Fiona M.F. Lun

Noninvasive prenatal diagnosis of fetal chromosomal aneuploidy by massively parallel genomic sequencing of DNA in maternal plasma

Chromosomal aneuploidy is the major reason why couples opt for prenatal diagnosis. Current methods for definitive diagnosis rely on invasive procedures, such as chorionic villus sampling and amniocentesis, and are associated with a risk of fetal miscarriage. Fetal DNA has been found in maternal plasma but exists as a minor fraction among a high background of maternal DNA. Hence, quantitative perturbations caused by an aneuploid chromosome in the fetal genome to the overall representation of sequences from that chromosome in maternal plasma would be small. Even with highly precise single molecule counting methods such as digital PCR, a large number of DNA molecules and hence maternal plasma volume would need to be analyzed to achieve the necessary analytical precision. Here we reasoned that instead of using approaches that target specific gene loci, the use of a locus-independent method would greatly increase the number of target molecules from the aneuploid chromosome that could be analyzed within the same fixed volume of plasma. Hence, we used massively parallel genomic sequencing to quantify maternal plasma DNA sequences for the noninvasive prenatal detection of fetal trisomy 21. Twenty-eight first and second trimester maternal plasma samples were tested. All 14 trisomy 21 fetuses and 14 euploid fetuses were correctly identified. Massively parallel plasma DNA sequencing represents a new approach that is potentially applicable to all pregnancies for the noninvasive prenatal diagnosis of fetal chromosomal aneuploidies.

Non-invasive prenatal assessment of Trisomy 21 by multiplexed maternal plasma DNA sequencing: large scale validity study

Objectives To validate the clinical efficacy and practical feasibility of massively parallel maternal plasma DNA sequencing to screen for fetal trisomy 21 among high risk pregnancies clinically indicated for amniocentesis or chorionic villus sampling. Design Diagnostic accuracy validated against full karyotyping, using prospectively collected or archived maternal plasma samples. Setting Prenatal diagnostic units in Hong Kong, United Kingdom, and the Netherlands. Participants 753 pregnant women at high risk for fetal trisomy 21 who underwent definitive diagnosis by full karyotyping, of whom 86 had a fetus with trisomy 21. Intervention Multiplexed massively parallel sequencing of DNA molecules in maternal plasma according to two protocols with different levels of sample throughput: 2-plex and 8-plex sequencing. Main outcome measures Proportion of DNA molecules that originated from chromosome 21. A trisomy 21 fetus was diagnosed when the z score for the proportion of chromosome 21 DNA molecules was >3. Diagnostic sensitivity, specificity, positive predictive value, and negative predictive value were calculated for trisomy 21 detection. Results Results were available from 753 pregnancies with the 8-plex sequencing protocol and from 314 pregnancies with the 2-plex protocol. The performance of the 2-plex protocol was superior to that of the 8-plex protocol. With the 2-plex protocol, trisomy 21 fetuses were detected at 100% sensitivity and 97.9% specificity, which resulted in a positive predictive value of 96.6% and negative predictive value of 100%. The 8-plex protocol detected 79.1% of the trisomy 21 fetuses and 98.9% specificity, giving a positive predictive value of 91.9% and negative predictive value of 96.9%. Conclusion Multiplexed maternal plasma DNA sequencing analysis could be used to rule out fetal trisomy 21 among high risk pregnancies. If referrals for amniocentesis or chorionic villus sampling were based on the sequencing test results, about 98% of the invasive diagnostic procedures could be avoided.

Maternal Plasma DNA Sequencing Reveals the Genome-Wide Genetic and Mutational Profile of the Fetus

