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Chromosomal Abnormalities Flashcards

0
Q

How many base pairs per octamer of histone?

A

166

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1
Q

How are histones stabilised?

A

h1 stabilises DNA wrapped around on tamer

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2
Q

What is epibenthic modification?

A

Demethylation, histone acetylation changes inactive chromatin to active chromatin
Methylation, histone deacetylation changes active chromatin to inactive chromatin

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3
Q

At what stage of mitosis are chromosomes analysed?

A

Metaphase

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4
Q

How are chromosomes isolated for analysis?

A

Spindle inhibitor - colcemid
Fix with 3:1 methanol : acetic acid
Stain with geimsa- banding
Light microscopy

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5
Q

What specimens can be used for chromosome analysis, and how long do they take to culture?

A 
Bone marrow 0–1 days
T lymphocytes 2-3 days
(Grow in suspension)
Amniotic fluid 7-21 days
(Grow on substrate)
Chorionic villus sampling 
Solid tissue
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6
Q

What are the relative sizes of chromosomes harvested from blood, bone marrow and amniotic fluid?
What is the implication?

A

Blood>amniotic fluid >bone marrow

Can detect smaller abnormalities in bigger chromosomes

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7
Q

Which chromosomes are acrocentric?

A

D and G groups - 13, 14, 15, 21, 22

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8
Q

Which of chromosomes 21 and 22 is larger?

A

22

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9
Q

What groups are chromosomes X and Y in?

A

X is C, Y is G

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10
Q

How are chromosomes stained?

A

Trypsin digests proteins of metaphase chromosomes
Stain with geimsa or leishman (romanowksi type dye)
AT rich = gene poor. Stain darkly
GC rich = gene rich. Stain lightly

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11
Q

What is c banding?

A

Stains heterochromatin at centromeres, 1, 9, 16 and Yq

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12
Q

Who do cytogenetic analysis?

A

Better management eg hormonal treatment of Klinefelter syndrome
Accurate diagnosis - account for phenotype/pregnancy loss
Assess future reproductive risks
Prenatal diagnosis

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13
Q

Why are people referred for cytogenetic testing?

A 
Birth defects
Abnormal sexual development
Prenatal diagnosis
Infertility
Recurrent fetal loss
Leukaemia ALL/AML/CML/myelodysplasia/myeloproliferative disorders
Solid tumours
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14
Q

What might prompt the offer of prenatal diagnosis?

A

Maternal serum screening - downs
First trimester biochemical markers and ultrasound
Familial hypercholesterolaemia
Abnormal ultrasound - cleft lip and palate, abnormal limbs/heart

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15
Q

Whats Williams syndrome?

A

Deletion 7q11.23

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16
Q

What’s DiGeorge syndrome?

A

Deletion 22q11.2

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17
Q

WHat are the viable aneuplodies?

A

Trisomy 21 - downs
Patau trisomy 13, Edwards +18
Turners monosomy X

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18
Q

What is polyploidy?

A

Gain of a whole haploid set of chromosomes

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19
Q

What causes polyploidy?

A

Polyspermy

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20
Q

What is the consequence of triploidy?

A

15% of miscarriages, 2-3% of pregnancies - die shortly after birth

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21
Q

WHat causes aneuplodies?

A

Meiosis non disjunction

Mitotic non disjunction- mosaicism

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22
Q

WHat is anaphase lag?

A

Chromosomes left behind due to defects in spindle formation or attachment to chromosomes. Mitosis or meiosis

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23
Q

What is the incidence of Down syndrome?

