Role of the progressive ankylosis gene in cartilage mineralization

Among the myriad of players in the calcification of cartilage, ANK is a relatively new entrant. It is a multipass transmembrane protein that regulates the transport of inorganic pyrophosphate between the cell and the extracellular space. Mutations in ANK result in two distinct calcification disorders: craniometaphyseal dysplasia and familial calcium pyrophosphate dihydrate deposition disease. The purpose of this review is to highlight recent work on the role of ANK in physiological and pathological calcification of articular and growth plate cartilage.

New information on the function of ANK suggests that the protein is part of a constellation of critical components that interact to regulate the elaboration of inorganic pyrophosphate. In addition to ANK, these components include alkaline phosphatase, the ectoenzyme PC-1, and osteopontin. ANK expression is also regulated by a variety of growth factors and cytokines that may further affect the transport of inorganic pyrophosphate and may be particularly relevant to the increased levels of expression of ANK in cartilage from chondrocalcinosis and osteoarthritis patients.

Additional studies will be required to understand the contribution of ANK in shaping the fine balance of components necessary for crystal deposition in degenerating articular cartilage. Furthermore, the precise role of inherited mutations in ANK on the elaboration of inorganic pyrophosphate, and the ultimate deposition of either basic calcium phosphate or calcium pyrophosphate dihydrate crystals, remains unclear.