Lecture 22: Introduction to prenatal genetics
Learning outcomes:
Describe the differences between prenatal diagnosis and prenatal screening
Prenatal screening uses maternal blood samples and ultrasonography so noninvasively assess the risk for certain common genetic disorders ( chromosomal aneuploidies), common birth defects (neural tube defects), and structural anomalies in pregnancies not otherwise known to be at an increased risk for these conditions.
Screening tests are designed to be inexpensive with sufficiently low risk to make them suitable for cost effective screening of all pregnant individuals in a population.
They do not provide diagnostic answers about whether an abnormality is present but are intended to find those with pregnancies at higher risk relative to the background and offer them follow up diagnostic testing.
Prenatal diagnosis is the term applied to performing genetic testing on a fetal sample to determine if the fetus is affected with a genetic disorder.
It is traditionally offered when the risk for a genetic disorder is increased because of the previous birth of an affected child, a family history of the disorder, a positive parental carrier test, or when a prenatal screening indicated an increased risk.
Prenatal diagnosis, which is meant to provide a definitive answer as to whether the fetus is affected, requires a procedure such as chorionic villus sampling or amniocentesis to directly acquire fetal or placental cells for analysis.
Describe the use of ultrasonography and analyte testing in prenatal screening
Prenatal screening has traditionally relied on ultrasonography combined with measurements of proteins and hormones (referred to as analytes) whose levels in maternal serum are altered when a fetus is affected by a trisomy or NTD.
Screening for neural tube defects (NTD)
An estimated 95% of infants with NTDs are born into families with no prior history.
The first developed noninvasive serum analyte based screening test measures the amount of maternal serum αfetoprotein (MSAFP) to identify pregnancies at increased risk for fetal open NTD, including open spina bifida and other disorders.
These are associated with high amounts of AFP in the amniotic fluid.
AFP is a fetal glycoprotein produced mainly in the liver, secreted into the fetal circulation, and excreted through the fetal kidneys.
AFP also leaks into the amniotic fluid when the fetal skin is breached,
Because AFP enters the maternal bloodstream via the placenta, membranes and maternal-fetal circulation, MSAFP is also elevated, which is the basis for using MSAFP measurements at ~16 weeks.
There is considerable overlap between the range of MSAFP concentrations in unaffected pregnancies and those where the fetus has an open NTD, and the sensitivity of MSAFP screening to detect an increased risk for fetal open NTDs depends on statistically defined cutoff values.
If the cutoff for an elevated concentration is 2.5 multiples of the median value in unaffected pregnancies, one can estimate that 80% of fetuses with NTD are detected and 20% remain undetected.
However, lowering the cutoff to improve sensitivity would be at the expense of reduced specificity, thereby increasing the number of unaffected pregnancies that would be interpreted as high risk.
Screening for down syndrome
MSAFP concentrations of unaffected pregnancies and Down syndrome pregnancies overlap too much for MSAFP to be a useful screening analyte on its own.
Ultrasonography screening
A number of findings on prenatal ultrasonography are associated with chromosomal aneuploidy, including trisomies 21, 18, and 13; 45,X; and many other abnormal karyotypes.
Fetal sex can be determined as early as 13 weeks.
Some fetal abnormalities are associated with aneuploidy. The fetus is more likely to have a chromosome disorder if multiple anomalies are detected.
Explain how Non-Invasive Prenatal Testing (NIPT) is used in prenatal screening
All individuals have fragmented DNA in their blood that is not contained in the nucleus of cells but free floating and can be assayed from plasma or serum.
The discover that during pregnancy maternal plasma contains fetal cell free DNA derived from trophoblast cells of the placenta, which have the same genome as the fetus, has drastically changed the approach to prenatal screening for fetal chromosomal abnormalities.
After 10 weeks of gestation, the proportion of cell free DNA in the maternal blood that is derived from trophoblast, referred to as fetal fraction, is ~5% to 20%.
This can be analyzed to noninvasively evaluate whether the fetus has aneuploidy.
This led to the rapid expansion of cell free DNA based noninvasive prenatal testing (NIPT) for trisomies 21, 13, and 18, with sensitivities approaching 99 for trisomy 21.
It still remains a screening test, not a diagnostic test.
A result that indicates that the fetus is at an increased risk for a chromosomal abnormality should be confirmed by diagnostic testing.
