Urine Uromodulin and Genetics of its Variation
Authors: Nora Franceschini, Thu H. Le
Journal: Journal of the American Society of Nephrology (2022)

Uromodulin, or Tamm-Horsfall protein, is a protein well known to nephrologists as the gel-like structure matrix found in urinary casts.1 The glycoprotein is produced by epithelial cells of the thick ascending limb (TAL) and the distal convoluted tubules of the nephron. It is detected in urine as a high-molecular weight polymer, and is found in low concentrations in the serum, in a monomeric form. Both serum and urine uromodulin concentrations correlate with kidney function, and may be markers of functional kidney mass and tubular health.2,3 In the urine, uromodulin acts as an inhibitor of calcium crystallization and is thought to have an immune role against uropathogenic bacteria. The uromodulin protein is commercially measured using ELISA. However, urinary uromodulin is more susceptible to degradation and quality-control laboratory conditions, and there is variability in its measurement.4 Genetic studies have identified rare coding mutations in the uromodulin gene (UMOD) associated with monogenic forms of CKD and UMOD noncoding (regulatory) common variants have been consistently associated with kidney function, CKD, and hypertension in genome-wide association studies (GWAS). Therefore, there is a great interest in identifying additional genetic factors that regulate uromodulin protein beyond the gene that encodes it. In this issue, Joseph et al.5 report new loci for urine uromodulin in a GWAS meta-analysis that included 13 studies and 29,512 participants of European ancestry. The study identified associations at the known UMOD locus and two new loci at KRT40 (raw uromodulin) and WDR72 (urine uromodulin adjusted by creatinine). WDR72 is a locus previously described for eGFR and more research is needed to examine if variants at this locus have pleiotropic effects on uromodulin and eGFR. The study provided some experimental support for the finding at the KRT40 locus by showing that the KRT40 encoded protein is weakly expressed and colocalizes with uromodulin in the TAL in both mouse and human kidney tissues. These findings contrast with those from the database of RNA sequencing analysis of microdissected rat kidney tubule segments (https://esbl.nhlbi.nih.gov/helixweb/Database/NephronRNAseq/All_transcripts.html), which showed no Krt40 transcript identified anywhere along the rat kidney tubule, and the Human Protein Atlas reporting no KRT40 protein expression in kidney tubules (https://www.proteinatlas.org/ENSG00000204889-KRT40/tissue/kidney#img). This raises the question of whether KRT40 kidney expression is species specific, or that the discordant findings are due to specificity of antibodies used. Nevertheless, the authors showed Krt40 downregulation modulated uromodulin expression and localization in mouse kidney cells. Specifically, knockdown of Krt40 resulted in more nuclear localization of uromodulin, increased intracellular accumulation of uromodulin, and decreased uromodulin in the apical medium, suggesting loss of KRT40 impairs uromodulin secretion. Knockdown of Krt40 also resulted in altered localization of ROMK to the basolateral side of the TAL, suggesting KRT40 may play a role in the polarized sorting of proteins in kidney tubular cells. Further studies are needed to build on these findings in human population genetic studies and functional experimental correlate to better delineate the effect of KRT40 in kidney health and disease. For example, prior human genetic studies and experimental models have uncovered that UMOD plays a role in salt-sensitive hypertension by modulating the activation of renal sodium cotransporter NKCC2.6 It follows that dysregulation of the KRT40-UMOD axis might affect blood pressure and salt sensitivity. The study found two independent single nucleotide polymorphisms (SNP) at the UMOD locus associated with urine uromodulin, including a SNP in the UMOD promoter and a SNP in the nearby PDILT gene. Despite impressive P values for associations with uromodulin, the SNPs explain only 0.1% of the trait variance, and the variance for the additional loci was not reported, but it is expected to be small. The same SNPs have been associated with eGFR in GWAS, although the results are less strong in non-European populations.7,8 Multiethnic GWAS meta-analyses of eGFR have identified at least four independent SNPs at the locus, suggesting extensive allelic heterogeneity.9 Important work is still needed to extend research to diverse (non-European) populations to fine-map causal variants, better understand the genetic risk across populations, and validate findings. Nongenetic factors accounting for the uromodulin protein variability in urine samples have been described, including increasing age, diabetes, and low eGFR, and the relationship may be related to functional kidney mass.2 There were some differences in the distribution of urine uromodulin across the 13 studies, even after urine uromodulin was adjusted by creatinine. Most studies used an ELISA assay to measure uromodulin in a spot urine sample. Some studies included individuals with CKD, which affects urine uromodulin. Accounting for the nongenetic determinants of the trait including technical factors related to sample storage and assays may help to reduce the heterogeneity in genetic estimates and improve gene discovery. Studies are needed to determine if there are differences in the levels of uromodulin protein across populations on the basis of ancestry and how these may affect the strength of uromodulin effect on disease risk. In summary, this is a step forward to understanding this complex protein that has continued to fascinate nephrologists for over a half a century. Disclosures N. Franceschini reports having an advisory or leadership role with the Editorial Boards of the American Journal of Physiology-Renal Physiology and the Contemporary Clinical Trials journal, as the National Heart, Lung and Blood Institute TOPMed kidney working group convener, member of the Women's Health Initiative Publication and Presentation Committee, and the co-Chair of the Women's Health Initiative Ancillary Committee. T.H. Le reports having an ownership interest in Goldilocks; and reports receiving research funding from the Renal Research Institute. Funding This work was supported in part by the National Institutes of Health grants R01 DK113632 (to T.H. Le), R01-MD012765, and R01 DK117445.