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Cell-free fetal DNA
DNA
(cffDNA) is fetal DNA
DNA
which circulates freely in the maternal blood. Maternal blood is sampled by venipuncture. Analysis of cff DNA
DNA
is a method of non-invasive prenatal diagnosis frequently ordered for pregnant women of advanced maternal age.

Contents

1 Background 2 Laboratory methods

2.1 Separation of cffDNA 2.2 Analysis of cffDNA

2.2.1 Quantitative real-time PCR 2.2.2 Nested PCR 2.2.3 Digital PCR 2.2.4 Shotgun sequencing 2.2.5 Mass spectrometry 2.2.6 Epigenetic modifications 2.2.7 mRNA

3 Applications

3.1 Prenatal sex determination 3.2 Single gene disorders 3.3 Hemolytic disease of the fetus and newborn 3.4 Aneuploidy 3.5 Preeclampsia

4 Future perspectives 5 See also 6 References

Background[edit]

Cell free fetal DNA
DNA
sheds into the maternal blood circulation

cff DNA
DNA
originates from placental trophoblasts.[1][2] Fetal DNA
DNA
is fragmented when placental microparticles are shed into the maternal blood circulation (figure 1).[3] cff DNA
DNA
fragments are approximately 200 base pairs (bp) in length. They are significantly smaller than maternal DNA
DNA
fragments.[4] The difference in size allows cff DNA
DNA
to be distinguished from maternal DNA fragments.[5][6] Approximately 11 to 13.4 percent of the cell-free DNA
DNA
in maternal blood is of fetal origin. The amount varies widely from one pregnant woman to another.[7] cff DNA
DNA
is present after five to seven weeks gestation. The amount of cff DNA
DNA
increases as the pregnancy progresses.[8] The quantity of cff DNA
DNA
in maternal blood diminishes rapidly after childbirth. Two hours after delivery, cff DNA
DNA
is no longer detectable in maternal blood.[9] Analysis of cff DNA
DNA
may provide earlier diagnosis of foetal conditions than current techniques. As cff DNA
DNA
is found in maternal blood, sampling carries no associated risk of spontaneous abortion.[10][11][12][13][14] cff DNA
DNA
analysis has the same ethical and practical issues as other techniques such as amniocentesis and chorionic villus sampling.[15] Some disadvantages of sampling cff DNA
DNA
include a low concentration of cff DNA
DNA
in maternal blood; variation in the quantity of cff DNA
DNA
between individuals; a high concentration of maternal cell free DNA
DNA
compared to the cff DNA
DNA
in maternal blood.[16] New evidence shows that cff DNA
DNA
test failure rate is higher, fetal fraction (proportion of fetal versus maternal DNA
DNA
in the maternal blood sample) is lower and PPV for trisomies 18, 13 and SCA is decreased in IVF pregnancies compared to those conceived spontaneously.[17] Laboratory methods[edit] A maternal peripheral blood sample is taken by venesection at about ten weeks gestation.[18] Separation of cffDNA[edit] Blood plasma
Blood plasma
is separated from the maternal blood sample using a laboratory centrifuge. The cff DNA
DNA
is then isolated and purified.[19] In 2007, a standardized protocol for doing this was written by Legler et al through an evaluation of the scientific literature. The highest yield in cff DNA
DNA
extraction was obtained with the "QIAamp DSP Virus Kit".[20] Addition of formaldehyde to maternal blood samples increases the yield of cffDNA. Formaldehyde stabilizes intact cells, and therefore inhibits the further release of maternal DNA. With the addition of formaldehyde, the percentage of cff DNA
DNA
recovered from a maternal blood sample varies between 0.32 percent and 40 percent with a mean of 7.7 percent.[21] Without the addition of formaldehyde, the mean percentage of cff DNA
DNA
recovered has been measured at 20.2 percent. However, other figures vary between 5 and 96 percent.[22][23] Recovery of cff DNA
DNA
may be related to the length of the DNA
DNA
fragments. Another way to increase the fetal DNA
DNA
is based on physical length of DNA
DNA
fragments. Smaller fragments can represent up to seventy percent of the total cell free DNA
DNA
in the maternal blood sample. Analysis of cffDNA[edit] In real-time PCR, fluorescent probes are used to monitor the accumulation of amplicons. The reporter fluorescent signal is proportional to the number of amplicons generated. The most appropriate real time PCR protocol is designed according to the particular mutation or genotype to be detected. Point mutations are analysed with qualitative real time PCR with the use of allele specific probes. insertions and deletions are analyzed by dosage measurements using quantitative real time PCR. cff DNA
