Chromosomal abberration Flashcards
Karyotype
• The number and structure of chromosomes
within a cell is called a karyotype
• Chromosomes are often rearranged in order of size and
position of the centromere to form a karyogram
Karyotyping method (basic)
- Blood, 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 with Giemsa
Pre-natal screening
Genetic analysis of unborn foetuses common for the diagnosis of:
• Autosomal aneuploidies (next week)
• Sex chromosome aneuploidies
• Chromosomal re-arrangements
• Previously relied on amniocentesis (AFT) or chorionic villus sampling
(CVS) & karyotyping
• Dangerous for unborn child – spontaneous abortion
• Both require the culturing of cells post-collection – slow
• Move towards DNA testing
How prenatal DNA testing is achieved via obtaining sample from maternal blood?
• During pregnancy foetal DNA is shed into the maternal blood stream • Apoptosis of placental cells during embryogenesis • Foetal DNA consists < 3-10% plasmaderived DNA • Purification of foetal DNA obtained by epigenetic patterns • Foetal DNA differently methylated
Detection of foetal aneuploidies
1. Quantitative PCR methods
• Example: Harmony® • Determination of copy number of aneuploidic markers • Typically directed towards common abnormalities (such as Chromosomes 13, 18, 21, X & Y)
Detection of foetal aneuploidies
2. Next-generation sequencing
• Shows promise for the detection of chromosomal
translocations
• Detection of common genetic mutations
Aneuploidy vs Euploidy
- Aneuploidy
- A 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)
- Examples:
- Triploidy – 3n
- Tetraploidy – 4n
- Polyploidy – 3n, 4n, 5n, 6n
Naming of aneuploids
Chromosome number, genotype
For example:
47, XXX
47, 21+
Aneuploids are the result of
non-disjunction of chromosomes during meiosis
1st meiosis: homologous chromosome separate
2nd meiosis: Sister chromosome separates
If ND occurs during meiosis I,
gamete carries different recombinant chromosomes
• Mendel - segregation
2 Gamete with no chromosome and 2 gamete with 2 chromosome
when fused with egg
50% monosomy and 50% Trisomy
If ND occurs during meiosis II,
gamete carries same recombinant chromosomes
• Useful to determine when the ND occurred
50% normal
25% monosomy
25% Trisomy
Gene dosage responsible for
abnormal phenotypes
Diploid individuals has 2 copies of every gene
Trisomy has three copies of every gene
Monosomy has only one copy of every gene
Departures from normal gene dosage consequences
- The abnormal phenotype is characteristic for each chromosome
- Monosomy generally results in the worst phenotype (compared to
trisomy) - Aneuploidy of larger chromosomes results in a more severe
abnormal phenotype - Severe imbalance of genes leads to inviability
Sex aneuploids
Generally better tolerated 18 different combinations are possible, but four are more common • Monosomy X – Turner syndrome • XXY – Klinefelter syndrome • Trisomy X – Triple X syndrome • XYY – Double Y syndrome
Non disjunction of sex chromosomes
Female meiosis
ND 1 = 50% XX and 50% 00
ND 2 = 50% X and 25% XX 25% 00
Non disjunction of sex chromosomes male meiosis
Normal : 4 gametes with 2 with X and 2 with Y
ND 1 = 50% XY 50% 00
ND 2= 25% XX 25% YY 50% 00
Why are sex aneuploids better tolerated?
• Two reasons:
1. X-inactivation
• XXX individuals will have two Barr bodies instead of one
• XXY will have one Barr Body
• Imprinting retains inactivated X chromosomes in subsequent cellular
generations
2. Y chromosome encodes only a few gene
So where do the abnormalities come from?
• Not the entire X chromosome is inactivated • Hypothesis: • Abnormalities due to excess/deficit gene dosage within pseudo-autosomal regions
Why are two fertile and the others not?
