Harmonization and streamlining of research oversight for pragmatic clinical trials.

The oversight of research involving human participants is a complex process that requires institutional review board review as well as multiple non-institutional review board institutional reviews. This multifaceted process is particularly challenging for multisite research when each site independently completes all required local reviews. The lack of inter-institutional standardization can result in different review outcomes for the same protocol, which can delay study operations from start-up to study completion. Hence, there have been strong calls to harmonize and thus streamline the research oversight process. Although the institutional review board is only one of the required reviews, it is often identified as the target for harmonization and streamlining. Data regarding variability in decision-making and interpretation of the regulations across institutional review boards have led to a perception that variability among institutional review boards is a primary contributor to the problems with review of multisite research. In response, many researchers and policymakers have proposed the use of a single institutional review board of record, also called a central institutional review board, as an important remedy. While this proposal has merit, the use of a central institutional review board for multisite research does not address the larger problem of completing non-institutional review board institutional review in addition to institutional review board review­and coordinating the interdependence of these reviews. In this article, we describe the overall research oversight process, distinguish between institutional review board and institutional responsibilities, and identify challenges and opportunities for harmonization and streamlining. We focus on procedural and organizational issues and presume that the protection of human subjects remains the paramount concern. Suggested modifications of institutional review board processes that focus on time, efficiency, and consistency of review must also address what effect such changes have on the quality of review. We acknowledge that assessment of quality is difficult in that quality metrics for institutional review board review remain elusive. At best, we may be able to assess the time it takes to review protocols and the consistency across institutions.

Loss of DMP1 causes rickets and osteomalacia and identifies a role for osteocytes in mineral metabolism.

The osteocyte, a terminally differentiated cell comprising 90%-95% of all bone cells, may have multiple functions, including acting as a mechanosensor in bone (re)modeling. Dentin matrix protein 1 (encoded by DMP1) is highly expressed in osteocytes and, when deleted in mice, results in a hypomineralized bone phenotype. We investigated the potential for this gene not only to direct skeletal mineralization but also to regulate phosphate (P(i)) homeostasis. Both Dmp1-null mice and individuals with a newly identified disorder, autosomal recessive hypophosphatemic rickets, manifest rickets and osteomalacia with isolated renal phosphate-wasting associated with elevated fibroblast growth factor 23 (FGF23) levels and normocalciuria. Mutational analyses showed that autosomal recessive hypophosphatemic rickets family carried a mutation affecting the DMP1 start codon, and a second family carried a 7-bp deletion disrupting the highly conserved DMP1 C terminus. Mechanistic studies using Dmp1-null mice demonstrated that absence of DMP1 results in defective osteocyte maturation and increased FGF23 expression, leading to pathological changes in bone mineralization. Our findings suggest a bone-renal axis that is central to guiding proper mineral metabolism.

ASARM peptides: PHEX-dependent and -independent regulation of serum phosphate.

Increased acidic serine aspartate-rich MEPE-associated motif (ASARM) peptides cause mineralization defects in X-linked hypophosphatemic rickets mice (HYP) and "directly" inhibit renal phosphate uptake in vitro. However, ASARM peptides also bind to phosphate-regulating gene with homologies to endopeptidases on the X chromosome (PHEX) and are a physiological substrate for this bone-expressed, phosphate-regulating enzyme. We therefore tested the hypothesis that circulating ASARM peptides also "indirectly" contribute to a bone-renal PHEX-dependent hypophosphatemia in normal mice. Male mice (n = 5; 12 wk) were fed for 8 wk with a normal phosphorus and vitamin D(3) diet (1% P(i) diet) or a reduced phosphorus and vitamin D(3) diet (0.1% P(i) diet). For the final 4 wk, transplantation of mini-osmotic pumps supplied a continuous infusion of either ASARM peptide (5 mg·day(-1)·kg(-1)) or vehicle. HYP, autosomal recessive hypophosphatemic rickets (ARHR), and normal mice (no pumps or ASARM infusion; 0.4% P(i) diet) were used in a separate experiment designed to measure and compare circulating ASARM peptides in disease and health. ASARM treatment decreased serum phosphate concentration and renal phosphate cotransporter (NPT2A) mRNA with the 1% P(i) diet. This was accompanied by a twofold increase in serum ASARM and 1,25-dihydroxy vitamin D(3) [1,25 (OH)(2)D(3)] levels without changes in parathyroid hormone. For both diets, ASARM-treated mice showed significant increases in serum fibroblast growth factor 23 (FGF23; +50%) and reduced serum osteocalcin (-30%) and osteopontin (-25%). Circulating ASARM peptides showed a significant inverse correlation with serum P(i) and a significant positive correlation with fractional excretion of phosphate. We conclude that constitutive overexpression of ASARM peptides plays a "component" PHEX-independent part in the HYP and ARHR hypophosphatemia. In contrast, with wild-type mice, ASARM peptides likely play a bone PHEX-dependent role in renal phosphate regulation and FGF23 expression. They may also coordinate FGF23 expression by competitively modulating PHEX/DMP1 interactions and thus bone-renal mineral regulation.

