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Bone and mineral metabolism in chronic kidney disease 

Bone and mineral metabolism in chronic kidney disease
Chapter:
Bone and mineral metabolism in chronic kidney disease
Author(s):

John Cunningham

DOI:
10.1093/med/9780199699254.003.0017
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date: 04 April 2020

Introduction

The papers chosen for this chapter bring us from the era when it was not understood why parathyroidectomized animals died of tetany, or how exposure to sunlight was capable of healing rickets, to more recent landmark events such as the identification of the calcium-sensing receptor and the demonstration that disorders of fibroblast growth factor 23 (FGF23) regulation and action are central to the pathogenesis of tumour-induced osteomalacia and other phosphate wasting syndromes. Along the way, we see outstanding, prescient accounts of the role of phosphate in the genesis of the mineral disturbances that accompany chronic kidney disease (CKD) and the consequences of a progressive reduction of nephron number leading to the ‘intact nephron hypothesis’, as espoused by Bricker and colleagues in 1960. The review by Stanbury from 1968 lamented the lack of focus on the skeleton up to that time and predicted a continuing lack of focus in the future—a fair point, in that over the past two or three decades the enormous research effort that has been drawn towards the parathyroids and the vitamin D system has probably been at the expense of the bone itself. In the early 1970s, two groups, led by Fraser and Kodicek and by DeLuca, demonstrated the structure of 1,25-dihydroxyvitamin D and the importance of the kidney as the principal site of its production. It was not long before Bricker, Coburn, and Norman reported the first studies of 1,25-dihydroxyvitamin D (calcitriol) used as a treatment in uraemic subjects, a paper demonstrating spectacular effects on hyperparathyroidism, as well as giving early warning of some of the less desirable tendency of vitamin D compounds to cause hypercalcaemia and hyperphosphataemia. Later, the group of Thadhani working at Harvard published the first of a series of papers looking at the effects of treatment with active vitamin D receptor ligands on the survival of patients with CKD, showing apparent benefit attributable to vitamin D compounds. The identification and cloning of the extracellular calcium-sensing receptor in bovine parathyroid tissue by Brown and colleagues in the early 1990s constituted another dramatic step forward and quickly led to the development of compounds capable of modifying the action of this receptor, one of which, cinacalcet, has already seen the light of day as a pharmaceutical agent used to treat hyperparathyroidism. The elapsed time between the cloning of the receptor and the first papers describing the action of calcimimetics was remarkably short. Finally, FGF23, something of a ‘new kid on the block’, was identified as the central cause of a variety of phosphate wasting syndromes. As an important phosphate-regulating hormone in both physiological and pathophysiological scenarios, extreme elevation of FGF23 as seen in CKD may prove to be a sensitive and potent marker for adverse cardiovascular outcomes in these patients.

One can only presume that this blistering pace of development will continue in the future, although, as in the past, it is difficult to predict from where the truly radical developments will emerge.

17.1 The extraction of a parathyroid hormone which will prevent or control parathyroid tetany and which regulates the level of blood calcium

Author

JB Collip

Reference

Journal of Biological Chemistry 1925, 63, 395–438.

Abstract

  1. 1. An extract has been made from the parathyroid glands of oxen by the use of which parathyroid tetany in dogs can be prevented or controlled.

  2. 2. The active principle in this extract produces its effect by causing the calcium content of the blood serum to be restored within normal limits.

  3. 3. A very close parallelism has been observed between the clinical condition of experimental animals and the calcium content of the blood serum. Coincident with the marked improvement observed following the use of the active extract a rise in blood calcium has been noted.

  4. 4. Overdosage effects have been observed and the blood findings in this condition invariably show a condition of hypercalcemia.

  5. 5. The symptoms of hypercalcemia are anorexia, vomiting, apathy, drowsiness verging into coma, and a failing circulation.

  6. 6. Hypercalcemia in parathyroidectomized dogs is a fatal condition if allowed to persist.

  7. 7. Sodium bicarbonate has been observed to reduce the calcium content of the blood serum in hypercalcemia.

  8. 8. Tetania parathyreopriva has been prevented or controlled in dogs receiving no preoperative preparation and which have been placed on a heavy meat diet immediately following recovery from the operative procedure. Animals in which tetany has been prevented from occurring by prophylactic treatment have been thrown into tetany by temporary withdrawal of the treatment.

  9. 9. One to two treatments per day are sufficient to prevent tetany in parathyroidectomized dogs.

  10. 10. The extract containing the active principle has been found to be effective by each of three modes of administration; namely, by the oral route, by intravenous injection, and by subcutaneous injection.

  11. 11. A rise in the level of blood calcium in. the normal dog has been observed following the injection of parathyroid extract.

Copyright 1925 by the American Society for Biochemistry and Molecular Biology. Reproduced from Collip JB, ‘The extraction of a parathyroid hormone which will prevent or control parathyroid tetany and which regulates the level of blood calcium’ Journal of Biological Chemistry, 63, 395–438.

Importance

Collip’s paper is remarkable in several respects. The studies described collectively represented an enormous step forward in the understanding of the regulation of extracellular fluid calcium concentration and the critical role of the parathyroids in this regulation. Total parathyroidectomy had long been known to lead to fatal tetany. Cross-circulation experiments had shown that blood from an animal in tetany increased the neural excitability in a normal limb. In 1907, a palliative effect of calcium on tetany was identified, and in 1913 low blood calcium in tetanic animals was documented, followed in 1918 by the establishment of a normal range of serum calcium (9.2–11.3 mg/dl, 2.3–2.8 mmol/l) to be compared with serum calcium in tetanic patients, which was typically about 4–6 mg/dl (1–1.5 mmol/l). Finally, it was found that parathyroidectomized animals could be kept alive for long periods when fed on calcium-enriched diets containing lactose—a combination now known to facilitate intestinal calcium absorption—with other studies at about the same time showing similar amelioration of tetany by intravenous administration of calcium. With considerable difficulty, Collip developed a bovine parathyroid extract capable of preventing tetany in totally parathyroidectomized animals (dogs) with a close parallelism between their clinical state and serum calcium concentration. It was further noted that excessive doses of the parathyroid extract led to hypercalcaemia.

