Hypomorphic variant of the slow allele of C3 associated with hypocomplementemia and hematuria.

This report describes the first instance of a hypomorphic variant (C3*s) of the most common C3 allele, C3 Slow, which was detected in a four-year-old Caucasian boy with hematuria. Analysis of C3 phenotypes, as determined by agarose electrophoresis, showed a hypomorphic C3 Slow in the patient and a maternal aunt. Serum C3 concentration was significantly reduced in the patient and his mother (610 and 750 micrograms/ml; normal +/- 1 SD = 1,240 +/- 240 micrograms/ml) and was at the lower limits of normal in the affected aunt (770 micrograms/ml). The mother's phenotype was C3 S (? Ss) and she was the presumed carrier, since the father (C3 FS) had neither an abnormal C3 S band nor a low C3 concentration (980 micrograms/ml). Total hemolytic complement was significantly reduced only in the patient (19 units/ml; normal = 38 +/- 16). Hypomorphic C3 variants should be considered in the evaluation of decreased serum C3 levels.