Mesangial glomerulonephropathy with decreased circulating C4 and predominant mesangial C4 deposition in association with one null gene at the C4B locus.

We report a case of mesangial glomerulonephropathy associated with decreased circulating C4 in a young man with recurrent microscopic hematuria and one null gene at the C4B locus. Mesangial deposits moderately reactive with anti-C4 and weakly reactive with anti-C3 and anti-IgA were found on renal biopsy. No evidence was found to support a diagnosis of IgA nephropathy or any other of the recently described mesangial glomerulonephropathies with immunoglobulin and complement deposition. This case apparently represents a unique, heretofore undescribed variant of mesangial glomerulonephropathy associated with mesangial C4 deposition and C4 hypocomplementemia.