Sequencing plasma DNA from a pregnant woman permits genome-wide scanning for the mutational status of the fetus prenatally and noninvasively. Maternal Plasma Yields Fetal Secrets Plasma from a pregnant woman is known to contain small amounts not only of maternal DNA but also of fetal DNA. Although only 10% of cell-free DNA in maternal plasma is of fetal origin, next-generation sequencing technology should enable sequencing of fetal DNA fragments that could then be assembled into a full genetic map (with the parental genomes as guides). The fetal genome then could be scanned for mutations prenatally and noninvasively. In a proof-of-concept study in a family where both parents carry mutations for the blood disease β-thalassemia, Lo and colleagues now use this approach to identify whether the fetus carries no, one, or even two β-thalassemia mutations. Using massively parallel sequencing, these investigators sequenced DNA in the plasma of the pregnant mother to 65-fold genome coverage. They demonstrated that the full maternal and fetal genomes were present in maternal plasma at constant proportions and that maternal and fetal DNA showed distinctive fragmentation patterns. Next, the authors assembled a complete fetal genomic map, using the paternal genotype and maternal haplotype (deduced from a chorionic villus sample) as guides. They then scanned the fetal genome to see whether the fetus had inherited β-thalassemia. β-Thalassemia is an autosomal recessive disease characterized by severe anemia and is caused by mutations in the HBB gene encoding the β subunit of hemoglobin. To inherit the disease, the fetus must carry mutations from both parents. The pregnant mother carried one HBB gene mutation, and the father carried a different mutation. The authors conducted genome-wide genotyping of maternal and paternal DNA derived from blood cells for ~900,000 single-nucleotide polymorphisms (SNPs) and divided the SNPs into five categories. From the maternal plasma DNA sequencing data, they searched for and found the paternal mutation inherited by the fetus. They then used relative haplotype dosage (RHDO) analysis to see whether the fetus had inherited the genomic region that contained the maternal mutation. They found that the fetus had not inherited the maternal mutation and thus was a heterozygous carrier for β-thalassemia. Although still at the proof-of-concept stage, this study shows that sequencing of maternal plasma DNA provides a way for noninvasive prenatal genome-wide scanning for genetic disorders. Cell-free fetal DNA is present in the plasma of pregnant women. It consists of short DNA fragments among primarily maternally derived DNA fragments. We sequenced a maternal plasma DNA sample at up to 65-fold genomic coverage. We showed that the entire fetal and maternal genomes were represented in maternal plasma at a constant relative proportion. Plasma DNA molecules showed a predictable fragmentation pattern reminiscent of nuclease-cleaved nucleosomes, with the fetal DNA showing a reduction in a 166–base pair (bp) peak relative to a 143-bp peak, when compared with maternal DNA. We constructed a genome-wide genetic map and determined the mutational status of the fetus from the maternal plasma DNA sequences and from information about the paternal genotype and maternal haplotype. Our study suggests the feasibility of using genome-wide scanning to diagnose fetal genetic disorders prenatally in a noninvasive way.

Digital PCR for the molecular detection of fetal chromosomal aneuploidy

Trisomy 21 is the most common reason that women opt for prenatal diagnosis. Conventional prenatal diagnostic methods involve the sampling of fetal materials by invasive procedures such as amniocentesis. Screening by ultrasonography and biochemical markers have been used to risk-stratify pregnant women before definitive invasive diagnostic procedures. However, these screening methods generally target epiphenomena, such as nuchal translucency, associated with trisomy 21. It would be ideal if noninvasive genetic methods were available for the direct detection of the core pathology of trisomy 21. Here we outline an approach using digital PCR for the noninvasive detection of fetal trisomy 21 by analysis of fetal nucleic acids in maternal plasma. First, we demonstrate the use of digital PCR to determine the allelic imbalance of a SNP on PLAC4 mRNA, a placenta-expressed transcript on chromosome 21, in the maternal plasma of women bearing trisomy 21 fetuses. We named this the digital RNA SNP strategy. Second, we developed a nonpolymorphism-based method for the noninvasive prenatal detection of trisomy 21. We named this the digital relative chromosome dosage (RCD) method. Digital RCD involves the direct assessment of whether the total copy number of chromosome 21 in a sample containing fetal DNA is overrepresented with respect to a reference chromosome. Even without elaborate instrumentation, digital RCD allows the detection of trisomy 21 in samples containing 25% fetal DNA. We applied the sequential probability ratio test to interpret the digital PCR data. Computer simulation and empirical validation confirmed the high accuracy of the disease classification algorithm.

Microfluidics Digital PCR Reveals a Higher than Expected Fraction of Fetal DNA in Maternal Plasma

