Type 1 neurofibromatosis (NF1), a disease involving ectodermal and mesodermal tissues, is autosomal dominant with an incidence of about 1 in 3,000 and a high mutation rate, The clinical manifestations of NF1 skeletal dysplasia are diverse and include scoliosis, lateral kyphosis, cervical spine lesions, spondylolisthesis, bone growth lesions, congenital arch and pseudarthrosis, subperiosteal bone proliferation, thinning of the bone cortex, short stature and large head deformity, and pterygoid dysplasia. Recently, many literature reported that NF1 patients, both adults and children, have significantly lower bone density and show different degrees of bone loss or osteoporosis, but their pathogenesis is still unclear, and the possible pathogenesis is reviewed in this paper as follows. 1. Abnormal function of neurofibromatosis protein The NF1 gene encodes a product of neurofibromatosis protein, which is expressed in a variety of cells and tissues. Neurofibromatosis protein inhibits growth factor expression and growth factor activation by acting as a GTPase-activating protein (Ras-GAP), a protein consisting of 2818 amino acids with a molecular weight of 327 kDa but a molecular weight of approximately 220-280 kDa on SDS-PAGE, which negatively regulates the activity of the intracellular signaling molecule P21-Ras (Ras), thereby inhibiting growth factor expression and induced cell proliferation. Haploinsufficiency or complete deletion of the NF1 gene leads to a quantitatively dependent increase in Ras activity, which then sequentially activates a large number of signaling pathways, including the mitogen-activated protein kinase (MAPK, mitogen-activated protein kinase) pathway and the phosphatidylinositol triphosphate kinase (PI-3K phosphatidylinositol-3-phosphate kinase) pathway. These signaling pathways can affect cell proliferation and differentiation in a cell type-specific manner. In NF1 patients, an initial single NF1 gene copy defect caused by a specific mutation or microdeletion can be followed by local inactivation of another functional NF1 allele. The transition from a single gene deletion to double inactivation of the allele is also referred to as loss of heterozygosity (LOH). specific mutations or complete deletions of the NF1 gene result in abnormal neurofibromatosis protein function, which may lead to abnormal changes in its regulation of bone development and remodeling as well as bone homeostasis. Loss of NF1 gene function in periosteal cells may cause thinning of the bone cortex due to dysfunction of periosteal osteoblasts as demonstrated by pseudarthrosis in NF1 patients. Neurofibromatosis proteins, in addition to Ras-GAP function, also regulate adenylate cyclase activity (cAMP) and protein kinase A (PKA) activity. cAMP and PKA are the main signaling pathways regulating osteoblast and osteoclast function, so haploinsufficient NF1 patients may have decreased bone formation (decreased osteoblast activity) and/or increased osteoclast activity, resulting in bone dysplasia or bone loss. In turn, bone dysplasia or impaired bone metabolism may occur. Serum 25-(OH)VD is reduced. 25-(OH)VD is converted to 1,25(OH)2VD in vivo, which increases intestinal absorption of calcium and phosphorus and plays an important role in bone formation. serum 25-(OH)VD is significantly lower in NF1 patients than in normal controls. However, the relationship between low serum 25-(OH)VD concentrations and reduced BMD in NF1 patients remains unclear.Tucker et al. measured serum serum 25-(OH)VD in 72 adult NF1 patients and 312 normal healthy subjects in summer and winter. Of the 312 controls, 56 had summer measurements and 256 had winter measurements. Among the normal controls, diseases and treatments affecting BMD were excluded. 56% of the NF1 patients (29/52 in winter; 38/68 in summer) had reduced serum 25-(OH)VD concentrations. Mean serum 25-(OH)VD concentrations were significantly and statistically lower in NF1 patients than in normal controls, both in winter and summer. Reduced serum 25-(OH)VD concentrations in NF1 patients may decrease calcium and phosphorus absorption, reduce calcium and phosphorus deposition in bone, and therefore lead to decreased BMD. in 2006, Lammert et al [8] studied the number of serum 25-hydroxyvitamin D and cutaneous neurofibromas in 55 NF1 patients and 58 healthy controls and found that serum 25-hydroxyvitamin D in NF1 patients values were significantly and statistically lower than those of normal controls. reduced serum 25-hydroxyvitamin D levels in NF1 patients may lead to lower BMD. in 2010, Seitz et al. clinically evaluated 14 adult NF1 patients with age- and sex-matched controls. The results showed that serum 25-(OH)-VD3 and BMD were both lower in NF1 patients than in controls. The results of this study suggest that reduced serum 25-(OH)-VD3 in NF1 patients promotes the development of skeletal lesions in NF1 patients. In vitro, VD treatment inhibits the growth of some cell lines, but activation of the Ras signaling pathway, as occurs in NF1 haploinsufficient patients, can hinder the inhibitory effect of VD on cell growth. In addition, both fibronectin and VD regulate cell proliferation, and the normal inhibitory cell proliferation and apoptosis-promoting effects of VD are decreased and VD deficiency may accelerate skeletal changes in NF1 patients. Increasing VD intake and calcium supplementation may increase bone mass in patients. 