DNA
may be detected by finding paternally inherited DNA
DNA
sequences via polymerase chain reaction (PCR).[24]>[25] Quantitative real-time PCR[edit] In 2010, Hill et al analyzed the sex-determining region Y gene (SRY) and the Y chromosome
Y chromosome
short tandem repeat "DYS14" in cff DNA
DNA
from 511 pregnancies using a quantitative real-time PCR (RT-qPCR). In 401 of 403 pregnancies where maternal blood was drawn at seven weeks gestation or more, both segments of DNA
DNA
were found.[26] Nested PCR[edit] In 2001, Al-Yatama et al evaluated the use of nested polymerase chain reaction (nested PCR) to determine sex by detecting a Y chromosome specific signal in the cff DNA
DNA
from maternal plasma. Nested PCR detected 53 of 55 male fetuses. The cff DNA
DNA
from the plasma of 3 of 25 women with female fetuses contained the Y chromosome-specific signal. The sensitivity of nested PCR in this experiment was 96 percent. The specificity was 88 percent.[27] Digital PCR[edit] Microfluidic
Microfluidic
devices allow the quantification of cff DNA
DNA
segments in maternal plasma with accuracy beyond that of real-time PCR. Point mutations, loss of heterozygosity and aneuploidy can be detected in a single PCR step.[28][29][30] Digital PCR can differentiate between maternal blood plasma and fetal DNA
DNA
in a multiplex fashion.[28] Shotgun sequencing[edit] High throughput shotgun sequencing using tools such as Solexa or Illumina, yields approximately 5 million sequence tags per sample of maternal serum. In 2008, Fan et al identified aneuploid pregnancies such as trisomy when testing at the fourteenth week of gestation. In 2010, fetal whole of genome mapping by parental haplotype analysis was completed using sequencing of cff DNA
DNA
from maternal serum.[13] Chiu et al. in 2010 studied 753 pregnant females, using a 2-plex massively parallel maternal plasma DNA
DNA
sequencing and trisomy was diagnosed with z-score greater than 3.[31] The sequencing gave sensitivity of 100 percent, specificity of 97.9 percent, a positive predictive value of 96.6 percent and a negative predictive value of 100 percent. Mass spectrometry[edit] Matrix-assisted laser desorption/ionization-time-of-flight mass spectrometry (MALDI-TOF MS) combined with single-base extension after PCR allows cff DNA
DNA
detection with single base specificity and single DNA
DNA
molecule sensitivity.[32] DNA
DNA
is amplified by PCR. Then, linear amplification with base extension reaction (with a third primer) is designed to anneal to the region upstream from the mutation site. One or two bases are added to the extension primer to produce two extension products from wild-type DNA
DNA
and mutant DNA. Single base specificity provides advantages over hybridization-based techniques using TaqMan
TaqMan
hydrolysis probes. In 2008, when assessing the technique, Ding et al found no false positives or negatives when looking for cff DNA
DNA
to determine fetal sex in sixteen maternal plasma samples.[32] In 2010, Akolekar et al correctly detected the sex of ninety of ninety-one male foetuses using MALDI-TOF mas spectrometry. The technique had accuracy, sensitivity and specificity of over 99 percent.[33] Epigenetic modifications[edit] Differences in gene activation between maternal and fetal DNA
DNA
can be exploited. epigenetic modifications (heritable modifications that change gene function without changing DNA
DNA
sequence) can be used to detect cffDNA.[34][35] The hypermethylated RASSF1A promoter is a universal fetal marker used to confirm the presence of cffDNA.[36] In 2012, White et al described a technique where cff DNA
DNA
was extracted from maternal plasma and then digested with methylation-sensitive and insensitive restriction enzymes. Then, real-time PCR analysis of RASSF1A, SRY, and DYS14 was done.[36] The procedure detected 79 out of 90 (88 percent) maternal blood samples where hypermethylated RASSF1A was present. mRNA[edit] m RNA
RNA
transcripts from genes expressed in the placenta are detectable in maternal plasma.[37] In this procedure, plasma is centrifuged so an aqueous layer appears. This layer is transferred and from it RNA
RNA
is extracted. RT-PCR
RT-PCR
is used to detect a selected expression of RNA. For example, Human placental lactogen
Human placental lactogen
(hPL) and beta-hCG m RNA
RNA
are stable in maternal plasma and can be detected. (Ng et al. 2002). This can help to confirm the presence of cff DNA
DNA
in maternal plasma.[16] Applications[edit] Prenatal sex determination[edit]

