C1q nephropathy: A variant of focal segmental glomerulosclerosis

Background. C1q nephropathy is a poorly understood and controversial entity with distinctive immunopathologic features. In order to better define the clinical-pathologic spectrum, we report the largest single-center series. Methods. Nineteen biopsies with C1q nephropathy were identified from among 8909 native kidney biopsies received from 1994 to 2002 (0.21%). Defining criteria included ( 1) dominant or co-dominant immunofluorescence staining for C1q, ( 2) mesangial electron dense deposits, and ( 3) no clinical or serologic evidence of systemic lupus erythematosus (SLE). Results. The 19 patients were predominantly African American (73.7%), female ( 73.7%), young adults and children ( range, 3 to 42 years; mean, 24.2 years). Presentation included nephrotic range proteinuria (78.9%), nephrotic syndrome (50%), renal insufficiency (27.8%), and hematuria (22.2%). No patient had hypocomplementemia or evidence of underlying autoimmune or infectious disease. Renal biopsy revealed focal segmental glomerulosclerosis (FSGS) in 17 ( including six collapsing and two cellular) and minimal-change disease (MCD) in two. All biopsies displayed co-deposits of immunoglobulin G (IgG), with more variable IgM (84.2%), IgA (31.6%), and C3 (52.6%). Foot process effacement varied from 20% to 100% (mean, 51%). Twelve of 16 patients with available follow-up received immunosuppressive therapy. One patient had complete remission of proteinuria and six had partial remission. Four patients with FSGS pattern had progressive renal insufficiency, including two who reached end-stage renal disease ( ESRD). Median time from biopsy to ESRD was 81 months. On multivariate analysis, the best correlate of renal insufficiency at biopsy and at follow-up was the degree of tubular atrophy and interstitial fibrosis ( P = 0.0495 and 0.0341, respectively). Conclusion. C1q nephropathy falls within the clinical-pathologic spectrum of MCD/FSGS. Although further studies are needed to determine the pathomechanism of C1q deposition, we hypothesize that it may be a non-specific marker of increased mesangial trafficking in the setting of glomerular proteinuria.