Limited Clinical Utility of Non-invasive Prenatal Testing for Subchromosomal Abnormalities

The use of massively parallel sequencing of maternal cfDNA for non-invasive prenatal testing (NIPT) of aneuploidy is widely available. Recently, the scope of testing has increased to include selected subchromosomal abnormalities, but the number of samples reported has been small. We developed a calling pipeline based on a segmentation algorithm for the detection of these rearrangements in maternal plasma. The same read depth used in our standard pipeline for aneuploidy NIPT detected 15/18 (83%) samples with pathogenic rearrangements > 6 Mb but only 2/10 samples with rearrangements < 6 Mb, unless they were maternally inherited. There were two false-positive calls in 534 samples with no known subchromosomal abnormalities (specificity 99.6%). Using higher read depths, we detected 29/31 fetal subchromosomal abnormalities, including the three samples with maternally inherited microduplications. We conclude that test sensitivity is a function of the fetal fraction, read depth, and size of the fetal CNV and that at least one of the two false negatives is due to a low fetal fraction. The lack of an independent method for determining fetal fraction, especially for female fetuses, leads to uncertainty in test sensitivity, which currently has implications for this technique’s future as a clinical diagnostic test. Furthermore, to be effective, NIPT must be able to detect chromosomal rearrangements across the whole genome for a very low false-positive rate. Because standard NIPT can only detect the majority of larger (>6 Mb) chromosomal rearrangements and requires knowledge of fetal fraction, we consider that it is not yet ready for routine clinical implementation.     

Noninvasive Prenatal Molecular Karyotyping from Maternal Plasma

Fetal DNA is present in the plasma of pregnant women. Massively parallel sequencing of maternal plasma DNA has been used to detect fetal trisomies 21, 18, 13 and selected sex chromosomal aneuploidies noninvasively. Case reports describing the detection of fetal microdeletions from maternal plasma using massively parallel sequencing have been reported. However, these previous reports were either polymorphism-dependent or used statistical analyses which were confined to one or a small number of selected parts of the genome. In this report, we reported a procedure for performing noninvasive prenatal karyotyping at 3 Mb resolution across the whole genome through the massively parallel sequencing of maternal plasma DNA. This method has been used to analyze the plasma obtained from 6 cases. In three cases, fetal microdeletions have been detected successfully from maternal plasma. In two cases, fetal microduplications have been detected successfully from maternal plasma. In the remaining case, the plasma DNA sequencing result was consistent with the pregnant mother being a carrier of a microduplication. Simulation analyses were performed for determining the number of plasma DNA molecules that would need to be sequenced and aligned for enhancing the diagnostic resolution of noninvasive prenatal karyotyping to 2 Mb and 1 Mb. In conclusion, noninvasive prenatal molecular karyotyping from maternal plasma by massively parallel sequencing is feasible and would enhance the diagnostic spectrum of noninvasive prenatal testing.     

Evaluation of non‐invasive prenatal testing to detect chromosomal aberrations in a Chinese cohort

The aim of this study was to evaluate the clinical feasibility of non‐invasive prenatal testing (NIPT) to detect foetal copy number variations (CNVs). Next‐generation sequencing for detecting foetal copy number variations (CNVs) was performed on the collected samples from 161 pregnancies with ultrasound anomalies and negative NIPT results for aneuploidy. The performance of NIPT for detecting chromosome aberrations was calculated. The sensitivity and specificity of NIPT for detecting CNVs > 1 Mb were 83.33% and 99.34%; the PPV and negative predictive rate (NPV) were 90.91% and 98.68%. Non‐invasive prenatal testing can be performed to detect chromosomal aberrations in first trimester with high performance for CNVs, and occasional discordant cases are unavoidable.     