BACKGROUNDnThe precise measurement of cell-free fetal DNA in maternal plasma facilitates noninvasive prenatal diagnosis of fetal chromosomal aneuploidies and other applications. We tested the hypothesis that microfluidics digital PCR, in which individual fetal-DNA molecules are counted, could enhance the precision of measuring circulating fetal DNA.nnnMETHODSnWe first determined whether microfluidics digital PCR, real-time PCR, and mass spectrometry produced different estimates of male-DNA concentrations in artificial mixtures of male and female DNA. We then focused on comparing the imprecision of microfluidics digital PCR with that of a well-established nondigital PCR assay for measuring male fetal DNA in maternal plasma.nnnRESULTSnOf the tested platforms, microfluidics digital PCR demonstrated the least quantitative bias for measuring the fractional concentration of male DNA. This assay had a lower imprecision and higher clinical sensitivity compared with nondigital real-time PCR. With the ZFY/ZFX assay on the microfluidics digital PCR platform, the median fractional concentration of fetal DNA in maternal plasma was > or =2 times higher for all 3 trimesters of pregnancy than previously reported.nnnCONCLUSIONSnMicrofluidics digital PCR represents an improvement over previous methods for quantifying fetal DNA in maternal plasma, enabling diagnostic and research applications requiring precise quantification. This approach may also impact other diagnostic applications of plasma nucleic acids, e.g., in oncology and transplantation.

Noninvasive prenatal diagnosis of monogenic diseases by digital size selection and relative mutation dosage on DNA in maternal plasma

Prenatal diagnosis of monogenic diseases, such as cystic fibrosis and β-thalassemia, is currently offered as part of public health programs. However, current methods based on chorionic villus sampling and amniocentesis for obtaining fetal genetic material pose a risk to the fetus. Since the discovery of cell-free fetal DNA in maternal plasma, the noninvasive prenatal assessment of paternally inherited traits or mutations has been achieved. Due to the presence of background maternal DNA, which interferes with the analysis of fetal DNA in maternal plasma, noninvasive prenatal diagnosis of maternally inherited mutations has not been possible. Here we describe a digital relative mutation dosage (RMD) approach that determines if the dosages of the mutant and wild-type alleles of a disease-causing gene are balanced or unbalanced in maternal plasma. When applied to the testing of women heterozygous for the CD41/42 (–CTTT) and hemoglobin E mutations on HBB, digital RMD allows the fetal genotype to be deduced. The diagnostic performance of digital RMD is dependent on interplay between the fractional fetal DNA concentration and number of DNA molecules in maternal plasma. To achieve fetal genotype diagnosis at lower volumes of maternal plasma, fetal DNA enrichment is desired. We thus developed a digital nucleic acid size selection (NASS) strategy that effectively enriches the fetal DNA without additional plasma sampling or experimental time. We show that digital NASS can work in concert with digital RMD to increase the proportion of cases with classifiable fetal genotypes and to bring noninvasive prenatal diagnosis of monogenic diseases closer to reality.

Noninvasive prenatal diagnosis of fetal trisomy 18 and trisomy 13 by maternal plasma DNA sequencing.

Massively parallel sequencing of DNA molecules in the plasma of pregnant women has been shown to allow accurate and noninvasive prenatal detection of fetal trisomy 21. However, whether the sequencing approach is as accurate for the noninvasive prenatal diagnosis of trisomy 13 and 18 is unclear due to the lack of data from a large sample set. We studied 392 pregnancies, among which 25 involved a trisomy 13 fetus and 37 involved a trisomy 18 fetus, by massively parallel sequencing. By using our previously reported standard z-score approach, we demonstrated that this approach could identify 36.0% and 73.0% of trisomy 13 and 18 at specificities of 92.4% and 97.2%, respectively. We aimed to improve the detection of trisomy 13 and 18 by using a non-repeat-masked reference human genome instead of a repeat-masked one to increase the number of aligned sequence reads for each sample. We then applied a bioinformatics approach to correct GC content bias in the sequencing data. With these measures, we detected all (25 out of 25) trisomy 13 fetuses at a specificity of 98.9% (261 out of 264 non-trisomy 13 cases), and 91.9% (34 out of 37) of the trisomy 18 fetuses at 98.0% specificity (247 out of 252 non-trisomy 18 cases). These data indicate that with appropriate bioinformatics analysis, noninvasive prenatal diagnosis of trisomy 13 and trisomy 18 by maternal plasma DNA sequencing is achievable.