This paper is also notable for the extraordinary amount of experimental detail in what, by modern standards at least, was an exceptionally long paper providing an important bedrock upon which the future understanding of parathyroid disorders was built. It was also in 1925 that the phosphaturic action of parathyroid hormone (PTH) extract on the kidney was identified. Among the most important later developments were the discovery by Chase and Auerbach (1967) of the role of adenylate cyclase and cyclic AMP as an intracellular second messenger in the mediation of PTH effects on its target organs, the structural analysis of PTH (Brewer and Ronan, 1970), and the demonstration that the biological activity resided at the amino terminal end (Potts et al., 1971), and with that the development of radioimmune assays sensitive enough to assay the hormone in clinical practice (Berson and Yalow, 1968). Without this earlier work, none of our understanding of the critical role of PTH in the maintenance of normal serum calcium in the face of developing CKD, and also the development of secondary hyperparathyroidism and its consequences in that clinical scenario, could have been realized.

References

Berson SA, Yalow RS (1968). Immunochemical heterogeneity of parathyroid hormone in plasma. J Clin Endocrinol Metab 28, 1037–1047. Brewer HB, Ronan R (1970). Bovine parathyroid hormone: amino acid sequence. Proc Nat Acad Sci USA 67, 1862–1869. Chase LR, Auerbach CD (1967). Parathyroid function and the renal excretion of 3′,5′-adenylic acid. Proc Nat Acad Sci USA 58, 518–525. Potts JT, Tregear GW, Keutmann H, et al. (1971). Synthesis of a biologically active N-terminal tetratriacontapeptide of parathyroid hormone. Proc Nat Acad Sci 68, 63–67.

17.2 The pathologic physiology of chronic Bright’s disease. An exposition of the ‘intact nephron hypothesis’

Authors

NS Bricker, PA Morrin, SW Kime Jr

Reference

American Journal of Medicine 1960, 28, 77–98.

Abstract

The course of advancing chronic renal disease is characterized by the development of a constellation of clinical, biochemical and physiologic derangements. These derangements may ultimately involve many organs and organ systems; however, the fundamental event underlying their development is the progressive destruction of nephrons. Although the causal relationship between intrinsic renal disease and the complex abnormalities of the uremic state was recognized by Bright more than twelve decades ago, the precise nature of the events leading from the initial destruction of nephrons to the picture of terminal uremia is yet to be fully understood. Until it becomes possible to prevent the various forms of chronic renal disease or to interrupt their inexorable progression, a major requisite to effective concepts of therapy is the clarification of the sequential events in pathologic physiology. In this regard it is essential to define clearly the functional capacity, range of operation and limitations of the diseased kidney. The present discussion consists of a review of recent experimental observations relating to these considerations.

Reprinted from The American Journal of Medicine, 28, Bricker NS, Morrin PA, Kime Jr SW, ‘The pathologic physiology of chronic Bright’s disease. An exposition of the ‘intact nephron hypothesis’’, 77–98, Copyright (1960), with permission from Elsevier.

Importance

A notable feature of this article is the extent to which, despite being published in a journal devoted principally to clinical internal medicine and its subspecialties, it delves deeply into renal physiology, and references most of the figures who contributed to the ‘state of the art’ of renal pathophysiology during the 15 years following World War II. Early in the paper, Bricker, who was a renal physiologist, stated that ‘…until it becomes possible to prevent the various forms of chronic renal disease or to interrupt their inexorable progression, a major requisite to effective concepts of therapy is the clarification of the sequential events in pathologic physiology. In this regard it is essential to define clearly the functional capacity, range of operation and limitations of the diseased kidney…’ This powerfully makes a general point that clinical medicine will rarely advance except when underpinned by an understanding of the relevant pathophysiology. The article reviewed, with great lucidity, the literature covering the renal handling of water, sodium, potassium, and phosphate. Central to the discussion was the notion that, in most pathologies associated with chronic renal insufficiency, there is a tendency for nephrons to ‘drop out’ in an ‘all-or-nothing’ fashion. Although direct measurement of single-nephron glomerular filtration rate (GFR) was still some way off, the necessity for residual nephrons to ‘up their game’ by increasing their GFR, and also adapting their handling of water and solute, was a central plank of Bricker’s thinking at this time. For example, a nephron that is already in a highly phosphaturic state under the influence (as we know now) of PTH and FGF23 has little capacity to reduce further the tubular reabsorption of phosphate, a concept that would have been readily appreciated in the early 1950s (Goldman and Bassett, 1954). Thus, the clinical sequelae of CKD are inevitably associated with a striking reduction in the adaptive capacity of the kidney on many fronts. The application of these principles to the management of moderate and severe renal failure in the pre-dialysis era focused initially on water, sodium, potassium, and later phosphate (Platt, 1952; Goldman and Bassett, 1954; Schwartz, 1955).

Bricker acknowledged that the observations of Bright (see papers 3.1 and 5.3), establishing the relationship between chronic renal disease and the uraemic state, set the stage for much of his work and this is reflected in his use of the term ‘chronic Bright’s disease’ in this article.

References

Goldman R, Bassett SH (1954). Phosphorus excretion in renal failure. J Clin Invest 33, 1623–1625. Platt R (1952). Structural and functional adaptation in renal failure. Brit Med J 1, 1313–1317. Schwartz B (1955). Potassium and the kidney. New Engl J Med 253, 601–608.