Triple X and Double Y are fertile
• During embryonic development, normal genotype restored
• Oocytes – 46, XX
• Spermatogonia – 46, XY
• One sex chromosome must be lost to develop germline
• ND or lagging during early mitosis
Turner syndrome (45, XO)
- Female
- 1/5000 – 10000 live births
- Mild phenotype
- Near normal intelligence
- Short stature
- Webbed neck
- Sterile (ovaries degenerate)
Mosaic Turner Syndrome
The missing X chromosome may not occur in the germline
• X chromosome could be lost in early foetal development
• Leads to only some cells being monosomic X (45, XO), while others are normal (46, XX)
• Called mosaicism
Klinefelter (47, XXY)
- Male
- 1/1000 live births
- Mild phenotype
- Slightly lower IQ
- Taller than average (long legs)
- 30% show breast development
- Sterile (devolved testes – no spermatogonia)
Triple X (47, XXX)
- Female
- 1/1000 live births
- Very mild phenotype – many women unaware
- Mild reduction in IQ
- Tend to be very tall
- Occasionally behavioural problems reported (i.e. ADHD)
- Fertile
Double Y (47, XYY)
- Male
- 1/1000 live births
- Very mild phenotype
- Very tall
- Rarely a slight reduction in IQ
- Learning difficulties reported (delayed speech)
- Rarely antisocial behaviour
Uniparental diploidy
• Uniparental diploidy – generation of diploid set of chromosomes from a single parent • i.e. sperm carries 46 chromosomes; egg carries 0 • Very rare event (requires multiple errors in meiosis in both parents) • Foetuses don’t develop correctly • Possibly due to genetic imprinting?
Uniparental disomy
• Uniparental disomy – inheritance of both chromosomes from a single parent e.g. normal sperm and egg which has • A chromosome is lost during an early mitotic division in the foetus • Only a 1/3 chance • Range of symptoms • Many go undiagnosed • Abnormalities arise – imprinting errors? Uniparental disomy (UPD) occurs when a person receives two copies of a chromosome, or part of a chromosome, from one parent and no copies from the other parent.
Prader-Willi syndrome
• Deletion of paternal 15q11-13
• Or uniparental disomy where both copies of Chr 15 are inherited
from the mother
• Maternal copies of this region are silent due to imprinting
• Conversely, maternal deletion of the same region –
Angelman syndrome
• Symptoms
• Poor muscle tone
• Insatiable appetite - obesity
• Cognitive delays
Autosomal aneuploidies
- Arise due to errors in meiosis
- Monosomy – one chromosomal homolog missing
- Trisomy – extra chromosome
- Normally not well tolerated
- Compared to sex aneuploids (Triple X, Turner)
- Why?
Autosomal Monosomies
(2n – 1)
• Autosomal monosomies are not tolerated in humans
• Exception Turner syndrome (but that’s a sex aneuploid which is tolerated better due
to X-inactivation)
• In humans, monosomic foetuses die in utero
• Generally better tolerated in the plant kingdom
• Monosomic tobacco & Jimson weed have been isolated
• Tend to be less viable and often sterile
how Monosomies unmask recessive alleles
- Lethal alleles can be tolerated in the host, if an intact non-lethal homolog available
- Similar to X-linked inherited traits being more common in males
Haploinsufficiency
• When a single copy of an allele is NOT sufficient to
perform biological function
Example:
• DiGeorge Syndrome (22q11.2 deletion syndrome)
• TBX1 mutations
Trisomies (2n + 1)
- Accumulation of an additional chromosome
- Generally better tolerated in humans than monosomy
- i.e: trisomies more likely to survive (0.3% live births)
- Survivability increased if trisomy is in small chromosome
- Trisomy in all chromosomes occur, but only 3 survive
- Trisomy accounts for 35% spontaneous abortions
Trisomy in plants
- Trisomic plants viable, but infertile
- Typically associated with phenotypic differences
- Trisomics of Jimson weed (Datura)
- Diploid number 24
- Trisomy of each chromosome results in a different capsule phenotype
Down Syndrome (Trisomy 21 or 47, 21+)
• Occurs in 1/800 live births • Phenotypically variable • Commonly presents with: • Short stature • Mental retardation • Epicanthic fold (give eye characteristic almond-shape) • Heart and nervous system abnormalities • Life expectancy not common beyond 60s
Down Syndrome Critical Region (DSCR)
• A hypothetical region on chromosome 21 thought to be involved in
the phenotypes observed
• 21q22.2 = DSCR
• In a mouse DS model, identified candidate genes
1. DYRK – reproduces dosage-sensitive learning defects in Drosophila and mice
2. DSCAAM – reproduces heart and nervous system defects
Maternal age effect and Down syndrome
Prevalence of Down Syndrome
births increases with maternal
age (at conception)
• In human females, meiosis starts in the foetus, but stops at diplotene
(prophase I)
• Meiosis only continues upon ovulation
• Therefore women in the late 30s early 40s produce eggs that are older (been arrested
in prophase I) for longer
• Spindle fibres less effective at older ages?
