Prenatal Diagnosis and Screening

Prenatal diagnostic and screening are meant to inform pregnant women and their partners to their fetus’s likelihood of birth abnormalities or genetic disorders, as well as give them with information about how to deal with the ambiguity. Many families that are known to be at high risk of having a kid with a significant birth defect decide against having children. Prenatal screening allows women to become pregnant with the knowledge that the scans will indicate whether or not the fetus has a disorder.

Prenatal diagnosis refers to the process of diagnosing a fetus who is currently regarded to be at elevated risk for a genetic disorder and determining whether or not the fetus is impacted by the condition. The increased risk is usually detected as a consequence of the birth of a prior child with the condition, a family history of the disorder, a positive parental carrier test, or a positive prenatal screening test. An invasive technique such as chorionic villus sampling (CVS) or amniocentesis to extract fetal cells or amniotic fluid for examination might be required for prenatal diagnosis.

Prenatal screening, on the other hand, has commonly been extended to testing for other specific birth complications, such as chromosomal aneuploidies, neural tube defects, and some developmental anomalies in pregnancy that are not thought to pose an increased risk of heart defects or genetic disorders. Screening tests were developed because such birth defects occur more often in births that are not deemed to be at high risk, and therefore prenatal screening could not have been given to the parents. Typically, screening tests are non-invasive based on obtaining a sample of maternal blood or on imaging, usually through ultrasonography or magnetic resonance imaging ( MRI). Typically, screening tests are designed to be cheap and small enough to make them ideal for screening all pregnant women in a population regardless their chance.

METHODS OF PRENATAL DIAGNOSIS

A) Invasive Testing

Amniocentesis

Invasive testing takes advantage of CVS or amniocentesis to provide fetal tissues. Amniocentesis refers to the process of injecting a needle into the amniotic membrane, and transabdominally collecting a sample of amniotic fluid. The amniotic fluid includes fetal cells, which can be grown for diagnostic testing. Ultrasonographic screening was commonly used before amniocentesis to determine fetal viability, gestational age (determining various biometric parameters such as head diameter, abdominal circumference and femur length), number of fetuses, volume of amniotic fluid, normality of fetal anatomical structures, and location of the fetus and placenta to allow an optimum position of the needle within the fetus. Amniocentesis is performed on an outpatient basis usually after the first day of the last menstrual cycle between the 16th and 20th week. In addition to fetal chromosome and genome analysis, the alpha-fetoprotein (AFP) concentration in amniotic fluid can be assessed to detect open neural tube defects (NTDs). AFP is a fetal glycoprotein that is predominantly formed in the liver, secreted into the fetal bloodstream, and excreted into the amniotic fluid via the renals. AFP travels through the placenta, amniotic membranes and maternal-fetal circulation via the maternal bloodstream. Hence, it can be tested either in (amniotic fluid AFP [AFAFP]) or in (maternal serum AFP [MSAFP]). Both assays are extremely useful for an transparent NTD risk evaluation but also for other purposes (see later discussion).

Amniocentesis

Chorionic Villus Sampling

CVS requires a transcervical or transabdominal biopsy of tissue from the chorion villi, usually between the 10th and 13th weeks of pregnancy. Chorionic villi are extracted from the trophoblast, the blastocyst’s extraembryonic portion, and are a ready source for biopsy of fetal tissue. As with amniocentesis, CVS uses ultrasonographic scanning to determine the best approach for sampling.

The key advantage of CVS relative to mid-trimester amniocentesis is that CVS allows the outcomes to be available at an early stage of birth, thus reducing the time of doubt and enabling termination in the first trimester, if chosen. Unlike amniocentesis, however, AFAFP can’t be checked at this point. Therefore, testing for a potential open NTD must be carried out using other approaches, including MSAFP screening, AFAFP amniocentesis, and ultrasonography.

 Preimplantation Genetic Diagnosis

Preimplantation genetic diagnosis ( PGD) refers to in vitro fertilization ( IVF) monitoring to identify embryos that are free from a particular genetic disorder prior to moving to the uterus. This technique was created in an attempt to provide an alternative abortion method for certain couples at serious risk for a particular genetic condition or aneuploidy in their children , allowing them to undergo pregnancy even though they advocate termination of pregnancy. Anomalies of chromosomes may also be observed using NGS.

