Prenatal Genetics Flashcards
What is a teratogen
Any substance, agent, or process that induces the formation of developmental abnormalities in a foetus
What are the 3 developmental periods
Preorganogenetic (conception – 2.5 weeks)
Active organogenesis (3-8 weeks)
Foetal period (>8 week)
Why do you need to consider the developmental period in relation to teratogens
The ability of a teratogen to cause malformations is dependent on the developmental stage of the embryo
The peak susceptibility to teratogens occurs 3-8 weeks post conception during which organ primordia are being formed (critical periods)
Some organs continue to develop after this period (notably the brain)
Through what extra-embryonic organ do teratogens need to pass to affect the foetus
The placenta
What are some examples of teratogens
Medicine - anticonvulsants
Drugs - alcohol
Infection - rubella, CMV
Medical - diabetes
Diet - folic acid/vit D deficiency
What consequences can anti-epileptic drugs have
Exposure in first trimester = higher risk
Behavioural and cognitive problems
Damage dependent on dose on how long it was taken throughout pregnancy
What is FACS
Foetal Anticonvulsant Syndrome
What is Foetal Anticonvulsant Syndrome
The term foetal anticonvulsant syndrome (FACS) is used when referring to children who have suffered adverse effects after being exposed to anti-epileptic drugs (AEDs) in utero
FACS encompasses major and minor congenital malformations, dysmorphic facial features, and learning or behavioural problems
What is the risk of malformation after exposure to antiepileptic drugs
2-3x greater
Risk increases with number and dose of anticonvulsant
What are the confounding variables associated with use of anti-epileptic drugs and pregnancy
Epilepsy and seizures - not good, which bring up comorbidities
Use of other teratogens
AED affects folic acid metabolism
AED users may be socially disadvantageous such as re. ability to work
Family history
IQ
What are the features of FACS
The anticonvulsant face (teratogenic face?), metopic ridge (suture in centre of forehead), ocular hypertelorism (wide spaced eyes), infraorbital grooves (under eye creases), depressed nasal bridge, long smooth philtrum, thin upper lip
Other features; nail hypoplasia, digital anomalies, developmental delay (almost invariably mild), behavioural problems
What is the relationship between AED’s and IQ
Recent study suggests VPA (sodium valproate) has a general effect on IQ
Malformation might not influence developmental delay
What malformations are increased due to exposure to AED’s
Cleft lip, malformations of the ear/neck/face, and spina bifida (nearly 15x more likely after exposure to AEDs)
Defects in neural tube, heart, limbs, genitourinary system and skin (VPA)
May rarely affect brain, eye, respiratory tract and abdominal wall (VPA)
Why is VPA dangerous of various AED drugs
It shows highest rate of malformation
Why can’t pregnant women simply stop taking AED’s
Having a seizure can be fatal
What are the mechanisms causing birth defects due to anticonvulsants/AED’s
Reduction of folic acid
Oxidative stress
Inhibition of histone deacetylase
Altered lipid metabolism
What are other teratogens
Warfarin Retinoic acid Tetracycline Thalidomide Carbimazole
Why is Warfarin a teratogen
Causes phenotype of nasal hypoplasia with stippling of epiphyses
Similar to phenotype seen with Vit K deficiency and chondrodysplasia punctata
Mechanism therefore likely to be through warfarin’s therapeutic mechanism – vitamin K dependent post-translational modification of various proteins
How is warfarin involved in vitamin K
ANS
What is foetal alcohol syndrome
Pre and postnatal growth retardation, characteristic face (cf FACs face), microcephaly, congenital heart disease, developmental delay, behavioural difficulties – autistic spectrum, food aversion
What effects can smaller quantities of alcohol cause
Alcohol related neurodevelopmental delay
What is rubella
Classical triad of cataracts, cardiac malformation and deafness - skin rash may be present at birth
What are the risks of malformation associated with rubella
Risk of malformation related to timing of exposure
First trimester – foetal loss or severely affected including neurodevelopmental problems (>80%)
Second or third trimester – variable outcome but possibility of hearing loss
What is cytomegalovirus
More common cause of intrauterine infection, mild illness – woman may not be aware of infection
In 80% will be no effect on developing foetus