17.3 Bone disease in uremia

Authors

SW Stanbury

Reference

American Journal of Medicine 1968, 44, 714–724.

Summary & Importance

While the focus of Landmarks in Nephrology is on original articles, here a review paper has been selected because it is far reaching and prescient. Stanbury opens with a key summary statement. ‘…it is necessary to emphasise that one cannot understand bone disease without studying the involved bone itself and that much of the confusion that enveloped azotemic osteodystrophy in the past stemmed from neglect of this…’ Furthermore ‘…that history may repeat itself in the osteodystrophy complicating chronic hemodialytic therapy unless systemic studies are made of biopsy specimens of bone using the many techniques that are now available…’ These predictions were right—the number of bone biopsies performed for diagnostic or research purposes in uraemic patients has always lagged behind that required to define the underlying pathology properly, and current clinical guidelines continue to lament the impoverished state of our knowledge of what is really going on in the bones of dialysis patients.

Stanbury’s influence in the field peaked around the inception of maintenance haemodialysis, a time at which the type of renal osteodystrophy seen moved from that associated with progressive CKD to that associated with long-term haemodialysis. Stanbury was a strong believer in the central importance of defective mineralization in the genesis of renal osteodystrophy (Stanbury and Lumb, 1962), in this respect finding himself, not for the only time, somewhat at odds with at least one of the other major opinion leaders of his time, Fuller Albright (Albright et al., 1937), who emphasized hyperparathyroid bone disease (‘azotemic hyperparathyroidism’). The studies of uraemic bone at the time preceded the full characterization of the vitamin D endocrine system, and vitamin D therapies were largely confined to extraordinarily high doses of native vitamin D2 (ergocalciferol).

The article also touched on another burning argument of the day—the potential role of metabolic acidosis in azotemic osteodystrophy and hence also that of correction of acidosis. The paper ranges over the important issues of vitamin D resistance, calcium balance, and also hyperparathyroidism. Stanbury also described what was almost certainly nodular hyperplasia of the parathyroid glands with ‘apparently adenomatous transformation in the glands’. He even referred to autonomous hyperparathyroidism (effectively the phenotype of primary hyperparathyroidism) following renal transplantation (Stanbury et al., 1960).

Throughout, Stanbury’s opinions are expressed strongly, suggesting that behind the scenes he was fighting significant academic battles. The paper is, however, a masterly and far-reaching discussion of the state of play that existed at that time.

References

Albright F, Drake TG, Sulkowitch HW (1937). Renal osteitis fibrosa cystica; report of a case with discussion of metabolic aspects. Bull Johns Hopkins Hosp 60, 377–399. Stanbury SW, Lumb GA (1962). Metabolic studies of renal osteodystrophy. I. Calcium, phosphorus and nitrogen metabolism in rickets, osteomalacia and hyperparathyroidism complicating chronic uraemia and in the osteomalacia of the adult Fanconi syndrome. Medicine 41, 1–34. Stanbury SW, Lumb GA, Nicholson WF (1960). Elective sub-total parathyroidectomy for renal hyperparathyroidism. Lancet 1, 793–799.

17.4 Calcium, phosphorus and bone in renal disease and transplantation

Authors

NS Bricker, E Slatopolsky, E Reiss, LV Avioli

Reference

Archives of Internal Medicine 1969, 123, 543–553.

Importance

The studies described in this paper gave rise to the concept that became known as the ‘trade-off hypothesis’. Building on their earlier work (Slatopolsky et al., 1968), the trade-off hypothesis focused specifically on the necessity for the kidney to adapt its handling of phosphate in the face of falling GFR and the mechanisms necessary for the kidney to achieve this. A powerful role for PTH was identified, with progressive elevation of PTH seen over time in dogs subjected to subtotal nephrectomy. This paper further demonstrated that the progressive reduction of the tubular reabsorption of phosphate, apparently driven by PTH, was greatly attenuated if the animals were subjected to a proportional reduction of dietary phosphate. The really significant outcome was the attenuation of secondary hyperparathyroidism and with that the attenuation of an important consequence of hyperparathyroidism, increased osteoclastic bone resorption. The other side of the coin was nicely demonstrated by Lemann’s group, who showed that profound renal phosphate retention was generated by healthy volunteers on low-phosphate diets (Dominguez et al., 1976).

We must remember that these studies were undertaken in the 1960s, and a role for vitamin D was probably not considered seriously at the time. Looking back, we can now see that FGF23 was almost certainly a significant component of these adaptive responses and that the modification to the tubular reabsorption of phosphate was probably driven by both PTH and FGF23 acting in concert (Gutierrez et al., 2005). An important clinical spin off from this work was heightened awareness of the importance of controlling hyperphosphataemia in patients with advanced CKD. Subsequent studies in man, using similar experimental protocols, yielded consistent, although not always identical, findings to those from the original animal work.

An additional piece of the jigsaw, again not appreciated at the time, was the impact of hyperphosphataemia on vitamin D metabolism and specifically the production of 1,25-dihydroxyvitamin D. In the 1970s, it was shown convincingly that phosphate was an important regulator of the 1-α‎ hydroxylase enzyme, and more recently a role for FGF23 in that regulation has been established, as FGF23, elevated in response to hyperphosphataemia, powerfully down regulates 1-α‎ hydroxylase, an action that is counter-regulatory to that of PTH (Gutierrez et al., 2005). The early implications of this work were discussed in detail in an excellent review in the New England Journal of Medicine by Bricker (1972), and a contemporary view is offered by Slatopolsky in Kidney International (2011).

