Inherited human complement C3 deficiency: An amino acid substitution in the β-chain (ASP⁵⁴⁹ to ASN) impairs C3 secretion

We recently described a case of hereditary complement C3 deficiency (C3D) in a New Zealand male who has a small amount of serum C3 (7 μg/ml), a normal size 5.2-kilobase C3 mRNA that is present in normal quantities, and a normal size M_r 180,000 proC3 molecule that is synthesized in normal amounts. Secretion of C3 from this patient's cells was greatly diminished, however, and an aberrant C3 trypsin cleavage profile indicated an abnormality in the proC3 structure. To determine the primary structure of the C3D proC3 molecule, the corresponding cDNA was cloned and sequenced in the present study, revealing a normal signal peptide, tetraarginine linker, and thiolester domain. One nucleotide substitution in exon 13 (G¹⁷⁰⁵AC to AAC) was found, however, that resulted in an amino acid change in a highly conserved region of the C3 β-chain (Asp⁵⁴⁹ to Asn). This substitution has not been described in any individual with either C3 Fast or C3 Slow phenotypes. Immunoprecipitation of C3 from L-cells transfected with full-length normal and C3D cDNAs demonstrated that C3 was secreted by the cells transfected with the normal C3 cDNA; however, only a C3 precursor was detected in the intracellular compartment of the cells transfected with the C3D cDNA and none detected extracellularly. Immunofluorescence studies revealed a perinuclear localization of C3 in the C3D transfectants, suggesting that transport of the mutant precursor C3 is arrested early in the secretory pathway. Allele-specific polymerase chain reaction analysis demonstrated that this New Zealand family is a compound heterozygous C3D kindred, with the Asn⁵⁴⁹ point mutation being inherited from the mother and a yet undescribed C3 defect being inherited from the father. Taken together, these data indicate that 1) C3 deficiency is caused in a New Zealand kindred by two distinct molecular genetic mutations, one being an amino acid substitution in a highly conserved region of the β-chain that results in impaired C3 secretion, and 2) the molecular basis of this deficiency has not been described in any other C3-deficient individual, providing additional evidence that multiple defects cause inherited C3 deficiency in humans.