What are the malformations associated with cytomegalovirus
May result in growth retardation, hearing loss, retinal pigmentation – visual problems, microcephaly/ventricular dilatation, brain calcification (as a consequence of inflammation)
More common as cause of delay than Down syndrome
Why can diabetes be a teratogen
Poorly controlled diabetes associated with increased risk of wide range of birth defects
Prevalence of pre-gestational diabetes in mothers of babies with birth defects is significantly increased (4 fold)
Some evidence that gestational diabetes also increases risk of birth defects
What birth defects can arise out of diabetes
Macrosomia
Cardiac defects – isomerism (flipped heart??)Neural tube defects – spina bifida, anencephaly
Cleft lip/palate
Limb defects/sacral agenesis
Renal abnormalities
What is the use of prenatal diagnosis
Used for prenatal and postnatal treatment
Preparation for delivery
Prognosis
Termination of pregnancy
Because people want to know
What are the challenges/drawbacks regarding prenatal diagnosis
Mostly no family history
US can only detect some abnormalities and phenotypes less specific than postnatally
Some problems only detectable late in pregnancy
DNA diagnosis can be very slow
Time is limited
Very stressful for parents
What are prenatal screening options
Before pregnancy - unusual, screen for recessive and X-linked disease
During pregnancy - blood grouping, HB electrophoresis, hepatitis AIDS, syphilis
US
Combined test
NIPT
Why is maternal blood testing important
To avoid rhesus disease and identify beta globin variants
What is rhesus disease
This occurs when a Rh-ve mother has a Rh+ve child
The affects occur for the second Rh+ve child where the maternal antibodies attac the RH+ve cells
Treated with anti-D immunoglobulin
How is rhesus disease treated
During pregnancy mother is treated with anti-D immunoglobulin
What does Hb electrophoresis identify
Identifies β globin variants - β-thalassaemia, sickle cell
Does not identify α globin variants which are diagnosed by microcytic picture in absence of anaemia
What is ultrasound used for
Foetal anomaly scan
Screening scan
What are the drawbacks of ultrasound
Gestation dependent
Difficulty with resolution
Polydactyly at 12 weeks
Structure not developed enough to identify
Cerebellar vermis
Can only identify by secondary abnormalities - e.g. bladder not visualised despite frequent urination = absent kidneys
What is the combined test
11 weeks + 3 days and 13 weeks + 5 days = NT + PAPP-A + βHCG + maternal age
NT = nuchal translucency
PAPP-A + βHCG = placental hormones
Taking into account maternal age
What is the significance of nuchal translucency
Indicates risk of trisomy 13,18,21, monosomy X, triploidy
Congenital heart disease
Needs trained individuals
What are the false negative rates for nuchal translucency
16% false negatives, 2.2% false positives
What are the placental hormones measured
PAPP-A + βHCG
What is the significance of placental hormones
Different ratios indicate different ratio’s - results expressed as MoM (multiple of the mean)
Down’s syndrome - less PAPP-A , increased βHCG
Patau’s and Edward’s - less of both PAPP-A + βHCG
What is the quadruple test
16-18 week test
AFP, βHCG uE3 (oestrogen),Inhibin A
AFP = initially noted to be high in babies with spina bifida
These babies also were noted to have a lemon shaped skull
Babies with Down’s syndrome have low AFP
Describe the non invasive prenatal testing
Free placental DNA in maternal circulation. (ffDNA)
Found in plasma fraction of blood
Majority of free DNA is of maternal origin (fmDNA)
↑ ffDNA with ↑ gestation (fetal fraction)
ffDNA shorter fragments than fmDNA
What are the uses of ffDNA
Foetal sexing - when child at risk of an X-linked disease e.g. DMD
If there is a Y chromosome in mothers blood it shows it is the ffDNA
X-linked disorders mainly affect males
Single gene disorders
Trisomy screening
Rhesus disease
High risk pregnancies - placental disease
How is NIPT used to diagnose single gene disorders
De Novo Mutations - suitable only if there are few genes associated with disease
Foetal sexing in X –Linked disorders
Dominant disorders with father affected
Recessive compound heterozygote mutations
What are the drawbacks of NIPT
High demand for Downs screening
Result for Down’s needs to be confirmed by invasive test
Women would like it for many other indications BUT not total reliable
DNA is of placental origin - what about placental mosaicism?
