The genetics of ficolins

Ficolins constitute a family of proteins whose biological role has been an enigma for many years. Over the past few years it has become evident that ficolins are part of the innate immune system and function as recognition molecules in the complement system. The 3 human ficolins, ficolin-1 (M-ficolin), ficolin-2 (L-ficolin) and ficolin-3 (H-ficolin or Hakata antigen) are encoded by the FCN1, FCN2 and FCN3 genes, respectively. Phylogenetic studies suggest that ficolins are of ancient origin. Ficolin-3 seems to be the most ancient molecule, from a phylogenetic perspective. Searches in databases and phylogenetic tree analysis demonstrate that the ficolin precursor has gone through an expansion involving independent duplication events in the different branches of the evolutionary tree. Of particular interest is the prediction that ficolin-1 appears to be present as an ortholog molecule. All human FCN genes are polymorphic. The FCN2 gene encoding ficolin-2, contains polymorphisms that affect ligand binding, while differences in the serum levels are associated with promoter polymorphisms. Recently, a frame-shift variation in the FCN3 gene was described, leading to ficolin-3 deficiency and defective complement activation. This FCN3 variation was also shown to be associated with immunodeficiency. This survey summarizes the current phylogenetic and inter-individual molecular understanding of the FCN genes.