Updated: September 30, 2007
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Phosphorus plays an important role in growth, development, bone formation, acid-base regulation, and cellular metabolism. Inorganic phosphorus exists primarily as the critical structural ion, phosphate (PO 4), which serves as a constituent of hydroxyapatite, the mineral basis of the vertebrate skeleton, and at the molecular level, providing the molecular backbone of DNA. Its chemical properties allow its use as a biological energy store as adenosine triphosphate. Additionally, phosphorus influences a variety of enzymatic reactions (e.g. glycolysis) and protein functions (e.g. the oxygen-carrying capacity of hemoglobin by regulation of 2,3-diphosphoglycerate synthesis). Finally, phosphorus is an important signaling moiety, as phosphorylation and dephosphorylation of protein structures serves as an activation signal. Indeed, phosphorus is one of the most abundant components of all tissues, and disturbances in its homeostasis can affect almost any organ system. Most phosphorus within the body is in bone (600-700 g), while the remainder is largely distributed in soft tissue (100-200 g). The plasma contains 11-12 mg/dL of total phosphorus (in both organic and inorganic states) in adults. Inorganic phosphorus (Pi) primarily exists as phosphate (PO 4), and is the commonly measured fraction, found in plasma at concentrations averaging 4 mg/dl in older children and adults. Plasma Pi concentrations values in children are higher, often up to 8 mg/dl in infants, and gradually declining throughout childhood to adult values. The organic phosphorus component is primarily found in phospholipids and is not routinely assessed, and comprises approximately two-thirds of the total plasma phosphorus (1). Thus the term “plasma phosphorus” generally is used when referring to plasma Pi concentrations, and because plasma Pi is nearly all in the form of the PO 4ion, the terms phosphorus and phosphate are often interchangeably used in the clinical chemistry laboratory.
The critical role that phosphorus plays in cell physiology has resulted in the development of elaborate mechanisms designed to maintain phosphate balance. These adaptive changes are manifest by a constellation of measurable responses, the severity of which is modified by the difference between metabolic Pi need and exogenous Pi supply. Such regulation maintains the plasma and extracellular fluid phosphorus within a relatively narrow range and depends primarily upon gastrointestinal absorption and renal excretion as mechanisms to effect homeostasis. Although investigators have recognized a variety of hormones which influence these various processes, in concert with associated changes in other metabolic pathways, the sensory system, the messenger and the mechanisms underlying discriminant regulation of Pi balance remain incompletely understood.
While long-term changes in Pi balance depend on these variables, short-term changes in Pi concentrations can occur due to redistribution between the extracellular fluid and either bone or cell constituents. Such redistribution results secondary to various mechanisms including: elevated levels of insulin and/or glucose; increased concentrations of circulating catecholamines; respiratory alkalosis; enhanced cell production or anabolism; and rapid bone remineralization.
The majority of ingested phosphorus is absorbed in the small intestine; hormonal regulation of this process plays only a minor role in normal Pi homeostasis. In contrast, the predominant site of regulation of Pi balance, is at the kidney level, where renal tubular reclamation of filtered Pi occurs in response to complex regulatory mechanisms. Thus the fate of Pi is generally renal elimination, incorporation into organic forms in proliferating cells, or deposition into the mineral phase of bone as hydroxyapatite. During times of severe phosphorus deprivation, the phosphate contained in bone mineral provides a source of phosphorus for the metabolic needs of the organism. The specific roles that the intestine and kidney play in this complex process are discussed below.
The small intestine is an important site for Pi absorption with transport greatest in the jejunum and ileum and less in the duodenum. In normal adults net Pi absorption is a linear function of dietary Pi intake. For a dietary Pi range of 4 to 30 mg/kg/day, the net Pi absorption averages 60 to 65% of the intake (2). Intestinal Pi absorption occurs via two routes (Figure 1), a cellularly mediated active transport mechanism and diffusional flux, largely through a paracellular shunt pathway (3). In this regard, several vitamin D responsive Na +-dependent phosphate cotransporters have been identified in intestinal brush border membranes, which have a high affinity for Pi binding (4-8). Energy for this electrochemical uphill process is provided by the sodium gradient, which is maintained by sodium-potassium ATPase. The phosphate incorporated into intestinal cells by this mechanism is ferried from the apical pole to the basolateral pole likely through restricted channels such as the microtubules. Exit of Pi from the enterocyte across the basolateral membrane and into the circulation is down electrical and perhaps concentration gradients. Although such active transport systems are responsive to 25(OH)D and 1,25(OH)2D (2,9), these hormones and systems play a relatively minor role in normal Pi homeostasis. Indeed, during vitamin D deficiency the percentage of phosphorus absorbed from the diet is reduced by only 15%.
Figure 1. Model of inorganic phosphate (HPO 4 =) transport in the intestine. At the luminal surface of the enterocyte the brush border membrane harbors a 1,25(OH)2D responsive 2Na +/ HPO 4 =transporter, which has high affinity for HPO 4 =. Energy for this sodium dependent phosphate transport is provided by an inward downhill sodium gradient, maintained by transport of Na +from the cell via a Na +/K +ATPase cotransporter at the basolateral membrane. The HPO 4 =incorporated into the enterocytes by this mechanism is incorporated into microtubules and ferried across the cell, where transfer to the circulation occurs down electrical and concentration gradients. The vast majority of HPO 4 =absorption occurs via the process of diffusional aborption across the intercellular spaces in the intestine.
The vast majority of Pi absorption occurs via the process of diffusional absorption. This results as a consequence of the relatively low Km of the active transport process (2 mM) and the luminal Pi content during feeding, which generally exceeds 5 mM throughout the intestine even during fasting (10-12). The diffusion is mediated through the paracellular space and, therefore, is primarily a function of Pi intake. Because most diets contain an abundance of Pi, the quantity absorbed always exceeds the need. Factors which may adversely influence the diffusional process are the formation of nonabsorbable calcium, aluminum or magnesium phosphate salts in the intestine and age, which reduces Pi absorption by as much as 50%.
The kidney is immediately responsive to changes in serum Pi levels or to dietary Pi intake. The balance between the rates of glomerular filtration and tubular reabsorption (13) determines net renal handling of Pi. Pi concentration in the glomerular ultrafiltrate is approximately 90% of that in plasma, as not all of the plasma Pi is ultrafilterable (14). Since the product of the serum Pi concentration and the glomerular filtration rate (GFR) approximates the filtered load of Pi, a change in the GFR may influence Pi homeostasis if uncompensated by commensurate changes in tubular reabsorption.
