Analysis of Complement Deficiency States. With Focus on Molecular Characterization of C4 and Properdin Deficiency

Sammanfattning: Complement deficiency is associated with an increased risk for invasive infection and immune complex disease. Screening for identification of complement-deficient patients is essential and a new ELISA-based procedure for detection of complement deficiency is described. Molecular characterization of a complete C4 deficiency and of three phenotypic forms of properdin deficiency is also reported. The C4-deficient person had two non-expressed C4 genes, one from each parent. Both the non-expressed genes displayed sequences identical to the allotype C4A3a. In the C4 gene belonging to the HLA-DR6 bearing haplotype inherited from the father, a two base pair insertion was found, which caused the non-expression. The reason for the failure of expression of the other C4 gene remains to be defined. The molecular genetic basis for properdin deficiency displayed heterogeneity. In families with properdin deficiency type I, different mutations leading to premature stop codons were identified. This correlates with the complete absence of circulating properdin. Males with low properdin serum level (deficiency type II), one Danish and one Swedish, showed two distinct point mutations, located on exon 8 causing a Gln316->Arg substitution and on exon 4 causing an Arg73->Trp substitution, respectively. Gel filtration of sera from the two males revealed abnormal properdin oligomer formation with a predominance of dimers. Expression of normal amounts of properdin at the cellular level suggested that the low serum properdin level might be due to rapid extracellular catabolism of abnormal properdin molecules. In a Dutch family with dysfunctional properdin (deficiency type III) a single point mutation was found on exon 9, causing a Tyr387->Asp substitution. The dysfunctional properdin did not bind to solid-phase C3. Studies with overlapping properdin peptides suggested that a conformational change of the molecule determined its loss of biological activity.