Vitamin D- dependent rickets (VDDR) types I and II (VDDR I and VDDR II) are rare, autosomal recessive disorders (OMIM #264700, #600081, #277440, %600785) that mimic vitamin D- deficiency rickets. However, there is no deficiency in cutaneous synthesis or accelerated loss of vitamin D. Patients are typically replete with vitamin D as shown by normal circulating levels of 25- hydroxyvitamin D.
VDDR I and II feature diminished biosynthesis of, and target tissue resistance to, 1,25- dihydroxyvitamin D, respectively. Because there is either disturbed conversion of 25- hydroxyvitamin D to 1,25- dihydroxyvitamin D (VDDR I) or peripheral resist ance to 1,25- dihydroxyvitamin D (VDDR II), serum levels of 1,25- dihydroxyvitamin D are low and high, respectively (Table 1). Nevertheless, both types of VDDR alter mineral homeostasis in a similar way. Dietary Ca2+ is malabsorbed, leading to hypocalcaemia, secondary hyperparathyroidism, and hypophosphataemia. Decreased extracellular fluid levels of Ca2+ and Pi together impair mineralization of skeletal matrix. Because the pathogenesis of VDDR I involves defective production of 1,25- dihydroxyvitamin D by the kidney, physiological doses of 1,25- dihydroxyvitamin D3 control the disorder. However, in VDDR II even enormous doses of 1,25- dihydroxyvitamin D3 may prove in effective and Ca2+ may need to be administered intravenously. Both VDDR I and II are understood at the gene level and as indicated next, now have more informative names.
Table1. Serum levels of vitamin D metabolites in disorders of vitamin D action, by aetiology
25- Hydroxyvitamin D, 1α- Hydroxylase Deficiency (Vitamin D- Dependent Rickets, Type I)
1,25- dihydroxyvitamin D deficiency can be defined as low circulating levels of this hormone despite normal or elevated (depending on preceding vitamin D therapy) concentrations of 25- hydroxyvitamin D. In theory, this situation could result from decreased production or increased clearance of 1,25- dihydroxyvitamin D. Indeed, decreased production of 1,25- dihydroxyvitamin D can be hereditary, but in other circum stances acquired. Acquired deficiency is usually explained by sys temic disease, such as chronic renal failure or acquired Fanconi’s syndrome, etc., which affect bone and mineral metabolism in multiple and complex ways. In contrast, increased clearance is uncommon and typically accompanies loss of other vitamin D metabolites, such as 25- hydroxyvitamin D, and would therefore fit within the definition of secondary vitamin D deficiency. The genetic entity discussed here (OMIM #264700) is now also called hereditary 1,25- dihydroxyvitamin D deficiency, and is an inborn error of metabolism featuring defective enzymatic biosynthesis of 1,25- dihydroxyvitamin D.
Prader and colleagues first characterized this disorder when they described two young children who showed all of the usual clinical features of vitamin D deficiency despite adequate intake of the vitamin. Complete remission depended upon continuous therapy with high doses of vitamin D— thus, the term ‘vitamin D- dependent rickets’. They had instead coined the term ‘pseudovitamin D deficiency’. Remission could, however, be achieved by physiological (microgram) doses of 1α- hydroxylated vitamin D metabolites. VDDR I is now understood at the molecular level and is, therefore, best described as 25- hydroxyvitamin D, 1α- hydroxylase deficiency.
Patients with 1α- hydroxylase deficiency appear healthy at birth. Subsequently, features consistent with nutritional rickets are usually noticed before 2 years of age, and often during the first 6 months of life. There is growth retardation and poor gross motor development. Muscle weakness, irritability, pneumonia, seizures, and failure to thrive are prominent findings.
Serum 1,25- dihydroxyvitamin D levels are low or undetectable despite normal levels of 25- hydroxyvitamin D. Malabsorption of dietary Ca2+ leads to hypocalcaemia, secondary hyperparathyroidism, and hypophosphataemia. Serum ALP activity is elevated.
