Lecture 18- Testing for genetic disorders Flashcards

1
Q

Purpose of Prenatal testing and screening

A

Often done if there is a concern of risk
Guides families in decisions around the pregnancy
-potential termination?
-Medical & psychological support

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2
Q

What is Prenatal Testing?

A

Screening tests and diagnostic test

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3
Q

Difference between screening and diagnostic tests

A

Screening:

  • assess RISK but not a definitive diagnostic test
  • non-invasive, low risk
    eg) maternal serum testing for downs syndrome

Diagnostic Tests

  • Test for specific chromosomal or genetic abnormality (usually via fetal cells/ DNA)
  • invasive test, carries some risk
    eg) gene mutation analysis for cystic fibrosis
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4
Q

What are the clinical indications to do prenatal testing for Chromosomal or single gene disorders

A
Single Gene disorders:
-family history
-one parent has AD disorder
-if both parents carriers 
-X linked
(risk prediction done from analysis of pedigree, may lead to specific diagnostic test).

Chromosomal Disorders

  • older mother pregnant
  • previous child with significant disorder
  • chemical/toxic disorders
  • Abnormal screening test result
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5
Q

Screening Stratagies

A
  • Ultrasound scan of pregnancy
  • Nuchal Translucency
  • Maternal Serum Testing
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6
Q

Screening Ultrasound

A

8 wks : check dates, confirm # babies
18wks: detailed scan of anomalies can be done

Safe, no risk to mother or fetus

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7
Q

Nuchal Translucency

A

collection/thickness of fluid under the skin at the back of your baby’s neck.
Measure by ultrasound ~10-13 wks
Noninvasive test
(combined with maternal serum testing to increase sensitivity and specificity for downs syndrome screening)

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8
Q

Maternal Serum Testing

A

Blood test
Performed ~15-17 wks
-testing for 2-4 feto-placental proteins1

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9
Q

Risk prediction for Down syndrome

A

Screening in 2nd trimester by –maternal age and maternal serum can identify 50-75% trisomy 21 pregnancies with a false + rate of 5%

-Fetal NT in 1st and maternal serum in the 2nd reported a detection rate of 85-90% trisomy 21 with a false + rate of 5%

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10
Q

Types of Diagnostic Tests

A

Access to fetal cells
May lead to specific diagnostic test
1) amniocentesis (weeks to culture)
2) Chronic villus test (hours)

These all take 3-4 days to do this testing

also percutaneous umbilical cord blood sampling
-fetoscopy & umbilical cord blood sampling
(done later, very rare to do, high chance of miscarriage)

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11
Q

Chorionic Villus Sampling

A

cells from developing placenta taken and tested
Preferred as done earlier ~10-12 wks
-obtain part of CV (fetal cells), and analyse chromosomes or DNA
-risk of miscarriage

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12
Q

Amniocentesis

A

15-16wks
Needle injected into uterus, collects amniotic fluid that contains fetal cells
-may need to culture cells before analysis
-Miscarriage risk ~1%

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13
Q

NEW SCREENING TEST

A

cell free fetal DNA in the maternal plasma
-non-invsive screening test for anueploidies
-still have to confirm with a diagnostic test
-Not routinely available in NZ
Detection Rate: 99%
False +: 0.9%

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14
Q

Issues with new screening test

A

Not comrehensive testing

  • only for common chromosome disorders
  • single gene disorders (need to know mutation)
  • risk of error

Risk (small) to fetus

Not all think its morally/ethically acceptable

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15
Q

Benefits when testing prenatal testing

A

Reassurance when results are normal
Advance warning for medical team
psychological preparation for babies arrival
-info, is termination an option?

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16
Q

Pre-Impantation Genetic diagnosis (PGD)

A

Uses IVF and tests for the genetic disorder before fertilised eggs are implanted.

  • Eggs fertilised ‘in-vitro’, tested in lab to determine if they are genetically affected.
  • Non-affected eggs implanted

Expensive, technically difficult

17
Q

Genetic Counselling

A

Essential part of the process and car of the patients

-Provide up to date info & support