Targeted Massively Parallel Sequencing of Maternal Plasma DNA Permits Efficient and Unbiased Detection of Fetal Alleles

BACKGROUNDnMassively parallel sequencing has recently been used in noninvasive prenatal diagnosis. The current costs of this technology are still relatively expensive, however, and sample throughput is still relatively low when it is used as a molecular diagnostic tool. Rather than nonselectively sequencing the genome, target enrichment provides a logical approach for more efficient and cost-effective massively parallel sequencing because it increases the proportion of informative data from the targeted region(s). Existing applications of targeted sequencing have mainly been qualitative analyses of genomic DNA. In this study, we investigated its applicability in enriching selected genomic regions from plasma DNA and the quantitative performance of this approach.nnnMETHODSnDNA was extracted from plasma samples collected from 12 pregnant women carrying female fetuses. The SureSelect Target Enrichment System (Agilent Technologies) was used to enrich for exons on chromosome X. Plasma DNA libraries with and without target enrichment were analyzed by massively parallel sequencing. Genomic DNA samples of the mother and fetus for each case were genotyped by microarray.nnnRESULTSnFor the regions targeted by the enrichment kit, the mean sequence coverage of the enriched samples was 213-fold higher than that of the nonenriched samples. Maternal and fetal DNA molecules were enriched evenly. After target enrichment, the coverage of fetus-specific alleles within the targeted region increased from 3.5% to 95.9%.nnnCONCLUSIONSnTargeted sequencing of maternal plasma DNA permits efficient and unbiased detection of fetal alleles at genomic regions of interest and is a powerful method for measuring the proportion of fetal DNA in a maternal plasma sample.

Systematic search for placental DNA-methylation markers on chromosome 21: Toward a maternal plasma-based epigenetic test for fetal trisomy 21

BACKGROUNDnThe presence of fetal DNA in maternal plasma represents a source of fetal genetic material for noninvasive prenatal diagnosis; however, the coexisting background maternal DNA complicates the analysis of aneuploidy in such fetal DNA. Recently, the SERPINB5 gene on chromosome 18 was shown to exhibit different DNA-methylation patterns in the placenta and maternal blood cells, and the allelic ratio for placenta-derived hypomethylated SERPINB5 in maternal plasma was further shown to be useful for noninvasive detection of fetal trisomy 18.nnnMETHODSnTo develop a similar method for the noninvasive detection of trisomy 21, we used methylation-sensitive single nucleotide primer extension and/or bisulfite sequencing to systematically search 114 CpG islands (CGIs)-76% of the 149 CGIs on chromosome 21 identified by bioinformatic criteria-for differentially methylated DNA patterns. The methylation index (MI) of a CpG site was estimated as the proportion of molecules methylated at that site.nnnRESULTSnWe identified 22 CGIs which were shown to contain CpG sites that were either completely unmethylated (MI = 0.00) in maternal blood cells and methylated in the placenta (MI range, 0.22-0.65), or completely methylated (MI = 1.00) in maternal blood cells and hypomethylated in the placenta (MI range, 0.00-0.75). We detected, for the first time, placental DNA-methylation patterns on chromosome 21 in maternal plasma during pregnancy and observed their postpartum clearance.nnnCONCLUSIONnTwenty-two (19%) of the 114 studied CGIs on chromosome 21 showed epigenetic differences between samples of placenta and maternal blood cells; these CGIs may provide a rich source of markers for noninvasive prenatal diagnosis.

Noninvasive Prenatal Methylomic Analysis by Genomewide Bisulfite Sequencing of Maternal Plasma DNA

BACKGROUNDnEpigenetic mechanisms play an important role in prenatal development, but fetal tissues are not readily accessible. Fetal DNA molecules are present in maternal plasma and can be analyzed noninvasively.nnnMETHODSnWe applied genomewide bisulfite sequencing via 2 approaches to analyze the methylation profile of maternal plasma DNA at single-nucleotide resolution. The first approach used maternal blood samples and polymorphic differences between the mother and fetus to analyze the fetal methylome across the genome. The second approach used the methylation profile of maternal blood cells and the fractional fetal DNA concentration in maternal plasma to deduce the placental methylomic profile from maternal plasma DNA-sequencing data.nnnRESULTSnBecause of the noninvasive nature of these approaches, we were able to serially assess the methylation profiles of fetal, placental, and maternal plasma with maternal blood samples collected in the first and third trimesters and after delivery. Gestation-related changes were observed. The fetal methylation profile deduced from maternal plasma data resembled that of the placental methylome, both on a genomewide level and per CpG site. Imprinted genes and differentially methylated regions were identified from the maternal plasma data. We demonstrated one potential clinical application of maternal plasma bisulfite sequencing with the successful detection of fetal trisomy 21.nnnCONCLUSIONSnWe successfully analyzed fetal and placental methylomes on a genomewide scale, noninvasively and serially. This development offers a powerful method for research, biomarker discovery, and clinical testing for pregnancy-related disorders.