The major site of phosphate reabsorption is the proximal convoluted tubule, at which 60% to 70% of reabsorption occurs (Figure 2). Along the proximal convoluted tubule the transport is heterogeneous, with greatest activity in the S1 segment. Further, increasing, but not conclusive, data supports the existence of a Pi reabsorptive mechanism in the distal tubule. Currently, however, conclusive proof for tubular secretion of Pi in humans is lacking (15).
Figure 2. Distribution of Pi reabsorption and hormone-dependent adenylate cyclase activity throughout the renal tubule. The renal tubule consist of a proximal convoluted tubule (PCT), composed of an S1, S2 and S3 segment, a proximal straight tubule (PST), also known as the S3 segment, the loop of Henle, the medullary ascending limb (MAL), the cortical ascending limb (CAL), the distal convoluted tubule (DCT) and three segments of the collecting tubule: the cortical collecting tubule (CCT); the outer medullary collecting tubule (OMCT); and the inner medullary collecting tubule (IMCT). Pi reabsorption occurs primarily in the PCT but is present is the PST and DCT, sites at which parathyroid hormone (PTH) dependent adenylate cyclase is localized. In contrast, calcitonin alters Pi transport at sites devoid of calcitonin dependent adenylate cyclase, suggesting that Pi reabsorption in response to this stimulus occurs by a distinctly different mechanism.
At all three sites of Pi reabsorption, the proximal convoluted tubule, proximal straight tubule and distal tubule, several investigators have mapped PTH-sensitive adenylate cyclase (Figure 2) (15,16). Not surprisingly, there is clear evidence that PTH decreases Pi reabsorption at these loci by a cAMP-dependent process, as well as a cAMP independent signaling mechanism. In contrast, calcitonin-sensitive adenylate cyclase maps to the medullary and cortical thick ascending limbs and the distal tubule (Figure 2) (17). Nevertheless, calcitonin inhibits Pi reabsorption in the proximal convoluted and proximal straight tubule by a cAMP-independent mechanism that may be mediated by a rise in intracellular calcium (18).
Investigations of the cellular events involved in Pi movement from the renal tubule luminal fluid to the peritubular capillary blood indicate that Pi reabsorption occurs principally by a unidirectional process that proceeds transcellularly. Entry of Pi into the tubular cell across the luminal membrane proceeds by way of a saturable active transport system that is sodium-dependent (analogous to the sodium-dependent co-transport in the intestine) (Figure 3). The rate of Pi transport is dependent on the magnitude of the Na +gradient maintained across the luminal membrane, which depends on the Na +/ATPase or sodium pump on the basolateral membrane. Further, the rate limiting step in transcellular transport is likely the Na +-dependent entry of Pi across the luminal membrane, a process with a low Km for luminal phosphate (~0.43M) which permits highly efficient transport.
Figure 3. Model of inorganic phosphate transcellular transport in the proximal tubule. At the brush border a Na +/H +exchanger and Na +/HPO 4 -co-transporters operate. The NaPi2a transporter is most abundant and is an electrogenic transporter with a 3:1 (Na: PO 4) stoichiometry. The less abundant NaPi2c transporter is electroneutral with a 2:1 (Na: PO 4) stoichiometry. The HPO 4 -that enters the cell across the luminal surface mixes with the intracellular pool of Pi and is transported across the basolateral membrane via an anion exchange mechanism. On the basolateral membrane there are Na +/HPO 4 -cotransporters and a Na +/K +ATPase. The ATPase pumps Na +out of the cell maintaining the inward downhill Na gradient, which serves as the driving force for luminal entry of Na +.
The phosphate that enters the tubule cell plays a major role in governing various aspects of cell metabolism and function and is in rapid exchange with intracellular phosphate. Under these conditions the relatively stable free P concentration in the cytosol implies that Pi entry into the cell across the brush border membrane must be tightly coupled with its exit across the basolateral membrane (Figure 3). The transport of phosphate across the basolateral membrane is apparently a passive process driven by an electrical gradient secondary to an anion exchange mechanism. However, several P transport pathways have been postulated, including Na +-Pi cotransport and an unspecific P leak, as well as anion exchange. In any case, the basolateral Pi transport serves at least two functions: 1) complete transcellular Pi reabsorption when luminal Pi entry exceeds the cellular Pi requirments; and 2) guaranteed basolateral Pi influx if apical Pi entry is insufficient to satisfy cellular requirements (19). Regulation of basolateral Pi transport however, is not well understood.
Pi entry into epithelium is believed to be performed by three classes of Na-Pi cotransporters (20-24) (type I, type II and type III Na-Pi cotransporters). The three families of Na-Pi cotransporters share no significant homology in their primary amino acid sequence and exhibit substantial variability in substrate affinity, pH dependence and tissue expression. The tissue expression, the relative renal abundance and overall transport characteristics of type I, II and III Na-Pi cotransporters suggest that the type IIa transporter plays a key role in brush-border membrane Pi flux. Indeed, changes in expression of the type IIa Na-Pi cotransporter protein parallel alterations in proximal tubular Pi handling, documenting its physiological importance (25,26). In addition, molecular and/or genetic suppression of the type IIa Na-Pi cotransporter supports its role in mediating brush-border membrane Na-Pi cotransport. Thus, intravenous injection of specific antisense oligonucleotides reduces brush-border membrane Na-Pi cotransport activity in accord with a decrease in type IIa cotransporter protein (27). In addition, disruption of the type IIa Na-Pi cotransporter gene (Npt2) in mice leads to a 70% reduction in brush-border Na-Pi cotransport rate and complete loss of the protein (28,29). Nevertheless, the recent finding of impaired renal tubular reabsorption of Pi in the setting of loss of function mutations in the the type IIc Na-Pi cotransporter (30) suggests a role for this transporter in maintenance of normal Pi homeostasis as well as the type IIa transporters.
Several hormones and metabolic pertubations are able to modulate phosphate reabsorption by the kidney. Among these PTH, PTHrP, calcitonin, TGFb, glucocorticoids and phosphate loading inhibit renal phosphate reclamation. In contrast, IGF-1, insulin, thyroid hormone, 1,25(OH)2D, EGF and phosphate deprivation (depletion) stimulate renal phosphate reabsorption. More recently the study of disorders of renal phosphate wasting has revealed important functions of FGF23, a novel member of the fibroblast growth factor family, with respect to renal Pi homeostasis. The common target for this hormonal regulation is the renal proximal tubular cell.
