Molecular characterization of Complement Factor I deficiency in two Spanish families.

Complement Factor I (CFI) is a regulator of the classical and alternative pathways. CFI has enzymatic activity and is able to cleave C3b and C4b. Homozygous Factor I deficiency is associated with infectious and/or autoimmune diseases. Here we describe the biochemical and genetic characterization in two Spanish families with complete Factor I deficiency. In Family 1, the propositus suffered from several episodes of meningitis for more than a year. Biochemical complement studies showed undetectable Factor I levels in the propositus and in her sister, while their parents and a brother had partial Factor I deficiency and were healthy. In Family 2, three out of five children were homozygous for Factor I deficiency, two of whom suffered from meningitis and the third one from several infections. The parents and the other two siblings were healthy and heterozygous for Factor I deficiency. Molecular studies showed that the two families had different mutations at exon 5 of the Factor I gene, which codifies for module LDLr1. One mutation corresponds to a 772G>A change at the donor splice site that was originally found in a family from Northern England. The second is a new missense mutation 739T>G, that generates a Cys to Gly change.

The mechanism of loss of CR1 during maturation of erythrocytes is different between factor I deficient patients and healthy donors.

During the in vivo maturation of erythrocytes, the number of CR1 per cell decreases by approximately two-thirds in 30 days. The CR1 loss is enhanced in several diseases such as SLE, AIDS, and particularly in factor I deficiency. Microvesicles enriched in CR1 and DAF are released from erythrocytes matured in vitro, leading to the same loss of both molecules. When comparing reticulocytes and erythrocytes, CR1 and DAF were lost similarly in 15 normal individuals, suggesting that vesiculation may be at the origin of CR1 loss in vivo. However, the enhanced loss of CR1 in 3 patients with factor I deficiency was contrasted with a normal loss of DAF, raising the possibility that, in this pathological condition, CR1 might be proteolytically cleaved, leaving small CR1 fragments on the erythrocytes. To answer this question, a rabbit polyclonal antibody was raised against the cytoplasmic (tail) domain of CR1, which recognised specifically CR1 of erythrocytes and urinary vesicles on Western blots. However, no CR1 fragments could be detected on erythrocytes of the factor I deficient patients although this antibody was able to recognise CR1 fragments after treatment of normal erythrocytes or urinary vesicles with elastase. These data suggest that cell surface domains rich in CR1, but not in DAF, are specifically lost in factor I deficiency.