Aberrant Phex function in osteoblasts and osteocytes alone underlies murine X-linked hypophosphatemia.

Patients with X-linked hypophosphatemia (XLH) and the hyp-mouse, a model of XLH characterized by a deletion in the Phex gene, manifest hypophosphatemia, renal phosphate wasting, and rickets/osteomalacia. Cloning of the PHEX/Phex gene and mutations in affected patients and hyp-mice established that alterations in PHEX/Phex expression underlie XLH. Although PHEX/Phex expression occurs primarily in osteoblast lineage cells, transgenic Phex expression in hyp-mouse osteoblasts fails to rescue the phenotype, suggesting that Phex expression at other sites underlies XLH. To establish whether abnormal Phex in osteoblasts and/or osteocytes alone generates the HYP phenotype, we created mice with a global Phex knockout (Cre-PhexDeltaflox/y mice) and conditional osteocalcin-promoted (OC-promoted) Phex inactivation in osteoblasts and osteocytes (OC-Cre-PhexDeltaflox/y). Serum phosphorus levels in Cre-PhexDeltaflox/y, OC-Cre-PhexDeltaflox/y, and hyp-mice were lower than those in normal mice. Kidney cell membrane phosphate transport in Cre-PhexDeltaflox/y, OC-Cre-PhexDeltaflox/y, and hyp-mice was likewise reduced compared with that in normal mice. Abnormal renal phosphate transport in Cre-PhexDeltaflox/y and OC-Cre-PhexDeltaflox/y mice was associated with increased bone production and serum FGF-23 levels and decreased kidney membrane type IIa sodium phosphate cotransporter protein, as was the case in hyp-mice. In addition, Cre-PhexDeltaflox/y, OC-Cre-PhexDeltaflox/y, and hyp-mice manifested comparable osteomalacia. These data provide evidence that aberrant Phex function in osteoblasts and/or osteocytes alone is sufficient to underlie the hyp-mouse phenotype.

Maintaining the trust of physicians and the public in the medical literature: report of a task force on scientific publishing of clinical trials.

In 2006, the American Society of Bone and Mineral Research and the Journal of Bone and Mineral Research convened a task force to consider whether and how to change our editorial policies to assure complete and unbiased reporting of clinical trials. We invited editors of journals that publish research on osteoporosis and disorders of bone and mineral metabolism and presidents of related societies to participate. The task force was charged to consider whether journals should (1) adopt the Principles for Protecting Integrity in the Conduct and Reporting of Clinical Trials published in 2006 by the American Association of Medical Colleges (AAMC) and should (2) require authors and sponsors of industry-funded clinical trials to provide a jointly signed letter that states that the authors had full access to all the data and analyses on which the manuscript was based. The AAMC Principles recommend that multicenter trials should designate a Lead Investigator, Steering Committee, and Publication and Analysis (P&A) Committee, which should consist of a majority of academic investigators who are not sponsor employees. The P&A Committee should have the right to access any data generated during a study and to conduct its own statistical analyses. A majority of task force members voted to support the AAMC Principles, to require a letter jointly signed by academic investigators and industry sponsor stating that the authors had access to the data on which the submission was based, and to recommend adoption of these requirements to their respective societies and journals. Broad-based adoption of the AAMC Principles and requirement of a jointly signed attestation of data access by journals that publish clinical trials in diseases of bone and mineral metabolism should improve the position of academic clinical investigators in their interactions with industry and other funding sources.

Low vitamin D status: time to recognize and correct a Wisconsin epidemic.