Investigations of classical PTH effects on proximal tubule phosphate transport indicate that both the cAMP-protein kinase A and the phospholipase C-protein kinase C signal transduction pathways modulate this process. The PTH mediated inhibition of phosphate reabsorption operates through the protein kinase C system at low hormone concentrations (10-8 to 10-10 M) and via protein kinase A at higher concentrations. More recently the mechanism by which these second messenger systems alter phosphate transport has become apparent. PTH, after interaction with its receptor effects a rapid and irreversible endocytosis of Pi transporters to the lysosomal compartment, where subsequent proteolytic degradation of the transporters occur. Recovery of Na-Pi cotransport activity following PTH inhibition requires protein synthesis, consistent with this observation. In concert with these findings, recent studies indicate that expression of the Npt2 protein at renal tubular sites is increased in parathyroidectomized rats and decreased after PTH treatment. In addition, Northern blot analysis of total RNA shows that the abundance of Npt2-specific mRNA is not changed by parathyroidectomy but is minimally decreased in response to administration of parathyroid hormone. These data indicate that parathyroid hormone regulation of renal Na-Pi cotransport is determined by changes in the abundance of Npt2 protein in the renal brush border membrane (31). Certain aspects of Pi homeostasis at the renal level, however, are not explained by actions of PTH. For instance, even in the setting where parathyroid glands have been removed, regulation of renal P transport by dietary P content still exists, implying that other mediators of this process are at work.
Although PTH has been the most well-documented physiologic regulator of renal Pi excretion, the recently described actions of FGF23, a novel member of the fibroblast growth factor family, suggest an important role for this hormone as an important regulator of Pi homeostasis. Although there are many gaps in our understanding of this new pathway, several points have been established:
-
Mice overexpressing FGF23 demonstrate increased renal Pi clearance and concomitant hypophosphatemia (32).
-
FGF23KO mice retain P at the kidney and are hyperphosphatemic (33) .
-
Administration to mice of an FGF23 neutralizing antibody increases serum Pi (34).
-
The presumed pathway for FGF23 action involves interaction with FGFRs on the basolateral surface of the renal tubular cell (35).
-
The FGF23/FGFR interaction is facilitated by yet another novel protein, klotho, which forms a ternary complex with FGF23 and FGFR allowing for signal transduction. It appears that klotho is necessary for this interaction to result in a biological response of FGF23, and is mediated through ERK signaling (36).
-
FGF23 levels are regulated in part by dietary Pi status, so that circulating levels increase during Pi loading and decrease during Pi deprivation (37).
The actions of FGF23 and other related proteins as mediators of disease are discussed in detail in the section on Pathophysiology of XLH (see below). Other potential regulators of renal Pi handling have been suggested. These include fragments of matrix extracellular glycoprotein (MEPE), secreted frizzled related protein-4 (sFRP4), stanniocalcin, and other FGFs, including FGF7 (38-41).
Indeed, repeated observations have confirmed that the balance between urinary excretion and dietary input of Pi is maintained not only in normal humans but in patients with hyper- and hypoparathyroidism. In fact, the renal tubule has an intrinsic ability to adjust Pi reabsorption rate according to dietary Pi intake and the body’s Pi supply and demand. Thus Pi reabsorption is increased under conditions of greater need, such as rapid growth, pregnancy, lactation and dietary restriction. Conversely, in times of surfeit, such as slow growth, chronic renal failure or dietary excess, renal Pi reabsorption is curtailed. Such changes in response to chronic changes in Pi availability are characterized by parallel changes in Na-phosphate cotransport activity, the Npt2 mRNA level and Npt2 protein abundance. In contrast, the acute adaptation to altered dietary Pi is marked by parallel changes in Na-phosphate cotransporter activity and Npt2 protein abundance in the absence of a change in Npt2 mRNA. Thus, in response to chronic conditions protein synthesis is requisite in the adaptive response, while under acute conditions the number of Npt2 cotransporters is rapidly changed by mechanisms independent of de novo protein synthesis, such as insertion of existing transporters in the apical membrane or internalization of existing transporters.
A variety of genetic diseases and disorders due to therapeutic agents and physiological adaptations affect phosphate homeostasis. Not surprisingly, since the kidney is the primary regulatory site for phosphate homeostasis, aberrant phosphate metabolism results most commonly from altered renal Pi handling. Moreover, the vast majority of the primary diseases are phosphate losing disorders in which renal Pi wasting and hypophosphatemia predominate and osteomalacia and rickets are characteristic presenting symptoms. Osteomalacia and rickets are disorders of calcification characterized by defects of bone mineralization in adults and bone and cartilage mineralization in youths. In osteomalacia, there is a failure to normally mineralize the newly formed organic matrix (osteoid) of bone. In rickets, a disease of children, there is not only abnormal mineralization of bone but defective cartilage growth plate calcification at the epiphyses as well. Apoptosis of chondrocytes in the hypertrophic zone is reduced, typically resulting in an expanded hypertrophic zone, delayed mineralization and vascularization of the calcification front, with an overall appearance of a widened and disorganized growth plate (42).
The remainder of this chapter reviews the pathophysiology of hypophosphatemic rachitic and osteomalacic disorders, and provides a systematic approach to the diagnosis and management of these diseases. The discussion will focus on disorders in which primary disturbances in phosphate homeostasis occur, emphasizing X-linked hypophosphatemic rickets/osteomalacia (XLH). We will also discuss other disorders including hereditary hypophosphatemic rickets with hypercalciuria (HHRH); autosomal dominant and autosomal recessive hypophosphatemic rickets (ADHR and ARHR); Dent's disease; and tumor induced osteomalacia (TIO).
Mineralization of bone is a complex process in which a calcium-phosphate mineral phase is deposited in a highly ordered fashion within the organic matrix (43). Apart from the availability of calcium and phosphorus, requirements for normal mineralization include: 1) adequate metabolic and transport function of chondrocytes and osteoblasts to regulate the concentration of calcium, phosphorus and other ions at the calcification sites; 2) the presence of collagen with unique type, number and distribution of cross-links, distinct patterns of hydroxylation and glycosylation and abundant phosphate content, which collectively facilitate deposition of mineral at gaps (or "hole zones") between the distal ends of collagen molecules; 3) a low concentration of mineralization inhibitors (such as pyrophosphates and proteoglycans) in bone matrix; and 4) maintenance of an appropriate pH of approximately 7.6 for deposition of calcium-phosphate complexes.
