Absence of renal 25-hydroxycholecalciferol-1-hydroxylase activity in a pig strain with vitamin D-dependent rickets

An outbreak of rickets in Corriedale sheep: Evidence for a genetic aetiology

CASE HISTORY

A skeletal disease characterised by lameness, limb deformities and reduced growth rate occurred over two successive years in lambs born on a commercial sheep farm in Marlborough. A genetic aetiology was considered likely following exclusion of other known causes of rickets and because of the progressive nature of the disease, even after affected animals were transferred to another property.

CLINICAL FINDINGS

Affected lambs appeared normal at birth but developed clinical signs during the first 2 months of life. The most severely affected animals either died or were euthanised within the first year of life, but some survived to breeding age. Serum biochemistry revealed hypocalcaemia, hypophosphataemia and increased concentrations of 1,25 dihydroxyvitamin D. The mean serum 25 hydroxyvitamin D concentration was similar to that of control lambs.

PATHOLOGICAL FINDINGS

Gross lesions included enlarged costochondral junctions, bilateral irregularity of articular surfaces on humeral heads due to collapse of subchondral bone, thickened cortices in long bones and irregular thickening of physeal cartilages. Microscopically, tongues of hypertrophic chondrocytes extended from physes into metaphyseal regions; metaphyseal trabeculae were thick, disorganised and often lined by wide osteoid seams. Osteoclastic activity was excessive both in cortical and trabecular bone.

DIAGNOSIS

Inherited rickets in Corriedale sheep. CLINICAL RELEVANCE AND CONCLUSIONS: This disease is likely to be present in several Corriedale sheep flocks in New Zealand and may have been misdiagnosed as arthritis or other diseases causing lameness and/or poor growth. A defect in end-organ responsiveness to 1,25 dihydroxyvitamin D is the likely mechanism. This disease of sheep may be a useful model for studying vitamin D metabolism and the treatment of inherited forms of rickets in human beings.

Molecular basis for pseudo vitamin D-deficiency rickets in the Hannover pig

The molecular basis for pseudo vitamin D deficiency rickets (PDDR) in the Hannover pig model was determined in the current study. Consistent with the inability of Hannover PDDR pigs to maintain ambient levels of 1,25-dihydroxyvitamin D (i.e., 1,25D), the bioactivation enzyme cytochrome P450C1 (or CYP27B1) was determined to contain coding-region deletions that rendered the enzyme ineffective due to frame-shift mutations and expression of a premature termination codon. Expression levels of P450C1mRNA were up-regulated in response to the low-1,25D high-parathyroid hormone state of the PDDR animals. In a complementary manner, cytochrome P450C24 mRNA was not detectable in PDDR pigs. Two different deletions were detected within the Hannover pig strain in which the P450C1 coding region contained either 173 bp or 329 bp deletions that resulted in the expression of non-sense products beginning within the I-helix region and extending through the truncated C-terminal domains. The boundaries for the deletion segments aligned with derived mRNA processing sites. This observation was consistent with an mRNA processing error as the causative factor for the coding-region deletions. Based upon the expression of a non-functional P450C1 enzyme, the Hannover pig model for PDDR was determined to be identical to the human disease in which enzyme-inhibitory mutations are the molecular basis for the calcium disorder.

Renal and hepatic 1 alpha-hydroxylation of 25-hydroxyvitamin D3 in piglets suffering from pseudo vitamin D-deficiency rickets, type I