As a result of low dietary intake and sun avoidance, low vitamin D status is endemic in Wisconsin. In a convenience sample of postmenopausal Wisconsin residents, 59% had suboptimal D status. Only recently, the medical community has begun to appreciate that low vitamin D status underlies multiple deleterious health consequences including skeletal fragility, muscle weakness, and a potential multitude of non-skeletal morbidities. At present, a routine recommendation indicates that at least 1000 IU of vitamin D3 (cholecalciferol) daily is indicated, although the true requirement may be greater. This review details vitamin D physiology and the prevalence of low vitamin D status in Wisconsin and elsewhere, and provides approaches to optimizing vitamin D status.

IRB Process Improvements: A Machine Learning Analysis.

OBJECTIVE: Clinical research involving humans is critically important, but it is a lengthy and expensive process. Most studies require institutional review board (IRB) approval. Our objective is to identify predictors of delays or accelerations in the IRB review process and apply this knowledge to inform process change in an effort to improve IRB efficiency, transparency, consistency and communication. METHODS: We analyzed timelines of protocol submissions to determine protocol or IRB characteristics associated with different processing times. Our evaluation included single variable analysis to identify significant predictors of IRB processing time and machine learning methods to predict processing times through the IRB review system. Based on initial identified predictors, changes to IRB workflow and staffing procedures were instituted and we repeated our analysis. RESULTS: Our analysis identified several predictors of delays in the IRB review process including type of IRB review to be conducted, whether a protocol falls under Veteran's Administration purview and specific staff in charge of a protocol's review. CONCLUSIONS: We have identified several predictors of delays in IRB protocol review processing times using statistical and machine learning methods. Application of this knowledge to process improvement efforts in two IRBs has led to increased efficiency in protocol review. The workflow and system enhancements that are being made support our four-part goal of improving IRB efficiency, consistency, transparency, and communication.

Calciphylaxis in the absence of end-stage renal disease.

OBJECTIVE: To report a case of calciphylaxis in the absence of renal failure in a patient with secondary hyperparathyroidism and low calcium/phosphorus product, in whom total parathyroidectomy resulted in relief of pain and healing of ulcerations. METHODS: We present the clinical, laboratory, and pathologic findings in a 62-year-old woman with calciphylaxis in the absence of end-stage renal disease. RESULTS: A 62-year-old woman presented with painful nonhealing bilateral calf ulcerations. Pathology examination of tissue specimens from surgical débridement revealed intravascular calcification, consistent with calciphylaxis. Laboratory investigation revealed normal renal function; however, hypocalcemia and hypophosphatemia were present--a corrected serum calcium level of 7.5 mg/dL (normal, 8.5 to 10.2) and a serum phosphorus value of 1.0 mg/dL (normal, 2.5 to 4.5). These abnormalities were likely due to vitamin D deficiency, evidenced by a 25-hydroxyvitamin D level of 14 ng/mL, which provoked an elevation of the serum parathyroid hormone (PTH) concentration, documented by an intact PTH of 213 pg/mL (normal, 15 to 65) and a whole PTH (1-84 PTH) of 70.6 pg/mL (normal, 7 to 36). Her quality of life was severely impaired, not only by the ulcerations but also by intractable pain that necessitated epidural analgesia during the hospitalization. The patient underwent total parathyroidectomy and transcervical thymectomy, with cryopreservation of parathyroid tissue. One year after the parathyroidectomy, the patient had no recurrence of calciphylaxis. CONCLUSION: This case suggests that despite the potential complex pathophysiologic aspects of calciphylaxis, even in the absence of both renal failure and an elevated calcium/phosphorus product, early parathyroidectomy in patients with appreciably increased PTH levels may improve wound healing and diminish pain.

Interobserver reproducibility of criteria for vertebral body exclusion.

