Severe osteopenia in CFTR-null mice

CFTR Modulates Hypothalamic Neuron Excitability to Maintain Female Cycle

Cystic fibrosis transmembrane conductance regulator (CFTR), known as an epithelial Cl- channel, is increasingly noted to be expressed in the nervous system, although whether and how it plays a role in neuronal excitability is unclear. Given the association of CFTR with fertility, we tested here possible involvement of CFTR in regulating hypothalamic neuron excitability. Patch-clamp and Ca2+ imaging showed that pharmacological inhibition of CFTR evoked electrical pulses and Ca2+ spikes in primary rat hypothalamic neurons, which was dependent on extracellular Cl-. Hypothalamic neurons in brain-slice preparations from adult female mice with CFTR mutation (DF508) exhibited significantly reduced electrical pulses as compared to the wild-type controls. Removal of extracellular Cl- eliminated hypothalamic electrical pulses in the wild-type brain slices, which was reversible by subsequent addition of Cl-. In adult female mice, Ca2+ indicator (GCaMP6s)-based fiber-photometry showed that hypothalamic Ca2+ activities in vivo were enhanced at the proestrus/estrus phase as compared to the diestrus phase of the female cycle. Such estrus-associated hypothalamic activities were largely diminished in DF508 female mice, together with delayed puberty and disturbed female cycles. Therefore, these findings suggest a critical role of CFTR in modulating hypothalamic neuron excitability, which may account for the disturbed female cycles and reduced female fertility associated with CFTR mutations.

Cystic Fibrosis Bone Disease: The Interplay between CFTR Dysfunction and Chronic Inflammation

Cystic fibrosis is a monogenic disease with a multisystemic phenotype, ranging from predisposition to chronic lung infection and inflammation to reduced bone mass. The exact mechanisms unbalancing the maintenance of an optimal bone mass in cystic fibrosis patients remain unknown. Multiple factors may contribute to severe bone mass reduction that, in turn, have devastating consequences in the patients' quality of life and longevity. Here, we will review the existing evidence linking the CFTR dysfunction and cell-intrinsic bone defects. Additionally, we will also address how the proinflammatory environment due to CFTR dysfunction in immune cells and chronic infection impairs the maintenance of an adequate bone mass in CF patients.

Endocrine Complications of Cystic Fibrosis

Endocrine comorbidities have become increasingly important medical considerations as improving cystic fibrosis (CF) care increases life expectancy. Although the underlying pathophysiology of CF-related diabetes remains elusive, the use of novel technologies and therapeutics seeks to improve both CF-related outcomes and quality of life. Improvements in the overall health of those with CF have tempered concerns about pubertal delay and short stature; however, other comorbidities such as hypogonadism and bone disease are increasingly recognized. Following the introduction of highly effective modulator therapies there are many lessons to be learned about their long-term impact on endocrine comorbidities.

Auxological and Endocrinological Features in Children and Adolescents with Cystic Fibrosis

Cystic fibrosis (CF) is a multisystem autosomal recessive disease caused by mutations that lead to deficient or dysfunctional CF transmembrane conductance regulator (CFTR) proteins. Patients typically present malnutrition resulting from the malabsorption of fundamental nutrients and recurring lung infections, with a progressive worsening of the respiratory function. For these reasons, the clinical management of CF requires a multidisciplinary team. From an endocrinological point of view, patients often present major complications, such as diabetes, bone disease, thyroid disorders, delayed growth and puberty, hypogonadism and infertility, which negatively affect their quality of life and, in some cases, significantly reduce life expectancy. These complications can arise as a direct result of CFTR dysfunction and/or as a consequence of a deterioration in the function of the organs affected. The objective of this review is to analyze all the possible endocrinological complications that can occur in patients with CF by evaluating the most recent papers in the literature.

Orthopedic Manifestations of Cystic Fibrosis

Cystic fibrosis (CF) is a relatively common disease seen in Whites of northern European descent. Classically, it was a lethal disease and uncommon for the orthopedic practitioner to interact with CF patients. Recent pharmaceutical breakthroughs targeting the CF transmembrane conductance regulator (CFTR) gene have significantly prolonged patient life expectancy. This makes it increasingly likely that orthopedic surgeons will encounter CF patients in their clinic. In this article, the authors discuss pertinent musculoskeletal manifestations of the CF population, including the increased risk of decreased bone mineral density and bone mineral content, muscle deconditioning, spinal kyphosis, fractures, and elevated systemic inflammation predisposing these individuals to CF-related arthralgia. The diagnoses are grouped into subspecialties (arthroplasty, pediatrics, spine, sports, and trauma) most likely to evaluate the patient. Additionally, the authors review treatment options for these conditions and discuss the need for these patients to be seen in the perioperative period by their CF care team for patient optimization due to their diminished pulmonary function. Interspersed with this literature review, the authors present 2 unique cases. The first case details a patient with pain over her spine due to multilevel spinous process bursitis caused by a high-frequency chest wall oscillation system, which masquerades as an infection. The second case is a non-contact midsubstance rectus femoris tear in an athlete. These cases highlight the need for increased vigilance for uncommon diagnoses in the CF patient population. [Orthopedics. 2021;44(3):e440-e445.].

Bone Cells Differentiation: How CFTR Mutations May Rule the Game of Stem Cells Commitment?

Cystic fibrosis (CF)-related bone disease has emerged as a significant comorbidity of CF and is characterized by decreased bone formation and increased bone resorption. Both osteoblast and osteoclast differentiations are impacted by cystic fibrosis transmembrane conductance regulator (CFTR) mutations. The defect of CFTR chloride channel or the loss of CFTRs ability to interact with other proteins affect several signaling pathways involved in stem cell differentiation and the commitment of these cells toward bone lineages. Specifically, TGF-, nuclear factor-kappa B (NF-B), PI3K/AKT, and MAPK/ERK signaling are disturbed by CFTR mutations, thus perturbing stem cell differentiation. High inflammation in patients changes myeloid lineage secretion, affecting both myeloid and mesenchymal differentiation. In osteoblast, Wnt signaling is impacted, resulting in consequences for both bone formation and resorption. Finally, CFTR could also have a direct role in osteoclasts resorptive function. In this review, we summarize the existing literature on the role of CFTR mutations on the commitment of induced pluripotent stem cells to bone cells.

The Effects of Ivacaftor on Bone Density and Microarchitecture in Children and Adults with Cystic Fibrosis

CONTEXT

Cystic fibrosis (CF) transmembrane conductance (CFTR) dysfunction may play a role in CF-related bone disease (CFBD). Ivacaftor is a CFTR potentiator effective in improving pulmonary and nutritional outcomes in patients with the G551D-CFTR mutation. The effects of ivacaftor on bone health are unknown.

OBJECTIVE

To determine the impact of ivacaftor on bone density and microarchitecture in children and adults with CF.

DESIGN

Prospective observational multiple cohort study.

SETTING

Outpatient clinical research center within a tertiary academic medical center.

PATIENTS OR OTHER PARTICIPANTS

Three cohorts of age-, race-, and gender-matched subjects were enrolled: 26 subjects (15 adults and 11 children) with CF and the G551D-CFTR mutation who were planning to start or had started treatment with ivacaftor within 3 months (Ivacaftor cohort), 26 subjects with CF were not treated with ivacaftor (CF Control cohort), and 26 healthy volunteers.

INTERVENTIONS

All treatments, including Ivacaftor, were managed by the subjects' pulmonologists.

