Disruption of Disulfide Bonds Is Responsible for Impaired Secretion in Human Complement Factor H Deficiency

Abstract

Factor H, a secretory glycoprotein composed of 20 short consensus repeat modules, is an inhibitor of the complement system. Previous studies of inherited factor H deficiency revealed single amino acid substitutions at conserved cysteine residues, on one allele arginine for cysteine 518 (C518R) and on the other tyrosine for cysteine 941 (C941Y) (Ault, B. H., Schmidt, B. Z., Fowler, N. L., Kashtan, C. E., Ahmed, A. E., Vogt, B. A., and Colten, H. R. (1997) J. Biol. Chem. 272, 25168 -25175). To ascertain if the phenotype, impaired secretion of factor H, is due to the C518R substitution or the C941Y substitution and to ascertain the mechanism by which secretion is impaired, we studied COS-1 and HepG2 cells transfected with wild type and several mutant factor H molecules. The results showed markedly impaired secretion of both C518R and C941Y factor H as well as that of factor H molecules bearing alanine or arginine substitutions at the Cys 518 -Cys 546 disulfide bond (C518A, C546A, C546R, C518A-C546A). In each case, mutant factor H was retained in the endoplasmic reticulum and degraded relatively slowly as compared with most other mutant secretory and membrane proteins that are retained in the endoplasmic reticulum. These data indicate that impaired secretion of the naturally occurring C518R and C941Y mutant factor H proteins is due to disruption of framework-specific disulfide bonds in factor H short consensus repeat modules.

Hemolytic uremic syndrome due to homozygous factor H deficiency

“…Since in the present case no circulating abnormal factor H protein was detected by Western blot, we propose that folding and secretion of this protein is impaired, resulting in its retention in the cytoplasm, as previously reported [12,13]. However, the genetic abnormality did not affect CFHL1 secretion because it was localized far from the complement factor H exon 10, at the 3 0 end of the gene.…”

Dense Deposit Disease and C3 Glomerulopathy

“…Intriguingly, an earlier report concerned a patient with severe Cfh deficiency in association with type 3 collagen glomerulopathy 82 (more likely representing chronic thrombotic microangiopathy 16 ) in whom a different homozygous missense mutation was identified at the same site on SCR 16 83 . The substitution of a cysteine residue (in this case with a tyrosine) altered the secondary structure of the Cfh protein, resulting in its nonsecretion 84 . A young girl was reported with macroscopic hematuria, very low C3 and Cfh levels, and endocapillary GN with predominant glomerular C3 85 .…”

Clinical Aspects and Molecular Basis of Primary Deficiencies of Complement Component C3 and its Regulatory Proteins Factor I and Factor H

“…They showed that these factor H mutants were retained in the endoplasmic reticulum. This suggests that the factor H deficiency in this patient was in fact due to impaired factor H secretion [117].…”