Analysis of the Size Distributions of Fetal and Maternal Cell-Free DNA by Paired-End Sequencing

Abstract

BACKGROUND Noninvasive prenatal diagnosis with cell-free DNA in maternal plasma is challenging because only a small portion of the DNA sample is derived from the fetus. A few previous studies provided size-range estimates of maternal and fetal DNA, but direct measurement of the size distributions is difficult because of the small quantity of cell-free DNA. METHODS We used high-throughput paired-end sequencing to directly measure the size distributions of maternal and fetal DNA in cell-free maternal plasma collected from 3 typical diploid and 4 aneuploid male pregnancies. As a control, restriction fragments of λ DNA were also sequenced. RESULTS Cell-free DNA had a dominant peak at approximately 162 bp and a minor peak at approximately 340 bp. Chromosome Y sequences were rarely longer than 250 bp but were present in sizes of <150 bp at a larger proportion compared with the rest of the sequences. Selective analysis of the shortest fragments generally increased the fetal DNA fraction but did not necessarily increase the sensitivity of aneuploidy detection, owing to the reduction in the number of DNA molecules being counted. Restriction fragments of λ DNA with sizes between 60 bp and 120 bp were preferentially sequenced, indicating that the shotgun sequencing work flow introduced a bias toward shorter fragments. CONCLUSIONS Our results confirm that fetal DNA is shorter than maternal DNA. The enrichment of fetal DNA by size selection, however, may not provide a dramatic increase in sensitivity for assays that rely on length measurement in situ because of a trade-off between the fetal DNA fraction and the number of molecules being counted.