Vitamin D-Dependent Rickets Type 2A

Mendelian MONDO:0010186 Pathograph 33 Show in embeddings browser Metabolic Bone Disorders Inborn Error of Metabolism

Vitamin D-dependent rickets type 2A is impaired calcitriol signaling caused usually by biallelic loss-of-function variants in VDR and, very rarely, by a heterozygous dominant-negative VDR variant. Allele-specific defects can impair receptor production, ligand binding, DNA binding, RXR heterodimerization, or coactivator recruitment. The resulting loss of VDR-regulated intestinal calcium and phosphate absorption causes secondary hyperparathyroidism, hypophosphatemia, and rickets, while intact hormone production and dysregulated receptor-mediated homeostasis make circulating calcitriol characteristically high. Hormone resistance is not uniformly complete: some genotypes respond to supraphysiologic active vitamin D plus calcium, whereas severe oral-treatment-refractory disease requires intravenous calcium. Alopecia reflects a separate ligand-independent hair-cycle function of VDR; it is frequent but not universal and is strongly genotype dependent.

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2
Inheritance
13
Pathophys.
15
Phenotypes
33
Pathograph
1
Genes
3
Medical Actions
2
Datasets
4
Models
1
Deep Research
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Inheritance

2
Autosomal recessive inheritance HP:0000007
The usual disease mechanism is a homozygous or compound-heterozygous pathogenic VDR genotype. Core skeletal and biochemical findings are consistently expressed among reported affected families and clinically ascertained series, while severity and alopecia are variably expressed; population penetrance among pathogenic-variant carriers is undetermined. Consanguinity increases the chance of homozygosity. Exceptionally, complete uniparental isodisomy of chromosome 12 can make one parental VDR allele homozygous and changes recurrence-risk counseling without constituting a new inheritance mode.
Autosomal recessive inheritance Penetrance: UNKNOWN Expressivity: VARIABLE
Show evidence (5 references)
PMID:2849209 SUPPORT Human Clinical
"Two families with affected children homozygous for this autosomal recessive disorder were studied for abnormalities in the intracellular vitamin D receptor (VDR) and its gene."
Establishes homozygous transmission in the founding molecular study.
PMID:28377956 SUPPORT Other
"Patients with HVDRR often have consanguinity in the family and display a number of clinical features including early onset rickets, hypocalcemia, and secondary hyperparathyroidism."
Supports consanguinity as a common risk context for the usual recessive disease while identifying the characteristic skeletal and biochemical phenotype.
PMID:38872968 SUPPORT Human Clinical
"As for initial laboratory tests, the PTH and Alkaline phosphatase (ALP) were high in all participants"
All 42 clinically ascertained patients had the characteristic biochemical disorder, documenting consistent expression among affected cohort members but not estimating penetrance among pathogenic-variant carriers.
+ 2 more references
Rare autosomal dominant inheritance HP:0000006
Rare heterozygous VDR alleles can act dominantly by inhibiting the remaining wild-type receptor. This is an exceptional inheritance mechanism rather than the usual pattern for VDDR2A.
Autosomal dominant inheritance
Show evidence (1 reference)
PMID:28377956 SUPPORT Human Clinical
"In conclusion, we found a dominant-negative mutant of VDR causing dominantly inherited HVDRR through a constitutive corepressor interaction, a mechanism similar to that in dominantly inherited thyroid hormone receptor mutations."
Establishes the rare dominant-negative mechanism and autosomal dominant transmission in a segregating family.
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Pathophysiology

