Endocervical fetal trophoblast for prenatal genetic diagnosis

Reappraisal of evolving methods in non-invasive prenatal screening: Discovery, biology and clinical utility

Non-invasive prenatal screening (NIPS) offers an opportunity to screen or determine features associated with the fetus. Earlier, prenatal testing was done with cytogenetic procedures like karyotyping or fluorescence in-situ hybridization, which necessitated invasive methods such as fetal blood sampling, chorionic villus sampling or amniocentesis. Over the last two decades, there has been a paradigm shift away from invasive prenatal diagnostic methods to non-invasive ones. NIPS tests heavily rely on cell-free fetal DNA (cffDNA). This DNA is released into the maternal circulation by placenta. Like cffDNA, fetal cells such as nucleated red blood cells, placental trophoblasts, leukocytes, and exosomes or fetal RNA circulating in maternal plasma, have enormous potential in non-invasive prenatal testing, but their use is still limited due to a number of limitations. Non-invasive approaches currently use circulating fetal DNA to assess the fetal genetic milieu. Methods with an acceptable detection rate and specificity such as sequencing, methylation, or PCR, have recently gained popularity in NIPS. Now that NIPS has established clinical significance in prenatal screening and diagnosis, it is critical to gain insights into and comprehend the genesis of NIPS de novo. The current review reappraises the development and emergence of non-invasive prenatal screen/test approaches, as well as their clinical application, with a focus, on the scope, benefits, and limitations.

Considerations for whole exome sequencing unique to prenatal care

Whole exome sequencing (WES) is increasingly being used in the prenatal setting. The emerging data support the clinical utility of prenatal WES based on its diagnostic yield, which can be as high as 80% for certain ultrasound findings. However, detailed practice and laboratory guidelines, addressing the indications for prenatal WES and the surrounding technical, interpretation, ethical, and counseling issues, are still lacking. Herein, we review the literature and summarize the most recent findings and applications of prenatal WES. This review offers specialists and clinical genetic laboratorians a body of evidence and expert opinions that can serve as a resource to assist in their practice. Finally, we highlight the emerging technologies that promise a future of prenatal WES without the risks associated with invasive testing.

Trophoblast retrieval and isolation from the cervix: origins of cervical trophoblasts and their potential value for risk assessment of ongoing pregnancies

BACKGROUND

Early during human development, the trophoblast lineage differentiates to commence placentation. Where the placenta contacts the uterine decidua, extravillous trophoblast (EVT) cells differentiate and invade maternal tissues. EVT cells, identified by expression of HLA-G, invade into uterine blood vessels (endovascular EVT), as well as glands (endoglandular EVT), and open such luminal structures towards the intervillous space of the placenta. Endoglandular invasion diverts the contents of uterine glands to the intervillous space, while glands near the margin of the placenta that also contain endoglandular EVT cells open into the reproductive tract. Cells of the trophoblast lineage have thus been recovered from the uterine cavity and endocervical canal. An emerging non-invasive technology [trophoblast retrieval and isolation from the cervix (TRIC)] isolates and examines EVT cells residing in the cervix to explore their origin, biology and relationship to pregnancy and fetal status.

OBJECTIVE AND RATIONALE

This review explores the origins and possible uses of trophoblast cells obtained during ongoing pregnancies (weeks 5-20) by TRIC. We hypothesize that endoglandular EVT cells at the margins of the expanding placenta enter the uterine cavity and are carried together with uterine secretion products to the cervix where they can be retrieved from a Papanicolaou (Pap) smear. The advantages of TRIC for investigation of human placentation and prenatal testing will be considered. Evidence from the literature, and from archived in utero placental histological sections, is presented to support these hypotheses.

SEARCH METHODS

We used 52 out of 80 publications that appeared between 1966 and 2017 and were found by searching the PubMed and Google Scholar databases. The studies described trophoblast invasion of uterine vessels and glands, as well as trophoblast cells residing in the reproductive tract. This was supplemented with literature on human placental health and disease.