The radiographic changes are in keeping with nutritional rickets. In addition to growth plate abnormalities and rachitic deformities, osteopaenia, and other features of secondary hyperparathyroidism are present. Undecalcified bone documents defective matrix mineralization and secondary hyperparathyroidism including osteoclastosis and peritrabecular fibrosis.
Early reports of affected siblings in inbred kindreds indicated that VDDR I is an autosomal recessive condition, especially prevalent in French- Canadians [46]. A founder effect seems to have occurred in this population and, in 1990, linkage studies mapped the disorder to chromosome l2q14. The molecular defect compromises the kidney’s mitochondrial cytochrome P450clα enzyme responsible for rate- limiting, hormonally regulated, 25- hydroxyvitamin D bioactivation to 1,25- dihydroxyvitamin D (i.e. 25- hydroxyvitamin D, lα- hydroxylase). Actually, this enzyme has several components, cytochrome P- 450D10t, ferredoxin, and ferredoxin reductase. A variety of mutations have been found in the P450clα gene (CYP27B1: OMIM 609506). French- Canadian patients are commonly homozygous for a 958ΔG defect in this single copy gene. None of these mutations engenders an enzyme with decreased (rather than absent) activity.
Serum concentrations of 25- hydroxyvitamin D are normal in VDDR I (elevated if pharmacological doses of vitamin D or 25- hydroxyvitamin D are given), yet 1,25- dihydroxyvitamin D levels are low, or remain only partially corrected by vitamin D or 25- hydroxyvitamin D therapy. Because pharmacological doses of vitamin D2 or D3 or 25- hydroxyvitamin D3 produce therapeutic responses in VDDR I similar to physiological (re placement) doses of 1,25- dihydroxyvitamin D3, it is apparent that 25- hydroxyvitamin D (or some metabolite) at sufficient levels can activate the VDR. Alternatively, perhaps enhanced local 1,25- dihydroxyvitamin D biosynthesis occurs with pharmacological doses of the prohormones.
The 1α- hydroxylase gene from more than 25 families with this disorder has been studied by site- directed mutagenesis and cDNA expression in transfected cells. All patients had homozygous mutations. Most French- Canadian patients have the same mutation causing a frame shift and a premature stop codon in the putative haem- binding domain. However, the same mutation was then observed in additional families of diverse origin. All other patients had either a base- pair deletion causing a premature termination codon upstream from the putative ferredoxin and haem- binding domains, or missense mutations. No 1α- hydroxylase activity was detected when the mutant enzyme was expressed in various cells. The 1α- hydroxylase gene sequence in keratinocytes and peripheral blood mononuclear cells is identical with the renal gene.
The differential diagnosis for VDDR I includes especially defects in the VDR- effector system, where serum concentrations of 1,25- dihydroxyvitamin D and the response to treatment with 1α- hydroxylated vitamin D metabolites are greatly different (Table 2).
Table2. Biochemical parameters of mineral and skeletal homeostasis in rickets/ osteomalacia, by aetiology
Clinical remission has followed daily, high- dose therapy with 1– 3 mg of vitamin D2, or with 0.2– 0.9 mg of 25- hydroxyvitamin D. Because there is no defect in hepatic conversion of vitamin D to 25- hydroxyvitamin D, vitamin D rather than 25- hydroxyvitamin D is cheap yet effective. However, a physiological (‘replacement’) dose of 1,25- dihydroxyvitamin D, 0.25– 1.0 µg daily, bypasses the lα- hydroxylase defect and provides an effective and direct treatment. Although 25- hydroxyvitamin D3 or 1,25- dihydroxyvitamin D3 therapy is expensive, it has advantages. The physiological half- lives of these metabolites are much shorter than vitamin D, and excessive dosing will respond more rapidly to temporary cessation of therapy. Most patients, however, can be managed with vitamin D, but follow- up is essential for any regimen.