13
Absent or Truncated VDR Protein
This allele-class branch applies when biallelic nonsense, frameshift, or other loss-of-function variants prevent production of a functional VDR protein. Loss of the receptor abolishes both ligand-activated transcription and the ligand-independent receptor function needed for hair cycling.
VDR hgnc:12679 HUGO Gene Nomenclature Committee (hgnc) Relation: this pathophysiological event involves this gene This pathophysiological event involves VDR (hgnc:12679). hgnc:12679 is a gene from the HUGO Gene Nomenclature Committee.
Genetic context VDR hgnc:12679 HUGO Gene Nomenclature Committee (hgnc) Relation: this genetic context concerns this gene This genetic context concerns VDR (hgnc:12679). hgnc:12679 is a gene from the HUGO Gene Nomenclature Committee. allele_type: Biallelic null or truncating variants variant_origin: GERMLINE functional_impact_category: LOSS_OF_FUNCTION
This branch is one alternative VDR defect and is not asserted for every affected individual.
Downstream
Impaired VDR-Regulated Transcription
Disrupted Ligand-Independent VDR Hair-Cycle Function
Show evidence (1 reference)
PMID:32596195 SUPPORT Other
"Alopecia is associated with VDR mutations that impair DNA binding, RXR heterodimerization, or production of the VDR, while mutations that alter VDR affinity for 1,25(OH)2D or disrupt coactivator interactions do not cause alopecia."
Distinguishes loss of receptor production from other allele-specific defects and links this class to loss of the hair-cycle function.
Impaired Calcitriol Binding by VDR
This allele-class branch applies to ligand-binding-domain variants that reduce or abolish calcitriol binding. Residual ligand affinity varies, so hormone resistance and responsiveness to supraphysiologic active vitamin D are graded rather than universally complete.
VDR hgnc:12679 HUGO Gene Nomenclature Committee (hgnc) Relation: this pathophysiological event involves this gene This pathophysiological event involves VDR (hgnc:12679). hgnc:12679 is a gene from the HUGO Gene Nomenclature Committee.
Genetic context VDR hgnc:12679 HUGO Gene Nomenclature Committee (hgnc) Relation: this genetic context concerns this gene This genetic context concerns VDR (hgnc:12679). hgnc:12679 is a gene from the HUGO Gene Nomenclature Committee. allele_type: Ligand-binding-domain variant variant_origin: GERMLINE functional_impact_category: LOSS_OF_FUNCTION
Includes partial and complete ligand-binding defects; not every allele retains residual activity.
vitamin D binding GO:0005499 ↓ DECREASED Gene Ontology (GO) Relation: this pathophysiological event involves this molecular function This pathophysiological event involves decreased vitamin D binding (GO:0005499). GO:0005499 is a molecular function from the Gene Ontology.
Downstream
Impaired VDR-Regulated Transcription
Show evidence (1 reference)
PMID:28377956 SUPPORT In Vitro
"The mutant had a dominant-negative effect on wild-type-VDR, and the ligand binding affinity of Q400LfsX7-VDR was completely impaired."
Demonstrates complete ligand-binding failure for one disease-associated VDR allele without generalizing that result to all VDDR2A variants.
Impaired VDR Binding to Vitamin D Response Elements
DNA-binding-domain variants can preserve hormone binding while reducing VDR affinity for vitamin D response elements. This is a specific allele-class mechanism, not the universal molecular lesion in VDDR2A.
VDR hgnc:12679 HUGO Gene Nomenclature Committee (hgnc) Relation: this pathophysiological event involves this gene This pathophysiological event involves VDR (hgnc:12679). hgnc:12679 is a gene from the HUGO Gene Nomenclature Committee.
Genetic context VDR hgnc:12679 HUGO Gene Nomenclature Committee (hgnc) Relation: this genetic context concerns this gene This genetic context concerns VDR (hgnc:12679). hgnc:12679 is a gene from the HUGO Gene Nomenclature Committee. allele_type: DNA-binding-domain variant variant_origin: GERMLINE functional_impact_category: LOSS_OF_FUNCTION
vitamin D response element binding GO:0070644 ↓ DECREASED Gene Ontology (GO) Relation: this pathophysiological event involves this molecular function This pathophysiological event involves decreased vitamin D response element binding (GO:0070644). GO:0070644 is a molecular function from the Gene Ontology.
Downstream
Impaired VDR-Regulated Transcription
Disrupted Ligand-Independent VDR Hair-Cycle Function
Show evidence (1 reference)
PMID:2849209 SUPPORT In Vitro
"Although the receptor displays normal binding of 1,25-dihydroxyvitamin D3 hormone, VDR from affected family members has a decreased affinity for DNA."
Directly isolates impaired DNA binding in the founding DNA-binding-domain kindreds while preserving the allele-specific framing.
Impaired VDR-RXR Heterodimerization
Some ligand-binding-domain variants disrupt formation of the VDR-RXR heterodimer required for response-element occupancy and transcription. This class is also associated with loss of ligand-independent hair-cycle function.
VDR hgnc:12679 HUGO Gene Nomenclature Committee (hgnc) Relation: this pathophysiological event involves this gene This pathophysiological event involves VDR (hgnc:12679). hgnc:12679 is a gene from the HUGO Gene Nomenclature Committee.
Genetic context VDR hgnc:12679 HUGO Gene Nomenclature Committee (hgnc) Relation: this genetic context concerns this gene This genetic context concerns VDR (hgnc:12679). hgnc:12679 is a gene from the HUGO Gene Nomenclature Committee. allele_type: RXR-heterodimerization-defective VDR variant variant_origin: GERMLINE functional_impact_category: LOSS_OF_FUNCTION
protein heterodimerization activity GO:0046982 ↓ DECREASED Gene Ontology (GO) Relation: this pathophysiological event involves this molecular function This pathophysiological event involves decreased protein heterodimerization activity (GO:0046982). GO:0046982 is a molecular function from the Gene Ontology.
Downstream
Impaired VDR-Regulated Transcription
Disrupted Ligand-Independent VDR Hair-Cycle Function
Show evidence (1 reference)
PMID:36705686 SUPPORT Human Clinical
"Among LBD-NT variants, those affecting RXR heterodimerization, but not those affecting ligand affinity, were associated with alopecia."
Separates RXR-heterodimerization defects from ligand-affinity defects and supports their different hair phenotype.
Impaired VDR Coactivator Recruitment
Activation-function-2 and other ligand-binding-domain variants can preserve receptor production, DNA binding, and RXR interaction yet fail to recruit transcriptional coactivators after calcitriol binding.
VDR hgnc:12679 HUGO Gene Nomenclature Committee (hgnc) Relation: this pathophysiological event involves this gene This pathophysiological event involves VDR (hgnc:12679). hgnc:12679 is a gene from the HUGO Gene Nomenclature Committee.
Genetic context VDR hgnc:12679 HUGO Gene Nomenclature Committee (hgnc) Relation: this genetic context concerns this gene This genetic context concerns VDR (hgnc:12679). hgnc:12679 is a gene from the HUGO Gene Nomenclature Committee. allele_type: Coactivator-recruitment-defective VDR variant variant_origin: GERMLINE functional_impact_category: LOSS_OF_FUNCTION
Animal models
Systemic Vdr Delta-AF2 Mouse Mouse partially recapitulates Directly models loss of AF2-dependent coactivator recruitment while retaining repressive signaling.
Downstream
Impaired VDR-Regulated Transcription
Show evidence (1 reference)
PMID:39164247 SUPPORT Model Organism
"We confirm that ligand-induced transcriptional activation was disabled because the mutant VDRΔAF2 protein was unable to interact with coactivators."
Demonstrates the atomic coactivator-recruitment defect in an AF2-deletion model; it does not imply that this mechanism applies to every human allele.
Dominant-Negative VDR Corepressor Retention
In the rare dominant branch, a heterozygous mutant receptor occupies VDREs, fails to release the NCoR corepressor after ligand exposure, and suppresses transcription by the coexpressed wild-type receptor.
VDR hgnc:12679 HUGO Gene Nomenclature Committee (hgnc) Relation: this pathophysiological event involves this gene This pathophysiological event involves VDR (hgnc:12679). hgnc:12679 is a gene from the HUGO Gene Nomenclature Committee.
Genetic context VDR hgnc:12679 HUGO Gene Nomenclature Committee (hgnc) Relation: this genetic context concerns this gene This genetic context concerns VDR (hgnc:12679). hgnc:12679 is a gene from the HUGO Gene Nomenclature Committee. allele_type: p.Q400LfsX7 (p.Gln400LeufsTer7) variant_origin: GERMLINE zygosity: HETEROZYGOUS functional_impact_category: DOMINANT_NEGATIVE