3. Cytological factors Bone metabolism is divided into two aspects: new bone formation (anabolism) and bone resorption (catabolism). These two aspects are accomplished by two types of mature cells, namely osteoblasts and osteoclasts. Bone homeostasis is the balance between skeletal matrix formation and resorption. Osteoclasts are derived from the bone marrow monocyte/macrophage system and successfully adhere to the bone matrix and resorb bone, while osteoblasts produce new bone matrix. Imbalances in bone morphology and remodeling can lead to pathological changes in bone structure and function. (1) Osteoblasts Studies have shown that the NF1 gene is expressed at high levels in growth plates and mature osteoblasts during endochondral osteogenesis. Both in intramembranous osteogenesis and endochondral osteogenesis, osteoblasts, osteocytes and osteocytes of mature bone are monitored for expression of neurofibromatosis proteins. Early in the pathogenesis, skeletal disease may arise from an impaired dynamic balance of endochondral osteogenesis and bone. On the other hand, reduced BMD in NF1 patients may be triggered by a failure of cell cycle control or impaired remodeling of mature bone. In NF1 patients, each of the mature bone cell types may have a role in BMD due to the widespread expression of neurofibromatosis proteins in osteoblasts, osteoclasts and osteocytes. Growth factor-mediated Ras-MAPK signaling counteracts osteogenic signaling and disrupts osteoblast differentiation. Indeed, NF1+/- mouse stromal stem cells show increased proliferation but impaired osteoblast differentiation. Thus, in the context of these studies, NF1 heterozygotes lead to increased Ras activity, which can increase osteoclast-mediated bone resorption and also decrease osteoblast-mediated bone formation. These two aspects affect bone homeostasis in NF1 patients. At the cytological level, in vitro osteoblasts show increased proliferation but decreased differentiation and mineralization. yu et al. found no significant differences in bone volume and structure in NF1+/- mice compared to wild-type controls, but there was a trend toward decreased bone formation. osteogenic progenitor cells in the femoral epiphysis of NF1+/- mice showed pre-mature apoptosis and high proliferation. osteogenic progenitor cells of NF1+/- mouse origin were activated by Ras signaling. Osteogenic differentiation of osteoprogenitor cells to osteoblasts was decreased. wu et al. found impaired osteogenic differentiation of mesenchymal stem cells and progenitor cells in NF1+/- mice. Different NF1 transgenic mouse models exhibited different phenotypes. nF1ob-/- mice lacked neurofibromatosis protein expression and exhibited increased bone formation without long bone arch changes. undifferentiated MSCs in the developmental limb of NF1prxl mice lacking neurofibromatosis protein exhibited tibial arch changes and high osteoporosis. Immunohistochemical detection of sections in NF1-/- embryonic developmental long bones showed osteoblasts lacking phosphorylated P44/42MAPK signaling. (2) Osteoclasts Abnormalities in osteoclast function and proliferation in NF1 patients, as well as altered proliferation and function of both osteoblasts and osteoclasts were observed in the NF1 mouse model. Increased serum TRAP5b concentrations suggest an increased number and activity of osteoclasts in NF1 patients. Recent histomorphometric studies of bone biopsies from NF1 patients showed a 4-fold increase in osteoblasts and a 10-fold increase in osteoclasts compared to age- and sex-matched controls.Stevenson et al. studied and analyzed urinary pyridine cross-linking products (urinary pyridine and deoxypyridine) from 51 children (5-19 years of age) with NF1. The control group was 99 normal healthy children. Multivariate analysis showed that Dpd and Dpd/Pyd were significantly and statistically increased in patients with NF1 with or without skeletal malformations. This suggests that bone resorption is significantly increased in NF1 patients and that the significant increase in bone resorption may be the result of proliferation or enhanced function of osteoclasts, which in turn leads to reduced BMD in NF1 patients. increased urinary excretion of urinary pyridine