Novel UMOD mutations in familial juvenile hyperuricemic nephropathy lead to abnormal uromodulin intracellular trafficking.

BACKGROUND: Familial juvenile hyperuricemic nephropathy (FJHN) is an autosomal dominant disorder characterized by hyperuricemia and progressive chronic kidney disease. Uromodulin gene (UMOD) mutations, leading to abnormalities of uromodulin intracellular trafficking contribute to the progress of the disease.

METHODS: We did UMOD screening in three Chinese FJHN families. We thus constructed mutant uromodulin express plasmids by site-mutagenesis from wild type uromodulin vector and transfected them into HEK293 (human embryonic kidney) cells. And then we detected uromodulin expression by western blot and observed intracellular distribution by immunofluorescence.

RESULTS: We found three heterozygous mutations. Mutation Val109Glu (c.326T/A; p.Val109Glu) and mutation Pro236Gln (c.707C/A; p.Pro236Gln) were newly indentified mutations in two distinct families (family F1 and family F3). Another previously reported UMOD mutation Cys248Trp (c.744C/G; p.Cys248Trp) was detected in family F2. Phenotypes varied both within the same family and between different families. Uromodulin expression is abnormal in the patient biopsy. Functional analysis of mutation showed that mutant types of uromodulin were secreted into the supernatant medium much less when compared with wild type. In mutant type uromodulin transfected cells, intracellular uromodulin localized less in the Golgi apparatus and more in endoplasmic reticulum(ER).

CONCLUSIONS: Our results suggested that the novel uromodulin mutations found in the Chinese families lead to misfolded protein, which was retained in the endoplasmic reticulum, finally contributed to the phenotype of FJHN.