Yoshitaka Yamanaka

Efficacy of growth hormone therapy for patients with skeletal dysplasia

Most patients with skeletal dysplasia show severe short stature. Surgical therapy has been attempted to correct bone deformities, but therapy for improving their severe short stature has been rarely attempted. We undertook a clinical trial of growth hormone (GH) therapy for patients with skeletal dysplasia accompanying severe short stature caused by achondroplasia (ACH), hypochondroplasia (HCH), pseudoachondroplasia (PSACH), spondyloepiphyseal dysplasia congenita (SED), or Schmid type metaphyseal dysplasia (MD). This study examined the efficacy of GH therapy on height increase and change of height SD score over a 1-year period in patients with skeletal dysplasia and showed a short-term efficacy for skeletal dysplasia. In ACH, HCH, and MD, GH had a significant effect on height gain. However, PSACH and SED showed no height gain efficacy; in cases of PSACH, height SD score was worse after therapy. Severe adverse events were not observed except in one SED case, in which scoliosis worsened and height did not increase. For patients with skeletal dysplasia, GH therapy is moderately effective for height gain. It is ineffective in cases with severe spinal deformities, however; although bone growth was promoted, the ligaments and matrix were too weak to support muscle tonus and the effects of gravity, resulting in worsened kyphosis and lordosis. These results clarify why GH therapy is ineffective for height gain. The pathogenic genes of skeletal dysplasia have recently been detected and consequently changes in bone formation have been investigated in detail. Careful consideration of indications for therapy and cautious observation during therapy are crucial when attempting to treat advanced bone deformities.

Growth Hormone Therapy in Achondroplasia

Achondroplasia is one of the most common causes of severe rhizomelic dwarfism. We have previously reported the growth-promoting effect of growth hormone (GH) in this disorder. In this expanded clinical study, dose dependency and the long-term effect of GH were also investigated. Prepubertal children with achondroplasia (82 males and 63 females) were randomly divided into 2 groups. Patients were treated with 0.5 IU/kg per week or 1.0 IU/kg per week subcutaneous recombinant human GH. Of 75 patients, the mutational analysis of fibroblast growth factor receptor-3 revealed that G1138A was detected in 70 and G1138C was found in 2. GH increased growth rate and height z score in a dose-dependent manner. GH also increased serum insulin-like growth factor (IGF)-I, IGF-binding protein-3 and osteocalcin. No adverse effects were observed in either group. We conclude that GH therapy is a useful method for improvement of severe growth retardation of achondroplasia.

Insulin-like Growth Factor-1 Rescues the Mutated FGF Receptor 3 (G380R) Expressing ATDC5 Cells From Apoptosis Through Phosphatidylinositol 3-Kinase and MAPK

An activated mutation in the FGFR3 gene causes ACH. To examine the effects of IGF‐1, which is an important mediator of GH, on apoptosis, we analyzed a chondrogenic cell line expressing the FGFR3 mutants. Our findings that IGF‐1 prevented the apoptosis through P13K and MAPK pathways may explain how GH treatment improves the disturbed bone growth in ACH.

FGFR3-related dwarfism and cell signaling

Skeletal development consists of the following steps: skeletal patterning, mesenchymal differentiation, bone growth, and homeostasis. In the early phase of embryogenesis, immature mesenchymal cells gather in the proper position, and the anteroposterior and dorsoventral axes are determined. This ‘‘skeletal patterning’’ is followed by ‘‘differentiation’’ of immature mesenchymal cells to chondrocytes or osteoblasts. In the appendicular skeleton, endochondral ossification takes part in elongation, and chondrocytes create the growth plate, a chondrocytic layer, and proliferate with high frequency in the longitudinal direction. Bone growth continues until the growth plate is closed at the time of pubertal maturation. In mature bones, a homeostasis process called bone remodeling occurs. Bone remodeling always renews old bone tissue by forming both osteoblasts and resorbing osteoclasts. Many essential genetic mechanisms for these bonegrowing steps have been described. Disorders of each bone formation step cause characteristic skeletal dysplasias. For example, polydactyly and split-hand are caused by patterning failure, dwarfism is caused by extraordinary differentiation and proliferation of chondrocytes in the growth plate, and osteoporosis and osteopetrosis are caused by abnormality of bone remodeling. Many functional genes are identified through genetic disorders of the skeleton and knockout mice. Skeletal disorders have been classified based on the responsible genes [1], clinical features [2], and molecular pathology and embryology [3]. According to the reviews, a total of 271 clinically different disorders and 75 responsible genes have been described. The most frequent genetic skeletal dysplasia is achondroplasia (ACH) (OMIM; #100800), caused by point mutations in the fibroblast growth factor receptor 3 (FGFR3; OMIM *134934) gene [4, 5]. Some other skeletal dysplasias are also caused by mutations located in the FGFR3, such as hypochondroplasia (HCH) (OMIM; #146000), thanatophoric dysplasia type I (TDI) (OMIM; #187600) and type II (TDII) (OMIM; #187601), and severe achondroplasia with developmental delay and acanthosis nigricans (SADDAN) (description in OMIM *134934.0015) [6–11]. The location of each mutation is unique. The phenotypes of those FGFR3-related skeletal dysplasias are similar to that of ACH, but the severity is quite different. Mutations of the FGFR3 gene have been reported to be responsible for craniofacial disorders and multiple myeloma, which are not a focus in this review. Many reports state all mutant FGFR3s are constitutively activated. Excessive D. Harada is a recepient of JSBMR Encouragement Award of 2004.

PTHrP rescues ATDC5 cells from apoptosis induced by FGF receptor 3 mutation.

An activation mutation in the FGFR3 gene causes ACH. The effects of the FGFR3 mutants on apoptosis were analyzed in a chondrogenic cell line. ACH chondrocytes exhibited marked apoptotic with downregulation of PTHrP expression. Rescue of these cells by PTHrP replacement implies a potential therapy for this disorder.

Molecular Basis for the Treatment of Achondroplasia

Achondroplasia (ACH), the most common form of short-limbed dwarfism, and its related disorders are caused by constitutively activated point-mutated fibroblast growth factor receptor 3 (FGFR3). Recent studies have provided a large body of evidence to prove chondrocyte proliferation and differentiation in these disorders. However, little is known about the possible effects of the FGFR3 mutants on apoptosis of chondrocytes. In the present study, we analyzed apoptosis using a chondrogenic cell line, ATDC5, expressing the FGFR3 mutants causing ACH and thanatophoric dysplasia, which is a more severe neonatal lethal form comprising type I and type II. We found that the introduction of these mutated FGFR3s into ATDC5 cells decreased mRNA expression of parathyroid hormone-related peptide (PTHrP) and induced apoptosis. Importantly, replacement of PTHrP prevented the apoptotic changes in ATDC5 cells expressing ACH mutant. Insulin-like growth factor (IGF)-I, which is an important mediator of growth hormone (GH), also reduced apoptosis in ATDC5 cells expressing ACH mutant. IGF-I prevented apoptosis through the phosphatidylinositol 3-kinase and mitogen-activated protein kinase pathways, indicating the mechanisms by which GH treatment improves disturbed bone growth in ACH.