High-resolution identification of chromosomal abnormalities using oligonucleotide arrays containing 116,204 SNPs

Mutation of the human genome ranges from single base-pair changes to whole-chromosome aneuploidy. Karyotyping, fluorescence in situ hybridization, and comparative genome hybridization are currently used to detect chromosome abnormalities of clinical significance. These methods, although powerful, suffer from limitations in speed, ease of use, and resolution, and they do not detect copy-neutral chromosomal aberrations--for example, uniparental disomy (UPD). We have developed a high-throughput approach for assessment of DNA copy-number changes, through use of high-density synthetic oligonucleotide arrays containing 116,204 single-nucleotide polymorphisms, spaced at an average distance of 23.6 kb across the genome. Using this approach, we analyzed samples that failed conventional karyotypic analysis, and we detected amplifications and deletions across a wide range of sizes (1.3-145.9 Mb), identified chromosomes containing anonymous chromatin, and used genotype data to determine the molecular origin of two cases of UPD. Furthermore, our data provided independent confirmation for a case that had been misinterpreted by karyotype analysis. The high resolution of our approach provides more-precise breakpoint mapping, which allows subtle phenotypic heterogeneity to be distinguished at a molecular level. The accurate genotype information provided on these arrays enables the identification of copy-neutral loss-of-heterozygosity events, and the minimal requirement of DNA (250 ng per array) allows rapid analysis of samples without the need for cell culture. This technology overcomes many limitations currently encountered in routine clinical diagnostic laboratories tasked with accurate and rapid diagnosis of chromosomal abnormalities.     

Chromosomal instability,tolerance of mitotic errors and multidrug resistance are promoted by tetraploidization in human cells

Up to 80% of human cancers, in particular solid tumors, contain cells with abnormal chromosomal numbers, or aneuploidy, which is often linked with marked chromosomal instability. Whereas in some tumors the aneuploidy occurs by missegregation of one or a few chromosomes, aneuploidy can also arise during proliferation of inherently unstable tetraploid cells generated by whole genome doubling from diploid cells. Recent findings from cancer genome sequencing projects suggest that nearly 40% of tumors underwent whole genome doubling at some point of tumorigenesis, yet its contribution to cancer phenotypes and benefits for malignant growth remain unclear. Here, we investigated the consequences of a whole genome doubling in both cancerous and non-transformed p53 positive human cells. SNP array analysis and multicolor karyotyping revealed that induced whole-genome doubling led to variable aneuploidy. We found that chromosomal instability (CIN) is a frequent, but not a default outcome of whole genome doubling. The CIN phenotypes were accompanied by increased tolerance to mitotic errors that was mediated by suppression of the p53 signaling. Additionally, the expression of pro-apoptotic factors, such as iASPP and cIAP2, was downregulated. Furthermore, we found that whole genome doubling promotes resistance to a broad spectrum of chemotherapeutic drugs and stimulates anchorage-independent growth even in non-transformed p53-positive human cells. Taken together, whole genome doubling provides multifaceted benefits for malignant growth. Our findings provide new insight why genome-doubling promotes tumorigenesis and correlates with poor survival in cancer.     

Molecular methods for rapid detection of aneuploidy

Rapid molecular biological methods for prenatal diagnosis of the most common aneuploidies, collectively known as rapid aneuploidy testing, are compared in this review. We discuss methodological problems and limitations of these various methods. All these techniques are believed to be accurate and carry a low risk of misdiagnosis, but they differ in terms of labour-intensity and amenability to automation and high throughput testing. The question how to apply them safely and economically in a clinical setting has not been answered yet. The discussed techniques are so far not used as stand-alone tests, but some of them are routinely applied as a preliminary test that shortens the waiting time for classic cytogenetic karyotyping. In the future, mainly because of economical reasons, these methods may replace cytogenetics in the category of patients who make up the majority of those currently offered prenatal karyotyping: patients with moderately increased risk and no abnormalities detected by ultrasound.     

Noninvasive Detection of Fetal Subchromosome Abnormalities via Deep Sequencing of Maternal Plasma

