Optimizing Preprocessing for Urinary Cell-Free DNA Preservation: Centrifugation, EDTA Concentration, Storage and Thawing Strategies

Technical Summary: Optimizing Preprocessing for Urinary Cell-Free DNA Preservation

Problem Statement
Urinary cell-free DNA (ucfDNA) represents a promising, noninvasive biomarker for liquid biopsies in the detection of various solid tumors and pathological conditions. However, the clinical translation of ucfDNA is currently hindered by a lack of standardized preprocessing protocols. Unlike blood, urine contains high levels of nucleases and microorganisms that rapidly degrade ucfDNA, which is inherently highly fragmented. Previous studies have utilized various preservatives (e.g., EDTA, commercial stabilizers) and storage methods, but inconsistencies remain regarding optimal EDTA concentrations, the timing of centrifugation (whole urine vs. supernatant), freezing temperatures, and thawing strategies. This study addresses the scarcity of standardized guidelines by systematically evaluating preanalytical variables to optimize ucfDNA yield and purity.

Methodology
The study utilized urine samples collected from healthy volunteers (HVs), which were pooled to minimize inter-individual variability and ensure detectable ucfDNA levels. The experimental design focused on four key preanalytical variables:

1. EDTA Concentration and Sample Type: Whole urine (WU) and urine supernatant (US) were treated with varying concentrations of EDTA (0, 5, 10, 20, and 40 mM). These samples were stored at room temperature (RT) for 1, 3, and 7 days to simulate clinical delivery delays.
2. Thawing Strategies: Frozen samples (Frozen Whole Urine [FWU], Frozen Urine Supernatant [FUS], and FUS with post-thawing secondary centrifugation [FUSS]) were subjected to three thawing methods: 4°C (slow), Room Temperature (RT), and 37°C (rapid). This was tested both with and without 10 mM EDTA.
3. Long-term Storage: EDTA-treated (10 mM) FUS samples were stored at -20°C and -80°C for periods ranging from one to three months.
4. Analysis: ucfDNA was extracted using the Quick-DNA™ Urine Kit. Quality was assessed via the Agilent 4200 TapeStation system, measuring total DNA (TDNA) yield, ucfDNA yield, and the "%cfDNA score" (the proportion of fragments between 50–700 bp relative to total DNA).

Key Results

• Degradation without Preservatives: Untreated urine (both WU and US) exhibited complete ucfDNA degradation within 24 hours at RT.
• EDTA in Whole Urine (WU): While EDTA (5–40 mM) prevented total degradation in WU for up to 7 days, it failed to prevent cellular lysis. Consequently, the %cfDNA scores decreased significantly over time due to the accumulation of high-molecular-weight (HMW) DNA from lysed cells. Higher EDTA concentrations (20–40 mM) correlated with lower ucfDNA yields after 7 days.
• EDTA in Urine Supernatant (US): Centrifugation prior to EDTA addition proved critical. US treated with 10 mM EDTA maintained ucfDNA yields and %cfDNA scores comparable to baseline for 7 days at RT. However, 40 mM EDTA impaired yields, suggesting an optimal concentration exists.
• Thawing Methods:
  • Frozen Whole Urine (FWU): Thawing at 4°C resulted in a >60% loss of ucfDNA yield, likely due to ice recrystallization and cell damage. Rapid thawing (RT/37°C) preserved yields better but still resulted in lower %cfDNA scores compared to US due to cellular lysis.
  • Frozen Urine Supernatant (FUS): FUS samples treated with 10 mM EDTA demonstrated robust stability across all thawing methods, maintaining baseline-equivalent yields and purity.
  • Secondary Centrifugation (FUSS): Post-thawing secondary centrifugation significantly reduced ucfDNA yields, particularly in samples thawed at 4°C (up to 80% loss), likely due to adsorption of DNA to the sediment.
• Storage Conditions: FUS samples with 10 mM EDTA stored at -80°C maintained baseline quality for 3 months. In contrast, samples stored at -20°C showed degradation after 2 months and complete degradation after 3 months.

Key Contributions and Significance
This study provides practical, evidence-based guidelines for the handling of urine samples intended for ucfDNA analysis, addressing the critical gap in preanalytical standardization. The authors claim the following specific contributions:

1. Centrifugation is Paramount: The primary determinant of ucfDNA stability is the removal of cells prior to storage. Storing whole urine, even with EDTA, leads to progressive cellular lysis and HMW DNA contamination, compromising sample purity.
2. Optimized Protocol: The study recommends adding 10 mM EDTA to urine supernatant (US) rather than whole urine. This allows for temporary storage at RT for up to 7 days or freezing at -20°C for less than one month.
3. Long-term Storage Requirements: For storage exceeding one month, ultra-low temperatures (-80°C or liquid nitrogen) are necessary to prevent degradation.
4. Thawing Efficiency: Rapid thawing at RT or 37°C is sufficient and effective for FUS samples, saving time without compromising quality. Conversely, slow thawing at 4°C is detrimental, especially for whole urine.
5. Elimination of Redundant Steps: The study concludes that post-thawing secondary centrifugation of FUS samples is unnecessary and counterproductive, as it leads to significant DNA loss.

Limitations
The authors note that the study relied on pooled urine from healthy volunteers, meaning baseline ucfDNA levels may differ in patient populations. Additionally, the use of pooled samples limited the number of biological replicates per group. The efficacy of EDTA concentrations in patient samples with varying ucfDNA levels remains to be further investigated.

Conclusion
By defining the optimal interplay between centrifugation, EDTA concentration, storage temperature, and thawing methods, this work offers a standardized workflow to maximize ucfDNA quality. The findings suggest that while EDTA is effective for preventing nuclease activity, its utility in whole urine is limited by cellular lysis, making the processing of urine supernatant the superior strategy for preserving high-quality ucfDNA for clinical applications.