References

Bricker NS (1972). On the pathogenesis of the uremic state. An exposition of the ‘trade-off hypothesis’. N Engl J Med 286, 1093–1099. Dominguez JH, Gray RW, Lemann J Jr (1976). Dietary phosphate deprivation in women and men: effects on mineral and acid balances, parathyroid hormone and the metabolism of 25-OH-vitamin D. J Clin Endocrinol Metab 43, 1056–1068. Gutierrez O, Isakova T, Rhee E et al. (2005). Fibroblast growth factor-23 mitigates hyperphosphatemia but accentuates calcitriol deficiency in chronic kidney disease. J Am Soc Nephrol 16, 2205–2215. Slatopolsky E (2011). The intact nephron hypothesis: the concept and its implications for phosphate management in CKD-related mineral and bone disorder. Kidney Int (Suppl.) 79, S3–S8. Slatopolsky E, Robson AM, Elkan I, Bricker NS (1968). Control of phosphate excretion in uremic man. J Clin Invest 47, 1865–1874.

17.5 Isolation and identification of 1,25-dihydroxycholecalciferol. A metabolite of vitamin D active in intestine

Authors

MF Holick, HK Schnoes, HF DeLuca, T Suda, RJ Cousins

Reference

Biochemistry 1971, 10, 2799–2804.

Abstract

A metabolite of vitamin D3, thought to be the “tissue-active” form of the vitamin in the intestine, has been isolated from chicken intestines in pure form as the monotriethylsilyl ether derivative. The structure of this metabolite has been identified as 1,25 dihydroxycholecalciferol by means of mass spectrometry, ultraviolet absorption spectophotometry, and specific chemical reactions.

Reprinted with permission from Holick MF, Schnoes HK, DeLuca HF et al, ‘Isolation and identification of 1,25-dihydroxycholecalciferol. A metabolite of vitamin D active in intestine’, Biochemistry, 10, 2799–2804. Copyright (1971) American Chemical Society.

Importance

It is difficult to overestimate the significance of the discovery and characterization of 1,25-dihydroxyvitamin D, the physiological hormonal form of vitamin D. This work was done by the group headed by Hector DeLuca at the University of Wisconsin and was enabled by crucial technical developments in 1966. High-specific-activity vitamin D compounds became available for the first time, allowing tracking of the pathways of vitamin D metabolism (Neville and DeLuca, 1966). This group subsequently spawned a large number of individuals who were to become prominent in the vitamin D field. Several of these are authors on the Biochemistry paper discussed here. This was the first detailed description of the chemical structure of the active form of vitamin D determined by mass spectrometry, along with evidence of its particular tissue effects. The hormone was extracted from the intestines of chicks labelled with tritiated vitamin D3. This paper is nicely complemented by that of Fraser and Kodicek (1970) working in Cambridge, UK, who demonstrated the synthesis of a metabolite (subsequently shown to be 1,25-dihydroxyvitamin D) by kidney homogenates. In the studies by Fraser and Kodicek, a number of organs were investigated empirically and only in the kidney was substantial production of the more polar 1,25-dihydroxyvitamin D compound seen. These authors also speculated that the failure of renal biosynthesis of 1,25-dihydroxyvitamin D was the immediate cause of vitamin D resistance seen in chronic renal failure. Writing later, Fraser modestly stated that ‘…despite its popularity, this Nature paper is not of special importance…’, an assertion with which many would disagree.

Nevertheless, these two papers are perfectly complementary and together constituted an enormous step forward in our understanding of the actions of vitamin D, and how these actions are modified in chronic renal disease. It is notable that these studies took place almost exactly half a century after the seminal observations of Mellenby (1919), who discovered that cod liver oil could cure rickets, and Huldschinsky (1919), who showed that ultraviolet irradiation of a single limb would cure rickets in all limbs, thereby satisfying the criteria for a hormone: synthesized at one site and active at a remote site.

Collectively, this work provided the basis for an emerging understanding of the regulation of vitamin D metabolism, its role in controlling skeletal and mineral homeostasis, and as a treatment for vitamin D-deficient states, particularly those associated with vitamin D resistance.

References

Fraser DR, Kodicek E (1970). Unique biosynthesis by kidney of a biologically active vitamin D metabolite. Nature 228, 764–766. Huldschinsky K (1919). Heilung von Rachitis durch kunstliche Hobensonne. Deutsch Med Wochenschr 41, 712–713. Mellanby E (1919). An experimental investigation on rickets. Lancet 1, 407–412. Neville PF, DeLuca HF (1966). The synthesis of [1, 2–3H] vitamin D3 and the tissue localization of a 0.25-μ‎g (10 IU) dose per rat. Biochemistry 5, 2201–2207.

17.6 Action of 1,25-dihydroxycholecalciferol, a potent kidney produced metabolite of vitamin D3, in uremic man

Authors

Brickman, AS, JW Coburn AW Norman

Reference

New England Journal of Medicine 1972, 287, 891–895.

Abstract

Only the kidney is capable of producing 1,25-dihydroxycholecalciferol (1,25diOHC), the probable active form of vitamin D. The possibility that parenchymal damage in chronic renal disease impairs production of 1,25diOHC and accounts for ‘vitamin-D resistant’ uremia prompted our evaluation of its effects in uremic man. Three patients with advanced renal failure showed significant responses to daily treatment with only 100 U (2.7 μ‎g) of 1,25diOHC for six to 10 days: serum calcium and phosphorus rose; intestinal radioactive calcium (47Ca) absorption increased by 30 to 220 per cent; and fecal calcium decreased by 25 to 71 per cent in those undergoing balance studies. In contrast, 40,000 U (1 mg) of vitamin D caused no change in serum calcium and phosphorus and had negligible effects on 47Ca absorption. Thus, 1,25diOHC is highly active in uremic man, and its impaired production may account for certain abnormalities of calcium homeostasis in uremia. The agent may hold future promise in management of disordered calcium metabolism in uremia.