The piglets examined suffer from rickets and have symptoms similar to those of classic pseudo vitamin D-deficiency rickets, type I (PVDRI), including plasma concentrations of 1 alpha, 25-dihydroxyvitamin D3 considerably lower than in healthy control piglets. It has been suggested that the rachitic piglets have a defective renal 1 alpha,25-dihydroxyvitamin D3 production. The present study shows that partially purified mitochondrial and microsomal cytochrome P450 from kidney and liver of both rachitic and control animals is able to catalyze 1 alpha-hydroxylation of 25-hydroxyvitamin D3. The renal mitochondrial 1 alpha-hydroxylase activity was higher in the rachitic piglets whereas the renal microsomal 1 alpha-hydroxylase activity was decreased. The immunodetectable levels in kidney of a mitochondrial 1 alpha-hydroxylase (CYP27) and a microsomal 1 alpha-hydroxylase (vitamin D3 25-hydroxylase) were correlated with the 1 alpha-hydroxylase activities. The results suggest that the renal microsomal 1 alpha-hydroxylase is affected by the rachitic condition. It is concluded that the primary genetic defect of systemic 1 alpha,25-dihydroxyvitamin D3 deficiency in the rachitic PVDRI piglets does not reside in a defective function or absence of renal mitochondrial 25-hydroxyvitamin D3 1 alpha-hydroxylase. From this, it may also be concluded that PVDRI in man and pig appear to be two different forms of the disease.

Osteomalacia

Osteomalacia is a generalized bone disorder characterized by impairment of mineralization, leading to accumulation of unmineralized matrix or osteoid in the skeleton. The classical clinical features of osteomalacia include musculoskeletal pain, skeletal deformity, muscle weakness and symptomatic hypocalcaemia. In childhood the features of osteomalacia are accompanied by rickets, with widening of the epiphyses and impaired skeletal growth. The major cause of osteomalacia is vitamin D deficiency, which is most often due to reduced cutaneous production of vitamin D in housebound elderly people, immigrants to Northern countries and women who adopt strict dress codes which prohibit exposure of uncovered skin. Vitamin D deficiency osteomalacia may also occur with malabsorption, liver disease and anticonvulsant therapy. Less commonly, osteomalacia may result from abnormal vitamin D metabolism, resistance to the action of vitamin D, hypophosphataemia or toxic effects on osteoblast function.

Cloning of the human 1 alpha,25-dihydroxyvitamin D-3 24-hydroxylase gene promoter and identification of two vitamin D-responsive elements

A genomic DNA clone for 1 alpha,25-dihydroxyvitamin D-3 (1,25-(OH)2D3) 24-hydroxylase was isolated from a human chromosome 20 library. It spans 2.42 kb, containing the first two exons, the first and part of the second introns, and a 1.26 kb 5'-flanking region. Putative transcription cis-elements were revealed throughout the 5'-flanking region, including TATA box, CAAT box, GC boxes, vitamin D-responsive elements (VDRE), AP1, and AP2 sites. In a CAT reporter gene expression assay, the 24-hydroxylase promoter with its 1.2 kb 5'-flanking sequence elicits a 1,25-(OH)2D3-induced transactivation activity. Gel mobility shift assays of those putative DREs have identified that two different elements can form specific complexes with porcine intestinal nuclear extract (PINE). The specificity of VDRE-PINE complexes was verified by supershift assay with VDR-specific monoclonal antibody VXIE10B6. The proximal element VDREp (-172/-143) consists of three direct repeat half-sites, GAGTCAgcgAGGTGAgcgAGGGCG, in anti-sense orientation. The distal element VDREd (-293/-273) consists of two direct repeat half-sites, GCGTTCaccGGGTGT, also in anti-sense orientation. Both VDREs can direct a reporter gene expression using a heterologous herpes simplex virus thymidine kinase (TK) promoter in a 1,25-(OH)2D3-dependent fashion. Further characterization of these VDREs in various constructs with either a native or TK promoter suggests that both VDREs are required for the optimal induction of 24-hydroxylase expression by 1,25-(OH)2D3.

