Hereditary Selective Deficiency of 1α,25(Oh)2D (MIM 264700)
Deficiency of 1α,25(OH)2D occurs as a component of several hereditary disorders (e.g., X-linked hypophosphatemia, some variants of Fanconi syndrome, and pseudohypoparathyroidism), but selective and simple (as defined earlier) deficiency of 1α,25(OH)2D is a distinctive state to be described later. This disorder (formerly called hereditary vitamin D dependency type I or hereditary pseudo-vitamin D deficiency type I) is an unusual cause of hereditary rickets.71,72 Patients appear normal at birth but have recognizable dysfunction between the ages of 2 and 24 months, suggesting lack of calciferol effect (see above) that began at the time of birth. Muscle weakness is prominent, radiographic features are striking, and responsivity to calciferols is complete (see below).
Serum shows low calcium, high PTH, but low or even undetectable 1α,25(OH)2D.73,74 The latter can be associated with normal or even modestly increased 25(OH)D [reflecting vitamin D supplementation and/or diminished clearance of 25(OH)D]. During therapy with vitamin D or 25(OH)D, serum 1α,25(OH)2D continues to be low or undetectable. During successful maintenance therapy with 1α-hydroxylated vitamin D metabolites, serum 1α,25(OH)2D is normal; random serum determinations may be hard to interpret during therapy with 1α,25(OH)2D3 because of the rapid turnover of this drug. Serum calcium and phosphate determinations in the partially treated patient also can be difficult to interpret. With partial treatment or early after discontinuation of treatment, secondary hyperparathyroidism can be associated with hypophosphatemia and a normal serum calcium level75 ; this can cause confusion with X-linked hypophosphatemia, particularly because some patients with that disorder also show secondary hyperparathyroidism.
Several sibships show features highly suggestive of autosomal recessive inheritance. No biochemical abnormalities have been recognized in obligate heterozygotes. The homozygous state is relatively common in the Saguenay region of Quebec (estimated gene frequency 0.02), and this has been attributed to a founder effect rather than to a high rate of consanguinity.76 Genetic linkage analysis in this population established a locus on the long arm of chromosome 12.77 It is of interest to note that the gene for 25(OH)D 1α-hydroxylase was mapped to chromosome 12q13.3.83a– 83c
Patients with this disorder have been treated successfully with all widely available calciferol analogues.75,78– 80 During the early phases of therapy (initial 3–6 months), they respond best to therapy with two to five times the expected long-term maintenance dose (Table 165-3) because of the high calcium requirements of the undermineralized skeleton. Long-term maintenance therapy is accomplished with any regimen that establishes normal circulating activity of metabolites that will activate vitamin D receptors. Based on the primary defect, it is obvious that physiologic replacement doses of 1α-hydroxylated metabolites are sufficient, whereas high doses of vitamin D or 25(OH)D are required to achieve remission (see Table 165-3). During successful treatment with vitamin D or 25(OH)D3, serum 25(OH)D levels are in the range of 250 ng/ml, but serum 1α,25(OH)2D may remain low or undetectable.73,74 This has several important implications. First, the degree of 1α-hydroxylase deficiency in these patients is severe; second, the indicated concentration of 25(OH)D is insufficient to be associated with normal activation of the vitamin D receptor. The second implication does not establish whether 25(OH)D is acting directly or through other metabolites [such as 5,6,-trans-25(OH)D]. During long-term maintenance therapy, the serum total 1α,25(OH)2D bioactivity is prevented by the deficiency of 1α-hydroxylase. Thus the patient must adapt to fluctuations in calcium availability through direct actions of PTH alone. Since intestinal fractional absorption of calcium cannot be regulated by endogenous mechanisms, all the external calcium balance must be regulated at the renal level. Thus these patients may show a more rapid fall of urine calcium or a rise of urine calcium at times of calcium deficiency or excess, respectively. The best way to minimize such fluctuations is to include a fixed calcium supplement (1000 mg/day as elemental calcium). Treatment must be continued indefinitely. Although relapses may be slow to develop after withdrawing treatment in an adult, relapses are inevitable and should be prevented.