The abnormal mineralization in the hypophosphatemic disorders, is due most likely to phosphopenia at calcification sites and, in some cases, paracrine inhibitory factors, which result in accumulation of unmineralized osteoid, a sine qua non for the diagnosis of osteomalacia. Since the resultant abundant osteoid is not unique, however, establishing the diagnosis of osteomalacia histopathologically requires demonstration that abnormal mineralization, and not increased production, underlies the pathologic abnormality (44, 45). Concordance of these events is manifest by an increase in the bone forming surface covered by incompletely mineralized osteoid, an increase in osteoid volume and thickness and a decrease in the mineralization front (the percentage of osteoid-covered bone-forming surface undergoing calcification) or the mineral apposition rate.
Inadequate growth plate cartilage mineralization in rickets is primarily observed in the hypertrophic zone of chondrocytes. Irregular alignment and more extensive disorganization of the growth plate may be evident with increasing severity of disease. Calcification in the interstitial regions of this hypertrophic zone is defective. Grossly, these changes result in increased thickness of the epiphyseal plate, and an increase in transverse diameter that often extends beyond the ends of the bone and causes characteristic cupping or flaring.
X-linked hypophosphatemic rickets/osteomalacia is the most common "vitamin D resistant" disease in man. The syndrome occurs as an X-linked dominant manifest by renal phosphate wasting and consequent hypophosphatemia (Table 1). Additional characteristic features of the disease include growth retardation, osteomalacia and rickets in growing children. The clinical expression of the disease is widely variable, ranging from a mild abnormality, the apparent isolated occurrence of hypophosphatemia, to severe bone disease. Evidence of a gene dose effect has been controversial, although most would agree that phenotypic differences between males (with a mutated gene on their only X chromosome) and females (who are heterozygous for the defective X-linked gene) are not striking. Generally, evidence of disease may be detected at or shortly after birth. However, features of the disease may not become apparent until age 6 to 12 months or older (46). The most common clinically evident manifestations of XLH are short stature and limb deformities. Growth abnormalities and limb deformities are both more evident in the lower extremities, since they represent the fastest growing body segment before puberty.
Table 1.
|
CALCIUM METABOLISM |
PHOSPHATE METABOLISM |
VITAMIN D METABOLISM |
|||||||
|---|---|---|---|---|---|---|---|---|---|
|
Serum Calcium |
Urine Calcium |
Serum PTH |
GI Calcium Absorption |
Serum Pi |
TmP/ GFR |
GI Pi Absorption |
Serum 25(OH)D |
Serum 1,25(OH)2D |
|
|
XLH, X-linked hypophosphatemia; ADHR, Autosomal dominant hypophosphatemic rickets; ARHR, Autosomal recessive hypophosphatemic rickets; TIO, Tumor-induced osteomalacia; XLHR, X-linked recessive hypophosphatemia (Dent's Disease); HHRH, Hereditary hypophosphatemic rickets with hypercalciuria. N, normal; ↓, decreased; ↑, increased, (↓), decreased relative to the serum phosphorus concentration; ?, unknown. |
|||||||||
|
XLH |
N |
↓ |
N, ↑ |
↓ |
↓ |
↓ |
↓ |
N |
(↓) |
|
ADHR |
N |
↓ |
N |
↓ |
↓ |
↓ |
↓ |
N |
(↓) |
|
ARHR |
N |
↓ |
N |
? |
↓ |
↓ |
? |
N |
(↓) |
|
TIO |
N |
↓ |
N |
↓ |
↓ |
↓ |
↓ |
N |
↓ |
|
XLRH |
N |
↑ |
N, ↓ |
↓ |
↓ |
↓ |
↓ |
N |
↓ |
|
HHRH |
N |
↓ |
N, ↓ |
↓ |
↓ |
↓ |
↓ |
N |
(↓) |
The majority of affected children exhibit clinical evidence of rickets (Figure 4), varying from enlargement of the wrists and/or knees to severe malalignment defects such as bowing or knock-knee deformities. (Figure 4). Such defects may result in waddling gait and leg length abnormalities (47). X-ray examination reveals expanded areas of non-mineralized cartilage in epiphyseal regions and lateral curvature of the femora and/or tibia. Severe secondary hyperparathyroidism as occurs in vitamin D deficiency is not present, however less striking elevations in circulating PTH occur in many patients naive to therapy. Other non-specific but typical findings include elevated serum alkaline phosphatase activity and osteocalcin levels. Serum alkaline phosphatase activity although usually elevated to 2-3 times the upper limit of normal in childhood, is generally less than the levels observed in overt vitamin D- and calcium-deficiency rickets.
Figure 4. Radiograph of the lower extremeties in a patient with X-linked hypophosphatemia. Bowing of the femurs is evident bilaterally. The distal femoral metaphysis is cupped, frayed and widened, radiographic features of an expanded and disorganized growth plate.
Additional signs of the disease may include delayed dentition and dental abscesses (48, 49), which are thought to arise from the limited mineralization of the dentine compartment of the tooth. An enlarged pulp chamber is evident on dental radiographs. Osteophytes, enthesopathy (50) and craniosynostosis are not uncommon. Strikingly absent are common features observed in vitamin D deficiency rickets, such as muscle weakness, tetany and convulsions.
Adults with XLH may be asymptomatic or present with severe bone pain. On clinical examination they often display evidence of post-rachitic deformities, such as bowed legs or short stature. However, radiographic or biochemical abnormalities typical of active bone disease are usually absent. In contrast, some adult patients present with "active" osteomalacia, characterized radiographically by pseudofractures, coarsened trabeculation, rarified areas and/or non-union fractures, and biochemically by elevated serum alkaline phosphatase activity. Symptoms at presentation may reflect the end-result of chronic changes, and may not correlate with apparent current activity of the disease. Many adults demonstrate progressive enthesopathy and bone overgrowth. Fusion of the sacroiliac joint(s) and severe symptomatic spinal stenosis are not uncommon (51).
In spite of marked variability in the clinical presentation of the disease, bone biopsy in affected children and adults universally reveals low turnover osteomalacia without osteopenia (Figure 5). Histomorphometry of biopsy samples invariably demonstrates a reduced rate of formation, diffuse patchy hypomineralization, a decrease in mineralizing surfaces and characteristic areas of hypomineralization of the periosteocytic lacunae (52).
Figure 5. Section from an undecalcified bone biopsy in an untreated patient with X-linked hypophosphatemia. The Goldner stain reveals mineralized bone (blue/green) and an abundance of unmineralized osteoid (red) covering a substantial portion of the surfaces. The width of the osteoid seams is substantially increased.
As previously noted, the primary biochemical abnormality of XLH is hypophosphatemia due to increased urinary phosphate excretion. Moreover, mild gastrointestinal phosphate malabsorption is present in the majority of patients, which may contribute to the evolution of the hypophosphatemia (Table 1) (53, 54).