The purpose of this study was to determine the deep sequencing and analytic conditions needed to detect fetal subchromosome abnormalities across the genome from a maternal blood sample. Cell-free (cf) DNA was isolated from the plasma of 11 pregnant women carrying fetuses with subchromosomal duplications and deletions, translocations, mosaicism, and trisomy 20 diagnosed by metaphase karyotype. Massively parallel sequencing (MPS) was performed with 25-mer tags at approximately 10⁹ tags per sample and mapped to reference human genome assembly hg19. Tags were counted and normalized to fixed genome bin sizes of 1 Mb or 100 kb to detect statistically distinct copy-number changes compared to the reference. All seven cases of microdeletions, duplications, translocations, and the trisomy 20 were detected blindly by MPS, including a microdeletion as small as 300 kb. In two of these cases in which the metaphase karyotype showed additional material of unknown origin, MPS identified both the translocation breakpoint and the chromosomal origin of the additional material. In the four mosaic cases, the subchromosomal abnormality was not demonstrated by MPS. This work shows that in nonmosaic cases, it is possible to obtain a fetal molecular karyotype by MPS of maternal plasma cfDNA that is equivalent to a chromosome microarray and in some cases is better than a metaphase karyotype. This approach combines the advantage of enhanced fetal genomic resolution with the improved safety of a noninvasive maternal blood test.     

Identification of de novo chromosomal markers and derivatives by spectral karyotyping

Despite major advances in molecular cytogenetics during the past decade and the important diagnostic role that fluorescence in situ hybridization (FISH) plays in the characterization of chromosomal abnormalities, the usefulness of this technique remains limited by the number of spectrally distinguishable fluorochromes or fluorochrome combinations. Overcoming this major limitation would allow one to use FISH to screen the whole human genome for chromosomal aberrations which, until recently, was possible only through conventional karyotyping. A recently described molecular cytogenetics technology, 24-color FISH using spectral karyotyping (SKY), permits the simultaneous visualization of all human chromosomes in 24 different colors. Most chromosomal aberrations detected during cytogenetic evaluation can be resolved using routine cytogenetic techniques alone or in combination with single- or dual-color FISH. However, some cases remain unresolved, in particular de novo supernumerary marker chromosomes and de novo unbalanced structural rearrangements. These findings cause particular diagnostic and counseling concerns when detected during prenatal diagnosis. The purpose of this report is to demonstrate the application of SKY in the characterization of these de novo structural chromosomal abnormalities. Eight cases are described in this report. SKY has considerable diagnostic applications in prenatal diagnosis because of its reliability and speed. The identification of the chromosomal origin of markers and unbalanced translocations provides the patient, physician, and genetic counselor with better predictive information on the phenotype of the carrier. Received: 2 June 1998 / Accepted: 16 June 1998     

An Improved Method to Extract DNA from 1 ml of Uncultured Amniotic Fluid from Patients at Less than 16 Weeks’ Gestation

The aim of this study was to develop an improved technique for DNA extraction from 1 ml of uncultured AF from patients with a gestational age less than 16 weeks and to allow the use of array-CGH without DNA amplification. The DNA extraction protocol was tested in a series of 90 samples including 41 of uncultured AF at less than 16 weeks of gestation. Statistical analyses were performed using linear regression. To evaluate the sensitivity and the specificity of array-CGH on 1 ml of uncultured AF, five samples with an abnormal karyotype (three with aneuploidy, two with structural abnormalities) and five with a normal karyotype were studied. This protocol was reproducible and we were able to show a great improvement with higher yield of DNA obtained from all patients, including those with a gestational age less than 16 weeks (p = 0.003). All chromosomal abnormalities were detected and characterized by array-CGH and normal samples showed normal profiles. This new DNA extraction protocol associated with array-CGH analysis could be used in prenatal testing even when gestational age is less than 16 weeks, especially in cases with abnormal ultrasound findings.     

From karyotyping to array-CGH in prenatal diagnosis

Conventional karyotyping detects chromosomal anomalies in up to 35% of pregnancies with fetal ultrasound anomalies, depending on the number and type of these anomalies. Extensive experience gained in the past decades has shown that prenatal karyotyping is a robust technique which can detect the majority of germline chromosomal anomalies. For most of these anomalies the phenotype is known. In postnatal diagnosis of patients with congenital anomalies and intellectual disability, array-CGH/SNP array has become the first-tier investigation. The higher abnormality detection yield and its amenability to automation renders array-CGH also suitable for prenatal diagnosis. As both findings of unclear significance and unexpected findings may be detected, studies on the outcome of array-CGH in prenatal diagnosis were initially performed retrospectively. Recently, prospective application of array-CGH in pregnancies with ultrasound anomalies, and to a lesser extent in pregnancies referred for other reasons, was studied. Array-CGH showed an increased diagnostic yield compared to karyotyping, varying from 1-5%, depending on the reason for referral. Knowledge of the spectrum of array-CGH anomalies detected in the prenatal setting will increase rapidly in the years to come, thus facilitating pre- and posttest counseling. Meanwhile, new techniques like non-invasive prenatal diagnosis are emerging and will claim their place. In this review, we summarize the outcome of studies on prenatal array-CGH, the clinical relevance of differences in detection rate and range as compared to standard karyotyping, and reflect on the future integration of new molecular techniques in the workflow of prenatal diagnosis.     

