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MGD > Chromosomal Abnormalities > Flashcards

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
Q

What are the symptoms of downs?

A 
Hypotonia
Intellectual disability 
Characteristic facial features
Heart defects
Increased incidence of leukaemia 
Increased incidence of early Alzheimer's
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25
Q

What is the cause of Edwards syndrome?

A

Trisomy 18 due to maternal meiosis two error

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26
Q

What is the prevalence of Edwards syndrome?

A

1 in 6000

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27
Q

What are the symptoms of Edwards syndrome?

A 
Prominent occiput
Rocker bottom feet
Lifespan 5-15 days
Small lower jaw
Low set ears
Overlapping fingers
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28
Q

What is patau syndrome?

A 
Trisomy 13
Death in neonatal period
Holoprosencephaly
Polydactylyl 
Congenital abnormalities
29
Q

What is the incidence of pataus syndrome?

A

1:12000

30
Q

What is the incidence of turners syndrome?

A

1:25000

31
Q

What are the symptoms of turners?

A

Short stature, neck webbing, infertility, heart defects, mild learning difficulties

32
Q

What is x inactivation?

A

Ensures only one x chromosome is active in any cell so males and females have same chromosome complement

33
Q

Why is monosomy of X a problem?

A

Pseudo autosomal regions at the ends of the chromosomes X and Y are essential for meiosis
SHOX gene in PAR, explains short stature

34
Q

What causes mosaicism?

A

Mitotic non disjunction

35
Q

What cell line is usually lost in mosaicism?

A

Monosomic cell lines

36
Q

What are the four UPDs?

A

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
Q

Why does UPD matter?

A

Some chromosomes are imprinted - chromosomes are expressed depending on their parental origin

38
Q

What is UPD15?

A

Prader willi/angel an

39
Q

What is Russell silver?

A

UPD 7

40
Q

What is beckwith weidemann?

A

UPD 11

41
Q

Ow does UPD arise?

A

Monosomy rescue
Trisomy rescue
Gamete complementation
Mitotic error

42
Q

How does trisomy rescue occur?

A

Disomic gamete -> trisomic conceptus

Lies a chromosome in post zygotic mitosis

43
Q

How do you test for UPD?

A

Use repetitive markers - micro satellites - to identify parental origin of chromosomes

44
Q

Which reciprocal translocation is not unique to a family?

A

Immanuel syndrome t(11;22)

45
Q

What gametes may someone affected with a reciprocal translocation produce and how is this determined!

A

Balanced or unbalanced

Pachytene diagram

46
Q

How can chromosomes be separated in a pachytene diagram, implications?

A

Alternate - balanced
Adjacent 1 homologous centromeres
Adjacent 2 non homologous centromeres
Both unbalanced

47
Q

How do you assess the impact of unbalanced segregation in reciprocal translocation?

A

Assess likely segregation

Literature search

48
Q

What’s a robertsonian translocation?

A

Two acrocentric chromosomes fused (13,14,15,21,22)

49
Q

How would you write in standard notation a female with robertsonian translocation of 14 and 21?

A

45,XY,der(14;21)(q10;q10)

50
Q

What probes are used in FISH?

A

Centromere probes
Telomere probes
Whole chromosome paints
Gene specific probes

51
Q

WhY use centromere probes?

A

Large, easy to see, use for copy number analysis

52
Q

Why use chromosome paints?

A

Identify rearrangements

53
Q

Why use prenatal aneuploidy analysis?

A

PND takes two weeks causes anxiety

FISH is quick 99% accuracy,common aneuplodies

54
Q

What FISH is used in leukaemias?

A

Interphase analysis- look for translocations, rearrangements, amplifications of genes

55
Q

HOw long are oligonucleotides in a microarray?

A

60bp

56
Q

How long should you hybridise in a microarray?

A

48hrs

57
Q

What is Cot-1 used for?

A

In microarrays to suppress repetitive sequences

58
Q

What is aCGH?

A

Array comparative genome hybridisation

Microarray

59
Q

Why use aCGH?

A

Check if an apparently balanced karyotype really is balanced
Investigate learning difficulties and congenital abnormalities
Investigate unbalanced karyotypes

60
Q

Hat different arrays are available?

A

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
Q

How are microarrays assessed?

A

3um laser

62
Q

WHat are the benefits of aCGH?

A

Can target many thousands of genes, can automate
Examine whole genome
Detailed information on genes in deleted/duplicated region
Target against known conditions

63
Q

WHat are the disadvantages of aCGH?

A

Expensive
Doesn’t detect balanced rearrangements
Might not detect mosaicism

64
Q

What can the outcomes of variation be?

A

Pathogenic, normal or uncertain - dilemma - no genes, assume not pathogenic

65
Q

What is NIPT?

A

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
Q

WhAt can you use NIPT for?

A 
Sexing
RhD status
Soon +21
X linked conditions excluding haemophilia 
Congenital adrenal hyperplasia
Achondroplasia
Thanatophoric dysplasia
67
Q

What is NGS?

A

Next generation sequencing - fragment genomic DNA, sequence in parallel
Realign to reference genome - full sequence

68
Q

How is NGS used in diagnosis?

A

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
Q

What is whole genome sequencing?

A

Look at all exons
Compare with parents - de novo mutations
Easier to interpret

MGD (6 decks)

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