Fibrocystin/polyductin (FPC): new functional insights into ARPKD pathogenesis revealed by informatics, comparative genomics, and model systems

Autosomal recessive polycystic kidney disease (ARPKD) is the prototype of the hepato-renal fibrocystic diseases, a subset of the broader ciliopathy disorders. As a severe form of PKD, ARPKD typically manifests in utero with 21% perinatal mortality and progressive loss of kidney function in most post-natal survivors. Congenital hepatic fibrosis is an invariant feature of ARPKD. PKHD1-encoded fibrocystin/polyductin (FPC) is a large 4074 amino acid glycoprotein that likely functions as a receptor molecule and appears to play a key role in maintaining differentiated renal tubular epithelium. The molecular mechanisms by which defects in FPC contribute to ARPKD pathogenesis are just beginning to be elucidated. FPC is a novel protein that likely evolved as vertebrates transitioned from aquatic to semi-terrestrial ecosystems. Full-length human FPC shares a phylogenetically conserved, multi-motif N-terminal region with its ancestral homolog, PKHD1L1, and contains a single-pass transmembrane domain and a novel C-terminal tail that harbors a ciliary targeting motif as well as mitochondrial and nuclear localization sequences. This review synthesizes the full range of recent experimental data about PKHD1/FPC to provide a current functional perspective about this complex protein. We also discuss the clinical relevance of these emerging functional insights for both kidney health and ARPKD pathogenesis.