UNLABELLED: We studied reproducibility of the ISCD vertebral exclusion criteria among four interpreters. Surprisingly, agreement among interpreters was only moderate, because of differences in threshold for diagnosing focal structural defects and choice of which vertebra among a pair discordant for T-score, area, or BMC to exclude. Our results suggest that reproducibility may be improved by specifically addressing the sources of interobserver disagreement. INTRODUCTION: Although DXA is widely used to measure vertebral BMD, its interpretation is subject to multiple confounders including osteoarthritis, aortic calcification, and scoliosis. In an attempt to standardize interpretation and minimize the impact of artifacts, the International Society for Clinical Densitometry (ISCD) established criteria for vertebral exclusion, including the presence of a focal structural defect (FSD), discrepancy of >1 SD in T-score between adjacent vertebrae, and a lack of increase in BMC or area from L1 to L4. Whereas the efforts of the ISCD represent an important advance in BMD interpretation, the interobserver reproducibility with application of these criteria is unknown. We hypothesized that there would be substantial agreement among four interpreters regarding application of the exclusion criteria and the final lumbar spine T-score. MATERIALS AND METHODS: Each interpreter read a set of 200 lumbar DXA scans obtained on male veterans, applying the ISCD vertebral body exclusion criteria. RESULTS: Surprisingly, agreement among interpreters was only moderate. Differences in interpretation resulted from differing thresholds for recognition of FSD and the choice of excluding the upper or lower vertebral body for the criteria requiring comparison between adjacent vertebrae. CONCLUSIONS: Despite their apparent simplicity, the ISCD vertebral exclusion criteria are difficult to apply consistently. In principle, appropriate refinement of the exclusion criteria may significantly improve interobserver agreement.

Recalculation of the NHANES database SD improves T-score agreement and reduces osteoporosis prevalence.

UNLABELLED: In attempt to improve diagnostic agreement between manufacturers, a recent software update incorporated NHANES III data in GE Lunar densitometers. As a result, the femur neck and trochanter T-scores were lowered, and osteoporosis prevalence was increased. Use of a recalculated young-normal SD for the GE Lunar-adjusted NHANES III database improved diagnostic agreement and is recommended. INTRODUCTION: Use of manufacturer-specific normative databases for T-score derivation leads to discordance in T-score values and differences in diagnostic classification. To address this issue, the International Committee for Standards in Bone Measurement (ICSBM) recommended the NHANES III database for femur T-score derivation. Acquired on Hologic (Hol) instruments, this database requires conversion equations for application to other DXA systems. NHANES III total femur (TF) conversions for GE Lunar (GE) have previously been available, and femoral neck (FN) and trochanter (TR) equations were reported recently. Per the ICSBM recommendation, GE Lunar incorporated these values into their female database. This should produce T-score and diagnostic agreement between Hol and GE instruments; however, this has not been evaluated. MATERIALS AND METHODS: We compared GE femur scans in 115 postmenopausal women using software before and after the NHANES III software update. Subsequently, T-scores derived from femur scans obtained on GE and Hol densitometers were compared in a different group of 89 postmenopausal women. RESULTS: The NHANES III software update had no effect on measured BMD (g/cm2) at any femur region. However, because of changes in values used for T-score calculation (increase in the mean young-normal BMD at the FN and TR and a reduction in SD at the TR), the T-scores were lower (mean, 0.48 and 0.68, respectively) at the FN and TR using post-NHANES III software. Consequently, this update increased femur osteoporosis prevalence in these 115 women from 7.8% to 18.3%. Comparison of GE with Hol total proximal femur T-scores revealed a minimal difference (<0.1) and equal diagnoses of osteoporosis. FN and TR differences were larger, with mean GE T-scores lower than Hol (p < 0.001) by 0.17 and 0.50, respectively, thereby introducing osteoporosis diagnostic disagreement (13 [GE] versus 9 [Hol]). Our evaluation suggested that this disparity resulted from direct application of published NHANES III SDs at the FN and TR. As such, we applied the conversion formulae to the NHANES III published Hologic data and found the FN and TR SDs were greater than assumed by GE. Using our recalculated SD to derive T-scores reduced the mean GE/Hol T-score difference to 0.03 at the FN and 0.32 at the TR and resolved osteoporosis diagnostic disagreement. CONCLUSION: The GE NHANES III software update leads to lower FN and TR T-scores than obtained with Hol or prior GE software. Recalculation of the young-normal SD reduces this difference and is recommended. Clinicians are advised to avoid using the TR for diagnosis or, at a minimum, use caution when making treatment decisions based solely on T-score at this site.

Transactivation of the epidermal growth factor receptor mediates parathyroid hormone and prostaglandin F2 alpha-stimulated mitogen-activated protein kinase activation in cultured transgenic murine osteoblasts.

