Lecture 18 Flashcards
Prenatal Screening and Diagnosis
Screening(low risk testing) for and diagnosis of genetic and congenital disorders(previous child with goncgenital disorder/chromosomal abnormality) (or high risk group) in established pregnancies (single jean disroder running through familt)
Guides family in decisions about the pregnancy
-might involve termination of pregnancy
-facilitate medical and psychological support
Majority of testing normal, provides reassurance
38year old female, 8 weeks pregnant, concerned about risk of having children with down symdrome
increasing maternal ages past 35, significant increase in risk of down syndrome
What advice should be given to this patient? -increasing age, increasing risk 0.8-1% chance
What strategies can be used to predict risk?
What prenatal screening or diagnostic procedures are available? Non-invasive screening tests(ultrasound), maternal serum testing. Can have more directive diagnositic test
Should these procedures be offered?
A couple have a son with severe haemophilia (an X chromosomes linked clotting disorder). They wish to have another child, but would not consider this if there was a risk of a second haemophiliac child
Single gene disorder
Haemophilia: coagulation disorder, deficiency in clotting factor 8
X chromosomes so does effect both females and males
mother carrier
What options are available to this family?
Risks: male 50% risk
female: not affected. but 50% chance of being a carrier
CV/amniocentesis –> test DNA of fetal cells –> potentially PGD (mutation dependant and timing)
Prenatal testing
Screening tests:
-assess risk but no definitive diagnostic test e.g. maternal serum testing for down syndrome
-non-invasive, low risk tests
-commonly used in pregnancies
Diagnostic tests:
-where test for a specific chromosomal or genetic abnormality e.g. gene mutation analysis for cystic fibrosis
-invasive test, carriers some risk(fetus)
-commonly have fetal cells/DNa, look specifically at gene
Clinical indications for Prenatal Testing: Single gene disorders
Single Gene disorders:
-Family history (autosomal or X-linked)
-both parents carrier e.g. cystic fibrosis
-female carrier e.g. haemophilia (X-linked)
-one parent has an autosomal dominant disorder
Only consider if serious genetic disorder- cystic fibrosis/severe haemophilia (e.g. not haematomacrosis autosomal recessive disorder of iron over efficiency 1/10 caucasian carriers. Very treatable non life threatening disorder if recognised early and treated correctly)
Clinical indications for Prenatal Testing: Considered at risk for constitutional/congenital chromosomal disorder
Considered at risk for constitutional/congenital chromosomal disorder
- older mother pregnant (most common)
- previous child with significant developmental or other disorder
- exposure to chemical or toxic agent
- abnormal screening test result
Screening occurs when either:
- Family history of single gene disorder
2. High suspicion of a significant chromosomal disorder
Screening strategies: Ultrasound scan of pregnancy
Ultrasounds scan of pregnancy
- at 8 weeks to check dates, confirm number of babies
- at 18 weeks detailed scan for abnormalies can be done e.g. anencephaly, cardiac abnormalities
- safe procedure- no risk to mother or fetus
Screening strategies: Nuchal Translucency
(chromosal abnormalities/high risk mothers)
-thickness of neck
Nuchal Translucency
-measure by ultra sound 10-13 weeks pregnancy
-non-invasive test
-combine with maternal serum testing to increase sensitivity and specificity for Down syndrome screening (increased nuchal translucency in trisomy 21)
-combine with maternal age and blood test to give risk prediction
Screening strategies: Maternal Serum Testing
Maternal Serum Testing
- incongunction with maternal age and ultrasound
- performed at 15-17 weeks
- test for 2-4 feto-placental proteins e.g. AFP, Beta-hCG, unconjugated estriol (uE3), Inhibin A and pregnancy associated plasma protein A (PAPP-A)
Risk prediction for down syndrome
Screening in the second trimester by maternal age and maternal serum testing can identify 50-75% of trisomy 21 pregnancies with a false positive of 5%
Screening for fetal NT in the first trimester and maternal serum biochemistry in the second trimester reported a detection rate of trisomy 21 of 85-90% with false positive rate of 5%
–> May lead to specific diagnostic test e.g. amniocentesis and karyotype analysis (look at fetal cells, and do FISH of karyotype analysis to confirm if pregnancy is affected by downsyndrome or not)
(screening test –> leads to more definitive diagnostic test)
Single gene disorders
