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Molecules, Genes, and Disease > L4.1 Chromosomal Abnormalities and Prenatal Genetic Testing > Flashcards

L4.1 Chromosomal Abnormalities and Prenatal Genetic Testing Flashcards

1
Q

Each chromosome consists of

A

proteins and one long molecule of DNA

a short arm (p) and long arm (q) connected by a
centromere

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2
Q

A chromatid is

A

one of two identical
halves of a replicated chromosome

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3
Q

A karyotype

A

is the actual picture of an individual’s
collection of chromosomes

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4
Q

Karyotypes

A

describe the number of chromosomes and what they look like (size bands and centromere placement)

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5
Q

Karyograms

A

the study of the whole set of chromosomes arranged in pairs by size and position of the centromere

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6
Q

chromosome ideogram

A

graphical or schematic representation of chromosomes

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7
Q

G banding

A

a technique used to produce a visible karyotype by staining condensed chromosomes

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8
Q

what are the dark and light bands on the chromosome ideogram numbered according to ?

A

the international convention

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9
Q

karyotyping

A

process of pairing and ordering all the chromosomes of an individual

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10
Q

karyotyping is also used for what?

A

detect changes in chromosome number and also more subtle structural changes such as chromosomal duplication, deletion, translocation or inversions.

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11
Q

what are karyotypes prepared from?

A

prepared from mitotic cells that have been arrested in the metaphase or prometaphase of the cell cycle when chromosomes assume their most condensed conformation’s

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12
Q

what types of tissue can be used to source karotypes?

A

peripheral blood, amniotic fluid, chronic villus specimens are used as the source of cells.

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13
Q

stages from sample to karyotype

A

-collect blood sample
-add phenomagglutnin and culture medium
- culture at 37 degrees for 3 days
-add colchicine and hypertonic saline
cells fixed
-spread cells onto slide by dropping
-digest with trypsin and stain with giesa
-analyse metaphase spread

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14
Q

indications for karyotyping

A

a. prenatal screening;
-> down syndrome with raised maternal age, family history of CA, abnormal ultrasound

b. birth defects;
-> malformations or metal impairment

c. Abnormal sexual development
-> Klinefelter or turner syndrome

d. Infertility; recurrent foetal loss
e. Leukaemia or other cancers

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15
Q

standard format for karyotyping

A

46, XX or 46, XY

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16
Q

chromosomal abnormalities

A
  1. Polyploidy
  2. Aneuploidy
  3. Chromosomal mutations
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17
Q

what is Polyploidy

A

more than two complete sets of chromosomes

-> most have an even set of chromosomes e.g. tetraploidy 4n

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18
Q

what is Aneuploidy

A

Aneuploidy is the presence of an abnormal number of chromosomes in a cell. usally 1 more or 1 less.

-> most common chromosomal abnormality and clinically significant

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19
Q

Types of chromosomal mutations

A

deletion
duplication
inversion
insertion
translocation

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20
Q

how does polyploidy occur in humans?

A

-> very rare in the form of triploid -> 69(XXX) chromosomes or tetraploid with -> 92 (XXXX) chromosomes

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21
Q

why does triploidy occur?

A

due to polyspermy - one oocyte being fertilised by more than 1 sperm.

observed in 1-2% of early miscarriages

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22
Q

how often does aneuploidy occur in pregnancies ?

A

3-4%

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23
Q

most common aneuploidies in humans is?

A

trisomy’s which represent 0.3% of all live births

with exceptions - trisomy’s do not appear to compatible with life.

