Genetics in clinical practice Flashcards
Define:
Chromosome
Gene
Chromosome= single molecule of DNA
Gene =DNA sequence containing genetic instructions for a protein / RNA - arranged along a chromosome, each containing 100s of genes
What is a karotype?
A visual way of seeing how human chromosomes are arranged - used to help determine the cause of a disease
- the chromosomes demonstrate clear banding patterns
What are the different components of a chromosome (in metaphase)?
Telomere- DNA and protein cap - ensures replication to tip - protecting genes embedded in the chromosome (no genes within this region). The shorter it is the closer the cells is to apoptosing
Light bands - replicate early in S phase, less condensed chromatin, transcriptionally active, gene and GC rich
Centromere - join sister chromatids - mitotic spindle attach to them
Dark bands - replicate late, contain condensed chromatid, AT rich
The differing chromatin within the dark and light bands causes them to stain differently
What are a couple of techniques to visualise chromosomes?
FISH - fluorescence in situ hybridisation
Array CGH - comparative genomic hybridisation
What is FISH?
Fluorescent DNA probes bind to parts of the chromosome with a high degree of sequence similarity (probes are artificially designed)- they bind to the DNA by hydrogen bonds
Used to detect and localise the presence or absence of specific DNA sequences
Can identify whether particular sequences are present (e.g. if a gene is deleted) and identify structural chromosomal variants
What is array CGH?
Comparing binding of test genomic DNA and normal reference DNA
Looking at very short fragments of DNA= more specific
DNA microarray contains probes representing genomic regions of interest - DNAs compete for probe sites on microarray
= yellow dots: normal
= both red and green dots mean differences
What are the different classifications of genetic disorders?
Single gene disorders - mutations in a single gene (often causing loss of function
Multifactorial diseases / common complex disorders - variants in genes causing alteration of function - alleles interacting with environment
Chromosome disorders - chromosomal imbalance causes alteration in gene dosage
Mitochondrial disorders - generally affect organ systems with high energy requirements - genes controlling function and structure of mitochondria are found in mitochondria and nuclear DNA
Somatic mutations - cause cancer - inactivation of both alleles “two hits” of a gene involved in growth required
What is the impact of chromosomal disorders?
Multiple organ systems affected at multiple stages in development
Growth retardation
Developmental delay and learning difficulties
What are the different types of chromosomal abnormalities?
Numerical - wrong number of chromosomes
- aneuploidy =monosomy, trisomy
- polyploidy = triploidy = 3 copies of every single chromosome
Structural
- translocations - reciprocal, robertsonian
- deletions, duplications and inversions
Different cell lines
- mosaicism
What is the international description of karotyping?
1) total number of chromsomes
2) sex chromosome
3) abnormalities / variants
What are the most frequent numerical abnormalities in live borns?
- autosomes
- sex chromosomes
- all chromosomes
Autosomes
- Down’s syndrome - trisomy 21
- Edward’s syndrome - trisomy 18
- Patau’s syndrome - trisomy 13
Sex chromosomes
- Turner’s syndrome - 45 X
- Klinefelter syndrome -47 XXY
All chromosomes
- Triploidy - 69
What are the genetic variations of Down’s syndrome?
95% have trisomy 21
4% have the extra copy of chromosome 21 because of a Robertsonian translocation- don’t have a full extra copy of 21 but enough to cause Down’s syndrome
1% have mosaicism - normal and trisomy cell lines - features are usually milder due to the presence of normal cells
What are the features specific to Down’s syndrome?
None
All individual features of chromosomal disorders can be found in members of the general population
- clues that a chromosomal disorder may be present include the presence of several anomalies in one person, together with growth delay and mental retardation
Examples:
- epicanthic folds
- depressed nasal bridge
- profound neonatal hypotonia
- single palmar crease
- Alzheimer’s disease
- Brushfield spots
1 in 700 births
> 60% spontaneously aborted
20% stillborn
What are the two hypotheses that having three copies of chromosome 21 cause the features of Down’s syndrome?
1) Gene dosage effect
- features of syndrome cause by 1.5x amount of specific gene products from chromosome 21
2) Amplified developmental instability
- features of syndrome caused by overall effect of imbalance on development
What is non-disjunction during meiosis I?