Applies to the rare dominant-negative allele class, not to unaffected heterozygous carriers of ordinary recessive alleles.
Downstream
Impaired VDR-Regulated Transcription
Show evidence (1 reference)
PMID:28377956 SUPPORT In Vitro
"Interestingly, Q400LfsX7-VDR had a strong interaction with corepressor NCoR and could interact with VDRE without the ligand."
Shows the corepressor retention and VDRE occupancy underlying the dominant-negative branch.
Impaired VDR-Regulated Transcription
The distinct allele-class defects converge on reduced ligand-activated VDR transcription. The degree of residual output differs among variants, which accounts for partial hormone responsiveness in some patients and profound resistance in others.
vitamin D receptor signaling pathway GO:0070561 ↓ DECREASED Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves decreased vitamin D receptor signaling pathway (GO:0070561). GO:0070561 is a biological process from the Gene Ontology.
Experimental models
Patient-Derived p.Arg30* VDR Dermal Fibroblasts PRIMARY CELL CULTURE Measures loss of the calcitriol-responsive transcriptome in patient cells.
Downstream
Reduced VDR-Dependent Enterocyte Calcium and Phosphate Transport Reduced transcription of intestinal VDR targets impairs active mineral absorption.
Elevated Circulating Calcitriol in VDR Resistance Reduced VDR output dysregulates receptor-mediated vitamin D homeostasis, contributing to elevated calcitriol despite deficient biological action.
Show evidence (1 reference)
PMID:30959822 SUPPORT In Vitro
"Roughly 4.5% of the transcriptome was regulated by 1,25D in CO fibroblasts, while MUT cells without a functional VDR were insensitive to 1,25D."
Patient-derived p.Arg30* fibroblasts directly demonstrate loss of the transcriptomic response to calcitriol.
Elevated Circulating Calcitriol in VDR Resistance
Calcitriol synthesis remains intact while mineral deficiency, secondary hyperparathyroidism, and dysregulated receptor-mediated vitamin D homeostasis favor a markedly elevated circulating 1,25-dihydroxyvitamin D concentration. This biochemical state is a clue to receptor resistance; it does not imply that every VDR allele has zero residual response to hormone.
vitamin D metabolic process GO:0042359 ↕ DYSREGULATED Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves dysregulated vitamin D metabolic process (GO:0042359). GO:0042359 is a biological process from the Gene Ontology.
Show evidence (1 reference)
PMID:42438800 SUPPORT Human Clinical
"Further evaluation revealed markedly elevated 1,25(OH)2D, and whole-exome sequencing identified a homozygous pathogenic VDR c.1027C>T (p.R343C) variant, confirming VDDR2A."
Links markedly elevated calcitriol to genetically confirmed VDDR2A.
Reduced VDR-Dependent Enterocyte Calcium and Phosphate Transport
Reduced VDR transcription in enterocytes decreases active calcium and phosphate uptake. Calcium may be low or maintained near normal by compensatory hyperparathyroidism, so hypocalcemia is not required for this mechanism or for diagnosis.
Enterocyte CL:0000584 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves Enterocyte (CL:0000584). CL:0000584 is a cell type from the Cell Ontology.
Intestinal Calcium Absorption GO:0070509 ↓ DECREASED Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves decreased Intestinal Calcium Absorption, annotated with calcium ion import (GO:0070509). GO:0070509 is a biological process from the Gene Ontology.
Downstream
Hypocalcemia Reduced active intestinal calcium uptake can lower circulating calcium.
Compensatory Secondary Hyperparathyroidism Reduced calcium delivery stimulates compensatory parathyroid hormone secretion even when that compensation maintains serum calcium near normal.
Defective Skeletal Mineralization Reduced calcium and phosphate delivery limits mineral substrate.
Show evidence (1 reference)
PMID:38872968 SUPPORT Human Clinical
"The primary defect caused by the mutant VDR gene is a decrease in intestinal calcium and phosphate absorption, which leads to decreased bone mineralization and rickets (3), despite high levels of 1,25(OH)2D (4)."
States the intestinal calcium and phosphate malabsorption this node models, and that it occurs despite elevated circulating calcitriol.
Compensatory Secondary Hyperparathyroidism
Reduced calcium absorption increases parathyroid hormone secretion. The response can preserve normocalcemia through renal calcium conservation and bone resorption while simultaneously promoting renal phosphate loss.
Downstream
Secondary hyperparathyroidism
PTH-Driven Renal Phosphate Wasting
Elevated Circulating Calcitriol in VDR Resistance Elevated PTH stimulates renal calcitriol production.
Show evidence (1 reference)
PMID:42438800 SUPPORT Human Clinical
"The preserved serum calcium level may reflect compensatory secondary hyperparathyroidism, in which markedly elevated PTH maintains normocalcemia through increased renal calcium reabsorption and bone resorption."
Explains why receptor resistance can produce marked secondary hyperparathyroidism without measured hypocalcemia.
PTH-Driven Renal Phosphate Wasting
Elevated parathyroid hormone reduces renal phosphate reabsorption, adding a phosphate deficit to impaired intestinal mineral uptake.
Downstream
Hypophosphatemia
Defective Skeletal Mineralization Phosphate wasting further reduces substrate for hydroxyapatite deposition.
Show evidence (1 reference)
PMID:42438800 SUPPORT Human Clinical
"In contrast, elevated PTH promotes renal phosphate wasting, contributing to hypophosphatemia."
Directly supports the PTH-to-phosphate-wasting step in the mineral pathway.
Defective Skeletal Mineralization
Early and often severe rickets with bowing of the weight-bearing limbs, delayed walking and growth failure.
Osteoblast CL:0000062 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves Osteoblast (CL:0000062). CL:0000062 is a cell type from the Cell Ontology.
Bone Mineralization GO:0030282 ↓ DECREASED Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves decreased Bone Mineralization (GO:0030282). GO:0030282 is a biological process from the Gene Ontology.
Bone UBERON:0001474 Uberon multi-species anatomy ontology (UBERON) Relation: this pathophysiological event occurs in this anatomical location This pathophysiological event occurs in Bone, annotated with bone element (UBERON:0001474). UBERON:0001474 is an anatomical location from the Uberon multi-species anatomy ontology.
Animal models
VDR-null mouse (targeted ablation of the second zinc finger) Mouse recapitulates Reproduces the full mineral and skeletal sequence, and shows the growth-plate lesion appearing before overt rickets.
Systemic Vdr Delta-AF2 Mouse Mouse partially recapitulates Reproduces severe undermineralization and dysmorphic growth plates.
Downstream
Rickets
Elevated circulating alkaline phosphatase
Bowing of the legs
Short stature
Delayed ability to walk
Bone pain
Enlargement of the costochondral junction
Show evidence (1 reference)
PMID:38872968 SUPPORT Human Clinical
"The median age at presentation was 15.5 months."
Records the early-infantile onset of the skeletal disease in the largest reported cohort.
Disrupted Ligand-Independent VDR Hair-Cycle Function
Hair distribution is initially normal, but receptor absence or variants that disrupt DNA binding or RXR heterodimerization can impair the unliganded VDR function required to reinitiate the hair cycle. Ligand-affinity and coactivator-recruitment defects can preserve this branch, explaining why alopecia is genotype dependent and absent in some affected individuals.
hair cycle GO:0042633 ↓ DECREASED Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves decreased hair cycle (GO:0042633). GO:0042633 is a biological process from the Gene Ontology.
Animal models
VDR-null mouse (targeted ablation of the second zinc finger) Mouse recapitulates Progressive alopecia develops from four weeks of age and is absent from dietary vitamin D deficiency, matching the human observation that alopecia tracks receptor loss rather than hormone deficiency.
Keratinocyte-targeted ligand-binding-null VDR transgene in VDR-null mice Mouse rescues Keratinocyte-restricted expression of a ligand-binding-dead receptor rescues the hair cycle, demonstrating the mechanism this node asserts.
Downstream
Alopecia
Show evidence (1 reference)
PMID:32596195 SUPPORT Other
"In addition, affected children are born with normal distribution of hair, and alopecia develops later due to failure of normal hair follicle cycling, which is dependent upon unliganded actions of the VDR."
Supports the postnatal, ligand-independent hair-cycle mechanism.
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Pathograph