While PGD was conducted several thousands of times worldwide by blastomer biopsy, it is not without controversy. First, a single cell’s molecular analysis is technologically challenging; precision ranges with false-positive rates of around 6% and false-negative levels of around 1%, much higher than for CVS or amniocentesis examination of specimens. The newly developed method for biopsy of blastocysts provides more cellular material, with an apparently greater precision.

B) Noninvasive Prenatal Diagnosis

Prenatal Diagnosis of Anomalies by Ultrasonography

Chromosomal aneuploidy is associated with a variety of fetal anomalies detected by ultrasound testing, including trisomy 21, trisomy 18, trisomy 13, 45,X and several other rare karyotypes. These anomalies can also occur in a chromosomally normal fetus as isolated findings. The likelihood of a chromosomally abnormal fetus increases dramatically when only one of many abnormalities is a fetal abnormality detected by ultrasonic examination.

Diagnostic ultrasonography can be useful for prenatal diagnosis of some single-gene conditions for which DNA testing is possible because a blood or tissue sample is impractical for DNA or biochemical research. Ultrasonography may also be helpful where the exposure of a genetic disease is unknown and there is no conclusive DNA-based research.

Ultrasonography may also recognize a variety of individual anomalies which can recur in families and which are considered to have multifactorial inheritance, including neural tube malformations. Often available in several clinics is prenatal echocardiography for a comprehensive evaluation of births at risk for congenital heart disease.

Ultrasound testing can be used as early as 13 weeks ‘ gestation to determine fetal sex. For certain women found to be at elevated risk, this assessment can be an significant prelude or supplement in the prenatal treatment of such X-linked recessive disorders (e.g., hemophilia)

PRENATAL SCREENING

Prenatal screening has traditionally relied both on ultrasonography and on the measurement of different proteins and hormones (called analytes) whose maternal serum levels are altered when a fetus is affected by a trisomy or an NTD.

Screening for Neural Tube Defects

The concentration of AFP in maternal serum is likely to be higher than normal when the fetus has an open NTD, just as we had seen in amniotic fluid before. This discovery forms the basis for using the 16-week Maternal Serum AFP (MSAFP) assessment as a screening tool for open NTDs. There is a substantial difference between the standard MSAFP distribution and the spectrum of concentrations observed when an active NTD is present in the fetus. While an elevated concentration of MSAFP is by no means unique to a pregnancy with an open NTD, fetal ultrasonography can discern many of the other causes of elevated MSAFP concentration from open NTDs.

Also, MSAFP is not perfectly sensitive, since its evaluation depends on statistically defined cutoff values. If an elevated concentration in pregnancies is characterized as two multiples of the median value without any abnormality that may raise the concentration of AFP, it can be calculated that 20 percent of fetuses with open NTDs remain undetected. Lowering the threshold to increase sensitivity, however, would come at the cost of decreased accuracy, thereby increasing the risk of false positivity.

Screening for Down Syndrome and Other Aneuploidies

About 70 percent of all children with severe autosomal trisomies are born to mothers who lack known risk factors, including advanced maternal age. A solution to this question was first proposed by the surprising finding that concentration of MSAFP was depressed in several.

First-Trimester Screening (Double Marker Screening)

First-trimester screening is typically conducted between 11 and 13 weeks of gestation and relies on the assessment of the amount of certain analytes in maternal serum combined with a tightly selective ultrasonographic test. The analytes used are plasma protein A (PAPP-A) linked with pregnancy and the hormone human chorionic gonadotropin ( hCG), either as a complete hCG or as its free β subunit. In all trisomies PAPP-A is depressed below the normal range; hCG (or free β-hCG) is elevated in trisomy 21 but depressed in other trisomies. Analyte measurements are paired with ultrasonographic measurement of nuchal translucency (NT), defined as the thickness of the echo-free area between the skin and the soft tissue overlying the dorsal part of the cervical spine caused by the fetal neck’s subcutaneous edema. An rise in NT is usually found in 21, 13, and 18 trisomies and in 45,X fetuses. Since NT varies with fetal age, it must be evaluated in relation to gestational age.