Table 165-3: Daily Calciferol Doses for Maintenance Treatment of Patients with Hereditary Defects in Calciferol Metabolism |Favorite Table|Download (.pdf) Table 165-3: Daily Calciferol Doses for Maintenance Treatment of Patients with Hereditary Defects in Calciferol Metabolism
|Calciferol Analogue ||In Deficient 25(OH)D 1α-Hydroxylase, μg/day ||In Generalized Resistance to 1α,25(OH)2D, μg/day |
|D3 or D2 ||500–3000 ||500–?* |
|25(OH)D3 ||30–200 ||30–?* |
|1α,25(OH)2D3 ||0.3–2 ||5–60† |
|1α(OH)D3 ||0.5–3 ||5–60† |
|Dihydrotachysterol ||150–1000 ||2000–20,000† |
The presumption is that hereditary selective deficiency of 1α,25(OH)2D results, in all cases, from defects in 25(OH)D 1α-hydroxylase. It seems inconceivable that accelerated clearance of 1α,25(OH)2D could produce this state. Deficient activity of the decidual enzyme 25(OH)D 1α-hydroxylase in cells isolated from the placenta of two women with this disease was reported.81 It is noteworthy that it was well documented that human decidual cells do produce 1α,25(OH)2D that was regulated by feedback mechanisms.82,83 The recent availability of sequence information for the 25(OH)D 1α-hydroxylase gene enabled screening for mutations in patients with hereditary selective deficiency of 1α,25(OH)2D and their obligate carriers. Close to 20 different mutations were documented in 26 kindreds with this disease.83a– 83e All patients were homozygous for the genetic defect, whereas the obligate carriers had one copy of the mutant allele. The cellular defect does not seem to impair 25(OH)D 24-hydroxylase. Serum levels of 24,25-(OH2)D are appropriate for the level of 25(OH)D, and cultured skin fibroblasts show a normal 25(OH)D 24-hydroxylase response to 1α,25(OH)2D3 .84 These data suggested nonidentity of the 25(OH)D 1α- and 24-hydroxylase enzyme systems.
Animal Models for Hereditary Selective Deficiency of 1α,25(Oh)2D.
Ploniat85 reported an autosomal recessive rachitic disorder in pigs. More recent studies86 have shown that the animals had hypocalcemic rickets responsive to “physiologic” doses of 1α,25(OH)2D or 1α(OH)D3. A similar trait was transferred to miniature pigs for detailed study. Direct assay of renal homogenates of homozygotes from both strains of pig established undetectable 25(OH)D3 1α-hydroxylase activity.87,88 Both strains also exhibited low circulating 24,25(OH)D and undetectable renal 25(OH)D3 24-hydroxylase activity. Furthermore, renal homogenate from a rachitic pig rendered normocalcemic by 1,25(OH)2D3 treatment showed no detectable 25(OH)D 24-hydroxylase activity. This seemed different from the findings of normal 25(OH)D 24-hydroxylase in affected humans.84 There are several possible explanations for this apparent inconsistency.84
States Resembling Hereditary Selective Deficiency of 1α,25(Oh)2D.
In several hereditary or acquired disorders, 25(OH)D 1α-hydroxylase deficiency is one component of a more complex disturbance. These disorders affecting the proximal renal tubule include X-linked hypophosphatemia89,90 (see Chap. 197), renal tubular acidosis91,92 (see Chap. 195), some forms of Fanconi syndrome93,94 (see Chap. 196), and tumor-associated osteomalacia95 (in which a humoral factor seems to cause impairment of 1α-hydroxylase and renal wasting of phosphate; see Chap. 197). Replacement of 1α,25(OH)2D is often an important component in therapy of these disorders.
