Neurofibromatosis Type 1 Gene Haploinsufficiency Reduces AP-1 Gene Expression without Abrogating the Anabolic Effect of Parathyroid Hormone

Abstract

Approximately 50% of neurofibromatosis type 1 (NF1) patients exhibit skeletal pathology, such as premature osteoporosis or pseudoarthroses. Loss of neurofibromin deregulates Ras signal transduction to affect generation of mitogen-activated protein kinase and Akt, both of which have been implicated in parathyroid hormone (PTH) anabolic mechanisms. Our aim was to determine if loss of neurofibromin impaired the anabolic effect of PTH on bone mass. Nf1 heterozygote (Nf1(+/-)) and wild type (Nf1(+/+)) mice were treated with recombinant human PTH(1-34) or vehicle once daily for 3-28 days. PTH enhanced mRNA expression of c-fos, junB, and fra2 in the distal femur metaphyses of both genotypes; expression of these transcripts was consistently lower in PTH-treated Nf1(+/-) mice. Despite lowered c-fos expression in Nf1(+/-) mice, PTH increased bone mass equivalently in both genotypes by 28 days. Ex vivo, Nf1 heterozygosity was associated with increased inducible osteoclasts in PTH-treated bone marrow cells and impairment of the actin stress fiber and cyclic adenosine monophosphate response to PTH in osteoprogenitors. Lower c-fos expression was previously thought to abrogate PTH responsiveness. Our results suggest crosstalk might occur between Ras signal transduction and the protein kinase A pathway in Nf1(+/-) mice. Ras signal transduction does not appear to be essential for the anabolic actions of PTH on bone. Because PTH was effective in the absence of Nf1, it may offer a useful approach to treat osteoporosis in NF1 patients.

Modeling bone morphogenetic protein and bisphosphonate combination therapy in wild‐type and Nf1 haploinsufficient mice

“…While the mechanism of PTH action is different to BMP-2, we found that the PTH axis is retained in Nf1 þ/À calvarial osteoblasts, as has been recently reported. 34 Consistent with Nf1 being a negative regulator of the small GTPase Ras, Nf1 þ/À cells displayed increased Ras activity in a pull-down assay, although Ras activity was further increased by BMP-2 treatment (Fig. 1E).…”

“…Systemic PTH has been proposed as an alternative anabolic agent for use in NF1, although primarily to target the premature osteoporosis seen in a subset of NF1 patients, rather than for orthopedic applications. 34 PTH is contraindicated in children due to the risk of malignancy in young rats treated with PTH. 35 Next we examined BMP-2-induced bone formation using by direct surgical implantation of rhBMP-2 in a resorbable CMC carrier; a technique that has been previously shown to robustly induce new bone in vivo.…”

Calvarial defects associated with neurofibromatosis Type 1

“…Although NF1 haploinsufficient mice have demonstrated reduced responsiveness to parathyroid hormone and abnormal levels of osteoblastic proliferation, apoptosis, and differentiation, the animals had similar bone density to normal mice. 45,46 These studies suggest that although the loss of a single allele increases the risk of osteoblast and osteoblastic progenitor malfunction, there are other pathways that preserve bone homeostasis.…”

Bone mineral density in children with neurofibromatosis 1

“…Although the skeletal involvement of NF1 is well known and osteoporosis has been reported in adults with NF1 (6–8), there is limited data in children. Elucidation of the bone mineral density (BMD) in children with NF1 will provide insight into the aetiology of bone abnormalities, and help in the development of potential therapeutic options (9). Herein, we present and discuss our findings in BMD in children with NF1.…”