Foetal fraction (amount of ffDNA) may be low
If very high NT despite normal NIPT you still require CVS if normal as high risk of other abnormalities
False +ve with mother suffering from cancer
What is amniocentesis
When the fluid surrounding the baby i samples after 6 weeks
What is chorionic villus sampling
Normally a transabdominal biopsy of the future placenta
What are the complications associated with CVS
Placental mosaicism = not representative
Trismic rescue to prevent trisomy 16 may occur in the foetus, but trisomy still seen in placental
What are the two chromosome analysis methods
QFPCR - trisomy 13,18,21
Karyotype - trisomy, triplication and robertsonian translocation
What is QFPCR in regards to prenatal diagnosis
Looks at polymorphic markers from the three trisomy related chromosomes
What are the potential results of qfPCR in trisomy investigation
The markers are seen as peaks
2 peaks = 2 alleles
If they are equal heights = normal biallelic
If one is larger = trsimic, biallelic
3 peaks = 3 alleles = trismic triallelic
One marker = uninformative
What is trismic rescue
The cell kicks out a copy of an extra chromosome - but this means that you could inadvertently result in both pairs of chromosomes being from a single parent
If trismic rescue results in loss of bi-parental inheritance for a chromosome, what are the consequences
If arised from meiotic I error the duplicates are different = heterodoxy
If arisen from meiotic II error = same = isodisomy = can lead to AR disease
Also it may lead to imprinting disorders e.g. if it occurs in Chr 11 or 15
What are the key maternal and paternal genes involved in Beckwith-Wiedemann Syndrome
Maternal alleles make H19 & CDKN1C
Paternal alleles make IGF2
How can you test for single gene disorders prenatally
Known family history = CVS/amniocentesis, testing is more accurate
New diagnosis -
Is there only one possible mutation or gene - single gene testing e.g. achondroplasia, cystic fibrosis
Is it in a specific ethnic group
Can you limit to gene panel - tuberous sclerosis
Unknown/large number of possibilities - exome/larger panels
What can be treated prenatally
Maternal drug Rx:
Fetal tachycardia
Fetal thyroid disease
Fetal Blood transfusion
Maternal IVIg :
Prevent platelet antibodies
Neonatal haemochromatosis
Fetal Myasthenia Gravis
What are three conditions where you can offer prenatal surgical treatment
Twin –Twin transfusion
Diaphragmatic hernia
Spina bifida
What delivery considerations need to be made
Delivery in a tertiary centre - congenital heart disease, exomphalos, diaphragmatic hernia
Mode of delivery - casaerean - hydrocephalus, achondroplasia, overgrowth syndromes
Prepare for postnatal complications and mentally prepare parents e.g. show pictures of cleft lip
Criteria for prenatal diagnosis for termination of pregnancy
Serious disease without effective treatment
Accurate SAFE test
Abortion is acceptable to the couple
<24 weeks legal for psychological/physical reasons
> 24 weeks only when there is significant risk of abnormality
Ethics
The status of handicapped individuals
The status of the foetus
Religious views
Cultural views
Personal views
What factors can increase risk of foetal abnormalities
Maternal age - increased risk of trisomy
Paternal age - certain single gene disorders
Drugs/medication
Family history/consanguinity
What is the difference between preimplantation genetic diagnosis and screening
Diagnosis - specific abnormality testing for severe genetic disorders
Screening - looking for a wide range of major disorders, used to increase IVF success rate
What is non invaive prenatal testing
Testing of ffDNA/cf foetal DNA from the maternal blood
Where do foetal cells and cell free DNA originate from
The placenta/trophoblast
When can you start and then no longer detect ffDNA
From 5 weeks until birth
Can you differentiate between foetal and maternal cfDNA
No
Maternal DNA is at higher quantities so you compare chromosome numbers to the majority - if there’s an outlier e.g. trisomy, and the mother has no symptoms it may be the foetus’s
What is the T21 detection rate using NIPT
- 2% detection rate
0. 09% false positive rate
Why use NIPT
Prevents unnecessary invasive tests
Makes choice for women easier whether they should take an invasive test or not
What is the foetal fraction
Amount of foetal DNA present in maternal blood
Why is it important to measure foetal fraction
False results can appear if this is not measured - needs to be above 6%
How to correct for foetal fraction
Consider it alongside maternal age
Is ffDNA considered alone
No, it is used alongside maternal age and results from the combined test
What may impact foetal fraction
Maternal height and weight, twin pregnancy
What technology is used for NIPT
NGS
What are the potential results of NIPT
Low risk = no further action
High risk (>95% risk for Down’s) = CVS/Amnio
Test failure - no call result
Logistics - posting, DNA loss, tube cracking thus DNS integrity lost
Low foetal fraction
High cell turnover
How can results be accessed
Portal for easy and secure exchange of patient results
Eliminate sample mixing
Sample tracking
Can find if there is a lost sample if tracking looks odd
Each lab has access to the portal to manage their own clinic users
Immediate update of the portal
Queries and documentation
What are the advantages of NIPT
Pragmatic and effective - detects majority of trisomy’s
NIPT can be done anywhere as it is simply sent to the lab
Safe and sensitive - not at all like having a needle inserted into the uterus
Cheaper than CVS/amnio
Results within 3-5 days
Keeps money in the NHS
What are the considerations regarding foetal sex determination
Sex determination is different from trisomy detection as this is mostly not medical in nature
Having this option widely at various stages gives rise for the option for population sex selection
In what situations may foetal sex determination be undertaken
X-linked disorder screening
Foetal sex aneuploidy screening - Turner’s syndrome
However, false positive rate is double that of other aneuploidies
What is foetal hydrops
Accumulation of fluid in 2 or more extra-vascular spaces
What are some of the causes of foetal hydrops
CVS Idiopathic - unknown Lymphatic Haematological - anaemia Chromosomal/syndromic Infection Twin-twin transfusion syndrome Thoracic leisons Inborn errors of metabolism Urinary tract malformations
What test can you use to identify FH by aetiology
Ultrasound
What are the benefits to identifying cause of foetal hydrops
Genetic cause - information and advice
Future patients - better know how to diagnose and prognosticate, determine risk for future children/family members and better management
How is data collated to improve investigation of foetal hydrops
Multi-centre data collection alongside 100K genomes project and exome data
What is the FOLD study
Investigation of foetal oedema (hydrops) and lymphatic disorders