Use the checkboxes to hide or show graph categories. Hover nodes for evidence and cross-linked metadata.
Pathograph: causal mechanism network for Vitamin D-Dependent Rickets Type 2A Interactive directed graph showing how pathophysiology mechanisms, phenotypes, genetic factors and variants, experimental models, environmental triggers, and treatments relate through causal and linked edges.
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Phenotypes

15
Endocrine 1
Secondary hyperparathyroidism HP:0000867 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Secondary hyperparathyroidism (HP:0000867). HP:0000867 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:38872968 SUPPORT Human Clinical
"Patients with HVDDR-II have rickets along with hypocalcemia, hypophosphatemia, secondary hyperparathyroidism, and elevated serum 1,25(OH)2D levels, which is a hallmark for diagnosing this disease."
Reports secondary hyperparathyroidism, the compensatory response to receptor-level calcium malabsorption.
Head and Neck 1
Enamel hypoplasia HP:0006297 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Enamel hypoplasia (HP:0006297). HP:0006297 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:38872968 SUPPORT Other
"The patient will present with the same characteristic features of nutritional rickets in the form of frontal bossing, bowing of the leg, growth failure, dental caries, and enamel hypoplasia (5)."
Supports enamel hypoplasia as part of the VDDR2A clinical spectrum but does not quantify its frequency in the reported cohort.
Integument 1
Alopecia VERY_FREQUENT HP:0001596 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Alopecia (HP:0001596). HP:0001596 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:36705686 SUPPORT Human Clinical
"On systematic review, alopecia was present in majority (81.5%) and preceded the onset of rickets."
Reports alopecia in 81.5% of 119 systematically reviewed probands, mapping to VERY_FREQUENT while showing that the finding is not universal.
Context-specific annotations (2)
Non-truncating ligand-binding-domain variants FREQUENT
Alopecia was reported in 51.4% of this allele class.
Show evidence (1 reference)
PMID:36705686 SUPPORT Human Clinical
"Alopecia was present in 51.4% of non-truncating (NT) ligand-binding domain (LBD) variants, whereas it was universal in truncating LBD and all DNA binding-domain (DBD) variants."
Quantifies the lower frequency in non-truncating LBD variants.
Truncating ligand-binding-domain or DNA-binding-domain variants OBLIGATE
Universal in these variant classes in the systematic review, not asserted as universal across VDDR2A.
Show evidence (1 reference)
PMID:36705686 SUPPORT Human Clinical
"Alopecia was present in 51.4% of non-truncating (NT) ligand-binding domain (LBD) variants, whereas it was universal in truncating LBD and all DNA binding-domain (DBD) variants."
Restricts the universal frequency claim to the reported truncating-LBD and DBD strata.
Limbs 1
Bowing of the legs OCCASIONAL HP:0002979 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Bowing of the legs (HP:0002979). HP:0002979 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:38872968 SUPPORT Human Clinical
"The most frequently reported symptoms are alopecia in 41 patients (97.6%), followed by short stature (height SDS<-2) in 36 patients (85.7%), bowing of legs in 9 patients (21.4%), delayed walking in 6 patients (14.3%), seizures in 4 patients (9.5%), and respiratory symptoms in 1 patient (2.4%) ("
Bowing of the legs occurred in 21.4% of this 42-patient referral cohort, mapping to OCCASIONAL.
Metabolism 3
Hypocalcemia HP:0002901 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Hypocalcemia (HP:0002901). HP:0002901 is a phenotype from the Human Phenotype Ontology.
Sequelae: Seizure
Show evidence (1 reference)
PMID:38872968 SUPPORT Human Clinical
"Patients with HVDDR-II have rickets along with hypocalcemia, hypophosphatemia, secondary hyperparathyroidism, and elevated serum 1,25(OH)2D levels, which is a hallmark for diagnosing this disease."
Reports hypocalcemia as part of the characteristic biochemical profile of the disorder.
Hypophosphatemia FREQUENT HP:0002148 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Hypophosphatemia (HP:0002148). HP:0002148 is a phenotype from the Human Phenotype Ontology.
Show evidence (2 references)
PMID:42438800 SUPPORT Human Clinical
"Hypophosphatemia is not consistently observed, occurring in around 58% of reported patients [9]."
The reported frequency maps to FREQUENT and avoids treating hypophosphatemia as universal.
PMID:38872968 SUPPORT Human Clinical
"Patients with HVDDR-II have rickets along with hypocalcemia, hypophosphatemia, secondary hyperparathyroidism, and elevated serum 1,25(OH)2D levels, which is a hallmark for diagnosing this disease."
Reports hypophosphatemia as part of the characteristic profile. Note the research artifact records presentations with hypophosphatemia but WITHOUT hypocalcemia, which can be mistaken for FGF23-mediated hypophosphatemic rickets - the differential this grouping exists to separate.
Elevated circulating alkaline phosphatase Elevated circulating alkaline phosphatase concentration HP:0003155 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Elevated circulating alkaline phosphatase concentration (HP:0003155). HP:0003155 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:38872968 SUPPORT Human Clinical
"HVDDR-II is characterized by early onset of rickets presentation and biochemical parameters such as low serum calcium levels, high parathyroid hormone, high alkaline phosphatase, and high 1,25(OH)2D levels (1)."
Reports elevated alkaline phosphatase among the characteristic biochemical parameters.
Musculoskeletal 4
Rickets HP:0002748 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Rickets (HP:0002748). HP:0002748 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:2849209 SUPPORT Human Clinical
"Hypocalcemic vitamin D-resistant rickets is a human genetic disease resulting from target organ resistance to the action of 1,25-dihydroxyvitamin D3."
Names rickets as the defining skeletal presentation of the disorder.
Muscle weakness HP:0001324 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Muscle weakness (HP:0001324). HP:0001324 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:20511055 SUPPORT Other
"Affected children have bone pain, muscle weakness, hypotonia and occasionally have convulsions due to the hypocalcemia."
An HVDRR-focused clinical review directly identifies muscle weakness without supplying a quantitative prevalence.
Hypotonia HP:0001252 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Hypotonia (HP:0001252). HP:0001252 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:20511055 SUPPORT Other
"Affected children have bone pain, muscle weakness, hypotonia and occasionally have convulsions due to the hypocalcemia."
Directly includes hypotonia in the HVDRR clinical spectrum without providing a quantitative prevalence.
Enlargement of the costochondral junction HP:0000920 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Enlargement of the costochondral junction (HP:0000920). HP:0000920 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:42438800 SUPPORT Human Clinical
"Radiographs demonstrated mild expansion of the anterior ribs at the costochondral junctions, flaring and fraying of metaphyses, and mild bowing of the femora, without fracture."
Directly supports costochondral-junction enlargement in VDDR2A. This uses HP:0000920; HP:0004464 is postauricular pit and is not a valid mapping for rachitic rosary.
Nervous System 2
Seizure OCCASIONAL HP:0001250 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Seizure (HP:0001250). HP:0001250 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:38872968 SUPPORT Human Clinical
"Alopecia was found in 97.6%, 21.4% presented with bowing legs, 14.3% with delayed walking, 9.5% with seizure, and 2.4% presented with respiratory failure, while a family history of the disease was positive in 71.4% of total patients."
Seizure at presentation in 9.5% of patients maps to OCCASIONAL (5-29%); seizures here are a consequence of hypocalcemia.
Delayed ability to walk OCCASIONAL HP:0031936 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Delayed ability to walk (HP:0031936). HP:0031936 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:38872968 SUPPORT Human Clinical
"The most frequently reported symptoms are alopecia in 41 patients (97.6%), followed by short stature (height SDS<-2) in 36 patients (85.7%), bowing of legs in 9 patients (21.4%), delayed walking in 6 patients (14.3%), seizures in 4 patients (9.5%), and respiratory symptoms in 1 patient (2.4%) ("
Delayed walking in 14.3% of this referral cohort maps to OCCASIONAL and exceeds the campaign's 10% inclusion threshold.
Constitutional 1
Bone pain HP:0002653 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Bone pain (HP:0002653). HP:0002653 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:40383122 SUPPORT Human Clinical
"Upon the initiation of IV calcium infusions, bone pain resolved, and the rickets healed within weeks."
Directly documents bone pain in genetically confirmed VDDR2A while not implying a disease-wide frequency from one case.
Growth 1
Short stature VERY_FREQUENT HP:0004322 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Short stature (HP:0004322). HP:0004322 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:38872968 SUPPORT Human Clinical
"The most frequently reported symptoms are alopecia in 41 patients (97.6%), followed by short stature (height SDS<-2) in 36 patients (85.7%), bowing of legs in 9 patients (21.4%), delayed walking in 6 patients (14.3%), seizures in 4 patients (9.5%), and respiratory symptoms in 1 patient (2.4%) ("
Short stature occurred in 85.7% of this 42-patient referral cohort, mapping to VERY_FREQUENT; the population context is retained in the explanation rather than treated as a universal disease-wide estimate.
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Genetic Associations

1
VDR pathogenic variants (Causative)
Gene: VDR hgnc:12679 gene page HUGO Gene Nomenclature Committee (hgnc) Relation: this disease-associated gene is this gene This disease-associated gene is VDR (hgnc:12679). hgnc:12679 is a gene from the HUGO Gene Nomenclature Committee. relationship_type: CAUSATIVE variant_origin: GERMLINE
Show evidence (11 references)
PMID:2849209 SUPPORT Human Clinical
"In each family, a different single nucleotide mutation was found in the DNA binding domain of the protein; one family near the tip of the first zinc finger (Gly----Asp) and one at the tip of the second zinc finger (Arg----Gly)."
Identifies the causal VDR variants and localizes them to the DNA-binding domain.
PMID:38872968 SUPPORT Human Clinical
"Molecular genetic testing of the VDR gene in our cohort identified six different gene variants c.885 C>A (p.Tyr295Ter), c.88 C>T (p.Arg30Ter), c.1036G>A (p.Val346Met), c.820C>T (p.Arg274Cys), c.803 T>C (p.Ile268Thr), and c.2T>G (p.Met1?)."
Documents the allelic spectrum in the largest reported cohort.
PMID:28377956 SUPPORT In Vitro
"The mutant had a dominant-negative effect on wild-type-VDR, and the ligand binding affinity of Q400LfsX7-VDR was completely impaired."
Functional testing identifies Q400LfsX7 as a dominant-negative causal VDR allele rather than treating the accompanying functionally normal R370H variant as causal.
+ 8 more references
💊

