Genetic Testing Flashcards
What happens in a normal pregnancy
Positive pregnancy test ( no longer confirmed at GP)
Book into antenatal care ( see midwife) at hospital where you will deliver
Nuchal scan - 10-14 weeks gestation Different tests dependent upon NHS Trust e.g. nuchal translucency, combined test etc.
Mid-trimester anomaly scan
Ultrasound examination is the main method for prenatal diagnosis of foetal abnormalities. All pregnant women should be offered routine ultrasound scans at 11-14 weeks and again at 20-22 weeks.
What also happens at the nuchal scan to identify risk
At nuchal scan, bloods are done to check hormone levels, maternal age is taken into account , sonographer looks at nuchal fold which is the back of the babys neck
Come up from a risk figure.
Risk of 1 in 150 or higher is considered high risk
Aims of 12 weeks scan
To date the pregnancy accurately.
To diagnose multiple pregnancy. To diagnose major foetal abnormalities. To diagnose early miscarriage. To assess the risks of Down Syndrome and other chromosomal abnormalities. Taking into account the maternal age, blood hormone levels, nuchal translucency thickness*, nasal bone, blood flow through the fetal heart and fetal abnormalities.
If there is an increased thickness at the back of the feral neck during the nuchal scan ( >3mm) this can indicate :
Chromosome abnormalities (e.g. Downs, Edwards, Patau, Turners) NT + maternal age detects up to 75% of Down syndrome with 5% false positive rate Birth defects: Cardiac anomalies Pulmonary defects (diaphragmatic hernia) Renal defects Abdominal wall defects Skeletal dysplasias
When is prenatal testing arranged
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 e.g. DS, CF If parent(s) carrier of chromosome rearrangement/abnormality or genetic condition, e.g. t(13;14), DMD, HD. Family history of genetic condition
Aims of prenatal testing
To inform and prepare parents for the birth of an affected baby/To be prepared for complications at or after birth
To allow in utero treatment Manage the remainder of the pregnancy To allow termination of an affected fetus
What other prenatal scans can be carried out in addition to nuchal scan
Feral MRI and cardiac scan ( usually at around 20 weeks)
Non invasive prenatal tests
Maternal blood test and Cell free feral DNA
Invasive prenatal tests
Chorionic villus sampling ( CVS) - takes a bit of tissue from placenta
Amniocentesis - takes fluid from around the developing baby
Cell free fetal DNA ( cffDNA)
Non-invasive prenatal diagnosis (NIPD) works by analysing the DNA fragments present in the maternal plasma during pregnancy (cell-free DNA).
Most of this DNA comes from the mother
10%-20% of it comes from the placenta, which is representative of the unborn baby (cell-free fetal DNA).
Cell-free fetal DNA (cffDNA) is first detectable from about 4 -5 weeks’ gestation, but cannot accurately be detected on testing until around 9 weeks
Why does cffDNA work
DNA from baby is shed from the placenta into mothers blood and so that is why a very small portion is found in the plasma of the mother
Eg. In trisomy 21 the amount of cfDNA for chromosome 21 is higher than in normal pregnancies
NIPD maternal blood test for which diseases
- Achondroplasia - testing is free
- Thanatophoric dysplasia ( can often be seen due to shorter limbs ) - testing is free
- Apert syndrome- testing is free
Find out the sex of the baby
Currently offered when there is a X-linked condition in the family e.g. DMD
Test detects SRY gene on Y chromosome, enabling us to determine if male or female fetus
If male -go on to prenatal test
If female - no invasive test required
NIPD offered privately visa NHS
Autosomal dominant single gene disorders inherited from the father or arisede novo
NF1
NIPD is also possible to alter management of pregnancies at risk of recessive conditions when the mother and father carry different altered genes.
- If the paternal alterationhas been inherited by the foetus, invasive prenatal testing can be offered to find the maternal gene - Cystic fibrosis – haplotyping (RHDO) can test for both maternal and paternal mutation
cffDNA testing for Aneuploidy ( NIPT)
Offered privately (Harmony) or via research studies
Harmony currently test for T13, T18, T21 and this identifies:
99% of fetuses with trisomy 21
97% of fetuses with trisomy 18
92% of fetuses with trisomy 13.
But not 100% accurate . May then need an invasive test to confirm
Limitations of NIPT and NIPD
Multiple pregnancies - It is not possible to tell which fetus the DNA is from when carrying twins/triplets etc. - so only used with singleton pregnancies
The relative proportion of cell-free fetal DNA is reduced in women with a high BMI as they have more of their own cell-free DNA. Although it is just a blood test, it has the same implications as an invasive test. Women may prepare themselves more for the implications of an invasive test result Women must consider the consequences of the results. Do they want this information? An invasive test may still be required to confirm an abnormal result.
Benefits of NIPT and NIPD
The number of invasive tests carried out is likely to reduce as a result
There is no increased risk of miscarriage. 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.
CVS
11-14 weeks
1-2% risk of miscarriage (lower in specialist centres e.g. UCH, Chelsea and Westminster appx 1 in ) - risk is higher because the foetus is smaller Transabdominal or transvaginal Takes sample of chorionic villi – part of developing placenta – same DNA as fetus Allows patient to have an earlier result than amnio - important for many patients re. TOP decision (termination of pregnancy)
Amniocentisis
From 16 weeks
Takes sample of amniotic fluid which contains fetal cell
Risks
Up to 1% risk of miscarriage
Infection Rh sensitisation
What test are done with the DNA sample
Test for the genetic disorder in question
Timing for results dependent upon condition
Karyotype if chromosomal abnormality in family
Results 2 weeks (dependent upon the cells growing)
QF-PCR for all:
We send samples to Guy’s Hospital laboratory
Looks for t13, 18 & 21 (plus sex chromosomes if sex chromosome disorder suspected)
Result within 24-48 hours
CGH array - comparative genomics hybridisation
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. This can help with interpretation
Neurosusceptibility loci: 1q21.1 dup, 16.11.2 dup, 15q11.2 del etc etc. Uncertainty regarding penetrance. Be wary about discussing PND/PGD
What is the process of comparative genomics hybridisation
1) extract genomic DNA from a test and a reference sample and label one with a red fluorescent dye and the other one with a green fluorescent dye
2) mix and hybridise to a microarray printed with thousands of oligonucleotide probes then wash
3) detect red and green signals using a fluorescence scanner
4) compute and report gains or losses in the test DNA using software
Trio exonerated sequence
Consider where fetus in previous pregnancy had significant anomalies e.g. heart, brain, skeletal or where baby has been born with developmental delay, dysmorphic features (and array normal)
Exome is the coding region of the genome. Take DNA from fetus/baby and parents
Trio refers to the parents and the baby
Good pick up (40%+) where referrals are appropriate
Started with Deciphering Developmental Disorders (DDD) study
Reproductive options when there is a known reproductive risk
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 (PGD)
