Diagnosis of fetal aneuploidy
Encyclopedia
Fetal aneuploidy
is not as common as other birth defects, affecting 9 in 1000 births and increasing with maternal age. Amniocentesis
and chorionic villus sampling
have historically been two of the most popular methods used to detect fetal aneuploidy. Since these procedures are invasive, they may result in an adverse effect on the health of the unborn fetus and/or the mother. Recently, there has been much research on the development of noninvasive methods to detect fetal aneuploidy.
, and direct preparation of chromosomes from chorionic villi
are all current methods being used that are the most effective for detecting fetal aneuploidy.
, where amniotic fluid is drawn from the uterus. These amniotic cells can be cultured as early as the second trimester of pregnancy. Both of these methods have risks because they are both invasive. Risks include infection of the amniotic fluid, leaking of the amniotic sac, and even miscarriage in the most serious cases. An advantage compared with analyzing maternal blood plasma for fetal nucleic acids is that amniotic fluid
is free from maternal nucleic acids.
in maternal blood plasma
. However, the key problem is that circulating fetal nucleated cells comprise only three to six percent of maternal blood plasma DNA. Therefore, two effective approaches have been developed that can be used for the detection of fetal aneuploidy. The first involves the measuring of the allelic ratio
of single nucleotide polymorphism
s (SNPs) in the mRNA coding region in the placenta
. The next approach is analyzing both maternal and fetal DNA and looking for differences in the DNA methylation
patterns.
Fetal cell DNA in maternal blood plasma is elevated in pregnancies complicated by fetal trisomy
21, 13 and 18, making it easier to acquire enough sample sizes of maternal blood plasma to test for fetal aneuploidy.
can used for detection of fetal aneuploidy using fetal DNA and RNA found in maternal blood plasma. Research has shown that digital PCR can be used to differentiate between normal and aneuploid DNA using fetal DNA in the maternal blood plasma.
A variation of the PCR technique called multiplex ligation-dependent probe amplification
(MLPA), targetting DNA, has been successively applied for diagnosing fetal aneuploidy as a chromosome- or gene-specific assay.
technology. This DNA was obtained from the blood plasma of eighteen pregnant women. This was followed by mapping the chromosome using the quantification of fragments. This was done using advanced methods in DNA sequencing resulting in the parallel sequencing of the fetal DNA. The amount of sequence tags mapped to each chromosome was counted. If there was a surplus or deficiency in any of the chromosomes, this meant that there was a fetal aneuploid. Using this method of shotgun sequencing, the successful identification of trisomy 21 (Down syndrome), trisomy 18 (Edward syndrome), and trisomy 13 (Patau syndrome) was possible. This method of noninvasive diagnosis is now starting to be heavily used and researched further.
Aneuploidy
Aneuploidy is an abnormal number of chromosomes, and is a type of chromosome abnormality. An extra or missing chromosome is a common cause of genetic disorders . Some cancer cells also have abnormal numbers of chromosomes. Aneuploidy occurs during cell division when the chromosomes do not separate...
is not as common as other birth defects, affecting 9 in 1000 births and increasing with maternal age. Amniocentesis
Amniocentesis
Amniocentesis is a medical procedure used in prenatal diagnosis of chromosomal abnormalities and fetal infections, in which a small amount of amniotic fluid, which contains fetal tissues, is sampled from the amnion or amniotic sac surrounding a developing fetus, and the fetal DNA is examined for...
and chorionic villus sampling
Chorionic villus sampling
Chorionic villus sampling , sometimes misspelled "chorionic villous sampling", is a form of prenatal diagnosis to determine chromosomal or genetic disorders in the fetus. It entails sampling of the chorionic villus and testing it...
have historically been two of the most popular methods used to detect fetal aneuploidy. Since these procedures are invasive, they may result in an adverse effect on the health of the unborn fetus and/or the mother. Recently, there has been much research on the development of noninvasive methods to detect fetal aneuploidy.
Current invasive practices
Currently, laboratory techniques are being developed that are more efficient and accurate than before for detecting fetal aneuploidy and other chromosomal aberrations. Interphase-fluorescence in situ hybridization (FISH), quantitative fluorescence polymerase chain reactionReal-time polymerase chain reaction
In molecular biology, real-time polymerase chain reaction, also called quantitative real time polymerase chain reaction or kinetic polymerase chain reaction , is a laboratory technique based on the PCR, which is used to amplify and simultaneously quantify a targeted DNA molecule...
, and direct preparation of chromosomes from chorionic villi
Chorionic villus sampling
Chorionic villus sampling , sometimes misspelled "chorionic villous sampling", is a form of prenatal diagnosis to determine chromosomal or genetic disorders in the fetus. It entails sampling of the chorionic villus and testing it...
are all current methods being used that are the most effective for detecting fetal aneuploidy.
Early diagnosis
During the first trimester of pregnancy, chorionic villi sampling has become an effective and common procedure where a piece of placental tissue is taken for examination. Another common method is amniocentesisAmniocentesis
Amniocentesis is a medical procedure used in prenatal diagnosis of chromosomal abnormalities and fetal infections, in which a small amount of amniotic fluid, which contains fetal tissues, is sampled from the amnion or amniotic sac surrounding a developing fetus, and the fetal DNA is examined for...
, where amniotic fluid is drawn from the uterus. These amniotic cells can be cultured as early as the second trimester of pregnancy. Both of these methods have risks because they are both invasive. Risks include infection of the amniotic fluid, leaking of the amniotic sac, and even miscarriage in the most serious cases. An advantage compared with analyzing maternal blood plasma for fetal nucleic acids is that amniotic fluid
Amniotic fluid
Amniotic fluid or liquor amnii is the nourishing and protecting liquid contained by the amniotic sac of a pregnant woman.- Development of amniotic fluid :...
is free from maternal nucleic acids.
