aneuploidys Flashcards
1
Q
Karyotype
A
- The number of a chromosome within a cell is called a karyotype
Chromosomes are rearranged in order of size and position of centromere to form a karyogram
2
Q
Karyotyping methods (basic)
A
- Blood, Amnionic Fluid (AFT) (or cvs) or bone marrow are common specimens
- Cells must be cultured in vitro, typically 3 days
- After incubation, colcemid is added
○ Arrests mitosis at metaphase
Cells fixed to slide and stained Giemsa
3
Q
FISH- staining
A
- Identifying small translocations using giemsa staining is almost impossible
- We use FISH staining
○ using probe DNA (labelled with dye)
○ Denature and hybridise
○ Where sequence is homologous to the probe DNA there is binding
○ Shine UV light
○
Probe bound to chromosome 1 that should come from homologous 3 - translocation
- We use FISH staining
4
Q
Pre-natal screening
A
- Genetic analysis for unborn foetus to diagnose aneuploidies or chromosomal rearrangement
- Previously relied on AFT or CVS and karyotyping
○ Dangerous
○ Require culturing - slow
Move towards DNA testing
- Previously relied on AFT or CVS and karyotyping
5
Q
Cell-free DNA
A
- Detection of foetal DNA in bloodstream - during pregnancy foetal DNA is shed
○ Apoptosis of placental cells during embryogenesis- Purification of foetal DNA obtained by epigenetic patterns
○ Foetal DNA primarily unmethylated
○ Maternal DNA displays unique epigenetic marker for the mother
Thus we can detect foetal DNA
- Purification of foetal DNA obtained by epigenetic patterns
6
Q
Detection of foetal aneuploidies
A
- Quantitative PCR methods
- E.g. Harmony
○ Probes that are unique to areas commonly aneuploidy
§ Chromosomes 13, 18, 21, x and y - Not good for rare (not in probe library)
2. Next-generation sequencing
We can detect copy number by number of contigs that come back for a chromosome result
- E.g. Harmony
7
Q
Terminology
A
- Aneuploidy ○ Loss or gain of a single chromosome ○ E.g. monosomy, trisomy, tetrasomy - Euploidy ○ An increase in a complete set of chromosomes (i.e. chromosome number doubles) (e.g.46 chr -> 92 chr) ○ E.g. § Triploidy - 3n § Tetraploidy - 4n Polyploidy - 3n, 4n, 5n, 6n
8
Q
Naming aneuploids
A
- Chromosome number, genotype
- E.g.
○ 47, XXX - 47 chromosomes in total, 3 x chromosomes
47, 21+ - 47 chromosomes in total, one extra chromosome 21
- E.g.
9
Q
Origins of aneuploidy
A
Non-disjunction (ND) during either meiosis 1 or 2
10
Q
Aneuploidy during meiosis 1
A
- If ND occurs during meiosis 1, gamete carries different recombinant chromosomes
○ Mendel - segregation
○
○ The bottom two left gametes have one too many chromosomes (called disomic)
○ The bottom two right gametes called empty
○
§ When combined with other parent can cause a trisomy (3 copies)
§ The other two are monosomy
○ 100% gametes are abnormal
§ 50% trisomy
50% monosomy
11
Q
Aneuploidy at meiosis 2
A
- If ND occurs during meiosis 1, gamete carries same recombinant chromosomes
- ○ One disomy, one empty and two normal
- ○ 25% trisomy
○ 25% monosomy
50% disomy (normal)
12
Q
Gene dosage effects
A
- Usually 2 copies are required for normal gene function
○ In some cases with monosomy you have haploinsufficiency (not enough of a gene product for normal phenotype)
In some cases having more than a disomy may lead to problems as well
13
Q
Departures from normal gene dosage
A
- Abnormal phenotype is characteristic for each chromosome
- Monosomy generally results in the worst phenotype (compared to trisomy)
- Aneuploidy of larger chromosomes results in more severe abnormal phenotype
- Severe imbalance of genes leads to inviability
○ Most autosomal aneuploidies aren’t tolerated in humans
Embryo doesn’t survive
14
Q
Sex aneuploidy
A
- Sex aneuploidies are better tolerated
- 4 most common (there is a total of 18)
○ Monosomy x - turner syndrome
○ XXY - Klinefelter syndrome
○ XXX - triple x syndrome
