Background: Chronic uric acid nephropathy (CUAN), defined as tophaceous deposits of UA in the kidney, is usually an incidental finding on kidney biopsies. Whether these gouty tophi contribute to the progression of chronic kidney disease (CKD) is subject of debate. We hypothesized that patients with renal UA tophi have more kidney fibrosis than patients with hyperuricemia or gout (conditions that are also associated with CKD). Furthermore, we sought to characterize a novel mouse model for CUAN by inducing a liver-specific knockout of Glut9 in mice that additionally received a high fat diet with the UA precursor inosine to elicit hyperuricemia and CKD.
Methods: Out of 81,200 diagnostic kidney biopsies, 199 cases with a history or clinical diagnosis of UA-related pathologies were retrieved of which 84 cases had gouty tophi in kidney biopsies. From the remaining 115 cases without gouty tophi, 84 biopsies from patients with documented hyperuricemia or gout were randomly selected for a case-control design. Both groups were compared to determine the prevalence of gouty tophi and to examine the pathology of morphological abnormalities associated with gouty tophi. Six-week old Alb-creERT2/Glut9lox/lox (ki/ki) or Alb-creERT2/Glut9lox/lox without active Cre (+/+, control) mice were injected with Tamoxifen. Both groups of mice were either fed a high fat diet + inosine or normal chow + inosine for 4 weeks. We assessed UA crystal deposition, macrophage infiltration, the extent of kidney damage, and glomerular filtration rate (GFR) on day 0 and 21 before sacrifice on day 22 followed by kidney flow cytometry, RT-PCR, (immune)-histology, microscopy, and urine colorimetric assays.
Results: Patients with renal gouty tophi had significantly more global glomerulosclerosis (48.1% vs. 33.8%, p=0.003), and moderate to severe interstitial fibrosis and tubular atrophy (81.1% vs. 64.1%, p=0.03) than patients without gouty tophi. Additionally, there was significantly more arteriosclerosis, proteinuria, and podocyte foot process effacement in the group with gouty tophi. The ki/ki mice on chow diet + inosine became hyperuricemic without renal impairment (serum UA: 15-20mg/dl, BUN: 68mg/dl, creatinine: 1.2mg/dl), whereas the ki/ki mice on high fat diet + inosine developed CUAN (serum UA: 15-20mg/dl, BUN: 125mg/dl, creatinine: 2.0mg/dl). Control +/+ mice on both diets did neither develop hyperuricemia nor kidney damage. CUAN caused a significant decline in GFR compared to hyperuricemic ki/ki mice or +/+ control mice, as well as increased mRNA levels of kidney injury and fibrosis markers, and infiltration of macrophages. Histological analysis of kidney sections from mice with CUAN illustrated tubular atrophy, casts and interstitial fibrosis associated with crystal granuloma.
Conclusions: Renal gouty tophi as foreign body granulomas are found at a frequency of 1 out of 1,000 consecutive kidney biopsies and associated with substantial kidney fibrosis, tubular atrophy, glomerulosclerosis, and arteriosclerosis. Challenging ki/ki mice with HFD + inosine resulted in the development of CUAN with crystal deposits. This animal model should be useful to perform interventional studies that could unravel the pathogenesis of granulomatous forms of CKD including CUAN.