Identification of CUBN variants in triplets with a 20-year history of proteinuria.

CUBN encodes cubilin, which plays a role in the reabsorption of glomerular-filtered albumin in the proximal tubule. CUBN-related proteinuria was recently established as a new disease concept and may be present in proteinuric cases that were previously undiagnosed either genetically or histologically. We herein report a case of triplets diagnosed with chronic benign proteinuria due to CUBN variants 20 years after its onset. The proband, the first child of triplets, tested positive for urinary protein several times during the neonatal period. A urine screening test at 3 years old was positive. Proteinuria persisted for years within a non-nephrotic range. Kidney biopsy at 8 years old revealed minor glomerular abnormalities. Renin-angiotensin system inhibitors were started for albumin-based proteinuria but were ineffective. Since the two other triplets had similar courses, analyses of the NPHS1/2 and WT1 genes were performed but revealed no abnormalities. The triplets transitioned to adult care at 15 years old. CUBN-related proteinuria was reported in 2020; therefore, we re-analyzed their DNA samples and identified compound heterozygous variants in CUBN in all three triplets. The molecular diagnosis of CUBN-related proteinuria will save patients from unnecessary treatments and concerns about renal prognosis.