Patau syndrome (Trisomy 13 or 47, 13+
• 1/15000 live births • A higher proportion die in utero • Risk increases with maternal age • Poor prognosis • Few survive beyond 1 year • Survivors affected with severe learning difficulties, psychomotor difficulties, cardiac abnormalities
Edward’s syndrome (Trisomy 18 or 47, 18+
• 1/8000 live births • Most foetuses result in spontaneous abortion • Few (5-10%) survive beyond 1 year • Common phenotypes: • Failure to thrive, microcephaly skull deformities, born with clenched hands
Polyploidies
Term used to describe instances where more than 2, complete sets of the genome are available
Polyploidies in animal and plants
• Stable polyploidy un-common in animals 1. Reproduction of polyploid leads to aneuploidy 2. Interferes with gene dosage • But polyploidy is seen in fish, lizards and amphibians • Batura toad(Bufotes baturnae) • Much more stable in plants
Autopolyploidy
• More than 2 sets of chromosomes all derived from one ancestral species
• Can happen naturally if:
1. Fusion of non-diploid gametes
2. The cell re-enters interphase after prophase I (mitosis) in early
embryonic development
• Can also be induced experimentally using colchicine
• Tolerated in plants and of important commercial significance
New species by autopolyploidy
Once produced, an autotetraploid is an “instant” new species
So tetraploid is reproductively isolated from its diploid ancestor
Diploid X Tetraploid = Triploid (sterile)
Allopolyploidy
the progeny will be fertile if……
• Generation of a polyploid individual from the fertilisation of two closely related but different species
• Progeny will be fertile if:
1. The progeny has a diploid number of chromosomes AND
2. There is sufficient similarity between genes for synapsis to occur
Allopolyploidy -Inter-species hybrid
Inter-species hybrid
• Often done to create progeny with the most desirable characteristics of both parents
• The hybrid has one set of chromosomes from each parent (e.g. AB)
• Allopolyploidy is tolerated well in plant species, used frequently in commercial crops
• Rarer in mammals and other animals
mule
Donkey = 62 chromosomes
Horse = 64 chromosomes
Mule = 63 chromosomes (infertile)
Creation of allopolyploids
how Interspecies hybrids can be made fertile
• Interspecies hybrids can be made fertile if made polyploidic
• Use colchicine to arrest mitosis during embryogenesis, then remove (chromosome doubling)
at fertlization AAABB after inducing it to re-enter interphase AA AA AA BB BB
• Generates a fertile amphidiploid
(or an allotetraploid)
Somatic cell hybrids
- The fusion of two somatic cells (from different species) into a single hybrid cell
- Generates a hybrid that contains the genetic material of BOTH species
- Valuable technique for mapping genes and determining genetic function
Somatic cell hybrids
Example:
• Mouse cell line defect in Thymidine kinase (TK gene) • Generate SCH with human cell line with functional TK gene • Redundant human chromosomes lost • Only chromosomal segment that complements mouse mutation remains
Endopolyploidy (somatic polyploidy)
• The situation where certain cells within a diploid organism become polyploidic
• A type of tissue mosaicism
• Can happen by:
1. Cells entering the mitotic cycle (prophase), without progressing through the other
stages – re-enter interphase
2. The cell can progress through the normal steps of mitosis, except that a single
nuclear membrane will form over all DNA during telophase
Endopolyploidy in humans
• Humans liver cells can be polyploidic • 3n, 4n or 8n • Sometimes chromosomes remain attached – called polytene chromosomes • Unclear why polyploidy happens • Produce high levels of gene products? • To generate larger cells?