OUTCOMES

The literature describes a variety of invasive routes taken by EVT cells at the fetal-maternal interface that could displace them into the reproductive tract. Since the 1970s, investigators have attempted to recover trophoblast cells from the uterus or cervix for prenatal diagnostics. Trophoblast cells from Pap smears obtained at 5-20 weeks of gestation have been purified (>95% β-hCG positive) by immunomagnetic isolation with nanoparticles linked to anti-HLA-G (TRIC). The isolated cells contain the fetal genome, and have an EVT-like expression profile. Similar EVT-like cells appear in the lumen of uterine glands and can be observed entering the uterine cavity along the margins of the placenta, suggesting that they are the primary source of cervical trophoblast cells. Cells isolated by TRIC can be used to accurately genotype the embryo/fetus by targeted next-generation sequencing. Biomarker protein expression quantified in cervical trophoblast cells after TRIC correlates with subsequent pregnancy loss, pre-eclampsia and fetal growth restriction. A key remaining question is the degree to which EVT cells in the cervix might differ from those in the basal plate and placental bed.

WIDER IMPLICATIONS

TRIC could one day provide a method of risk assessment for maternal and fetal disease, and reveal molecular pathways disrupted during the first trimester in EVT cells associated with placental maldevelopment. As perinatal interventions emerge for pregnancy disorders and inherited congenital disorders, TRIC could provide a key diagnostic tool for personalized precision medicine in obstetrics.

Cell-free fetal DNA and maternal serum analytes for monitoring embryonic and fetal status

Biomarkers can be employed for screening for fetal genetic disorders, identifying individuals at sufficiently high risk for a confirmatory invasive procedure. In this article we discuss prenatal genetic aneuploidy screening. Maternal serum analytes and ultrasound have long been routinely offered, providing detection rates of 80% to 93% for trisomy 21; however, an invasive procedure (false-positive) must be performed in 5% of pregnancies screened. Recovering fetal cells from maternal blood initially proved inconsistent, but new methods recovering trophoblasts offer promise for detecting disorders using a single fetal cell, analogous to preimplantation genetic diagnosis. Current emphasis is focused predominantly on cell-free fetal DNA, which accounts for 5% to 10% of total cell-free DNA in maternal blood. Analysis of maternal blood results in detection rates of over 99% for fetal trisomy 21, and also very high rates for trisomy 18 and sex chromosomal abnormalities. Such detection rates are substantively higher than with maternal serum analytes, and are accompanied by a much lower (<1%) false-positive rate.

Prenatal diagnosis of trisomy 21 through detection of trophoblasts in cervical smears

BACKGROUND

Fetal cells exfoliate in the uterine cavity during early pregnancy and are a potential source of material for NIPD.

AIMS

This study was designed to test the hypothesis that fetal cells obtained from the uterine cervix during the first trimester of pregnancy could be utilized for prenatal diagnosis of chromosomal aneuploidy.

STUDY DESIGN

Fetal cells retrieved from the distal endocervical canal during the first trimester of pregnancy were hybridized with chromosome 21 specific FISH probes and analyzed with an automated fluorescence microscope.

SUBJECTS AND OUTCOME MEASURES

Cells with 3 copies of chromosome 21 were detected in 5 out of 5 trisomy 21 pregnancies.

RESULTS

The number of trisomic cells detected ranged from 1 to 27 with a median value of 5.

CONCLUSIONS

FISH-based scanning can identify trisomy 21 pregnancies by analysis of routine cervical brushings. The approach offers the potential for non-invasive prenatal diagnosis as early as 5 weeks gestation.

Transcervical retrieval of fetal cells in the practice of modern medicine: a review of the current literature and future direction

OBJECTIVE

To review published methods for transcervical collection of fetal cells and to assess the potential of this approach for application in prenatal diagnosis.