MAIN OUTCOME MEASURES

Bone microarchitecture by high-resolution peripheral quantitative computed tomography (HR-pQCT), areal bone mineral density (aBMD) by dual-energy X-ray absorptiometry (DXA) and bone turnover markers at baseline, 1, and 2 years.

RESULTS

Cortical volume, area, and porosity at the radius and tibia increased significantly in adults in the Ivacaftor cohort. No significant differences were observed in changes in aBMD, trabecular microarchitecture, or estimated bone strength in adults or in any outcome measures in children.

CONCLUSIONS

Treatment with ivacaftor was associated with increases in cortical microarchitecture in adults with CF. Further studies are needed to understand the implications of these findings.

Cystic fibrosis bone disease treatment: Current knowledge and future directions

Bone disease is a frequent complication in adolescents and adults with cystic fibrosis (CF). Early detection and monitoring of bone mineral density and multidisciplinary preventive care are necessary from childhood through adolescence to minimize CF-related bone disease (CFBD) in adult CF patients. Approaches to optimizing bone health include ensuring adequate nutrition, particularly intake of calcium and vitamins D and K, addressing other secondary causes of low bone density such as hypogonadism, encouraging weight bearing exercise, and avoiding bone toxic medications. Of the currently available anti-resorptive or anabolic osteoporosis medications, only bisphosphonates have been studied in individuals with CF. Future studies are needed to better understand the optimal approach for managing CFBD.

CFTR deletion affects mouse osteoblasts in a gender-specific manner

Advancements in research and care have contributed to increase life expectancy of individuals with cystic fibrosis (CF). With increasing age comes a greater likelihood of developing CF bone disease, a comorbidity characterized by a low bone mass and impaired bone quality, which displays gender differences in severity. However, pathophysiological mechanisms underlying this gender difference have never been thoroughly investigated. We used bone marrow-derived osteoblasts and osteoclasts from Cftr^+/+ and Cftr^-/- mice to examine whether the impact of CF transmembrane conductance regulator (CFTR) deletion on cellular differentiation and functions differed between genders. To determine whether in vitro findings translated into in vivo observations, we used imaging techniques and three-point bending testing. In vitro studies revealed no osteoclast-autonomous defect but impairment of osteoblast differentiation and functions and aberrant responses to various stimuli in cells isolated from Cftr^-/- females only. Compared with wild-type controls, knockout mice exhibited a trabecular osteopenic phenotype that was more pronounced in Cftr^-/- males than Cftr^-/- females. Bone strength was reduced to a similar extent in knockout mice of both genders. In conclusion, we find a trabecular bone phenotype in Cftr^-/- mice that was slightly more pronounced in males than females, which is reminiscent of the situation found in patients. However, at the osteoblast level, the pathophysiological mechanisms underlying this phenotype differ between males and females, which may underlie gender differences in the way bone marrow-derived osteoblasts behave in absence of CFTR.

CFTR-deficient pigs display alterations of bone microarchitecture and composition at birth

BACKGROUND

The lack of cystic fibrosis transmembrane conductance regulator (CFTR) function causes cystic fibrosis (CF), predisposing to severe lung disease, reduced growth and osteopenia. Both reduced bone content and strength are increasingly recognized in infants with CF before the onset of significant lung disease, suggesting a developmental origin and a possible role in bone disease pathogenesis. The role of CFTR in bone metabolism is unclear and studies on humans are not feasible. Deletion of CFTR in pigs (CFTR ^-/- pigs) displays at birth severe malformations similar to humans in the intestine, respiratory tract, pancreas, liver, and male reproductive tract.

METHODS

We compared bone parameters of CFTR ^-/- male and female pigs with those of their wild-type (WT) littermates at birth. Morphological and microstructural properties of femoral cortical and trabecular bone were evaluated using micro-computed tomography (μCT), and their chemical compositions were examined using Raman microspectroscopy.

RESULTS

The integrity of the CFTR ^-/- bone was altered due to changes in its microstructure and chemical composition in both sexes. Low cortical thickness and high cortical porosity were found in CFTR ^-/- pigs compared to sex-matched WT littermates. Moreover, an increased chemical composition heterogeneity associated with higher carbonate/phosphate ratio and higher mineral crystallinity was found in CFTR ^-/- trabecular bone, but not in CFTR ^-/- cortical bone.

CONCLUSIONS

The loss of CFTR directly alters the bone composition and metabolism of newborn pigs. Based on these findings, we speculate that bone defects in patients with CF could be a primary, rather than a secondary consequence of inflammation and infection.

Evaluation of bone disease in patients with cystic fibrosis and end-stage lung disease

Objective:

Bone disease is a common comorbidity in patients with cystic fibrosis (CF). We sought to determine risk factors and identify potential biochemical markers for CF-related bone disease (CFBD) in a unique cohort of CF patients with end-stage lung disease undergoing lung transplantation (LTx) evaluation.

Methods:

All of the CF patients who were evaluated for LTx at our center between November of 1992 and December of 2010 were included in the study. Clinical data and biochemical markers of bone turnover, as well as bone mineral density (BMD) at the lumbar spine and femoral neck, were evaluated. Spearman’s rho and multivariate logistic regression analysis were used.

Results:

A total of 102 adult CF patients were evaluated. The mean age was 28.1 years (95% CI: 26.7-29.5), and the mean body mass index was 17.5 kg/m² (95% CI: 17.2-18.2). Mean T-scores were −2.3 and −1.9 at the lumbar spine and femoral neck, respectively, being lower in males than in females (−2.7 vs. −2.0 at the lumbar spine and −2.2 vs. −1.7 at the femoral neck). Overall, 52% had a T-score of < −2.5 at either skeletal site. The homozygous Phe508del genotype was found in 57% of patients without osteoporosis and in 60% of those with low BMD. Mean T-scores were not particularly low in patients with severe CFTR mutations. Although the BMI correlated with T-scores at the femoral neck and lumbar spine, serum 25-hydroxyvitamin D and parathyroid hormone levels did not.

Conclusions:

CFBD is common in CF patients with end-stage lung disease, particularly in males and patients with a low BMI. It appears that CF mutation status does not correlate with CFBD. In addition, it appears that low BMD does not correlate with other risk factors or biochemical parameters. The prevalence of CFBD appears to have recently decreased, most likely reflecting increased efforts at earlier diagnosis and treatment.

Reduced bone length, growth plate thickness, bone content, and IGF-I as a model for poor growth in the CFTR-deficient rat

Background

Reduced growth and osteopenia are common in individuals with cystic fibrosis (CF). Additionally, improved weight and height are associated with better lung function and overall health in the disease. Mechanisms for this reduction in growth are not understood. We utilized a new CFTR knockout rat to evaluate growth in young CF animals, via femur length, microarchitecture of bone and growth plate, as well as serum IGF-I concentrations.

Methods

Femur length was measured in wild-type (WT) and SD-CFTR^tm1sage (Cftr-/-) rats, as a surrogate marker for growth. Quantitative bone parameters in Cftr-/- and WT rats were measured by micro computed tomography (micro-CT). Bone histomorphometry and cartilaginous growth plates were analyzed. Serum IGF-I concentrations were also compared.

Results

Femur length was reduced in both Cftr-/- male and female rats compared to WT. Multiple parameters of bone microarchitecture (of both trabecular and cortical bone) were adversely affected in Cftr-/- rats. There was a reduction in overall growth plate thichkness in both male and female Cftr-/- rats, as well as hypertrophic zone thickness and mean hypertrophic cell volume in male rats, indicating abnormal growth characteristics at the plate. Serum IGF-I concentrations were severely reduced in Cftr-/- rats compared to WT littermates.

