The gradual elimination of risky procedures used to sample fetal material for prenatal diagnosis has been an important objective in medicine. It is often stated that more fetuses are lost due to such invasive procedures (amniocentesis and chorionic villus sampling) than are identified as carrying a chromosomal abnormality. Noninvasive prenatal diagnosis (NIPD) 2 has developed substantially since the discovery in 1997 that appreciable amounts of free fetal DNA occur in maternal plasma (1). This discovery rapidly paved the way for the detection of paternally inherited alleles in maternal blood, including, most notably, Rh D blood group, fetal sex, and single-gene disorders. For more than a decade now, NIPD has been used routinely for pregnancies in which paternally inherited alleles require detection because their inheritance would indicate a clinical condition (for example, hemolytic disease of the fetus and newborn, congenital adrenal hyperplasia, hemophilia) [for reviews, see (2,–,7)].
NIPD has been more challenging for conditions in which analysis of maternal alleles is required owing to the ubiquitous presence of free maternal DNA in plasma. This situation still represents the most important technical obstacle to achieving routine NIPD for the common chromosomal abnormalities, such as aneuploidy. In 2008, however, there appeared a brace of reports that applied the emerging next-generation sequencing (NGS) technologies to the analysis of trisomies with DNA extracted from maternal plasma (8, 9). Several groups confirmed this breakthrough, and last year investigations of a very large cohort proved the efficacy of NGS as applied to chromosome counting in maternal plasma samples (10). Several consortia [e.g., the European Commission consortium EuroGentest (http://www.eurogentest.org) and the UK RAPID consortium (http://www.rapid.nhs.uk)] are driving the implementation of this technology into clinical practice. Thus, the exquisite resolution of NGS and its ability to sequence a population of DNA molecules prove it has sufficient discriminatory power to define chromosome copy number.
In this issue of Clinical Chemistry, van den Oever et al. describe the application of single-molecule NGS (with the Helicos BioSciences platform) to the detection of trisomy 21 (11). NGS has developed substantially over the past 5 years, but most methods require the preliminary amplification of genomic DNA sequences, a process that is prone to sequence-specific artifacts, notably for regions of high GC content. Single-molecule NGS obviates this requirement, and the authors have compared the efficacy of this approach with that of amplification-based NGS (Illumina's Genome Analyzer II) by using identical maternal plasma samples obtained from mothers with (control) euploid and trisomy 21 fetuses. The greater sensitivity achieved with the Helicos platform is clearly advantageous because it may permit early diagnosis, which the authors of this study reported as being successful at as early as 9 weeks, 3 days gestation. The work clearly shows that further refinement of NGS will pave the way for routine use of NIPD for aneuploidy. Achievement of that goal will reduce appreciably the numbers of invasive prenatal diagnostic procedures and potentially banish the techniques of amniocentesis and chorionic villus sampling to the history books.
↵2 Nonstandard abbreviations:
- noninvasive prenatal diagnosis;
- next-generation sequencing
Author Contributions: All authors confirmed they have contributed to the intellectual content of this paper and have met the following 3 requirements: (a) significant contributions to the conception and design, acquisition of data, or analysis and interpretation of data; (b) drafting or revising the article for intellectual content; and (c) final approval of the published article.
Authors' Disclosures or Potential Conflicts of Interest: Upon manuscript submission, all authors completed the Disclosures of Potential Conflict of Interest form. Potential conflicts of interest:
Employment or Leadership: None declared.
Consultant or Advisory Role: N.D. Avent, Grifols.
Stock Ownership: None declared.
Honoraria: None declared.
Research Funding: N.D. Avent, National Institute of Health Research (UK) and European Commission Framework 7.
Expert Testimony: None declared.
- Received for publication January 14, 2012.
- Accepted for publication January 18, 2012.
- © 2012 The American Association for Clinical Chemistry