Prenatal diagnosis of pure partial monosomy 18p associated with holoprosencephaly and congenital heart defects

We applied CMA to detect chromosomal variations during a prenatal diagnosis and detected a 4.5 Mb pure microdeletion at 18p11.3 that was not detected by conventional karyotyping. Fluorescent in situ hybridization (FISH) analysis was performed to confirm the deletion. Accurate breakpoints of the deletion in this patient were used to build correlations between monosomy 18p and the concomitant phenotypes, particularly holoprosencephaly (HPE), which is rarely reported in monosomy 18p11.3.     

Cryptic and Complex Chromosomal Aberrations in Early-Onset Neuropsychiatric Disorders

Structural variation (SV) is a significant component of the genetic etiology of both neurodevelopmental and psychiatric disorders; however, routine guidelines for clinical genetic screening have been established only in the former category. Genome-wide chromosomal microarray (CMA) can detect genomic imbalances such as copy-number variants (CNVs), but balanced chromosomal abnormalities (BCAs) still require karyotyping for clinical detection. Moreover, submicroscopic BCAs and subarray threshold CNVs are intractable, or cryptic, to both CMA and karyotyping. Here, we performed whole-genome sequencing using large-insert jumping libraries to delineate both cytogenetically visible and cryptic SVs in a single test among 30 clinically referred youth representing a range of severe neuropsychiatric conditions. We detected 96 SVs per person on average that passed filtering criteria above our highest-confidence resolution (6,305 bp) and an additional 111 SVs per genome below this resolution. These SVs rearranged 3.8 Mb of genomic sequence and resulted in 42 putative loss-of-function (LoF) or gain-of-function mutations per person. We estimate that 80% of the LoF variants were cryptic to clinical CMA. We found myriad complex and cryptic rearrangements, including a “paired” duplication (360 kb, 169 kb) that flanks a 5.25 Mb inversion that appears in 7 additional cases from clinical CNV data among 47,562 individuals. Following convergent genomic profiling of these independent clinical CNV data, we interpreted three SVs to be of potential clinical significance. These data indicate that sequence-based delineation of the full SV mutational spectrum warrants exploration in youth referred for neuropsychiatric evaluation and clinical diagnostic SV screening more broadly.     

Prenatal diagnosis using fetal cells from the maternal circulation.

All current methods of fetal karyotyping are invasive and carry a definite, albeit small, procedure-related risk. Because of this and testing costs, only women older than 35 years who have a greater risk for fetal aneuploidy are currently offered prenatal testing. But this detects only 20% to 25% of fetuses with Down syndrome. It would be a tremendous advance to find a noninvasive technique for prenatal diagnosis that carries no procedure-related risk and could be offered to all pregnant women. We describe a possible technique for noninvasive prenatal diagnosis that aims to identify fetal cells in the peripheral maternal circulation and successfully garner them for prenatal testing. Early attempts at fetal karyotyping were hampered by inaccurate diagnostic methods and cumbersome cell-counting techniques. Today, improved capabilities of identifying and enriching for fetal cells, coupled with sensitive methods of analysis such as the polymerase chain reaction, bring renewed enthusiasm to this task. Many technical issues, as well as serious questions regarding the test''s utility, still exist, however, and must be explored and answered before the capture of fetal cells in the maternal circulation translates into reality for noninvasive prenatal diagnosis.     