Conclusions

Despite absence of overt lung or pancreatic disease, reduced growth and bone content were readily detected in young Cftr-/- rats. Reduced size of the growth plate and decreased IGF-I concentrations suggest the mechanistic basis for this phenotype. These findings appear to be intrinsic to the CFTR deficient state and independent of significant clinical confounders, providing substantive evidence for the importance of CFTR on maintinaing normal bone growth.

Overexpression of RANKL in osteoblasts: a possible mechanism of susceptibility to bone disease in cystic fibrosis

Bone fragility and loss are a significant cause of morbidity in patients with cystic fibrosis (CF), and the lack of effective therapeutic options means that treatment is more often palliative rather than curative. A deeper understanding of the pathogenesis of CF-related bone disease (CFBD) is necessary to develop new therapies. Defective CF transmembrane conductance regulator (CFTR) protein and chronic inflammation in bone are important components of the CFBD development. The receptor activator of nuclear factor kappa-B ligand (RANKL) and osteoprotegerin (OPG) drive the regulation of bone turnover. To investigate their roles in CFBD, we evaluated the involvement of defective CFTR in their production level in CF primary human osteoblasts with and without inflammatory stimulation, in the presence or not of pharmacological correctors of the CFTR. No major difference in cell ultrastructure was noted between cultured CF and non-CF osteoblasts, but a delayed bone matrix mineralization was observed in CF osteoblasts. Strikingly, resting CF osteoblasts exhibited strong production of RANKL protein, which was highly localized at the cell membrane and was enhanced in TNF (TNF-α) or IL-17-stimulated conditions. Under TNF stimulation, a defective response in OPG production was observed in CF osteoblasts in contrast to the elevated OPG production of non-CF osteoblasts, leading to an elevated RANKL-to-OPG protein ratio in CF osteoblasts. Pharmacological inhibition of CFTR chloride channel conductance in non-CF osteoblasts replicated both the decreased OPG production and the enhanced RANKL-to-OPG ratio. Interestingly, using CFTR correctors such as C18, we significantly reduced the production of RANKL by CF osteoblasts, in both resting and TNF-stimulated conditions. In conclusion, the overexpression of RANKL and high membranous RANKL localization in osteoblasts are related to defective CFTR, and may worsen bone resorption, leading to bone loss in patients with CF. Targeting osteoblasts with CFTR correctors may represent an effective strategy to treat CFBD. Copyright © 2016 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

Trabecular and cortical bone deficits are present in children and adolescents with cystic fibrosis

Osteopenia and increased fracture rates are well-recognized in adults with CF, but neither the specific contributions of cortical and trabecular bone deficits to bone fragility nor their presence in youth with CF are well-characterized. This study sought to characterize cortical and trabecular volumetric bone mineral density (vBMD), geometry, and biomechanical competence in children with CF and determine their relationship to growth, body composition, and disease severity. Peripheral quantitative computerized tomography (pQCT) measures of total, cortical, and trabecular vBMD, cortical, muscle, and fat cross-sectional areas (CSA), periosteal and endosteal circumferences, and the polar unweighted section modulus (Zp) of the tibia were converted to age- and tibial length-adjusted Z-scores in 97 CF and 199 healthy children (aged 8-21y). Effects of body composition and pulmonary function (forced expiratory volume in 1s, FEV1) upon pQCT outcomes were determined using linear regression. Children with CF (FEV1%-predicted: 84.4+19.7) had lower weight-, height-, BMI-, and whole body lean mass (LBM)-Z and tibial length. Females with CF had lower (p<0.01) total and trabecular vBMD; cortical, muscle, and fat CSA; Zp and periosteal circumference than females in the healthy reference group. These bone differences persisted after adjustment for BMI-Z and to a great extent following adjustment for muscle CSA. Males with CF had lower (p<0.01) cortical, muscle, and fat CSA and their trabecular vBMD deficit approached significance (p=0.069). Deficits were attenuated by adjustment for BMI-Z and to a greater extent adjustment for muscle CSA-Z. The relationship between FEV1%-predicted and pQCT outcomes persisted only in males following adjustment for age and BMI-Z. The CF cohort had lower tibial muscle CSA than expected for their LBM. In this relatively healthy, young CF cohort, deficits in trabecular and multiple cortical bone parameters were present. In females, deficits were greater at older ages and were not fully explained by alterations in body composition. In males worsening pulmonary function was associated with greater deficits that was not explained by increasing age or compromised nutritional status. The occurrence of these differences in CF youth highlights the importance of instituting measures to optimize peak bone mass early in the course of CF.

Young adults with cystic fibrosis have altered trabecular microstructure by ITS-based morphological analysis

Young adults with cystic fibrosis have compromised plate-like trabecular microstructure, altered axial alignment of trabeculae, and reduced connectivity between trabeculae that may contribute to the reduced bone strength and increased fracture risk observed in this patient population.

INTRODUCTION

The risk of fracture is increased in patients with cystic fibrosis (CF). Individual trabecular segmentation (ITS)-based morphological analysis of high-resolution peripheral quantitative computed tomography (HR-pQCT) images segments trabecular bone into individual plates and rods of different alignment and connectivity, which are important determinants of trabecular bone strength. We sought to determine whether alterations in ITS variables are present in patients with CF and may help explain their increased fracture risk.

METHODS

Thirty patients with CF ages 18-40 years underwent DXA scans of the hip and spine and HR-pQCT scans of the radius and tibia with further assessment of trabecular microstructure by ITS. These CF patients were compared with 60 healthy controls matched for age (±2 years), race, and gender.

RESULTS

Plate volume fraction, thickness, and density as well as plate-plate and plate-rod connectivity were reduced, and axial alignment of trabeculae was lower in subjects with CF at both the radius and the tibia (p < 0.05 for all). At the radius, adjustment for BMI eliminated most of these differences. At the tibia, however, reductions in plate volume fraction and number, axially aligned trabeculae, and plate-plate connectivity remained significant after adjustment for BMI alone and for BMI and aBMD (p < 0.05 for all).

CONCLUSIONS

Young adults with CF have compromised plate-like and axially aligned trabecular morphology and reduced connectivity between trabeculae. ITS analysis provides unique information about bone integrity, and these trabecular deficits may help explain the increased fracture risk in adults with CF not accounted for by BMD and/or traditional bone microarchitecture measurements.

[Insights into cystic fibrosis-related bone disease]

With the increasing life expectancy of patients with cystic fibrosis (CF), prevalence of late complications such as CF-related bone disease (CFBD) has increased. It was initially described in 24% of the adult population with CF and has also been reported in the pediatric population. CFBD is multifactorial and progresses in different steps. Both decreased bone formation and increased bone resorption (in different amounts) are observed. CFBD is likely primitive (directly related to the CFTR defect itself), but is also worsened by acquired secondary factors such as lung infections, chronic inflammation, denutrition, vitamin deficiency, and decreased physical activity. CFBD may be clinically apparent (i.e., mainly vertebral and costal fractures), or clinically asymptomatic (therefore corresponding to abnormalities in bone density and architecture). CFBD management mainly aims to prevent the occurrence of fractures. Prevention and regular monitoring of bone disease as early as 8 years of age is of the utmost importance, as is the control of possible secondary deleterious CFBD factors. New radiological tools, such as high-resolution peripheral quantitative computed tomography, allow an accurate evaluation of cortical and trabecular bone micro-architecture in addition to compartmental density; as such, they will likely improve the assessment of the bone fracture threat in CF patients in the near future.

