Jianguang Zhang

Maternal Mosaicism Is a Significant Contributor to Discordant Sex Chromosomal Aneuploidies Associated with Noninvasive Prenatal Testing

BACKGROUND In the human fetus, sex chromosome aneuploidies (SCAs) are as prevalent as the common autosomal trisomies 21, 18, and 13. Currently, most noninvasive prenatal tests (NIPTs) offer screening only for chromosomes 21, 18, and 13, because the sensitivity and specificity are markedly higher than for the sex chromosomes. Limited studies suggest that the reduced accuracy associated with detecting SCAs is due to confined placental, placental, or true fetal mosaicism. We hypothesized that an altered maternal karyotype may also be an important contributor to discordant SCA NIPT results. METHODS We developed a rapid karyotyping method that uses massively parallel sequencing to measure the degree of chromosome mosaicism. The method was validated with DNA models mimicking XXX and XO mosaicism and then applied to maternal white blood cell (WBC) DNA from patients with discordant SCA NIPT results. RESULTS Sequencing karyotyping detected chromosome X (ChrX) mosaicism as low as 5%, allowing an accurate assignment of the maternal X karyotype. In a prospective NIPT study, we showed that 16 (8.6%) of 181 positive SCAs were due to an abnormal maternal ChrX karyotype that masked the true contribution of the fetal ChrX DNA fraction. CONCLUSIONS The accuracy of NIPT for ChrX and ChrY can be improved substantially by integrating the results of maternal-plasma sequencing with those for maternal-WBC sequencing. The relatively high frequency of maternal mosaicism warrants mandatory WBC testing in both shotgun sequencing- and single-nucleotide polymorphism-based clinical NIPT after the finding of a potential fetal SCA.

Confined placental origin of the circulating cell free fetal DNA revealed by a discordant non-invasive prenatal test result in a trisomy 18 pregnancy

BACKGROUND Non-invasive prenatal testing (NIPT) by massively parallel sequencing is a useful clinical test for the detection of common fetal aneuploidies. While the accuracy of aneuploidy detection can approach 100%, results discordant with the fetus are occasionally reported. In this study we investigated the basis of a discordant T21 positive and T18 negative NIPT result associated with a T18 fetus confirmed by karyotyping. METHODS Massively parallel sequencing was used to detect fetal DNA in maternal circulating plasma. The parental origin and nature of the fetal and placental aneuploidies were investigated by quantitative fluorescent PCR of short tandem repeat (STR) sequences and by copy number variation (CNV) sequencing. RESULTS There was no evidence of T21 maternal mosaicism, T21 microchimerism or a vanishing twin to explain the discordant NIPT result. However, examination of multiple placental biopsies showed both T21 and T18 mosaicism, including one confined region with a significantly higher proportion of T21 cells. Based on fetal DNA fractions and average mosaicism levels, the effective T21 and T18 fetal DNA fractions should have been sufficient for the detection of both trisomies. CONCLUSIONS In this pregnancy, we speculate that confined placental region(s) with higher proportions of T21 cells were preferentially releasing fetal DNAs into the maternal circulation. This study highlights placental mosaicism as a significant risk factor for discordant NIPT results.

Noninvasive Prenatal Testing for Wilson Disease by Use of Circulating Single-Molecule Amplification and Resequencing Technology (cSMART)

BACKGROUND Noninvasive prenatal testing (NIPT) for monogenic diseases by use of PCR-based strategies requires precise quantification of mutant fetal alleles circulating in the maternal plasma. The study describes the development and validation of a novel assay termed circulating single-molecule amplification and resequencing technology (cSMART) for counting single allelic molecules in plasma. Here we demonstrate the suitability of cSMART for NIPT, with Wilson Disease (WD) as proof of concept. METHODS We used Sanger and whole-exome sequencing to identify familial ATP7B (ATPase, Cu(++) transporting, β polypeptide) gene mutations. For cSMART, single molecules were tagged with unique barcodes and circularized, and alleles were targeted and replicated by inverse PCR. The unique single allelic molecules were identified by sequencing and counted, and the percentage of mutant alleles in the original maternal plasma sample was used to determine fetal genotypes. RESULTS Four families with WD pedigrees consented to the study. Using Sanger and whole-exome sequencing, we mapped the pathogenic ATP7B mutations in each pedigree and confirmed the probands original diagnosis of WD. After validation of cSMART with defined plasma models mimicking fetal inheritance of paternal, maternal, or both parental mutant alleles, we retrospectively showed in second pregnancies that the fetal genotypes assigned by invasive testing and NIPT were concordant. CONCLUSIONS We developed a reliable and accurate NIPT assay that correctly diagnosed the fetal genotypes in 4 pregnancies at risk for WD. This novel technology has potential as a universal strategy for NIPT of other monogenic disorders, since it requires only knowledge of the parental pathogenic mutations.

