Lecture 26 - Genetic Disorders Flashcards by Eleanor Nicklason

In general, what are the categories of genetic disorders?

Chromosomal abnormalities
Single gene disorders
Polygenic / multifactorial disorders

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What is the name for abnormal chromosome number disorders?

Aneuploidies

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What is the mechanism of aneuploidies?

Missassortment (non-disjunction) in meiosis

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What are the sub-categories of aneuploidies?

Trisomy: extra single chromosome

Monosomy: deletion of a single chromosome

Polyploidy: multiples of haploid number of chromosomes

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What is an example of polyploidy?

Triploidy: human is 3n instead of 2n

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What is the mechanism of structural abnormalities?

Rearrangement or deletion
During crossing over

examples:
• Translocations
• Inversions
• Duplications

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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

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Describe Down syndrome

• Collection of facial and physical features

Associated with:
 • Intellectual disability
 • Congenital heart disease
 • Congenital GIT abnormalities
 • Leukaemia
 • Immunological defects
 • Premature ageing

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Describe the premature ageing seen in Down syndrome

Alzheimer-like dementia

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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

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Is trisomy 21 seen in all cases of Down syndrome?

No, only in 95% of cases

95%: Trisomy 21

5%: Unbalanced translocations

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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

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How many chromosomes do humans have?

46:
22 autosomes
2 sex chromosomes

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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

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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

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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

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In which phase of the cell cycle is FISH performed?

Interphase

The cells are non-dividing

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What proportion of DS cases are inherited unbalanced translocations?

1-2%

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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

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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

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What are SNPs?

Single nucleotide polymorphisms

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What are CNVs?

Copy number variations

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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 1. PCR of β-globin chain gene 2. Cut with NcoI (restriction enzyme) 3a. No mutation: • Cutting 3b. Mutation: • No cutting 4. 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 ```

1. CVS: chorionic villus sampling • Invasive; risk of miscarriage • Placental tissue 2. Amniocentesis • Amniotic fluid containing sloughed off foetal cells • Invasive, risk of miscarriage • Ultrasound 3. 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

1. Ultrasound 2. Maternal serum screening 3. 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?

1. Groups with higher carrier frequencies in ethnic groups e.g. • Ashkenazi Jewish high school students for Tay Sachs 2. 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)