Gains Made Toward Treatment of Rare Bone Disease

Diagnosed in toddlers, X-linked hypophosphatemia (XLH) is the most common form of heritable rickets, in which soft bones bend and deform, and tooth abscesses develop because infections penetrate soft teeth that are not properly calcified. Re­searchers at McGill Uni­ver­sity and the Federal Univer­sity of Sao Paulo have identified that osteopontin, a major bone and tooth substrate protein, plays a role in XLH. Their discovery may pave the way to effectively treating this rare disease.
The findings were made by the laboratories of Dr. Marc McKee, a professor in the Faculty of Den­tistry and the department of anatomy and cell biology at McGill University (Canada), and of Dr. Nilana M. T. Barros, a professor at the Federal Uni­versity of São Paulo (Brazil). The team built upon previous research that had shown that mutations in the single gene called phosphate regulating endopeptidase homolog, X-linked (PHEX) are responsible for causing XLH. “XLH is caused in part by renal phosphate wasting, which is the urinary loss from the body of phosphate, an important building block of bones and teeth, along with calcium,” says Dr. McKee. “In pursuing other factors that might contribute to XLH, we used a variety of research methods to show that PHEX enzymatic activity leads to an essentially complete degradation of os­teopontin in bones.” This loss of osteopontin, a known po­tent inhibitor of mineralization (or calcification) in the skeleton and dentition, normally allows bones and teeth to mineralize and thus harden to meet the biomechanical demands placed on them. In XLH patients lacking functional PHEX enzyme, osteopontin and some of its smaller potent inhibitory peptides are retained and accumulate within the bone. This prevents their hardening and leads to soft, deformed bones such as bowed legs (or knock-knees) seen in toddlers. While not life-threatening, this decreased mineralization of the skeleton (osteomalacia), along with the soft teeth, soon leads to a waddling gait, short stature, bone and muscle pain, weakness and spontaneous tooth abscesses. The fact that these symptoms are only partially improved by the standard treatment with phosphate—which improves circulating phosphate levels—prompted the research­ers to look for local factors within the bone that might be blocking mineralization in these patients. “With this new identification of osteopontin as a substrate protein for PHEX,” says Dr. Barros, “we can begin to develop an enzyme-replacement therapy to treat XLH patients who have nonfunctional PHEX, much as has been done using a different enzyme to treat another rare bone disease called hypo­phosphatasia.”
The results of this latest re­search by Drs. McKee and Barros were published in the March 2013 issue of the Jour­nal of Bone and Mineral Research.
(Source: Mc­Gill Uni­versity news release, February 19, 2013)