Copyright 1972 Massachusetts Medical Society. Reproduced with permission from Brickman, AS, Coburn JW, Norman AW, ‘Action of 1,25-dihydroxycholecalciferol, a potent kidney produced metabolite of vitamin D3, in uremic man’, New England Journal of Medicine, 287, 891–95.

Importance

This publication, a landmark in that it was the first description of the use of 1,25-dihydroxyvitamin D as treatment for the vitamin D resistance of uraemia, built on the early understanding that calcitriol was the probable hormonal form of vitamin D, and that failure of its synthesis underpinned the vitamin D resistance seen in uraemia. It also reflected the early demonstration that this hormone localized to certain classical target tissues of vitamin D (Norman et al., 1971) and that the kidney was the organ responsible for synthesizing almost all of the circulating 1,25-dihydroxyvitamin D (Fraser and Kodicek; 1970). Only three patients with advanced renal failure were studied, in whom large doses (1 mg) of native vitamin D had already been shown to have negligible effects on calcium absorption or serum calcium and phosphorus. In contrast, the 1,25-dihydroxyvitamin D, when given at doses of only 2.7 μ‎g daily for 6–10 days, elevated serum calcium and phosphorus and decreased faecal calcium. Confirmatory studies followed in short order, one of these utilizing the pro-drug alfacalcidol, which generated similar biological responses in uraemic patients (Davie et al., 1976). The arrival of increasingly reliable assays for PTH was soon applied to studies of this type, and showed a striking reduction of PTH in patients treated with 1,25-dihydroxyvitamin D or alfacalcidol (Brownjohn et al., 1977). These medications proved highly effective in the treatment of hyperparathyroidism, by far the commonest and most damaging form of metabolic bone disease in uraemia at that time. Substantial skeletal healing was documented, manifest by radiological improvement and reduction of elevated alkaline phosphatase. On the down side, the narrow therapeutic window of active vitamin D compounds quickly became apparent with hypercalcaemia and hyperphosphataemia recognized as important adverse events. At that time, there was no perception that PTH could be oversuppressed, and the general mantra was ‘low is good and lower is better’. So spectacular were the early responses to the new active vitamin D compounds that the secondary problem of augmenting intestinal phosphate absorption was not fully appreciated, not least because most of the early studies were of short duration. Even now, some 40 years later, there remains a surprising amount of uncertainty as to how best to deploy these compounds (Wetmore and Quarles, 2008).

References

Brownjohn AM, Goodwin FJ, Hately W, Marsh FP, O’Riordan JL, Papapoulos SE (1977). 1-alpha-hydroxycholecalciferol for renal osteodystrophy. Brit Med J 2, 721–723. Davie MWJ, Chalmers TM, Hunter UO, Pelc B, Kodicek E (1976). 1-Alphahydroxycholecalciferol in chronic renal failure. Studies of the effect of oral doses. Ann Intern Med 84, 281–285. Fraser DR, Kodicek E (1970). Unique biosynthesis by kidney of a biologically active vitamin D metabolite. Nature 228, 764–766. Norman AW, Myrtle JF, Midgett RJ, Nowicki HG, Williams V, Popjak G (1971). 1, 25-dihydroxy cholecalciferol: identification of the proposed active form of vitamin D3 in the intestine. Science 173, 51–54. Wetmore JB, Quarles LD (2008). Calcimimetics or vitamin D analogs for suppressing parathyroid hormone in end-stage renal disease: time for a paradigm shift? Nat Clin Pract Nephrol 5, 24–33.

17.7 The dialysis encephalopathy syndrome. Possible aluminum intoxication

Authors

AC Alfrey, GR LeGendre, WD Kaehny

Reference

New England Journal of Medicine 1976, 294, 184–188.

Abstract

The aluminum content of muscle, bone and brain was measured in control subjects and in uremic patients on dialysis who had been maintained on phosphate-binding aluminum gels. The mean muscle aluminum was 14.8 ppm, and the trabecular-bone aluminum 98.5 ppm in the patients on dialysis, as compared with 1.2 and 2.4 in control subjects (P 〈 0.05). Brain gray-matter aluminum values in a group of uremic patients on dialysis who died of a neurologic syndrome of unknown cause were 25 ppm as compared with 6.5 ppm in a group of uremic patients on dialysis who died of other causes and 2.2 ppm in control subjects. The fact that brain gray-matter aluminum was higher in all patients with the dialysis-associated encephalopathy syndrome than any of the control subjects or other uremic patients on dialysis suggests that this syndrome may be due to aluminum in intoxication.

Copyright 1976 Massachusetts Medical Society. Reproduced with permission from Alfrey AC, LeGendre GR, Kaehny WD, ‘The dialysis encephalopathy syndrome. Possible aluminum intoxication’, New England Journal of Medicine, 294, 184–8.

Importance

During the early 1970s, an hitherto unrecognized syndrome of rapidly progressive encephalopathy emerged in patients with renal failure. The paper by Alfrey and colleagues provided convincing, if not conclusive, evidence that aluminium was the principle aetiological factor. Brain aluminium content was increased substantially in affected patients and was much higher than those in ‘control’ uraemic subjects without encephalopathy. These, in turn, were much higher than the brain aluminium content in non-uraemic individuals.

