Karyotype & Aneuploidy Flashcards
What are the types of chromosomal abnormalities
Numerical chromosomal abnormalities May be a result from:
- non-disjunction during meiosis
- non- disjunction during mitosis
- Mosaicism
Structural chromosomal abnormalities May arise from -deletions -duplications -inversions -translocations Isochromosom3s -ring chromosomes
How is a karyotype generated?
- Blood sample added to culture medium
- White blood cells, lymphocytes, divide in the presence of phytohemagglutinin
- Dividing cells arrested in metaphase with spindle inhibitor colchicine
- Cells swollen with hypoton8c solution
- Spread onto glass slide and fix
- Stain and examine with microscope
What is a karyotype?
These are analyzed by microscopy
- only relatively large alterations will be observable by karyotype
- banding pattern must be altered
Can detect deletions, duplications, inversions, translocations, other abnormalities
Deletions larger then 5 Mb are usually by standard karyotype (this is by microscopy)
-Deletions that aren’t visible by karyotype are called “Microdeletion”
Classify chromosome by structure
- Total. Length of the chromosome is equal on both ends of the chromosome from the centromere- metacentric
- Sub metacentric- p are shorter than q arms
- Acrocentric- only satellites on the p side
What are used to classify chromosomes?
- Total length of the chromosome
- position of the centromere
- Presence of satellites
How can we classify chromosomes by G-banding pattern?
Ideogram of the G-banding pattern of metaphase chromosomes stained with Geimsa
Resolution: can’t detect deletions or insertions smaller than 5Mb
What are FISH?
FISH are big; ASO probes are small
- typically to get a good signal, they are about 100 kB or bigger
- This allows a lot of fluorescent tags to be packed into the probe
With the correct probe FISH can detect:
- numerical chromosome aberrations
- deletions
- translocations
- gene amplification(very important in cancer characterization)
FISH May detect many “Microdeletions” because FISH is of higher resolution than G-band karyotype
-As long as the deletion is larger than the probe being used, the deletion can be detected
Contrast interphase FISH and metaphase FISH
Interphase FISH- rapid test, avoids need for cell culture, chromosomes are de condensed in the nucleus
Meta9hsse- must first culture the cells, usually done @fter a G-band karyotype (condensed metaphase chromosomes), gives location information because the banding pattern would be known
What is a chromosomal painting?
Chromosomal painting, spectral karyotype, SKY FISH
-Each chromosome probed so that it fluorescent a different color
Can show translocations and rearrangements
Very useful in cancer genetics- genetic stability in advanced cancer
Summarize chromosomal abnormalities
Chromosomal abnormalities account fir a large number of spontaneous abortions
-(non induced embryonic or fetal death or passage of products of conception before. 20 weeks gestation)
Chromosomal abnormalities May be
- numerical
- structural
What are the numerical chromosomal abnormalities?
Euploidy(polyploidy) (number of chromosomes is in multiples of 23)
- Diploidy- normal human compliment of chromosomes
- triploidy- contain three copies of each chromosome(69)- not compatible with life
- tetraploidy- lethal- contain four copies of each chromosome (92)
Aneuplody-
- Monosomy: loss of a chromosome
- Nit compatible with life (except Monosomy X: Turner syndrome)
- Trisomy: presence of an additional chromosome (only 3 are compatible with life)
What is aneuploidy?
Chromosome number isn’t a multiple of 23
- Monosomy: loss of a chromosome. Nit compatible with life (except in Turner syndrome)
- Trisomy: presence of an additional chromosome (frequently confused with triploidy(69 chromosomes)
Autosomal trisomy- Edward syndrome (18), Down syndrome, Patau syndrome(13)
Sex chromosome aneuploidy- (45,X) Turner, 47, XXY) kleinfekter sundrome
What are the risk factors of trisomy 21?
Most common live born autosomal trisomy
Risk factors: increased maternal age, increases risk of meiotic nondisjunction during oogenesis (most common is meiosis 1 nondisjunction)
WhAt are the features of trisomy 21?
- intellectual” disability
- short stature
- depressed nasal bridge, unslanting palpebral fissures, epicantal folds
- Cingenitak heart defects
- single palmar crease
- develop changesssimilar to Ahlzeimer’s disease at a relatively young age.
- One if the genetic factors responsible fir Ahlzeimer’s is localized to chromosome 21(amyloid precursor protein (APP)
What is the main use of FISH probes?
Will work with interphase stage cells to allow rapid return of results
Prenatal diagnosis with maternal serum screening and ultrasound
Karyotype analysis or FISH with fetal cells is confirmatory
What are the genetic causes of down syndrome?
