Third Department of Internal Medicine, Kagoshima University Faculty of Medicine.
We report two sisters with hereditary deficiency of the third complement component (C3) and a homozygous mutation at C3303G (Tyr1081Stop) of the gene. They developed systemic lupus erythematosus-like symptoms during adolescence. Their C3 were not detected in serum immunochemically. Their mother and a brother had half of the normal C3 levels and a heterozygous mutation in the same position. Western blot analysis of murine L-cells transfected with the mutant C3 cDNA showed no C3 protein, however mRNA was detectable using reverse-transcriptase polymerase chain reaction. To the best of our knowledge, this is the first report of C3 deficiency due to a stop codon in the gene.