[No abstract]

Summary

Skeletal abnormalities with delayed bone age and decreased linear bone growth are commonly found in children with prolonged juvenile hypothyroidism. However, rachitic bone abnormalities have not been previously reported in children with acquired hypothyroidism. Here, we present a case of newly found rickets in an 8-year-old female with untreated acquired hypothyroidism secondary to Hashimoto's thyroiditis. Laboratory finding for abnormalities in calcium/phosphorus homeostasis and hormones that regulate skeletal health was normal. Her radiographic anomalies resolved with levothyroxine treatment alone, suggesting that hypothyroidism was the etiology of the rickets. To our knowledge, this is the first case report of rickets associated with long-standing severe acquired hypothyroidism that resolved exclusively with thyroid repletion.

Learning points

Thyroid hormone plays an important role in bone mineralization. Prolonged hypothyroidism can result in rachitic bone abnormalities noted on radiographs. Hypothyroidism should be considered in the evaluation of a child with rickets.

To investigate the role of IGF-I signaling in osterix (OSX)-expressing cells in the skeleton, we generated IGF-I receptor (IGF-IR) knockout mice ((OSX)IGF-IRKO) (floxed-IGF-IR mice × OSX promoter-driven GFP-labeled cre-recombinase [(OSX)GFPcre]), and monitored postnatal bone development. At day 2 after birth (P2), (OSX)GFP-cre was highly expressed in the osteoblasts in the bone surface of the metaphysis and in the prehypertrophic chondrocytes (PHCs) and inner layer of perichondral cells (IPCs). From P7, (OSX)GFP-cre was highly expressed in PHCs, IPCs, cartilage canals (CCs), and osteoblasts (OBs) in the epiphyseal secondary ossification center (SOC), but was only slightly expressed in the OBs in the metaphysis. Compared with the control mice, the IPC proliferation was decreased in the (OSX)IGF-IRKOs. In these mice, fewer IPCs invaded into the cartilage, resulting in delayed formation of the CC and SOC. Immunohistochemistry indicated a reduction of vessel number and lower expression of VEGF and ephrin B2 in the IPCs and SOC of (OSX)IGF-IRKOs. Quantitative real-time PCR revealed that the mRNA levels of the matrix degradation markers, MMP-9, 13 and 14, were decreased in the (OSX)IGF-IRKOs compared with the controls. The (OSX)IGF-IRKO also showed irregular morphology of the growth plate and less trabecular bone in the tibia and femur from P7 to 7 weeks, accompanied by decreased chondrocyte proliferation, altered chondrocyte differentiation, and decreased osteoblast differentiation. Our data indicate that during postnatal bone development, IGF-I signaling in OSX-expressing IPCs promotes IPC proliferation and cartilage matrix degradation and increases ephrin B2 production to stimulate vascular endothelial growth factor (VEGF) expression and vascularization. These processes are required for normal CC formation in the establishment of the SOC. Moreover, IGF-I signaling in the OSX-expressing PHC is required for growth plate maturation and osteoblast differentiation in the development of the metaphysis.

Skeletal development is regulated by the coordinated activity of signaling molecules that are both produced locally by cartilage and bone cells and also circulate systemically. During embryonic development and postnatal bone remodeling, receptor tyrosine kinase (RTK) superfamily members play critical roles in the proliferation, survival, and differentiation of chondrocytes, osteoblasts, osteoclasts, and other bone cells. Recently, several molecules that regulate RTK signaling have been identified, including the four members of the Sprouty (Spry) family (Spry1-4). We report that Spry2 plays an important role in regulation of endochondral bone formation. Mice in which the Spry2 gene has been deleted have defective chondrogenesis and endochondral bone formation, with a postnatal decrease in skeletal size and trabecular bone mass. In these constitutive Spry2 mutants, both chondrocytes and osteoblasts undergo increased cell proliferation and impaired terminal differentiation. Tissue-specific Spry2 deletion by either osteoblast- (Col1-Cre) or chondrocyte- (Col2-Cre) specific drivers led to decreased relative bone mass, demonstrating the critical role of Spry2 in both cell types. Molecular analyses of signaling pathways in Spry2(-/-) mice revealed an unexpected upregulation of BMP signaling and decrease in RTK signaling. These results identify Spry2 as a critical regulator of endochondral bone formation that modulates signaling in both osteoblast and chondrocyte lineages.