In contrast, the serum calcium concentration in affected subjects is normal despite gastrointestinal malabsorption of calcium. However, as a consequence of this defect, urinary calcium is often decreased. Circulating PTH levels may be normal to modestly elevated in naïve patients, but treatment with phosphate salts may aggravate this tendency such that persistent secondary hyperparathyroidism may occur. Prior to the initiation of therapy, serum 25(OH)D levels are normal, and serum 1,25(OH)2D levels are in the low normal range (55, 56). The paradoxical occurrence of hypophosphatemia and normal serum calcitriol levels in affected subjects is consistent with aberrant regulation of synthesis of this metabolite (due to decreased 25(OH)D-1a-hydroxylase activity) and its clearance (due to increased 25(OH)D-24-hydroxylase activity), findings that have been demonstrated in the Hyp mouse, the murine homologue of the human disease (57, 58).
With the recognition that hypophosphatemia is the definitive marker for XLH, Winters et al (59) and Burnett et al (60) discovered that this disease is transmitted as an X-linked dominant disorder. Analysis of data from 13 multigenerational pedigrees identified PHEX (for phosphate regulating gene with homologies to endopeptidases located on the X chromosome) as the gene mutated in XLH (61). PHEX is located on chromosome Xp22.1, and encodes a 749-amino acid protein with three putative domains: 1) a small aminoterminal intracellular tail; 2) a single, short transmembrane domain; and 3) a large carboxyterminal extracellular domain, containing ten conserved cysteine residues and a HEXXH pentapeptide motif, which characterizes many zinc metalloproteases. Further studies have revealed that PHEX is homologous to the M13 family of membrane-bound metalloproteases, or neutral endopeptidases. M13 family members, including neutral endopeptidase 24.11 (NEP), endothelin-converting enzymes 1 and 2 (ECE-1 and ECE-2), the Kell blood group antigen (KELL), neprilysin-like peptide (NL1), and endothelin converting enzyme-like 1 (ECEL1), degrade or activate a variety of peptide hormones. In addition, like other neutral endopeptidases, immunofluorescent studies have revealed a cell-surface location for PHEX in an orientation consistent with a type II integral membrane glycoprotein (62). It has been demonstrated that certain missense mutations in PHEX that substitute a highly conserved cysteine residue will interfere with normal trafficking of the molecule to the plasma membrane (63). Thus it appears that one mechanism associated with the pathophysiology of XLH is to prevent PHEX from locating to the cell membrane.
Phex is predominantly expressed in bones (in osteoblasts/osteocytes) and teeth (in odontoblasts/ameloblasts) (64-67); mRNA, protein or both have also been found in lung, brain, muscle, gonads, skin and parathyroid glands. Subcellular locations appear to be the plasma membrane, endoplasmic reticulum and Golgi organelle. Immunohistochemistry studies suggest that Phex is most abundant on the cell surface of the osteocyte. In sum, the ontogeny of Phex expression suggests a possible role in mineralization in vivo.
The work of several groups has documented PHEX mutations in >160 patients (68-76). Mutations are scattered throughout the 749-amino acid extracellular domain, encoded by exons 2-22, and are diverse, consisting of deletions, insertions and duplications, as well as splice site, nonsense and missense mutations.
The location of Phex expression in bone cells have led to the hypothesis that diminished PHEX/Phex expression in bone initiates the cascade of events responsible for the pathogenesis of XLH. In order to confirm this possibility, several investigators have used targeted over-expression of Phex in attempts to normalize osteoblast mineralization, in vitro, and rescue the Hyp phenotype in vivo (77-79). Results from these studies fail to support the widely held opinion that abnormal PHEX/Phex function in mature osteoblasts triggers the hypophosphatemia. However, partial rescue of the mineralization defect in Hyp mice occurs, suggesting that local effects of the PHEX mutation may play some role in the mineralization process, but cannot completely restore the skeleton to normality. In sum, it seems that the temporal and developmental expression of either the osteocalcin or type I collagen promoter-driven Phex expression may not mimic endogenous Phex regulation. Thus, limitation of Phex expression to the mature osteoblast appears insufficient to completely rescue the phenotype
The primary inborn error in XLH results in an expressed abnormality of the renal proximal tubule that impairs Pi reabsorption. The immediate cause of this abnormality is the decreased abundance of the NPT2a mRNA and immunoreactive protein in the proximal convoluted tubule cells (80, 81). The identification of a hypophosphatemic factor, FGF23, isolated from tumors causing a similar hypophosphatemic syndrome, tumor-induced osteomalacia, raised the possibility that this factor could also mediate the hypophosphatemia of XLH. Indeed mean circulating FGF23 concentrations are greater in XLH patients than in control samples, further providing evidence to this effect. Animal studies of renal cross-transplantation between Hyp and normal mice resulted in neither transfer of the mutant phenotype with introduction of Hyp kidney to a normal host, nor its correction with introduction of a normal kidney to a Hyp host. These findings are most consistent with humoral mediation of the Pi wasting in the disease (82). Moreover classical parabiosis experiments suggested that a cross-circulating factor could mediate renal phosphate wasting (83).
One natural hypothesis derived from this new information would be that PHEX (a member of the M13 family of zinc-dependent type II cell surface membrane metalloproteinases) could serve as a processor of a phosphaturic hormone such as FGF23. However, it does not appear that FGF23 is a substrate for PHEX, and the nature of the role PHEX plays in this pathway is not clear. Exploration of diseases related to XLH have resulted in identification of factors that may be important elements of the pathway that relates PHEX to reduced renal tubular reabsorption of Pi. Autosomal Dominant Hypophosphatemic Rickets (ADHR) results from mutations in FGF23 that result in an apparent gain of function of the protein (84). These mutations disrupt an RXXR protease recognition site, and thereby protect FGF23 from proteolysis, resulting in reduced clearance and elevating circulating levels, which likely leads to Pi wasting. FGF23 has been identified as a product of tumors causing Tumor-Induced Osteomalacia (TIO)(85). Transgenic mice which overexpress FGF23, exhibit retarded growth, hypophosphatemia, decreased serum 1,25(OH)2D levels and rickets/osteomalacia, all features of XLH. The recent description of Autosomal Recessive Hypophosphatemic Rickets (86), due to homozygous loss of function mutations in DMP1 have introduced further complexities. DMP1 is a matrix protein of the SIBLING (small integrin binding ligand N-glycated) family, and, like PHEX and FGF23 has been primarily identified in osteocytes. Furthermore, FGF23 levels are elevated in patients with ARHR, and in mice with biallelic disruption of DMP1.