Hereditary Generalized Resistance to 1α,25(Oh)2D (MIM 277440)
Hereditary generalized resistance to 1α,25(OH)2D (also called vitamin D dependency type II or pseudo-vitamin D deficiency type II) was first recognized in 1977 and is a rare disorder. There are fewer than 50 known kindreds (a partial list in refs.9 and 96– 132 and personal communications). The clinical features are almost identical to those in hereditary selective deficiency of 1α,25(OH)2D with the exception that hereditary generalized resistance to 1,25(OH)2D has been associated with alopecia in about half the kindreds. Patients with hereditary generalized resistance to 1α,25(OH)2D appear normal at birth but develop the clinical and biochemical features of calciferol deficiency (see above) with hypocalcemia and rickets over the first 2 to 8 months of life. In many cases, hair loss occurs between the ages of 2 and 12 months. The hair loss may be complete (Fig. 165-5) or incomplete; sometimes there is selective sparing of the eyelashes. Light microscopic examination of a scalp biopsy showed normal numbers and morphology of hair follicles in a patient with total alopecia.98 Alopecia occurs in patients with the most severe resistance to 1α,25(OH)2D (see below).133 Without therapy, this disorder leads to inanition, severe skeletal deformity, recurrent respiratory infections, and death. Although therapy with calciferols can sustain complete biochemical remission in some patients, the alopecia does not improve. Other ectodermal defects have been reported in small numbers of patients and have an uncertain relation to the syndrome; these include oligodentia98 and papular skin rash.98,134 All patients suffer from the consequences of intestinal malabsorption of calcium. Attempts to show vitamin D receptor–mediated dysfunctions outside the intenstine in vivo have so far been inconclusive. Basal and stimulated concentrations of insulin, thyrotropin, prolactin, growth hormone, and testosterone have been normal (aside from deficiencies in insulin stimulation attributable to hypocalcemia).135 Bone biopsies have shown normal or increased numbers of osteoclasts (suggesting that the vitamin D receptor is not essential for osteoclast formation), but their resportive activity was suggested to be impaired.136
Sisters (aged 7 and 3) with alopecia and rickets from hereditary generalized resistance to 1α,25(OH)2D. (Used with permission from Rosen et al. 97)
In several cases, early postnatal development was apparently normal, and dysfunction was not evident until late in childhood101 or even in adulthood99 [in the latter case, serum 1α,25(OH)2D was not measured, so alternate etiologies such as noncompliance were not excluded]. These patients did not show alopecia and responded to high doses of calciferols, indicating a mild variant of the syndrome. None showed clear features of a genetic etiology (that is to say, there was no parental consanguinity and no affected siblings; unfortunately, cultured cells were not evaluated in any).
Several patients have shown unexplained fluctuations in disease severity. Two patients without any clear calcemic response to calciferols experienced lessening of secondary hyperparathyroidism and improved bone mineralization around the ages of 7 to 9. One patient showed a prolonged remission of biochemical and radiographic abnormalities that subsequently seemed completely unresponsive to much higher doses of calciferols.104 Another patient showed amelioration of resistance to 1α,25(OH)2D following a brief trial of 24,25(OH)2D.98
Measurement of calciferol metabolites in plasma usually provides useful information for diagnosis. Serum concentrations of 1α,25(OH)2D are 50 to 1000 pg/ml (normal in children is 30–100 pg/ml) before treatment. During treatment with calciferols, typical concentrations are 200 to 10,000 pg/ml133 (Fig. 165-6).
Relations between serum concentrations of calcium and 1α,25(OH)2D in patients with generalized resistance to 1α,25(OH)2D. Stippled area is normal range for calcium. Solid curve is theoretical normal relation between calcium and 1α,25(OH)2D. Each symbol depicts data from one patient with generalized resistance to 1α,25(OH)2D: (A) Without calciferol therapy. (B) During calciferol therapy. Solid circle, hair normal; open circle, alopecia. (Used with permission from Marx et al. 133)
There usually have been strong suggestions of autosomal recessive transmission (parental consanguinity, etc.). Most patients have been recognized in a broad region centered on the Mediterranean shores, and this may relate to a high consanguinity rate in the receptor-disease source population.137 No clinical abnormalities have been reported in obligate heterozygotes.
Some patients show complete remission while receiving high doses of calciferols109 (see Table 165-3). The presence or absence of alopecia is one simple predictor of potential for response to therapy.133 Virtually all patients with normal hair can sustain remission when given high doses of analogues not requiring 1α-hydroxylation. Among patients with alopecia, approximately half have not responded to the highest doses of calciferols available; half have shown satisfactory calcemic response, but the dose requirement [deduced from serum 1,25(OH)2D during therapy] is typically tenfold higher than in patients with normal hair (see Fig. 165-6). Maintenance treatment is based on four considerations: (1) the most mildly affected patients can be treated with calciferols [vitamin D3, vitamin D2, 25(OH)D3] that provide substrate for a high renal secretion of 1α,25(OH)2D, (2) more severely affected patients may respond only to extremely high doses of analogues [1α,25(OH)2D, 1α-(OH)D3, dihydrotachysterol] that do not require 1α-hydroxylation, (3) some patients may not respond to maximal doses of any calciferols, and (4) the role of calcium supplements is different in each of the prior three therapy categories (see below).
