S4: Pre-symptomatic and prenatal diagnosis

Question 1

What would prompt a referral of a pregnant lady to genetics service?

Answer 1

Prenatally women may present saying things such as they have had an abnormal screening test in the pregnancy, they are pregnant and have a family history of a genetic disorder or they have had recurrent miscarriages.

Question 2

What would the clinician at the genetic services ask about to pregnant lady?

Answer 2

• Metabolic disorders
• Cystic fibrosis, Huntington’s, Hypercholesterolemia.
• If there is consanguinity in the family, as this increases risk of recessive disease.
• Any other genetic disorder that runs in the family.

Question 3

Describe routine screening tests in UK that will be done during pregnancy

Answer 3

• Haemoglobin electrophoresis in all women are done as well as their partners if from high risk ethnic origin. This is to rule out sickle cell disease or beta-thalassemia.
• Prenatal diagnosis/screening is testing for diseases or conditions in a foetus/embryo before it is born. The aim is to detect birth defects e.g. neural tube defects and genetic disorders. It may also be used to determine the sex of the baby.
• Ultrasound scans are also standard screening in pregnancy. All women regardless of their age have a risk of having a child with physical or cognitive disability. Nuchal translucency at 11-13 weeks and detailed anomaly scan at 20-23 weeks looking at heart and kidneys etc. 12 week early dating ultrasound scan. - 28 weeks foetal MRI.

Question 4

Describe the nuchal translucency test during a pregnant women’s ultrasound scan

Answer 4

Nuchal translucency is done at about 11-13 weeks and looks at the back of the neck of the foetus where there is a collection of fluid. This is at the end of the first trimester. The thickness of the nuchal is correlated to genetic abnormalities and problems during pregnancy, thus it is used to estimate the risk of abnormality esp. Down’s, Turners, chromosomal abnormalities, congenital heart diseases.

• The chance of there being a disorder increases with increasing nuchal thickness. The upper limit of being normal is less than 3.5mm.
• The thicker it is the increased risk the foetus has a chromosomal abnormality and/or an adverse outcome.

Question 5

What further investigations done if nuchal is thicker than 3.5 mm?

Answer 5

If the nuchal is thicker than 3.5mm a foetal echo will be done to investigate further at the 12 week scan.
A combined risk score is calculated that includes
- Nuchal (mm).
- PAPP-A levels (used to screen for Down’s when low and this substance is produced by placenta).
- Beta hCG (pregnancy hormone).
- Maternal age.
The combined risk score includes a background risk if we include age only, it can give us an adjusted risk when we include the nuchal, nasal bone, PAPP-A and beta hCG. Used to calculate the risk of Trisomy 21, 13 and 18. If the adjusted risk for an abnormality is less than 1 in 150, an NIPT (non-invasive pre-natal testing) will be done or an invasive test.

Question 6

Describe Early dating ultrasound (12 week scan)

Answer 6

• To see how far along the pregnancy is.
• Importance of scan is to check multiple pregnancy and to see if the baby is viable,
• Foetal heart seen at about 6 weeks.
• Gestational age can be accurately assessed.
• Detection of multiple pregnancy.

Question 7

Describe Ultrasound at 16 -20 Weeks

Answer 7

If the pregnancy is progressing normally an ultrasound will be done at 16-20 weeks. By now the baby is larger so a more detailed scan is possible to look at various structures to see if they are developing normally or if there are abnormalities.
Common things detected at this stage:
- Problems with kidneys.
- Uncommon facial profile.
- Foetal sexing is possible at this stage but not 100% reliable.
- Also congenital heart disease can be detected, the structures of the brain (e.g. corpus callosum and ventricles) can be seen, bone length analysed, a renal scan and look at abdominal structures.
There might be false positives or false negatives in the scan (this is inevitable).

Question 8

What are the two main types of ultrasound abnormality?

Answer 8

• Isolated structural abnormalities are where there is only one specific body structure that is abnormal e.g. enlarged heart ventricles (ventriculomegally) need to be followed up and managed possibly by other specialists. The prognosis is also found out.
• Multiple structural abnormalities very often tend to be caused by genetics (chromosomal in origin) or due to environmental teratogens e.g. thalidomide, warfarin, alcohol. Infections can also cause multiple abnormalities like Zica and TORCH. A set of structural abnormalities become a syndrome E.g. Tuners syndrome , female, raised nuchal, L sided heart defect, renal defect.

Question 9

Describe 28 weeks foetal MRI

Answer 9

From 28 weeks a foetal MRI is available and this is the most detailed imaging that can be done in pregnancy. Note however the cut-off point for termination is 24wks unless there are severe abnormalities. Also offered if brain abnormality is suggested in 20 week scan so MRI offered to mother to check baby in the womb.