trisomy represent about 35% of spontaneous abortions

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24
Q

aneuploidy is the most common cause of

A

of the nondisjunction of chromosomes during meiosis

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25
the nondisjunction of chromosomes during meiosis is:
is the failure of homologous chromosomes to separate properly during meiotic cell division
26
age for testing for foetal chromosome abnormalities
35+
27
viable trisomy's are restricted to only a few human chromosomes; name the conditions and the chromosome number
1. Down syndrome - TRISOMY 21 - viable 2. Edwards syndrome - TRISOMY 18 - lethal 3. Patau Syndrome - TRISOMY 13 - lethal
28
What might be a reason that trisomy 21 is the only viable one?
because the number of protein coding sequences predicted for chromosome 21 is the smallest of any chromosome with the exception of Y chromosome thus the addition of the additional copy of chromosome 21 would be predicted to pertub the normal equilibrium
29
3 types of genetic variations that can cause Down syndrome
Trisomy 21 - 95% of cases - affects 1 in 800 to 100 births Mosaic Down syndrome: rare form of down syndrome where a person has only some cells with an extra copy of chromosome 21 Translocation down syndrome - when a portion of chromosome 21 becomes attached or translocated onto another chromosome before or at conception
30
Down syndrome features
distinct facial features: flattened face small head short neck protruding tongue upward slanting eye lids small ears poor muscle tone
31
More common features of down syndrome
- broad, short hands with a single crease in the palm - relatively short fingers - small hands and feet excessive flexibility - tiny white spots on the coloured parts of the iris - Bushfield's spots - short height
32
symptoms
mild to moderate Intellectual disability (low IQ) congenital heart disease haematological malignancies hypothyroidism gastrointestinal: lack of nerves in the colon (constipation) infertility eye disorders: strabismus, refractive errors, cataracts hearing disorders in 38-78%
33
Edwards syndrome stats
occurs in around 1 in 6000 births modal lifespan 5-15 days 5-10% of infants survive longer than a year
34
symptoms of Edwards syndrome
unusually small head back of head is prominent ears are malformed and low set mouth and jaw are small clubfeet hands clenched as fists heart defects and abnormalities of other organs that develop before birth
35
Patau syndrome stats
1 in 10,000 births 1 in 27,000 live births 5-10% live past their first year small percentage reach adulthood multiple congenital abnormalities
36
symptom's of Patau syndrome
severe intellectual disabilities congenital heart defects brain or spinal cord abnormalities very small or poorly developed eyes low set ears cleft lip polydactyly hypertonia
37
humans are able to tolerate extra sex chromosomes than autosomes why?
-> this tolerance most likely relates to both X -> inactivation and to the small number of genes on the Y chromosome -> affected individuals generally show reduced sex development and fertility but often have normal life spans and symptoms can be treated with hormones.
38
types of sex chromosome aneuploidies
Turner 45, X Triple X syndrome; 47, XXX Klinefelter syndrome; 47, XXY XYY syndrome; 47 XYY
39
X chromosome inactivation
40
Chromosomal translocation
chromosomes sometimes break and stick to another chromosome the breakpoints on the two chromosomes often lie between genes ->a person with this translocation will have the full complement of genes; no phenotype -> risk of passing one of the derivative chromosomes to offspring -> offspring would be unbalanced and present with a phenotype
41
Robertsonian translocation
results from the breakage of two acrocentric chromosomes numbers; numbers 12,14,15,21,22 or close to their centromeres, with subsequent fusion of their long arms to form one chromosome
42
The total chromosome number in a Robertsonian translocation carrier is thus reduced to....
45
43
Are Robertsonian translocation carriers normal?
yes because there is no gain or loss of important genetic material
44
incidence of Robertsonian translocation
1 in 1000 - most common fusion is 13 and 14
45
what is a Philadelphia chromosome
results from reciprocal translocation between chromosomes 9 and 22
46
what does the reciprocal translocation in a Philadelphia chromosome cause?
the translocation creates a FUSION gene; BRC-ABL that encodes for a fusion protein BRC-ABL that is oncogenic
47
first line of therapy for individuals with Philadelphia chromosomes BRC-ABL
BRC-ABL tyrosine-kinase inhibitors
48
deletion - chromosome abnormalities example
chronic lymphocytic leukaemia - deletion of the short arm on chromosome 17 is found in 5-8% of patients and is assoicated with rapid disease progression
49
visualising chromosomes - what technique?
FISH - fluorescence in situ hybridization
50
what is FISH
-> cytogenetic localisation of DNA sequences
51
outline the process of FISH
-> produces a red fluorescent signal at the sites of a specific DNA sequence; in this case; a 150kb segment of chromosome 1 -> several probes each corresponding to a defined genomic segment can be simultaneously analysed and ordered with respect to each other using multicolour FISH
