Defective Bone Mineralization Research Articles

Prenatal diagnosis of familial lethal hypophosphatasia using imaging, blood enzyme levels, chorionic villus sampling and archived fetal tissue

Hypophosphatasia is an inheritable disorder characterized by defective bone mineralization and a deficiency of tissue-nonspecific alkaline phosphatase (TNSALP) activity. Screening for mutations in the TNSALP gene allows genetic counseling and prenatal diagnosis of the disease in families with severe forms of hypophosphatasia. A 33-year-old, gravida 4, para 3 Japanese woman was referred to Nagoya City University Hospital for prenatal genetic counseling because of two previous occurrences of fetal bone anomalies. The molecular examination showed that the fetus was homozygous for the TNSALP gene mutation c.1559delT, each parent being heterozygous. Genetic counseling was offered and at the next pregnancy, chorionic villus sampling was performed, whereupon genetic analysis confirmed that the fetus did not carry the familial mutation c.1559delT. Postnatal molecular genetic analysis using the cord tissue can provide a diagnosis of lethal hypophosphatasia and prenatal genetic diagnosis of the TNSALP gene allows time for parental counseling and delivery planning.

Bone proteins PHEX and DMP1 regulate fibroblastic growth factor Fgf23 expression in osteocytes through a common pathway involving FGF receptor (FGFR) signaling.

Fibroblastic growth factor 23 (FGF23) is a circulating phosphaturic hormone. Inactivating mutations of the endopeptidase PHEX or the SIBLING protein DMP1 result in equivalent intrinsic bone mineralization defects and increased Fgf23 expression in osteocytes. The mechanisms whereby PHEX and DMP1 regulate Fgf23 expression are unknown. We examined the possibility that PHEX and DMP1 regulate Fgf23 through a common pathway by analyzing the phenotype of compound Phex and Dmp1 mutant mice (Hyp/Dmp1(-/-)). Compared to single-mutant littermates, compound-mutant Hyp/Dmp1(-/-) mice displayed nonadditive elevations of serum FGF23 (1912 ± 183, 1715 ± 178, and 1799 ± 181 pg/ml), hypophosphatemia (P(i): 6.0 ± 0.3, 5.8 ± 0.2, and 5.4 ± 0.1 mg/dl), and severity of rickets/osteomalacia (bone mineral density: -36, -36, and -30%). Microarray analysis of long bones identified gene expression profiles implicating common activation of the FGFR pathway in all the mutant groups. Furthermore, inhibiting FGFR signaling using SU5402 in Hyp- and Dmp1(-/-)-derived bone marrow stromal cells prevented the increase in Fgf23 mRNA expression (129- and 124-fold increase in Hyp and Dmp1(-/-) vs. 1.3-fold in Hyp+SU5402 and 2.5-fold in Dmp1(-/-)+SU5402, P<0.05). For all analyses, samples collected from nonmutant wild-type littermates served as controls. These findings indicate that PHEX and DMP1 control a common pathway regulating bone mineralization and FGF23 production, the latter involving activation of the FGFR signaling in osteocytes.

Hypophosphatasia in a child with widened anterior fontanelle: lessons learned from late diagnosis and incorrect treatment

Hypophosphatasia is characterized by deficiency of serum alkaline phosphatase with defective bone and teeth mineralization. We report on an 11-month-old boy who developed a complex clinical picture characterized by bulging anterior fontanelle, growth failure, nephrocalcinosis and impaired bone mineralization during high-dose calcium and vitamin D supplementation. This therapy had been started 5 months earlier for a presumed diagnosis of nutritional rickets established on the grounds of isolated widened anterior fontanelle. However, laboratory investigations revealed reduced alkaline phosphatase levels associated with hypercalcemia, hypercalciuria, low PTH and normal 25-hydroxy vitamin D levels. Genetic testing detected a compound heterozygote for the novel mutation (c.262G>A) and the described mutation (c.920C>T) in the ALPL gene. High calcium and vitamin D supplementation should not be started in the presence of isolated signs of nutritional rickets without assessing calcium-phosphate metabolism. In fact, in rare bone-mineralizing disorders, this combined therapy might induce severe clinical complications.