Copy Number Variation Sequencing for Comprehensive Diagnosis of Chromosome Disease Syndromes

Detection of chromosome copy number variation (CNV) plays an important role in the diagnosis of patients with unexplained clinical symptoms and for the identification of chromosome disease syndromes in the established fetus. In current clinical practice, karyotyping, in conjunction with array-based methods, is the gold standard for detection of CNV. To increase accessibility and reduce patient costs for diagnostic CNV tests, we speculated that next-generation sequencing methods could provide a similar degree of sensitivity and specificity as commercial arrays. CNV in patient samples was assessed on a medium-density single nucleotide polymorphism array and by low-coverage massively parallel CNV sequencing (CNV-seq), with mate pair sequencing used to confirm selected CNV deletion breakpoints. A total of 10 ng of input DNA was sufficient for accurate CNV-seq diagnosis, although 50 ng was optimal. Validation studies of samples with small CNVs showed that CNV-seq was specific and reproducible, suggesting that CNV-seq may have a potential genome resolution of approximately 0.1 Mb. In a blinded study of 72 samples with known gross and submicroscopic CNVs originally detected by single nucleotide polymorphism array, there was high diagnostic concordance with CNV-seq. We conclude that CNV-seq is a viable alternative to arrays for the diagnosis of chromosome disease syndromes.

Validation of Copy Number Variation Sequencing for Detecting Chromosome Imbalances in Human Preimplantation Embryos

ABSTRACT Chromosome aneuploidies commonly arise in embryos produced by assisted reproductive technologies and represent a major cause of implantation failure and miscarriage. Currently, preimplantation genetic diagnosis (PGD) is performed by array-based methods to identify euploid embryos for transfer to the patient. We speculated that a combination of next-generation sequencing technologies and sophisticated bioinformatics would deliver a more comprehensive and accurate methodology to improve the overall efficacy of embryo testing. To meet this challenge, we developed a high-resolution copy number variation (CNV) sequencing pipeline suitable for single-cell analysis. In validation studies, we showed that CNV-Seq was highly sensitive and specific for detection of euploidy, aneuploidy, and segmental imbalances in 24 whole genome amplification samples from PGD embryos that were originally diagnosed by gold standard array comparative genomic hybridization. In addition, CNV-Seq was capable of detecting, mapping, and accurately quantifying terminal chromosome imbalances down to 1 Mb in size originating from abnormal segregation of translocation chromosomes. These validation studies indicate that CNV-Seq displays the hallmarks of an accurate and reliable embryo test with the potential to further improve the overall efficacy of PGD.

The Performance of Whole Genome Amplification Methods and Next-Generation Sequencing for Pre-Implantation Genetic Diagnosis of Chromosomal Abnormalities

Reliable and accurate pre-implantation genetic diagnosis (PGD) of patients embryos by next-generation sequencing (NGS) is dependent on efficient whole genome amplification (WGA) of a representative biopsy sample. However, the performance of the current state of the art WGA methods has not been evaluated for sequencing. Using low template DNA (15 pg) and single cells, we showed that the two PCR-based WGA systems SurePlex and MALBAC are superior to the REPLI-g WGA multiple displacement amplification (MDA) system in terms of consistent and reproducible genome coverage and sequence bias across the 24 chromosomes, allowing better normalization of test to reference sequencing data. When copy number variation sequencing (CNV-Seq) was applied to single cell WGA products derived by either SurePlex or MALBAC amplification, we showed that known disease CNVs in the range of 3-15 Mb could be reliably and accurately detected at the correct genomic positions. These findings indicate that our CNV-Seq pipeline incorporating either SurePlex or MALBAC as the key initial WGA step is a powerful methodology for clinical PGD to identify euploid embryos in a patients cohort for uterine transplantation.

Detection of Chromosomal Aneuploidy in Human Preimplantation Embryos by Next-Generation Sequencing

ABSTRACT Embryos produced by assisted reproductive technologies are commonly associated with a high level of aneuploidy. Currently, 24-chromosome profiling of embryo biopsy samples by array-based methods is available to identify euploid embryos for transfer that have a higher potential for implantation and development to term. From a laboratory and patient perspective, there is a need to explore the feasibility of developing an alternative method for routine aneuploidy assessment of embryos that would be more comprehensive, cost-effective, and efficient. We speculated that aneuploidy could be readily assessed in test single-cell biopsy samples by first performing whole genome amplification followed by library generation, massively parallel shot-gun sequencing, and finally bioinformatics analysis to quantitatively compare the ratio of uniquely mapped reads to reference cells. Using Down syndrome as an example, the copy number change for chromosome 21 was consistently 1.5-fold higher in multiple cell and single-cell samples with a 47,XX,+21 karyotype. Applying the validated sequencing strategy to 10 sister blastomeres from a single human embryo, we showed that the aneuploidy status called by sequencing was consistent with short tandem repeat allelic profiling. These validation studies indicate that aneuploidy detection using sequencing-based methodology is feasible for further improving the practice of preimplantation genetic diagnosis.

