New Use of old PAP test: Prenatal Genetic Screening

photo courtesy:Wikimedia commons 

Noninvasive prenatal testing (NIPT) used for antenatal detection of fetal aneuploidy is expanding worldwide. NIPT is the most recent addition to the growing armament of methods available for diagnosing chromosomal malformations. The test detects the fetal circulating DNA in maternal blood.

In USA, cell-free DNA analysis became clinically available in 2011 and the American College of Obstetricians and Gynecologists and the Society for Maternal–Fetal Medicine recommended it as a screening option for women at increased risk of fetal aneuploidy. Its use has since been expanded as a screening tool in general obstetric population.[1]

Fetal circulating cell free DNA is derived mostly from placenta and is present in maternal blood for testing as early as 10 weeks of gestation. Different laboratories have validated different techniques and mainly rely on next-generation sequencing technologies and advanced bioinformatic analyses.[2]

Cell free DNA is commonly used to screen for only the common trisomies and, if requested, sex chromosome composition. The sensitivity and specificity as well as the negative predictive value of the method is >99% for trisomy 21 (Down syndrome), with slightly lower performance for trisomy 13 and 18.[3]

However, this test is sometimes limited by low fetal cell fraction in the collected sample. The other two most commonly used test are both invasive. (amniocentesis and chorionic villus sampling).

A recent paper published by Jain et al in Science translational medicine describes a method for still earlier detection of trophoblastic cells, carrying fetal DNA by Papanicolaou smear. It is possible to analyze fetal DNA as early as 5 weeks without any of the invasive tests.[4]

Incidence of fetal aneuploidies are on the rise because of more and more women delaying pregnancies till late thirties or early forties.

The new technique was shown to be effective in correctly picking up cell free DNA from cells collected from maternal endocervical canal in 20 consecutive samples.

Chandni V. Jain, PhD, a research associate in the laboratory of Sascha Drewlo, PhD, and D. Randall Armant, PhD, and colleagues at Wayne State University School of Medicine, Detroit, Michigan, devised a "nuclear isolation protocol" to exclude most maternal DNA from ECC samples.[5]

The investigator quoted “a straightforward alternative that uses a Pap smear to capture intact fetal trophoblast cells in numbers sufficient for next-generation sequencing as early as 5 weeks of gestation." 

The average fetal cell free DNA fraction was 92.2% ± 6.5%, way more than 4% to 10% obtained from maternal blood at 10 weeks for Noninvasive prenatal testing (NIPT).

This test has got tremendous commercial potential as it utilizes existing technology in a novel way.

The limitation at present are small sample size (20) and placental mosaicism.

James Byrne, MD, maternal-fetal medicine specialist and chair of the Department of Obstetrics and Gynecology at the Santa Clara Valley Medical Center and affiliated clinical professor, Stanford University School of Medicine, California opined that the technique is simple, safe and will be accepted widely by patients and physician once it is endorsed by national organization like American Congress of Obstetricians and Gynecologists and Society for Maternal and Fetal Medicine.

Larger studies with large sample size are awaited before it can be used for screening of all pregnant women, who are at average risk. It’s also very suitable for genetic testing, which targets high-risk groups, such as women over age 35 or with a family history of a detectable genetic or chromosomal condition.

A perfect example of new use of old technology. 

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[1] http://www.acog.org/Resources-And-Publications/Committee-Opinions/Committee-on-Genetics/Cell-free-DNA-Screening-for-Fetal-Aneuploidy

[2] Zimmermann B, Hill M, Gemelos G, Demko Z, Banjevic M, Baner J, et al. Noninvasive prenatal aneuploidy testing of chromosomes 13, 18, 21, X, and Y, using targeted sequencing of polymorphic loci. Prenat Diagn 2012;32:1233–41

[3] http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0156088

[4] http://stm.sciencemag.org/content/8/363/363re4

[5] Prenatal Genetic Screening Through Pap Smear?. Medscape. Nov 02, 2016.