Disorders of phosphate metabolism--pathomechanisms and management of hypophosphataemic disorders

Disorders of phosphate metabolism

Phosphate in both inorganic and organic form is essential for several functions in the body. Plasma phosphate level is maintained by a complex interaction between intestinal absorption, renal tubular reabsorption, and the transcellular movement of phosphate between intracellular fluid and bone storage pools. This homeostasis is regulated by several hormones, principally the parathyroid hormone, 1,25-dihydroxyvitamin D and fibroblast growth factor 23. Abnormalities in phosphate regulation can lead to serious and fatal complications. In this review phosphate homeostasis and the aetiology, pathophysiology, clinical features, investigation and management of hypophosphataemia and hyperphosphataemia will be discussed.

Comparison of surgical treatments of tumor-induced osteomalacia in different locations in the lower limbs: A retrospective study

Tumor-induced osteomalacia (TIO) is a rare disease that behaves benignly. Very few reports about the features of the responsible tumors according to anatomical locations have been presented.In this retrospective study of 53 patients with TIO-associated tumors in the foot/ankle, tibia and femur, we compared preoperative, postoperative, and follow-up courses, including alkaline phosphatase, phosphorus, and fibroblast growth factor 23, to compare the characteristics of TIO-associated tumors in these 3 locations (level of evidence: therapeutic level III).Patients in the foot/ankle group had longer disease courses and therefore a significantly higher complication rate (P < .001). All TIO-associated tumors in the foot/ankle group involved soft tissue (P = .021), whereas most lesions in the tibia group involved bone, and therefore had much higher concentrations of alkaline phosphatase (P = .020). Additionally, serum phosphorus took much longer to normalize after surgery in the foot/ankle group than that in the other 2 groups (P = .004). Consequently, symptom remission was much better in the tibia and femur groups (P = .008). Moreover, the Ki 67 index in TIO-associated tumors was significantly higher in the foot/ankle group (P < .001) and the recurrence rate in this group was markedly higher (P = .002).The TIO-associated tumors in the foot/ankle are characteristically of occult onset, more soft-tissue involvement, and more readily recurrence. More knowledge and examinations are necessary to enable early diagnosis, radical treatments, and minimize recurrence. New therapies are welcomed and needed.

Hyperphosphatemia in Renal Failure

The recent recognition that hyperphosphatemia is a strong predictor of survival on dialysis has rekindled interest in the regulation and control of serum phosphate. In incipient renal failure hyperphosphatemia is prevented by increased fractional renal phosphate excretion mediated via an increase in parathyroid hormone and the novel phosphaturic hormone FGF-23 (and possibly others). At a glomerular filtration rate of approximately 30 ml/min this compensatory mechanism fails and hyperphosphatemia ensues. Pre-dialytic serum phosphate concentrations of >6 mg/dl increase cardiac mortality presumably to a large extent, but not exclusively, via promoting vascular calcification. It has recently been recognized that vascular calcification is not only a passive precipitation process following transgression of the critical Ca-x-P product, but is an active process accompanied by expression of osteoblastic bone markers in the vessel wall. Because of the recent recognition of the relation between vascular calcification and serum phosphate as well as serum calcium, there is a need for novel calcium-free phosphate binders. Currently sevelamer and lanthanum carbonate have been introduced and trivalent iron preparations are under development.

Vitamin D metabolism and rickets in domestic animals: a review

Rickets and osteomalacia are increasing in prevalence in people because of cultural practices, breast-feeding, decreased sun exposure, and increased sunscreen usage. Several hereditary forms of rickets owing to either renal phosphate wasting or defects in vitamin D metabolism are also reported in people. Rickets is well recognized in domestic animals, but published reports are not always supported by microscopic findings, and diagnoses based on clinical signs and radiology are unreliable. Most cases in domestic animals are caused by dietary deficiency of either vitamin D or phosphorus, but occasional inherited forms are reported in pigs, sheep, cats, and dogs. There is variation between species in susceptibility to dietary vitamin D and phosphorus deficiency and in the ability to manufacture vitamin D in their skin. A number of mouse models have been discovered or created to study human skeletal diseases and skeletal homeostasis. With the discovery that vitamin D is involved in not only calcium and phosphorus homeostasis but also in the immune system and cancer, there is great potential for new and existing animal models to generate valuable information about vitamin D and its many functions. This review presents an overview of vitamin D metabolism and rickets in domestic and laboratory animals and makes comparisons where appropriate with the disease in humans.