Recent data suggest that G protein-coupled receptors (GPCRs), including those for PTH and prostaglandins (PGs), contribute to the proliferation and differentiation of osteoblasts in vivo. To understand how these signals are transduced, we studied activation of the ERK1/2 MAPK cascade in cultures of differentiating TMOb murine osteoblasts. In TMOb cells, stimulation of endogenous Gs/Gq-coupled PTH receptors, Gq-coupled PGF2 alpha receptors, and Gi/Gq-coupled lysophosphatidic acid receptors, but not Gs-coupled PGE2 receptors, caused a rapid 5- to 10-fold increase in ERK1/2 phosphorylation. GPCR-stimulated ERK1/2 activation coincided with increased tyrosine phosphorylation of epidermal growth factor (EGF) receptors and was blocked by the EGF receptor inhibitor, tyrphostin AG1478, and the metalloprotease inhibitor, batimastat, suggesting that the response involved transactivation of EGF receptors through the proteolytic release of an EGF receptor ligand. To further examine the mechanism of PTH-stimulated EGF receptor transactivation, we employed COS-7 cells expressing the rat PTH receptor. Here, stimulation with PTH(1-34) caused proteolysis of hemagglutinin epitope-tagged heparin binding-EGF, increased tyrosine autophosphorylation of EGF receptors, and AG1478-sensitive ERK1/2 activation. When PTH receptor-expressing COS-7 cells were placed in a mixed culture with cells lacking the PTH receptor but expressing a green fluorescent protein-tagged ERK2, stimulation with PTH(1-34) induced phosphorylation of green fluorescent protein-ERK2 that was abolished by either batimastat or tyrphostin AG1478. These data suggest that autocrine/paracrine cross-talk between EGF receptors and Gi- or Gq/11-coupled GPCRs represents the predominant mechanism of GPCR-mediated activation of ERK1/2 in cultured TMOb osteoblasts.

Abnormal regulation of renal 25-hydroxyvitamin D-1alpha-hydroxylase activity in X-linked hypophosphatemia: a translational or post-translational defect.

The hyp mouse exhibits abnormal metabolic/hormonal regulation of renal 25(OH)D-1alpha-hydroxylase activity. Whether this results from aberrant transcriptional regulation of the 1alpha-hydroxylase gene, CYP27B1, remains unknown. To investigate this possibility, we compared phosphate and parathyroid hormone effects on renal proximal convoluted tubule and thyrocalcitonin effects on proximal straight tubule enzyme activity and mRNA expression in normal and hyp mice. We assayed 25(OH)D-1alpha-hydroxylase activity by measuring 1,25(OH)2D production and mRNA by ribonuclease protection. Phosphate-depleted mice exhibited a 3-fold increment of 25(OH)D-1alpha-hydroxylase activity compared with normals, whereas hyp mice displayed no enhanced enzyme function. Phosphate-depleted mice concurrently displayed a 2-fold increase in mRNA transcripts; in contrast, despite failure to alter enzyme activity, hyp mice exhibited a similar increment in mRNA transcripts. Parathyroid hormone stimulation of normal mice increased 25(OH)D-1alpha-hydroxylase activity 10-fold, while eliciting only a 2-fold increment in hyp mouse enzyme function. This disparity occurred despite increments of mRNA transcripts to comparable levels (22.2 +/- 3.5- vs. 19.9 +/- 1.8-fold). The dissociation between phosphate- and parathyroid hormone-mediated transcriptional activity and protein function was not universal. Thus, thyrocalcitonin stimulation of normal and hyp mice resulted in comparable enhancement of mRNA transcripts and enzyme activity. These observations indicate that abnormal regulation of vitamin D metabolism in hyp mice occurs in the proximal convoluted tubule and results, not from aberrant transcriptional regulation, but from a defect in translational or post-translational activity.

Evidence for abnormal translational regulation of renal 25-hydroxyvitamin D-1alpha-hydroxylase activity in the hyp-mouse.