Medical Actions

3
High-Dose Active Vitamin D Plus Oral Calcium for Residual Responders
Category: Therapeutic Action: PharmacotherapyNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is Pharmacotherapy (NCIT:C15986). NCIT:C15986 is a clinical intervention from the NCI Thesaurus. NCIT:C15986
Agent: calcitriol CHEBI:17823 Chemical Entities of Biological Interest (CHEBI) Relation: this treatment uses this therapeutic agent This treatment uses calcitriol (CHEBI:17823). CHEBI:17823 is a therapeutic agent from Chemical Entities of Biological Interest. alfacalcidol CHEBI:31186 Chemical Entities of Biological Interest (CHEBI) Relation: this treatment uses this therapeutic agent This treatment uses alfacalcidol (CHEBI:31186). CHEBI:31186 is a therapeutic agent from Chemical Entities of Biological Interest. calcium(2+) CHEBI:29108 Chemical Entities of Biological Interest (CHEBI) Relation: this treatment uses this therapeutic agent This treatment uses calcium(2+) (CHEBI:29108). CHEBI:29108 is a therapeutic agent from Chemical Entities of Biological Interest.
Patients with residual VDR responsiveness, particularly some non-truncating ligand-binding-domain variants and milder presentations without alopecia, may respond to supraphysiologic calcitriol or alfacalcidol together with oral calcium. Biochemical response, growth, and radiographic healing guide continued use; response cannot be inferred from alopecia alone.
Mechanism Target:
MODULATES Impaired VDR-Regulated Transcription — Supraphysiologic active vitamin D can increase signaling through a receptor with residual function.
Show evidence (2 references)
PMID:42438800 SUPPORT Human Clinical
"High-dose calcitriol therapy resulted in progressive biochemical improvement and catch-up growth."
Genetically confirmed p.R343C VDDR2A responded clinically and biochemically, disproving universal calcitriol nonresponse.
PMID:36705686 SUPPORT Human Clinical
"Overall response to oral therapy was highest in LBD-NT (46.4%) as compared to 7.6% in LBD-truncating and 19% in DBD-NT variants."
Quantifies genotype-stratified oral response and supports a response-guided rather than universal treatment claim.
BYPASSES Reduced VDR-Dependent Enterocyte Calcium and Phosphate Transport — High oral calcium concentrations increase passive calcium uptake while active vitamin D exploits any residual receptor activity.
Show evidence (1 reference)
PMID:40219799 SUPPORT Human Clinical
"They both are being managed on high doses of oral calcium supplements and activated form of vitamin D."
Documents oral calcium plus activated vitamin D management in two genetically characterized siblings.
High-Dose Oral Calcium
Category: Therapeutic Action: PharmacotherapyNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is Pharmacotherapy (NCIT:C15986). NCIT:C15986 is a clinical intervention from the NCI Thesaurus. NCIT:C15986
Agent: calcium(2+) CHEBI:29108 Chemical Entities of Biological Interest (CHEBI) Relation: this treatment uses this therapeutic agent This treatment uses calcium(2+) (CHEBI:29108). CHEBI:29108 is a therapeutic agent from Chemical Entities of Biological Interest.
High-dose oral elemental calcium is an initial bypass strategy when it can restore normocalcemia and suppress secondary hyperparathyroidism. It may also permit transition away from intravenous calcium, but passive absorption is limited and requires close biochemical and urinary-calcium monitoring.
Mechanism Target:
BYPASSES Reduced VDR-Dependent Enterocyte Calcium and Phosphate Transport — A large luminal calcium load increases VDR-independent passive absorption.
Show evidence (1 reference)
PMID:32596195 SUPPORT Other
"Management during the first few months of life may require only administration of high doses of oral calcium (5–6 g/m2 body surface of elemental calcium). If oral therapy is able to restore normocalcemia and reverse secondary hyperparathyroidism it may not be necessary to initiate intravenous..."
Supports oral-first calcium treatment when biochemical targets respond.
Intravenous Calcium for Maximal-Oral-Treatment-Refractory Disease
Category: Therapeutic Action: PharmacotherapyNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is Pharmacotherapy (NCIT:C15986). NCIT:C15986 is a clinical intervention from the NCI Thesaurus. NCIT:C15986
Agent: calcium(2+) CHEBI:29108 Chemical Entities of Biological Interest (CHEBI) Relation: this treatment uses this therapeutic agent This treatment uses calcium(2+) (CHEBI:29108). CHEBI:29108 is a therapeutic agent from Chemical Entities of Biological Interest.
Severe disease that fails maximal oral calcium and active vitamin D requires prolonged intravenous calcium to bypass intestinal absorption entirely and replenish skeletal calcium deficits. Central-line complications make this a refractory-disease strategy, not evidence that intravenous therapy is universally preferable.
Mechanism Target:
BYPASSES Reduced VDR-Dependent Enterocyte Calcium and Phosphate Transport — Intravenous delivery completely bypasses the receptor-dependent intestinal route.
Show evidence (2 references)
PMID:32596195 SUPPORT Other
"Those patients who fail to respond to maximal doses of calciferol will require intravenous infusions of calcium (1,000 mg elemental calcium per 24 h infused over 12 h), which can be used even by the youngest children with VDDR2a."
Restricts intravenous calcium to patients who fail maximal hormone-based treatment.
PMID:36705686 SUPPORT Human Clinical
"All patients had poor overall response to oral high-dose calcium and calcitriol and most required intravenous calcium."
Documents escalation to intravenous calcium in an oral-refractory genetically confirmed cohort.
🔬

Biochemical Markers

2
1,25-dihydroxyvitamin D (calcitriol) (Elevated)
Context: Markedly elevated calcitriol in a child with rickets, high PTH, and high alkaline phosphatase is a major clue to VDR resistance and contrasts with type-1 defects in calcitriol synthesis. It is not sufficient by itself to distinguish VDDR2A from the VDDR2B phenocopy, and interpretation must account for treatment and assay context; molecular VDR testing establishes type 2A.
Show evidence (1 reference)
PMID:38872968 SUPPORT Human Clinical
"Patients with HVDDR-II have rickets along with hypocalcemia, hypophosphatemia, secondary hyperparathyroidism, and elevated serum 1,25(OH)2D levels, which is a hallmark for diagnosing this disease."
Names the elevated calcitriol level explicitly as the diagnostic hallmark of the disorder.
Parathyroid hormone (Elevated)
Context: Secondary hyperparathyroidism driven by receptor-level calcium malabsorption.
Show evidence (1 reference)
PMID:38872968 SUPPORT Human Clinical
"HVDDR-II is characterized by early onset of rickets presentation and biochemical parameters such as low serum calcium levels, high parathyroid hormone, high alkaline phosphatase, and high 1,25(OH)2D levels (1)."
Reports elevated parathyroid hormone among the characteristic biochemical parameters.
🔬

Diagnosis

2
Biochemical and Radiographic Evaluation for VDR Resistance
Evaluate a child with clinical or radiographic rickets using serum calcium, phosphate, alkaline phosphatase, PTH, 25-hydroxyvitamin D, and 1,25-dihydroxyvitamin D. Markedly elevated 1,25-dihydroxyvitamin D with high PTH and alkaline phosphatase supports receptor resistance; hypocalcemia and alopecia are not required. Explicit differentials include nutritional rickets, the synthetic defects VDDR1A and VDDR1B, the biochemical phenocopy VDDR2B, and FGF23-mediated renal phosphate-wasting disorders such as X-linked hypophosphatemia (XLH). Normocalcemic, hypophosphatemic VDDR2A can initially mimic an FGF23-mediated disorder; markedly elevated 1,25-dihydroxyvitamin D and PTH prompt evaluation for receptor resistance, while VDR testing distinguishes type 2A from type 2B. During follow-up, monitor calcium, phosphate, PTH, alkaline phosphatase, and radiographic healing separately because PTH can remain elevated after phosphate normalization and radiographic healing.
diagnostic procedure NCIT:C18020 NCI Thesaurus (NCIT)
Results: Radiographic rickets with elevated PTH, alkaline phosphatase, and 1,25-dihydroxyvitamin D; phosphate is often low and calcium may be low or normal, and biochemical indices may normalize asynchronously during treatment.
Show evidence (5 references)
PMID:42438800 SUPPORT Human Clinical
"Initial evaluation showed normocalcemia, hypophosphatemia, hyperparathyroidism, markedly elevated alkaline phosphatase, and radiographic rickets, leading to an initial diagnosis of hypophosphatemic rickets."
Demonstrates the diagnostic pitfall and that a normal serum calcium does not exclude VDDR2A.
PMID:38365463 SUPPORT Other
"Nutritional vitamin D deficiency is the leading cause of rickets and osteomalacia worldwide and responds well to vitamin D supplementation. Inherited disorders of vitamin D metabolism (vitamin D-dependent rickets, VDDR) account for a small proportion of calcipenic rickets/osteomalacia. Defective..."
Names nutritional rickets and VDDR1A, VDDR1B, and VDDR2B as explicit biochemical and mechanistic differentials.
PMID:32596195 SUPPORT Other
"Subjects who have hypophosphatemia and normal serum levels of PTH, calcium, and 25(OH)D are likely to have phosphopenic rickets; a normal or elevated tubular reabsorption of phosphorus (TRP) will be found in subjects with inadequate intake or absorption of phosphorus. By contrast, patients with..."
Supports the explicit XLH and renal-phosphate-wasting differential and shows how PTH and tubular phosphate handling separate major rickets axes.
+ 2 more references
VDR Molecular Genetic Testing
Sequencing of VDR by a rickets panel, exome, or genome confirms VDDR2A when pathogenic biallelic variants are found; segregation and functional data are especially important when considering the exceptional heterozygous dominant-negative mechanism.
molecular genetic testing NCIT:C19770 NCI Thesaurus (NCIT)
Results: Pathogenic VDR variant(s) in a disease-consistent allelic state confirm type 2A.
Show evidence (1 reference)
PMID:42438800 SUPPORT Human Clinical
"Further evaluation revealed markedly elevated 1,25(OH)2D, and whole-exome sequencing identified a homozygous pathogenic VDR c.1027C>T (p.R343C) variant, confirming VDDR2A."
Directly demonstrates molecular confirmation after biochemical recognition.
📊