[Reduced bone density and bone pain :osteomalacia with hypophospatemia and hypophosphaturia]

Two patients aged 24 and 64 years presented at our hospital with similar symptoms including bone pain and muscle weakness. Basic laboratory tests and urinary diagnostics, bone densitometry and bone histology revealed severe osteomalacia with renal phosphate wasting. After the exclusion of other causes an extensive tumor search was performed due to suspected tumor-induced osteomalacia. In one patient a mesenchymal tumor was found in the thigh and completely resected. After surgery the patient showed a rapid recovery from osteomalacia. Because the search was unsuccessful in the other patient phosphorus supplementation in combination with calcitriol was started. Despite continuing renal phosphate wasting a significant increase in bone mineral density was observed.

[Metabolic bone diseases]

Osteomalacia is caused by impaired vitamin D receptor (VDR) signaling, calcium deficiency, and altered bone mineralization. This can be due to insufficient sunlight exposure, malabsorption, reduced D hormone activation in chronic kidney disease, and rare alterations of VDR signaling and phosphate metabolism. Leading symptoms are bone pain, muscular cramps, and increased incidence of falls in the elderly. The adequate respective countermeasures are to optimize the daily intake of calcium and vitamin D3 and to replace active D hormone and phosphate if deficient. Osteoporosis is characterized by bone fragility fractures upon minor physical impact. Indications for diagnosis and treatment can be established by estimating the absolute fracture risk, taking into account bone mineral density, age, gender, and individual risk factors. Exercise, intervention programs to avoid falls, and specific drugs are capable of substantially reducing fracture risk even in the elderly. Secondary osteoporosis primarily requires both bone-altering medications and effective treatment of underlying diseases.

PHEX gene mutations and genotype-phenotype analysis of Korean patients with hypophosphatemic rickets

X-linked hypophosphatemic rickets (XLH) results from mutations in the PHEX gene. Mutational analysis of the PHEX gene in 15 unrelated Korean patients with hypophosphatemic rickets revealed eight mutations, including five novel mutations, in nine patients: two nonsense mutations, two missense mutations, one insertion, and three splicing acceptor/donor site mutations. Of these, c.64G>T, c.1699C>T, c.466_467 insAC, c.1174-1G>A, and c.1768+5G>A were novel mutations. To analyze the correlation between genotype and phenotype, phenotypes were compared between groups with and without a mutation, in terms of mutation location, mutation type, and sex. Skeletal disease tended to be more severe in the group with a mutation in the C-terminal half of the PHEX gene, but no genotype-phenotype correlation was detected in other comparisons. Further extensive studies of the PHEX gene mutations and analyses of the genotype-phenotype relationships are required to understand PHEX function and the pathogenesis of XLH.

[Primary and secondary osteoporosis. The important role of internal medicine in its differential diagnosis]

Bone metabolism as an important part of internal medicine is covered by endocrinologists, rheumatologists and nephrologists. Primary osteoporosis is an inheritable metabolic bone disease, which can be strongly modified by lifestyle, ageing and underlying diseases. Chronic inflammatory diseases, disorders of metabolism and nutritional deficits enhance the risk. Secondary osteoporosis is caused by endocrinological disorders and drugs such as glucocorticoids. It is the task of internists to clinically recognise and diagnose prominent individual risk factors for primary osteoporosis and underlying diseases for secondary osteoporosis. The key competence of internal medicine in metabolism, hormone-related disorders, malignant diseases and in handling complex medical treatment modalities represents an indispensable segment of an interdisciplinary network approach in patient care, research and teaching. This network includes orthopaedic surgery, paediatrics and gynaecology in addition to other specialties.