Microarray analysis of cell-free fetal DNA in amniotic fluid: a prenatal molecular karyotype

Metaphase karyotype analysis of fetal cells obtained by amniocentesis or chorionic villus sampling is the current standard for prenatal cytogenetic diagnosis, particularly for the detection of trisomy 21. We previously demonstrated that large quantities of cell-free fetal DNA (cffDNA) are easily extracted from amniotic fluid (AF). In this study, we explored potential clinical applications of AF cffDNA by testing its ability to hybridize to DNA microarrays for comparative genomic hybridization (CGH) analysis. cffDNA isolated from 11 male fetuses showed significantly increased hybridization signals on SRY and decreased signals on X-chromosome markers, compared with female reference DNA. cffDNA isolated from six female fetuses showed the reverse when compared with male reference DNA. cffDNA from three fetuses with trisomy 21 had increased hybridization signals on the majority of the chromosome 21 markers, and cffDNA from a fetus with monosomy X (Turner syndrome) had decreased hybridization signals on most X-chromosome markers, compared with euploid female reference DNA. These results indicate that cffDNA extracted from AF can be analyzed using CGH microarrays to correctly identify fetal sex and aneuploidy. This technology facilitates rapid screening of samples for whole-chromosome changes and may augment standard karyotyping techniques by providing additional molecular information.     

多重荧光定量PCR技术快速诊断21三体及18三体方法的建立及临床应用

利用收集的20例21三体、3例18三体DNA样本及40例正常人DNA样本,选择多对21、18号染色体短串联重复序列分子标记,建立多重荧光定量PCR检测技术用于21三体、18三体的快速产前诊断;利用建立的方法对165例产前诊断病例及4例消化道畸形新生儿进行检测,并与核型分析结果相比较。169例病例中共诊断21三体4例,18三体1例,所有病例均在1～3d得到结果,无漏诊和误诊,并利用建立的多重荧光定量PCR技术为5例核型分析失败的病例提供了明确的产前诊断。对于同期B超检查胎儿结构异常的22例胎儿,则采用传统的核型分析,检出1例(45,X),1例(47,XXY)。结果表明建立的多重荧光定量PCR技术可快速、准确地诊断21三体和18三体,减轻核型分析需时过长给孕妇带来的焦虑,可用于血清学筛查21三体、18三体高风险者及高龄孕妇,也适用于对其他遗传性疾病如遗传性耳聋进行产前诊断时并行检测以排除21三体和18三体。多重荧光定量PCR技术结合传统核型分析可更好的满足产前诊断的临床需求。     

Applying high‐throughput sequencing to identify and evaluate foetal chromosomal deletion and duplication

The present study aimed to estimate the clinical performance of non‐invasive prenatal testing (NIPT) based on high‐throughput sequencing method for the detection of foetal chromosomal deletions and duplications. A total of 6348 pregnant women receiving NIPT using high‐throughput sequencing method were included in our study. They all conceived naturally, without twins, triplets or multiple births. Individuals showing abnormalities in NIPT received invasive ultrasound‐guided amniocentesis for chromosomal karyotype and microarray analysis at 18‐24 weeks of pregnancy. Detection results of foetal chromosomal deletions and duplications were compared between high‐throughput sequencing method and chromosomal karyotype and microarray analysis. Thirty‐eight individuals were identified to show 51 chromosomal deletions/duplications via high‐throughput sequencing method. In subsequent chromosomal karyotype and microarray analysis, 34 subchromosomal deletions/duplications were identified in 26 pregnant women. The observed deletions and duplications ranged from 1.05 to 17.98 Mb. Detection accuracy for these deletions and duplications was 66.7%. Twenty‐one deletions and duplications were found to be correlated with the known abnormalities. NIPT based on high‐throughput sequencing technique is able to identify foetal chromosomal deletions and duplications, but its sensitivity and specificity were not explored. Further progress should be made to reduce false‐positive results.     

