Background: Circulating nucleic acids (CNAs) have been shown to have diagnostic utility in human diseases. The aim of this study was to sequence and organize CNAs to document typical profiles of circulating DNA in apparently healthy individuals.
Methods: Serum DNA from 51 apparently healthy humans was extracted, amplified, sequenced via pyrosequencing (454 Life Sciences/Roche Diagnostics), and categorized by (a) origin (human vs xenogeneic), (b) functionality (repeats, genes, coding or noncoding), and (c) chromosomal localization. CNA results were compared with genomic DNA controls (n = 4) that were subjected to the identical procedure.
Results: We obtained 4.5 x 10(5) sequences (7.5 x 10(7) nucleotides), of which 87% were attributable to known database sequences. Of these sequences, 97% were genomic, and 3% were xenogeneic. CNAs and genomic DNA did not differ with respect to sequences attributable to repeats, genes, RNA, and protein-coding DNA sequences. CNA tended to have a higher proportion of short interspersed nuclear element sequences (P = 0.1), of which Alu sequences were significant (P < 0.01). CNAs had a significantly lower proportion of L1 and L2 long interspersed nuclear element sequences (P < 0.01). In addition, hepatitis B virus (HBV) genotype F sequences were found in an individual accidentally evaluated as a healthy control.
Conclusions: Comparison of CNAs with genomic DNA suggests that nonspecific DNA release is not the sole origin for CNAs. The CNA profiling of healthy individuals we have described, together with the detailed biometric analysis, provides the basis for future studies of patients with specific diseases. Furthermore, the detection of previously unknown HBV infection suggests the capability of this method to uncover occult infections.