Related Datasets

2
Effects of 1,25-dihydroxyvitamin D (1,25D) on global gene expression of human fibroblasts with (CO) or without (MUT) a functional VDR geo:GSE127314
Microarray profiling of primary skin fibroblasts from one homozygous VDR p.Arg30* VDDR2A patient and an age- and sex-matched control after vehicle or 10 nM calcitriol for 24 hours. Fifteen quality-controlled arrays represent four control-vehicle, four control-calcitriol, three mutant-vehicle, and four mutant-calcitriol samples.
human MICROARRAY n=15 Affymetrix GeneChip Human Gene 2.0 ST Array
primary skin fibroblast CL:0000057 Cell Ontology (CL) Relation: this dataset samples this sample type This dataset samples primary skin fibroblast, annotated with fibroblast (CL:0000057). CL:0000057 is a sample type from the Cell Ontology.
Conditions: homozygous VDR p.Arg30* patient fibroblasts matched control fibroblasts vehicle or 10 nM calcitriol for 24 hours
PMID:30959822
GEO metadata and accession were independently resolved with just verify-datasets on 2026-08-20.
Show evidence (1 reference)
PMID:30959822 SUPPORT In Vitro
"The array data were submitted to the National Center for Biotechnology Information Gene Expression Omnibus (http://www.ncbi.nlm.nih.gov/geo) under series accession no. GSE127314."
Directly identifies the public dataset accession in the source publication.
Coactivator-independent vitamin D receptor signaling impairs intestinal calcium transport in mice, leading to severe rickets, which is not prevented by a diet high in calcium, phosphate, and lactose. geo:GSE260988
Bulk RNA sequencing of duodenum and kidney from 8-week-old female wild-type, Vdr-null, and systemic VdrΔAF2 mice maintained on a high-calcium, high-phosphate, high-lactose diet. Each tissue contributes six wild-type, four null, and four VdrΔAF2 samples, for 28 samples total.
mouse BULK RNA SEQ n=28 QuantSeq expression profiling on...
duodenum UBERON:0002114 Uberon multi-species anatomy ontology (UBERON) Relation: this dataset samples this sample type This dataset samples duodenum (UBERON:0002114). UBERON:0002114 is a sample type from the Uberon multi-species anatomy ontology. kidney cortex UBERON:0001225 Uberon multi-species anatomy ontology (UBERON) Relation: this dataset samples this sample type This dataset samples kidney cortex, annotated with cortex of kidney (UBERON:0001225). UBERON:0001225 is a sample type from the Uberon multi-species anatomy ontology.
Conditions: Vdr wild-type Vdr knockout systemic Vdr Delta-AF2 high-calcium, high-phosphate, high-lactose rescue diet
PMID:39164247
GEO metadata and accession were independently resolved with just verify-datasets on 2026-08-20.
Show evidence (1 reference)
PMID:39164247 SUPPORT Model Organism
"In addition, RNA-sequencing analysis of duodenum and kidney revealed a decreased expression of VDR target genes in systemic VdrΔAF2 mice, which was not observed in Vdr-/- mice."
Confirms the tissues, sequencing modality, and mechanistic comparison represented by the dataset.
🧫

Experimental Models

1
Patient-Derived p.Arg30* VDR Dermal Fibroblasts PRIMARY_CELL_CULTURE
Primary skin fibroblasts from one individual homozygous for the severe p.Arg30* VDR allele were compared with age- and sex-matched control fibroblasts after vehicle or 10 nM calcitriol exposure for 24 hours. The model directly tests a human disease genotype but is limited to one donor and a dermal surrogate rather than enterocytes, kidney, growth plate, or bone.
homozygous VDR p.Arg30* patient fibroblasts age- and sex-matched control fibroblasts vehicle or calcitriol exposure
fibroblast CL:0000057 Cell Ontology (CL) Relation: this experimental model uses this cell type This experimental model uses fibroblast (CL:0000057). CL:0000057 is a cell type from the Cell Ontology.
Organism
human NCBITaxon:9606 NCBI Taxonomy (NCBITaxon) Relation: this experimental model is built in this organism This experimental model is built in human, annotated with Homo sapiens (NCBITaxon:9606). NCBITaxon:9606 is an organism from the NCBI Taxonomy.
Cell source
Primary patient-derived skin fibroblasts and matched control fibroblasts
Culture
Two-dimensional monolayer culture exposed to vehicle or 10 nM 1,25-dihydroxyvitamin D for 24 hours
Publication
PMID:30959822
Pathograph links
Impaired VDR-Regulated Transcription recapitulates fidelity: moderate Measures loss of the calcitriol-responsive transcriptome in patient cells.
Limitations: A single patient line and a fibroblast surrogate cannot reproduce intestinal mineral transport or skeletal disease.
Show evidence (1 reference)
PMID:30959822 SUPPORT In Vitro
"Roughly 4.5% of the transcriptome was regulated by 1,25D in CO fibroblasts, while MUT cells without a functional VDR were insensitive to 1,25D."
Directly measures the impaired transcriptional response represented by the target node.
Show evidence (1 reference)
PMID:30959822 SUPPORT In Vitro
"We analyzed the transcriptomic response to 1,25D in fibroblasts bearing a severe homozygous hereditary vitamin D resistant rickets-related p.Arg30* VDR mutation (MUT) and in control fibroblasts (CO)."
Establishes the disease genotype, human primary-cell system, and matched comparator.
🐁

Animal Models

3
VDR-null mouse (targeted ablation of the second zinc finger)
The reference model of this disorder, engineered to reproduce the human DNA-binding-domain lesion. Mice are normal at birth and develop hypocalcemia, hyperparathyroidism, rickets and osteomalacia on the human timeline, together with progressive alopecia that dietary vitamin D deficiency does not cause.
Species
Mouse
Genotype
Vdr knockout, second zinc finger of the DNA-binding domain deleted
Publication
PMID:9275211
Pathograph links
Defective Skeletal Mineralization recapitulates fidelity: high Reproduces the full mineral and skeletal sequence, and shows the growth-plate lesion appearing before overt rickets.
Limitations: The engineered allele removes the second zinc finger specifically; human disease alleles are heterogeneous across the DNA-binding and ligand-binding domains, so the model represents the DNA-binding class rather than the whole allelic spectrum.
Readouts
Hypertrophic chondrocyte zone width in the growth plate
Direction: INCREASED
Interpretation: Expansion of the hypertrophic zone is the growth-plate correlate of the mineralization defect, and it precedes radiographic rickets in this model.
Show evidence (1 reference)
PMID:9275211 SUPPORT Model Organism
"Rickets and osteomalacia are seen by day 35; however, as early as day 15, there is an expansion in the zone of hypertrophic chondrocytes in the growth plate."
Reports the histological measurement and its timing relative to overt rickets.
Show evidence (1 reference)
PMID:9275211 SUPPORT Model Organism
"They become hypocalcemic at 21 days of age, at which time their parathyroid hormone (PTH) levels begin to rise."
Establishes that the model reproduces the hypocalcemia and secondary hyperparathyroidism that drive the mineralization failure in patients.
Disrupted Ligand-Independent VDR Hair-Cycle Function recapitulates fidelity: high Progressive alopecia develops from four weeks of age and is absent from dietary vitamin D deficiency, matching the human observation that alopecia tracks receptor loss rather than hormone deficiency.
Limitations: Alopecia is present in some but not all human kindreds, so the model reproduces the alopecic subset rather than the disorder uniformly.
Show evidence (1 reference)
PMID:9275211 SUPPORT Model Organism
"In contrast to animals made vitamin D deficient by dietary means, and like some patients with VDDR II, these mice develop progressive alopecia from the age of 4 weeks."
Contrasts receptor ablation with dietary deficiency, isolating the receptor-dependent, hormone-independent origin of the alopecia.
Show evidence (1 reference)
PMID:9275211 SUPPORT Model Organism
"We have generated a mouse model of VDDR II by targeted ablation of the second zinc finger of the VDR DNA-binding domain."
Establishes that the model was engineered specifically to reproduce the human DNA-binding-domain lesion, which is what makes it informative for this disorder rather than for vitamin D deficiency generally.
Keratinocyte-targeted ligand-binding-null VDR transgene in VDR-null mice
A rescue experiment that separates the two arms of the disorder. Restoring a receptor that cannot bind hormone, in keratinocytes only, restores normal hair cycling, showing that the hair-follicle function of the receptor is ligand independent. As a transgenic mouse rescue, it does not establish universal treatment nonresponse of alopecia across human VDDR2A genotypes.
Species
Mouse
Genotype
Vdr knockout carrying a keratinocyte-targeted VDR transgene with a hormone-binding-domain mutation abolishing ligand binding
Publication
PMID:15591533
Pathograph links
Disrupted Ligand-Independent VDR Hair-Cycle Function rescues fidelity: high Keratinocyte-restricted expression of a ligand-binding-dead receptor rescues the hair cycle, demonstrating the mechanism this node asserts.
Limitations: A transgenic rescue rather than a disease model; it establishes sufficiency of the receptor protein for hair cycling but does not model any human genotype.
Readouts
Hair cycling in keratinocyte-rescued VDR-null mice
Direction: RESTORED
Interpretation: Normal hair cycling despite an inability to bind hormone localizes the alopecia mechanism to a ligand-independent receptor function.
Show evidence (1 reference)
PMID:15591533 SUPPORT Model Organism
"Keratinocyte-specific expression of a VDR transgene with a mutation in the hormone-binding domain that abolishes ligand binding restores normal hair cycling in VDR null mice"
Reports the rescue and the direction of the effect.
Show evidence (1 reference)
PMID:15591533 SUPPORT Model Organism
"These studies demonstrate that the effects of the VDR on the hair follicle are ligand independent and point to novel molecular and cellular actions of this nuclear receptor."
States the conclusion that grounds the ligand-independent framing of this node.
Show evidence (1 reference)
PMID:15591533 SUPPORT Model Organism
"To determine which functional domains of the VDR are required for hair cycling, mutant VDR transgenes were targeted to the keratinocytes of VDR null mice."
States the design of the transgenic rescue, which is what makes this system informative for the alopecia node specifically.
Systemic Vdr Delta-AF2 Mouse
The mutant receptor cannot recruit coactivators and therefore cannot mount ligand-induced transcription, but it retains coactivator-independent repressive activity. Mice develop severe undermineralized short bones and dysmorphic growth plates. This is an allele-mechanism model rather than a generic proxy for all VDDR2A: its phenotype is more severe than Vdr null and is not corrected by the high-calcium/phosphate/lactose rescue diet that normalizes the null model.
Species
Mouse
Genotype
Systemic deletion of the Vdr activation-function-2 domain (VdrΔAF2)
Publication
PMID:39164247
Pathograph links
Impaired VDR Coactivator Recruitment partially recapitulates fidelity: moderate Directly models loss of AF2-dependent coactivator recruitment while retaining repressive signaling.
Limitations: The engineered deletion retains AF2-independent repression and is more severe than Vdr absence, so its diet response must not be generalized to all human VDR variants or used to negate human calcium responsiveness.
Show evidence (1 reference)
PMID:39164247 SUPPORT Model Organism
"We confirm that ligand-induced transcriptional activation was disabled because the mutant VDRΔAF2 protein was unable to interact with coactivators."
Directly validates the coactivator-recruitment defect represented by the model.
Defective Skeletal Mineralization partially recapitulates fidelity: moderate Reproduces severe undermineralization and dysmorphic growth plates.
Limitations: Model-specific retained repression makes the skeletal phenotype more severe than Vdr null and resistant to a rescue diet; this does not predict universal failure of calcium treatment in patients.
Show evidence (1 reference)
PMID:39164247 SUPPORT Model Organism
"Interestingly, a rescue diet that is high in calcium, phosphate, and lactose, normalized this phenotype in Vdr-/-, but not in VdrΔAF2 mice."
Captures the model-specific rescue-diet divergence and its key limitation.
Show evidence (1 reference)
PMID:39164247 SUPPORT Model Organism
"Systemic VdrΔAF2 mice developed short, undermineralized bones with dysmorphic growth plates, a bone phenotype that was more pronounced than that of systemic Vdr knockout (Vdr-/-) mice."
Defines the systemic model and its more-severe-than-null skeletal phenotype.
{ }

