Nephrotic-range proteinuria in an infant with thin basement membrane nephropathy

Thin basement membrane nephropathy (TBMN) with heterozygous COL4A3/COL4A4 mutations is considered to be a cause of benign familial hematuria. The disease has been believed to have excellent prognosis and TBMN in early childhood is rarely associated with nephrotic-range proteinuria. Furthermore, the presence of proteinuria in patients with TBMN is associated with autosomal-dominant Alport syndrome, which has poorer prognosis in later life. We present an infant case of nephrotic-range proteinuria associated with TBMN caused by heterozygous COL4A4 mutation. A previously healthy 3-year-old boy developed microhematuria and nephrotic-range proteinuria. Renal pathology simply revealed thinning of the glomerular basement membrane (GBM) and mutational analysis revealed a novel heterozygous mutation in COL4A4. He was treated with lisinopril for 1.5 years, which resolved his proteinuria and hematuria. At the most recent follow-up at 6.5 years of age, urinalysis and kidney function were completely normal, without requiring medication. However, transient but repeated moderate to nephrotic-range proteinuria and microscopic hematuria occurred in association with other illnesses. This case highlights the spectrum of phenotypes that may be apparent in an infant with TBMN. Thinning of the GBM can cause transient nephrotic-range proteinuria, particularly in the early stages of TBMN.