A

1 in 650-1000

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24
What are the symptoms of downs?
``` Hypotonia Intellectual disability Characteristic facial features Heart defects Increased incidence of leukaemia Increased incidence of early Alzheimer's ```
25
What is the cause of Edwards syndrome?
Trisomy 18 due to maternal meiosis two error
26
What is the prevalence of Edwards syndrome?
1 in 6000
27
What are the symptoms of Edwards syndrome?
``` Prominent occiput Rocker bottom feet Lifespan 5-15 days Small lower jaw Low set ears Overlapping fingers ```
28
What is patau syndrome?
``` Trisomy 13 Death in neonatal period Holoprosencephaly Polydactylyl Congenital abnormalities ```
29
What is the incidence of pataus syndrome?
1:12000
30
What is the incidence of turners syndrome?
1:25000
31
What are the symptoms of turners?
Short stature, neck webbing, infertility, heart defects, mild learning difficulties
32
What is x inactivation?
Ensures only one x chromosome is active in any cell so males and females have same chromosome complement
33
Why is monosomy of X a problem?
Pseudo autosomal regions at the ends of the chromosomes X and Y are essential for meiosis SHOX gene in PAR, explains short stature
34
What causes mosaicism?
Mitotic non disjunction
35
What cell line is usually lost in mosaicism?
Monosomic cell lines
36
What are the four UPDs?
Isodisomy - 2 identical chromosomes from 1 parent Heterodisomy - 2 different chromosomes from one parent Segmental UPD - only part of chromosome involved Acquired UPD - solid tumours and leukaemias
37
Why does UPD matter?
Some chromosomes are imprinted - chromosomes are expressed depending on their parental origin
38
What is UPD15?
Prader willi/angel an
39
What is Russell silver?
UPD 7
40
What is beckwith weidemann?
UPD 11
41
Ow does UPD arise?
Monosomy rescue Trisomy rescue Gamete complementation Mitotic error
42
How does trisomy rescue occur?
Disomic gamete -> trisomic conceptus | Lies a chromosome in post zygotic mitosis
43
How do you test for UPD?
Use repetitive markers - micro satellites - to identify parental origin of chromosomes
44
Which reciprocal translocation is not unique to a family?
Immanuel syndrome t(11;22)
45
What gametes may someone affected with a reciprocal translocation produce and how is this determined!
Balanced or unbalanced | Pachytene diagram
46
How can chromosomes be separated in a pachytene diagram, implications?
Alternate - balanced Adjacent 1 homologous centromeres Adjacent 2 non homologous centromeres Both unbalanced
47
How do you assess the impact of unbalanced segregation in reciprocal translocation?
Assess likely segregation | Literature search
48
What's a robertsonian translocation?
Two acrocentric chromosomes fused (13,14,15,21,22)
49
How would you write in standard notation a female with robertsonian translocation of 14 and 21?
45,XY,der(14;21)(q10;q10)
50
What probes are used in FISH?
Centromere probes Telomere probes Whole chromosome paints Gene specific probes
51
WhY use centromere probes?
Large, easy to see, use for copy number analysis
52
Why use chromosome paints?
Identify rearrangements
53
Why use prenatal aneuploidy analysis?
PND takes two weeks causes anxiety | FISH is quick 99% accuracy,common aneuplodies
54
What FISH is used in leukaemias?
Interphase analysis- look for translocations, rearrangements, amplifications of genes
55
HOw long are oligonucleotides in a microarray?
60bp
56
How long should you hybridise in a microarray?
48hrs
57
What is Cot-1 used for?
In microarrays to suppress repetitive sequences
58
What is aCGH?
Array comparative genome hybridisation | Microarray
59
Why use aCGH?
Check if an apparently balanced karyotype really is balanced Investigate learning difficulties and congenital abnormalities Investigate unbalanced karyotypes
60
Hat different arrays are available?
Cancer gene arrays (copy number and loss of heterozygosity LOH) Chromosome/gene specific (X/DMD) Oligo arrays and SNPs (copy number and LOH) Exon arrays - small intergenic copy number changes WHole genome screen - copy number changes
61
How are microarrays assessed?
3um laser
62
WHat are the benefits of aCGH?
Can target many thousands of genes, can automate Examine whole genome Detailed information on genes in deleted/duplicated region Target against known conditions
63
WHat are the disadvantages of aCGH?
Expensive Doesn't detect balanced rearrangements Might not detect mosaicism
64
What can the outcomes of variation be?
Pathogenic, normal or uncertain - dilemma - no genes, assume not pathogenic
65
What is NIPT?
Non invasive prenatal testing Cell free fetal DNA in maternal plasma (5%) at nine weeks gestation Technically challenging -research. Early in pregnancy. Non invasive
66
WhAt can you use NIPT for?
``` Sexing RhD status Soon +21 X linked conditions excluding haemophilia Congenital adrenal hyperplasia Achondroplasia Thanatophoric dysplasia ```
67
What is NGS?
Next generation sequencing - fragment genomic DNA, sequence in parallel Realign to reference genome - full sequence
68
How is NGS used in diagnosis?
Generate lots of data - hard to analyse Useful for NIPT, copy number analysis, mutation scanning, epigenetic analysis, expression analysis, tumour profiling, RNA sequencing Reduces cost compared to Sanger sequencing
69
What is whole genome sequencing?
Look at all exons Compare with parents - de novo mutations Easier to interpret

MGD (6 decks)

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