The clinical utility of next-generation sequencing for identifying chromosome disease syndromes in human embryos.

Next-generation sequencing is emerging as a reliable and accurate technology for pre-implantation genetic diagnosis (PGD) of aneuploidies and translocations. The aim of this study was to extend the clinical utility of copy number variation sequencing (CNV-Seq) to the detection of small pathogenic copy number variations (CNVs) associated with chromosome disease syndromes. In preliminary validation studies, CNV-Seq was highly sensitive and specific for detecting small CNV in whole-genome amplification products from three replicates of one and five cell samples, with a resolution in the order of 1-2 Mb. Importantly, the chromosome positions of all CNV were correctly mapped with copy numbers similar to those measured in matching genomic DNA samples. In seven clinical PGD cycles where results were obtained for 34 of 35 blastocysts, CNV-Seq identified 18 blastocysts with aneuploidies, one with an aneuploidy and a 4.98 Mb 5q35.2-qter deletion associated with Sotos syndrome, one with a 6.66 Mb 7p22.1-pter deletion associated with 7p terminal deletion syndrome and 14 with no detectable abnormalities that were suitable for transfer. On the basis of these findings, CNV-Seq displays the hallmarks of a comprehensive PGD technology for detection of aneuploidies and CNVs that are known to affect the development and health of patients embryos.

Maternal X chromosome copy number variations are associated with discordant fetal sex chromosome aneuploidies detected by noninvasive prenatal testing.

BACKGROUND The sensitivity and specificity of noninvasive prenatal testing (NIPT) for detection of sex chromosome aneuploidies (SCAs) compared to common autosomal trisomies are significantly lower. We speculated that in addition to altered maternal X chromosome karyotype, maternal X chromosome copy number variations (CNVs) may also contribute to discordant NIPT SCA results. METHODS Clinical NIPT was performed for pregnant women at a single hospital. Copy number variation sequencing (CNV-Seq) was used to identify and quantitate the copy number of maternal X chromosome CNVs for each positive SCA pregnancy. RESULTS Two out of 25 SCA positive NIPT samples had slightly abnormal ChrX/ChrY z-scores and were referred for invasive test confirmation. However, fetal karyotypes were found to be normal. CNV-Seq analysis of the maternal white blood cell DNA archived from the original two NIPT blood samples identified small CNVs spanning the STS gene, which is associated with X-linked ichthyosis. Correcting for the altered plasma levels of X chromosome DNA caused by the two CNVs and, taking into consideration the phenotypic consequences for X-linked disease, both fetuses were diagnosed as normal. CONCLUSIONS Maternal DNA sequencing is recommended for all positive NIPT SCA results to avoid unnecessary referral for invasive testing and also to evaluate the risk to the fetus of X-linked disease.

A pregnancy with discordant fetal and placental chromosome 18 aneuploidies revealed by invasive and noninvasive prenatal diagnosis

This study investigated a pregnancy where the fetus was diagnosed with monosomy 18p by invasive amniocentesis and karyotyping. Additional noninvasive prenatal diagnosis, which detects fetal chromosome abnormalities in the circulating cell-free plasma DNA originating from the placenta revealed a related 18p monosomy/18q trisomy, suggesting confined placental mosaicism. Based on recent observations of chromosomal instability in the early preimplantation embryo, this study speculates on the possible embryonic origin(s) of these related but discordant chromosome 18 aneuploidies in the placental and fetal tissues. The findings highlight the potential for both false-positive and -negative noninvasive prenatal diagnosis results in pregnancies where there is either confined placental mosaicism or placental mosaicism. The study investigated a pregnancy involving a fetus with a chromosome disease syndrome called monosomy 18p where part of the short arm of chromosome 18 was missing in the fetal tissues. Using non-invasive prenatal diagnosis which detects fetal chromosome abnormalities in the circulating cell free plasma DNA originating from the placenta, we also detected monosomy 18p as well a related chromosome 18 abnormality involving duplication of the long arm of chromosome 18. This suggested confined placental mosaicism where the constitution of the chromosomes are different between fetal and placental tissues. We speculated that these related chromosome 18 abnormalities arose during preimplantation embryo development, leading to the formation of different chromosome abnormalities observed in the placental and fetal tissues of this pregnancy. Our findings highlight the potential for both false positive and negative non-invasive prenatal diagnosis test results in pregnancies where there is confined placental mosaicism.