Hormone-nuclear receptor interactions in health and disease. Vitamin D resistance

Tissue resistance to vitamin D, or vitamin D-dependent rickets (VDDR), can be classified as two separate conditions--VDDR type I and VDDR type II--both of which present with the classical clinical, radiological and biochemical features of rickets despite adequate vitamin D intake. VDDR II can also be associated with alopecia, for reasons that are not clear. The two syndromes result from distinct disorders of vitamin D metabolism or action. Both are inherited in an autosomal recessive fashion. VDDR I is caused by decreased production of the active form of vitamin D, 1,25-dihydroxycholecalciferol, with the proposed defect being in the gene encoding the enzyme 1 alpha-hydroxylase. VDDR II results from mutations in the gene for the intracellular receptor for 1,25-dihydroxycholecalciferol (vitamin D receptor), resulting in changes in hormone or DNA binding, depending on the mutation. These mutations are analogous to those affecting receptors for other steroid-thyroid hormones, which have also been shown to cause resistance to hormone action.

Evidence for vitamin D-independent active calcium absorption in newborn piglets

The role of 1,25-dihydroxycholecalciferol (calcitriol) for intestinal calcium (Ca2+) absorption was studied in newborn (< 1 week old) and weaned piglets (> 6 weeks old). In both groups, normal piglets and piglets suffering from inherited pseudo vitamin D-deficiency rickets, type I (PVDRI) were used. In this inherited disorder, renal production of calcitriol is absent. Plasma samples were assayed for calcitriol and total Ca, and dissociation constants (Kd) and maximum binding capacities (Bmax) of intestinal calcitriol receptors were determined under equilibrium conditions at 4 degrees C. Unidirectional Ca(2+)-flux rates were measured across stripped duodenal mucosae in Ussing chambers in the absence of electrochemical gradients. The plasma calcitriol concentrations of neonatal (26.5 +/- 7.1 pg/ml, n = 11; mean +/- SEM) and weaned PVDRI piglets (18.8 +/- 5.7 pg/ml, n = 8) were unphysiologically low and differed significantly from control animals (83.6 +/- 14.8 pg/ml, n = 8, and 86.9 +/- 9.6 pg/ml, n = 11, respectively). However, newborn PVDRI piglets had normal plasma Ca levels at least during the first days of life. They became hypocalcemic and developed clinical symptoms of rickets during the following weeks. In newborn PVDRI and control piglets, Bmax was significantly lower (84 +/- 28 fmol/mg protein and 127 +/- 55 fmol/mg protein, n = 9, respectively) than in weaned piglets (741 +/- 82 fmol/mg protein, n = 9, and 778 +/- 121 fmol/mg protein, n = 8, respectively). Significant net Ca(2+)-fluxes were found in both newborn PVDRI and control piglets (88.8 +/- 25.1 nmol.cm-2 x h-1, n = 6, and 86.5 +/- 10.5 nmol.cm-2 x h-1, n = 9, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of hypocalcemia/hypophosphatemia on porcine bone and dental hard tissues in an inherited form of type 1 pseudo-vitamin D deficiency rickets

The effects of Vitamin D deficiency rickets on the formation of mineralized dental tissues were studied in a breed of pigs which had moderate and marked hypocalcemia because of an inherited defect in the renal production of the biologically-active Vitamin D metabolites. Affected piglets developed classical symptoms of rickets which were fatal unless Vitamin D supplements were given. The dissected mandibles of homozygous (rickets) and heterozygous (normal) pigs were photographed and radiographed. Compared with those of normal pigs, the mandibles of homozygous pigs demonstrated slowed development/eruption of permanent teeth, under-mineralized bone, underdeveloped dentin (enlarged pulp chambers), interglobular dentin, and enamel hypoplasia. Enamel defects in rachitic pig teeth were difficult to observe radiographically, but could be detected visually and by SDS-PAGE analysis of the enamel protein components of developing and maturing enamel. There was significant retention of amelogenins in the enamel maturation zones of developing molars taken from rachitic pigs, but there was no obvious difference in the patterns of amelogenin processing.

