Kohji Miura

An Overgrowth Disorder Associated with Excessive Production of cGMP Due to a Gain-of-Function Mutation of the Natriuretic Peptide Receptor 2 Gene

We describe a three-generation family with tall stature, scoliosis and macrodactyly of the great toes and a heterozygous p.Val883Met mutation in Npr2, the gene that encodes the CNP receptor NPR2 (natriuretic peptide receptor 2). When expressed in HEK293A cells, the mutant Npr2 cDNA generated intracellular cGMP (cyclic guanosine monophosphate) in the absence of CNP ligand. In the presence of CNP, cGMP production was greater in cells that had been transfected with the mutant Npr2 cDNA compared to wild-type cDNA. Transgenic mice in which the mutant Npr2 was expressed in chondrocytes driven by the promoter and intronic enhancer of the Col11a2 gene exhibited an enhanced production of cGMP in cartilage, leading to a similar phenotype to that observed in the patients. In addition, blood cGMP concentrations were elevated in the patients. These results indicate that p.Val883Met is a constitutive active gain-of-function mutation and elevated levels of cGMP in growth plates lead to the elongation of long bones. Our findings reveal a critical role for NPR2 in skeletal growth in both humans and mice, and may provide a potential target for prevention and treatment of diseases caused by impaired production of cGMP.

Overgrowth syndrome associated with a gain-of-function mutation of the natriuretic peptide receptor 2 (NPR2) gene

The signal pathway of the C‐type natriuretic (CNP) and its receptor, natriuretic peptide receptor 2 (NPR2) is involved in the longitudinal growth of long bones. Loss of function mutations at NPR2 cause acromesomelic dysplasia, type Maroteaux, while overproduction of CNP by chromosomal translocation and a gain‐of‐function mutation at NPR2 have been reported to be responsible for an overgrowth syndrome in three cases and one family, respectively. We identified a four‐generation family with an overgrowth syndrome characterized by tall stature, macrodactyly of the great toes, scoliosis, coxa valga and slipped capital femoral epiphysis, similar to those previously reported in association with CNP/NPR2 overactivity. The serum level of amino‐terminal proCNP was normal in the proband. A novel missense mutation of NPR2, c.1462G>C (p.Ala488Pro) was found to co‐segregate with the phenotype in this family. In vitro transfection assay of the mutant NPR2 revealed overactivity of the mutant receptor at baseline as well as with the ligand. This overgrowth syndrome caused by a gain‐of‐function mutation at NPR2 should be differentiated from Marfan or related syndromes, and may be categorized along with the overgrowth syndrome caused by overproduction of CNP due to its phenotypical similarity as overgrowth CNP/NPR2 signalopathy.

A human skeletal overgrowth mutation increases maximal velocity and blocks desensitization of guanylyl cyclase-B☆

C-type natriuretic peptide (CNP) increases long bone growth by stimulating guanylyl cyclase (GC)-B/NPR-B/NPR2. Recently, a Val to Met missense mutation at position 883 in the catalytic domain of GC-B was identified in humans with increased blood cGMP levels that cause abnormally long bones. Here, we determined how this mutation activates GC-B. In the absence of CNP, cGMP levels in cells expressing V883M-GC-B were increased more than 20 fold compared to cells expressing wild-type (WT)-GC-B, and the addition of CNP only further increased cGMP levels 2-fold. In the absence of CNP, maximal enzymatic activity (Vmax) of V883M-GC-B was increased 15-fold compared to WT-GC-B but the affinity of the enzymes for substrate as revealed by the Michaelis constant (Km) was unaffected. Surprisingly, CNP decreased the Km of V883M-GC-B 10-fold in a concentration-dependent manner without increasing Vmax. Unlike the WT enzyme the Km reduction of V883M-GC-B did not require ATP. Unexpectedly, V883M-GC-B, but not WT-GC-B, failed to inactivate with time. Phosphorylation elevated but was not required for the activity increase associated with the mutation because the Val to Met substitution also activated a GC-B mutant lacking all known phosphorylation sites. We conclude that the V883M mutation increases maximal velocity in the absence of CNP, eliminates the requirement for ATP in the CNP-dependent Km reduction, and disrupts the normal inactivation process.