Hereditary Resistance to 1,25- Dihydroxyvitamin D (Vitamin D- Dependent Rickets, Type II)
This disorder was characterized in 1978 when a patient with features of ‘pseudovitamin D deficiency’ (see earlier) was found in stead to have high serum levels of 1,25- dihydroxyvitamin D. Thus, ‘hereditary resistance to 1,25- dihydroxyvitamin D’ or VDDR II refers to this condition (OMIM #277440). Autosomal recessive inheritance is well established, and parental consanguinity has been reported in approximately 50% of cases.
There is a striking clustering of patients around the Mediterranean, including patients reported form Europe and America who originated from the same area [7, 8]. A notable exception is a cluster of kindreds from Japan . Obligate heterozygotes do not have clinical or biochemical manifestations. Patients appear normal at birth, but typically then develop features resembling vitamin D deficiency during the first year of life. Although several sporadic cases first manifested skeletal disease as late as their teenage years or middle age, they represent the mildest form of the disease and had complete remission when treated with vitamin D or its active metabolites. It was unclear if the adult- onset patients belong to this entity. In general, the earlier the presentation, the more severe the clinical and biochemical features.
Hypocalcaemia causes secondary hyperparathyroidism, hypophosphataemia, and elevated serum ALP activity. However, 1,25- dihydroxyvitamin D levels are elevated, sometimes as much as 10- fold [5– 8]. This abnormality reflects peripheral resistance to 1,25- dihydroxyvitamin D causing malabsorption of dietary Ca2+ and the subsequent combined effects of three activators of renal 25- hydroxyvitamin D, lα- hydroxylase activity: hypocalcaemia, increased circulating PTH, and hypophosphataemia together with di minished feedback inhibition by 1,25- dihydroxyvitamin D on the kidney 1α- hydroxylase.
The radiographic and histological findings of VDDR II resemble those of nutritional rickets, as described before, including growth plate disturbances, rachitic deformities, osteopaenia, fractures, and evidence of secondary hyperparathyroidism.
A peculiar feature, appearing in more than half of the affected individuals, is total alopecia or sparse hair. Alopecia usually manifests during the first year of life and there may be additional ectodermal anomalies including oligodontia, epidermal cysts, and cutaneous milia. In a patient with total alopecia, hair follicles were present.
Alopecia seems to be a marker for a more severe form of the dis ease, as judged by earlier onset, severity of the clinical features, pro portion of patients who do not respond to treatment with high doses of vitamin D or its active metabolites, and the extremely elevated serum levels of 1,25- dihydroxyvitamin D during therapy. Although some patients with alopecia achieve clinical and biochemical remission of their bone disease, none have shown hair growth. The notion that total alopecia reflects a defective VDR- effector system is supported by the fact that alopecia has only been associated with hereditary defects in the VDR system (i.e. with end- organ resistance to the action of the hormone. Indeed, hair follicles normally contain the VDR).
Patients with VDDR II with normal hair can respond fully to high doses of bioactive vitamin D metabolites. However, only some with total alopecia do so. Remarkably, some patients with VDDR II may no longer need 1,25- dihydroxyvitamin D3 therapy, or require lower doses, later in life. A VDR- positive mild variant has been reported in Columbia, South America (OMIM % 600785).
The nature of the resistance to 1,25- dihydroxyvitamin D and ab errations in the VDR/ effector system have been elucidated. A variety of VDR, or post- VDR, defects block the peripheral action of 1,25- dihydroxyvitamin D. There can be absence of the VDR, diminished binding capacity or binding affinity of the VDR for 1,25- dihydroxyvitamin D, or failure of the 1,25- dihydroxyvitamin D– VDR complex to localize to the nucleus or bind to DNA [8] . Patients without hormone or DNA binding by the VDR are the most difficult to treat . A mouse model has been developed by targeted ablation of the VDR gene.