Several patients have shown remissions while receiving high doses of vitamin D2 or 25(OH)D3 9,96 ; they respond to this because their tissue resistance is only moderate, and they can produce sufficient 1α,25(OH)2D endogenously if presented with a high level of substrate for 1α-hydroxylation. It is uncertain if this mechanism of therapy requires pathologic elevations of PTH; it seems possible that near-normal parathyroid function may be sufficient because of deficient feedback suppression of 1α-hydroxylation (Fig. 165-7) (resulting from the defect of vitamin D receptors in the proximal renal tubule). In this group, calcium supplements may have little or no role, since serum concentrations of both PTH and 1α,25(OH)2D can compensate for fluctuations in calcium availability. High levels of 1α,25(OH)2D in an affected female can permit a normal pregnancy138 ; this is important because of concern that a high level of 1α,25(OH)2D may disturb fetal tissues (see “Calciferol Excess States,” below).
Abnormal regulation of 1α,25(OH)2D in serum of a patient with hereditary generalized resistance to 1α,25(OH)2D. Serum calcium, phosphate, and PTH had been normalized by treatment with 1α(OH)D3. 1α(OH)D3 was stopped for 2 days, and the patient received 25(OH)D3 as sole therapy. Stippled zones indicate normal ranges. Note logarithmic scales for calciferols. Serum 1α,25(OH)2D reaches extremely high concentrations without the usual stimuli (low calcium, high PTH, low phosphate) for its production. PHOSPH, phosphorus. (Used with permission from Marx et al. 109)
Patients unable to produce sufficient 1α,25(OH)2D endogenously [because of a requirement for particularly high 1α,25(OH)2D concentrations] may still respond to extraordinarily high doses of analogues not requiring 1α-hydroxylation [i.e., 1α,25(OH)2D3 , 1α-(OH)D3, dihydrotachysterol]. Patients in this group requiring therapy that bypasses 1α-hydroxylation should receive fixed calcium supplements (1000 mg/day elemental calcium) for the same reasons as patients with hereditary selective deficiency of 1α,25(OH)2D [see “Hereditary Selective Deficiency of 1α,25(OH)2D,” above].
Some patients may have little or no response to maximal calciferol doses. But it is difficult to identify these patients. There are no widely available indexes of intestinal responsivity to calciferols; fractional calcium absorption is the most relevant index, but this test may be unavailable because of the inconvenience of balance studies or because of radiation exposure from calcium-47. It is possible to measure this parameter with stable calcium isotopes.139 Most studies have relied on the calcemic response to a therapeutic trial (Fig. 165-8). These trials may be unsatisfactory, first, because of the time required for repletion of undermineralized bones (i.e., as long as the bones retain the increased calcium input, this will not result in normalization of serum calcium). Second, therapeutic trials with high calciferol doses are limited by available drug formulations and drug cost. Patients with an undetectable response to calciferols can receive substantial benefit if large amounts of calcium can be delivered to the bloodstream. The most rapid way to accomplish this is by intravenous infusions136,139– 141 ; high calcium doses (1000 mg elemental calcium per day infused over 12 h) can be tolerated even by young children with this disorder. Since normal positive calcium balance during childhood growth is approximately 300 mg/day, and since the total deficit may be several hundred grams of elemental calcium, such infusions must be given repeatedly over many months to accomplish significant results. This form of therapy requires methods similar to those used in hyperalimentation programs. Another way to increase calcium input to the bloodstream is to increase net absorption independent of calciferols. This can be accomplished by increasing calcium intake to the point of intolerance.134 Unfortunately, the upper limit of oral intake is around 6000 mg/day and requires great cooperation; in the absence of calciferol bioeffect, the fractional calcium absorption (virtually all of which is retained) is approximately 10 percent, implying that up to 600 mg/day can be delivered to the bloodstream by oral supplements. With high doses of calcium intravenously or orally, the serum phosphate level may decrease further; oral phosphate supplements should then be added. The utility of therapy with intravenous or oral calcium confirms the centrality of the intestine as a target tissue for 1α,25(OH)2D.