Bone disease in cystic fibrosis: new pathogenic insights opening novel therapies

Mutations within the gene encoding for the chloride ion channel cystic fibrosis transmembrane conductance regulator (CFTR) results in cystic fibrosis (CF), the most common lethal autosomal recessive genetic disease that causes a number of long-term health problems, as the bone disease. Osteoporosis and increased vertebral fracture risk associated with CF disease are becoming more important as the life expectancy of patients continues to improve. The etiology of low bone density is multifactorial, most probably a combination of inadequate peak bone mass during puberty and increased bone losses in adults. Body mass index, male sex, advanced pulmonary disease, malnutrition and chronic therapies are established additional risk factors for CF-related bone disease (CFBD). Consistently, recent evidence has confirmed that CFTR plays a major role in the osteoprotegerin (OPG) and COX-2 metabolite prostaglandin E2 (PGE2) production, two key regulators in the bone formation and regeneration. Several others mechanisms were also recognized from animal and cell models contributing to malfunctions of osteoblast (cell that form bone) and indirectly of bone-resorpting osteoclasts. Understanding such mechanisms is crucial for the development of therapies in CFBD. Innovative therapeutic approaches using CFTR modulators such as C18 have recently shown in vitro capacity to enhance PGE2 production and normalized the RANKL-to-OPG ratio in human osteoblasts bearing the mutation F508del-CFTR and therefore potential clinical utility in CFBD. This review focuses on the recently identified pathogenic mechanisms leading to CFBD and potential future therapies for treating CFBD.

Genetic deletion of keratin 8 corrects the altered bone formation and osteopenia in a mouse model of cystic fibrosis

Patients with cystic fibrosis (CF) display low bone mass and alterations in bone formation. Mice carrying the F508del genetic mutation in the cystic fibrosis conductance regulator (Cftr) gene display reduced bone formation and decreased bone mass. However, the underlying molecular mechanisms leading to these skeletal defects are unknown, which precludes the development of an efficient anti-osteoporotic therapeutic strategy. Here we report a key role for the intermediate filament protein keratin 8 (Krt8), in the osteoblast dysfunctions in F508del-Cftr mice. We found that murine and human osteoblasts express Cftr and Krt8 at low levels. Genetic studies showed that Krt8 deletion (Krt8(-/-)) in F508del-Cftr mice increased the levels of circulating markers of bone formation, corrected the expression of osteoblast phenotypic genes, promoted trabecular bone formation and improved bone mass and microarchitecture. Mechanistically, Krt8 deletion in F508del-Cftr mice corrected overactive NF-κB signaling and decreased Wnt-β-catenin signaling induced by the F508del-Cftr mutation in osteoblasts. In vitro, treatment with compound 407, which specifically disrupts the Krt8-F508del-Cftr interaction in epithelial cells, corrected the abnormal NF-κB and Wnt-β-catenin signaling and the altered phenotypic gene expression in F508del-Cftr osteoblasts. In vivo, short-term treatment with 407 corrected the altered Wnt-β-catenin signaling and bone formation in F508del-Cftr mice. Collectively, the results show that genetic or pharmacologic targeting of Krt8 leads to correction of osteoblast dysfunctions, altered bone formation and osteopenia in F508del-Cftr mice, providing a therapeutic strategy targeting the Krt8-F508del-CFTR interaction to correct the abnormal bone formation and bone loss in cystic fibrosis.

Trends in bone mineral density in young adults with cystic fibrosis over a 15 year period

BACKGROUND

Improvements in clinical care have led to increased life expectancy in patients with cystic fibrosis (CF) over the past several decades. Whether these improvements have had significant effects on bone health in patients with CF is unclear.

METHODS

This is a cross-sectional study comparing clinical characteristics and bone mineral density (BMD) measured by dual energy X-ray absorptiometry (DXA) in adults with CF evaluated in 1995-1999 to age-, race-, and gender-matched patients with CF evaluated in 2011-2013 at the same center on calibrated DXA machines.

RESULTS

The cohorts were similar in terms of age, BMI, pancreatic insufficiency, presence of F508del mutation, and reproductive history. In the most recent cohort, pulmonary function was superior, and fewer patients had vitamin D deficiency or secondary hyperparathyroidism. Areal BMD measures of the PA spine, lateral spine, and distal radius were similarly low in the two cohorts.

CONCLUSIONS

Although pulmonary function and vitamin D status were better in patients in the present-day cohort, areal BMD of the spine was reduced in a significant number of patients and was no different in patients with CF today than in the late 1990s. Further attention to optimizing bone health may be necessary to prevent CF-related bone disease.

Increased NF-κB Activity and Decreased Wnt/β-Catenin Signaling Mediate Reduced Osteoblast Differentiation and Function in ΔF508 Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) Mice

The prevalent human ΔF508 mutation in the cystic fibrosis transmembrane conductance regulator (CFTR) is associated with reduced bone formation and bone loss in mice. The molecular mechanisms by which the ΔF508-CFTR mutation causes alterations in bone formation are poorly known. In this study, we analyzed the osteoblast phenotype in ΔF508-CFTR mice and characterized the signaling mechanisms underlying this phenotype. Ex vivo studies showed that the ΔF508-CFTR mutation negatively impacted the differentiation of bone marrow stromal cells into osteoblasts and the activity of osteoblasts, demonstrating that the ΔF508-CFTR mutation alters both osteoblast differentiation and function. Treatment with a CFTR corrector rescued the abnormal collagen gene expression in ΔF508-CFTR osteoblasts. Mechanistic analysis revealed that NF-κB signaling and transcriptional activity were increased in mutant osteoblasts. Functional studies showed that the activation of NF-κB transcriptional activity in mutant osteoblasts resulted in increased β-catenin phosphorylation, reduced osteoblast β-catenin expression, and altered expression of Wnt/β-catenin target genes. Pharmacological inhibition of NF-κB activity or activation of canonical Wnt signaling rescued Wnt target gene expression and corrected osteoblast differentiation and function in bone marrow stromal cells and osteoblasts from ΔF508-CFTR mice. Overall, the results show that the ΔF508-CFTR mutation impairs osteoblast differentiation and function as a result of overactive NF-κB and reduced Wnt/β-catenin signaling. Moreover, the data indicate that pharmacological inhibition of NF-κB or activation of Wnt/β-catenin signaling can rescue the abnormal osteoblast differentiation and function induced by the prevalent ΔF508-CFTR mutation, suggesting novel therapeutic strategies to correct the osteoblast dysfunctions in cystic fibrosis.

Compromised bone microarchitecture and estimated bone strength in young adults with cystic fibrosis

CONTEXT

Young adults with cystic fibrosis (CF) are at risk for low bone density and fractures, but the underlying alterations in bone microarchitecture that may contribute to their increased fracture risk are currently unknown.

OBJECTIVE

The main goal of this study was to use high-resolution peripheral quantitative computed tomography (HR-pQCT) to characterize the bone microarchitecture, volumetric bone mineral density (vBMD), and estimated strength of the radius and tibia in young adults with CF compared with healthy volunteers.

DESIGN AND SETTING

This was a cross-sectional study at an outpatient clinical research center within a tertiary academic medical center.