Source YAML

click to show 
name: Vitamin D-Dependent Rickets Type 2A
synonyms:
- VDDR2A
- Hereditary vitamin D-resistant rickets
- HVDRR
- Hereditary 1,25-dihydroxyvitamin D-resistant rickets
creation_date: "2026-08-19T00:00:00Z"
category: Mendelian
description: >-
  Vitamin D-dependent rickets type 2A is impaired calcitriol signaling caused
  usually by biallelic loss-of-function variants in VDR and, very rarely, by a
  heterozygous dominant-negative VDR variant. Allele-specific defects can impair
  receptor production, ligand binding, DNA binding, RXR heterodimerization, or
  coactivator recruitment. The resulting loss of VDR-regulated intestinal
  calcium and phosphate absorption causes secondary hyperparathyroidism,
  hypophosphatemia, and rickets, while intact hormone production and dysregulated
  receptor-mediated homeostasis make circulating calcitriol characteristically
  high. Hormone resistance is not
  uniformly complete: some genotypes respond to supraphysiologic active vitamin
  D plus calcium, whereas severe oral-treatment-refractory disease requires
  intravenous calcium. Alopecia reflects a separate ligand-independent hair-cycle
  function of VDR; it is frequent but not universal and is strongly genotype
  dependent.
disease_term:
  preferred_term: vitamin D-dependent rickets, type 2A
  term:
    id: MONDO:0010186
    label: vitamin D-dependent rickets, type 2A
parents:
- Metabolic Bone Disorders
- Inborn Error of Metabolism
inheritance:
- name: Autosomal recessive inheritance
  inheritance_term:
    preferred_term: Autosomal recessive inheritance
    term:
      id: HP:0000007
      label: Autosomal recessive inheritance
  penetrance: UNKNOWN
  expressivity: VARIABLE
  description: >-
    The usual disease mechanism is a homozygous or compound-heterozygous
    pathogenic VDR genotype. Core skeletal and biochemical findings are
    consistently expressed among reported affected families and clinically
    ascertained series, while severity and alopecia are variably expressed;
    population penetrance among pathogenic-variant carriers is undetermined.
    Consanguinity increases the chance of homozygosity. Exceptionally, complete
    uniparental isodisomy of chromosome 12 can make one parental VDR allele
    homozygous and changes recurrence-risk counseling without constituting a new
    inheritance mode.
  evidence:
  - reference: PMID:2849209
    reference_title: Point mutations in the human vitamin D receptor gene associated with hypocalcemic rickets.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Two families with affected children homozygous for this autosomal recessive
      disorder were studied for abnormalities in the intracellular vitamin D
      receptor (VDR) and its gene.
    explanation: >-
      Establishes homozygous transmission in the founding molecular study.
  - reference: PMID:28377956
    reference_title: "Hereditary 1,25-dihydroxyvitamin D-resistant rickets (HVDRR) caused by a VDR mutation: A novel mechanism of dominant inheritance."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: >-
      Patients with HVDRR often have consanguinity in the family and display a
      number of clinical features including early onset rickets, hypocalcemia,
      and secondary hyperparathyroidism.
    explanation: >-
      Supports consanguinity as a common risk context for the usual recessive
      disease while identifying the characteristic skeletal and biochemical
      phenotype.
  - reference: PMID:38872968
    reference_title: 'Clinical characteristics and long-term management for patients with vitamin D-dependent rickets type II: a retrospective study at a single center in Saudi Arabia.'
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      As for initial laboratory tests, the PTH and Alkaline phosphatase (ALP)
      were high in all participants
    explanation: >-
      All 42 clinically ascertained patients had the characteristic biochemical
      disorder, documenting consistent expression among affected cohort members
      but not estimating penetrance among pathogenic-variant carriers.
  - reference: PMID:36705686
    reference_title: "Genotypic Spectrum and its Correlation with Alopecia and Clinical Response in Hereditary Vitamin D Resistant Rickets: Our Experience and Systematic Review."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Alopecia was present in 51.4% of non-truncating (NT) ligand-binding domain
      (LBD) variants, whereas it was universal in truncating LBD and all DNA
      binding-domain (DBD) variants.
    explanation: >-
      Demonstrates allele-class-dependent expressivity of alopecia rather than a
      universal hair phenotype.
  - reference: PMID:26153892
    reference_title: "Detection of Hereditary 1,25-Hydroxyvitamin D-Resistant Rickets Caused by Uniparental Disomy of Chromosome 12 Using Genome-Wide Single Nucleotide Polymorphism Array."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Genome-wide SNP array of the family (the patient and her parents) showed
      complete maternal isodisomy of chromosome 12.
    explanation: >-
      Establishes complete maternal chromosome-12 isodisomy as an exceptional
      route to VDR homozygosity.
- name: Rare autosomal dominant inheritance
  inheritance_term:
    preferred_term: Autosomal dominant inheritance
    term:
      id: HP:0000006
      label: Autosomal dominant inheritance
  description: >-
    Rare heterozygous VDR alleles can act dominantly by inhibiting the remaining
    wild-type receptor. This is an exceptional inheritance mechanism rather than
    the usual pattern for VDDR2A.
  evidence:
  - reference: PMID:28377956
    reference_title: "Hereditary 1,25-dihydroxyvitamin D-resistant rickets (HVDRR) caused by a VDR mutation: A novel mechanism of dominant inheritance."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      In conclusion, we found a dominant-negative mutant of VDR causing
      dominantly inherited HVDRR through a constitutive corepressor interaction,
      a mechanism similar to that in dominantly inherited thyroid hormone
      receptor mutations.
    explanation: >-
      Establishes the rare dominant-negative mechanism and autosomal dominant
      transmission in a segregating family.
genetic:
- name: VDR pathogenic variants
  gene_term:
    preferred_term: VDR
    term:
      id: hgnc:12679
      label: VDR
  association: Causative
  relationship_type: CAUSATIVE
  variant_origin: GERMLINE
  notes: >-
    Most affected individuals have homozygous or compound-heterozygous VDR
    variants; complete maternal chromosome-12 isodisomy is a rare alternative
    route to homozygosity. Representative alleles span non-truncating
    ligand-binding-domain variants such as p.Ile268Thr and p.Arg343Cys,
    DNA-binding-domain variants such as p.Val26Met, the RXR-binding-domain
    p.Arg343His variant, splice-site variants, truncating variants, and
    compound-heterozygous combinations. Functional consequences include absent
    or truncated receptor, impaired ligand binding, impaired DNA binding,
    defective RXR heterodimerization, and defective coactivator recruitment;
    residual activity, alopecia, and treatment response vary by allele class.
    Exceptionally, heterozygous dominant-negative alleles such as p.Q400LfsX7
    inhibit wild-type VDR through constitutive corepressor retention and cause
    dominant disease.
  evidence:
  - reference: PMID:2849209