Osteoporosis following organ transplantation: pathogenesis, diagnosis and management

Organ transplantation has become popular for the management of various chronic illnesses. With the advent of modern immunosuppressive treatments, the longevity of transplant recipients has increased. Consequently, morbid complications such as osteoporosis and bone fractures are seen at an increasing frequency in this population. In most transplant recipients, bone mineral density (BMD) falls shortly after transplantation. However, bone fracture rate plateaus in all except for post-renal transplant patients. Although the underlying pathophysiologic mechanism for this difference is not fully understood, potential mechanisms for sustained bone loss in renal transplant recipients may be persistent phosphorus wasting and defective bone mineralization. Current treatment regimens are based on studies in a small numbers of subjects with BMD as the primary outcome. Although BMD is recognized as a gold standard in the assessment of bone fracture risk, to date, its association with bone fracture risk in the general post-transplant population is not robust. Therefore, randomized controlled trials with bone fracture as the primary end point are crucial. The development of noninvasive bone markers in distinguishing bone turnover and bone mineralization status is also pivotal since skeletal lesions are heterogeneous in various organ transplantations. The elucidation of these underlying skeletal lesions is necessary for the consideration of selective treatment in this population.

Kbus/Idr, a mutant mouse strain with skeletal abnormalities and hypophosphatemia: Identification as an allele of 'Hyp'

BackgroundThe endopeptidase encoded by Phex (phosphate-regulating gene with homologies to endopeptidases linked to the X chromosome) is critical for regulation of bone matrix mineralization and phosphate homeostasis. PHEX has been identified from analyses of human X-linked hypophosphatemic rickets and Hyp mutant mouse models. We here demonstrated a newly established dwarfism-like Kbus/Idr mouse line to be a novel Hyp model.MethodsHistopathological and X-ray examination with cross experiments were performed to characterize Kbus/Idr. RT-PCR-based and exon-directed PCR screening performed to identify the presence of genetic alteration. Biochemical assays were also performed to evaluate activity of alkaline phosphatase.ResultsKbus/Idr, characterized by bone mineralization defects, was found to be inherited in an X chromosome-linked dominant manner. RT-PCR experiments showed that a novel mutation spanning exon 16 and 18 causing hypophosphatemic rickets. Alkaline phosphatase activity, as an osteoblast marker, demonstrated raised levels in the bone marrow of Kbus/Idr independent of the age.ConclusionsKbus mice should serve as a useful research tool exploring molecular mechanisms underlying aberrant Phex-associated pathophysiological phenomena.

Release of Alkaline Phosphatase Caused by PIGV Mutations In Patients with Hyperphosphatasia-Mental Retardation Syndrome (HPMR), a Recently Found Second Inherited GPI Anchor Deficiency.

