C4 null phenotypes among lupus erythematosus patients are predominantly the result of deletions covering C4 and closely linked 21-hydroxylase A genes.

Two genes, C4A and C4B, encoding the fourth component of the complement system are linked to the HLA complex. C4 defects or C4 'null' genes can predispose to an autoimmune disease, lupus erythematosus (LE). We have used Southern blotting techniques to analyse genomic DNA from 23 patients with LE and from healthy controls, to evaluate the molecular basis of the C4 null phenotypes. In addition to the high frequencies of C4 null phenotypes and HLA-B8. DR3 antigens, confirming earlier results, we observed that among the patients both the C4A and C4B null phenotypes mostly resulted from gene deletions. Among the controls only the C4A null phenotypes were predominantly the result of gene deletions. In all cases these C4 gene deletions also extended to a closely linked pseudogene, 21-hydroxylase A (21-OHA). Altogether, 52% of the patients and 26% of the controls carried a C4/21-OHA deletion.