BACKGROUND. Proteinuria is considered an unfavorable clinical condition that accelerates renal and cardiovascular disease. However, it is not clear whether all forms of proteinuria are damaging. Mutations in CUBN cause Imerslund-Gräsbeck syndrome (IGS), which is characterized by intestinal malabsorption of vitamin B12 and in some cases proteinuria. CUBN encodes for cubilin, an intestinal and proximal tubular uptake receptor containing 27 CUB domains for ligand binding. METHODS. We used next-generation sequencing for renal disease genes to genotype cohorts of patients with suspected hereditary renal disease and chronic proteinuria. CUBN variants were analyzed using bioinformatics, structural modeling, and epidemiological methods. RESULTS. We identified 39 patients, in whom biallelic pathogenic variants in the CUBN gene were associated with chronic isolated proteinuria and early childhood onset. Since the proteinuria in these patients had a high proportion of albuminuria, glomerular diseases such as steroid-resistant nephrotic syndrome or Alport syndrome were often the primary clinical diagnosis, motivating renal biopsies and the use of proteinuria-lowering treatments. However, renal function was normal in all cases. By contrast, we did not found any biallelic CUBN variants in proteinuric patients with reduced renal function or focal segmental glomerulosclerosis. Unlike the more N-terminal IGS mutations, 37 of the 41 proteinuria-associated CUBN variants led to modifications or truncations after the vitamin B12-binding domain. Finally, we show that 4 C-terminal CUBN variants are associated with albuminuria and slightly increased GFR in meta-analyses of large population-based cohorts. CONCLUSION. Collectively, our data suggest an important role for the C-terminal half of cubilin in renal albumin reabsorption. Albuminuria due to reduced cubilin function could be an unexpectedly common benign condition in humans that may not require any proteinuria-lowering treatment or renal biopsy.

Hepatitis C virus (HCV) infection is associated with tremendous morbidity and mortality due to liver complications. HCV infection is also associated with many extrahepatic manifestations including cardiovascular diseases, glucose metabolism impairment, cryoglobulinemia vasculitis, B cell non-Hodgkin lymphoma and chronic kidney disease (CKD). Many studies have shown a strong association between HCV and CKD, by reporting (i) an increased prevalence of HCV infection in patients on haemodialysis, (ii) an increased incidence of CKD and proteinuria in HCV-infected patients, and (iii) the development of membranoproliferative glomerulonephritis secondary to HCV-induced cryoglobulinemia vasculitis. HCV seropositivity is found to be associated with an increased relative risk for all-cause and cardiovascular mortality in the dialysis population. HCV seropositivity is linked to lower patient and graft survival after kidney transplantation. Such poor HCV-associated prognosis should have encouraged clinicians to treat HCV in CKD patients. However, due to frequent side effects and the poor efficacy of interferon-based treatments, very few HCV dialysis patients have received HCV medications until now. The emergence of new direct acting, interferon-free antiviral treatment, leading to HCV cure in most cases with a satisfactory safety profile, will shortly modify the management of HCV infection in CKD patients. In patients with a glomerular filtration rate (GFR) >30ml/min, the choice of DAA is not restricted. In those with a GFR <30 and >15ml/min, only paritaprevir/ritonavir/ombitasvir/dasabuvir or a grazoprevir plus elbasvir regimen are approved. In patients with end stage renal disease (GFR <15ml/min or dialysis), current data only allows for the use of a grazoprevir plus elbasvir combination. No doubt these data will be modified in the future with the advent of new studies including larger cohorts of HCV patients with renal impairment.