Chromosomal Abnormalities I Flashcards
What are histones?
Histones are highly positively charged proteins attracted to negatively charged DNA
They give DNA support to wrap around
What is a chromosome?
An organized package of DNA found in the cell nucleus
Describe the structure of chromosomes
Chromosomes usually exists as chromatin
DNA double helix binds to histones
What is a nucleosome?
DNA wrapped around octamer of histones form nucleosome
What are features of euchromatin?
Extended state, dispersed through nucleus
Allows gene expression
Describe heterochromatin
Highly condensed, genes not expressed
Why is DNA loosely packed?
DNA usually loosely packed to enable TF to bind for protein expression
When is DNA tightly coiled?
During cell division DNA is complexed with various proteins and undergoes several levels of compaction through coiling and supercoiling
How many chromosomes do we have?
Humans have 23 homologous pairs of chromosomes
- 1 from each parent
What is locus?
Locus: any given position on a chromosome
Can be used to define a single base or a genomic region
Describe loci on homologous chromosomes
Homologous chromosomes have identical loci of genes with varying allelic forms - loci should match up; significant in meiosis (metaphase) where chromosomes line up
What enables genetic variation?
Dominant and recessive forms of genes give genetic variation
Outline how chromosomes go from single chromatids to sister chromatids
During interphase of cell cycle a single chromatid is produced after cell division has occurred in G1
S1 duplicates DNA products from G1 to produce identical sister chromatids
Describe what occurs during G1 of cell cycle?
G1 = Cell makes a variety of proteins needed for DNA replication
What happens in S phase?
S = synthesis; chromosomes are replicated so that each chromosome now consists of two sister, identical chromatids
What occurs in cell cycle phase of G2?
G2 – synthesis of proteins especially microtubules
Some cells don’t replicate; some are senescent. Undergoes error checks
How do loci and gene content differ in chromatids and homologous chromosomes?
Loci and gene content is identical in sister chromatids
In homologous chromosomes gene loci and content is also similar but may have varying allelic forms (dominant / recessive)
Describe the normal human karyotype
Humans have 23 pairs of chromosomes
22 pairs autosomes, 1 pair sex chromosomes XX or XY
What is the best time to view chromosomes under microscopes?
Sister chromatids in metaphase of meiosis are easier to visualise under a microscope as they’re more condensed
What are metacentric chromosomes?
Metacentric
- p & q arms even length
- 1-3, 16-18
What are submetacentric chromosomes?
Submetacentric
- p arm shorter than q
- 4-12, 19-20, X
What are accrocentric chromosomes?
Acrocentric
- Long q, small p
- p contains no unique DNA
- 13-15, 21-22, Y
Why do all cells not have 23 perfect pairs of chromosomes?
Not all cells have 23 pairs of perfect chromosomes e.g:
- Mutations: trisomy abnormalities
- Gametes have 23 single chromosomes (haploid)
What are the 2 types of chromosomal changes that can occur?
There are 2 categories of genome changes that can occur:
- Numerical
- Structural
What is haploid?
HAPLOID: one set of chromosomes (n=23) as in a normal gamete.
What is a diploid no.?
DIPLOID: cell contains two sets of chromosomes (2n=46; normal in human)
What does polyploid mean?
POLYPLOID: multiple of the haploid number (e.g. 4n=92)
What is meant by aneuploid?
ANEUPLOID: chromosome number which is not an exact multiple of haploid number - due to extra or missing chromosome(s) (e.g. 2n+1=47)
What are the common chromosomal numerical abnormalities?
Trisomy
Monosomy
Mosaicism
What is disjunction?
Pulling apart at anaphase = disjunction
What are the 2 phases of meiosis
Meiosis I
Meiosis II
Describe what occurs in meiosis I
Meiosis I - allelic recombination
- Generates variations
- Important homologous chromosomes line up correctly at equator for recombination to occur
What happens during in meiosis II?
Meiosis II - chromatids disjunction
- Haploid daughter cells with genetic variation
What happens during mitosis?
Single round of cell division
Produces diploid daughter cells with identical genetic info
What is the mechanism responsible for aneuploidy
Primary mechanism = nondisjunction; chromosomes don’t separate correctly
Describe how aneuploidy arises in meiosis I
- Nondisjunction in M1 = both chromosomes end up in
same daughter cell - M2 occurs normally but a daughter cell will end up
with 2 copies of nondisjunction chr. ⇒ disomic - Another daughter cell will lack both normal chr. pairs ⇒
nullisomic
How does aneuploidy in Meiosis II arise?
