Causes of alternative pathway dysregulation in dense deposit disease

Background and objectives This study was designed to investigate the causes of alternative pathway dysregulation in a cohort of patients with dense deposit disease (DDD). Design, setting, participants, & measurements Thirty-two patients with biopsy-proven DDD underwent screening for C3 nephritic factors (C3Nefs), factor H autoantibodies (FHAAs), factor B autoantibodies (FBAAs), and geneticvariants in CFH.C3Nefs were detected by: ELISA,C3 convertase surface assay (C3CSA), C3CSA with properdin (C3CSAP), two-dimensional immunoelectrophoresis (2DIEP), and immunofixation electrophoresis (IFE). FHAAs and FBAAs were detected by ELISA, and CFH variants were identified by Sanger sequencing. Results Twenty-fivepatients(78%) were positivefor C3Nefs. Three C3Nef-positivepatientswerealso positive for FBAAs and one of these patients additionally carried two novel missense variants in CFH. Of the seven C3Nef-negativepatients, one patient was positiveforFHAAs andtwopatientscarriedCFH variants that maybecausally related to their DDD phenotype. C3CASP was the most sensitive C3Nef-detection assay. C3CASP and IFE are complementary because C3CSAP measures the stabilizing properties of C3Nefs, whereas IFE measures their expected consequence-breakdown of C3b. Conclusions A test panel that includes C3CSAP, IFE, FHAAs, FBAAs, and genetic testing for CFH variants will identify a probable cause for alternative pathway dysregulation in approximately 90% of DDD patients. Dys-regulation is most frequently due to C3Nefs, although some patients test positive for FHAAs, FBAAs, and CFH mutations. Defining the pathophysiology of DDD should facilitate the development of mechanism-directed therapies.