Lecture 26 - Genetic Disorders Flashcards
In general, what are the categories of genetic disorders?
- Chromosomal abnormalities
- Single gene disorders
- Polygenic / multifactorial disorders
What is the name for abnormal chromosome number disorders?
Aneuploidies
What is the mechanism of aneuploidies?
Missassortment (non-disjunction) in meiosis
What are the sub-categories of aneuploidies?
Trisomy: extra single chromosome
Monosomy: deletion of a single chromosome
Polyploidy: multiples of haploid number of chromosomes
What is an example of polyploidy?
Triploidy: human is 3n instead of 2n
What is the mechanism of structural abnormalities?
- Rearrangement or deletion
- During crossing over
examples:
• Translocations
• Inversions
• Duplications
Why would genetic testing be carried out?
Presence of certain clinical indications:
- Problems with early growth and development
- Stillbirth and neonatal death
- Fertility problems
- Family history
- Neoplasia
- Pregnancy in older women
Describe Down syndrome
• Collection of facial and physical features
Associated with: • Intellectual disability • Congenital heart disease • Congenital GIT abnormalities • Leukaemia • Immunological defects • Premature ageing
Describe the premature ageing seen in Down syndrome
Alzheimer-like dementia
What is the classical cytogenic analysis of Down syndrome?
Karyotyping:
• Presence of three chromosome 21’s
Process:
- Lymphocytes isolated
- Grown in culture, diving
- Arrested in metaphase (chromosomes are the most dense)
- Staining
- Observation
Is trisomy 21 seen in all cases of Down syndrome?
No, only in 95% of cases
95%: Trisomy 21
5%: Unbalanced translocations
What is FISH?
What is it used for?
Fluorescence in situ hybridisation
Used for detection of trisomy 21:
- Cells taken from foetus
- Fluorescently marked probe for a given chromosome
- Interphase (non-dividing) cells given the probe
- Observation of 3 coloured dots → three of the given chromosome
How many chromosomes do humans have?
46:
22 autosomes
2 sex chromosomes
Describe the process of balanced translocation
Compare with unbalanced translocation
- Balanced
• No net gain or loss of genetic material
• Reciprocal translation between two non-homologous chromosomes
e.g. 21 and 3:
• Breakage in both chromosomes
• The fragment rejoins on the other chromosome
- Unbalanced
• Net gain or loss of genetic material
Describe Robertsonian translocation
Does it result in a disease phenotype?
Translocation between two acrocentric chromosomes
Short arms are lost
Only one centromere
Phenotype?
• Depends on whether the translocation is balanced or unbalanced
What are acrocentric chromosomes?
Chromosome in which the centromere is very close to the end of the chromosome
There are only 5 acrocentric chromosome in humans • 13 • 14 • 15 • 21 • 22
In which phase of the cell cycle is FISH performed?
Interphase
The cells are non-dividing
What proportion of DS cases are inherited unbalanced translocations?
1-2%
What are inherited DS cases due to?
Describe this
Unbalanced translocation
One of the parents is a carrier of unbalanced translocation
i.e. one of the gametes has two versions of a given chromosome (21)
e.g. Parent 1° gamete: (14;21) & 21 Parent 2° gamete: 14 & 21 -- fertilisation -- Zygote: (14;21), 21, 14, 21
The zygote has three copies of Chromosome 21
What are ‘DNA chips’?
Describe their use
Microarrays
Can look at:
• SNPs
• Chromosome n°
• CNVs
Process:
• Glass slide
• Imprinting short DNA sequences onto slide (oligonucleotides)
• Patient’s DNA sample hybridised onto oligonucleotides on the slide
What are SNPs?
Single nucleotide polymorphisms
What are CNVs?
Copy number variations
What are CMAs?
Describe their use
Chromosome microarrays
Allows discovery of micro deletions that are too small to be seen in FISH or karyotyping
Wolf-Hirschhorn syndrome:
• Microdeletion at the terminal region of Ch 4
• The read out shows dip at the very end
• Hybridisation of the patient’s DNA and ‘reference’ DNA (this is a controversial idea)
• If there is an imbalance of the hybridisation, we know that there is a microdeletion
• This is too small to be seen in FISH or karyotyping
What are some of the ways chromosome abnormalities are analysed?
- FISH
- Karyotyping
- Microarrays
- CMAs
Give examples of single gene disorders that are:
• Autosomal dominant
• Autosomal recessive
• X-linked recessive
Autosomal dominant:
• Huntington
• Osteogenesis imperfecta
Autosomal recessive: • CF • Thalassaemias • Sickle cell disease • Hereditary haemochromatosis
X-linked recessive:
• Duchenne muscular dystrophy
• Haemophilia
From whom are the mitochondria inherited from?
The mother
What is:
• Penetrance
• Variable expressivity
• Genetic heterogeneity?
A disorder may be ‘single gene’
However, there is variable severity of the phenotype
Environmental factors etc.