PARTICIPANTS

Thirty young adults with CF, 18 to 40 years of age, were evaluated and compared with 60 healthy volunteers matched by age (±2 years), gender, and race.

MAIN OUTCOME MEASURES

The primary outcomes were HR-pQCT-derived cortical and trabecular vBMD, bone microarchitecture, and estimates of bone strength.

RESULTS

At the radius and tibia, young adults with CF had smaller bone cross-sectional area and lower vBMD. Cortical and trabecular microarchitecture were compromised at both sites, most notably involving the trabecular bone of the tibia. These differences translated into lower estimated bone strength both at the radius and tibia. After accounting for body mass index differences, young adults with CF had lower bone area and estimated bone strength at the radius and had compromised trabecular microarchitecture and lower total and trabecular vBMD and estimated bone strength at the tibia. Alterations in trabecular bone density and microarchitecture and estimated strength measures of the tibia were also greater than expected based on dual-energy x-ray absorptiometry-derived areal BMD differences.

CONCLUSIONS

Young adults with CF have compromised bone microarchitecture and lower estimated bone strength at both the radius and tibia, even after accounting for their smaller body size. These skeletal deficits likely explain the higher fracture risk observed in young adults with CF.

Bisphosphonates for osteoporosis in people with cystic fibrosis

BACKGROUND

Osteoporosis is a bone mineralisation disorder occurring in about one third of adults with cystic fibrosis. Bisphosphonates can increase bone mineral density and decrease the risk of new fractures in post-menopausal women and people receiving long-term oral corticosteroids.

OBJECTIVES

To assess the effects of bisphosphonates on the frequency of fractures, bone mineral density, quality of life, adverse events, trial withdrawals, and survival in people with cystic fibrosis.

SEARCH METHODS

We searched the Cystic Fibrosis and Genetic Disorders Group Trials Register of references (identified from electronic database searches and handsearches of journals and abstract books) on 13 January 2014.Additional searches of PubMed were performed on 13 January 2014.

SELECTION CRITERIA

Randomised controlled trials of at least six months duration studying bisphosphonates in people with cystic fibrosis.

DATA COLLECTION AND ANALYSIS

Two authors independently selected trials and extracted data. Trial investigators were contacted to obtain missing data.

MAIN RESULTS

Nine trials were identified and seven (with a total of 237 adult participants) were included.Data were combined (when available) from six included studies in participants without a lung transplant. Data showed that there was no significant reduction in fractures between treatment and control groups at 12 months, odds ratio 0.72 (95% confidence interval 0.13 to 3.80). No fractures were reported in studies with follow-up at 24 months. However, in patients taking bisphosphonates after six months the percentage change in bone mineral density increased at the lumbar spine, mean difference 4.61 (95% confidence interval 3.90 to 5.32) and at the hip or femur, mean difference 3.35 (95% confidence interval 1.63 to 5.07); but did not significantly change at the distal forearm, mean difference -0.49 (95% confidence interval -2.42 to 1.45). In patients taking bisphosphonates, at 12 months the percentage change in bone mineral density increased at the lumbar spine, mean difference 6.10 (95% confidence interval 5.10 to 7.10) and at the hip or femur, mean difference 4.35 (95% confidence interval 2.99 to 5.70). At 24 months, in patients treated with bisphosphonates the percentage change in bone mineral density also increased at the lumbar spine, mean difference 5.49 (95% confidence interval 4.38 to 6.60) and at the hip or femur, mean difference 6.05 (95% confidence interval 3.74 to 8.36). There was clinical heterogeneity between studies and not all studies reported all outcomes. Bone pain was the most common adverse event with intravenous agents. Flu-like symptoms were also increased in those taking bisphosphonates.In participants with a lung transplant (one study), intravenous pamidronate did not change the number of new fractures. At axial sites, bone mineral density increased with treatment compared to controls: percentage change in bone mineral density at lumbar spine, mean difference 6.20 (95% confidence interval 4.28 to 8.12); and femur mean difference 7.90 (95% confidence interval 5.78 to 10.02).

AUTHORS' CONCLUSIONS

Oral and intravenous bisphosphonates increase bone mineral density in people with cystic fibrosis. Severe bone pain and flu-like symptoms may occur with intravenous agents. Additional trials are needed to determine if bone pain is more common or severe (or both) with the more potent zoledronate and if corticosteroids ameliorate or prevent these adverse events. Additional trials are also required to further assess gastrointestinal adverse effects associated with oral bisphosphonates. Trials in larger populations are needed to determine effects on fracture rate and survival.

Cystic fibrosis-related bone disease: insights into a growing problem

PURPOSE OF REVIEW

This review will describe the clinical significance, pathogenesis and treatment of cystic fibrosis related bone disease (CFBD).

RECENT FINDINGS

CFBD continues to increase as the life expectancy of individuals with cystic fibrosis increases. According to clinical guidelines, individuals with cystic fibrosis should be initially screened at the age of 18 years via dual-energy x-ray absorptiometry, if not done so previously. The underlying pathogenesis of CFBD appears to be multifactorial, but increasing data imply a direct impact by the cystic fibrosis transmembrane conductance regulator (CFTR). CFTR deficiency and/or dysfunction impair osteoblast activity and differentiation, and indirectly promote osteoclast formation. Unfortunately, once diagnosed with CFBD, few cystic fibrosis tested medical therapies exist.

SUMMARY

CFBD is an increasingly recognized complication that has a significant impact on the overall health of the individual. Recommendations to identify patients with cystic fibrosis who are at risk for fracture using dual-energy x-ray absorptiometry have been established. Therapeutic agents directly studied in patients with cystic fibrosis are limited to bisphosphonates, although other potential treatment agents exist. Finally, an improved understanding of the pathologic mechanisms will aid in the study and development of therapies.

Osteoblast CFTR inactivation reduces differentiation and osteoprotegerin expression in a mouse model of cystic fibrosis-related bone disease

Low bone mass and increased fracture risk are recognized complications of cystic fibrosis (CF). CF-related bone disease (CFBD) is characterized by uncoupled bone turnover—impaired osteoblastic bone formation and enhanced osteoclastic bone resorption. Intestinal malabsorption, vitamin D deficiency and inflammatory cytokines contribute to CFBD. However, epidemiological investigations and animal models also support a direct causal link between inactivation of skeletal cystic fibrosis transmembrane regulator (CFTR), the gene that when mutated causes CF, and CFBD. The objective of this study was to examine the direct actions of CFTR on bone. Expression analyses revealed that CFTR mRNA and protein were expressed in murine osteoblasts, but not in osteoclasts. Functional studies were then performed to investigate the direct actions of CFTR on osteoblasts using a CFTR knockout (Cftr−/−) mouse model. In the murine calvarial organ culture assay, Cftr−/− calvariae displayed significantly less bone formation and osteoblast numbers than calvariae harvested from wildtype (Cftr+/+) littermates. CFTR inactivation also reduced alkaline phosphatase expression in cultured murine calvarial osteoblasts. Although CFTR was not expressed in murine osteoclasts, significantly more osteoclasts formed in Cftr−/− compared to Cftr+/+ bone marrow cultures. Indirect regulation of osteoclastogenesis by the osteoblast through RANK/RANKL/OPG signaling was next examined. Although no difference in receptor activator of NF-κB ligand (Rankl) mRNA was detected, significantly less osteoprotegerin (Opg) was expressed in Cftr−/− compared to Cftr+/+ osteoblasts. Together, the Rankl:Opg ratio was significantly higher in Cftr−/− murine calvarial osteoblasts contributing to a higher osteoclastogenesis potential. The combined findings of reduced osteoblast differentiation and lower Opg expression suggested a possible defect in canonical Wnt signaling. In fact, Wnt3a and PTH-stimulated canonical Wnt signaling was defective in Cftr−/− murine calvarial osteoblasts. These results support that genetic inactivation of CFTR in osteoblasts contributes to low bone mass and that targeting osteoblasts may represent an effective strategy to treat CFBD.

