Non-invasive fetal sex determination: impact on clinical practice

Non-invasive tool for foetal sex determination in early gestational age

Free foetal DNA in maternal blood during early pregnancy is an ideal source of foetal genetic material for non-invasive prenatal diagnosis. The aim of this study was to evaluate the use of free foetal DNA analysis at early gestational age as pretest for the detection of specific Y-chromosome sequences in maternal plasma of women who are carriers of X-linked disorders, such as haemophilia. Real-time quantitative PCR analysis of maternal plasma was performed for the detection of the SRY or DYS14 sequence. A group of 208 pregnant women, at different gestational periods from 4 to 12 weeks, were tested to identify the optimal period to obtain an adequate amount of foetal DNA for prenatal diagnosis. Foetal gender was determined in 181 pregnant women sampled throughout pregnancy. Pregnancy outcome and foetal gender were confirmed using karyotyping, ultrasonography or after birth. The sensitivity, which was low between 4th and 7th week (mean 73%), increased significantly after 7+1th weeks of gestation (mean 94%). The latter sensitivity after 7+1th week of gestation is associated to a high specificity (100%), with an overall accuracy of 96% for foetal gender determination. This analysis demonstrates that foetal gender determination in maternal plasma is reliable after the 9th week of gestation and it can be used, in association with ultrasonography, for screening to determine the need for chorionic villus sampling for prenatal diagnosis of X-linked disorders, such as haemophilia.

Fetal sex determination using circulating cell-free fetal DNA (ccffDNA) at 11 to 13 weeks of gestation

OBJECTIVE

To examine the performance of a mass spectrometry-based detection platform using three Y-chromosome sequences for fetal sex determination from circulating cell-free fetal DNA (ccffDNA) in maternal blood in the first trimester of pregnancy.

METHODS

We extracted ccffDNA for the determination of fetal sex from stored maternal plasma obtained at 11 to 13 weeks' gestation from singleton pregnancies with documented fetal gender. Mass spectrometry was used to examine 236 specimens for the presence of three Y-chromosome sequences (SRY, DBY and TTTY2). The sample was classified as male, female or inconclusive depending on the detection of three, one/none and two sequences, respectively.

RESULTS

Three (1.3%) of the 236 cases were classified as invalid due to the absence of a well-defined spectral peak for TGIF and 22 (9.3%) were reported as inconclusive. In the 211 cases with a valid result, the fetal sex was correctly identified in 90 of 91 male babies and 119 of 120 female babies giving an accuracy of 99.1% and sensitivity and specificity for prediction of male fetuses of 98.9 and 99.2%, respectively.

CONCLUSION

Fetal sex determination can be accurately determined from maternal ccffDNA in the first trimester of pregnancy using mass spectrometry analysis.

Molecular prenatal diagnosis: the impact of modern technologies

Originally prenatal diagnosis was confined to the diagnosis of metabolic disorders and depended on assaying enzyme levels in amniotic fluid. With the development of recombinant DNA technology, molecular diagnosis became possible for some genetic conditions late in the 1970s. Here we briefly review the history of molecular prenatal diagnostic testing, using Duchenne muscular dystrophy as an example, and describe how over the last 30 years we have moved from offering testing to a few affected individuals using techniques, such as Southern blotting to identify deletions, to more rapid and accurate PCR-based testing which identifies the precise change in dystrophin for a greater number of families. We discuss the potential for safer, earlier prenatal genetic diagnosis using cell free fetal DNA in maternal blood before concluding by speculating on how more recent techniques, such as next generation sequencing, might further impact on the potential for molecular prenatal testing. Progress is not without its challenges, and as cytogenetics and molecular genetics begin to unite into one, we foresee the main challenge will not be in identifying the genetic change, but rather in interpreting its significance, particularly in the prenatal setting where we frequently have no phenotype on which to base interpretation.

Cell-free fetal DNA in the maternal serum and plasma: current and evolving applications

PURPOSE OF REVIEW

Free fetal nucleic acids, found in the plasma of every pregnant woman, have made a substantial impact on prenatal diagnosis. The past decade has seen the introduction of routine noninvasive prenatal diagnosis (NIPD) using DNA extracted from maternal plasma for a number of clinical complications of pregnancy, notably feto-maternal blood group incompatibility, fetal sexing and exclusion/detection of single-gene disorders. It appears that mass-scale analysis of all RhD-negative pregnant women will be adopted to conserve stocks of prophylactic anti-D and avoid the administration of a blood product unnecessarily. For the majority of prenatal diagnostic procedures, the assessment of trisomy, particularly trisomy 21, is the highest priority. Because RHD genotyping, fetal sexing and analysis of single-gene disorders all depend on the detection of paternally inherited alleles, they were relatively simple to adapt on the basis of PCR analysis of DNA obtained from maternal plasma. However, for assessment of chromosome copy number, this is not so straightforward.

RECENT FINDINGS

The assessment of polymorphisms among placentally expressed mRNAs found in maternal plasma has enabled the detection of trisomy 21 fetuses using a combination of reverse transcriptase PCR and mass spectrometry to define allelic ratios of maternally and paternally inherited single nucleotide polymorphisms. Interesting recent developments also include the finding that direct sequence analysis of maternal plasma extracted DNA using 'next-generation' DNA sequencers can differentiate between normal and trisomy fetuses.

SUMMARY

NIPD using nucleic acids obtained from maternal plasma and serum is now a clinical reality, particularly in the management of hemolytic disease of the fetus and newborn. Recent advances signal that NIPD for common aneuploidies will soon be possible.

Prenatal testing for Down syndrome: comparison of screening practices in the UK and USA

Prenatal testing for Down Syndrome is a topic covered in every genetic counselor's training as it constitutes the main workload of genetic counselors in prenatal settings. Most Western countries nowadays offer some type of testing for Down Syndrome. However, practices vary according to country with regards to what tests are offered, insurance coverage and the legal situation concerning the option of terminating an affected pregnancy. In view of the growing interest in international genetic counseling issues, this article aims to compare prenatal testing practices in two English-speaking countries: the United Kingdom and the United States of America. A case will be presented to highlight some of the differences in practice. The topic underlines important implications for genetic counseling practice, such as patients' understanding of testing practices, risk perception, counseling provision and impact of prenatal testing results.

High throughput non-invasive determination of foetal Rhesus D status using automated extraction of cell-free foetal DNA in maternal plasma and mass spectrometry

PURPOSE

To examine the potential high throughput capability and efficiency of an automated DNA extraction system in combination with mass spectrometry for the non-invasive determination of the foetal Rhesus D status.

METHODS

A total of 178 maternal plasma samples from RHD-negative pregnant women were examined, from which DNA was extracted using the automated Roche MagNA Pure system. Presence of the foetal RHD gene was detected by PCR for RHD exon 7 and subsequent analysis using the Sequenom MassArray mass spectrometric system.

RESULTS

We determined that as little as 15 pg of RHD-positive genomic DNA could be detected in a background of 585 pg of RHD-negative genomic DNA. The analysis of the clinical samples yielded a sensitivity and specificity of 96.1 and 96.1%, respectively.

CONCLUSION

Our study indicated that automated DNA extraction in combination with mass spectrometry permits the determination of foetal Rhesus D genotype with an accuracy comparable to the current approaches using real-time PCR.