Quantification of Ca(2+)-ATPases in porcine duodenum. Effects of 1,25(OH)2D3 deficiency

Previous studies have identified a calmodulin-stimulated ATP-dependent Ca2+ pump as the major Ca2+ efflux pathway in enterocytes. Here, we developed methods to quantify the number of Ca2+ pumps in basolateral and intracellular membranes from porcine duodenum. By the use of a pig strain with a genetic defect in renal 1 alpha-hydroxylase, we were able to investigate the influence of 1,25(OH)2D3-deficiency on the number of Ca(2+)-ATPases in porcine duodenum. The amount of Ca(2+)-ATPase in isolated basolateral membranes was 5.5 +/- 0.7 micrograms/mg protein, while the Vmax of ATP-dependent Ca2+ transport into inside-out resealed basolateral membrane vesicles was 2.6 +/- 0.4 nmol/mg protein per min. From these data we estimated roughly about 95 x 10(3) plasma membrane Ca2+ pump sites per enterocyte. In addition, the amount of intracellular Ca(2+)-ATPase in microsomal fractions was 0.41 +/- 0.02 microgram/mg protein. Comparison of these parameters between control and rachitic animals showed that Ca2+ pump capacities in both basolateral membranes and microsomal fractions of porcine duodenum are not influenced by 1,25(OH)2D3-deficiency. In conclusion, stimulatory effects of 1,25(OH)2D3 on intestinal Ca2+ transport most likely result from specific effects on apical influx and facilitation of cytosolic Ca2+ diffusion by Ca(2+)-binding proteins and not from an increase in Ca2+ pumping capacity in basolateral membranes.

Effects of calcitriol on stimulation of ion transport in pig jejunal mucosa

1. Active sodium (Na+) and chloride (Cl-) fluxes were studied in vitro in Ussing-type chambers with stripped jejunal mucosa of piglets which suffered from pseudo-vitamin D deficiency rickets, type I. The piglets are devoid of renal calcitriol (1,25-dihydroxy vitamin D3) production and have only small amounts of calbindin in their jejunal enterocytes. 2. In the presence of 0.01 mM-indomethacin non-stimulated short-circuit current (Isc), transepithelial potential difference (PD), tissue conductance (Gt) and unidirectional Na+ (JNa) and Cl- fluxes (JCl) were not affected by the low calcitriol (LC) concentration in plasma. 3. Adding 10 mM-theophylline to the serosal solution in the presence of 0.01 mM-indomethacin caused significantly greater increases in Isc in LC mucosa than in mucosa of vitamin D3-treated and control piglets with normal calcitriol (NC) concentrations in plasma. Omission of indomethacin significantly increased Isc stimulation provoked by theophylline with LC and NC mucosa. The increase, however, was significantly greater with LC than with NC mucosa. 4. Omission of calcium (Ca2+) from the serosal bathing solution significantly depressed Isc stimulation by 10 mM-theophylline in indomethacin-treated LC and NC mucosa. But depression was greater with LC than with NC mucosa. 5. Blocking Ca2+ entry into the cytosol by adding either 0.1 mM-TMB-8 or 0.5 mM-d,l-verapamil to the serosal bathing solution abolished the difference in Isc response to theophylline between indomethacin-treated LC and NC mucosa due to greater depression of Isc in LC than in NC mucosa. 6. The combined effects of theophylline and A23187 on Isc stimulation were calcitriol dependent. In the presence of indomethacin this dependence was only significant when A23187 was given prior to theophylline. In the absence of indomethacin the combined effects of A23187 and theophylline on Isc were always significantly greater in LC than in NC mucosa, irrespective of the order of adding the two agents. 7. Addition of theophylline stimulated net Na+ and Cl- secretion in indomethacin-treated LC and NC mucosa. The increases of net Na+ and Cl- fluxes fully accounted for the rise of Isc with NC mucosa but accounted only partly for the increase in Isc with LC mucosa. This resulted in significant increase in theophylline-stimulated residual ion flux (JR) in LC mucosa which probably resulted from enhanced secretion of bicarbonate.(ABSTRACT TRUNCATED AT 400 WORDS)