The ATP-sensitive potassium channel (KATP) functions as a metabo-electric transducer in regulating insulin secretion from pancreatic β-cells. The pancreatic KATP channel is composed of a pore-forming inwardly-rectifying potassium channel, Kir6.2, and a regulatory subunit, sulphonylurea receptor 1 (SUR1). Loss-of-function mutations in either subunit often lead to the development of persistent hyperinsulinemic hypoglycemia of infancy (PHHI). PHHI is a rare genetic disease and most patients present with immediate onset within the first few days after birth. In this study, we report an unusual form of PHHI, in which the index patient developed hyperinsulinemic hypoglycemia after 1 year of age. The patient failed to respond to routine medication for PHHI and underwent a complete pancreatectomy. Genotyping of the index patient and his immediate family members showed that the patient and other family members with hypoglycemic episodes carried a heterozygous novel mutation in KCNJ11 (C83T), which encodes Kir6.2 (A28V). Electrophysiological and cell biological experiments revealed that A28V hKir6.2 is a dominant-negative, loss-of-function mutation and that KATP channels carrying this mutation failed to reach the cell surface. De novo protein structure prediction indicated that this A28V mutation reoriented the ER retention motif located at the C-terminal of the hKir6.2, and this result may explain the trafficking defect caused by this point mutation. Our study is the first report of a novel form of late-onset PHHI that is caused by a dominant mutation in KCNJ11 and exhibits a defect in proper surface expression of Kir6.2.

Mechanical loading of the skeleton, as achieved during daily movement and exercise, preserves bone mass and stimulates bone formation, whereas skeletal unloading from prolonged immobilization leads to bone loss. A functional interplay between the insulin-like growth factor 1 receptor (IGF1R), a major player in skeletal development, and integrins, mechanosensors, is thought to regulate the anabolic response of osteogenic cells to mechanical load. The mechanistic basis for this cross-talk is unclear. Here we report that integrin signaling regulates activation of IGF1R and downstream targets in response to both IGF1 and a mechanical stimulus. In addition, integrins potentiate responsiveness of IGF1R to IGF1 and mechanical forces. We demonstrate that integrin-associated kinases, Rous sarcoma oncogene (SRC) and focal adhesion kinase (FAK), display distinct actions on IGF1 signaling; FAK regulates IGF1R activation and its downstream effectors, AKT and ERK, whereas SRC controls signaling downstream of IGF1R. These findings linked to our observation that IGF1 assembles the formation of a heterocomplex between IGF1R and integrin β3 subunit indicate that the regulation of IGF1 signaling by integrins proceeds by direct receptor-receptor interaction as a possible means to translate biomechanical forces into osteoanabolic signals.

BACKGROUND:Hyperkyphosis, an excessive anterior curvature in the thoracic spine, is associated with reduced health status in older adults. Hyperkyphosis is highly prevalent, more common in older women than men. There is no standard intervention to reduce age-related hyperkyphosis. Sex differences in response to a kyphosis-specific exercise intervention are not known. METHODS:We conducted a randomized controlled trial of a targeted kyphosis-specific exercise and postural training program on the primary outcome Cobb angle of kyphosis, and investigated whether the magnitude of change differed between men and women. One hundred twelve participants aged ≥60 years with kyphosis ≥40° were enrolled and randomized to exercise or waitlist control, and 101 participants had analyzable baseline and follow-up radiographs for Cobb angle measurements. A group intervention including 10 participants per group was delivered by a physical therapist, 1-h, twice a week for 3-months. Controls were placed on a waitlist for 3 months before receiving a delayed intervention. Primary outcome was change from baseline to 3-months in Cobb angle measured from standing lateral spine radiographs. Secondary outcomes included change over 3-months in kyphometer-measured kyphosis, physical function and quality of life. Groups were combined for analysis after both received the intervention, and sex differences in response to the intervention were tested with ANOVA. RESULTS:Participants (60 women, 41 men) were 70.0 (SD = 5.7) years old with mean Cobb angle 55.9 (SD = 12.2) degrees at baseline. The active group had higher baseline modified Physical Performance Test scores than control, p = 0.03. Men had greater baseline kyphometer-measured kyphosis, p = 0.09, and higher bone mineral density (BMD), spine strength, more vertebral fractures and diffuse idiopathic skeletal hyperostosis (DISH) than women, p ≤ 0.01. There was no statistically significant difference between groups in change in Cobb at 3-months, p = 0.09, however change in kyphometer-measured kyphosis differed by 4.8 (95% CI:-6.8,-2.7) degrees, p < 0.001, favoring the active group. There were no differences between men and women in change in either kyphosis measurement after intervention, p > 0.1. CONCLUSIONS:A 3-month targeted spine strengthening exercise and posture training program reduced kyphometer-measured, but not radiographic-measured kyphosis. Despite sex differences in baseline kyphosis, BMD, spine strength, fractures and DISH, sex did not affect treatment response. TRIAL REGISTRATION:ClinicalTrials.gov Identifier: NCT01766674.