Bisphosphonates for osteoporosis in people with cystic fibrosis

BACKGROUND

Osteoporosis is a bone mineralisation disorder occurring in about one third of adults with cystic fibrosis. Bisphosphonates can increase bone mineral density and decrease the risk of new fractures in post-menopausal women and people receiving long-term oral corticosteroids.

OBJECTIVES

To assess the effects of bisphosphonates on the frequency of fractures, bone mineral density, quality of life, adverse events, trial withdrawals, and survival in people with cystic fibrosis.

SEARCH METHODS

We searched the Cystic Fibrosis and Genetic Disorders Group Trials Register of references (identified from electronic database searches and handsearches of journals and abstract books) on 15 February 2012.Additional searches of PubMed were performed on 14 May 2011.

SELECTION CRITERIA

Randomised controlled trials of at least six months duration studying bisphosphonates in people with cystic fibrosis.

DATA COLLECTION AND ANALYSIS

Two authors independently selected trials and extracted data. Trial investigators were contacted to obtain missing data.

MAIN RESULTS

Nine trials were identified and seven (with a total of 237 adult participants) were included.Data were combined (when available) from six included studies in participants without a lung transplant. Data showed that there was no significant reduction in fractures between treatment and control groups at 12 months, odds ratio 0.72 (95% confidence interval 0.13 to 3.80). No fractures were reported in studies with follow-up at 24 months. However, in patients taking bisphosphonates after six months the percentage change in bone mineral density increased at the lumbar spine, mean difference 4.61 (95% confidence interval 3.90 to 5.32) and at the hip or femur, mean difference 3.35 (95% confidence interval 1.63 to 5.07); but did not significantly change at the distal forearm, mean difference -0.49 (95% confidence interval -2.42 to 1.45). In patients taking bisphosphonates, at 12 months the percentage change in bone mineral density increased at the lumbar spine, mean difference 6.10 (95% confidence interval 5.10 to 7.10) and at the hip or femur, mean difference 4.35 (95% confidence interval 2.99 to 5.70). At 24 months, in patients treated with bisphosphonates the percentage change in bone mineral density also increased at the lumbar spine, mean difference 5.49 (95% confidence interval 4.38 to 6.60) and at the hip or femur, mean difference 6.05 (95% confidence interval 3.74 to 8.36). There was clinical heterogeneity between studies and not all studies reported all outcomes. Bone pain was the most common adverse event with intravenous agents. Flu-like symptoms were also increased in those taking bisphosphonates.In participants with a lung transplant (one study), intravenous pamidronate did not change the number of new fractures. At axial sites, bone mineral density increased with treatment compared to controls: percentage change in bone mineral density at lumbar spine, mean difference 6.20 (95% confidence interval 4.28 to 8.12); and femur mean difference 7.90 (95% confidence interval 5.78 to 10.02).

AUTHORS' CONCLUSIONS

Oral and intravenous bisphosphonates increase bone mineral density in people with cystic fibrosis. Severe bone pain and flu-like symptoms may occur with intravenous agents. Additional trials are needed to determine if bone pain is more common or severe (or both) with the more potent zoledronate and if corticosteroids ameliorate or prevent these adverse events. Additional trials are also required to further assess gastrointestinal adverse effects associated with oral bisphosphonates. Trials in larger populations are needed to determine effects on fracture rate and survival.

Cystic fibrosis growth retardation is not correlated with loss of Cftr in the intestinal epithelium

Maldigestion due to exocrine pancreatic insufficiency leads to intestinal malabsorption and consequent malnutrition, a mechanism proposed to cause growth retardation associated with cystic fibrosis (CF). However, although enzyme replacement therapy combined with increased caloric intake improves weight gain, the effect on stature is not significant, suggesting that growth retardation has a more complex etiology. Mouse models of CF support this, since these animals do not experience exocrine pancreatic insufficiency yet are growth impaired. Cftr absence from the intestinal epithelium has been suggested as a primary source of growth retardation in CF mice, a concept we directly tested by generating mouse models with Cftr selectively inactivated or restored in intestinal epithelium. The relationship between growth and functional characteristics of the intestines, including transepithelial electrophysiology, incidence of intestinal obstruction, and histopathology, were assessed. Absence of Cftr exclusively from intestinal epithelium resulted in loss of cAMP-stimulated short-circuit current, goblet cell hyperplasia, and occurrence of intestinal obstructions but only slight and transient impaired growth. In contrast, specifically restoring Cftr to the intestinal epithelium resulted in restoration of ion transport and completely protected against obstruction and histopathological anomalies, but growth was indistinguishable from CF mice. These results indicate that absence of Cftr in the intestinal epithelium is an important contributor to the intestinal obstruction phenotype in CF but does not correlate with the observed growth reduction in CF.

CFTR induces extracellular acid sensing in Xenopus oocytes which activates endogenous Ca²⁺-activated Cl⁻ conductance

The cystic fibrosis transmembrane conductance regulator (CFTR) produces a cyclic adenosine monophosphate (cAMP)-dependent Cl⁻ conductance of distinct properties that is essential for electrolyte secretion in human epithelial tissues. However, the functional consequences of CFTR expression are multifaceted, encompassing much more than simply supplying a cellular cAMP-regulated Cl⁻ conductance. When we expressed CFTR in Xenopus oocytes, we found that extracellular acidic pH activates a Ca²⁺-dependent outwardly rectifying Cl⁻ conductance that does not reflect CFTR activity. The proton-activated Cl⁻ conductance showed biophysical and pharmacological features of a Ca²⁺-dependent Cl⁻ conductance, most likely mediated by Xenopus TMEM16A. In contrast to the effects of extracellular acidification, intracellular acidification did not activate an endogenous Cl⁻ conductance. Proton/CFTR-mediated activation of human TMEM16A was also detected in HEK293 cells. The gating mutant G551D-CFTR conferred proton sensitivity, while deltaF508-CFTR enabled proton activation of TMEM16A only in Xenopus oocytes, which, unlike HEK293 cells, allow deltaF508-CFTR to be trafficked to the cell membrane. Activation of TMEM16A by lysophosphatidic acid was enhanced in the presence of CFTR but was additive with activation by extracellular protons. Because expression of CFTR-E1474X did not confer proton sensitivity, we propose that CFTR translocates a proton receptor to the plasma membrane via its PDZ-binding domain.