AbstractAbstract 2031Recent whole exome sequencing of three sibs of non-consanguineous parents demonstrated PIGV mutations in Hyperphosphatasia-Mental Retardation syndrome (HPMR), an autosomal recessive disease characterized by mental retardation and elevated serum alkaline phosphatase (ALP) levels (Eur J Med Genet, vol.53, issue2, p85, 2010). PIGV is the second mannosyltransferase essential for glycosylphosphatidylinositol (GPI) biosynthesis. Mutations found in four families caused amino acid substitutions A341E, A341V, Q256K and H385P. We have shown that these mutant PIGV proteins are unstable and the mutant cDNAs restored only subnormal GPI biosynthetic activity after transfection into PIGV deficient CHO cells. The backbone of GPI-anchor is synthesized in the endoplasmic reticulum (ER) and is transferred to the proteins which have GPI attachment signal at the C-terminal. It is known that GPI-anchored proteins are not expressed on the surface of GPI deficient cells due to degradation within the cells or secretion. ALP, a GPI anchored protein, was efficiently secreted into medium from PIGV deficient CHO cells, in which incomplete GPI bearing one mannose was accumulated. In contrast, ALP was degraded in PIGL, DPM2 or PIGX deficient CHO cells, in which GPIs lacking mannose were accumulated. Secretion of ALP required GPI transamidase that cleaves the C-terminal GPI attachment signal peptide and replaces it with GPI. It seems that GPI transamidase is activated by GPI bearing at least one mannose, cleaving the hydrophobic signal peptide and resulting in secretion of soluble ALP. It is well known that hypophosphatasia is caused by a deficiency of liver-, bone-, kidney-type alkaline phosphatase due to mutations in the tissue-nonspecific alkaline phosphatase (ALPL) gene. This inherited disorder is characterized by defective bone mineralization and some patients develop epilepsy caused by the disturbance of pyridoxal-5-phosphate (PLP) dependent metabolism of neurotransmitters due to the defect in ALPL which converts PLP to pyridoxal (PL) thereby facilitating passive uptake of this freely diffusible form at the cell surface. Some of the HPMR patients have seizures and the patients with previously reported inherited GPI deficiency caused by defective PIGM encoding the first mannosyltransferase (Nature Med. 12, p846 2006) also have seizures. This common symptom may be caused by the loss of membrane bound ALPL. On the other hand, patients with HPMR do not have the portal vein thrombosis seen in the patients with PIGM deficiency. The affected cell types and the degree of deficiencies in various GPI-anchored proteins might be the cause of the variability of the clinical features among GPI deficiencies. Disclosures:No relevant conflicts of interest to declare.

Clinical, pathological and genetic evaluations of Chinese patients with autosomal-dominant hypophosphatasia

Objectives Hypophosphatasia (HPP) is an inherited disorder characterised by defective bone and tooth mineralisation and deficient serum and bone alkaline phosphatase activity, and it results from mutations in alkaline phosphatase ( ALPL) encoding tissue-nonspecific alkaline phosphatase (TNAP). The objective of the present work was to explore the correlations between genotype and phenotype in a Chinese family affected by autosomal-dominant HPP. Design We examined all individuals of a HPP family by clinical and radiographic examinations as well as laboratory assays. Furthermore, a prematurely exfoliated tooth was observed histopathologically. Based on the clinical and pathological manifestations, the causative gene ALPL was selected for further analysis and screened for mutations. Results The proband presented the characteristic clinical features of childhood HPP such as rachitic skeletal changes, early loss of primary teeth, and short root anomalies of the permanent teeth. Histopathological evaluation of a tooth revealed a “shell” structure, severe mineralisation defects of dentin, and an absence of cementum. The patient's mother and grandfather were clinically diagnosed with adult HPP. The family showed autosomal-dominant moderate hypophosphatasia. DNA sequencing and analysis revealed a novel missense mutation (c.251A>T) in exon4 of ALPL. This mutation (p.E84V) is located in the secondary structure of TNAP's homodimer interface, and it was predicted to have a dominant negative effect. Conclusion Our findings suggest the missense transversion (c.251A>T, p.E84V) should be responsible for the HPP phenotype in this Chinese family.

Prenatal Diagnosis for a Novel Splice Mutation of PHEX Gene in a Large Han Chinese Family Affected with X-Linked Hypophosphatemic Rickets

X-linked hypophosphatemia (XLH) is the most common form of heritable rickets characterized by X-linked dominant inheritance, renal phosphate wasting, hypophosphatemia, and defective bone mineralization. Inactivating mutations of the PHEX gene located at Xp22.1 have been linked with this disease. Ethnic distribution of such mutations seems widespread but only a few mutations in the Chinese population have been reported to date. We report on a large Han Chinese family affected with XLH rickets, which included 13 patients from four generations. Polymerase chain reaction and direct sequencing were performed for all exons and intron-exon boundaries of the PHEX gene. The effect of nucleotide changes was analyzed using bioinformatic software. Prenatal diagnosis was performed on umbilical cord blood at the 20th gestational week. A novel G-->A splice mutation in intron 7 (c.849+1G>A) was identified in all patients from the family. As confirmed by reverse-transcription (RT)-polymerase chain reaction (PCR), the mutation has rendered loss of a normal splice donor site (c.849+1G) while activating a cryptic one at c.849+519G, which resulted in addition of 518 nucleotides to the mature RNA. Prenatal diagnosis had excluded the fetus for carrying the same mutation. A healthy boy was born later. A novel splice mutation c.849+1G>A in the PHEX gene is responsible for XLH in the studied family. Further studies may enhance our understanding of the role of this mutation in the pathogenesis of XLH.

