MBL2, FCN1, FCN2 and FCN3—The genes behind the initiation of the lectin pathway of complement

Mannose-binding lectin (MBL) and the ficolins (Ficolin-1, Ficolin-2 and Ficolin-3) are soluble collagen-like proteins that are involved in innate immune defence. They bind sugar structures or acetylated compounds present on microorganisms and on dying host cells and they initiate activation of the lectin complement pathway in varying degrees. Common variant alleles situated both in promoter and structural regions of the human MBL gene (MBL2) influence the stability and the serum concentration of the protein. Although not as thoroughly investigated as the MBL2 gene polymorphisms the ficolin genes (FCNs) also exhibit genetic variations affecting both the serum concentration, stability and binding capacity of the corresponding proteins. Epidemiological studies have suggested that the genetically determined variations in MBL serum concentrations influence the susceptibility to and the course of different types of diseases, while the importance of the ficolins in general and the genetic variation in the FCNs genes in particular is still largely unresolved. This overview will summarize the current molecular knowledge of the human MBL2, FCN1, FCN2 and FCN3 genes.