Pigs and humans with cystic fibrosis have reduced insulin-like growth factor 1 (IGF1) levels at birth

People with cystic fibrosis (CF) exhibit growth defects. That observation has been attributed, in part, to decreased insulin-like growth factor 1 (IGF1) levels, and the reduction has been blamed on malnutrition and pulmonary inflammation. However, patients with CF already have a reduced weight at birth, a manifestation not likely secondary to poor nutrition or inflammation. We found that, like humans, CF pigs were smaller than non-CF littermates and had lower IGF1 levels. To better understand the basis of IGF1 reduction, we studied newborn pigs and found low IGF1 levels within 12 h of birth. Moreover, humerus length and bone mineral content were decreased, consistent with less IGF1 activity in utero. These findings led us to test newborn humans with CF, and we found that they also had reduced IGF1 levels. Discovering lower IGF1 levels in newborn pigs and humans indicates that the decrease is not solely a consequence of malnutrition or pulmonary inflammation and that loss of cystic fibrosis transmembrane conductance regulator function has a more direct effect. Consistent with this hypothesis, we discovered reduced growth hormone release in organotypic pituitary slice cultures of newborn CF pigs. These findings may explain the long-standing observation that CF newborns are smaller than non-CF babies and why some patients with good clinical status fail to reach their growth potential. The results also suggest that measuring IGF1 levels might be of value as a biomarker to predict disease severity or the response to therapeutics. Finally, they raise the possibility that IGF1 supplementation beginning in infancy might be beneficial in CF.

The cystic fibrosis transmembrane conductance regulator (CFTR) is expressed in maturation stage ameloblasts, odontoblasts and bone cells

Patients with cystic fibrosis (CF) have mild defects in dental enamel. The gene mutated in these patients is CFTR, a Cl(-) channel involved in transepithelial salt and water transport and bicarbonate secretion. We tested the hypothesis that Cftr channels are present and operating in the plasma membranes of mouse ameloblasts. Tissue sections of young mouse jaws and fetal human jaws were immunostained with various anti-Cftr antibodies. Specificity of the antibodies was validated in Cftr-deficient murine and human tissues. Immunostaining for Cftr was obtained in the apical plasma membranes of mouse maturation ameloblasts of both incisor and molar tooth germs. A granular intracellular immunostaining of variable intensity was also noted in bone cells and odontoblasts. In Cftr-deficient mice the incisors were chalky white and eroded much faster than in wild type mice. Histologically, only maturation ameloblasts of incisors were structurally affected in Cftr-deficient mice. Some antibody species gave also a positive cytosolic staining in Cftr-deficient cells. Transcripts of Cftr were found in maturation ameloblasts, odontoblasts and bone cells. Similar data were obtained in forming human dentin and bone. We conclude that Cftr protein locates in the apical plasma membranes of mouse maturation ameloblasts. In mouse incisors Cftr is critical for completion of enamel mineralization and conceivably functions as a regulator of pH during rapid crystal growth. Osteopenia found in CF patients as well as in Cftr-deficient mice is likely associated with defective Cftr operating in bone cells.

Bone mineral and body composition alterations in paediatric cystic fibrosis patients

BACKGROUND

With the increased life span of cystic fibrosis (CF) patients, CF-related bone diseases could have an increased prevalence and morbidity in this group. In children, previous retrospective and prospective studies have yielded conflicting results on bone mineralization.

OBJECTIVE

To monitor body composition and bone mineral status of children with CF.

MATERIALS AND METHODS

We reviewed the dual-energy X-ray absorptiometry (DXA) data of 161 children with CF (age 10 +/- 4.8 years). Total body bone mineral content (BMCt), total lean tissue mass (LTMt) and total fat mass (FMt) were measured and compared to expected data calculated from ideal weight for height (Wi; e.g. BMCti, LTMti, FMti). The bt (BMCt/BMCti), lt (LTMt/LTMti) and ft (FMt/FMti) ratios were used as quantitative variables.

RESULTS

Low bt ratio was found at all ages (mean bt ratio 0.94 +/- 0.10; P < 0.001), even in children <6 years of age. However, the children's BMCt was satisfactorily adapted to their weight. lt and ft ratios were not constant across age groups. Children <10 years had 8% reduction of their lt ratio, maintaining normal levels thereafter. The opposite trend was found for ft ratio. Poor clinical, nutritional status and vitamin A levels were correlated with bt and lt ratios.

CONCLUSION

Our results indicate that children with CF could have early alterations in their bone status and that lt and ft ratios did not have constant values across ages. Interpreting DXA data using this approach is suitable in children with CF.

Cystic fibrosis transmembrane conductance regulator (CFTR) regulates the production of osteoprotegerin (OPG) and prostaglandin (PG) E2 in human bone

Bone loss is an important clinical issue in patients with cystic fibrosis (CF). Whether the cystic fibrosis transmembrane conductance regulator (CFTR) plays a direct role in bone cell function is yet unknown. In this study, we provide evidence that inhibition of CFTR-Cl(-) channel function results in a significant decrease of osteoprotegerin (OPG) secretion accompanied with a concomitant increase of prostaglandin (PG) E(2) secretion of primary human osteoblast cultures (n=5). Our data therefore suggest that in bone cells of CF patients, the loss of CFTR activity may result in an increased inflammation-driven bone resorption (through both the reduced OPG and increased PGE(2) production), and thus might contribute to the early bone loss reported in young children with CF.

Functional expression of cystic fibrosis transmembrane conductance regulator in mouse chondrocytes

1. Cystic fibrosis transmembrane conductance regulator (CFTR) is well known for its role in the cystic fibrosis (CF). Recent studies have shown that CF patients and CFTR-deficient mice exhibit a severe abnormal skeletal phenotype, indicating that CFTR may play a role in bone development and pathophysiological processes. However, it is not known whether CFTR has a direct or indirect effect on bone formation. The aim of the present study was to detect the expression and function of CFTR in mouse chondrocytes. 2. Reverse transcription-polymerase chain reaction, western blotting and immunofluorescence were used to characterize the expression of CFTR in primary isolated mouse chondrocytes. Expression of CFTR mRNA and protein was detectable in mouse chondrocytes. Importantly, whole-cell patch-clamp analysis demonstrated that CFTR in mouse chondrocytes is functional as a cAMP-dependent Cl(-) channel that is inhibited by CFTRinh-172. 3. Thus, the results of the present study demonstrate that functional CFTR is expressed in mouse chondrocytes, which offers essential evidence for the potential direct role of CFTR in physiological and pathological processes of bone.

Bisphosphonates for osteoporosis in people with cystic fibrosis

BACKGROUND

Osteoporosis is a bone mineralisation disorder occurring in about one third of adults with cystic fibrosis (CF). Bisphosphonates can increase bone mineral density (BMD) and decrease the risk of new fractures in post-menopausal women and people receiving long-term oral corticosteroids.

OBJECTIVES

To assess the effects of bisphosphonates on the frequency of fractures, BMD, quality of life, adverse events, trial withdrawals, and survival in people with CF.

SEARCH STRATEGY

We searched the CF and Genetic Disorders Group Trials Register of references (identified from electronic database searches and handsearches of journals and abstract books) on 29 October 2008.Additional searches of Pubmed were performed on 01 November 2008.

SELECTION CRITERIA

Randomised controlled trials of at least six months duration studying bisphosphonates in people with CF.

DATA COLLECTION AND ANALYSIS

Two authors independently selected trials and extracted data. Trial investigators were contacted to obtain missing data.