Question 10

Steps for prenatal diagnosis of known disorder

Answer 10

• Determine if the disorder is in a single gene or chromosome. This is because we need different technologies to pick these things up.
• With known disorders time is of the essence as it needs to be picked up earlier. The dating scan can be done from 6 weeks (but more accurate at 8-9 weeks), CVS can be done 11-14 weeks. Amniocentesis 15-17 weeks.
• Relationship to affected person with genetic disease
  and family tree and calculated risk using probability.
• Mode of inheritance: Previous child/pregnancy with ‘sporadic’ genetic disorder is low risk (but not zero so still refer) as de novo condition. This is due to germ line mosaicism as a few of sperm in father and eggs in mother may carry the same genetic change that can affect the child - this cannot be quantified or measured. So usually a generic risk is given such as 1-2%.
• An early dating ultrasound can be done. Whether the genetic condition shows up on the early scans e.g. antenatal depends on the condition. Some conditions will be identifiable on scans early on in gestation, whereas others may not be seen until much later or even after birth.
• If there is a known mutation that runs in the family, we can do the molecular prenatal diagnosis but this can only be done once we are able to get hold of the foetal DNA and there are risks.
• Potentially there are other options to diagnose antenatally, we can do biochemical diagnosis of metabolic conditions e.g. by analysing enzymes in the amniotic fluid.

Question 11

Describe single gene triplet repeat expansion disorders

Answer 11

• E.g. Fragile X syndrome, Huntington syndrome, myotonic dystrophy.
• Size of repeat varies from normal - intermediate-affected.
• Unstable and may expand when transmitted from particular parent. This means the expansion does not stay the same size when transmitted.
• May lead to genetic ‘anticipation’ within families. Earlier age of onset with increased number of repeats.

Question 12

What causes fragile x syndrome?

Answer 12

• Fragile x affects men, >200 repeats in FMR1 (fragile part of x chromosome).
• Women usually carry and risk passing on to their sons (pre mutation carriers).

Question 13

What causes Huntington’s Chorea?

Answer 13

• AD inheritance.
• (CAG)n repeat (intron 1 of Huntington gene on chromosome 4).
• Neurodegeneration, chorea (involuntary), psychiatric manifestations .
• Normal < 30 repeats.
• Affected > 39 repeats.
• Particularly large expansions when paternally inherited (there are male to male transmission).
• Many people choose not to know as no cure for this condition.

Question 14

What causes Myotonic dystrophy (type 1)?

Answer 14

• Myotonic dystrophy is part of a group of inherited disorders called muscular dystrophy. There are two types of myotonic dystrophy.
• AD inheritance.
• (CTG)n repeat in DMPK gene.
• Myotonia (cant release a grip), diabetes, cataracts, facial weakness, cardiac conduction problems, sensitive to anaesthetics (malignant hypothermia).
• Normal 4-37 repeats.
• Affected individuals have >50 repeats.
• Expansion increased particularly if maternally inherited. Risk of congenital myotonia (floppy baby).

Question 15

What are recurrent miscarriages?

Answer 15

• Individual or couple with X 3 miscarriages and occurs in 1% couples.
• Array CGH first line tested on products of conception to see any chromosomal loss or gains and balanced vs. unbalanced translocations .
• Will be done on very early zygote.

Question 16

Describe Non-Invasive Pre-natal Testing (NIPT)

Answer 16

• NIPT involves obtaining short fragments of free fetal DNA from the maternal circulation and testing these. Thus the test is non-invasive as just involves taking blood from the mother.
• Looking at single nucleotide polymorphism are used to distinguish the maternal and foetal DNA.
• The free-fetal DNA can be used for fetal sexing and this can be helpful for diagnosis of X-linked conditions e.g. Duchenne if they appear to run in the family This fetal sexing is available from 9 weeks onwards.
• We can also do fetal chromosome analysis to diagnose aneuploidy e.g. trisomy 21, 18, 13. We do this by QF-PCR of the DNA picked up in the NIPT.
  We work it out by looking at the ratio between chromosomes. If chromosome 21 appears more often than we see other chromosomes appearing we will see the ratio will be unequal between chromosome 21 and other chromosomes.
• A positive predictive value is a percentage that tells us how many of individuals who test positive for the condition actually have the condition. For example for Down’s a PPV of 33% means that of all the individuals that test positive for the fetus having Down’s, only 33% will actually have Down’s.
• Thus these risks do not tell us a yes or no answer, only the level of risk that a child will have a chromosomal abnormality.Therefore more invasive tests will be required if risk is high.

Question 17

Describe Non invasive prenatal diagnosis (NIPD)

Answer 17

• Dominant (paternally inherited).
• De novo.
• Skeletal dysplasias
  • Achondroplasia
  • Thanatophoric dysplasia
• Craniosynostosis
  FGFR3 related e.g. Apert, crouzon.