Absent calcemic response during a long therapeutic trial with calciferols. Calciferol therapy is in upper panel: 1α(OH) refers to 1α(OH)D3. Stippled zones indicate normal ranges. Not only is hypocalcemia persistent, but also secondary hyperparathyroidism persists (high PTH and low phosphate), and very high serum levels of 1α,25(OH)2D are documented. PHOSPH, phosphorus. (Used with permission from Marx et al. 109)
Detailed studies of the interactions of calciferol dose with calcium dose have not been done. However, it seems possible that some patients (e.g., those with 90 percent deficiency in hormone-binding capacity104,109) would have a diminished maximal response to calciferols; such patients with partial responses may obtain unique benefits from combinations of calciferols and calcium, both at high doses.
As in the calciferol deficiency states, total-body calcium requirements are highest at the onset of treatment. Thus the doses of calcium, the doses of calciferols, and even the type of approach judged necessary to initiate therapy (e.g., intravenous calcium) may not prove the same as those used for maintenance therapy.
Cells from patients with hereditary generalized resistance to 1α,25(OH)2D have been used to characterize the defect presumed to be present in all target tissues. Because of the widespread expression of the 1α,25(OH)2D effector system in many tissues, these studies have been possible with skin fibroblasts in most studies, keratinocytes,105 bone cells,142 peripheral lymphocytes,144,145 and virally transformed lymphocytes.146 These cells were used to assess most of the steps in 1,25(OH)2D action from cellular uptake to bioresponse and to elucidate the molecular aberrations in the hormone-receptor protein and the nuclear DNA that encodes for it.116– 121,123– 132 The latter became feasible with cloning and sequencing of the human VDR chromosomal gene.
Several methods have been used to characterize the hormone-receptor interaction, including binding capacity and affinity of [3H]1,25(OH)2D3 to intact cells, nuclei, or high-salt-soluble extract cytosol,102,103,106 measurements of receptor content by monoclonal antibodies,117,119,124 and characterization of the hormone-receptor complex on continuous sucrose gradient and heterologous DNA-cellulose columns.111,112,117
Based on the hormone-receptor-nuclear interaction, three different classes of intracellular defects have been identified in these cells, as presented below:
Hormone binding defects
No (negative) hormone binding. Unmeasurable specific binding of [3H]1,25(OH)2D3 to either high-salt-soluble cell extract and/or intact cells or nuclei106,107 is the most common abnormality observed. In the majority of these patients, high concentrations of 1,25(OH)2D in serum or culture medium did not evoke a biologic or biochemical response in vivo or in vitro (see below).
Decreased maximal capacity of hormone binding. This abnormality has been reported in only one kindred.104 Cell extracts showed a hormone-binding capacity only 10 percent of normal but a hormone-binding affinity that was normal. The patient did not respond to prolonged treatment with high doses of active vitamin D metabolites.
Decreased affinity of hormone binding. A selective abnormality in hormone-binding affinity with normal hormone-binding capacity has been suggested but not proven conclusively in one kindred.106 An additional patient, recently described, had a modest decrease of the affinity of the receptor for 1,25(OH)2D3 when measured at 0°C.130 A complete remission of the disease in all these patients could be achieved with high doses of active calciferol metabolites.106,130
Defective receptor translocation to nucleus. Extracts of cells from two kindreds have shown normal capacity and affinity of hormone binding; however, high-affinity uptake of hormone into the nucleus of intact cells was undetectable.102,106 The receptors from both kindreds showed a normal affinity for nonspecific DNA.112 Analysis by immunocytology showed that vitamin D receptors from these patients accumulated along the nuclear membrane after addition of 1α,25(OH)2D3 , unlike normal receptors, which rapidly translocated from cytoplasm into nucleus.147 Recently, two additional unrelated patients were described with a lowered 1,25(OH)2D3 retention in intact cells incubated at 37°C.129 All these patients were treated successfully with high doses of vitamin D and its active metabolites.96,102,129
Defective receptor binding to DNA. With cells from several kindreds the 1α,25(OH)2D receptor showed abnormal elution from nonspecific DNA111,112,117 ; in each the receptor eluted from the DNA at lower salt concentration than normal (Fig. 165-9). No biologic response to high doses of vitamin D or its active metabolites either in vivo or in vitro (see below) was documented in almost all patients with this type of defect.98,111,112,117
Elution of vitamin D receptor from a column of DNA-cellulose. Shaded area shows location of elution peak for normal receptors. Solid circle, elution profile for receptors from patient with hereditary resistance to 1α,25(OH)2D and receptors with a DNA-binding defect. (Used with permission from Liberman et al.112 )
To summarize, no patient has met the strict criteria for a possible prereceptor or postreceptor defect. Wherever detailed testing of vitamin D receptor properties has been done, abnormalities have been found. The class of cellular defect has shown no correlation with clinical features. Rather, these patients seem to fit along one continuous spectrum of severity of disease.
