Complement 4 gene deletion in patients with IgA nephropathy and Henoch-Schönlein nephritis.

The fourth component of complement (C4), especially B isotype, has been said to be deficient in the IgA nephropathy and Henoch-Schönlein nephritis. However, the association between these diseases and C4 deficiency was questioned recently, and the usual C4 allotyping method is unable to discriminate the C4 deficiency from the C4 duplication. So by combining the DNA restriction fragment length polymorphism with the usual C4 allotyping, we tried to determine whether the deficiency of C4 can be demonstrated in the DNA level. We found that the frequency of C4 gene deletion was increased, although the frequency of null phenotype was not different from the control. From these results we can say that C4 gene deletion is a genetic risk factor in these diseases, at least in the Japanese population.