5.2 - Chromosomal Abnormalities Flashcards
1
Q
Normal human karyotype
A
- karyotype - chromosome count of an individual
- contains 22 pairs of autosomal chromosomes and one pair of sex chromosomes (allosomes)
- females: two X chromosomes denoted by 46,XX
- males: have both an X and a Y chromosome denoted by 46,XY
- chromosomes karyotyped during metaphase - chromosomes condensed and comprises two chromatids
2
Q
How do you prepare a karyotype?
A
- collect 5ml heparinised venous blood (peripheral) - can also use amniotic cells, CVS (chorionic villus sample)
- isolate white cells
- culture in presence of phytohaemagglutinin - stimulates T lymphocyte growth/differentiation
- after 48h, add colchicine - causes mitotic arrest at metaphase
- place in hypotonic saline
- place on slide
- fix and stain with Gisema
- cut out individual chromosomes and arrange into karyotype
3
Q
What is DNA compaction?
A
- DNA does not exist as a simple double helix but is compacted around histones and further condensed into chromatin - what we see in karyotype
- proteins bound to the chromatin affect its regulation
- the 3D genome is important
4
Q
What is an ideogram?
A
- a way of showing a chromosome based on its G-banded architecture
- chromosomes have some common structural features
- Gisema staining leaves a recognisable pattern of bands (light and dark numbered bands)
- p-arm - shorter, top arm
- q-arm - longer, bottom arm
5
Q
How are bands formed and numbered?
A
- bands are caused by differentially staining - Gisema staining causes some to be darker than others as they take up more of the stain
- bands were originally identified with low level of resolution = only few bands were visible per chromosome e.g. 1, 2, 3
- improved technology = more bands visible which are named as sub-bands e.g. 11, 12, 21, 22, 23 etc
- further improvements = sub-sub-bands e.g. 11.1, 11.2 etc
- improved resolution helped identify smaller aberrations
- number of bands are measured in bphs (bands per haploid set)
- bands do not represent genes or families of genes
- there are regions of different compaction:
- dark (heterochromatin) = more compact, fewer genes
- light (euchromatin) = more open, more genes
- now this banding is often done during prophase not metaphase because chromosomes are less compact so get more detail from karyotype
6
Q
What is fluorescent staining?
A
- another staining method where chromosomes are stained fluorescent with a specific marker or individual portions of chromosomes are stained by individual markers
6
Q
Standard nomenclature
A
- p - short arm
- q - long arm
- pter - tip of short arm
- qter - tip of long arm
- cen - centromere
- del - deletion
- der - derivative chromosome (contains extra material)
- dup - duplication
- ins - insertion
- inv - inversion
- t - translocation
- +/- before chromosome number - gain/loss whole chromosome
- +/- after chromosome number - gain/loss part of chromosome
7
Q
What is aneuploidy?
A
- abnormal number of chromosomes - having a number that is not 46 or a multiple of it
- why is this an issue? the genome has developed such that having two copies of the chromosome is sufficient
- we can go about missing a single gene, but missing a chromosome means losing a large set of genes
- aneuploidy causes syndromes
- can have too many chromosomes (e.g. trisomy) or insufficient (e.g. monosomy)
8
Q
What is the purpose of meiosis?
A
- to achieve reduction from diploid (2n=46) to haploid (n=23)
- to ensure genetic variation in the gametes
- enables random assortment of homologues and recombination
9
Q
What is non-disjunction?
A
- non-disjuncture results in uneven number of chromosomes in daughter cells
- occurs when chromosomes are not split properly between daughter cells and can occur in meiosis I (all daughter cells affected) or meiosis II (half daughter cells affected)
- always results in either +1 or -1 chromosomes - when fertilised results in trisomy or monosomy
10
Q
Sex chromosome aneuploidy
A
- the most common form of aneuploidy
- affects 1/400 males and 1/650 females
Why is sex chromosome imbalance tolerated whereas most other chromosome imbalances are not? - due to X-inactivation of excess X chromosomes - in females, only one X chromosome is active (one is inactivated)
- also due to the low gene content of Y chromosome
Why if inactivated does abnormal number X/Y have effect? - both X and Y chromosomes have PAR (pseudo-autosomal region) which still produces genes and does not get inactivated - gives sex chromosomes aneuploidy phenotype
11
Q
Trisomy 21
A
- second most common form of aneuploidy
- trisomy of chromosome 21 (Down’s syndrome)
- most cases result from maternal non-disjunction
- risk of Down’s syndrome increases as age of mother increases, however 75% of cases born to mothers under 35 - this is because 90% of children are born to mothers of this age
12
Q
Why is there a maternal age effect on aneuploidy?
A
- due to the vulnerability of oogenesis
- primary oocytes are produced before birth then arrest in prophase I until puberty = finite number of primary oocytes and one primary oocyte –> one ovum
- secondary oocytes are then produced and arrest in metaphase II, and meiosis is only completed if the oocyte is fertilised
- the older the mother, the longer the oocyte has been paused in meiosis = increases chance of non-disjunction due to degradation of factors which hold homologous chromatids together - older you are, more disorganised chromatid arrangement
13
Q
Is there a paternal age effect on aneuploidy?