52
Using FISH to detect chromosomal abnormalities in interphase nuclei - CHARCOT MARIE
the duplication of a small portion of chromosome 17 that causes CHARCOT MARIE TOOTH SYNDROME is evident by the appearance of three rather than 2 red signals in this nucleus.
53
Using FISH to detect chromosomal abnormalities in interphase nuclei - Philadelphia chromosome
is evident by the appearance of the close juxtaposition of one pair of green and red signals
54
visualising chromosomes
- spectral karyotyping and multicolour FISH paint each human chromosome's in one of 24 colours.
55
M-FISH
a 24 colour karyotyping technique and is the method of choice for studying the arrangement of intrachromosomal rearrangements
56
screening
is a program applied to a whole population, or large numbers of asymptomatic but potentially at risk individuals
57
testing
is applied to either symptomatic individuals to establish diagnosis or asymptomatic individuals with a positive screening test
58
Screening programmes offered during pregnany
infectious diseases inherited conditions 11 physical conditions (20 week scan) down syndrome, Edwards syndrome, pataus syndrome
59
screening offered for the new born
physical exam hearing screening blood spot screening
60
Young person and adult screening programmes
eye problems in individuals with diabetes abdominal aortic aneurysms
61
cancer screening programmes
cervical screening breast screening bowel screening
62
New born blood spot DAY 5 -> heel prick test what does it look for?
PKU congenital hypothyroidism CF MCADD HCU MSUD GA1 IVA
63
New-born hearing screening
automated otoacoustic emission test -> quick and non invasive Earpiece placed into baby ear and gentle clicking noises are played. -> if results are unclear offered a second test
64
AABR test - further hearing test
3 sensors on baby's head and neck, soft headphones are placed over baby's ears and gentle clicking noises are played
65
New-born Physical Examination
- offered within 72 hours after birth examine: eyes, heart, hips, testicles, back, hands, feet, palate, anus
66
Screening in pregnancy - conditions looked for and treatment
Gestational diabetes - OGTT Preeclampsia - Low dose aspirin Mental health disorders - specialist services blood group rhesus factor - Anti D injection
67
Screening for infectious diseases during pregnancy?
-> blood test at week 10 3 Infections: Hep B, HIV, and syphilis treatment to prevent transmission
68
Screening for 11 physical conditions - what happens
week 20 scan -> detailed look at bones, heart, brain, spinal cord, face, kidneys, and abdomen ->sonographer looks for the 11 rare conditions
69
11 rare conditions at 20 week scan
anencephaly open spina bifida cleft lip diaphragmatic hernia gastroschisis exomphalos cardiac abnormalities: TGA, AVSD, TOF, HLHS bilateral renal agenesis lethal skeletal dysplasia Edwards syndrome Patau's syndrome
70
What does a diaphragmatic hernia look like on a scan?
there are multiple air filled loops in the left hemi thorax with mediastinal shift to the right and ipsilateral lung hypoplasia
71
Spina bifida on a scan
failure of closure of the vertebral arch at level of L5
72
Down syndrome, Patau's syndrome, and Edwards syndrome testing:
COMBINED TEST at 10-14 weeks -> Ultrasound scan to check for Nuchal translucency (fluid at the back of the baby's neck) -> Blood test: Beta HCG - serum pregnancy assoicated plasma protein A Combined results of both tests calculate risk for all 3 conditions
73
what happens if the sonographer is unable to see the Nuchal translucency measurement ?
at 14-20 weeks they will have: -> blood test (alpha feta protein, BHCG, unconjugated estriol and inhibin A) ->downs only ->less accurate
74
what is the outcome of the foetal anomaly screening ?
higher risk result: offered a diagnostic test to find out for certain whether the baby has down, Edwards, or Patau's syndrome
75
the use of alpha-fetoprotein (AFP) or USS for prenatal screening for neural tube defects
USS is more effective than AFP The level of AFP is an indicator of potential NTD
76
List the genetic diagnostic tests:
Tests and biopsies of the foetus PCR tests karyotyping new developments
77
Foetal DNA screening - who's DNA is obtain and how is foetal DNA isolated?
Parental and sibling DNA collected from blood or saliva BABYS DNA OBTAINED: amniotic fluid cells chorion villus biopsy foetal DNA in mother blood
78
Amniocentesis - when is it performed, what is done?
performed at 15-20 weeks ultrasound guidance cells must be recovered and may need to be cultered for 2 weeks
79
Amniocentesis miscarriage rate
0.5-1%
80
Chorion villus biopsy- what is it and when is it done?
Performed at 10-13 weeks gestation ultrasound guided ->transcervical or transabdominal foetal villi must be seerated from maternal tissue
81
Chorion villus biopsy miscarriage rate
2%
82
NIPT - Non-invasive prenatal testing : what is it ?
Foetal DNA can be isolated from the mothers blood along with the mothers own DNA If the mother and father have been typed for particular disease causing mutations the foetal status can be derived from sequencing the isolated DNA -> can be offered to women who had a higher change results form combined or quadruple test

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