference between APOE3 (common allele) and APOE4 (changes an amino acid in protein)
polygenetic factors - twin studies
· Concordance - the degree to which a trait is seen in 2 individuals
genetics of schizophrenia
· High correlation between risk of developing schizophrenia and genetic relationship (Gottesman, 1991)
· Large genetic component, but not purely genetic.
genetics of schizophrenia 2
· GWAS study of over 150k people (36,000 with schizophrenia)
· Lots of the genes to do with synaptic transmission, glutamate and dopamine (dopamine D2 receptor)
genes to behaviour
· Environment can change gene expression (Epigenetics)
· Genes can alter how we interact and react to a particular environment.
· Difficult:
- to define genetic and non-genetic factors
- to understand the interactions among these many factors
- to follow the steps between gene expression and behaviour
- to allow for individual differences
- as no particular combination of genes and experiences is ever replicated exactly.
· limited experimental control and ethical issues in human studies.
animal models
· Similarity of genes and biological function with humans.
· Conservation of behaviour with humans.
· Create inbreed strains of animals that are genetically identical.
· Control environmental conditions.
· Manipulate genes: mutate or remove particular gene, or insert copy of a human gene.
animal models 2
· Mice:
- Known genome
- Relatively short life cycle
· The mouse genome:
- 22,000 genes
- 20 chromosome pairs.
· Why Mouse?
- 99% of mouse genes have homologues in man
- Similarly orgainsed brain, and behavioural traits common across mammalian species.
- Wealth of background information on biological processes and well defined behavioural tests, including models of disease states.
- Targeted mutagenesis: mutate a particular gene and look for subtle changes in behaviour.
genetic mutations
- mouse genes mapped onto the human chromosomes
genetic studies using rodents
· Inbred strains and BXD recombinant lines
· Mutagenesis and Knockouts
· Genetic tools for neuroscience
inbred strains
· Inbred strains will have different genetics, neurobiology and behaviour
· Comparison of these strain can tell us about how genetics influence behaviour
BXD recombinant breeding
· Which mice show high preference for alcohol?
· What C57 genes do they have ?
· Genetics: QTLs for activity mapped to chromosomes 2, 5, 12, 13, and X.
genetically engineered mouse models
· Knockout mice:
- What happens in absence of specific gene
· Knock-in mice:
- Introduce a specific mutation
- ‘humanised’ mice
· Transgenic mice:
- Reporter constructs to tag cells
- Constructs to target cell specific or time specific gene manipulations
from human condition to animal model and back - Rett syndrome
· Progressive neurodevelopmental disorder almost exclusively affecting females, leading to profound mental impairment.
· Mutation in the gene MeCP2. This “transcriptional repressor” turns off the expression of unwanted genes during synapse formation.
· Disease a result of inactive MeCP2
· Mouse with knockout of MePC2 display similar symptoms to Rett Syndrome
· Inducible KNOCKIN… gene switched off during development… reverts to wildtype (normal) form with drug treatment… can symptoms be reversed?