A
- in spermatogenesis there isn’t an equivalent to oocyte mitotic arrest
- primary spermatocytes undergo around 23 mitotic divisions per year and potentially accumulate defects
- paternal age is not a risk factor for increased aneuploidy, but does affect a subset of single gene disorders caused by point mutations in FGFR2, FGFR3 and RET including Apert, Crouzon and Pfeiffer syndromes
- thought to be enhanced by ‘selfish spermatogonial selection’ resulting from advantage over neighbouring wild type cells
- paternal smoking is a risk factor for aneuploidy (causes 80% 45X, 46% 47XXY, 100% 47XYY, 8% 47+21)
14
Q
What is the effect of aneuploidy on pregnancy risk?
A
- aneuploidy causes 5% of still births and 50% of spontaneous abortions
- aneuploidy occurs in 5% of all clinically recognised pregnancies
- trisomy of all chromosomes has been detected prenatally
- most trisomies aren’t compatible with life except 21, 18, 13
- monosomy is very poorly tolerated
- estimated 50% of preimplantation embryos have some degree of aneuploidy which are then lost before implantation/before pregnancy is clinically recognised
15
Q
What is mosaicism?
A
- presence of two or more populations of cells with different genotypes
- e.g. females are often examples due to X inactivation where one X chromosome is randomly turned off early in development so tissues have different expressions of X chr encoded genes - but not TRUE mosaicism as each cell has same genotype
- mosaicism can arise from two mechanisms:
- non disjuncture during early development
- loss of extra chromosome in early development (if embryo starts of as aneuploid)
- they result in generally milder phenotype and some lethal aneuploidy is survivable if mosaic
- most common mosaic is 46,XX/45,X and 46,XY/45,X - where 1 set of sex chromosomes has been lost in development
- everyone is thought to be mosaic to a certain degree as loss of some genetic material occurs often
16
Q
How is chromosomal crossover a cause for structural chromosome abnormalities?
A
- occurs in prophase I and increases genetic diversity
- pairs of chromosomes align, chiasma form and crossover occurs - can occur at a single point where large terminal chromosome portions are swapped or in middle of chromosomal material in a double crossover
- 1 to 3 times per chromosome per meiosis
- however sometimes goes wrong
17
Q
What single chromosome abnormalities are there?
A
Deletion - loss of genes
- can be the result of unequal crossover, or breaks in chromosome during meiosis and loss of material
- can occur at chromosome ends or middle
Duplication -
- most often caused by unequal crossover where material from one chromosome is transferred to another
- 1 chr suffers from deletion and other has duplication insertion of genetic material
Inversions -
- occur in middle of chromosomes or around centromere
- breaks in chromosome, inversion, reinserted piece of DNA
- carriers often unaffected but can cause reproductive problems as children with deletions/insertions born
- paracentric inversion - does not occur in centromere
- pericentric inversion - occurs in centromere
18
Q
What are two chromosome abnormalities?
A
- genetic material transfer between non-homologous chromosome pairs
- if transfer is unidirectional, insertion occurs
- can also have mutual exchange of material - translocation
- if this is balanced, it does not affect carrier but may cause problems in offspring
- can cause partial trisomy or monosomy e.g. Cri-du-chat syndrome
- can occur in somatic cells
19
Q
Inheritance of chromosomal abnormalities
A
- many chromosomal abnormalities are de-novo (occur in individual gametes during oogenesis/spermatogenesis and parents not affected by condition)
- e.g. two chromosomes have balanced translocation - child can inherit both normal chromosomes (no issue), or both chromosomes with translocated bits (no issue, balanced translocation carrier themself) but if they inherit extra material from one –> unbalanced = problems
20
Q
Chromosomal deletions
A
- microscopic - detected easily by microscope e.g. Cri-du-chat syndrome 46,XY,del(5p) - means loss of whole short arm of chr 5
- microdeletion - seen in high resolution banding / require molecular genetic approaches to detect - despite name, large numbers of genes deleted (20+) e.g. Velocardiofacial/DiGeorge syndrome 22q11.2 del
- deletions cause a constellation of symptoms which are characteristic
21
Q
Williams syndrome
A
- 7q11.23 deletion
- long philtrum, short upturned nose, arched eyebrows, supravalvular aortic stenosis, friendly social personality with lack of social anxiety
- phenotypes caused by imbalance of genes - deletion from one chromosome - which are unrelated apart from their location
- deletion too small to detect using normal karyotyping - use targeted FISH (fluorescent in situ hybridisation)
- lack of elastin on affected chromosome
22
Q
7q11.23 duplication syndrome
A
- opposite of Williams syndrome
- delayed speech development, autistic behaviour that affects social interaction and communication, dilatation of aorta, flat eyebrows, broad nose and short philtrum
- duplications usually have a milder phenotype than corresponding deletions
23
Q
Robertsonian translocation
A
- classes of chromosomes - metacentric (short and long arms equal and centromeres in middle of chr), submetacentric (short arm shorter than long arm), acrocentric (short arm shortened down to residual stump)
- affects 1/1000 people
- occurs between acrocentric chromosomes
- can be homologous or non-homologous
- most commonly between 13 and 14, 14 and 15, 14 and 21
- most people show no effects - silent in carriers
- can cause problems in offspring
- loss of both short arms of 2 acrocentric chromosomes and they translocate to produce a derivative chromosome (long arms join to form this, short arms lost as a fragment)