1α,25(OH)2D3 Bioeffect in Patients' Cells.
Several assays have tested posttranscriptional actions of 1α,25(OH)2D3 in cells from these patients. In all assays, each patient's cells have shown severely deficient responses. The most extensively tested response is 1,25(OH)2D3 induction of 25(OH)D3 24-hydroxylase activity in cultured skin fibroblasts.148,149 In general, patients with milder disease (normal hair, calcemic response to high doses of calciferols) show inducible 24-hydroxylase with supraphysiologic concentrations of 1α,25(OH)2D3 (Fig. 165-10), but patients with the severest disease (alopecia, no calcemic response to maximal doses of calciferols) show no 24-hydroxylase response to maximal concentrations of 1α,25(OH)2D3. Five of six obligate heterozygotes showed no abnormality107 ; the sixth showed a 50 percent decrease in hormone-binding capacity and a similar decrease in maximal induction of 24-hydroxylase. Similar severe defects have been identified with other bioassays or different cells, including inhibition of cell proliferation by 1α,25(OH)2D (cultured skin fibroblasts, keratinocytes,105 peripheral mononuclear cells144,145), 1,25(OH)2D3-mediated stimulation of 24-hydroxylase and of osteocalcin secretion in osteoblast-like bone cells,142 and rapid stimulation of cGMP accumulation by 1,25(OH)2D3 in fibroblasts.39
25(OH)D3 24-hydroxylase in skin fibroblasts preincubated with indicated concentrations of 1α,25(OH)2D3 . (A) Normal subjects. (B) Patients with hereditary generalized resistance to 1α,25(OH)2D. Patients 1A and 2B each showed a satisfactory calcemic response to high doses of 1α,25(OH)2D3, but patients 3 and 7 showed no calcemic response. Shaded area in B indicates normal response. (Modified with permission from Gamblin et al. 149)
If the predictive therapeutic value of the in vitro bioresponse to 1,25(OH)2D3 could be substantiated, it may eliminate the need for expensive and time-consuming therapeutic trials with high doses of vitamin D and its metabolites (as discussed before).
Studies on the molecular defects have used isolation, amplification, and sequencing of genomic VDR DNA, as well as cloning and sequencing of VDR cDNA, recreation of the mutant VDR in vitro, and testing transcriptional activity of patients and artificially created mutant VDR. Mutations in the VDR gene have been identified in almost every patient with this disease who was investigated (Fig. 165-11). Four of these genetic alterations resulted in a nonsense change that introduced a stop condon predicting a truncated VDR that lacks hormone-binding or both hormone- and DNA-binding domains.124,131,132
Schematic presentation of the homozygous mutation in the vitamin D receptor in patients with hereditary generalized resistance to 1α,25(OH)3D. The asterisks depict sites of amino acid substitutions due to point mutations and codon changes using the numbering system of Baker et al.143
Recently, two unrelated patients were described in whom two different point mutations led to a frameshift in translation resulting in a premature stop codon that would produce a VDR lacking both DNA- and hormone-binding sites in one patient and the ligand-binding site in the second.128,131
Missense VDR mutations were documented in cells derived from almost all other patients or kindreds examined. The functional characterization of the patient's VDR reflected the localization of the point mutation: (1) mutations localized to the DNA-binding, N-terminal, and zinc-finger region111,118,120,123,125– 127 (these mutations typically are found in residues conserved across the entire receptor family, and most lie within α-helices on the C-terminal side of the zinc fingers, which are involved in DNA base recognition and phosphate backbone interaction, respectively), (2) mutations localized to the C-terminal hormone-binding domain confirming defects in ligand binding,119– 121 and (3) mutations in a subregion of the C-terminal domain that affect heterodimerization of the VDR and RXR.129– 131 In two of these patients, some impairment of hormone binding to whole cells was observed at 37°C with no abnormality in 1α,25(OH)2D3 high-affinity binding to cytosol at 4°C (the so-called functional defect in receptor translocation to nucleus, described before). These receptors do not bind RXRs with normal affinity, and in cotransfection assays with normal exogenous RXR, the mutant VDR could be rescued, although it required higher concentrations of 1α,25(OH)2D3. This showed that, as expected, the hormone-binding and heterodimerization functions of VDR are not entirely separable.