At meiosis I all the chromosomes go into only 1 cell and therefore after meiosis II you have 2 cells which are diploid not haploid, and 2 nullisomic gametes
What is non-disjunction during meiosis II?
Split normally at meiosis I but after meiosis II you end up with a disomic cell and nullisomic cell and 2 normal monosomic gametes
Why is aneuploidy associated with increased maternal age?
WRONG NUMBER OF CHROMOSOMES
- stock of oocytes is ready by 6 months gestation
- Each remains in maturation arrest at the crossing over stage until ovulation
May be lengthy interval between onset and completion of meiosis (up to 50 years later)
Accumulating “wear and tear” may damage the cell’s spindle formation and repair mechanisms predisposing to non-disjunction
What is Edward’s syndrome?
1 in 3,000 births
Multiple malformations (esp. heart and kidneys)
Clenched hands with overlapping fingers
Very poor prognosis - majority of babies due in first few weeks - if they do survive there is severe developmental delay
What Patau syndrome?
1 in 5,000 births
Multiple malformations
Affects midline structures particularly- incomplete lobation of brain, cleft lip, congenital heart disease
Very poor prognosis - majority of babies due in first few weeks - if they do survive there is severe developmental delay
What are 2 conditions caused by anomalies of sex chromosome number?
Klinefelter syndrome
- 47 XXY
- 1 in 1,000 males
- poorly developed secondary sexual characteristics in some (lack of testosterone)
- infertility, tall
Turner’s syndrome
- 45 X
- 1 in 5,000 female
- 99% are lost spontaneously in pregnancy
- short stature
- primary amenorrhoea
- congenital heart disease (coarction of aorta)
Why do people with triploidy tend to survive development?
Tend to be mosaic
What does it mean by genomic imprinting “parent of origin” effect?
Maternal and paternal contributions are required for normal development
Some human genes behave differently depending from which parent they have been inherited
They carry an “imprint” (a mark or pattern)
What genes are imprinted?
Not all genes it tends to be the one that are developmentally important
- limited number of individual genes or small chromosomal regions proven to be imprinted
Some imprinted genes are expressed only from the paternally and others from the maternally inherited chromosome
What is the epigenetic phenomenon?
DNA methylation during gamete formation which is then preserved in all cells of the offspring
What is the genetic conflict hypothesis?
e.g. intrauterine growth promoted by paternal alleles but restricted by maternal alleles a plausible explanation
What does DNA methylation do?
it marks the gene so that it is not used
What chromosomes can be involved in robertsonian translocation?
A centric fusion (Robertsonian) translocation can involve any of the acrocentric chromosomes - 13, 14, 15, 21, 22
e.g. breakage of two acrocentric chromosomes at or close to their centromeres - subsequent fusion of their long arms - short arms lost
Why in practice is the number of liveborn children with Down’s syndrome (Robertsonian) less than what is predicted (1 in 3)?
due to loss through spontaneous miscarriage
What is a reciprocal translocation?
Breakage of 2 non-homologous chromosomes with exchange of the fragments (balanced)
Any chromosome can be involved in reciprocal translocation
What does the size and position of chromosome segments involved in translocation effect?
- the pairing of the chromosomes at meiosis
- frequency of different forms of the translocation in the gametes
- likelihood of the conceptus with that abnormality developing to term
What are chromosomal translocations?
Exchange of material between 2 chromosomes
- breaks in arms: reciprocal
- whole chromosome join end to end: Robertsonian
Individual clinically normal if no gain or loss of material
Clinical effects related to imbalance of genes involved
May cause chromosomally abnormal baby, miscarriage, stillbirth, infertility
What happens at large chromosome deletions / duplications?
Unequal crossing over in meiosis following mispairing (often as repetitive sequences)
What is mosaicism usually the result of?
result of an error in mitosis
Define: somatic mosaicism and gonadal mosacism
Somatic mosaicism: two populations of cells in the body - exposure to carcinogens
- non-disjunction in mitosis in somatic cells
Gonadal mosaicism: two populations of cells only in the gonads
- arisen from an error in one cell whilst the gonad was being populated