First Trimester Screening

 Nuchal TranslucencyPAPP-ABeta-HCG
Trisomy 21
Trisomy 18
Trisomy 13

Second-Trimester Screening (Triple Marker and Quadruple Markers)

Second-trimester screening is usually conducted by evaluating hCG in conjunction with three other analytes: MSAFP, unconjugated estriol(uE3), and inhibin A. The test panel is called a quadruple panel. Both of these compounds in all trisomies with the exception of hCG, which is elevated in trisomy 21 but depressed in the other trisomies, and inhibin A, which is elevated in trisomy 21 but not greatly impaired in the other trisomies, are depressed below normal. There are a variety of factors that can influence rates of these analytes, including age, alcohol, IVF pregnancy, and maternal diabetes, and laboratories that typically compensate for these variables. Extremely low levels of unconjugated estriol may indicate rare genetic conditions, such as deficiency in steroid sulfatase or Smith-lemli-Opitz syndrome.

Second Trimester Screening

 uE3AFPBeta-HCGInhibin A
Trisomy 21
Trisomy 18
Trisomy 13
NTD↑↑

Noninvasive Prenatal Screening by Analysis of Cell-Free Fetal DNA (cffDNA)

The area of prenatal screening and obstetrical genetics is being revolutionized by the convergence of two significant developments in genomics, one biological and the other computational, to develop a modern prenatal screening technique known as non-invasive prenatal screening (NIPS) (sometimes referred to as non-invasive prenatal research, NIPT). The scientific finding is that after 7 weeks of birth, a pregnant woman ‘s blood incorporates fetal DNA, which is not found in a cell’s nucleus but flows naturally in the maternal bloodstream. Approximately 2 to 10% of the non-cellular DNA in maternal blood is derived from placental trophoblasts and is therefore of fetal origin. While combined with maternal DNA, this cell-free fetal DNA offers a snapshot of the fetal genome that is suitable for study without the need for an intrusive technique.The technological breakthrough is the development of high-throughput sequencing methods which allow millions of individual DNA molecules to be sequenced into a mixture.

NIPT allows for highly precise, noninvasive monitoring of pregnancies for common autosomal and sex chromosome aneuploidies, with sensitivities and specificities near 99 percent for trisomy 21. Cell-free fetal DNA in maternal serum has also been used to genotype the fetus at the Rh locus and determine fetal sex. Further advancements in cell-free DNA research will make noninvasive testing for many other genetic diseases, including many single-gene disorders, eligible for clinical treatment in the future.

Non-invasive prenatal monitoring based on cffDNA using next-generation sequencing (NGS) is a highly sensitive and precise method for fetal aneuploidy screening. NIPT employs a variety of genetic tools, including microarray-guided sequencing and whole-genome sequencing. Since the advent of NGS technology, NIPT has also been used in many sequencing systems, including a sequencing semi-conductor platform (Ion torrent sequencing) and the Illumina sequencing platform. Despite being built on separate sequencing concepts, both the Ion Torrent and Illumina sequencing systems performed admirably in detecting T13, T18, and T21.

References:

  • Nussbaum RL, McInnes RR, Willard HF. THOMPSON & THOMPSON GENETICS IN MEDICINE, EIGHTH EDITION. Vol 6. 8th ed. Philadelphia: Elsevier Inc.; 2016. doi:10.3126/jkmc.v6i1.18578
  • Cogulu O. Next Generation Sequencing as a Tool for Noninvasive Prenatal Tests. In: Clinical Applications for Next-Generation Sequencing. Elsevier Inc.; 2016:171-188. doi:10.1016/B978-0-12-801739-5.00009-X
  • Curnow KJ, Sanderson RK, Beruti S. Noninvasive Detection of Fetal Aneuploidy Using next Generation Sequencing. Vol 1885. Second. (Levy B, ed.). New York, NY: Springer New York; 2019. doi:10.1007/978-1-4939-8889-1_22
  • Xue Y, Zhao G, Li H, et al. Non-invasive prenatal testing to detect chromosome aneuploidies in 57,204 pregnancies. Mol Cytogenet. 2019;12(1):1-7. doi:10.1186/s13039-019-0441-5

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