Others, principally groups in Sheffield (see paper 12.6) and Newcastle (Ward et al., 1978) also described this syndrome and reported that patchy distribution of cases was associated with variations in water borne aluminium. The Newcastle and Sheffield groups further identified aluminium as the principle aetiological factor in an unusually symptomatic form of osteomalacia found in dialysis patients. Although aluminium was present in serum at extremely low concentration, it was selectively taken up at the mineralization front in bone; studies by Faugere and Malluche (1986) elegantly showed that the patchy linear deposition of aluminium coincided precisely with areas of mineralization failure. These findings were of enormous importance to dialysis patients who had been facing a frightening epidemic of uncertain aetiology without a coherent preventative strategy. The effects on clinical practice were dramatic, and centred initially around the routine use of deionized water for dialysis and subsequently the progressive withdrawal of aluminium-based phosphate binders in the face of clear evidence that oral aluminium could be a significant and sometimes dominant contributor to overall aluminium burden (Felsenfeld et al., 1982). The initial move to calcium-based phosphate binders itself brought difficulties with recurrent hypercalcaemia seen quite commonly and a later appreciation that high oral calcium intake probably contributed to accelerated vascular calcification and possibly to increased morbidity and mortality (Braun et al., 1996). Thus, the withdrawal of a toxic phosphate binder was immediately followed by the introduction of another suboptimal agent, albeit a less toxic one.

References

Braun J, Oldendorf M, Moshage W, Heidler R, Zeitler E, Luft FC (1996). Electron beam computed tomography in the evaluation of cardiac calcification in chronic dialysis patients. Am J Kidney Dis 27, 394–401. Faugere MC, Malluche HH (1986). Stainable aluminum and not aluminum content reflects bone histology in dialyzed patients. Kidney Int 30, 717–722. Felsenfeld AJ, Gutman RA, Llach F, Harrelson JM (1982). Osteomalacia in chronic renal failure: a syndrome previously reported only with maintenance dialysis. Am J Nephrol 2, 147–154. Ward MK, Feest TG, Ellis HA, Parkinson IS, Kerr DNS (1978). Osteomalacic dialysis osteodystrophy: evidence for a water-borne aetiological agent, probably aluminium. Lancet 1, 841–845.

17.8 Cloning and characterization of an extracellular Ca2+-sensing receptor from bovine parathyroid

Authors

EM Brown, G Gamba, D Riccardi, M Lombardi, R Butters, O Kifor, A Sun, MA Hediger, J Lytton, SC Hebert

Reference

Nature 1993, 366, 575–580.

Abstract

Maintenance of a stable internal environment within complex organisms requires specialized cells that sense changes in the extracellular concentration of specific ions (such as Ca2+). Although the molecular nature of such ion sensors is unknown, parathyroid cells possess a cell surface Ca2+-sensing mechanism that also recognizes trivalent and polyvalent cations (such as neomycin) and couples by changes in phosphoinositide turnover and cytosolic Ca2+ to regulation of parathyroid hormone secretion. The latter restores normocalcaemia by acting on kidney and bone. We now report the cloning of complementary DNA encoding an extracellular Ca2+-sensing receptor from bovine parathyroid with pharmacological and functional properties nearly identical to those of the native receptor. The novel approximately 120K receptor shares limited similarity with the metabotropic glutamate receptors and features a large extracellular domain, containing clusters of acidic amino-acid residues possibly involved in calcium binding, coupled to a seven-membrane-spanning domain like those in the G-protein-coupled receptor superfamily.

Reprinted by permission from Macmillan Publishers Ltd: Nature, 366, 575–580, Brown E M, Gamba G, Riccardi D et al, ‘Cloning and characterization of an extracellular Ca2+-sensing receptor from bovine parathyroid’, copyright (1993).

Importance

When looking at the development of our understanding of parathyroid physiology and pathophysiology, there are three defining issues that have set the framework for this field. The first was the appreciation that the parathyroid glands dominated the control of extracellular fluid calcium concentration, and that without them lethal tetany supervened. Secondly, the identification of PTH as the functionally critical output from the parathyroid glands, coupled with the ability to measure it reliably and to understand the function of its receptor in target tissue, generated an enormous acceleration in our understanding of the role of this gland and its hormone. Finally, the work described in this Nature paper by Brown and colleagues defined the structure of the calcium-sensing receptor (CaSR) on parathyroid and other cells and simultaneously advanced our understanding of the functionality of this receptor. This paper effectively fired the starting gun for an extraordinarily rapid development of understanding of the physiology and pathophysiology of the parathyroid glands and the development of therapeutic tools based on their ability to manipulate the behaviour of the CaSR. These developments have moved apace, with the rapid identification of activating and inactivating mutations of the CaSR underlying, respectively, the clinical syndromes of autosomal dominant hypocalcaemia and familial hypocalciuric hypercalcaemia (Pollak et al., 1993). Immune-mediated diseases may also operate via the CaSR, which may be a target of cell- and antibody-mediated attack in idiopathic autoimmune hypoparathyroidism (Kifor et al., 2004). This paper described the cloning and characterization of the CaSR from bovine parathyroid tissue. Related studies showed that extracellular divalent and trivalent cations operated through a cell-surface-sensing mechanism that demonstrably coupled to changes in cytosolic calcium and phosphoinositide turnover, which regulated PTH secretion in a manner that, in the case of low extracellular calcium concentration, restored normocalcaemia by actions on bone and kidney (Brown and MacLeod, 2001).

Another crucial and remarkably rapid development of the work described in this paper has been the development of small molecules targeting the CaSR. Prominent among these have been the phenylalkylamine derivatives with calcimimetic activity, which act as positive allosteric modifiers of the CaSR and increase its sensitivity to its ligands. Calcimimetics have now found an important role in the treatment of hyperparathyroid disorders (Steddon and Cunningham, 2005).