MAIN OUTCOME MEASURE(S)

Retrospective analysis of efforts at prenatal diagnosis with trophoblast cells shed into the lower uterine pole that accumulate within the cervical mucus at the level of the internal os.

RESULT(S)

Minimally invasive techniques that include cervical mucus aspiration, cervical swabbing, and cervical or intrauterine lavage can be used to retrieve trophoblast cells during the first trimester for diagnostic purposes, including for prenatal genetic analysis. Fetal cells have been identified in these specimens with success rates that vary from 40% to 90%. The disparity in reported success rates can be a function of gestational age, collection method, operator variability, detection sensitivity, or pregnancy status. Molecular approaches have been devised to determine fetal sex and identify aneuploidies. Antibody markers have proven useful to select trophoblast cells for genetic analysis and to demonstrate that the abundance of recoverable fetal cells diminishes in abnormal gestations, such as in ectopic pregnancy or blighted ovum.

CONCLUSION(S)

Transcervical collection of fetal cells offers several avenues for prenatal diagnosis that with further refinement could one day provide valuable information for the management of ongoing pregnancies.

The presence of trophoblastic cells in intrauterine lavage samples: lack of correlation with maternal and obstetric characteristics

OBJECTIVES

To investigate the correlation between maternal, obstetric and sample characteristics and the quality (i.e. yield of trophoblastic cells) of intrauterine lavage (IUL) samples.

METHODS

We collected 202 IUL samples from women scheduled for first trimester termination of pregnancy (TOP). Trophoblastic cells were isolated from IUL samples and used for DNA analysis by a multiplex quantitative fluorescent polymerase chain reaction (QF-PCR) assay. A multivariate logistic regression analysis was performed, and a p<0.05 was considered statistically significant.

RESULTS

The presence of trophoblastic cells in IUL samples was documented in 151/202 cases (74.7%). Blood contamination of IULs was the only characteristic to positively correlate with the presence of trophoblasts (p=0.039; OR: 1.99; 95% CI: 1.03-3.82).

CONCLUSIONS

The correlation between the presence of contaminating blood and trophoblastic cells would indirectly confirm the hypothesis that IUL might act as a mini-CVS. The high yield rate of trophoblasts irrespective of maternal characteristics and past obstetric history would support the clinical use of this sampling technique, provided that its safety is clearly defined.

Prenatal diagnosis of common aneuploidies in transcervical samples using quantitative fluorescent-PCR analysis

AIM

The aim of this study was to test the feasibility of diagnosing common fetal chromosomal aneuploidies using quantitative fluorescent (QF)-PCR on transcervical cell (TCC) samples collected in the first trimester of pregnancy by means of intrauterine lavage (IUL).

METHODS

A total of 181 TCC samples were retrieved from pregnant women between 5 and 12 weeks of gestation, immediately before elective termination of pregnancy, at which time corresponding placental tissue and maternal blood specimens were also obtained. Isolation of trophoblastic cells by micromanipulation was attempted in all TCC samples. Micromanipulated specimens were analyzed by multiplex QF-PCR, including short tandem repeats for the chromosomes X, Y, 21, 18, and 13.

RESULTS

The micromanipulation was successful in 152 of 181 cases (84.8%) where chorionic villous filaments and/or cell clumps of seeming trophoblastic origin could be isolated. All 152 samples were tested by QF-PCR analysis and peaks of paternal origin could be documented in all cases. Two cases of trisomy 21 and two cases of monosomy X0 were detected by means of QF-PCR assay, in accordance with the results obtained in corresponding placental samples.

CONCLUSION

This study provides evidence that the use of multiplex QF-PCR amplification of selected microsatellites could be applied to micromanipulated TCC samples and in particular to IUL samples, which often contain trophoblastic cells, for the detection of chromosomal aneuploidies. The approach described in this study appears, therefore, a very promising tool toward non-invasive prenatal genetic diagnosis in the early stage of gestation.