Uromodulin storage diseases: Clinical aspects and mechanisms

The recent discovery of mutations in the uromodulin gene (UMOD) in patients with medullary cystic kidney disease type 2 (MCKD2), familial juvenile hyperuricemic nephropathy (FJHN), and glomerulocystic kidney disease (GCKD) provides the opportunity for a revision of pathogenic aspects and puts forth the basis for a renewed classification. This review focuses on clinical, pathological, and cell biology advances in UMOD-related pathological states, including a review of the associated clinical conditions described to date in the literature. Overall, 31 UMOD mutations associated with MCKD2 and FJHN (205 patients) and 1 mutation associated with GCKD (3 patients) have been described, with a cluster at exons 4 and 5. Most are missense mutations causing a cysteine change in uromodulin sequence. No differences in clinical symptoms between carriers of cysteine versus polar residue changes have been observed; clinical phenotypes invariably are linked to classic MCKD2/FJHN. A common motif among all reports is that many overlapping symptoms between MCKD2 and FJHN are present, and a separation between these 2 entities seems unwarranted or redundant. Cell experiments with mutant variants indicated a delay in intracellular maturation and export dynamics, with consequent uromodulin storage within the endoplasmic reticulum (ER). Patchy uromodulin deposits in tubule cells were found by means of immunohistochemistry, and electron microscopy showed dense fibrillar material in the ER. Mass spectrometry showed only unmodified uromodulin in urine of patients with UMOD mutations. Lack of uromodulin function(s) is associated with impairments in tubular function, particularly the urine-concentrating process, determining water depletion and hyperuricemia. Intracellular uromodulin trapping within the ER probably has a major role in determining tubulointerstitial fibrosis and renal failure. We propose the definition of uromodulin storage diseases for conditions with proven UMOD mutations.