In sum, a variety of recent findings suggest that enhanced FGF23 activity is common to several of the phosphate-wasting disorders. In particular, those disorders that share the combined defects of inappropriate circulating levels of 1,25(OH)2D and renal tubular Pi wasting seem to be mediated by increased FGF23 levels. This coincidence of findings holds for XLH, ADHR, ARHR, and TIO, and are consistent with the notion that FGF23 is a direct regulator of Pi homeostasis at the renal level, and also has the effect of down-regulating metabolism of vitamin D to its active form. The teleological appeal to this argument stems from the provision of 2 major Pi regulating hormones in the body: firstly, PTH (primarily responsive to serum Ca levels), which also serves to increase Ca levels via an increase in circulating 1,25(OH)2D, and secondly, FGF23 (primarily responsive to Pi), which counters PTH’s calcemic effect by reducing 1,25(OH)2D levels (Figure 6).
Figure 6. Scheme for the speculated pathophysiology of XLH, ARHR, TIO, and ADHR. Upper panel, osteocytes, comprising a network of connected cells embedded in mineralized bone are the cellular source of PHEX (which is mutated in XLH), DMP1 (which is mutated in ARHR), and FGF23 (which is found in high concentrations in all four of these hypophosphatmic disorders). It follows that loss of PHEX or DMP1 results in increased FGF23 production/secretion by mechanisms that are not currently understood. Circulating FGF23 concentrations may also occur secondary to the increased production associated with various tumors. Lower panel, circulating FGF23 interacts with an FGF receptor (FGFR) on the basolateral surface of the proximal renal tubular cell. Klotho, produced by the distal renal tubule in both membrane bound and secretory forms is necessary for the FGF23/FGFR interaction. Signalling through this pathway results in a decrease in Npt2 mRNA, thereby reducing the abundance of Pi cotransporters on the apical membrane and the well-described impairment of renal tubular Pi reabsorption. Likewise 25(OH)D-1a-hydroxylase mRNA is decreased and synthesis of 1,25(OH)2D is impaired. In XLH and ARHR, increased production of FGF23 occurs in the skeleton; in TIO, increased production of FGF23 occurs in tumors; in ADHR, enhanced activity of FGF23 occurs as a result of the specific mutations that retard its metabolic clearance.
Further circumstantial evidence for the central role of FGF23 in the Pi-regulating process comes from the investigation of another group of rare disorders of Pi homeostasis in which renal Pi conservation is excessive in the setting of increased circulating Pi levels. This group of disorders, known as hyperphosphatemic tumoral calcinosis (HTC), is manifest clinically by precipitation of amorphous calcium-phosphate crystals in soft tissues. This phenomenon is thought to result from an increase in the ambient Ca x phosphate solubility product, and occurs as a direct result of enhanced renal tubular reabsorption of Pi (87). In addition, circulating 1,25(OH)2D levels are in the high-normal to high range. Thus the precise converse of primary metabolic derangements occurs, as compared to the XLH-related of diseases. HTC has been shown to directly result from loss of function mutations in either of 2 proteins, FGF23, or GALNT3, a glycosylating enzyme that appears to be necessary for appropriate O-glycosylation of FGF23 (88-90). Patients with HTC have low intact FGF23 levels in both cases. FGF23 knockout mice develop a hyperphosphatemic, calcifying phenotype with elevated 1,25(OH)2D levels (33), similar to the premature aging mouse with disruption of the klotho gene (91, 92). Indeed the klotho protein has been shown to serve as an essential co-factor in the receptor activation of the FGF receptor FGFR1 when FGF23 serves as the activating ligand (36), and, as predicted, the klotho knock out mouse demonstrates hyperphosphatemia and elevated 1,25(OH)2D levels (91).
The overall physiologic importance of this regulating system will require further study. It is not clear how PHEX or DMP1 result in elevated FGF23 levels. The intriguing aspect of the osteocyte as a potential central cell in this pathway also bears further study. One possible interpretation of these findings is that the osteocyte network throughout the skeleton may be a central sensor of skeletal mineral demand. The coordination of certain specific matrix proteins may play a role in the local regulation of phosphate supply and mineralization. It follows that genetic disruption of this pathway may result in the profound systemic disturbances observed in the diseases described above.
A generation ago, physicians employed pharmacological doses of vitamin D as the cornerstone for treatment of XLH. However, long-term observations indicate that this therapy fails to cure the disease and poses the serious problem of recurrent vitamin D intoxication and renal damage. Indeed, such treatment results only in incomplete healing of the rachitic abnormality, while hypophosphatemia and impaired growth remain. Similar unresponsiveness prevails upon use of 25(OH)D.
With the recognition that phosphate depletion is an important contributor to impaired skeletal mineralization, physicians began to devise treatment strategies that employed oral phosphate supplementation to compensate for the renal phosphate wasting and thereby increasing the available Pi to the mineralizing skeleton. Pharmacologic amounts of vitamin D were used in combination with phosphate supplements to counter the exacerbation of hyperparathyroidism observed in this setting. Such combination therapy was found to be more effective than either administering vitamin D or phosphate alone. With the recognition that circulating 1,25(OH)2D levels are not appropriately regulated in XLH, the use of this metabolite in combination with phosphate was subsequently used to treat the disease (55, 93-95). The newer treatment strategy directly addresses the combined calcitriol and phosphorus deficiency characteristic of the disorder. Although this combination therapy has become the conventional therapy for XLH, complete healing of the skeletal lesions is usually not the case, and late complications of the disease are persistent and often debilitating.
In children the goal of therapy is to improve growth velocity, normalize any lower extremity defects, and heal the attendant bone disease. Generally the treatment regimen includes a period of titration to achieve a maximum dose of 1,25(OH) 2D 3(Rocaltrol® or calcitriol), 20-50 ng/kg/day in two divided doses, and phosphorus, 1-2 gms/day in 3-5 divided doses. Occasionally patients will prove refractory to this therapy and maximally tolerated amounts of 1,25(OH) 2D 3and phosphorus are required with daily dose limits of 3 mcg and 2.5 gms, respectively.
Use of 1,25(OH) 2D 3/phosphorus combination therapy involves a significant risk of toxicity. Hypercalcemia, hypercalciuria, renal calcinosis, and hyperparathyroidism can be sequelae of unmonitored therapy. Detrimental effects on renal function were particularly common prior to the frequent monitoring now generally employed with this therapy. Indeed, hypercalcemia, severe nephrocalcinosis and/or diminished creatinine clearance necessitates appropriate dose adjustment, and in some cases discontinuation of therapy. Throughout the treatment course careful attention to renal function, as well as serum and urine calcium is extremely important. Nevertheless, in spite of these varied complications of therapy, treatment of XLH often proceeds with limited interruptions. Moreover, the improved outcome of this therapeutic intervention, compared to that achieved by previous regimens, justifies the aggressive approach that constitutes this current therapy.