Explain the differences between non-invasive genetic screening and invasive genetic diagnosis using amniocentesis and CVS, including their advantages and risks
Indications for amniocentesis and chorionic villus sampling
Previous child with a chromosomal abnormality
Presence of a chromosomal abnormality in a parent
Family history of a genetic disorder for which testing is available
Increased risk of an affected child suggested by prenatal screening (positive test from screening)
Couples who wish to have invasive screening
Amniocentesis
A needle is inserted transabdominally under continuous ultrasound visualization into the amniotic sac to remove a sample of amniotic fluid.
The amniotic fluid contains fetal cells that can be cultured or from which DNA can be prepared without culture and used for diagnostic tests.
Typically performed between 16 and 20 weeks
Fetal chromosome by karyotype and microarray are the standard genetic tests.
The major complications is a 1 in 300 to 1 in 500 chance of miscarriage.
Chorionic Villus Sampling
A small amount of placental villi is removed between 10-14 weeks. Chorionic villi consist of a mesenchymal core that contains capillaries and are covered by a layer of trophoblast cells, which are derived from the extraembryonic part of the early developing embryo and are a ready source of fetal tissue.
It is also performed under continuous ultrasound, and it can be performed transabdominally or transcervically.
The major advantage is that results are available at an early stage of pregnancy, thus reducing the period of uncertainty and allowing termination to be performed earlier.
Chromosome analysis with karyotype or microarray is the same as amniocentesis.
The rate of miscarriage is also about 1 in 300 to 1 in 500.
Understand confined placental mosaicism and how it can affect prenatal genetic testing
About 1% of CVS tests yield ambiguous results because of chromosomal mosaicism.
Depends on when the mutation occurred
This is an advantage of amniocentesis
Particularly concerning for trisomies involving chromosomes besides 21, 13, and 18 since they are usually inviable. The placenta tends to be more tolerant of aneuploidy.
Generalized mosaicism
Affects both placenta and fetus
Confined placental mosaicism with normal and abnormal cell lineages present
Fetus is fine but placenta is mosaic
Confined placental mosaicism with only an abnormal call lineage present
Fetus is fine and placenta is fully aneuploid
Mosaicism confined to embryo
Placenta is fine but fetus is mosaic
Understand what is meant by sensitivity and specificity of genetic tests and how sensitivity and specificity are calculated
Sensitivity
Ability of test to correctly identify those with disorder
True positive (individuals with disorder) (= true positive plus false negative)
Specificity
Ability of test to correctly identify those without the disorder
True negative (individuals without the disorder) (=false positive + true negative)
Describe the indications for prenatal testing and range of testing options available to women at various stages of their pregnancy
Carrier screening of couple
Any time before or during pregnancy
Prenatal screening (noninvasive)
First ultrasound exam and analyte blood tests during first trimester: 10-13 weeks
Additional blood tests and ultrasound during second trimester: 15-22 weeks
NIPT: 10 weeks or later
Prenatal diagnosis
CVS during first trimester: 10-13 weeks
Amniocentesis during second trimester (15 weeks or later)
Describe how preimplantation genetic diagnosis is conducted following in vitro fertilization
Preimplantation genetic testing (PGT) is performed on in vitro fertilized embryos prior to embryo transfer.
PGT offers couples at significant risk for a specific genetic disorder or aneuploidy in their offspring an option to manage reproductive risks that avoid pregnancy termination.
In the most commonly used approach, blastocyst biopsy, women have to undergo IVF.
Oocytes are fertilized in vitro and cultured for 5 to 6 days until the blastocyst stage.
At that time 5-10 cells are retrieved from the trophectoderm, which develops into the future placenta, without disrupting the inner cell mass, which will develop into the fetus.
The embryos that are not found to carry the genetic abnormality in question can then be transferred and allowed to implant.
Demonstrate familiarity with the factors that should be considered during prenatal genetic counseling
The risk that the fetus will be affected
The nature of the specific disorder
The risks and limitations of the procedures to be used
The time required before a report can be issued
The possible need for another procedure if result is inconclusive
Elective pregnancy termination versus other options
Key terms
Prenatal genetic testing
Prenatal screening
Prenatal diagnosis
Analyte
Ultrasonography
Neural tube defect
Spina Bifida
Alpha-fetoprotein
Amniocentesis
Chorionic Villus Sampling (CVS)
Confined placental mosaicism
Non-invasive prenatal testing (NIPT)
Sensitivity
Specificity
Preimplantation genetic diagnosis