Binding properties of plasma vitamin D-binding protein and intestinal 1,25-dihydroxyvitamin D3 receptor in piglets with pseudo-vitamin D-deficiency rickets, type I: treatment effects with pharmacological doses of vitamin D3

The effective treatment of the rachitic symptoms of pseudo-vitamin D-deficiency rickets, type I (PVDRI) by massive doses of vitamin D3 was examined. For this purpose, the affinities and the maximum binding capacities (Bmax) of the plasma vitamin D-binding protein (DBP) and of the intestinal 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3) receptor for vitamin D3, 25-hydroxyvitamin D3 (25-OHD3) and 1,25-(OH)2D3, were investigated in normal piglets and in rachitic piglets that suffered from PVDRI. The piglets were 5 to 10 weeks old and of both sexes. The Bmax of plasma DBP for 25-OHD3 was 6.77 +/- 0.45 microM for PVDRI piglets and 7.30 +/- 0.41 microM for control piglets and showed no differences between the two groups. Equilibrium association constants (Ka) of DBP for 25-OHD3 were 4.3 x 10(8) M-1 for PVDRI piglets and 4.0 x 10(8) M-1 for controls and showed also no differences between the two groups. Similarly the Ka of DBP for 1,25-(OH)2D3 was also the same for rachitic and control piglets (1.45 x 10(7) and 1.54 x 10(7) M-1, respectively). Due to the lower circulating concentration of 1,25-(OH)2D3 in the plasma of rachitic piglets compared to that of controls its free metabolite index was significantly lower in rachitic (0.42 +/- 0.05 x 10(-5)) than in control piglets (3.63 +/- 0.30 x 10(-5)). The Kd and Bmax of the intestinal nuclear receptor for 1,25-(OH)2D3 of rachitic and control piglets were 0.31 +/- 0.05 and 0.33 +/- 0.05 nM and 674 +/- 103 and 719 +/- 123 fmol/mg protein, respectively, and were also not different between the two groups of piglets. It was concluded from these observations that the rachitic symptoms of PVDRI piglets resulted solely from the lower free 1,25-(OH)2D3 concentration in plasma compared to that of normal piglets. The relative affinities of the intestinal 1,25-(OH)2D3 receptor for vitamin D3 and 25-OHD3 were also measured. It was found that 50% displacement of 1,25-(OH)2D3 from the intestinal receptor of PVDRI and control piglets required a 220,000- and 245,000-fold excess of the free concentration of vitamin D3, respectively, and a 20- to 42- and 23- to 71-fold excess of the free concentration of 25-OHD3, respectively.(ABSTRACT TRUNCATED AT 400 WORDS)

Calcitriol treatment in vitamin D-dependent and vitamin D-resistant rickets

Use of 1,25(OH)2D3 (calcitriol) can be of benefit in the treatment of two hereditary types of rickets and osteomalacia, vitamin D dependency type I (VDD1) and X-linked hypophosphatemic vitamin D-resistant rickets (HPDR). VDD1 is due to inadequate activation of 25(OH)D to 1,25(OH)2D, leading to very low circulating levels of 1,25(OH)2D in plasma; the basic abnormality appears to be an alteration in renal 1 alpha-hydroxylase activity. In VDD1, replacement therapy with calcitriol results in complete correction of the abnormal phenotype. By contrast, in HPDR, plasma levels of 25(OH)D and 1,25(OH)2D are in the normal range, although it has been demonstrated that the ability of patients to produce 1,25(OH)2D under conditions of stress is impaired. When started early in life, the use of phosphate salts in HPDR generally results in healing of rickets, normal growth, and correction of lower limb deformities. However, osteomalacia is not corrected by treatment with phosphate, either alone or in combination with vitamin D. By pharmacologically increasing the level of 1,25(OH)2D3 in these patients, there is often a dramatic improvement in the appearance of the trabecular surface, leading to correction of the osteomalacic component of HPDR; in addition, the secondary hyperparathyroidism observed in previous patients treated with phosphate and vitamin D is easier to control. Closed medical follow-up allows the prevention of renal damage that could result from long-term administration of calcitriol.

