5.7 - Genetic Testing

Question 1

What happens in a normal pregnancy?

Answer 1

• positive pregnancy test - no longer confirmed at GP
• book into antenatal care - see midwife
• nuchal scan at 10-14 weeks gestation - different tests depending on NHS Trust e.g. nuchal translucency, combined test etc
• maternal age taken into account and risk of Down’s syndrome given - anything more than 1/150 considered high risk, anything below is low risk and no further testing done
• mid-trimester anomaly scan - 20-22 weeks gestation
• ultrasound examination is the main method for prenatal diagnosis of foetal abnormalities

Question 2

What are the aims of the nuchal scan (12 weeks)?

Answer 2

• to date the pregnancy accurately
• to diagnose multiple pregnancy
• to diagnose major foetal abnormalities
• to diagnose early miscarriage
• to assess the chance of Down’s syndrome and other chromosomal abnormalities
• takes into account maternal age, blood hormone levels, nuchal translucency thickness, nasal bone, blood flow through the foetal heart and foetal abnormalities

Question 3

What is nuchal translucency (NT)?

Answer 3

• done at 10-14 weeks
• it is the thickness of fluid at back of foetal neck
• screening test and not diagnostic
• can indicate a range of genetic syndromes
• combined with maternal blood markers and maternal age

Question 4

What can a NT increased >3mm indicate?

Answer 4

• chromosome abnormalities e.g. Down’s, Edwards, Patau, Turners –> NT and maternal age detects up to 75% of Downs syndrome with 5% false positive rate
• birth defects e.g. cardiac anomalies, pulmonary defects (diaphragmatic hernia), renal defects, abdominal wall defects
• skeletal dysplasias

Question 5

What is the aim of anomaly scan?

Answer 5

Look for structural abnormalities specifically in the heart, brain, spinal cord, face, kidney and abdomen to measure the lengths of the bones

Question 6

What other scans are available?

Answer 6

• foetal MRI if more detailed scan needed
• foetal cardiac scan offered to look at blood flow through baby heart to diagnose cardiac conditions

Question 7

When is prenatal testing offered?

Answer 7

• following abnormal findings at nuchal scan or mid-trimester scan
• following results of combined test which give an increased risk of chromosomal abnormality
• if previous pregnancy/child was affected with a condition e.g. Down’s syndrome, CF
• if parent(s) is a carrier of chromosome rearrangement or genetic condition e.g. t(13;14), DMD, Huntington’s
• if there is a family history of a genetic condition

Question 8

What are the aims of prenatal testing?

Answer 8

• to inform and prepare parents for the birth of an affected baby
• to manage the remainder of the pregnancy
• to be prepared for complications at or after birth
• to allow termination of an affected foetus - this can be after 24 weeks
• to allow in utero treatment

Question 9

How does non-invasive prenatal diagnosis (NIPD) work?

Answer 9

• non-invasive prenatal diagnosis (NIPD) works by analysing DNA fragments present in the maternal plasma during pregnancy (cell-free DNA)
• most of this DNA comes from the mother - 10-20% comes from the placenta, which is representative of the baby (cffDNA)
• cffDNA first detectable from around 4-5 weeks gestation but cannot be accurately detected on testing until around 9 weeks
• maternal blood test of 20ml of her blood (large amount)
• achondroplasia, thanatophoric dysplasia and apert syndrome can be detected from cffDNA in this blood

Question 10

How can NIPD detect dominant and recessive disorders?

Answer 10

• detects autosomal dominant single gene disorders inherited from the father e.g. NF1
• NIPD is also possible to alter management of pregnancies at risk of recessive conditions when the mother and father carry different mutations in the same gene
• if the paternal mutation has been inherited by the foetus, invasive prenatal testing can be offered to look for maternal mutation - if baby not inherited paternal mutation then we know baby won’t be affected so further tests unnecessary
• e.g. for CF - haplotyping (RHDO) can test for both maternal and paternal mutation

Question 11

How is NIPD used for foetal sexing?

Answer 11

• currently offered when there is an X-linked condition in the family e.g. DMD
• the test detects SRY gene on Y chromosome, enabling us to determine if male or female foetus
• if male, we go on to prenatal test (as they more commonly get DMD)
• if female, then no invasive test required

Question 12

What is cffDNA testing for aneuploidy (NIPT)?

Answer 12

• offered after high-risk combined screen / privately (Harmony) / via research studies
• currently tests for trisomy 13, 18 & 21 which identifies 92%, 97% & 99% of foetuses with those trisomies respectively
• done using non-invasive prenatal test (NIPT) which is not diagnostic - invasive test recommended if issue identified

Question 13

What are the limitations of NIPD and NIPT?

Answer 13

• multiple pregnancies - not possible to tell which foetus the DNA is from when carrying twins etc
• women with high BMI - relative proportion of cffDNA is reduced as they have more cell-free DNA = may need a second sample after 9 weeks
• women not prepared for implications - despite only blood test, same implications as invasive test e.g. can identify genetic problem in baby - do they want this info?
• an invasive test may still be required to confirm an abnormal result

Question 14

What are the benefits of NIPD and NIPT?