XYY - double Y syndrome
- 4 most common (there is a total of 18)
15
Q
Male and female aneuploidy are different
A
- Females only have x chromosomes
○ Non-disjunction in females only results in the case of meiosis 1 or 2 (look above)- In males there is more variety
○ In M1 you get gametes that carry both X&Y or neither (Klinefelter syndrome)
○ In M2 you get disomy x (triple x syndrome) or disomy Y (double Y syndrome) depending on which doesn’t segregate
For XXXX, XXXY, XXYY, XX - you need multiple non-disjunction events in both parents - rare
- In males there is more variety
16
Q
Why are sex-chromosomes better tolerated
A
- X-inactivated
○ XXX individuals will have two Barr bodies instead of one
○ XXY will have one Barr body
○ Increase Barr bodies to make gene dosage normal- Y chromosome encodes only a few genes
○ Only for sperm viability or spermatogenesis
Not critical
- Y chromosome encodes only a few genes
17
Q
Where do the abnormalities come from
A
- Not the entire x chromosome is inactivated
Abnormalities due to excess/deficit gene dosage with PAR1 and to a lesser extent in PAR2
18
Q
Alterations in sex chromosome number does not necessarily make the person sterile
A
klinefelter and turner syndrome are infertile - In Klinefelter most likely the testes don’t develop
- Why are two fertile
§ Possibly during embryonic development, normal genotype is restored
Possibly one sex chromosome must be lost to develop germline
19
Q
Turner syndrome (45, XO)
A
- Female (missing SRY)
- Near normal intelligence
- Short
- Webbed neck
Sterile
20
Q
Mosaic Turner syndrome
A
- In germline one chromosome is lost
- In some areas of tissue the cells come from precursor where one chromosome was last
21
Q
Klinefelter (47, XXY)
A
- Male
- Slightly lower IQ
- Taller
- 20% breast dev
- Sterile
22
Q
Triple x (47, XXX)
A
- Female
- Very mild - most don’t know they have it
- Mild reduction in IQ
- Tend to be very tall
- Occasionally behavioural problems
Fertile
23
Q
Double Y (47, XYY)
A
- Male
- Very mild
- Rarely a slight reduction in IQ
- Learning difficulties
Rarely antisocial behaviour
24
Q
Uniparental diploidy
A
- Generation of diploid set of chromosomes from a single parent
- i.e. sperm carries 46 chromosomes and egg carries 0
- Very rare requires many errors in both parents- Foetuses don’t develop correctly
- Typically dead, or with severe morbidity
- Possibly due to genetic imprinting
Maintain epigenetic markers of parents
- Foetuses don’t develop correctly
25
Q
Uniparental disomy
A
- Inheritance of both chromosomes from a single parent
- For example; you have multiple copies of a chromosome in the egg, but no copies in a normal sperm both copies would then come from one parent- A chromosome is lost during early mitotic division in foetus
- Many go undiagnosed
Abnormalities - imprinting errors?
26
Q
Prader-Willi syndrome
A
- Deletion of paternal 15q11-13
- Or uniparental disomy where both copies of Chr 15 come from the mother
- Maternal copies of this region are silent due to imprinting- Conversely, Angelman syndrome - maternal deletion of the same region
- Symptoms
- Poor muscle tone
- Insatiable appetite
Cognitive delays
27
Q
Autosomal aneuploidy
A
Autosomal aneuploidies have the same proportions as sex aneuploidies
28
Q
Why are they so badly tolerated
A
- Autosomal Monosomies (2n-1) ○ Not tolerated in humans ○ Die in utero ○ Better tolerated in plants Tend to be less viable and less sterile
29
Q
Monosomies unmask recessive alleles
A
- In empty (seen above) they will have one chromosome (i.e. monosomic) - the phenotype will be determined by that chromosome (dominant or recessive)
○ Tendency to shoer greater expression of recessive phenotypes- Lethal alleles can be tolerated if non-lethal homolog available
Traits more common in males
- Lethal alleles can be tolerated if non-lethal homolog available
