Supplementary Material for: Chronic allograft injury by recurrent crystalline nephropathy of Adenine Phosphoribosyl transferase Deficiency, even after early initiation of XOR inhibitor

Introduction: Adenine phosphoribosyl　transferase (APRT) deficiency is a rare genetic disorder of purine metabolism that results in excessive renal excretion of poorly soluble 2,8-dihydroxyadenine (DHA), leading to urolithiasis, crystalline nephropathy, and renal failure. Recurrence after renal transplantation usually occurs in the early post-transplantation period and is sometimes the initial indication for the disease. Allograft survival is poor without adequate treatment with xanthine oxidase (XOR) inhibitors to reduce DHA levels. Case presentation: A 49-year-old Japanese man with a history of recurrent renal stones presented with mild renal dysfunction three months after renal transplantation. Allograft biopsy revealed numerous brown crystals within the tubules and active interstitial inflammation. Febuxostat, a high-dose XOR inhibitor, was immediately prescribed. APRT deficiency was confirmed through urine metabolome analysis. The patient’s renal function improved, and the febuxostat dose was subsequently reduced. One-year allograft biopsy demonstrated markedly reduced inflammation and crystals; however, an expanded scarred area with focal active inflammation and a few small crystals were observed. Conclusion: XOR inhibitors effectively reduce the crystals of recurrent APRT deficiency; however, they cannot prevent chronic injury. Diagnosing recurrent crystalline nephropathy and initiating XOR inhibitors as early as possible is essential for improving allograft survival.