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 complement system, were studied in skin fibroblasts from an H-deficient child who has chronic hypocomplementemic renal disease, In normal fibro blasts, factor H transcripts of 4.3 and 1.8 kilobase pairs (kb) encode a 155-kDa protein containing short consensus repeat (SCR) domains 1-20 and a 45-kDa protein which contains SCRs 1-7, respectively. The patient's fibroblasts expressed normal amounts of the 4.3- and 1.8-kb messages constitutively and after tumor necrosis factor-alpha/interferon-gamma stimulation. Lysates of [S-35]methionine-labeled fibroblasts from the patient contained the 155- and 45-kDa H polypeptides, but secretion of the 155-kDa protein was blocked; the 45-kDa protein was secreted with normal kinetics, The patient's plasma lacked the 155-kDa protein but contained the small form of H. Moreover, in fibroblasts the retained 155-kDa factor H protein was not degraded, even after 12 h, Immunoflourescent staining and confocal microscopic imaging of the patient's fibroblasts indicated that factor H was retained in the endoplasmic reticulum, Sequence analysis of reverse transcription-polymerase chain reaction products (the entire coding region) and genomic DNA revealed a T1679C substitution on one allele and a G2949A substitution on the other (C518R mutation in SCR 9 and C991Y mutation in SCR 16, respectively), Both mutations affect conserved cysteine residues characteristic of SCR modules and therefore predict pro found changes in the higher 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 this and other plasma proteins with SCR motifs.