Acromesomelic dysplasia, type maroteaux caused by novel loss-of-function mutations of the NPR2 gene: Three case reports

The C‐type natriuretic peptide (CNP)–natriuretic peptide receptor 2 (NPR2) signaling pathway plays an important role in chondrocyte development. Homozygous loss‐of‐function mutations of the NPR2 gene cause acromesomelic dysplasia, type Maroteaux (AMDM). The aim of this study was to identify and characterize NPR2 loss‐of‐function mutations in patients with AMDM. The NPR2 gene was sequenced in three Korean patients with AMDM and functional analysis of the mutated proteins was performed in vitro. Five novel NPR2 mutations were found in the three patients: two compound heterozygous mutations [c.1231T>C (Tyr411His) and c.2761C>T (Arg921X) in Patient 1 and c.1663A>T (Lys555X) and c.1711‐1G>C (M571VfsX12) in Patient 3] and a homozygous mutation [c.2762G>A (Arg921Gln) in Patient 2]. Serum NT‐proCNP concentration was significantly increased in each patient compared to control subjects. Cells transfected with the expression vector of each mutant except those found in Patient 3 showed a negligible or a markedly low cGMP response after treatment with CNP. HA‐tagged wild‐type (wt) and HA‐mutant NPR2 were expressed at comparable levels: there were two bands of ∼130 and ∼120 kDa in wt and Arg921Gln, a single ∼120 kDa band in Tyr411His, and a single ∼110 kDa in the nonsense mutant. With respect to subcellular localization, Arg921Gln as well as wt‐NPR2 reached the cell surface, whereas Tyr411His and Arg921X mutants did not. The Tyr411His and Arg921X NPR2 proteins were co‐localized with an endoplasmic reticulum (ER) marker and failed to traffic from the ER to the Golgi apparatus. These results are consistent with deglycosylation experiments. Tyr411His and Arg921X NPR2 are complete loss‐of‐function mutations, whereas Arg921Gln behaves as a receptor for CNP with limited function.

A Japanese Male Patient with ‘Fibular Aplasia, Tibial Campomelia and Oligodactyly’: An Additional Case Report

We report a male infant with FATCO syndrome, an acronym for fibular aplasia, tibial campomelia, and oligosyndactyly. Courtens et al. reported an infant with oligosyndactyly of the left hand, complete absence of the right fibula, bowing of the right tibia, and absence of the right fifth metatarsal and phalanges. They noted 5 patients with similar clinical features, and proposed the FATCO syndrome. Our patient had a left-sided cleft lip, cleft palate, oligosyndactyly of the right hand and bilateral feet, and bilateral anterior bowing of the limbs associated with overlying skin dimpling. Radiographs showed a short angulated tibia with left fibular aplasia and right fibular hypoplasia. We consider our case the 6th patient with FATCO syndrome, and the cleft lip and palate, not reported in the previous 5 patients, may allow us to further understand the development of the extremities and facies.

Successful induction of sclerostin in human-derived fibroblasts by 4 transcription factors and its regulation by parathyroid hormone, hypoxia, and prostaglandin E2

Sclerostin, coded by SOST, is a secretory protein that is specifically expressed in osteocytes and suppresses osteogenesis by inhibiting WNT signaling. The regulatory mechanism underlying SOST expression remains unclear mainly due to the absence of an adequate human cell model. Thus, we herein attempted to establish a cell model of human dermal fibroblasts in order to investigate the functions of sclerostin. We selected 20 candidate transcription factors (TFs) that induce SOST expression by analyzing gene expression patterns in the human sarcoma cell line, SaOS-2, between differentiation and maintenance cultures using microarrays. An effective set of TFs to induce SOST expression was sought by their viral transduction into fibroblasts, and a combination of four TFs: ATF3, KLF4, PAX4, and SP7, was identified as the most effective inducer of SOST expression. Quantitative PCR demonstrated that the expression levels of SOST in fibroblasts treated with the 4 TFs were 199- and 1439-fold higher than those of the control after 1-week and 4-week cultures, respectively. The level of sclerostin in the conditioned medium, as determined by ELISA, was 21.2pmol/l 4weeks after the transduction of the 4 TFs. Interestingly, the production of Dickkopf1 (DKK1), another secreted inhibitor of WNT signaling, was also increased by transduction of these 4 TFs. Parathyroid hormone (PTH) significantly suppressed the induced SOST by 38% and sclerostin by 82% that of the vehicle. Hypoxia increased the induced SOST by 62% that of normoxia. Furthermore, prostaglandin E2 (PGE2) increased SOST expression levels to 16-fold those of the vehicle. In conclusion, the efficient induction of SOST expression and sclerostin production was achieved in human dermal fibroblasts by the transduction of ATF3, KLF4, PAX4, and SP7, and the induced SOST and sclerostin were regulated by PTH, hypoxia, and PGE2. This model may contribute to elucidating the regulatory mechanisms underlying SOST expression and advancing drug development for metabolic bone diseases.

Serum fibroblast growth factor 23 is a useful marker to distinguish vitamin D-deficient rickets from hypophosphatemic rickets.