Alteraciones del metabolismo óseo en la cirugía bariátrica

Obesity is an ever-increasing disease in our environment, and a major risk factor for the development of other chronic diseases that increase morbidity and mortality. Bariatric surgery is an effective treatment for obesity, since it not only allows a significant and sustained loss of weight, but also an important reduction of its co-morbidities. However, this treatment is not free of complications, since along with weight loss, frequent nutritional and metabolic deficiencies have been described. These complications are particularly frequent when mixed surgical procedures with a restrictive and malabsortive component such as Roux-en-Y gastric bypass and biliopancreatic diversion are performed. The nutritional deficiencies observed include, to a greater or lesser extent, malabsorption of fat and liposoluble vitamins, as well as other micronutrients such as calcium. As a result, disorders of bone mineral metabolism with skeletal manifestations that include bone mass reduction, increased bone turnover and defective bone mineralization may develop.

Whole-body MRI in the childhood form of hypophosphatasia

Hypophosphatasia (HPP) is a rare inborn error of bone metabolism caused by various defects in the gene coding for the tissue-nonspecific alkaline phosphatase (TNSAP). It results in a reduced activity of the TNSAP and elevated concentrations of its substrates, including inorganic pyrophosphate. Clinical features of HPP include defective bone mineralisation with bone deformities, fractures and chronic non-bacterial osteomyelitis. Renal damage due to calcification, craniosynostosis and dental abnormalities with premature loss of dentition are further complications. Until now, detailed descriptions of whole-body magnetic resonance imaging (WB-MRI) in HPP do not exist. Here, we analysed WB-MRIs of 4 children with the childhood form of HPP. Deformities and defects of the long bones could be seen. All patients showed radiological lesions in the metaphyses of the long bones predominantly in the lower extremities being consistent with hyperaemia and oedema. Differential diagnosis includes an inflammatory process being active in these locations.

Assessment of Bone Mineral Density and Risk Factors in Children Completing Treatment for Acute Lymphoblastic Leukemia

Reduced bone mineral density and increased fracture risk have been reported in children with cancer. In this study, we aimed to determine the growth and bone mineral density (BMD) of the children off chemotherapy for acute lymphoblastic leukemia, and the probable risk factors. The age, anthropometric measurements, lumbar spine BMDs were recorded in 70 children. The risk factors on BMD; daily calcium intake, the time interval from the completion of the chemotherapy, cranial radiotherapy, cumulative steroid dose, decrease in physical activity were investigated. Serum calcium, phosphate, alkaline phosphates, magnesium, insulin-like growth factor-1 (IGF-1) and 25 (OH) vitamin D levels were determined. The mean height percentile at the time of diagnosis was decreased from the value of 53 to a value of 47 at the beginning of the study (P=0.071). Of them; 44% had osteoporosis, 41% had osteopenia, and the rest had normal BMD. BMD z-scores were decreased during the first 2 years from the completion of the treatment. There was a positive correlation between BMD z-scores and daily calcium intake (CC=0.366, P=0.0015). A negative correlation was determined between the time spent on TV and computers and BMD z-scores (CC=-0.464, P=0.0019). Serum IGF-1 and 25 (OH) vitamin D levels of patients were significantly lower than controls (P=0.033). Our data revealed that 85% of the survivors had bone mineralization defect. BMDs and z scores were decreased during the first 2 years from the completion of the treatment and then gradually began to increase. The most important risk factor for decreased BMD was low daily calcium intake. Therefore, patients and their families should be encouraged to take sufficient amount of calcium. Prophylactic vitamin D may also be supplemented.