- Nondisjunction in M2 = chr. pulled apart and entered
one gamete as opposed to being split between two - One gamete will be disomic and another nullisomic
What are the products of fertilisation of nondisjunction products?
Fertilisation occurs producing:
- Trisomies (3 copies of chr.)
- Monosomies (single chr. copy)
- Disomies (2 copies of chr.) normal
Name the classic autosomal trisomies
Trisomies 13, 18 and 21 are classic autosomal trisomies (Patau’s, Edwards & Down’s)
Why do we not see autosomal trisomies other than 13,18 and 21?
Other chromosomal meiotic trisomies can occur but those pregnancies don’t continue to full term
What is the DNA content in mitotic nondisjunction?
Mitotic NDJ; majority of cells 2n some 2n+1 = mosaic
How does mitotic NDJ occur?
Zygote generated with correct no. of diploid chr. But at some point during mitotic cell division nondisjunction occurs
Describe the chromosome content of mitotic NDJ daughter cells
The daughter cell produces 3 copies of chr. while another daughter cell will have just one copy
What is the consequence of mitotic nondisjunction?
As these cells continue to divide, all cells originating from the trisomic daughter cell will also be trisomic and same for the monosomic daughter cells
What is the fate of monosomies?
Typically the monosomic cells won’t be maintained and are destroyed
What is the fate of trisomies?
Some trisomic cells are maintained and so some of the zygotic cells will be trisomic and others will be standard disomic ⇒ mosaicism will present as Down’s (less severe than meiotic Trisomy 21)
What is mosaicism?
The presence of 2 or more genetically different cell line derived from a single zygote
What are the 2 mosaicism mechanisms?
Postzygotic nondisjunction
- Mitotic nondisjunction
- All 2n to mixture of 2n and 2n+1
Anaphase lag
- Trisomic rescue
- All 2n+1 become a mixture of 2n+1 and 2n
What is the clinical relevance of mosaicism?
Mosaic phenotype thought to be less severe
Difficult to assess:
- What are proportions of different cell types?
- Which tissues/organs affected?
Give examples of mosaicism disorders
Down ~2% (trisomy 21 / normal)
Klinefelter 15% (46 XY / 47 XXY)
Turner <25%
How do mitotic and meiotic nondisjunction disorders differ?
Sex chromosomal abnormalities are typically far less severe than autosomal chromosomal abnormalities
What is a common monosomy?
Relatively common sex chromosome monosomy = Turner’s
How does full monosomies arise?
Full monosomy arise by NDJ
What causes partial monosomies?
Partial monosomy (microdeletion syndromes) far more common – mechanism different
Describe the clinical significance of monosomies and trisomies
Trisomies are detrimental to health, but monosomies are far worse and more lethal
Describe turner mosaicism
45X / 46XY mosaicism - usually normal male (90%) but with potential for gonadoblastoma
What is variant turners syndrome?
Mosaicism with ring or isochromosome Xq results in variant Turner Syndrome
What are the possible combinations for Turners
45X0 or mosaic (45X0/46XX)
Outline the incidence of turners
Incidence 1/4000 female births
What are the effects of turners?
- Raised nuchal translucency
- Short stature
- Infertility secondary to gonadal dysgenesis
- Intellectually normal
At birth:
- oedema of hands and feet
- Neck webbing
- ?coarctation (narrowing) of aorta
- ?renal malformation
What is nuchal translucency?
Nuchal translucency is a collection of fluid under the skin at the back of your baby’s neck
What is cystic hygroma?
A cystic hygroma is a collection fluid-filled sacs known as cysts that result from a malformation in the lymphatic system
How does Turners XO arise?
Nullisomic gametes due to NDJ fertilised with a sperm carrying an X chromosome will be XO (Turners)
How does YO turners arise?
Nullisomic gametes fertilised with a sperm carrying a Y chromosome will be YO
What is the result of disomic gametes fertilising to form Turners
Disomic gametes fertilise to become either XXY or XXX trisomies
What are the possible nullisomic Turners combinations
Nullisomic gametes
+X chr = XO = Turner’s (physically female)
+Y chr = YO = lethal
Outline the possible disomic combinations in Turners
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
What are the autosomal numerical abnormalities?
Trisomy 13, 18, 21
What are the common sex chromosome abnormalities?
