S100A9 (S100 calcium binding protein A9)

Note S100A9 belongs to the S100/calgranulin family of small non-ubiquitous cytoplasmic Ca-binding proteins of EF-hand type. The proteins were referred to "S100" because of their solubility in saturated ammonium sulphate solution. Sixteen of 21 members are localised in a cluster on human chromosome 1q21. The clustered organization of these S100 genes is conserved during evolution (Ridinger et al., 1998). A comparison between man and mouse has shown that during evolution, the colinearity of the S100 gene cluster has been destroyed by some inversions. However, the colocalization of the myeloid expressed S100 genes such as S100A8, S100A9, and S100A12 is conserved. It has been speculated, that the structural integrity of that part of the locus is necessary for the coordinated expression of these genes (Nacken et al., 2001). Remarkably, the S100 gene cluster is located in close proximity to a region which has been frequently rearranged in human cancer (Carlsson et al., 2005) and to the epidermal differentiation complex (EDC) (Mischke et al., 1996). EDC is a cluster of genes on chromosome 1q21 encoding proteins that fulfil important functions in terminal differentiation in the human epidermis, including filaggrin, loricrin and others. In addition, linkage analyses have identified a psoriasis susceptibility region, the PSORS4 locus, that is close to the S100 gene cluster (Hardas et al., 1996; Semprini et al., 2002). These data are important indications for the involvement of S100 genes in inflammatory as well as neoplastic disorders. It has been speculated that the rearrangements result in a deregulated expression of S100 genes associated with neoplasia.