While such combined therapy often improves growth velocity, refractoriness to the growth-promoting effects of treatment can be encountered in children who present with markedly short stature prior to 4 years of age. For that reason the use of recombinant growth hormone as additional treatment has been suggested (96). Although positive effects have been observed in young patients with XLH with particularly impaired stature, this approach has not been universally recommended.
Indications for combined therapy in adults with XLH are less clear. The occurrence of intractable bone pain and refractory non-union fractures often respond to treatment with calcitriol and phosphorus (97). However, data remain unclear regarding the effects of treatment on fracture incidence (which may not be increased in untreated patients), enthesopathy and dental abscesses. Therefore, the decision to treat affected adults must be individualized.
Given the limitations with even currently advised treatment for XLH, the quest for new and better therapies for XLH continues. The recent description of correction of serum P levels and improved bony growth in Hyp mice treated with a neutralizing antibody to FGF23 raise the possibility that measures to inhibit action of this suspected mediator of disease will have a role in the treatment of XLH in the future (34).
Several studies have documented autosomal dominant inheritance of a hypophosphatemic disorder similar to XLH (98, 99). The phenotypic manifestations of this disorder include the expected hypophosphatemia due to renal phosphate wasting, lower extremity deformities, and rickets/osteomalacia. Affected patients also demonstrate normal serum 25(OH)D levels, while maintaining inappropriately normal serum concentrations of 1,25(OH)2D, in the presence of hypophosphatemia, all hallmarks of XLH (Table 1). PTH levels are normal. Long-term studies indicate that a few of the affected female patients demonstrate delayed penetrance of clinically apparent disease and an increased tendency for bone fracture, uncommon occurrences in XLH. In addition, among patients with the expected biochemical features documented in childhood, rare individuals lose the renal phosphate-wasting defect after puberty. As noted above, specific mutations in FGF23 in the 176-179 amino acid residue sequence have been discovered in patients with ADHR (84). These mutations disrupt an RXXR furin protease recognition site, and the resultant mutant molecule is thereby protected from proteolysis, and resultant elevated circulating levels of FGF23 are the likely cause of the renal Pi wasting.
An apparent forme fruste of ADHR (autosomal dominant) hypophosphatemic bone disease has many of the characteristics of XLH and ADHR, but recent reports indicate that affected children display no evidence of rachitic disease. Because this syndrome is described in only a few small kindreds, and radiographically evident rickets is not universal in children with familial hypophosphatemia, these families may have ADHR. Further observations are necessary to discriminate this possibility.
Two recent reports describe families with phosphate wasting rickets inherited in an autosomal recessive manner (86, 100). These patients have been found to have the same constellation of progressive rachitic deformities seen in both XLH and ADHR. Moreover the biochemical phenotype is manifest by the same measures of hypophosphatemia, excess urinary Pi losses, and aberrant vitamin D metabolism (normal circulating 25-OHD and 1,25(OH)2D levels, despite ambient hypophosphatemia) as observed in both XLH and ADHR. In addition to the expected phenotypic features, and in contrast to XLH, spinal radiographs of patients with ARHR reveal noticeably sclerotic vertebral bodies. In addition to the enlarged pulp chamber characteristic of teeth in individuals with XLH, enamel hypoplasia can be evident in heterozygotes. Of particular interest is the identification of elevated levels of FGF23 in the affected individuals.
The identification of a progressive mineralization defect associated with hypophosphatemia in DMP1 knockout mice led to the consideration of homozygous loss of function in this candidate gene as the cause of ARHR. Indeed this has proven to be the case. Thus the role of the osteocyte product, DMP1, appears as either part of the PHEX-FGF23 pathway, or at least can affect circulating FGF23 levels, perhaps independently of PHEX. These observations suggest that the osteocyte plays a central role in mineral homeostasis.
Experience with long-term follow-up is not widespread in ARHR and therapeutic response or guidelines have not been definitively established.
Rickets and/or osteomalacia has been associated with various types of tumors (87). In many cases, the metabolic disturbances improved or completely disappeared upon removal of the tumor, indicating a causal role of the tumor. Affected patients generally present with bone and muscle pain, muscle weakness, rickets/osteomalacia and occasionally recurrent fractures of long bones. Biochemistries include hypophosphatemia secondary to renal phosphate wasting and normal serum levels of calcium and 25(OH)D. Serum 1,25(OH)2D is often overtly low or is otherwise inappropriately normal in the setting of hypophosphatemia (Table 1). Aminoaciduria and/or glucosuria may be present. Radiographic abnormalities include generalized osteopenia, pseudofractures and coarsened trabeculae, as well as widened epiphyseal plates in children. The histologic appearance of trabecular bone in affected subjects most often reflects the presence of a low turnover osteomalacia. In contrast, bone biopsies from the few patients who have tumors that secrete a nonparathyroid hormone factor(s), which activates adenylate cyclase, exhibit features of enhanced bone turnover, including an increase in osteoclast and osteoblast number.
The large majority of patients with this syndrome harbor tumors of mesenchymal origin, including primitive-appearing, mixed connective tissue lesions, osteoblastomas, nonossifying fibromas and ossifying fibromas. In addition tumors of epidermal and endodermal derivation have been implicated as causal of the disease. Indeed, the observation of tumor-induced osteomalacia concurrent with breast carcinoma, prostate carcinoma oat cell carcinoma, small cell carcinoma, multiple myeloma and chronic lymphocytic leukemia supports this conclusion. In addition, the occurrence of osteomalacia in patients with widespread fibrous dysplasia of bone, neurofibromatosis and linear nevus sebaceous syndrome could be related to a similar mechanism as with the more classic mesenchymal cell tumors. Although proof of a causal relationship in these disorders has been precluded in general by an inability to surgically excise the multiplicity of lesions, in one case of fibrous dysplasia, removal of virtually all of the abnormal bone did result in appropriate biochemical and radiographic improvement.
Although this syndrome is relatively rare compared to XLH, the importance in its understanding of hypophosphatemia has been very important. The study of these tumors eventually led to the identification and isolation of FGF23 (32, 101), which has become a central factor in the entire class of disorders and represents a novel regulatory system affecting Pi homeostasis.
