HUMAN FACTOR-H DEFICIENCY - MUTATIONS IN FRAMEWORK CYSTEINE RESIDUES AND BLOCK IN H-PROTEIN SECRETION AND INTRACELLULAR CATABOLISM

The synthesis and secretion of factor H, a regulatory protein of the c omplement system, were studied in skin fibroblasts from an H-deficient child who has chronic hypocomplementemic renal disease, In normal fib ro blasts, factor H transcripts of 4.3 and 1.8 kilobase pairs (kb) enc ode a 155-kDa protein containing short consensus repeat (SCR) domains 1-20 and a 45-kDa protein which contains SCRs 1-7, respectively. The p atient's fibroblasts expressed normal amounts of the 4.3- and 1.8-kb m essages constitutively and after tumor necrosis factor-alpha/interfero n-gamma stimulation. Lysates of [S-35]methionine-labeled fibroblasts f rom the patient contained the 155- and 45-kDa H polypeptides, but secr etion of the 155-kDa protein was blocked; the 45-kDa protein was secre ted with normal kinetics, The patient's plasma lacked the 155-kDa prot ein but contained the small form of H. Moreover, in fibroblasts the re tained 155-kDa factor H protein was not degraded, even after 12 h, Imm unoflourescent staining and confocal microscopic imaging of the patien t's fibroblasts indicated that factor H was retained in the endoplasmi c reticulum, Sequence analysis of reverse transcription-polymerase cha in reaction products (the entire coding region) and genomic DNA reveal ed a T1679C substitution on one allele and a G2949A substitution on th e other (C518R mutation in SCR 9 and C991Y mutation in SCR 16, respect ively), Both mutations affect conserved cysteine residues characterist ic of SCR modules and therefore predict pro found changes in the highe r order structure of the 155-kDa factor H protein, These data provide the first description of a molecular mechanism for factor H deficiency and yield important insights into the normal secretory pathway for th is and other plasma proteins with SCR motifs.