Cellular and Molecular Mechanisms of Kidney Injury in 2,8-Dihydroxyadenine Nephropathy
Authors: Klinkhammer Barbara Mara , Djudjaj Sonja , Kunter Uta , Palsson Runolfur , Edvardsson Vidar Orn , Wiech Thorsten , Thorsteinsdottir Margret , Hardarson Sverrir , Foresto-Neto Orestes , Mulay Shrikant R. , Moeller Marcus Johannes , Jahnen-Dechent Wilhelm , Floege Jürgen , Anders Hans-Joachim , Boor Peter 
Published: February 2020

Significance Statement
Lack of well characterized experimental models of 2,8-dihydroxyadenine nephropathy—resulting from formation of 2,8-dihydroxyadenine crystals within renal tubules due to a rare hereditary deficiency of adenine phosphoribosyltransferase in humans (or excessive adenine load in animals)—has hindered achieving a better understanding of underlying disease mechanisms. The authors demonstrate that crystal formation, deposition, and clearance, as well as the resulting renal tubular injury, inflammation, fibrosis, and loss of kidney function, are virtually identical in experimental rodent models induced by an adenine-enriched diet and in patients with adenine phosphoribosyltransferase deficiency. These models are thus suitable to study cellular mechanisms, such as crystal clearance (via a reparative process the authors call extratubulation), or to identify relevant molecular pathways, such as TNF receptor 1–dependent crystal retention, that might inform the development of novel treatments.

Background
Hereditary deficiency of adenine phosphoribosyltransferase causes 2,8-dihydroxyadenine (2,8-DHA) nephropathy, a rare condition characterized by formation of 2,8-DHA crystals within renal tubules. Clinical relevance of rodent models of 2,8-DHA crystal nephropathy induced by excessive adenine intake is unknown.

Methods
Using animal models and patient kidney biopsies, we assessed the pathogenic sequelae of 2,8-DHA crystal-induced kidney damage. We also used knockout mice to investigate the role of TNF receptors 1 and 2 (TNFR1 and TNFR2), CD44, or alpha2-HS glycoprotein (AHSG), all of which are involved in the pathogenesis of other types of crystal-induced nephropathies.

Results
Adenine-enriched diet in mice induced 2,8-DHA nephropathy, leading to progressive kidney disease, characterized by crystal deposits, tubular injury, inflammation, and fibrosis. Kidney injury depended on crystal size. The smallest crystals were endocytosed by tubular epithelial cells. Crystals of variable size were excreted in urine. Large crystals obstructed whole tubules. Medium-sized crystals induced a particular reparative process that we term extratubulation. In this process, tubular cells, in coordination with macrophages, overgrew and translocated crystals into the interstitium, restoring the tubular luminal patency; this was followed by degradation of interstitial crystals by granulomatous inflammation. Patients with adenine phosphoribosyltransferase deficiency showed similar histopathological findings regarding crystal morphology, crystal clearance, and renal injury. In mice, deletion of Tnfr1 significantly reduced tubular CD44 and annexin two expression, as well as inflammation, thereby ameliorating the disease course. In contrast, genetic deletion of Tnfr2, Cd44, or Ahsg had no effect on the manifestations of 2,8-DHA nephropathy.

Conclusions
Rodent models of the cellular and molecular mechanisms of 2,8-DHA nephropathy and crystal clearance have clinical relevance and offer insight into potential future targets for therapeutic interventions.