Fragmentomics of urinary cell-free DNA in nuclease knockout mouse models

Urinary cell-free DNA (ucfDNA) is a potential biomarker for bladder cancer detection. However, the biological characteristics of ucfDNA are not well understood. We explored the roles of deoxyribonuclease 1 (DNASE1) and deoxyribonuclease 1-like 3 (DNASE1L3) in the fragmentation of ucfDNA using mouse models. The deletion of Dnase1 in mice (Dnase1^-/-) caused aberrations in ucfDNA fragmentation, including a 24-fold increase in DNA concentration, and a 3-fold enrichment of long DNA molecules, with a relative decrease of fragments with thymine ends and reduction of jaggedness (i.e., the presence of single-stranded protruding ends). In contrast, such changes were not observed in mice with Dnase1l3 deletion (Dnase1l3^-/-). These results suggested that DNASE1 was an important nuclease contributing to the ucfDNA fragmentation. Western blot analysis revealed that the concentration of DNASE1 protein was higher in urine than DNASE1L3. The native-polyacrylamide gel electrophoresis zymogram showed that DNASE1 activity in urine was higher than that in plasma. Furthermore, the proportion of ucfDNA fragment ends within DNase I hypersensitive sites (DHSs) was significantly increased in Dnase1-deficient mice. In humans, patients with bladder cancer had lower proportions of ucfDNA fragment ends within the DHSs when compared with participants without bladder cancer. The area under the curve (AUC) for differentiating patients with and without bladder cancer was 0.83, suggesting the analysis of ucfDNA fragmentation in the DHSs may have potential for bladder cancer detection. This work revealed the intrinsic links between the nucleases in urine and ucfDNA fragmentomics.

Cell-free DNA (cfDNA) in various bodily fluids, for example blood plasma and urine, is of great importance for noninvasive cancer detection and noninvasive prenatal testing. Many emerging studies on the fragmentation of plasma DNA (i.e., fragmentomics) have received much recent interest. However, the fragmentomics in urinary cfDNA (ucfDNA) remained much less explored. In this study, we explored the biological links between ucfDNA fragmentation and DNA nucleases, using mice for which either the Dnase1 or Dnase1l3 gene was genetically inactivated. Interestingly, we found that the deletion of Dnase1, but not Dnase1l3, caused dramatic alterations of ucfDNA fragmentation patterns, including the elevation of DNA concentration, lengthening of fragment size, disruptions of ucfDNA end motifs (i.e., nucleotide sequences at fragment end) and the jagged ends (i.e., the single-stranded protruding ends). The proportion of fragment ends within DNase I hypersensitive sites (DHSs) was greatly increased in mice with the Dnase1 deletion. Such ucfDNA fragmentation pattern surrounding DHSs holds potential for classifying the human subjects with and without bladder cancer. The analysis combining various fragmentomic features could further improve the performance for bladder cancer detection, with an AUC of 0.91. This study has shed mechanistic insights into fragmentomics of ucfDNA in urine and has opened up new possibilities for applying ucfDNA fragmentomics in a clinical context.