Hyp-mice exhibit abnormal regulation of 25-hydroxyvitamin D [25(OH)D]-1alpha-hydroxylase activity. Previous observations suggest such aberrant modulation is posttranscriptional. To investigate this possibility further, we examined whether hyp-mice manifest abnormal translation of 25(OH)D-1alpha-hydroxylase mRNA. We compared phosphate, parathyroid, and calcitonin effects on renal 25(OH)D-1alpha-hydroxylase protein as well as mRNA and enzyme activity in normal and hyp-mice. We assayed protein by Western blots, mRNA by real-time RT-PCR, and enzyme activity by measuring 1,25-dihydroxyvitamin D production. Although phosphate-depleted mice exhibited enhanced enzyme function, with significantly increased mRNA and protein expression, hyp-mice comparably increased mRNA but failed to augment enzyme activity, concordant with an inability to increase protein expression. PTH stimulation increased mRNA and protein expression as well as enzyme activity in normal mice but in hyp-mice, despite effecting mRNA enhancement, did not increment enzyme function or protein. The inability of hypophosphatemia and PTH to increase 25(OH)D-1alpha-hydroxylase activity and protein expression in hyp-mice was not universal because calcitonin stimulation was normal, suggesting proximal convoluted tubule localization of the defect. These data, in accord with absent undue enhancement of protein expression in hyp-mice treated with protease inhibitors, establish that abberrant regulation of vitamin D metabolism results from abnormal translational activity.

Use of the lowest vertebral body T-score to diagnose lumbar osteoporosis in men: is "cherry picking" appropriate?

In this study, we hypothesized that use of the lowest T-score among four lumbar vertebral bodies would lessen the impact of degenerative arthritis and other artifacts on diagnostic categorization at this site and increase study sensitivity, classifying more men with prior fracture as osteoporotic than the other two methods of lumbar spine analysis. Bone density studies of 533 male veterans measured between January and October 2002 were reviewed to determine diagnostic classification using the L1-L4 average, International Society for Clinical Densitometry (ISCD)-determined, and lowest lumbar vertebral body T-score. We calculated sensitivity and specificity of the three methods of spine analysis, using spine osteoporosis to indicate a positive test and prior fracture as the true indicator of osteoporosis. The lowest lumbar T-score performed with similar sensitivity and specificity to that of the lowest hip or wrist T-score in the ability to classify men with prior fracture as osteoporotic, whereas the average L1-L4 and ISCD-determined T-scores performed with lower sensitivity, but better specificity. In conclusion, this retrospective study suggests that use of the lowest vertebral body T-score among men increases diagnostic sensitivity of lumbar spine bone mass measurement. Prospective studies are needed to determine which of these three methods of lumbar spine analysis best predicts future fragility fracture in men and women.

Thallium-pertechnetate subtraction scanning in the preoperative localization of an ectopic undescended parathyroid gland.

Although bilateral exploration is highly effective in the treatment of primary hyperparathyroidism, minimally invasive parathyroidectomy has evolved into the procedure of choice when a single parathyroid lesion can be localized preoperatively. In this article, we discuss the utilization of thallium-pertechnetate subtraction scanning (TPSS) after technetium Tc-99m sestamibi scintigraphy failed to localize an ectopic parathyroid adenoma. Subsequently, radioguided resection of an undescended parathyroid adenoma inferior to the left submandibular gland was performed with surgical cure after a single procedure. This case report illustrates the importance of TPSS as a second-line modality in preoperative adenoma localization, thereby using minimally invasive techniques to successfully treat this patient's primary hyperparathyroidism.

Are Wisconsin physicians knowledgeable about male osteoporosis?

It is unknown if recent research defining the relatively frequent occurrence and pathophysiology of male osteoporosis has been conveyed to clinicians. To assess if physicians have incorporated such information, 5646 licensed Wisconsin physicians received a 1-page survey consisting of 14 statements regarding general knowledge, diagnosis, and treatment of osteoporosis in men. Twenty-six percent (1488) responded; 69% (1033) were family physicians or internists, 7% (106) orthopedic surgeons, and 5% (63) endocrinologists or rheumatologists. Of these physicians, 61% to 78% recognized that osteoporosis is not rare in men. Most (68%-97%) agreed that low-trauma fracture or corticosteroid therapy initiation are indications for bone mass measurement. Treating osteoporosis at a T-score of -2.5 was accepted by 79% to 89% of responders. Overall these physicians: (1) recognize osteoporosis as an important disease in men; (2) accept corticosteroid therapy and prior low-trauma fracture as indications for bone mass measurement; and (3) utilize T-scores in treatment decisions. These data suggest that knowledge regarding male osteoporosis has been conveyed to practicing physicians. Evaluation of methods by which physicians will translate this knowledge into action, thereby optimizing patient care, are needed.