Regardless of the tumor cell type, the lesions at fault for the syndrome are often small, difficult to locate and present in obscure areas which include the nasopharynx, jaw, sinuses, the popliteal region and the suprapatellar area. In any case, a careful and thorough examination is necessary to document/exclude the presence of such a tumor. Indeed, CT and/or MRI scan of a clinically suspicious area should be undertaken. Recently newer imaging techniques such as octreotide scintigraphy or PET scans have been used to successfully identify tumors that remained unidentified by other means of localization. Others have suggested directing imaging to anatomic regions defined by step-ups in FGF23 concentrations from selective venous sampling.
TIO is a result of Pi wasting secondary to circulating factor(s) secreted by causal tumors. Although the leading candidate for the cause of TIO is FGF23, a variety of other factors have been considered as a potential part of the cascade that can lead to renal Pi wasting including: 1) FRP4 (frizzled related protein 4) (39), a secreted protein with phosphaturic properties, 2) FGF7, which has been identified in TIO tumors and has been shown to inhibit renal Pi transport (41), 3) the SIBLING protein, MEPE (matrix extracellular phosphglycoprotein), which has been reported to generate fragments (ASARM peptide) with potential Pi wasting capacity (38), and 4) the SIBLING protein, DMP1, which has now been implicated in ARHR, and has been shown to be in particularly high abundance in TIO tumors (32, 86, 101, 102). It is also possible that these or other tumor products may have direct effects on the mineralization function of the skeleton.
In contrast to these observations, patients with TIO secondary to hematogenous malignancy manifest abnormalities of the syndrome due to a distinctly different mechanism. In these subjects the nephropathy induced with light chain proteinuria or other immunoglobulin derivatives results in the decreased renal tubular reabsorption of phosphate characteristic of the disease. Thus, light-chain nephropathy must be considered a possible mechanism for the TIO syndrome.
The first and foremost treatment of TIO is complete resection of the tumor. However, recurrence of mesenchymal tumors, such as giant cell tumors of bone, or inability to resect completely certain malignancies, such as prostatic carcinoma, has resulted in development of alternative therapeutic intervention for the syndrome. In this regard, administration of 1,25(OH)2D alone or in combination with phosphorus supplementation has served as effective therapy for TIO. Doses of calcitriol required range from 1.5-3.0 µg/d, while those of phosphorus are 2-4 g/d. Although little information is available regarding the long-term consequences of such treatment, the high doses of medicine required raise the possibility that nephrolithiasis, nephrocalcinosis and hypercalcemia may frequently complicate the therapeutic course. Indeed, hypercalcemia secondary to parathyroid hyperfunction has been documented in at least five treated subjects. All of these patients received phosphorus as part of a combination regimen, which may have stimulated parathyroid hormone secretion and exacerbated the path to parathyroid autonomy. Thus, careful assessment of parathyroid function, serum and urinary calcium and renal function are essential to ensure safe and efficacious therapy.
The initial description of X-linked recessive hypophosphatemic rickets involved a family in which males presented with rickets or osteomalacia, hypophosphatemia, and a reduced renal threshhold for phosphate reabsorption. In contrast to patients with XLH, affected subjects exhibited hypercalciuria, elevated serum 1,25(OH)2D levels (Table 1), and proteinuria of up to 3 g/day. Patients also developed nephrolithiasis and nephrocalcinosis with progressive renal failure in early adulthood. Female carriers in the family were not hypophosphatemic and lacked any biochemical abnormalities other than hypercalciuria. Three related syndromes have been reported independently: X-linked recessive nephrolithiasis with renal failure, Dent's disease, and low-molecular-weight proteinuria with hypercalciuria and nephrocalcinosis. These syndromes differ in degree from each other, but common themes include proximal tubular reabsorptive failure, nephrolithiasis, nephrocalcinosis, progressive renal insufficiency, and, in some cases, rickets or osteomalacia. Identification of mutations in the voltage-gated chloride-channel gene CLCN5 in all four syndromes has established that they are phenotypic variants of a single disease and are not separate entities (103,104). However, the varied manifestations that may be associated with mutations in this gene, particularly the presence of hypophosphatemia and rickets/osteomalacia, underscore that environmental differences, diet, and/or modifying genetic backgrounds may influence phenotypic expression of the disease.
This rare autosomal recessive disease is marked by hypophosphatemic rickets with hypercalciuria (105). Initial symptoms of the disorder generally manifest between 6 months to 7 years of age and usually consist of bone pain and/or deformities of the lower extremities. Such deformities may include genu varum or genu valgum or anterior bowing of the femur and coxa vara. Additional disease features include short stature, and radiographic signs of rickets or osteopenia. In contrast to XLH, muscle weakness is often elicited as a presenting symptom. Affected patients may exhibit these symptoms and features of the disease in variable combination and in a mild or severe form. In contrast to other diseases in which renal phosphate transport is limited, patients with HHRH exhibit increased 1,25(OH)2D production. The resultant elevated serum calcitriol levels enhance the gastrointestinal calcium absorption, which in turn increases the filtered renal calcium load and inhibits PTH secretion. Collectively these events produce the hypercalciuria observed in affected patients (Table 1). Although initially not thought to be part of the syndrome, kidney stones have been reported in several patients.
In general, the severity of the bone mineralization defect correlates inversely with the prevailing serum Pi concentration. Relatives of patients with evident HHRH may exhibit an additional mode of disease expression (106). These subjects manifest hypercalciuria and hypophosphatemia, but the abnormalities are less marked and occur in the absence of discernible bone disease, which would suggest a mild phenotype in the heterozygous state with certain mutations.
After mutations in the candidate NPT2 gene encoding Na-Pi2a transporters, were excluded as causal to HHRH, the mutated gene in HHRH was identified as one of the lesser abundant renal tubular Na-Pi cotransporters, Na-Pi2c (30, 107). As would be predicted by the isolated loss of function of a Pi transporter, reduced serum Pi and increased renal Pi losses occur. However unlike the findings in XLH, Pi wasting does not coexist with limitations in 1,25(OH)2D production, and the system retains its capacity to increase 1,25(OH)2D levels in response to the ambient hypophosphatemia.
Patients with HHRH have been treated successfully with high-dose phosphorus (1 to 2.5 g/day in five divided doses) alone. In response to therapy, bone pain disappears and muscular strength improves substantially. Moreover, the majority of treated subjects exhibit accelerated linear growth, and radiologic signs of rickets are completely absent within several months. Despite this favorable response, limited studies indicate that such treatment does not completely heal the associated osteomalacia. Therefore, further studies are necessary to determine if phosphorus alone is truly sufficient for this disorder.