Perspectives on Donor-Derived Cell-Free DNA in Kidney Transplant Recipients with Systemic Lupus Erythematosus.

Abstract

Many kidney transplant recipients undergo graft surveillance with AlloSure® donor-deived cell-free DNA (dd-cfDNA) (CareDx Inc.). Published data show that 75% of patients with stable grafts have dd-cfDNA less than 0.4%. In our experience, patients with systemic lupus erythematosus (SLE) have higher levels of dd-cfDNA, with fluctuations that cannot be explained by graft injury. We retrospectively evaluated 19 patients with SLE who underwent kidney transplantation (68% female, median age 40 years). Seventy-five dd-cfDNA values were obtained over a 3-year period; median= 0.39% (interquartile range: 0.21%-0.95%). Six allograft biopsies were performed during the study period (Table 1). Five showed no rejection, associated dd-cfDNA levels= 0.24%, 0.26%, 0.43%, 2.9%, 6.9%. One biopsy revealed antibody-mediated rejection (AMR), dd-cfDNA= 0.85%. The published median for AMR is 1.8%, and values less than 1% have a negative predictive value of 96%. Variations in dd-cfDNA can be used to detect rejection; relative change value (RCV) of >61% has been associated with high probability of graft injury. Two patients demonstrated significant variation in dd-cfDNA (Figure 1). The First patient increased from 0.5% to 2.9% (RCV= 480%) and the second patient rose from 0.31% to 6.9% (RCV= 2125%). Both patients had normal biopsies at the time of peak dd-cfDNA. Cell-free DNA (cfDNA) has been described in the pathophysiology of SLE and other inflammatory diseases. Elevations in cfDNA have been associated with SLE and have been proposed as a diagnostic biomarker. Systemic inflammation results in the release of cfDNA into circulation; fluctuations in dd-cfDNA may be related to lupus flares, or other SLE-related physiology, however, this does not appear to be the reason based on review of the patients’ charts. Published data doesn’t show clear correlation between disease flares and cfDNA levels in SLE. Because AlloSure is measured as a fraction (donor-derived/total circulating cfDNA), we investigated that changes in the total circulating cfDNA may lead to inappropriately high or low values in the setting of SLE. The total cfDNA values for our patients had no correlation between total cfDNA and the reported AlloSure values. Patients with large fluctuations in AlloSure had consistently low total cfDNA, suggesting that the source was the allograft. This requires attention since false positive values can lead to unnecessary biopsies. This small cohort of 6 biopsies could be outliers by chance, or by other factors not related to SLE. However, given the relationship between SLE and cfDNA, it is worth further investigation. Elevations in cfDNA are a hallmark of SLE, where both hematopoietic and nonhematopoietic cells contribute to cfDNA levels. Levels of cfDNA in circulation are affected by genetic and systemic factors. DNAse family of enzymes naturally degrade cfDNA and are essential for regulating the levels in circulation. Patients with single-nucleotide polymorphisms (SNPs) in nucleases DNAse1L3, DNAse2, and TREX1 were highly associated with SLE. Thus, changes in the level of cfDNA digesting enzymes among the patients might be a factor responsible for the aberrant dd-cfDNA levels observed. An alternative could be a deficit in the normal cfDNA clearance mechanisms. In healthy persons, cfDNA has a half-life of between 16 min and 2 h, with clearance effected via the reticuloendothelial system in the spleen and liver or via passive urinary secretion mediated by the kidney. Functional deficit in any of the 3 organ systems could contribute to the observed dd-cfDNA elevations in SLE. Future basic science studies or clinical collaborations could greatly benefit this population of kidney transplant recipients.