cross-linking products in NF1ob-/- mice indicates enhanced osteoclast function and increased bone resorption. yang et al [13] found an increased number of multinucleated osteoclasts in NF1+/- mice. NF1+/- mouse osteoblasts and osteoclast progenitors were sensitive to limited concentrations of M-CSF and NF-kB ligand receptor activator (RANKL) levels.NF1+/- mouse osteoblasts showed increased phosphorylation of P21-Ras-GTP and AKt in response to M-CSF stimulation, which resulted in increased osteoclast acquisition of survival, proliferation, migration, adhesion and lysis activity. These increased functions of osteoclasts in NF1+/- mice after ovariectomy resulted in severe bone loss compared to wild-type mice. In addition, osteoclasts differentiated from NF1 patients in vitro in culture showed increased Ras/PI3K activity and increased lysis activity, similar to the performance of osteoclasts from NF1+/- mice. This study suggests that increased osteoclast activity and numbers may contribute to bone loss and bone loss in NF1 patients, and that Ras is an important cellular signaling mediator that links growth factor and cytokine signaling via MAPK and PI3K pathways. Recent cell culture models reveal that PI3K increases RANKL-induced osteoclast formation, which is associated with a high level of Ras activity. This was confirmed in in vitro cultures of bone marrow cells from NF1+/- mice and in serum mononuclear cells from patients with NF1. Tucker et al. found increased serum PTH concentrations, increased bone TRAP5b and urinary deoxypyridinoline hinge products, and increased 24-hour urinary calcium and urinary phosphorus excretion in >10% of patients. These findings suggest that NF1 patients may have increased osteoclast activity leading to accelerated bone catabolism. Bisphosphonate agents that inhibit osteoclast function may be effective in the treatment of such patients. (3) Fibroblasts and mast cells Fibroblast growth factor activates the Ras/MAPK pathway in mice, and inactivation of the SHP2-Ras-MAPK pathway leads to increased bone formation after increased osteoclast activity, suggesting impaired signaling between osteoclasts and osteoblasts. In addition, proliferation and invasion of fibroblasts from NF1 patients into bone may hinder the implantation of osteogenic progenitor cells during bone repair. In vitro cultured NF1-/- fibroblasts exhibit unrestricted proliferation and abnormal collagen synthesis, suggesting that high proliferation and abnormal bone formation may also exist in osteoblasts deficient in the NF1 gene. Haploinsufficient NF1 also increases mast cell proliferation, survival and clone formation due to CKITL (c-kit receptor) ligand action. Since mastocytosis is associated with osteoporosis, high proliferation of mast cells may lead to bone loss in NF1 patients. Parosteal mast cells are closely associated with increased bone turnover, bone marrow fibrosis and bone formation. 4. Biomechanical and physical activity factors Mechanical alterations associated with the muscles of haploinsufficient NF1 patients may further affect bone homeostasis. In some NF1 patients the intermuscular compartment volume decreases, which can cause bone loss. Although dystrophic skeletal alterations are the more definitive causative factor, it has been suggested that decreased muscle strength around the spine may contribute to scoliosis formation. Although it remains unclear whether primary bone loss in NF1 patients or decreased physical activity following skeletal deformities in NF1 patients leads to bone loss, decreased physical activity in NF1 patients may also be a factor in decreased bone mineral density. Increased physical activity or activity may improve bone mass reduction. 5. Vascular factors Although there is evidence that not all atrophic bone discontinuities have impaired blood supply, impaired vascular ingrowth or lack of blood supply affects bone healing in patients with NF1, especially in patients with tibial pseudarthrosis. Reduced vascularity in the adjacent periosteum or generalized thickening of the vascular wall around the tibial pseudarthrosis has been reported. Both of these conditions impede bone healing and bone volume increase. Studies in NF+/- mice have shown increased angiogenesis due to increased cell migration and the effects of pro-angiogenic factors. However, this study only shows that fracture revascularization is not disrupted and that immature and excessive vascular thickening may have affected bone repair and bone mass maintenance. In conclusion, reduced bone mass in patients with type I neurofibromatosis may result from abnormal function or decreased expression of neurofibromatosis proteins; abnormal function of osteoblasts, osteoclasts, fibroblasts and mast cells; and a combination of biomechanical factors or vascular factors.