Question 18

List types invasive prenatal tests

Answer 18

We can also use invasive tests in prenatal diagnosis of chromosomal abnormalities or infections of the foetus.

• CVS.
• Amniocentesis.

Question 19

Describe Chorionic villous sampling (CVS)

Answer 19

• Chorionic villous sampling (CVS) and is available from 11 weeks and is where a sample of chorionic villi are removed from the placenta. This can be done through abdominal wall or through the vagina/cervix (now almost aleays through transabdominal).
• CVS has a miscarriage rate of about 0.5-1% and there is maternal contamination during the procedure, the latter meaning that we get some of the mothers tissue that contaiminates the foetal DNA. There can also be placental mosaicism, where the placental cellular make up has slightly different DNA than the foetus.
• As a result of this procedure there is also a risk of transverse limb defects.
  Although there are these risks, it is better than abnormalities are picked up early and thus termination can be arranged earlier if wanted.

Question 20

Describe Amniocentesis

Answer 20

• Amniocentesis is available from 16 weeks. Amniocentesis involves extraction of amniotic fluid from the amniotic sac as it contains some foetal tissue.
  There is a lower risk of miscarriage and most tests will be an accurate reflection of the fetus’s genetics. As well as allowing analysis of the baby’s chromosomes it can also be used for other biochemical tests.

Question 21

Describe foetal chromosomal analysis

Answer 21

Array CGH is a common method of foetal chromosomal analysis, that could be done early on after some scans e.g. nuchal. A karyotype could also be done, this is useful for visualising where a big translocation or deletion has been. QF-PCR also useful for foetal chromosomal analysis.

Question 22

What are foetal single gene tests?

Answer 22

• These are tests where we are looking at a single gene to see if it is abnormal, this will be done when there is a known genetic condition that runs in the family e.g. Huntington’s, Sickle cell
• Any couple that already has an affected child with germline mosiacism (i.e. parents don’t have it, as mutated gametes the cause) are at risk. Therefore testing just the parents wouldn’t be as helpful
• Should also be done with autosomal recessive disorders and X-linked disorders e.g. Fragile X.

Question 23

Describe Preimplantation Genetic Diagnosis (PGD)

Answer 23

• PGD is a type of genetic profiling of embryos and is used to ensure that parents who have a genetic condition in their family do not pass it on to their offspring.
  The embryo is made by IVF, then at the 6-8 cell stage, a single cell is taken and screened for the known genetic condition e.g. Huntington’s, Adrenoleukodystrophy.
  In this way we look for the embryo that doesn’t have the mutated gene and this is the one we can implant so the child doesn’t have this condition.
• More controversial is genetic screening for BRCA1 and hearing loss (some families with hearing loss want to have a child with hearing loss also).
• PGD is offered on the NHS but there are certain criteria (e.g. haven’t had a previous child), you can be referred.

Question 24

Describe prenatal exclusion testing

Answer 24

• Parent at 50% risk but does not want to know their status and does not want an affected child.
• Grandparental haplotype.
• Foetus with high risk haplotype at 50% risk of being unaffected.

Question 25

Ethical and Legal Implications of prenatal diagnosis

Answer 25

• Firstly the timing of the diagnosis is very important, ideally it should be as early as possible. This is because diagnosis of a serious condition before 24 weeks generally means a less traumatic termination and easier legally speaking.
  Post-24 weeks termination is much more difficult to legally allow and will only be allowed if there is significant risk of the child having a severe medical condition. It is also more traumatic for the mother.
• Another ethical question is what should we offer pre-symptomatic diagnosis for adult onset conditions and hearing loss?
• There are also cultural and religious sensitivities to pre-natal diagnosis and termination.
• The technology is available of fetal genome sequencing, but ethically is this right?

Question 26

Describe Presymptomatic genetic testing

Answer 26

• Pre-symptomatic testing/predictive is genetic testing before a person develops any symptoms of disease. This may be seen when an individual knows a family member developed a certain condition and they want to know if they will develop it.
• Pre-symptomatic testing often just gives us a likelihood of developing the disease as penetrance can vary. Also pre-symptomatic testing often does not indicate the disease severity (expressivity) or the age of onset (that can vary due to anticipation for example).
• Not routinely offered to children for adult onset conditions

Question 27

Ethical and legal implications of presymptomatic genetic testing

Answer 27

• Variants of uncertain significance (i.e. differences that we don’t know if they will cause anything, these are difficult to manage).
• Incidental finding (e.g. by chance when doing an array may find individual has other things, e.g. cancer gene).
• Non-paternity.
• Monozygotic twins (identical genetics, so does one want to know and not the other?).
• Insurance (e.g. may make it more expensive.
• Relationships.