Risk prediction from analysis of pedigree
May proceed to specific diagnostic test if requested by the family
(cystic fibrosis)-autosomal recessive
Diagnositic Tests
Chorionic Villus Sampling
Amniocentesis
-Obtain fetal cells for further analysis (e.g. karyotype, FISH studies or single gene mutation analysis (hemophilia or cystic fibrosis mre specific)
Chorionic Villus Sampling
CV sampling
10-12 weeks of pregnancy (earlier)
Ultra sound guidance/transvaginal biopsies part of chorionic villus
Obtain fetal cells - analyze chromosomes or DNA
(throng of tissue allows for good cell counts and good yeild of tissue and DNa)(biochemical/genetic tests and karyotyping)
Risk of miscarriage approx 1% (invasive) -must be discused
Aniocentesis
Amniocentesis
15-16 weeks of pregnancy (later)
Ultra sound guidance/transabdominal
collect amniotic fluid which contains fetal cells
(centrifugation, biochemical/genetic tests and karyotyping)
may need to culture cells before analysis (usually for a week to get sufficient cells-delay-lengthier process)
miscarriage risk approx 1%
Analysis of fetal tissue
- chromosomes studies - looking for anauploudiy
- FISH studies for anapleudy (3 signals )
3.If looking for single gene disorder: cystic fibrosis, haemophilia, huntingtons disease= requires DNA sequencing (snaga sequencing, changes in individual base pairs)- PCR type testing, amplify interested sequence
alot of work and analysis- 3-4 days work
Diagnostic Tests: Cordocentesis
Cordocentesis: Percutaneous umbilical cord blood sampling
testing for heamoglobinopothy
-want to analyse in more phenotypic level (look for protein/blood abnormality)
-done later in pregnancy
-Rarely done. carries more risk (miscarriage)
Diagnositc Tests: Fetoscopy and umbilical cord blood sampling
Fetoscopy and umbilical cord blood sampling -Performed late in pregnancy (under ultrasound) -Miscarriage risk 2-10% Obtain fetal blood which can be analysed for: Karyotype and FISH Gene mutation analysis Metabolic disorders Anemia
Detection of cell free fetal DNA in maternal plasma
- Cell-free DNA in maternal plasma (circulation)
- Aneuploidies - massively parallel sequencing of total DNA present in maternal plasma
- Alignment of sequencing reads to human genome sequence and determination of relative chromosome representation
- Detection of aneuploidy (e.g. trisomy 21)
(other strategy: separate out plasma buffy coat of maternal blood (WBC), compare patterns between plasma DNA and Buffy coat DNA)
Non-invasive screening for aneuploidies
Confirm with diagnostic e.g. amniocentesis
Not routinely available in NZ at present
Higher accuracy, sensitivity and specificity
-higher than combination of all other current direction
Trisomy 21: Detection rate 99.2%. False +ve rate 0.90%
Trisomy 18: Detection rate 91.0%. False +ve rate 0.13%
Trisomy 13: Detection rate 90.3%. False +ve rate 0.23%
-potential use for cancer, found in plasma
Likely change in clinical pregnancy tests
All woman: Offer Prenatal Screening Testing
- -: If High Risk: Offer NIPT (irculating fetal DNA)
- -: If positive: Offer Prenatal Diagnostic Testing
- implications: people that resort to invasive testing will be a very high risk group (most benefit from test -risk worthwhile)
Issues to consider
Not comprehensive testing:
-only test for common chromosome disorders (not testing for all congenital disorders)
-single gene disorders (need to know mutation)
-risk of error e.g. clerical, laboratory testing
Risk (small) to fetus
Not morally or ethically acceptable to all
Potential benefits of prenatal testing
Reassurance when results normal
Psychological preparation for arrival of affected baby
Advance warning for medical team
Provision of additional information for couple where termination is an option
Preimplantation Genetic Diagnosis (PGD)
- Egg donor is given fertility drugs (sample single cell from egg during 6-8 cell blastocyst stage without affecting eggs viability)
- Multiple eggs are produced (invitro)
- Eggs are fertilized to produce embryos
- Embryos analyzed for genetic defects (FISH)
- Only healthy embryos are injected into uterus
- Mother gives birth to genetically healthy baby
(fertilized eggs (zygotes) –> tested for presence of disaese-causing genes –> healthy zygotes implanted and brought to term)
-more acceptable ethically, less invasive, less stressful
-expensive, technically difficult, reimplantation success rate 25-30%
- serious option for heamophilia families e.g.
Counselling
Essential part of the process and care of these patients
Provides up to date information and support for the patient/family/couple
Non-directive
-discuss Options
-entire process