Preproenkephalin (Penk) is Expressed in Differentiated Osteoblasts, and its Deletion in Hyp Mice Partially Rescues Their Bone Mineralization Defect

Although our understanding of the molecular mechanisms controlling osteoblast differentiation and function is steadily increasing, there are still many open questions, especially regarding the regulation of bone matrix mineralization. For instance, while there is hallmark evidence for the importance of the endopeptidase Phex, whose inactivation in Hyp mice or human patients causes X-linked hypophosphatemic rickets, it is still largely unknown how Phex controls bone mineralization since a physiological substrate for its endopeptidase activity has not been identified yet. Using a genome-wide expression analysis comparing primary calvarial osteoblasts, we have identified preproenkephalin (Penk) as a gene that is selectively expressed in mineralized cultures. Since a role of enkephalin in the regulation of bone remodeling has been suggested previously and since Leu-enkephalin is known to be cleaved by Phex, we analyzed whether Penk expression in osteoblasts is physiologically relevant. Through skeletal analysis of a Penk-deficient mouse model, we found that Penk expression is dispensable for bone development and remodeling since we could not detect any defect following nondecalcified bone histology and histomorphometry compared to wild-type littermates. When Penk was deleted in Phex-deficient Hyp mice, however, we observed a significant reduction of the osteoid enrichment at 24 weeks of age, whereas their disturbance of mineral homeostasis was not affected by the additional absence of the Penk gene. Taken together, our data provide the first in vivo analysis concerning the role of Penk in osteoblasts.

Lack of Sustained Response to Teriparatide in a Patient with Adult Hypophosphatasia

Hypophosphatasia (HPP) is a rare genetic disorder characterized by low serum alkaline phosphatase (ALP) and defective bone mineralization predisposing to poorly healing pseudofractures and fractures. Experience with teriparatide in HPP is limited. A 53-yr-old woman was diagnosed with HPP on the basis of repeatedly low serum ALP (6-8 IU/liter; normal, 30-120 IU/liter), high urine phosphoethanolamine (PEA) and serum pyridoxal 5'-phosphate (PLP) concentrations, and pseudofractures on the lateral aspect of both proximal femurs. Teriparatide (20 microg/d sc) was initiated 4 months after surgery for a painful nonhealing left femoral fracture sustained after minimal trauma. The patient carried two missense mutations at exons 6 and 11 (Ala176Thr and Val423Ala) and one polymorphism at exon 12 (Val522Ala) of the tissue nonspecific ALP gene (TNSALP). Pain resolved and mobility improved with teriparatide treatment. Serum ALP doubled, and both urine PEA and serum PLP decreased. Markers of bone remodeling increased markedly. Comparison of bone biopsy before and 5 months after teriparatide revealed increased amounts of osteoid and osteoblast numbers. After 8 months, there was complete healing of the pseudofracture of the right femur, and bony callus was apparent on the left. Despite good compliance, serum ALP and PLP and urine PEA returned to baseline with between 8 and 13 months of treatment. This is the first bone biopsy report of teriparatide response in adult HPP. In contrast to the two previously reported cases, biochemical response to teriparatide was unsustained, suggesting that response may be variable depending on the TNSALP gene mutation.

Bone mineralization defects and vitamin D deficiency: Histomorphometric analysis of iliac crest bone biopsies and circulating 25-hydroxyvitamin D in 675 patients