If untreated, most patients with VDDR II die in early child hood [5– 8]. However, good control of the disorder is possible with therapy, especially in individuals without alopecia. Depending upon severity, VDDR II may require calciferols that enhance endogenous production of 1,25- dihydroxyvitamin D, administration of high doses of both calciferols and Ca2+ to compensate for the target tissue resistance to 1,25- dihydroxyvitamin D, or high doses of Ca2+ alone (given orally or intravenously) to circumvent the target cell 1,25- dihydroxyvitamin D resistance. Whereas most patients may respond to very high oral doses of 1,25- dihydroxyvitamin D3 (10– 40 µg daily), some can have clinical, radiographic, and biochemical corrections with high doses of vitamin D2 or 25- hydroxyvitamin D3. Some patients have unexplained disease fluctuation.
Before therapy, serum 1,25- dihydroxyvitamin D concentrations range from the upper normal limit to markedly elevated. With vitamin D treatment, they may reach the highest levels found in any living system (≥100 times the upper normal limit).
The near ubiquity of a similar if not identical VDR- effector system among various cell types helped clarify the nature of the intracellular and molecular defects in these patients. A defect that compromises RXR heterodimerization with the VDR (essential for nuclear localization and probably for recognition of the vitamin D responsive element in the DNA as well) was characterized in several kindreds with and without alopecia. In kindreds with defects in VDR binding to DNA, different single nucleotide mutations in the DNA- binding region were found. All point mutations affected the region of the two zinc fingers of the VDR essential for functional inter action of the hormone receptor complex with DNA. Interestingly, all altered amino acids are highly conserved in the steroid receptor superfamily. In all such patients, no response followed very high doses of vitamin D or its active 1α- hydroxylated metabolites.
Normal hair is usually associated with milder and usually complete clinical and biochemical remission on high doses of vitamin D or its metabolites. In contrast, only ~50% of patients with alopecia have satisfactory clinical and biochemical remission to high doses of vitamin D or its active 1α- hydroxylated metabolites, and the dose requirement is about 10- fold higher than those with normal hair.
It seems that defects characterized as deficient hormone binding affinity and deficient heterodimerization with RXR achieve remis sion on high doses of vitamin D or its active 1α- hydroxylated metabolites. Most with other defects could not be cured.
Typical clinical and biochemical features (Table 1) support the diagnosis. The issue becomes more complicated when the clinical features are atypical, i.e. late onset, sporadic cases, and normal hair. Failure of a therapeutic trial with Ca2+ and/ or physiological replacement doses of vitamin D or its active metabolites may support the diagnosis but now mutation analysis should reveal the aetiology and pathogenesis.
Based on the clinical and biochemical features, the following additional disease states should be considered: (1) extreme Ca2+ deficiency, and (2) severe vitamin D deficiency, because during the initial stages of vitamin D therapy in children with severe vitamin D- deficient rickets, the biochemical picture may resemble 1,25- dihydroxyvitamin D resistance, but this is a transient condition differentiated by the history of vitamin D deficiency and the final therapeutic response to vitamin D.
An adequate therapeutic trial must include vitamin D at sufficient doses to maintain high serum concentrations of 1,25- dihydroxyvitamin D because patients can produce high serum 1,25- dihydroxyvitamin D levels if supplied with substrate. If high serum levels are not achieved, 1α- hydroxylated vitamin D metabolites should be given in daily doses up to 6 µg/ kg weight or a total of 30– 60 µg and up to 3 g of elemental Ca2+ orally daily; therapy must continue for a period sufficient to mineralize the abundant osteoid (usually 3– 5 months). Therapy may be considered a failure if no change in the clinical, radiological, or biochemical parameters occurs while serum 1,25- dihydroxyvitamin D concentrations are maintained at approximately 100 times average normal values. Several patients have shown unexplained fluctuations in response to therapy or at presentation of the disease.
In some patients unresponsive to vitamin D or its metabolites, clinical and biochemical remission, including catch- up growth, ac companied administration of large amounts of Ca2+ achieved by long- term (months) intracaval infusions of up to 1000 mg of Ca2+ daily. Alternatively, increasing oral Ca2+ intake was used success fully in only very few patients and this approach was limited by dose and patient tolerability.