Rapid diagnosis
Cell culturing usually takes up to one to two weeks in the case of chorionic villi sampling and amniocentesis. Fluorescence in situ hybridization (FISH) is used for rapid diagnosis. It detects the presence of specific DNA sequences by using fluorescent probes that bind to the DNA sequences. FISH involves three basic steps. The first step entails the fixation of a sample on a slide. The second step is the hybridization of a labeled probe to fragments of DNA. The last step is the detection of the tagged DNA sequence.Noninvasive diagnosis
Due to the detection of fetal cells and fetal DNA circulating in maternal blood, noninvasive diagnosis of fetal aneuploidy is becoming more promising.Maternal plasma nucleic acid analysis
The development of a variety of screening methods for fetal aneuploidy and other chromosomal aberrations is now a prominent research area because of the discovery of circulating fetal nucleic acidNucleic acid
Nucleic acids are biological molecules essential for life, and include DNA and RNA . Together with proteins, nucleic acids make up the most important macromolecules; each is found in abundance in all living things, where they function in encoding, transmitting and expressing genetic information...
in maternal blood plasma
Blood plasma
Blood plasma is the straw-colored liquid component of blood in which the blood cells in whole blood are normally suspended. It makes up about 55% of the total blood volume. It is the intravascular fluid part of extracellular fluid...
. However, the key problem is that circulating fetal nucleated cells comprise only three to six percent of maternal blood plasma DNA. Therefore, two effective approaches have been developed that can be used for the detection of fetal aneuploidy. The first involves the measuring of the allelic ratio
Allele
An allele is one of two or more forms of a gene or a genetic locus . "Allel" is an abbreviation of allelomorph. Sometimes, different alleles can result in different observable phenotypic traits, such as different pigmentation...
of single nucleotide polymorphism
Single nucleotide polymorphism
A single-nucleotide polymorphism is a DNA sequence variation occurring when a single nucleotide — A, T, C or G — in the genome differs between members of a biological species or paired chromosomes in an individual...
s (SNPs) in the mRNA coding region in the placenta
Placenta
The placenta is an organ that connects the developing fetus to the uterine wall to allow nutrient uptake, waste elimination, and gas exchange via the mother's blood supply. "True" placentas are a defining characteristic of eutherian or "placental" mammals, but are also found in some snakes and...
. The next approach is analyzing both maternal and fetal DNA and looking for differences in the DNA methylation
DNA methylation
DNA methylation is a biochemical process that is important for normal development in higher organisms. It involves the addition of a methyl group to the 5 position of the cytosine pyrimidine ring or the number 6 nitrogen of the adenine purine ring...
patterns.
Fetal cell DNA in maternal blood plasma is elevated in pregnancies complicated by fetal trisomy
Trisomy
A trisomy is a type of polysomy in which there are three copies, instead of the normal two, of a particular chromosome. A trisomy is a type of aneuploidy .-Description and causes:...
21, 13 and 18, making it easier to acquire enough sample sizes of maternal blood plasma to test for fetal aneuploidy.
History of development
- 2000: It was found that in pregnant women carrying male fetuses, the mRNA transcribed from the Y chromosome was found in maternal blood plasma.
- 2002: Plasma RNA is found to be stable.
- 2003: Fetal RNA in maternal bloods plasma was found to have its origins in the placenta.
- 2004: mRNA markers found in the placenta can be detected in the maternal blood plasma using microarrays.
- 2007: Using microarrays, a specific mRNA from the placenta was found in maternal blood plasma. This mRNA disappeared from the maternal blood plasma following delivery of the baby.
Digital PCR
Recently, it has been proposed that digital PCRPolymerase chain reaction
The polymerase chain reaction is a scientific technique in molecular biology to amplify a single or a few copies of a piece of DNA across several orders of magnitude, generating thousands to millions of copies of a particular DNA sequence....
can used for detection of fetal aneuploidy using fetal DNA and RNA found in maternal blood plasma. Research has shown that digital PCR can be used to differentiate between normal and aneuploid DNA using fetal DNA in the maternal blood plasma.
A variation of the PCR technique called multiplex ligation-dependent probe amplification
Multiplex ligation-dependent probe amplification
Multiplex ligation-dependent probe amplification is a variation of the polymerase chain reaction that permits multiple targets to be amplified with only a single primer pair. Each probe consists of a two oligonucleotides which recognise adjacent target sites on the DNA...
(MLPA), targetting DNA, has been successively applied for diagnosing fetal aneuploidy as a chromosome- or gene-specific assay.
Shotgun sequencing
Fetal cell DNA has been directly sequenced using shotgun sequencingShotgun sequencing
In genetics, shotgun sequencing, also known as shotgun cloning, is a method used for sequencing long DNA strands. It is named by analogy with the rapidly-expanding, quasi-random firing pattern of a shotgun....
technology. This DNA was obtained from the blood plasma of eighteen pregnant women. This was followed by mapping the chromosome using the quantification of fragments. This was done using advanced methods in DNA sequencing resulting in the parallel sequencing of the fetal DNA. The amount of sequence tags mapped to each chromosome was counted. If there was a surplus or deficiency in any of the chromosomes, this meant that there was a fetal aneuploid. Using this method of shotgun sequencing, the successful identification of trisomy 21 (Down syndrome), trisomy 18 (Edward syndrome), and trisomy 13 (Patau syndrome) was possible. This method of noninvasive diagnosis is now starting to be heavily used and researched further.