Parathyroid hormone (PTH) is only one measurable index of skeletal health, and we reasoned that a histomorphometric analysis of iliac crest biopsies would be another and even more direct approach to assess bone health and address the required minimum 25-Hydroxyvitamin D [25(OH)D] level. A cohort from the northern European population with its known high prevalence of vitamin D deficiency therefore would be ideal to answer the latter question. We examined 675 iliac crest biopsies from male and female individuals, excluding all patients who showed any signs of secondary bone diseases at autopsy. Structural histomorphometric parameters, including osteoid indices, were quantified using the Osteomeasure System according to ASBMR standards, and serum 25(OH)D levels were measured for all patients. Statistical analysis was performed by Student's t test. The histologic results demonstrate an unexpected high prevalence of mineralization defects, that is, a pathologic increase in osteoid. Indeed, 36.15% of the analyzed patients presented with an osteoid surface per bone surface (OS/BS) of more than 20%. Based on the most conservative threshold that defines osteomalacia at the histomorphometric level with a pathologic increase in osteoid volume per bone volume (OV/BV) greater than 2% manifest mineralization defects were present in 25.63% of the patients. The latter were found independent of bone volume per trabecular volume (BV/TV) throughout all ages and affected both sexes equally. While we could not establish a minimum 25(OH)D level that was inevitably associated with mineralization defects, we did not find pathologic accumulation of osteoid in any patient with circulating 25(OH)D above 75 nmol/L. Our data demonstrate that pathologic mineralization defects of bone occur in patients with a serum 25(OH)D below 75 nmol/L and strongly argue that in conjunction with a sufficient calcium intake, the dose of vitamin D supplementation should ensure that circulating levels of 25(OH)D reach this minimum threshold (75 nmol/L or 30 ng/mL) to maintain skeletal health.

Gain-of-function mutation in FGFR3 in mice leads to decreased bone mass by affecting both osteoblastogenesis and osteoclastogenesis

Achondroplasia (ACH) is a short-limbed dwarfism resulting from gain-of-function mutations in fibroblast growth factor receptor 3 (FGFR3). Previous studies have shown that ACH patients have impaired chondrogenesis, but the effects of FGFR3 on bone formation and bone remodeling at adult stages of ACH have not been fully investigated. Using micro-computed tomography and histomorphometric analyses, we found that 2-month-old Fgfr3(G369C/+) mice (mouse model mimicking human ACH) showed decreased bone mass due to reduced trabecular bone volume and bone mineral density, defect in bone mineralization and increased osteoclast numbers and activity. Compared with primary cultures of bone marrow stromal cells (BMSCs) from wild-type mice, Fgfr3(G369C/+) cultures showed decreased cell proliferation, increased osteogenic differentiation including up-regulation of alkaline phosphatase activity and expressions of osteoblast marker genes, and reduced bone matrix mineralization. Furthermore, our studies also suggest that decreased cell proliferation and enhanced osteogenic differentiation observed in Fgfr3(G369C/+) BMSCs are caused by up-regulation of p38 phosphorylation and that enhanced Erk1/2 activity is responsible for the impaired bone matrix mineralization. In addition, in vitro osteoclast formation and bone resorption assays demonstrated that osteoclast numbers and bone resorption area were increased in cultured bone marrow cells derived from Fgfr3(G369C/+) mice. These findings demonstrate that gain-of-function mutation in FGFR3 leads to decreased bone mass by regulating both osteoblast and osteoclast activities. Our studies provide new insight into the mechanism underlying the development of ACH.

Odontohypophosphatasia - A Case Report

Hypophosphatasia is a rare disease characterized by low levels of tissue non-specific alkaline phosphatase. It is characterized by defective bone and teeth mineralization. The symptoms are highly variable which ranges from stillbirth with defective mineralization of bone to early loss of tooth. The severity of disease is not directly related to serum alkaline phosphate levels. Some cases present with premature loss of only deciduous teeth without any bone disease. This condition is known as odontohypophosphatasia. Here we report a case of 15 year old female who presented only dental findings of hypophosphatasia but had no appreciable bone defects seen.