Background/Aims: Vitamin D-deficient rickets (DR) has recently re-emerged among developed countries. Vitamin D deficiency can influence biochemical results of patients with fibroblast growth factor 23 (FGF23)-related hereditary hypophosphatemic rickets (HR), making differential diagnosis difficult. In the present study we evaluated the utility of serum FGF23 levels in the diagnosis of DR and during its treatment. Methods: The study group comprised 24 children with DR and 8 children with HR. Serum FGF23 levels and bone metabolism-related measurements were assessed. Results: Serum FGF23 levels in patients with DR were less than 19 pg/ml, while those in patients with HR were more than 57 pg/ml. There were significant differences in serum levels of calcium, phosphate, parathyroid hormone, and 1,25-dihydroxyvitamin D, as well as tubular maximum phosphate reabsorption per glomerular filtration rate between patients with DR and HR, but these values were not fully mutually exclusive. In addition, serum FGF23 and phosphate levels were increased following treatment. Conclusion: Serum FGF23 level is the most critical biochemical marker for distinguishing DR from HR and might be a good indicator of biochemical response to the intervention. Serum FGF23 levels show utility for the diagnosis of DR and in the assessment of its response to treatment.

Two Japanese familial cases of Caffey disease with and without the common COL1A1 mutation and normal bone density, and review of the literature

Caffey disease, also known as infantile cortical hyperostosis, is a rare bone disease characterized by acute inflammation with swelling of soft tissues and hyperostosis of the outer cortical surface in early infancy. The common heterozygous mutation of the COL1A1 gene, p.Arg1014Cys, has been reported in patients with Caffey disease. However, its pathogenesis remains to be elucidated, and the reason for the incomplete penetrance and transient course of the disease is still unclear. In the present study, we performed mutation analysis of the COL1A1 and COL1A2 genes and measured bone mineral density in two Japanese familial cases of Caffey disease. The index case and two clinically healthy members of one family carry the common heterozygous mutation; in contrast, no mutation in COL1A1 or COL1A2 was identified in the affected members of the second family. In addition, we found normal bone mineral density in adult patients of both families who have had an episode of cortical hyperostosis regardless of the presence or absence of the common p.Arg1014Cys mutation. Conclusion: The results reveal that Caffey disease is genetically heterogeneous and that affected and unaffected adult patients with or without the common COL1A1 mutation have normal bone mineral density.

A Male Patient with Humoral Hypercalcemia of Malignancy (HHM) with Leukocytosis Caused by Cutaneous Squamous Cell Carcinoma Resulting from Recessive Dystrophic Epidermolysis Bullosa.

Recessive dystrophic epidermolysis bullosa (RDEB) is a severe skin disorder. Although the patients are at risk for cutaneous squamous cell carcinoma (SCC), no case of cutaneous SCC derived from RDEB with humoral hypercalcemia of malignancy (HHM) has been reported. We present the first case report of a male patient with HHM with leukocytosis caused by cutaneous SCC resulting from RDEB. A 20-yr-old Japanese male patient with RDEB; the diagnosis was confirmed by electron microscopic examination, suffered an intractable skin ulcer and hypercalcemia and leukocytosis. PTH-rP, SCC antigen and Granulocyte colony-stimulating factor (G-CSF) levels were elevated. The histological diagnosis of the skin lesion was made well-differentiated SCC. Immunohistochemical staining showed the expression of PTH-rP in atypical tumor cells. For the control of hypercalcemia before an amputation, we used zoledronate safely and could control the serum Ca concentration in the normal range. After the amputation of his right leg including SCC, leukocytosis improved immediately and PTH-rP in blood decreased to the normal range. One month after the amputation, local recurrence of cutaneous SCC and multiple lung metastases were observed. PTH-rP increased gradually associated with hypercalcemia. Although the patient reached an unfortunate turning point about 4 mo after the amputation, we propose that zoledronate is an effective and safe treatment for HHM with cardiorenal complications.

Constitutively active form of natriuretic peptide receptor 2 ameliorates experimental pulmonary arterial hypertension

We recently found a constitutively active mutant of natriuretic peptide receptor 2 (caNPR2; V883M), which synthesizes larger amounts of cyclic guanosine monophosphate (cGMP) intracellularly without any ligand stimulation than existing drugs. The aim of this study was to investigate the therapeutic effects of gene transduction using caNPR2 for pulmonary arterial hypertension (PAH). In vitro gene transduction into human pulmonary arterial smooth muscle cells using Sendai virus (SeV) vectors carrying caNPR2 induced 10,000-fold increases in the synthesis of cGMP without ligand stimulation, and the proliferation of caNPR2-expressing cells was significantly attenuated. The PAH model rats generated by hypoxia and the administration of SU5416 were then treated with SeV vectors through a direct injection into the left pulmonary artery. Right ventricular systolic pressure was significantly decreased 2 weeks after the treatment, while systemic blood pressure remained unchanged. Histological analyses revealed that the medial wall thickness and occlusion rate of pulmonary arterioles were significantly improved in caNPR2-treated lungs. Neither the systemic integration of virus vectors nor side effects were observed. The massive stimulation of cGMP synthesis by gene therapy with caNPR2 was safe and effective in a PAH rat model and, thus, has potential as a novel therapy for patients with severe progressive PAH.