X染色体变异对男性精神发育迟滞致病性的研究进展

精神发育迟滞(旧称智力低下)作为儿科神经科常见的一组疾患,具有高度的遗传和表型异质性,大约25%~50%的精神发育迟滞是由遗传因素引起的,其中X染色体基因/基因组变异占25%~30%,导致X连锁的精神发育迟滞。X连锁的精神发育迟滞患者占所有精神发育迟滞患者的10%~15％以上,约20%~25％的男性精神发育迟滞归因于X连锁的精神发育迟滞。精神发育迟滞男女患病比例为1.3:1,这与男性只有一条X染色体的遗传背景有关。随着新一代基因组检测技术的快速发展和临床应用,尤其是全外显子测序、高深度测序、X染色体深度测序和全基因组芯片杂交,这些大大改善了精神发育迟滞患者的X染色体基因/基因组变异检出。本文综述了致精神发育迟滞的X染色体基因组/基因变异特点、其对男性精神发育迟滞的致病性,以及如何采用新测序技术提高检出率,旨在促进科研人员认识X染色体变异在男性精神发育迟滞的致病性,拓宽精神发育迟滞遗传病因的认识,同时也为遗传咨询和产前诊断提供理论依据。     

染色体微阵列分析技术在2600例流产物中的应用

染色体微阵列分析(chromosomal microarray analysis, CMA)是一种通过对染色体进行全基因组扫描来筛查染色体数目和结构异常的检测技术,是儿科和产前遗传诊断的常规工具,已被应用于流产病因分析。本研究应用CMA技术在全基因组水平分析引起流产的染色体异常情况,并评估该技术在临床流产中的应用价值。对收集的2600例流产样本进行CMA技术检测,成功检测了2505例,成功率高达96.3%,其中1021例用CytoScan Optima芯片进行检测,1211例用CytoScan 750K芯片进行检测,273例用CytoScan HD芯片进行检测。利用这3种芯片共检出967例(38.60%)样本发生染色体异常,其中通过CytoScan Optima芯片检出506例(50.00%),CytoScan 750K芯片检出388例(32.00%),CytoScan HD芯片检出73例(26.74%)。在967例染色体异常中,有801例(82.83%)发生染色体数目异常,94例(9.72%)发生染色体结构异常,56例(5.79%)发生嵌合体,16例(1.65%)检出纯合区域。本研究结果表明,CMA可应用于临床流产物的遗传学诊断,是一种可靠、稳定、高分辨的技术,其检测结果能够对再生育风险评估提供指导。     

MLPA-微阵列技术在Y染色体异常检测中的初步应用

摘要: 为了探讨MLPA-微阵列技术用于检测性染色体异常的可行性和精确性, 针对Y染色体上的3个基因TSPY(p11.2)、PRY(q11)和RBMY(q11.2)设计MLPA探针, 应用MLPA-微阵列技术对15例已知染色体核型的样品进行检测, 将检测结果与各样品核型分析和PCR的检测结果进行对照和比较。结果表明, MLPA-微阵列技术对上述各基因位点的检测结果与样品染色体核型基本吻合, 特别是对二例核型分析没有获得染色体结构异常信息的样品, MLPA-微阵列技术检测出Y染色体微小的缺失或指示某些未知染色体片段的信息, 并与PCR检测结果完全相符。表明文章报道的MLPA-微阵列技术能够检测核型分析无法分辨的微小变化和异常, 显示MLPA-微阵列技术在染色体异常分析中具有很高的检测效率和准确性, 相对于染色体核型分析具有明显的优势, 在临床染色体病诊断中具有较大的应用前景。     

Effect of maternal age on the frequency of cytogenetic abnormalities in human oocytes

The cytogenetic investigation of human oocytes was initiated in the Sixties, and for the last four decades, this field of research has never stopped progressing as new technologies appear. Numerous karyotyping studies and molecular cytogenetic studies have been reported to date, providing a large body of data on the incidence and the distribution of chromosomal abnormalities in human female gametes, but also displaying a great variability in results, which may be essentially attributable to the technical limitations of these in situ methods when applied to human oocytes. Essentially, the most relevant analyses have led to the estimate that 15-20% of human oocytes display chromosome abnormalities, and they have emphasized the implication of both whole chromosome nondisjunction and chromatid separation in the occurrence of aneuploidy in human oocytes. The effect of advanced maternal age on the incidence of aneuploidies has also been investigated in human oocytes. Most previous studies have failed to confirm any relationship between maternal age and aneuploidy frequency in human oocytes, whereas the more recent reports based on large samples of oocytes or polar bodies have provided evidence for a direct correlation between increased aneuploidy frequency and advanced maternal age, and have clarified the contribution of the various types of malsegregation in the maternal age-dependent aneuploidies.