INHERITED HUMAN-COMPLEMENT C3 DEFICIENCY - AN AMINO ACID SUBSTITUTIONIN THE BETA-CHAIN (ASP(549) TO ASN) IMPAIRS C3 SECRETION

We recently described a case of hereditary complement C3 deficiency (C 3D) in a New Zealand male who has a small amount of serum C3 (7 mu g/m l), a normal size 5.2-kilobase C3 mRNA that is present in normal quant ities, and a normal size M(r) 180,000 proC3 molecule that is synthesiz ed in normal amounts. Secretion of C3 from this patient's cells was gr eatly diminished, however, and an aberrant C3 trypsin cleavage profile indicated an abnormality in the proC3 structure. To determine the pri mary structure of the C3D proC3 molecule, the corresponding cDNA was c loned and sequenced in the present study, revealing a normal signal pe ptide, tetra-arginine linker, and thiolester domain. One nucleotide su bstitution in exon 13 (G(1705)AC to AAC) was found, however, that resu lted in an amino acid change in a highly conserved region of the C3 be ta-chain (Asp(549) to Asn). This substitution has not been described i n any individual with either C3 Fast or C3 Slow phenotypes. Immunoprec ipitation of C3 from L-cells transfected with full-length normal and C 3D cDNAs demonstrated that C3 was secreted by the cells transfected wi th the normal C3 cDNA; however, only a C3 precursor was detected in th e intracellular compartment of the cells transfected with the C3D cDNA and none detected extracellularly. Immunofluorescence studies reveale d a perinuclear localization of C3 in the C3D transfectants, suggestin g that transport of the mutant precursor C3 is arrested early in the s ecretory pathway. Allele-specific polymerase chain reaction analysis d emonstrated that this New Zealand family is a compound heterozygous C3 D kindred, with the Asn(549) point mutation being inherited from the m other and a yet undescribed C3 defect being inherited from the father. Taken together, these data indicate that 1) C3 deficiency is caused i n a New Zealand kindred by two distinct molecular genetic mutations, o ne being an amino acid substitution in a highly conserved region of th e beta-chain that results in impaired C3 secretion, and 2) the molecul ar basis of this deficiency has not been described in any other C3-def icient individual, providing additional evidence that multiple defects cause inherited C3 deficiency in humans.