X-linked genetic disorder

The analysis of cff DNA
DNA
from a sample of maternal plasma allows the determination of fetal gender. Whether the sex of the fetus is male or female allows the determination of the risk of a particular X-linked recessive genetic disorder in a particular pregnancy, especially where the mother is a genetic carrier of the disorder.[38] Most X-linked diseases are evident in males because of the lack of the second X-chromosome that can compensate for the disease allele. Common X-linked recessive disorders include Duchenne muscular dystrophy, fragile X syndrome and haemophilia. In comparison to obstetric ultrasonography which is unreliable for sex determination in the first trimester and amniocentesis which carries a small risk of miscarriage, sampling of maternal plasma for analysis of cff DNA
DNA
is without risk.[39] The main targets in the cff DNA
DNA
analysis are the gene responsible for the sex-determining region Y protein (SRY) on the Y chromosome
Y chromosome
and the DYS14 sequence.[40][41]

Congenital adrenal hyperplasia

In congenital adrenal hyperplasia, the adrenal cortex lacks appropriate corticosteroid synthesis, leading to excess adrenal androgens and affects female fetuses.[42] There is an external masculinization of the genitalia in the female fetuses.[43] Mothers of at risk fetuses are given dexamethasone at 6 weeks gestation to suppress pituitary gland release of androgens.[44] If analysis of cff DNA
DNA
obtained from a sample of maternal plasma lacks genetic markers found only on the Y chromosome, it is suggestive of a female fetus. However, it might also indicate a failure of the analysis itself ( a false negative result). Paternal genetic polymorphisms and sex-independent markers may be used to detect cffDNA. An high degree of heterozygosity of these markers must be present for this application.[45]

Paternity testing

Prenatal DNA
DNA
paternity testing is commercially available. The test can be performed at nine weeks gestation.[citation needed] Single gene disorders[edit] Autosomal dominant and recessive single gene disorders which have been diagnosed prenatally by analysing paternally inherited DNA
DNA
include cystic fibrosis, beta thalassemia, sickle cell anemia, spinal muscular atrophy, and myotonic dystrophy.[24][40] Prenatal diagnosis of single gene disorders which are due to an autosomal recessive mutation, a maternally inherited autosomal dominant mutation or large sequence mutations that include duplication, expansion or insertion of DNA sequences is more difficult.[46] In cffDNA, fragments of 200 – 300 bp length involved in single gene disorders are more difficult to detect. For example, the autosomal dominant condition, achondroplasia is caused by the FGFR3 gene point mutation.[47] In 2007, a study of two pregnancies with a fetus with achondroplasia found a paternally inherited G1138A mutation from cff DNA
DNA
from a maternal plasma sample in one and a G1138A de novo mutation from the other.[47] In studies of the genetics of Huntington's chorea
Huntington's chorea
using q RT-PCR
RT-PCR
of cff DNA
DNA
from maternal plasma samples, CAG repeats have been detected at normal levels (17, 20 and 24).[48] cff DNA
DNA
may also be used to diagnose single gene disorders.[15]Developments in laboratory processes using cff DNA
DNA
may allow prenatal diagnosis of aneuploidies such as trisomy 21 (Down's syndrome) in the fetus.[49][29] Hemolytic disease of the fetus and newborn[edit] Incompatibility of fetal and maternal RhD antigens is the main cause of Hemolytic disease of the newborn.[50] Approximately 15 percent of Caucasian women, 3 to 5 percent of black Africa
Africa
women and less than 3 percent of Asian women are RhD negative.[51] Accurate prenatal diagnosis is important because the disease can be fatal to the newborn and because treatment including intramuscular immunoglobulin (Anti-D) or intravenous immunoglobulin can be administered to mothers at risk.[52] In 2010, Cardo et al reported that PCR to detect RHD (gene)
RHD (gene)
gene exons 5 and 7 from cff DNA
DNA
obtained from maternal plasma between 9 and 13 weeks gestation gave a high degree of specificity, sensitivity and diagnostic accuracy (>90 percent) when compared to RhD determination from newborn cord blood serum.[50] In 2013, Aykute et al found similar results targeting exons 7 and 10.[53] In 2015, Svobodova et al reported that droplet digital PCR in fetal RhD determination was comparable to a routine real-time PCR technique.[54] Routine determination of fetal RhD status from cff DNA
DNA
in maternal serum allows early management of at risk pregnancies while decreasing unnecessary use of Anti-D by over 25 percent.[55] Aneuploidy[edit]

