Haploinsufficiency of parathyroid hormone-related peptide (PTHrP) results in abnormal postnatal bone development

Although apparently phenotypically normal at birth, mice heterozygous for inactivation of the gene encoding parathyroid hormone-related peptide (PTHrP) develop haplotype insufficiency by 3 months of age. In addition to histologic and morphologic abnormalities similar to those seen in homozygous mutants, heterozygous animals demonstrated alterations in trabecular bone and bone marrow. These included metaphyseal bone spicules which were diminished in volume, irregularly distributed, and less well developed than those seen in age-matched controls as well as bone marrow, which contained an inordinate number of adipocytes. A substantial reduction in PTHrP mRNA was detected in heterozygous tissue, while circulating parathyroid hormone (PTH) and calcium concentrations were normal. Thus, while a physiologic concentration of PTH was capable of maintaining calcium homeostasis, it was incapable of compensating for PTHrP haploinsufficiency in developing bone. In normal animals, both PTHrP and the PTH/PTHrP receptor were expressed predominantly in chondrocytes situated throughout the proliferative zone of the tibial growth plate. In the metaphysis, the PTH/PTHrP receptor was identified on osteoblasts and preosteoblastic cells situated in the bone marrow, while PTHrP was expressed only by osteoblasts. These observations indicate that postnatal bone development involves susceptible pathways that display exquisite sensitivity to critical levels of PTHrP and imply that the skeletal effects of PTH are influenced by locally produced PTHrP. Moreover, identification of both the ligand and its N-terminal receptor in metaphyseal osteoblasts and their progenitors suggests an autocrine/paracrine role for the protein in osteoblast differentiation and/or function. Impairment in this function as a consequence of PTHrP haploinsufficiency may critically influence the course of bone formation, resulting in altered trabecular architecture and perhaps low bone mass and increased bone fragility.