Total plasma cfDNA methylation in kidney transplant recipients provides insight into acute allograft rejection pathophysiology
Acute rejection threatens kidney allograft longevity. Cell-free DNA (cfDNA) is a real-time marker of organ injury and immune response. DNA methylation is an epigenetic marker that regulates gene expression. We aim to elucidate differential methylation of total plasma cfDNA between pediatric kidney transplant recipients in the presence compared to the absence of acute rejection. In doing so, we hope to exploit the property of cfDNA as a real-time biomarker and build on available testing to identify genes and processes participating in acute allograft rejection pathophysiology in kidney transplantation. Twenty plasma cfDNA samples from pediatric kidney transplant recipients were collected at the time of allograft biopsy. Using whole genome bisulfite sequencing (WGBS), differentially methylated cytosines were identified in presence vs absence of acute rejection. Separate analyses were performed comparing those with borderline rejection to those with rejection, and to those without rejection. Differentially methylated cytosines were then assessed for gene associations and pathway enrichments. Acute rejection was present in 7 biopsies, borderline rejection in 4 biopsies, and no rejection in 9 biopsies. In the comparison of acute rejection to non-rejection biopsies, there were 34,356 differentially methylated cytosines corresponding to 1,269 associated genes, and 533 enriched pathways. These numbers were all substantially greater (4x-13x) than the comparisons made between acute rejection against those with borderline rejection, and between non-rejection against borderline rejection. Prominently enriched pathways between samples of individuals with and without acute rejection were related to immune cell regulation, inflammatory response, lipid metabolism, and tryptophan-kynurenine metabolism. Our data suggest methylation plays a role in development of or response to acute kidney allograft rejection. Specifically, differentially methylated pathways associated with acute rejection include those related to immune and inflammatory responses. Overall design: WGBS was performed on cell-free DNA isolated from plasma of pediatric kidney transplant recipients. Differentially methylated cytosines (DMCs) were compared between individuals with acute allograft rejection, borderline rejection, and no evidence of rejection.