Are Cubilin (CUBN) Variants at the Heart of Urinary Albumin Excretion?
Authors: John F. O'Toole, John R. Sedor
Journal: Journal of the American Society of Nephrology (2011)

In this issue of JASN, Böger et al.1 report the results of a meta-analysis from approximately 63,000 individuals of European ancestry with genotype information identifying a susceptibility locus for the quantitative trait known as urinary albumin-to-creatinine ratio (UACR) and the clinical diagnosis microalbuminuria. A single variant within a protein-coding exon of CUBN, which encodes the proximal tubular epithelial cell apical protein, cubilin, reached genome-wide significance. The association of a cubilin variant with UACR and microalbuminuria resonates with existing knowledge about cubilin function from genetic, experimental animal, and in vitro studies. CUBN mutations have been identified in patients with Imerslund-Grasbeck syndrome (IGS).2 This rare autosomal recessive disease is characterized by megaloblastic anemia and a variable degree of proteinuria. Cubilin was first identified as the intrinsic factor-cobalamin (IF-Cbl) receptor in the ileal mucosa and kidney of the rat,3 and the IGS mutations linked to CUBN disrupt the cubilin binding of IF-Cbl.4 Subsequent genetic studies of patients with IGS and without CUBN mutations have identified mutations in AMN5 that encodes a protein, amnionless, which is required for the localization of cubilin to the apical surface of epithelial cells.6 Taken together, these studies identify a molecular pathway to explain the IGS phenotype. Megaloblastic anemia in patients with IGS results from defects in the intestinal absorption of IF-Cbl. The variable degree of proteinuria, averaging 750 mg/d, in these patients is related to cubilin's role in the proximal tubular reabsorption of albumin.7 Murine models of Cubn deletion demonstrate a sixfold increase in urinary albumin excretion, a degree of albuminuria that is roughly equivalent to 200 mg/d in humans.8 The quantity of albumin filtered through the normal glomerulus is a contentious issue9,10; however, under normal conditions, filtered albumin is efficiently reabsorbed in the proximal tubule.11 Filtered albumin binds to an amino terminal domain of cubilin. Megalin directly interacts with two noncontiguous sites of the cubilin protein,12 and the megalin-cubilin-albumin complex is endocytosed by the cell. Megalin and cubilin are recycled to the apical membrane, and albumin is delivered to the lysosomal compartment.13 These data demonstrate that CUBN mutations do result in impairments of proximal tubular albumin reabsorption but not nephrotic-range proteinuria and are consistent with the study by Böger et al.,1 in which the degree of albuminuria is very modest. The variant identified in this report1 results in the conservative, nonsynonymous amino acid substitution I2984V. This residue falls within a protein domain previously reported to participate in the megalin–cubilin protein interaction.11 The identification of a coding variant within a functional protein domain suggests that the associated variant is causing increased albumin excretion. The cubilin protein, encoded by the variant gene, may be unable to bind megalin, a required event for normal endocytosis of albumin. However, the conservative nature of the substitution and the fact that bioinformatic algorithms predict the change to be benign with respect to function require this hypothesis to be experimentally proved. Further work will be needed to determine whether a variant cubilin with an I2984V substitution increases albuminuria. The CUBN single-nucleotide polymorphism reported by Böger et al.1 accounts for only 0.15% of the variance in UACR, but because this variant is common, it would account for a large population-attributable risk for albuminuria. Regardless of whether the associated variant contributes to abnormal albumin excretion, its identification points to some interesting possibilities from a genetic perspective. First, if it is causal but has only a minor contribution to the variance in albuminuria, then CUBN should be scanned for rare variants not genotyped on current platforms that may have larger effects. Second, if it is not causal, then it must be viewed as a positional marker of a genomic region harboring the causal variant. The true causal variant may not have been identified because it was not genotyped in this study but lies in close proximity to the sentinel single-nucleotide polymorphism responsible for the association signal. The causal variant could be located in the CUBN gene itself or reside in a nearby gene or regulatory region and may possibly have a more substantial effect size. As whole-genome sequencing accelerates and rare variants are more densely mapped,14 the data generated may clarify these possibilities. Although successful gene mapping studies are provocative, the greatest challenge remains determining the clinical importance of associated variants. In this particular case, the identification of genetic variants regulating albumin excretion is of interest because of the epidemiologic association of albuminuria with renal failure and cardiovascular events. The authors note that they did not find this variant to be associated with estimated GFR, suggesting that cubilin-mediated mechanisms of abnormal albuminuria do not have a direct impact on glomerular function. The association of a CUBN variant with UACR does not discount the importance of glomerular mechanisms of albuminuria, which is clearly a complex phenotype with contributions from multiple genetic and environmental factors. Unfortunately, the authors do not comment on whether this CUBN variant is associated with an increased cardiovascular disease risk, although many of the cohorts used in their analysis were used to establish the association of UACR with cardiovascular events and mortality.15,16 The findings presented in this issue of JASN1 reinforce the previously established role of cubilin in proximal tubular albumin reabsorption and suggest that CUBN variants may modify the degree of albuminuria in the general population. Experimental work will define whether the CUBN mutation associated with UACR is causal or is a positional marker pointing to the true variant. Its relation with cardiovascular risk remains an open question. The association between the amount of albumin in the urine and renal progression17,18 or heart disease19 has been repeatedly and robustly demonstrated. The mechanism for this epidemiologic connection remains speculative. Measuring the association of CUBN variants with cardiovascular disease risk phenotypes and studying their effects on cubilin function may unravel this mystery and move the statistical association of CUBN with UACR toward clinical utility. DISCLOSURES None.