MAIN RESULTS

Seven trials were identified and five (with a total of 145 adult participants) were included.Data were combined when available from four included studies in participants without a lung transplant. This showed that there was no significant reduction in fractures between groups. However, after six months, the percentage change in BMD increased in those on bisphosphonates at the lumbar spine, mean difference (MD) 4.61 (95% confidence interval (CI) 3.90 to 5.32) and at the hip, MD 3.35 (95% CI 1.63 to 5.07); but did not significantly change at the distal forearm, MD -0.49 (95% CI -2.42-1.45). There was clinical heterogeneity between studies and not all studies reported all outcomes. Bone pain was the most common adverse event with intravenous agents. Flu-like symptoms were also increased in those taking bisphosphonates.In participants with a lung transplant (one study), intravenous pamidronate did not change the number of new fractures. At axial sites, BMD increased with treatment compared to controls: percentage change in bone mineral density at lumbar spine, MD 6.20 (95% CI 4.28 to 8.12) and femur MD 7.90 (95% CI 5.78 to 10.02).

AUTHORS' CONCLUSIONS

Oral and intravenous bisphosphonates increase BMD in people with CF. Severe bone pain and flu-like symptoms may occur with intravenous agents. Additional trials are needed to determine if bone pain is more common or severe (or both) with the more potent zoledronate and if corticosteroids ameliorate or prevent these adverse events. Trials in larger populations are needed to determine effects on fracture rate and survival.

Update on cystic fibrosis-related bone disease: a special focus on children

A high prevalence of low bone mineralization is documented in adult patients with cystic fibrosis (CF). Osteopenia is present in up to 85% of adult patients and osteoporosis in 10% to 34%. In children, study results are discordant probably because of comparisons to different control populations and corrections for bone size in growing children. Malnutrition, inflammation, vitamin D and vitamin K deficiency, altered sex hormone production, glucocorticoid therapy, and physical inactivity are well known risk factors for poor bone health. Puberty is a critical period for bone mineralization and requires a careful follow-up to achieve optimal bone peak mass. Strategies for optimizing bone health, such as monitoring bone mineral density (BMD) and providing preventive care are necessary from childhood through adolescence to minimize CF-related bone disease in adult CF patients.

Relation of bone mineral density with clinical and laboratory parameters in pre-pubertal children with cystic fibrosis

To study bone mineral density (BMD) of pre-pubertal cystic fibrosis (CF) children, and its relation with clinical and laboratory parameters, we enrolled 16 CF (8 girls) (4-8 years), and 16 control children (8 girls) (4-8 years). After anthropometric measurements, BMD, serum calcium, phosphorus, total alkaline phosphatase (ALP), 25-hydroxy vitamin D (25-OHD), parathyroid hormone, osteocalcin, tumor necrosis factor (TNF)-alpha, soluble TNF-alpha receptor 2 (sTNFR2), and soluble IL-2 receptor (sIL-2R) levels, and urinary calcium and hydroxyproline excretions were assessed. Disease severity of CF patients was determined with Shwachman-Kulczycki clinical and Brasfield radiological scoring systems.The mean Shwachman-Kulczycki and Brasfield scores of CF patients were indicating well-controlled disease. The anthropometric measurements, mean BMD values, and serum calcium, phosphorus and parathyroid hormone levels were within normal range and similar in both groups. Serum osteocalcin levels were lower, and ALP and 25-OHD levels were higher in CF. Although 24-hr urinary calcium excretions was higher in CF patients, hydroxyproline excretions were similar in both groups. There was no difference between two groups for the serum levels of sIL-2R, TNF-alpha and sTNFR2. Children with low vertebral z-scores had higher serum sIL-2R levels in both groups, but the same relation could not be shown for TNF-alpha and sTNFR2.We may speculate that younger, healthier and well-nourished patients with CF may have normal BMD, but the bone disease develop as patients get older because of the other contributing factors. Future well-designed longitudinal studies with large cohorts might show a relation with BMD and cytokines in CF.

Murine model for cystic fibrosis bone disease demonstrates osteopenia and sex-related differences in bone formation

As the incidence of cystic fibrosis (CF) bone disease is increasing, we analyzed CF transmembrane conductance regulator (CFTR) deficient mice (CF mice) to gain pathogenic insights. In these studies comparing adult (14 wk) CF and C57BL/6J mice, both bone length and total area were decreased in CF mice. Metaphyseal trabecular and cortical density were also decreased, as well as diaphyseal cortical and total density. Trabecular bone volume was diminished in CF mice. Female CF mice revealed decreased trabecular width and number compared with C57BL/6J, whereas males demonstrated no difference in trabecular number. Female CF mice had reduced mineralizing surface and bone formation rates. Conversely, male CF mice had increased mineralizing surface, mineral apposition, and bone formation rates compared with C57BL/6J males. Bone formation rate was greater in males compared with female CF mice. Smaller bones with decreased density in CF, despite absent differences in osteoblast and osteoclast surfaces, suggest CF transmembrane conductance regulator influences bone cell activity rather than number. Differences in bone formation rate in CF mice are suggestive of inadequate bone formation in females but increased bone formation in males. This proanabolic observation in male CF mice is consistent with other clinical sex differences in CF.

TNAP, TrAP, ecto-purinergic signaling, and bone remodeling

Bone remodeling is a process of continuous resorption and formation/mineralization carried out by osteoclasts and osteoblasts, which, along with osteocytes, comprise the bone multicellular unit (BMU). A key component of the BMU is the bone remodeling compartment (BRC), isolated from the marrow by a canopy of osteoblast-like lining cells. Although much progress has been made regarding the cytokine-dependent and hormonal regulation of bone remodeling, less attention has been placed on the role of extracellular pH (pH(e)). Osteoclastic bone resorption occurs at acidic pH(e). Furthermore, osteoclasts can be regarded as epithelial-like cells, due to their polarized structure and ability to form a seal against bone, isolating the lacunar space. The major ecto-phosphatases of osteoclasts and osteoblasts, acid and alkaline phosphatases, both have ATPase activity with pH optima several units different from neutrality. Furthermore, osteoclasts and osteoblasts express plasma membrane purinergic P2 receptors that, upon activation by ATP, accelerate bone osteoclast resorption and impair osteoblast mineralization. We hypothesize that these ecto-phosphatases help regulate [ATP](e) and localized pH(e) at the sites of bone resorption and mineralization by pH-dependent ATP hydrolysis coupled with P2Y-dependent regulation of osteoclast and osteoblast function. Furthermore, osteoclast cellular HCO3(-), formed as a product of lacunar V-ATPase H(+) secretion, is secreted into the BRC, which could elevate BRC pH(e), in turn affecting osteoblast function. We will review the existing data addressing regulation of BRC pH(e), present a hypothesis regarding its regulation, and discuss the hypothesis in the context of the function of proteins that regulate pH(e).

Chronic pediatric inflammatory diseases: effects on bone

In children, chronic inflammatory diseases present a significant challenge to long-term skeletal health. These conditions are often associated with poor appetite and suboptimal overall nutrition, altered nutrient utilization, delayed puberty, inactivity, and reduced muscle mass, all of which can alter bone metabolism. In addition, bone cell activity is susceptible to the effects of the immune response that characterizes these diseases. Moreover, drugs used to treat these maladies, notably glucocorticoids, may have negative effects on bone formation and on linear growth in developing children. As a result, predicted peak bone mass may not be achieved, and fracture risk may be increased in the short term or in the future. Studies using primarily dual energy X-ray absorptiometry have documented that deficits in bone mass are common in these diseases. However, there are wide variations in the prevalence of low bone mass, largely due to differences in the characteristics of each study population. Recent studies provide insight into the pathogenesis of decreased bone mass in these conditions. In this paper we will provide an overview of the effects of chronic inflammatory conditions on bone mass in children. We will also present relevant data from adult patients, when pediatric data are scant or not available.