References

Brown EM, MacLeod RJ (2001). Extracellular calcium sensing and extracellular calcium signaling. Physiol Rev 81, 239–297. Kifor O, McElduff A, LeBoff MS, et al. (2004). Activating antibodies to the calcium-sensing receptor in two patients with autoimmune hypoparathyroidism. J Clin Endocrinol Metab 89, 548–556. Pollak MR, Brown EM, Chou YH, et al. (1993). Mutations in the human Ca2+-sensing receptor gene cause familial hypocalciuric hypercalcemia and neonatal severe hyperparathyroidism. Cell 75, 1297–1303. Steddon S, Cunningham J (2005). Calcimimetics and calcilytics—fooling the calcium receptor. Lancet 365, 2237–2239.

17.9 Survival of patients undergoing hemodialysis with paricalcitol or calcitriol therapy

Authors

M Teng, M Wolf, E Lowrie, N Ofsthun, JM Lazarus, R Thadhani

Reference

New England Journal of Medicine 2003, 349, 446–456.

Abstract

  • Background: Elevated calcium and phosphorus levels after therapy with injectable vitamin D for secondary hyperparathyroidism may accelerate vascular disease and hasten death in patients undergoing long-term hemodialysis. Paricalcitol, a new vitamin D analogue, appears to lessen the elevations in serum calcium and phosphorus levels, as compared with calcitriol, the standard form of injectable vitamin D.

  • Methods: We conducted a historical cohort study to compare the 36-month survival rate among patients undergoing long-term hemodialysis who started to receive treatment with paricalcitol (29,021 patients) or calcitriol (38,378 patients) between 1999 and 2001. Crude and adjusted survival rates were calculated and stratified analyses were performed. A subgroup of 16,483 patients who switched regimens was also evaluated.

  • Results: The mortality rate among patients receiving paricalcitol was 3417 per 19,031 person-years (0.180 per person-year), as compared with 6805 per 30,471 person-years (0.223 per person-year) among those receiving calcitriol (P〈0.001). The difference in survival was significant at 12 months and increased with time (P〈0.001). In the adjusted analysis, the mortality rate was 16 percent lower (95 percent confidence interval, 10 to 21 percent) among paricalcitol-treated patients than among calcitriol-treated patients. A significant survival benefit was evident in 28 of 42 strata examined, and in no stratum was calcitriol favored. At 12 months, calcium and phosphorus levels had increased by 6.7 and 11.9 percent, respectively, in the paricalcitol group, as compared with 8.2 and 13.9 percent, respectively, in the calcitriol group (P〈0.001). The two-year survival rate among patients who switched from calcitriol to paricalcitol was 73 percent, as compared with 64 percent among those who switched from paricalcitol to calcitriol (P=0.04).

  • Conclusions: Patients who receive paricalcitol while undergoing long-term hemodialysis appear to have a significant survival advantage over those who receive calcitriol. A prospective, randomized study is critical to confirm these findings.

Copyright 2003 Massachusetts Medical Society. Reproduced with permission from Teng M, Wolf M, Lowrie E et al, ‘Survival of patients undergoing hemodialysis with paricalcitol or calcitriol therapy’, New England Journal of Medicine, 349, 446–456.

Importance

This paper proved to be the first of an exciting cascade of studies that appeared to show a survival advantage for maintenance haemodialysis patients according to the type of vitamin D treatment used, and subsequently whether any vitamin D at all had been given. A historical cohort design was used to demonstrate that, in patients who had received paricalcitol, the adjusted mortality rate was 15% lower than that in calcitriol-treated patients. Subsequent studies by the same Harvard-based group used a similar design to compare survival in patients who had, and had not, at any time received injectable active vitamin D treatment with calcitriol or paricalcitol (Teng et al., 2005). Here, substantially lower mortality was seen in patients who had received active vitamin D treatment, with the somewhat counterintuitive observation that this benefit appeared to be maintained even in those with low PTH, high calcium, and high phosphate, in whom active vitamin D treatment would normally be relatively contraindicated.

These studies sparked an enormous interest in the role of vitamin D beyond its classical actions related to bone and mineral metabolism, and in particular focused on vitamin D action on the heart and vasculature. Studies of a similar design in South American cohorts showed broadly similar results in patients treated with oral active vitamin D compounds, demonstrating that the effect was not confined to those who were treated parenterally. These latter studies also showed an inverse dose response relationship, whereby the highest mortality was seen in patients who had received no vitamin D and the lowest mortality seen in those who had received the smallest doses of vitamin D, with higher dose vitamin D-treated patients occupying an intermediate position (Naves-Diaz et al., 2008).

Much work has flowed directly and indirectly from these observations. Experimental studies using vitamin D receptor and 1α‎-hydroxylase knockout mice (Zhou et al., 2008) and clinical studies in patients with CKD (London et al., 2007) suggested that activation of the vitamin D receptor is associated with higher levels of vascular health. Compelling work in a human ex vivo model have suggested a U-shaped relationship between vitamin D exposure and vascular calcification, whereby no exposure, and high exposure, to active vitamin D are both more likely to be associated with vascular calcification than intermediate-level exposure (Schroff et al., 2010). Interesting though these observations are, there remains a distinct paucity of high-quality clinical trial data to support them, and current management recommendations are still underpinned by a mixture of belief and fact.

References

London GM, Guérin AP, Verbeke FH, et al. (2007). Mineral metabolism and arterial functions in end-stage renal disease: potential role of 25-hydroxyvitamin D deficiency. J Am Soc Nephrol 18, 613–620. Naves-Diaz M, Alvarez-Hernandez D, Passlick-Deetjen J, et al. (2008). Oral active vitamin D is associated with improved survival in hemodialysis patients. Kidney Int 74, 1070–1078. Schroff R, Knott C, Rees L (2010). The virtues of vitamin D—but how much is too much? Ped Nephrol 25, 1607–1620. Teng M, Wolf M, Ofsthun MN, et al. (2005). Activated injectable vitamin D and hemodialysis survival: a historical cohort study. J Am Soc Nephrol 16, 1115–1125. Zhou C, Lu F, Cao K, Xu D, Goltzman D Miao D (2008). Calcium-independent and 1,25(OH) 2D3-dependent regulation of the renin–angiotensin system in 1α‎-hydroxylase knockout mice. Kidney Int 74, 170–179.