Placental transport of calcium and phosphorus in pigs

The role of vitamin D in regulating placental transport of calcium and phosphate in pigs was studied using the "Hannover Pig Strain" which suffers from pseudo-vitamin D-deficiency rickets, type I. Sows and fetuses of normal phenotype (heterozygotes) and of homozygote phenotype which suffered from clinical symptoms of rickets were used. The homozygote animals are devoid of renal 25-cholecalciferol-1-hydroxylase. In the rachitic sows which normally depend on treatment with pharmacological doses of vitamin D3 at intervals of four to six weeks, vitamin D-treatment was discontinued two months before conception. Ca, P and 1.25-(OH)2D3 concentrations in plasma of sows and fetuses (Aa. umb., V. umb.) were measured in samples obtained at term during cesarian section. Mean concentrations of Ca and P in heterozygote sows at parturition were 2.34 mmol/l and 2.33 mmol/l, respectively, and were significantly higher than in homozygote sows, 1.58 mmol/l and 1.26 mmol/l, respectively. Ca concentrations in plasma of the umbilical vein of fetuses from both homozygote and heterozygote sows were normal, however, (3.23 mmol/l and 2.96 mmol/l, respectively) and statistically not different from each other. No significantly different P concentrations in arterial plasma of fetuses from heterozygote or homozygote sows were seen, either. The concentrations of Ca and P in the arterial umbilical plasma were significantly higher (p less than 0.001) than in venous plasma of both homozygote and heterozygote fetuses, indicating net placental transfer of these elements in both genotypes. The concentration of 1.25-(OH)2D3 in hypocalcemic sows at term (25.5 +/- 8.25 pg/ml) was significantly lower than that in the normocalcemic (heterozygote) sows (84.1 +/- 25.6 pg/ml). The 1.25-(OH)2D3 concentration in arterial plasma of fetuses from homozygote sows was only 46% that of fetuses from normocalcemic sows which showed transfer of calcitriol from mother to fetus. It is concluded from these studies that in pigs, renal production and physiological concentrations of 1.25-(OH)2D3 in either mother or fetus is not essential for the maintainance of the Ca and P homeostasis in the fetal-maternal system.

Phosphate transport in brush-border membranes from control and rachitic pig kidney and small intestine

1. Na-Pi co-transport was analysed using renal cortical and small intestinal brush-border membrane vesicles which were isolated from control (normal, heterozygotes) and rachitic piglets (homozygotes). 2. A kinetic analysis of Na-dependent initial linear uptake of Pi was performed using vesicles obtained from control animals. The results suggest similar kinetic properties for the renal and small intestinal co-transport system. (i) A sigmoidal dependence on Na concentration of Pi uptake suggests the involvement of more than one Na ion in the co-transport. (ii) Increasing Na concentration leads to an increase in the apparent affinity of the transport system for Pi and has minimal effect on the apparent Vmax (maximum velocity of uptake). (iii) Increasing pH leads to an increase in Pi transport rate. 3. The kinetic characteristics of the Na-Pi co-transport system in vesicles obtained from rachitic animals were similar to those in controls. The apparent Vmax, but not the apparent Km (Michaelis constant) for Na and Pi, is reduced in intestinal and renal brush-border membranes isolated from rachitic animals as compared to control animals. Injection of vitamin D3, three days prior to killing of rachitic litter-mates, increased the Na-Pi uptake rate in the brush-border membrane vesicles isolated from these piglets. 4. It is concluded that intestinal and renal brush-border membranes from piglets contain a similar Na-Pi co-transport system and that in vitamin-D-dependent rickets the number of operating transport units is reduced in both membranes.