30. Chromosomal Abnormalities I

Question 1

RECAP: Describe the condensation of DNA

Answer 1

Histones are highly positively charged proteins that are attracted to the negative charge of DNA. Imagine this like coiling a garden hose up; in takes up less room to store than if you leave the garden hose out stretched. But it requires energy to coil that hose up and essentially that is what histones supply. They give the DNA a support to wrap aroundA chromosome is an organized package of DNA found in the nucleus of the cell. Different organisms have different numbers of chromosomes. Humans have 23 pairs of chromosomes–22 pairs of numbered chromosomes, called autosomes, and one pair of sex chromosomes, X and Y. Each parent contributes one chromosome to each pair so that offspring get half of their chromosomes from their mother and half from their father.

Question 2

What are the 2 states in which chromosomes can exists?

Answer 2

Chromosomes usually exists as chromatin

• DNA double helix bounds to histones
• Octamer of histones form nucleosome

Euchromatin

• Extended state, dispersed through nucleus
• Allows gene expression

Heterochormatin
- Highly condensed, genes not expressed

Question 3

Why are chromosomes sometimes shown with a single chromatid and sometimes with with two?

Answer 3

Due to the fact that there will be two chromatids during cell replication

Question 4

Describe the different types of chromosomes that can be classifies from a karyotype

Answer 4

Metacentric

• p & q arms even length
• 1-3, 16-18

Submetacentric

• p arm shorter than q
• 4-12, 19-20, X

Acrocentric

• Long q, small p
• p contains no unique DNA
• 13-15, 21-22, Y

Question 5

What are the two different types of chromosomal changes and how can you detect them?

Answer 5

1) NUMERICAL
- Can detect through traditional karyotyping, FISH, QF-PCR, NGS

2) STRUCTURAL
- Can detect through traditional karyotyping, FISH

Question 6

Define terms common in numerical abnormalities

Answer 6

HAPLOID:
one set of chromosomes (n=23) as in a normal gamete.

DIPLOID:
cell contains two sets of chromosomes (2n=46; normal in human)

POLYPLOID:
multiple of the haploid number (e.g. 4n=92)

ANEUPLOID:
chromosome number which is not an exact multiple of haploid number - due to extra or missing chromosome(s) (e.g. 2n+1=47)

Question 7

List some example of aneuploid numerical abnormalities

Answer 7

Trisomy

Monosomy

Mosaicism

Question 8

How does aneuploidy arise?

Answer 8

The primary mechanism is non-disjunction, where the homologues don’t pull apart during metaphase and both go into the same cell

If this occurs during meiosis I, you end up with two diploid cells.
If this occurs in meiosis II, we will end up with 2 haploid cells and one diploid cell

Question 9

What are the 2 mechanisms of mosaicism?

Answer 9

Post-zygotic nondisjunction, i.e. mitotic non-disjunction = All 2n to mixture of 2n and 2n+1

Anaphase lag, i.e. trisomic rescue = All 2n+1 to mixture of 2n+1 and 2n

Question 10

What is anaphase lag?

Answer 10

Anaphase lag describes a delayed movement during anaphase, where one homologous chromosome in meiosis or one chromatid in mitosis fails to connect to the spindle apparatus, or is tardily drawn to its pole and fails to be included in the reforming nucleus. Instead, the chromosome forms a micronucleus in the cytoplasm and is lost from the cell.

The lagging chromosome is not incorporated into the nucleus of one of the daughter cells, resulting in one normal daughter cell and one with monosomy.

Anaphase lag is one of several causes of aneuploidy and one of several causes of mosaicism. Anaphase lag can also cause a rescue of the daughter cell if the cell was originally trisomy.

Question 11

What is the clinical relevance of a mosaic phenotype?

Answer 11

The mosaic phenotype is thought to be less severe.

However, it is difficult to assess:

• what are the proportions of the different cell types?
• which tissues/organs are affected?

Examples of mosaic phenotypes include:

• Down
• Klinefelter
• Turner

Question 12

Describe the different types of monosomies

Answer 12

Autosomal are very very rare, found one case report from 1967

Relatively common sex chromosome monosomy = Turner’s

Full monosomy arise by NDJ

Partial monosomy (microdeletion syndromes) far more common – mechanism is different to non-disjunction.

Question 13

Considering how Turner’s arises (45,X), what are some possible combinations of chromosomes in monosomies?

Answer 13

•Nullisomic gametes

• • X chr = XO = Turner’s (physically female)
• • Y chr = lethal

•Disomic gametes
 - XX
   • + X chr = XXX = triple X syndrome
   • + Y chr = XXY = Klinefelter’s (physically male)
 - XY
   •+ X chr = XXY = Klinefelter’s
   • +Y chr = XYY = XYY syndrome

Question 14

Describe chorionic villus sampling for prenatal diagnosis

Answer 14

Occurs at 11-14 weeks

Miscarriage rate 0.5% to 1%

maternal contamination

transverse limb defects

Question 15

Describe amniocentesis for prenatal diagnosis

Answer 15

> 16 weeks
extraction of amniotic fluid
Biochemical diagnosis possible
miscarriage risk (0.5-1%)

Question 16

How does G-banding work?

Answer 16

Stain sample with Giemsa stain. Giemsa highlights heterochromatic regions which are less likely to contain genes. But the crucial thing is that the banding can be used to differentiate between chromosomes ant to compare chromosomes.

Metaphase

Line-up based on

• Size
• Banding
• Centromere position

Question 17

How can quantitative fluorescence PCR be used to identify numerical abnormalities?

Answer 17

We can design primers for a specific microsatellite on chromosome 21 (for Down’s, for e.g.) and amplify the region using PCR.

We would then essentially see how big the regions are, and how many copies there are.

Question 18

What are some ways in which we can get a prenatal diagnosis?

Answer 18

•Invasive

• Amniocentesis (14-20 wks, amniotic fluid)
• Chorionic villus sampling (CVS) (11-14 wks, placental cells)

•Non-invasive

• Cell free foetal DNA (cffDNA): DNA fragments in maternal plasma (10 wks onwards)
• Actually for trisomies still need confirmation with amnio/CVS