How are single gene disorders tested for?
Pathology settings:
• PCR + RFLP analysis
Increasingly used:
• Microarrays
• Panels of gene mutations
• DNA sequencing
Which gene is mutated in β-thalassaemia?
Describe this gene
β-globin chain genes
Mutation in transcription start site:
T → C
→ Decreased transcription of β-globin chain gene
→ Decreased (or lack of) β-globin chain production in those affected
Describe the structure of adult Haemoglobin
Compare this with β-thalassaemia
Adult form of Haemoglobin:
“heterotetramer’
• Two β-globin chains
• Two α-globin chains
In β-thalassaemia:
‘homotetramer’
• Four α-globin chains
What is bad about the Hb in β-thalassaemia?
α4 homotetramers lead to the destruction of RBCs
→ anaemia
What is the main clinical feature of β-thalassaemia?
Anaemia
Describe how β-thalassaemia is tested for
PCR + RFLP analysis
- PCR of β-globin chain gene
- Cut with NcoI (restriction enzyme)
3a. No mutation:
• Cutting
3b. Mutation:
• No cutting
- Gel electrophoresis:
a. No disease: 250 bp & 900 bp fragments
b. Disease: 1200 bp fragments
Describe polygenic disorders
Additive contribution of several genes to the disease phenotype
Input of each of the genes not necessarily equal
Variables are continuous
Give examples of continuously variable phenotypes
Height Body weight Cholesterol levels Diabetes Asthma Heart disease
Compare hereditary component of monogenic and polygenic disorders
Monogenic: high hereditary component
Polygenic: low hereditary component
How are genes responsible for polygenic disorders identified initially?
GWAS
SNPs that are shared with much greater frequency among individuals with the same phenotype than among others
How are complex genetic diseases tested for?
Same as single-gene conditions
Why is genetic testing performed?
- Clinical diagnosis
- Carrier testing
- Pre-symptomatic / predictive testing
Why is prenatal diagnosis performed?
- Help making informed choices
- Reassurance with normal results
- Risk information
- Choice of termination
- Psychological preparation
- Planning of delivery and care
Describe the following tests: • CVS • Amniocentesis • PGD • NIPT
- CVS: chorionic villus sampling
• Invasive; risk of miscarriage
• Placental tissue - Amniocentesis
• Amniotic fluid containing sloughed off foetal cells
• Invasive, risk of miscarriage
• Ultrasound - PGD: pre-implantation genetic diagnosis
• 1 or 2 cells taken from dividing zygote
• Only can be done in IVF
• Unaffected embryos implanted
4. NIPT: non-invasive pre-natal testing • Foetal cells and DNA/RNA in maternal blood • Introduced in 2013 • Based on next generation sequencing • Marketed as genetic screening
Compare how TOP is performed in CVS and Amnio
CVS:
• Aspiration under general anaesthetic
Amnio:
• Prostaglandin induction of labour
Compare risk of miscarriage in CVS and amnio
CVS: about 1%
Amnio: about 0.5%
What is the incidence of Down syndrome?
1 in 660
What are the pros/cons of FISH and Karyotyping
FISH:
• Much faster than Karyotyping
• Not as definitive as karyotyping
Karyotyping:
• Slower, because cells must be cultured and grown up
What is genetic screening used for?
Which people are screened?
Are they effective?
Identification of individuals at high risk of having or transmitting a specific genetic disorder
Who?
• General population
• Groups at risk
Efficacy:
• Not always definitive
• False positive and negatives occur
• Not all genes and mutations can be picked up
When would PCR+RFLP analysis be performed?
When the mutation is known, and the mutation is suspected in a family
This is because this is a very specific test
e.g. B-thalassaemia
Compound heterozygosity?
Two different mutations leading to disease
Why is genetic screening carried out?
- Prevention of disease
- For early treatment
- Future reproductive options
- Decrease social and financial burdens
What are the criteria of screening for a genetic condition?
As defined by WHO:
Must be:
• Important, severe, common health problem
• Preventable or treatable
• Screening is simple, safe, reliable, acceptable and relatively inexpensive
• Education and counselling available
What type of population genetic screening is carried out in Victoria?
- Pre-natal
- Newborn
- Pre-conception carrier screening
Describe how pre-natal screening can be carried out
- Ultrasound
- Maternal serum screening
- NIPT screening
Which diseases does newborn screening look for?
- CF
- Phenylketonuria (PKU)
- Congenital hypothyroidism
- Some rare metabolic conditions
When is pre-conception carrier screening carried out?
- Groups with higher carrier frequencies in ethnic groups
e.g.
• Ashkenazi Jewish high school students for Tay Sachs
- ad hoc
• CF
• Haemoglobinopathies
What is personalised medicine?
How is it carried out?
Individualised care based on genotype
How:
• Whole genome sequencing
• others… (new technologies)