Answer 14

• the number of invasive tests carried out is likely to reduce as a result
• no increased risk of miscarriage (unlike invasive)
• less expertise is required to perform a blood test than an invasive test
• in many cases we can offer NIPD/NIPT earlier than traditional invasive testing, thereby getting a result much earlier

Question 15

What is invasive testing?

Answer 15

• offered if there is a ‘known risk’
• two types: chorionic villus sampling (CVS) where placenta tissue is taken, and amniocentesis where fluid from around developing baby is taken
• molecular, cytogenetic and biochemical tests done on these samples and ultrasound guidance used with both
• offered on an outpatient basis - come in for appointment, do test and go home same day

Question 16

What is CVS?

Answer 16

• chorionic villus sampling
• done at 11-14 weeks
• transabdominal or transvaginal (usually former)
• takes sample of chorionic villi - part of developing placenta, same DNA as foetus
• allows patient to have an earlier result than amniocentesis - important for many patients e.g. for abortion decisions
• risks: 1-2% risk of miscarriage, infection, Resus (Rh) sensitisation

Question 17

What is amniocentesis?

Answer 17

• happens from 16 weeks
• takes sample of amniotic fluid which contains foetal cells
• risks: up to 1% risk of miscarriage, infection, Rh sensitisation

Question 18

What tests are done with the DNA sample?

Answer 18

• test for the genetic disorder in question - timing for results depends on condition
• karyotype if chromosomal abnormality in family - results in two weeks (dependent on cells growing)
• QF-PCR looking for t13, 18 & 21 (plus sex chromosomes if sex chromosome disorder suspected) - results within 24-48 hours

Question 19

What is a CGH array?

Answer 19

• if there are concerns on 20-week scan the gold standard is to offer CGH array
• looks for small/large imbalances in chromosomes (picks up microdeletions and duplications)
• if something found on array we standardly test parents to see if either is a carrier - can help with interpretation (change is inherited or new?)
• neuro-susceptibility loci (changes in chromosomes that can cause developmental delay and other features in some but not all with changes) e.g. 1q21.1 dup –> uncertainty regarding penetrance

Question 20

What is prenatal exome?

Answer 20

• consider where foetus has significant abnormalities e.g. heart, brain, skeletal, dysmorphic features = trio-exome testing done
• exome is coding region of genome - take DNA from foetus and parents to confirm where issue came from e.g. if baby has genetic condition and both parents found to be carriers
• urgent test - results approx 1 week
• allows efficient diagnosis of genetic condition in utero
• does not always give full answers if results too complicated to interpret / diagnosis isn’t made - important when counselling

Question 21

What options are there for family planning if there is known reproductive risk?

Answer 21

• conceive naturally, no prenatal testing
• conceive naturally, have prenatal testing
• use of egg and/or sperm donors - helps reduce risk e.g. if both parents are carriers then using a donor means only one parent is a carrier
• adoption
• choose not to have children
• pre-implantation genetic diagnosis (PGD) - uses IVF with an additional step to genetically test embryo before implantation, particularly used by people who do not want termination of pregnancy (TOP)

Question 22

What are the rules against egg and sperm donation?

Answer 22

• no longer anonymous, children conceived have the right to contact donor when 18
• best to go through a UK HFEA licensed fertility centre - conform to strict medical, ethical and legal standards
• can privately find own donor
• some couples may consider going abroad to find donor

Question 23

What is the process of pre-implantation genetic diagnosis?

Answer 23

1. stimulation of ovaries - women take medicine to produce many eggs
2. egg collection
3. insemination - using intracytoplasmic sperm injection (single sperm injected into centre of each egg, used for conditions caused by a single faulty gene to reduce amount of non-embryo DNA that could increase risk of wrong diagnosis)
4. fertilisation
5. embryo biopsy - at blastocyst stage
6. embryo testing - on one cell
7. embryo transfer - of those that have not inherited genetic condition
8. pregnancy test

Question 24

What disorders is PGD used to test for?

Answer 24

• translocation carriers
• Huntington’s disorder (HD)
• DMD - only implant female embryos where mutation in family is unknown
• CF

Question 25

What are the eligibility criteria for PGD?

Answer 25

- female partner under age 39 - female partner has a BMI of 19-30 - both partners are non-smokers - couple are living together in a stable relationship (longer than a year) - no living unaffected children from the relationship - known risk of having a child affected by a 'serious' genetic condition (at least 10%) - female partner has hormone levels that suggest she will respond to treatment - accurate genetic test available - no welfare concerns for unborn child - license required from the HFEA for each genetic condition / indication - eligible couples usually funded for three rounds of PGD

Question 26

What is the role of a genetic counsellor in prenatal testing?

Answer 26

- arrange & explain CVS, amniocentesis, PGD, cffDNA - facilitate decision-making - give results - see patients in clinic following a diagnosis - arrange termination if necessary - discuss recurrence risks and plans for future pregnancies

Question 27

What facilitates decision making?

Answer 27

- previous experience - family situation - religion - personal beliefs - psychosocial situation - balancing miscarriage risk with genetic risk - dealing with indecision - couples do not always agree