XO, XXY, XYY
How do we identify and test for numerical abnormalities?
Access blood and culture mitotic (metaphase) cells to identify abnormal karyotypes
Which cell type is used to produce a karytotype?
Pretty much any nucleated cell can be used – e.g. skin, bone marrow, amniotic cells, CVS, but generally lymphocytes are used
Describe chorionic villus sampling
- Done at 11-14 weeks
- Miscarriage rate 0.5% to 1%
- Risk of maternal cell contamination
- Risk of transverse limb defects
Outline amniocentesis prenatal diagnosis technique
- Carried out at >16 weeks
- Extraction of amniotic fluid
- Biochemical diagnosis possible
- Miscarriage risk (0.5-1%)
What is G-banding?
Using a Giemsa stain on chromosomes in Metaphase
What is the G-banding line up dependent on?
Line-up based on
- Size
- Banding due to chromatin makeup
- Centromere position
What does the giemsa stain highlight?
Giemsa highlights heterochromatic regions which are less likely to contain genes
What is the used of G-banding?
The banding is used to differentiate between chromosomes and to compare chromosomes
What is identified from a karyotype?
When identifying individuals with abnormal karyotypes need to outline the following:
- No. of chromosomes
- Sex
- Abnormal chr.
Describe the features of FISH
Fluorescent in situ hybridisation
Cultured cells, metaphase spread
Microscopic (5-10Mb)
Describe how FISH is carried out
- Fluorescent probe
- Denature probe and target DNA
- Mix probe and target DNA
- Probe binds to target
Why is FISH a long process?
because it uses cultured cells, this takes longer than QF-PCR
What is FISH?
Fluorescence in situ hybridization (FISH) is a molecular diagnostic technique utilizing labeled DNA probes to detect or confirm gene or chromosome abnormalities
What is the significance of microsatellites in QF-PCR
Microsatellites are typically not associated with disease - just normal genetic variation but can be used in a diagnostic manner in QF-PCR
Why are microsatellites used in QF-PCR?
We all have copies of these microsatellites but have differing alleles as these are length variations
Designed to be quicker in diagnosing than FISH
Describe invasive foetal testing techniques
Invasive
- Amniocentesis (14-20 wks, amniotic fluid)
- Chorionic villus sampling (CVS) (11-14 wks, placental cells)
What are the non-invasive foetal testing methods?
Non-invasive
- Cell free foetal DNA (cffDNA): DNA fragments in maternal plasma (10 wks onwards)
- For trisomies still need confirmation with amnio/CVS
Outline the mechanisms responsible for Downs (trisomy 21)
95% NDJ (usually maternal meiosis I)
5% Robertsonian translocation involving chr.21
~2% mosaic
‘Older Egg Model’ – maternal age effect
Outline features of Trisomy 21 ‘Downs’
- 1 in 650-1000 live births
- Most common cause of mental retardation
- Hypotonia, particularly in newborn period
- Developmental delay
- Cardiac abnormalities
- GI abnormalities
- Acute Lymphocytic Leukaemia/Acute Myeloid Leukaemia – 10-20 x relative risk
- Conductive hearing loss
- Features of Alzheimer’s >40 years
Outline features of Trisomy 13 {Pataus)
~1 in 10 000 live births Midline defects - Hypotelorism - Holoprosencephaly - Midline cleft lip/palate - Scalp defects Post axial polydactyly Heart defects/renal abnormalities
Outline features of Trisomy 18 (Edwards)
- Incidence ~1 in 6000 live births
- Intrauterine growth retardation
- Micrognathia
- Cleft lip +/- palate
- Short palpebral fissures
- Fixed flexion deformities of fingers
- Heart defect >95%
- Inguinal/diaphragmatic herniae
- Renal malformations
Outline the survival rates in Edwards
Survival rates in Edwards Syndrome:
- 30% die by 1 month
- 50% die by 2 months
- 90% by 1 year
What caues kleinefelters?
NDJ paternal meiosis I (50%), others NDJ maternal meiosis or zygotic mitotic error (mosaic)
Variants: 48,XXYY, 48,XXXY etc
Outline features of kleinefelters
- May present prenatally, during childhood with behavioural problems, or adulthood with infertility
- Tall stature
- Eunuchoid body habitus
- Some behavioural and minor learning difficulties
- Lack of secondary sexual characteristics – treat with testosterone
- Infertility
