Cell-Free DNA Levels of Twins and Sibling Pairs Indicate Individuality and Possible Use As a Personalized Biomarker

Background: Cell-free DNA (cfDNA) in the human blood circulation has been under investigation since its initial observation in 1948. Plasma cfDNA is known to be significantly elevated in diseased people. Due to possible variation in the population, evaluating cfDNA as a non-invasive biomarker at disease onset alone may not be sensitive enough to accurately diagnose diseases, particularly early stage cancers on a personal level. Methods: To understand the source and level of variation, we quantified cfDNA from a total of 365 plasma samples (twins, siblings, unrelated individuals, and cancer patients) by a fluorometer (ng/ml) and/or real-time PCR (copies/ml). We evaluated associations between individuals' genetic and environmental background and their plasma cfDNA levels. Findings: We found a high correlation (r =0.77; p < 0.0001) in plasma cfDNA levels between identical twins (n=39). However, the correlation was gradually reduced to moderate (r=0.47; p=0.016) between fraternal (dizygotic) twins (n=13) and low correlation (r=0.28; p=0.043) between sibling pairs (n=26). Moreover, while age had no effect, gender significantly influenced the individual's plasma cfDNA level. As expected, cfDNA was also significantly increased in both breast (p<0.0001) and ovarian (p<0.0001) cancer patients compared to healthy individuals. Interpretation: Our study clearly demonstrated that both genome and environmental factors modulate the individual's cfDNA level suggesting that its diagnostic sensitivity may be improved only if the person's cfDNA level is known prior to disease presentation. Funding Statement: This work was supported by the Kingdom of Saudi Arabia National Science, Technology and Innovation Plan (NSTIP) Strategic Technologies in the Kingdom-Award No: 10-BIO951-20. Declaration of Interests: The authors declare no competing interests. Ethics Approval Statement: This study was approved by the Research Ethics Committee (RAC) of the King Faisal Specialist Hospital and Research Center (KFSH&RC) (RAC#2130037). All methods have been carried out in accordance with relevant guidelines and regulations. All subjects were properly informed about the study and their written consents were obtained. For subjects who were under age 18 years, informed consents were provided by either a parent or a legal guardian.