Circulating cell-free DNA undergoes significant decline in yield after prolonged storage time in both plasma and purified form.

OBJECTIVES: Circulating DNA (cirDNA) is generally purified from plasma that has been biobanked for variable lengths of time. In long-term experiments or clinical trials, the plasma can be stored frozen for up to several years. Therefore, it is crucial to determine the stability of cirDNA to ensure confidence in sample quality upon analysis. Our main objective was to determine the effect of storage for up to 2 years on cirDNA yield and fragmentation. METHODS: We stored frozen EDTA plasma and purified cirDNA from 10 healthy female donors, then quantified cirDNA yield at baseline, and at regular intervals for up to 2 years, by qPCR and Qubit. We also compared cirDNA levels in non-haemolysed and haemolysed blood samples after 16 months of storage and tested the effect of varying DNA extraction protocol parameters. RESULTS: Storage up to two years caused an annual cirDNA yield decline of 25.5% when stored as plasma and 23% when stored as purified DNA, with short fragments lost more rapidly than long fragments. Additionally, cirDNA yield was impacted by plasma input and cirDNA elution volumes, but not by haemolysis. CONCLUSIONS: The design of long-term cirDNA-based studies and clinical trials should factor in the deterioration of cirDNA during storage.

Comparison of total and endometrial circulating cell-free DNA in women with and without endometriosis.

RESEARCH QUESTION: Do women with laparoscopically confirmed endometriosis have higher plasma concentrations of circulating cell-free DNA (cirDNA) than those without endometriosis? DESIGN: Prospective study of women aged 18-45 years undergoing benign gynaecological laparoscopy at two tertiary hospitals. Venous blood was collected immediately before surgery, and women were allocated to the endometriosis or control groups based on surgical findings. Total plasma cirDNA and cirDNA integrity were measured by quantitative polymerase chain reaction (qPCR) targeting short (115 bases) and long (247 bases) ALU segments. Endometrial-derived cirDNA was measured by qPCR of bisulfite-treated cirDNA using primers selective for a FAM101A sequence uniquely unmethylated in endometrial tissue. Five cirDNA parameters were compared between the control and endometriosis cohorts: total cirDNA concentration, long-stranded cirDNA concentration, integrity ratio, endometrial cirDNA concentration and endometrial cirDNA proportion. RESULTS: Twenty-eight endometriosis and 15 control samples were included. Women with and without endometriosis had cirDNA concentrations of 2.24 ± 0.89 ng/ml and 2.56 ± 0.92 ng/ml, respectively. Analysis by phenotype of endometriosis revealed a significantly higher endometrial cirDNA concentration in women with superficial disease (n = 10) compared with deep endometriosis (n = 18) (mean difference 0.14 ng/ml; 95% CI 0.15 to 0.26; P = 0.025), but not with controls. CONCLUSIONS: No significant differences were found in any of the cirDNA parameters between women with and without endometriosis. The low statistical power and heterogenous pelvic pathology in the control group render it difficult to determine whether the negative results reflect a true lack of increase in cirDNA in endometriosis.

The influence of biological and lifestyle factors on circulating cell-free DNA in blood plasma.

Research and clinical use of circulating cell-free DNA (cirDNA) is expanding rapidly; however, there remain large gaps in our understanding of the influence of lifestyle and biological factors on the amount of cirDNA present in blood. Here, we review 66 individual studies of cirDNA levels and lifestyle and biological factors, including exercise (acute and chronic), alcohol consumption, occupational hazard exposure, smoking, body mass index, menstruation, hypertension, circadian rhythm, stress, biological sex and age. Despite technical and methodological inconsistences across studies, we identify acute exercise as a significant influence on cirDNA levels. Given the large increase in cirDNA induced by acute exercise, we recommend that controlling for physical activity prior to blood collection is routinely incorporated into study design when total cirDNA levels are of interest. We also highlight appropriate selection and complete reporting of laboratory protocols as important for improving the reproducibility cirDNA studies and ability to critically evaluate the results.

Total and endothelial cell-derived cell-free DNA in blood plasma does not change during menstruation.

Assays measuring cell-free DNA (cfDNA) in blood have widespread potential in modern medicine. However, a comprehensive understanding of cfDNA dynamics in healthy individuals is required to assist in the design of assays that maximise the signal driven by pathological changes, while excluding fluctuations that are part of healthy physiological processes. The menstrual cycle involves major remodelling of endometrial tissue and associated apoptosis, yet there has been little investigation of the impact of the menstrual cycle on cfDNA levels. Paired plasma samples were collected from 40 healthy women on menstruating (M) and non-menstruating (NM) days of their cycle. We measured total cfDNA by targeting ALU repetitive sequences and measured endothelial-derived cfDNA by methylation-specific qPCR targeting an endothelium-unique unmethylated CDH5 DNA region. CfDNA integrity and endothelial cfDNA concentration, but not total cfDNA, are consistent across time between NM and M. No significant changes in total (ALU-115 p = 0.273; ALU-247 p = 0.385) or endothelial cell specific (p = 0.301) cfDNA were observed, leading to the conclusion that menstrual status at the time of diagnostic blood collection should not have a significant impact on the quantitation of total cfDNA and methylation-based cancer assays.

Circulating cell-free DNA from plasma undergoes less fragmentation during bisulfite treatment than genomic DNA due to low molecular weight.

BACKGROUND: Methylation patterns in circulating cell-free DNA are potential biomarkers for cancer and other pathologies. Currently, bisulfite treatment underpins most DNA methylation analysis methods, however, it is known to fragment DNA. Circulating DNA is already short, and further fragmentation during bisulfite treatment is of concern, as it would potentially reduce the sensitivity of downstream assays. METHODS: We used high molecular weight genomic DNA to compare fragmentation and recovery following bisulfite treatment with 2 commercially available kits (Qiagen). The bisulfite treated DNA was visualised on an agarose gel and quantified by qPCR. We also bisulfite treated, visualised and quantitated circulating DNA from plasma. RESULTS: There was no difference in DNA fragmentation between the two kits tested, however, the Epitect Fast kit gave better recovery than the standard Epitect kit, with the same conversion efficiency. We also found that bisulfite treated circulating DNA migrates as distinct bands on agarose gels, suggesting that, in contrast to genomic DNA, it remains largely intact following treatment. Bisulfite treatment of 129 and 234 base PCR products confirmed that this was due to the short length of the circulating DNA fragments. Compared to double stranded DNA, bisulfite treated single stranded DNA gives a very weak signal on gel electrophoresis. CONCLUSIONS: DNA fragmentation during bisulfite treatment does not contribute to loss of sensitivity in methylation analysis of circulating DNA. The absence of DNA fragments below approximately 170 bases from agarose gel images of purified circulating DNA raises the possibility that these fragments are single stranded following the DNA extraction step.