Successfully treated C3 glomerulopathy in which protein and genetic analyses were useful for diagnosis.

C3 glomerulopathy is a rare disease that results in nephritis due to complement dysregulation and is characterized by C3 deposition in the glomerulus. Dysregulation of the alternative pathway underlies the pathogenesis, but activation of the terminal pathway is also common. The disease is often caused by acquired rather than genetic factors, i.e., autoantibodies against C3 or C5 converting enzyme (convertase) and other complement-related proteins. We report a case of C3 glomerulopathy diagnosed by renal biopsy that responded well to corticosteroids and went into complete remission within two months. Analysis of complements and complement-related proteins revealed a low level of C3 and a high level of soluble terminal pathway protein complex (sC5b-9). Under genetic analysis about complement-related genes, no pathogenic variant was observed. Based on these findings, we diagnosed this patient with C3 glomerulopathy with autoantibodies. Corticosteroids had a marked effect, which also supports this speculation. Analyses of complements and complement-related proteins, and genetic variants may be useful in understanding the pathogenesis of C3 glomerulopathy and in selecting treatment options.