In every kindred in which more than one patient was examined, the same genetic defects were identified. Obligatory heterozygotes were found to have one normal and one mutant allele. The same mutation was observed in some unrelated patients.
Animal Models for Hereditary Generalized Resistance to 1α,25(Oh)2D.
A state resembling hereditary generalized resistance to 1α,25(OH)2D is present in New World primates (marmosets and tamarins). Osteomalacia sometimes develops in these animals in captivity, and they are known to have high nutritional requirements for calciferols.150 New World primates have high circulating concentrations of 1α,25(OH)2D.151 Intestinal and other cells form these animals have shown deficient calcitriol-binding capacity (in comparison with cells from Old World primates)152,153 and deficient calcitriol-binding affinity.154 These New World primates also exhibit hereditary generalized resistance to the true steroid hormones, including glucocorticoids, estrogens, and progestogens.155 Thus their special metabolic features appear to involve elements shared by many of the nuclear-active steroids and secosteroids. Extracts from New World primate cells are capable of inhibiting 1α,25(OH)2D binding to Old World primate vitamin D receptors.156 Nuclear extracts from β-lymphoblastoid cell lines of New World primates contain a VDRE-binding protein(s) that is capable of inhibiting normal VDR-RXR heterodimer binding to the VDRE.157
Recently, VDR knockout mice have been created by targeted ablation of the first zinc finger158 or the second zinc finger.159 Only the homozygote mice were affected. Although phenotypically normal at birth, they become hypocalcemic and develop secondary hyperparathyroidism, rickets, osteomalacia and progressive alopecia. The mice with ablation of the first zinc finger are infertile and die within 15 weeks after birth, whereas the mice with ablation of the second zinc finger survive at least until 6 months.
States Resembling Hereditary Generalized Resistance to 1α,25(Oh)2D.
There are multiple causes of rickets or osteomalacia in which calciferol metabolism is normal or is abnormal only as an appropriate response to a primary disturbance in mineral flux (see Fig. 165-4). These include hereditary and acquired causes. Rickets or osteomalacia with high circulating 1α,25(OH)2D is found in generalized resistance to 1α,25(OH)2D and in two additional states (calcium deficiency and phosphate deficiency).
Severe deficiency of calcium has been recognized as a common dysfunction in Bantu adolescents, who consume a diet severely deficient in calcium.160 Of course, calcium repletion cures all abnormalities. Osteopetrosis (marble-bones disease) results from a spectrum of defects in osteoclast function. Both in humans and in animal models of this disease, serum 1α,25(OH)2D is increased, and subtle histologic changes of osteomalacia have been noted.161 At least one patient has been treated with a low-calcium diet plus high doses of 1α,25(OH)2D with apparent improvement in osteoclast function.162 Other patients with osteopetrosis have not responded to similar treatment, and at least one cellular defect (carbonic anhydrase II deficiency) unlikely to be overcome by 1α,25(OH)2D has been discovered.162
Severe deficiency of phosphate also can cause rickets with high serum levels of 1α,25(OH)2D. In hereditary hypophosphatemic rickets with hypercalciuria, the primary renal loss of phosphate causes osteomalacia and activation of the renal 1α-hydroxylase.163 High 1α,25(OH)2D results in absorptive hypercalciuria; therefore, parathyroid function is suppressed in this condition, unlike in hereditary generalized resistance to 1α,25(OH)2D.
Survivors of extreme prematurity can pass through a phase when their growing bones (deprived of the placental pump) are severely deficient in both calcium and phosphate, resulting in neonatal rickets with high serum 1α,25(OH)2D.164,165 In this group, immaturity of the intestinal response to 1α,25(OH)2D may contribute to the disturbance.166
Deficient Bone Mineralization with Normal Calcium and Phosphate in Serum.
There are several causes of deficient bone mineralization with otherwise normal calcium and phosphate fluxes. These include hypophosphatasia,62 the chondrodystrophies (which can disturb epiphyseal function), and skeletal accumulation of aluminum, bisphosphonates, or fluoride.64