Osteocyte regulation of phosphate homeostasis and bone mineralization underlies the pathophysiology of the heritable disorders of rickets and osteomalacia.

Although recent studies have established that osteocytes function as secretory cells that regulate phosphate metabolism, the biomolecular mechanism(s) underlying these effects remain incompletely defined. However, investigations focusing on the pathogenesis of X-linked hypophosphatemia (XLH), autosomal dominant hypophosphatemic rickets (ADHR), and autosomal recessive hypophosphatemic rickets (ARHR), heritable disorders characterized by abnormal renal phosphate wasting and bone mineralization, have clearly implicated FGF23 as a central factor in osteocytes underlying renal phosphate wasting, documented new molecular pathways regulating FGF23 production, and revealed complementary abnormalities in osteocytes that regulate bone mineralization. The seminal observations leading to these discoveries were the following: 1) mutations in FGF23 cause ADHR by limiting cleavage of the bioactive intact molecule, at a subtilisin-like protein convertase (SPC) site, resulting in increased circulating FGF23 levels and hypophosphatemia; 2) mutations in DMP1 cause ARHR, not only by increasing serum FGF23, albeit by enhanced production and not limited cleavage, but also by limiting production of the active DMP1 component, the C-terminal fragment, resulting in dysregulated production of DKK1 and ß-catenin, which contributes to impaired bone mineralization; and 3) mutations in PHEX cause XLH both by altering FGF23 proteolysis and production and causing dysregulated production of DKK1 and ß-catenin, similar to abnormalities in ADHR and ARHR, but secondary to different central pathophysiological events. These discoveries indicate that ADHR, XLH, and ARHR represent three related heritable hypophosphatemic diseases that arise from mutations in, or dysregulation of, a single common gene product, FGF23 and, in ARHR and XLH, complimentary DMP1 and PHEX directed events that contribute to abnormal bone mineralization.

Hexa-D-arginine treatment increases 7B2•PC2 activity in hyp-mouse osteoblasts and rescues the HYP phenotype.

Inactivating mutations of the "phosphate regulating gene with homologies to endopeptidases on the X chromosome" (PHEX/Phex) underlie disease in patients with X-linked hypophosphatemia (XLH) and the hyp-mouse, a murine homologue of the human disorder. Although increased serum fibroblast growth factor 23 (FGF-23) underlies the HYP phenotype, the mechanism(s) by which PHEX mutations inhibit FGF-23 degradation and/or enhance production remains unknown. Here we show that treatment of wild-type mice with the proprotein convertase (PC) inhibitor, decanoyl-Arg-Val-Lys-Arg-chloromethyl ketone (Dec), increases serum FGF-23 and produces the HYP phenotype. Because PC2 is uniquely colocalized with PHEX in osteoblasts/bone, we examined if PC2 regulates PHEX-dependent FGF-23 cleavage and production. Transfection of murine osteoblasts with PC2 and its chaperone protein 7B2 cleaved FGF-23, whereas Signe1 (7B2) RNA interference (RNAi) transfection, which limited 7B2 protein production, decreased FGF-23 degradation and increased Fgf-23 mRNA and protein. The mechanism by which decreased 7B2•PC2 activity influences Fgf-23 mRNA was linked to reduced conversion of the precursor to bone morphogenetic protein 1 (proBMP1) to active BMP1, which resulted in limited cleavage of dentin matrix acidic phosphoprotein 1 (DMP1), and consequent increased Fgf-23 mRNA. The significance of decreased 7B2•PC2 activity in XLH was confirmed by studies of hyp-mouse bone, which revealed significantly decreased Sgne1 (7B2) mRNA and 7B2 protein, and limited cleavage of proPC2 to active PC2. The expected downstream effects of these changes included decreased FGF-23 cleavage and increased FGF-23 synthesis, secondary to decreased BMP1-mediated degradation of DMP1. Subsequent Hexa-D-Arginine treatment of hyp-mice enhanced bone 7B2•PC2 activity, normalized FGF-23 degradation and production, and rescued the HYP phenotype. These data suggest that decreased PHEX-dependent 7B2•PC2 activity is central to the pathogenesis of XLH.