Vitamin D deficiency implicated in bone mineralization defects

Vitamin D deficiency implicated in bone mineralization defects

Hypophosphatasie

Hypophosphatasia (HP) is an inborn error of bone metabolism transmitted predominantly as an autosomal-recessive trait. It is characterized by a reduced activity of the tissue-nonspecific isoenzyme of alkaline phosphatase (TNSAP) and elevated concentrations of its substrates, including pyrophosphates. Clinical symptoms include defective bone mineralisation with bone deformities, fractures and as recently defined chronic non-bacterial osteomyelitis. Renal damage due to calcification, craniosynostosis and dental abnormalities with premature loss of dentition are further symptoms, which have been described as characteristic in the ESPED inquiry of 2004. Knowledge about the mechanisms underlying cell activation leading to inflammation and tissue destruction is still limited in HP. Recent investigations have provided evidence that calcium pyrophosphate crystals are essentially involved in activating inflammatory signal transduction pathways via different receptors of the innate immune system. Laboratory assays, genetic counselling and testing, and radiologic imaging can confirm the diagnosis. Because symptoms are highly variable in their clinical expression, patients should be followed by a HP-experienced multidisciplinary team (paediatrician, radiologist, orthopedist, neurosurgeon, dentist). At the moment symptomatic support and treatment is most important because a causative therapy, e. g. enzyme replacement therapy, is not yet available.

The calcimimetic AMG 641 abrogates parathyroid hyperplasia, bone and vascular calcification abnormalities in uremic rats

Calcimimetics and vitamin D sterols reduce serum parathyroid hormone (PTH) in patients with secondary hyperparathyroidism receiving dialysis, a disease state associated with parathyroid hyperplasia, vascular calcification, bone disease, and increased mortality. The aim of this study was to determine the effects of the research calcimimetic AMG 641 (Amgen, Inc., Thousand Oaks, CA) or calcitriol (Sigma Aldrich Corporation, St. Louis, MO) on vascular calcification in a rodent model of progressive uremia with accompanying secondary hyperparathyroidism induced by dietary adenine. Treatment effects on parathyroid gland hyperplasia and bone loss were also investigated. Rats were treated daily with vehicle, calcitriol (10 ng), AMG 641 (3 mg/kg), or no treatment during the 4 week period the animals were fed adenine. The uremia-induced increases in serum PTH levels were significantly attenuated by both AMG 641 (> 90%) and calcitriol (~ 50%). AMG 641 significantly reduced calcium–phosphorus product (Ca × P) and significantly attenuated the development of both parathyroid hyperplasia and vascular calcification. In addition, AMG 641 prevented the defects in trabecular bone volume, trabecular number, and bone mineralization, as well as increases in trabecular spacing in this rodent model of secondary hyperparathyroidism. Calcitriol (10 ng/rat) decreased osteoid surface/bone surface, but had no effects on other bone parameters, or parathyroid hyperplasia (likely due to the lower PTH suppressive effect of calcitriol at the dose used in this study). However, this dose of calcitriol significantly exacerbated vascular calcification. These results suggest that calcimimetics can reduce the development of vascular calcification, parathyroid hyperplasia and bone abnormalities associated with secondary hyperparathyroidism.

Impaired gastric acidification negatively affects calcium homeostasis and bone mass

Activation of osteoclasts and their acidification-dependent resorption of bone is thought to maintain proper serum calcium levels. Here we show that osteoclast dysfunction alone does not generally affect calcium homeostasis. Indeed, mice deficient in Src, encoding a tyrosine kinase critical for osteoclast activity, show signs of osteopetrosis, but without hypocalcemia or defects in bone mineralization. Mice deficient in Cckbr, encoding a gastrin receptor that affects acid secretion by parietal cells, have the expected defects in gastric acidification but also secondary hyperparathyroidism and osteoporosis and modest hypocalcemia. These results suggest that alterations in calcium homeostasis can be driven by defects in gastric acidification, especially given that calcium gluconate supplementation fully rescues the phenotype of the Cckbr-mutant mice. Finally, mice deficient in Tcirg1, encoding a subunit of the vacuolar proton pump specifically expressed in both osteoclasts and parietal cells, show hypocalcemia and osteopetrorickets. Although neither Src- nor Cckbr-deficient mice have this latter phenotype, the combined deficiency of both genes results in osteopetrorickets. Thus, we find that osteopetrosis and osteopetrorickets are distinct phenotypes, depending on the site or sites of defective acidification.