17.10 Cloning and characterization of FGF23 as a causative factor of tumor-induced osteomalacia.

Authors

T Shimada, S Mizutani, T Muto, T Yoneya, R Hino, S Takeda, Y Takeuchi, T Fujita, S Fukumoto, T Yamashita

Reference

Proceedings of the National Academy of Sciences USA 2001, 98, 6500–6505.

Abstract

Tumor-induced osteomalacia (TIO) is one of the paraneoplastic diseases characterized by hypophosphatemia caused by renal phosphate wasting. Because removal of responsible tumors normalizes phosphate metabolism, an unidentified humoral phosphaturic factor is believed to be responsible for this syndrome. To identify the causative factor of TIO, we obtained cDNA clones that were abundantly expressed only in a tumor causing TIO and constructed tumor-specific cDNA contigs. Based on the sequence of one major contig, we cloned 2,270-bp cDNA, which turned out to encode fibroblast growth factor 23 (FGF23). Administration of recombinant FGF23 decreased serum phosphate in mice within 12 h. When Chinese hamster ovary cells stably expressing FGF23 were s.c. implanted into nude mice, hypophosphatemia with increased renal phosphate clearance was observed. In addition, a high level of serum alkaline phosphatase, low 1,25-dihydroxyvitamin D, deformity of bone, and impairment of body weight gain became evident. Histological examination showed marked increase of osteoid and widening of growth plate. Thus, continuous production of FGF23 reproduced clinical, biochemical, and histological features of TIO in vivo. Analyses for recombinant FGF23 products produced by Chinese hamster ovary cells indicated proteolytic cleavage of FGF23 at the RXXR motif. Recent genetic study indicates that missense mutations in this RXXR motif of FGF23 are responsible for autosomal dominant hypophosphatemic rickets, another hypophosphatemic disease with similar features to TIO. We conclude that overproduction of FGF23 causes TIO, whereas mutations in the FGF23 gene result in autosomal dominant hypophosphatemic rickets possibly by preventing proteolytic cleavage and enhancing biological activity of FGF23.

Reproduced with permission from Shimada T, Mizutani S, Muto T et al, ‘Cloning and characterization of FGF23 as a causative factor of tumor-induced osteomalacia’, Proc Natl Acad Sci,98, 6500–5. Copyright (2001) National Academy of Sciences, U.S.A.

Importance

This study established disturbed phosphate regulation by FGF23 as the underlying cause of tumour-induced osteomalacia. This condition, in which a tumour (usually of mesenchymal origin) is associated with a renal phosphate wasting syndrome that is cured by complete resection of the tumour, had for some time been thought to be the result of a circulating phosphaturic factor. A similar scenario was thought to exist in some other phosphate wasting conditions, notably X-linked hypophosphataemia (Bowe et al., 2001). Much experimental work had been conducted in the latter, facilitated by the existence of a murine homologue (the Hyp mouse). Cross-circulation and transplantation studies using Hyp mice eventually established the role of a humoral factor—from the transplantation studies, it was apparent that the abnormality resided in the animal and not in the transplanted kidney (Nesbitt et al., 1992). In this paper, cDNA clones extracted from a tumour apparently causing phosphate wasting osteomalacia were found to encode FGF23. Further support for the role of FGF23 was obtained from implantation of Chinese hamster ovary cells expressing FGF23 into nude mice, who developed hypophosphataemia within hours. The mouse phenotype also included low plasma calcitriol concentrations, especially when taken in the context of the prevailing hypophosphataemia, which, in other circumstances, would be expected to upregulate calcitriol production. This phenotype is remarkably similar to that of human X-linked hypophosphataemia, as well as that seen in the Hyp mouse.

This study provided an excellent example of the careful study of a single patient with a precisely defined phenotype, generating a large leap forward in our understanding of a clinical condition. Later, the same group established the critical role of Klotho in reconstituting the FGF receptor 1, enabling activity of FGF23 on selected cell types by this method (Urakawa et al., 2006). FGF23 knockout mice and patients with hyperphosphataemic tumoural calcinosis are hyperphosphataemic and have high renal tubular phosphate reabsorption and high serum calcitriol levels (Lyles et al., 1988). The later identification of an association between FGF23 and cardiovascular morbidity and mortality in the CKD population (Faul et al., 2011) further drew together the linked roles of phosphate, vitamin D metabolism, and FGF23 in the maintenance or otherwise of vascular health.

References

Bowe AE, Finnegan R, Jan de Beur SM, et al. (2001). FGF-23 inhibits renal tubular phosphate transport and is a PHEX substrate. Biochem Biophys Res Commun 284, 977–981. Lyles KW, Halsey DL, Friedman NE, Lobaugh B (1988). Correlations of serum concentrations of 1, 25-dihydroxyvitamin D, phosphorus, and parathyroid hormone in tumoral calcinosis. J Clin Endocrinol 67, 88–92. Nesbitt T, Coffman TM, Griffiths R, Drezner MK (1992). Crosstransplantation of kidneys in normal and Hyp mice. Evidence that the Hyp mouse phenotype is unrelated to an intrinsic renal defect. J Clin Invest 89, 1453–1459. Faul C, Amaral AP, Oskoeui B, et al. (2011). FGF23 induces left ventricular hypertrophy. J Clin Invest 121, 4393–4408. Urakawa I, Yamazaki Y, Shimada T, et al. (2006). Klotho converts canonical FGF receptor into a specific receptor for FGF23. Nature 444, 770–774.