Anabolic actions of parathyroid hormone in a hypophosphatasia mouse model

SUMMARY: Hypophosphatasia, the rare heritable disorder caused by TNAP enzyme mutations, presents wide-ranging severity of bone hypomineralization and skeletal abnormalities. Intermittent PTH (1-34) increased long bone volume in Alpl(-/-) mice but did not alter the skull phenotype. PTH may have therapeutic value for adults with TNAP deficiency–associated osteoporosis. INTRODUCTION: Hypophosphatasia is the rare heritable disorder caused by mutations in the tissue non-specific alkaline phosphatase (TNAP) enzyme leading to TNAP deficiency. Individuals with hypophosphatasia commonly present with bone hypomineralization and skeletal abnormalities. The purpose of this study was to determine the impact of intermittent PTH on the skeletal phenotype of TNAP-deficient Alpl(-/-) mice. METHODS: Alpl-/- and Alpl+/+ (wild-type; WT) littermate mice were administered PTH (1-34) (50 µg/kg) or vehicle control from days 4 to 12 and skeletal analyses were performed including gross measurements, micro-CT, histomorphometry, and serum biochemistry. RESULTS: Alpl(-/-) mice were smaller with shorter tibial length and skull length compared to WT mice. Tibial BV/TV was reduced in Alpl(-/-) mice and daily PTH (1-34) injections significantly increased BV/TV and BMD but not TMD in both WT and Alpl(-/-) tibiae. Trabecular spacing was not different between genotypes and was decreased by PTH in both genotypes. Serum P1NP was unchanged while TRAcP5b was significantly lower in Alpl(-/-) vs. WT mice, with no PTH effect, and no differences in osteoclast numbers. Skull height and width were increased in Alpl(-/-) vs. WT mice, and PTH increased skull width in WT but not Alpl(-/-) mice. Frontal skull bones in Alpl(-/-) mice had decreased BV/TV, BMD, and calvarial thickness vs. WT with no significant PTH effects. Lengths of cranial base bones (basioccipital, basisphenoid, presphenoid) and lengths of synchondroses (growth plates) between the cranial base bones, plus bone of the basioccipitus, were assessed. All parameters were reduced (except lengths of synchondroses, which were increased) in Alpl(-/-) vs. WT mice with no PTH effect. CONCLUSION: PTH increased long bone volume in the Alpl(-/-) mice but did not alter the skull phenotype. These data suggest that PTH can have long bone anabolic activity in the absence of TNAP, and that PTH may have therapeutic value for individuals with hypophosphatasia-associated osteoporosis.