Cloning and analysis of the gene encoding the human neonatal Fc receptor.

The neonatal Fc receptor, FcRn, is expressed in human placental syncytiotrophoblast, capillary endothelium, intestinal epithelium, and other tissues. By analogy with its role in the mouse, human FcRn is expected to transport maternal IgG to the foetus, and protect circulating IgG from catabolism. The larger subunit of FcRn is homologous to the alpha chains of the major histocompatibility complex (MHC) class I proteins, but is encoded outside the MHC on chromosome 19. We report the isolation of clones encoding the alpha chain of human FcRn from chromosome 19-specific libraries. The sequence revealed a similar organization to classical and non-classical MHC, and MHC-related genes. Compared with classical MHC class I genes, the human FcRn alpha chain gene has expanded by acquiring many repetitive sequences in its introns, including multiple Alu elements in the fourth intron. Primer extension analysis showed that there are two transcription initiation sites in the upstream flanking sequence.