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Genetic Testing Flashcards

1
Q

What is the main method for prenatal diagnosis of fetal abnormalities? When is it offered?

A

Ultrasound

Offered routine ultrasound scans at 11-14 weeks and again at 20-22 weeks

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2
Q

What is the prenatal procedure?

A 
\+ve pregnancy test
book into antenatal care
nuchal scan (10-14 weeks gestation) 
mid-trimester anomaly scan (20 weeks)
ultrasound
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3
Q

Aims of 12 week scan

A 
Date pregnancy accurately
Diagnose multiple pregnancy
Diagnose major foetal abnormalities
Diagnose early miscarriage
Assess risks of down syndrome + other chromosomal abnormalities
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4
Q

Why might a Nuchal Translucency (NT) scan be taken and when are they taken?

A

To read the thickness of fluid at the back of foetal neck

10-14 weeks

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5
Q

A thickness of fluid at the back of fetal neck of above 3mm can indicate what?

A

Chromosome abnormalities

Birth defects: Cardiac anomalies, Pulmonary defects, Renal defects, Abdominal wall defects

Skeletal dysplasias

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6
Q

Is Nuchal Translucency a screening test or a diagnostic?

A

Screening

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7
Q

When is prenatal testing arranged? (5)

A

Following abnormal findings at nuchal scan or mid-trimester scan

Following results of combined test which give an increased risk of Down Syndrome

If previous pregnancy affected with a condition

If parent is a carrier of chromosome rearrangement or genetic condition

Family history of genetic condition

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8
Q

What are the aims of prenatal testing?

A

To inform and prepare parents for the birth of an affected baby

To allow in utero treatment

Manage remainder of pregnancy

To be prepared for complication at or after birth

To allow termination of an affected fetus

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9
Q

Why is a Fetal MRI needed?

A

For more detailed scan to see organs more clearly

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10
Q

Why is a fetal cardiac scan needed?

A

To look at flow of blood through body’s heart to diagnose cardiac conditions

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11
Q

What are different types of prenatal testing and give examples

A
  1. SCANNING = Ultrasound, MRI
  2. NON-INVASIVE = maternal blood test, cffDNA
  3. INVASIVE = CVS, amnioscentesis
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12
Q

What is cffDNA?

A

Cell-free fetal DNA

Short DNA fragments of baby

Floats around in mother’s blood from placenta & composed of baby’s DNA

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13
Q

When can cffDNA be detected?

A

First 4-5 weeks of gestation but cannot accurately be detected on testing until 9 weeks

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14
Q

What is NIPD?

A

Non-invasive prenatal diagnosis

Maternal blood test at around 9 weeks of pregnancy

  • Analyse DNA fragments present in maternal plasma
  • 10-20% comes from placenta and representative of unborn baby

Trisomy 21 = cffDNA for chromosome 21 is higher than standard pregnancies

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15
Q

For which genetic conditions is NIPD free for?

A

Achondroplasia

Thanatophoric dysplasia

Apert syndrome

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16
Q

What gene is detected to determine the sex of the fetus?

A

SRY gene on the Y chromosome

17
Q

For Cystic Fibrosis, what test is used to detect both maternal and paternal mutation in the fetal DNA?

A

Haplotyping

18
Q

What is the difference between NIPD and NIPT?

A

NIPT is not as useful when coming to a decision and so further invasive techniques may be required

19
Q

What are the limitations of NIPD and NIPT?

A

Not possible in multiple pregnancies to tell which DNA came from which fetus

Relative proportion of cffDNA is reduced in women with high BMI so 2nd sample may be needed after 9 weeks

Women may not be as prepared for the result as they would be for an invasive test

Invasive test may still be required to confirm an abnormal result

20
Q

What are the benefits of NIPT and NIPD?

A

Number of invasive tests carried out is likely to reduce as a result

No increased risk of miscarriage

Less expertise required to perform blood test than invasive test

NIPD/NIPT can be offered earlier than traditional invasive testing, so result given earlier

21
Q

What is maternal blood test used for?

A

serum markers in blood can detect increased risk of trisomy 21, 18 and/or neural tube defects

22
Q

What is CVS? When?

A

Chorionic Villus Sampling (11-14 weeks)

Sample of chorionic villi (part of developing placenta with same DNA as fetus) is taken transabdominally or transvaginally

Allowing patient to have result earlier than amniocentesis

23
Q

What is Amniocentesis? When? Risk?

A

Sample of amniotic fluid which contains fetal cells is taken

16 weeks

Up to 1% risk of miscarriage, Infection, Rh Sensitisation

24
Q

What tests are done with a DNA sample?

A

Test for genetic disorder in question specifically

Karyotype if chromosome abnormality is present in the family history

QF_PCR for all
Looks for t13, 18 and 21

25
Q

What is CGH array?

A

If there are concerns on 20 week scan, CGH array is offered

Look for small/large imbalances in chromosomes (identifies microdeletions and duplications)

26
Q

If something is found on the array, what happens next?

A

Standardly test parents to see if either is a carrier to help with interpretation

27
Q

What are the 4 steps in carrying out a CGH array?

A

Extract and Label DNA

Hybridise & Wash

Scan

Analyse & Data

28
Q

What is Trio Exome and what does it allow?

A

Exome is taken from the baby and parents

Efficient diagnosis of genetic condition in utero

Consider where foetus in previous pregnancy had significant anomalies

29
Q

What is an Exome?

A

coding region of DNA/chromosome/genome

30
Q

Why are there fewer egg and sperm donors?

A

Donations are no longer anonymous and children conceived have the right to contact the donor when 18

31
Q

What are the 2 stages when a couple want to adopt?

A

Registration and checks

Assessment and approval

32
Q

Options for family planning if known reproductive risk

A
  • conceive naturally, no prenatal testing
  • conceive naturally, have prenatal testing
  • use of egg and/or sperm donors
    adoption
  • choose not to have children
  • pre-implantation genetic diagnosis (PIGD)
33
Q

What is Pre-implantation Genetic Diagnosis?

A

PGD uses IVF with an additional step to genetically test the embryo before implantation

34
Q

Who is PGD particularly used by?

A

People who do not want TOP (termination of pregnancy)

35
Q

Describe the process of PGD?

A
  1. Stimulation of ovaries
  2. Egg collection
  3. Insemination - intracytoplasmic sperm injection (ICSI) of only a single sperm to reduce contamination from more sperm cells
  4. Fertilisatiton
  5. Embryo biopsy
  6. Embryo testing - at blastocyst stage
  7. Embryo transfer - of those that did not inherit the genetic condition into the womb
  8. Pregnancy test
36
Q

For which disorders is PGD used for?

A

Translocation carriers

HD
DMD - only implant female embryos where mutation in family is unknown

CF

37
Q

What are some of the roles of a Genetic Counsellor in prenatal testing?

A

Arrange and explain CVS, amniocentesis, PGD, cffDNA

Facilitate decision-making

Give results

See patients in clinic following a diagnosis in utero

Arrange termination if necessary

Discuss recurrence risks and plans for future pregnancies

// (13 decks)

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