Trisomy 21- due to non-disjunction during meiosis.
Down syndrome May also result from R9bertsonian translocation
Some children may be mosaic- some cells gave trisomy 21; som3 other cells may have normal karyotupe
What are the features of edwards syndrome?
Genetic mechanism is nondisjunction during oogenesis
Features:
-Clenched fist, overlapping of fingers
- Rocker bottom feet
- Congenital heart defects
- Low set ears , small lower jaw (micrognathia)
- Microcephaly
- Intelkectual disability
What is the list of features of Patau syndrome (+13)?
Most common genetic mechanism is nondisjunction during oogenesis
Features
- polydactyly
- cleft lip and palate
- microphthalmia
- Microcephaly
- intellectual disability
- cardiac anomalies (VSD or ASD)
What are the common features of (45,X)/Turner syndrome?
X chromosome Monosomy- usually due to nondisjunction during spermatogenesis
- Short stature
- webbed neck, cystic hygrometer at birth (neck swelling)
- Primary amenorrhea/no menstruation
- Gonadal dysgenesis
- ‘Streak’ ovaries
- Lymphoedema of hands and feet
Explain mosaic Turner syndrome
Many TS patients are mosaic
- Some of their cells are 45, X
- Other cells are 46,XX and 47,XXX (indicate mitotic nondisjunction during embryogenesis)
Cells with 45,X: have no Barr body (inactivated X-chromosome )(female with NO Barr bidy)
-Isichromosome X will also lead to Turner syndrome
Contrast he not your and mosaic Turner syndrome
Mosaic Turner syndrome-
- fertilized egg, 46,XX
- Mitotic nondisjunction during embryogenesis
- Some cells are 45,X others are 46,XX and 47, XXX
- during a post-fertilization mitosis, one of the X chromosomes evicted during cell division
Phenotypic-
- fertilized egg, 46, XX
- Mitotic nondisjunction during embryogenesis
- Some cells are 45, X others are 46, XY
- Mosaic 45,X/46,XY are recommended fir oophorectomy due to risk of gonadoblastoma
- during post fertilization mitosis, the Y chromosome is evicted during cell division
What are the features of kleinfekters?
Genetic mechanism responsible for kleinfelter syndrome is nondisjunction during meiosis
-Some individuals may be mosaic (46XY/47XXY/48/XXXY)
Features:
- Gyecomastia
- Female distribution 9f hair
- Infertility and testicular atrophy due to low levels of testosterone
- Feminization of features
What are the probability of each abeuplofy occurring?
Trisomy 21- 1/830 live births
47, XXY(kleinfelter)- 1/1000 live births
45,X(Turner) -1/4000 live births
Trisomy 18- 1/7,50l live births
Trisomy 13- 1/22,700 live births
What is non-disjunction ?
The most common type in females is meiosis 1
- Individuals with down syndrome usually have two maternal chromosomes 21 and one paternal chromosome 21
- If two maternal chromosomes aren’t identical to each other, the non-disjunction occurred at meiosis 1 (that is they have different alleles )
- If the two maternal chromosomes are identical, the non-disjunction occurred at meiosis 2(sister chromatids shown as two blue alleles)
How can we draw out which type of nondisjunction occurred?
-47,XXY kleinfekter syndrome may be caused by nondisjunction during meiosis 1 or meiosis 2 in the mom, or nondisjunction during meiosis 1 in dad
- 47, XYY fetus is the result of meiosis 2 nondisjunction in dad
- May not be caused by meiotic non-disjunction in the mom (hint: thus person is also)
- 47, XXX females may be the result of meiosis 1 or 2 nondisjunction in moms or meiosis 2 non-disjunction in dad
- may not be caused by meiosis 1 nondisjunction in dad
What is Mosaicism?
The presence of two or more populations of cells in one individual with different genotypes
Mixed somatic/germline mosaic
Some percentage of the cells in a person body are of one genotype, and other cells are of another genotype
Confined germline mosaic
All cells in the body are of one genotype
Cells of the gametes are something else (leads to high risk of having children with a genetic disorder)
What are the two ways could be mosaic for down syndrome ?
- Non-disjunction May occur as a post-zygotic event during embryonic development; that is mitotically (after fertilization of a normal ovum and normal sperm)
This will lead to 2 cell types:
- one continues as normal karyotype
- the other with trisomy 21
- Trisomy rescue- nondisjunction May occur in the ovum during oogenesis, and fertilization by a normal sperm loads to trisomy 21 in the conception
- During a post-fertilization mitosis, one of the extra chromosone 21 is evicted during cell division
- Now the fetus has 2 cell types, one with normal karyotype, and one that is trisomy 21