Sex chromosomes

Analysis of maternal serum cff DNA
DNA
by high-throughput sequencing can detect common fetal sex chromosome aneuploidies such as Turner's syndrome, Klinefelter's syndrome
Klinefelter's syndrome
and triple X syndrome but the procedure's positive predictive value is low.[56]

Trisomy
Trisomy
21

Fetal trisomy of chromosome 21 is the cause of Down's syndrome. This trisomy can be detected by analysis of cff DNA
DNA
from maternal blood by massively parallel shotgun sequencing (MPSS).[57] Another technique is digital analysis of selected regions (DANSR).[57] However, such tests show inconsistent degrees of sensitivity and specificity and therefore may be best used to confirm a positive maternal screening test such as ultrasound markers of the condition.[57].[58]

Trisomy
Trisomy
13 and 18

Analysis of cff DNA
DNA
from maternal plasma with MPSS looking for trisomy 13 or 18 is possible[59] Factors limiting sensitivity and specificity include the levels of cff DNA
DNA
in the maternal plasma; maternal chromosomes may have mosaicism.[60] A number of fetal nucleic acid molecules derived from aneuploid chromosomes can be detected including SERPINEB2 mRNA, clad B, hypomethylated SERPINB5 from chromosome 18, placenta-specific 4 (PLAC4), hypermethylated holocarboxylase synthetase (HLCS) and c21orf105 m RNA
RNA
from chromosome 12.[61] With complete trisomy, the mRNA alleles in maternal plasma isn't the normal 1:1 ratio, but is in fact 2:1. Allelic ratios determined by epigenetic markers can also be used to detect the complete trisomies. Massive parallel sequencing and digital PCR for fetal aneuploidy detection can be used without restriction to fetal-specific nucleic acid molecules. (MPSS) is estimated to have a sensitivity of between 96 and 100%, and a specificity between 94 and 100% for detecting Down syndromeIt can be performed at 10 weeks of gestational age.[62] One study in the United States estimated a false positive rate of 0.3% and a positive predictive value of 80% when using cff DNA
DNA
to detect Down syndrome.[63] Preeclampsia[edit] Preeclampsia
Preeclampsia
is a complex condition of pregnancy involving hypertension and proteinuria usually after 20 weeks gestation.[64] It is associated with poor cytotrophoblastic invasion of the myometrium. Onset of the condition between 20 and 34 weeks gestation, is considered "early".[65] Maternal plasma samples in pregnancies complicated by preeclampsia have significantly higher levels of cffDNA that those in normal pregnancies.[66][67][68] This holds true for early onset preeclampsia.[65] Future perspectives[edit] New generation sequencing may be used to yield a whole genome sequence from cffDNA. This raises ethical questions.[69] However, the utility of the procedure may increase as clear associations between specific genetic variants and disease states are discovered.[70][71] See also[edit]

Triple test Quad test

References[edit]

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