    reference_title: Point mutations in the human vitamin D receptor gene associated with hypocalcemic rickets.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      In each family, a different single nucleotide mutation was found in the DNA
      binding domain of the protein; one family near the tip of the first zinc
      finger (Gly----Asp) and one at the tip of the second zinc finger
      (Arg----Gly).
    explanation: >-
      Identifies the causal VDR variants and localizes them to the DNA-binding
      domain.
  - reference: PMID:38872968
    reference_title: 'Clinical characteristics and long-term management for patients with vitamin D-dependent rickets type II: a retrospective study at a single center in Saudi Arabia.'
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Molecular genetic testing of the VDR gene in our cohort identified six
      different gene variants c.885 C>A (p.Tyr295Ter), c.88 C>T (p.Arg30Ter),
      c.1036G>A (p.Val346Met), c.820C>T (p.Arg274Cys), c.803 T>C (p.Ile268Thr),
      and c.2T>G (p.Met1?).
    explanation: >-
      Documents the allelic spectrum in the largest reported cohort.
  - reference: PMID:28377956
    reference_title: "Hereditary 1,25-dihydroxyvitamin D-resistant rickets (HVDRR) caused by a VDR mutation: A novel mechanism of dominant inheritance."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      The mutant had a dominant-negative effect on wild-type-VDR, and the ligand
      binding affinity of Q400LfsX7-VDR was completely impaired.
    explanation: >-
      Functional testing identifies Q400LfsX7 as a dominant-negative causal VDR
      allele rather than treating the accompanying functionally normal R370H
      variant as causal.
  - reference: PMID:15308610
    reference_title: Hereditary 1,25-dihydroxyvitamin D resistant rickets due to a mutation causing multiple defects in vitamin D receptor function.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      In conclusion, we describe a novel mutation, I268T, in the VDR
      ligand-binding domain that alters ligand binding, retinoid X receptor
      heterodimerization, and coactivator binding.
    explanation: >-
      Establishes p.Ile268Thr as a ligand-binding-domain allele and documents
      its experimentally measured multiple functional defects.
  - reference: PMID:19815438
    reference_title: "Hereditary 1,25-dihydroxyvitamin D-resistant rickets with alopecia resulting from a novel missense mutation in the DNA-binding domain of the vitamin D receptor."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      We found that the child was homozygous for a unique missense mutation in
      the VDR gene that converted valine to methionine at amino acid 26 (V26M)
      in the VDR DNA-binding domain (DBD).
    explanation: >-
      Provides a representative DNA-binding-domain missense allele.
  - reference: PMID:29127362
    reference_title: Functional Analysis of VDR Gene Mutation R343H in A Child with Vitamin D-Resistant Rickets with Alopecia.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      A novel homozygous missense R343H mutation in the exon 9 of VDR residing
      in the retinoid X receptor (RXR)-binding domain was identified.
    explanation: >-
      Provides the p.Arg343His example and its location at the RXR-binding
      interface.
  - reference: PMID:42438800
    reference_title: A case of vitamin D-dependent rickets type 2A presenting with hypophosphatemia without hypocalcemia.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Genetic testing using whole-exome sequencing identified a homozygous
      pathogenic variant in the VDR (c.1027C>T, p.R343C), confirming the
      diagnosis of VDDR2A. Both parents were found to be heterozygous carriers.
    explanation: >-
      Provides the p.Arg343Cys example and recessive segregation.
  - reference: PMID:42438800
    reference_title: A case of vitamin D-dependent rickets type 2A presenting with hypophosphatemia without hypocalcemia.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: >-
      The VDR c.1027C>T (p.R343C) variant results in a missense substitution of
      arginine, a polar amino acid, to cysteine, a neutral and slight polar amino
      acid, at position 343, which is located within the LBD of the VDR [12].
    explanation: >-
      Locally establishes p.Arg343Cys as an LBD allele without inferring a
      specific interface defect or functional consequence from this case.
  - reference: PMID:38872968
    reference_title: 'Clinical characteristics and long-term management for patients with vitamin D-dependent rickets type II: a retrospective study at a single center in Saudi Arabia.'
    supports: SUPPORT
    evidence_source: OTHER
    snippet: >-
      Several types of variants have been identified in the VDR gene as the
      leading cause of HVDDR-II, such as non-sense, substitutions, insertions,
      duplications, deletions, and splice-site (2).
    explanation: >-
      Documents the broad causal variant classes, including splice-site alleles.
  - reference: PMID:19049339
    reference_title: "Compound heterozygous mutations in the vitamin D receptor in a patient with hereditary 1,25-dihydroxyvitamin D-resistant rickets with alopecia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      In summary, we describe a patient with compound heterozygous mutations in
      the VDR that results in HVDRR with alopecia.
    explanation: >-
      Establishes compound heterozygosity as a disease-causing allelic state.
  - reference: PMID:26153892
    reference_title: "Detection of Hereditary 1,25-Hydroxyvitamin D-Resistant Rickets Caused by Uniparental Disomy of Chromosome 12 Using Genome-Wide Single Nucleotide Polymorphism Array."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      This is the first report of HVDRR caused by UPD, and the third case of
      complete UPD of chromosome 12, in the published literature.
    explanation: >-
      Separates the exceptional route to homozygosity from the usual biparental
      recessive inheritance.
pathophysiology:
- name: Absent or Truncated VDR Protein
  description: >-
    This allele-class branch applies when biallelic nonsense, frameshift, or
    other loss-of-function variants prevent production of a functional VDR
    protein. Loss of the receptor abolishes both ligand-activated transcription
    and the ligand-independent receptor function needed for hair cycling.
  role: trigger
  biological_scale: MOLECULAR
  genes:
  - preferred_term: VDR
    term:
      id: hgnc:12679
      label: VDR
  genetic_context:
    gene:
      preferred_term: VDR
      term:
        id: hgnc:12679
        label: VDR
    allele_type: Biallelic null or truncating variants
    variant_origin: GERMLINE
    functional_impact_category: LOSS_OF_FUNCTION
    description: This branch is one alternative VDR defect and is not asserted for every affected individual.
  evidence:
  - reference: PMID:32596195
    reference_title: Diagnosis and Management of Vitamin D Dependent Rickets.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: >-
      Alopecia is associated with VDR mutations that impair DNA binding, RXR
      heterodimerization, or production of the VDR, while mutations that alter
      VDR affinity for 1,25(OH)2D or disrupt coactivator interactions do not
      cause alopecia.
    explanation: >-
      Distinguishes loss of receptor production from other allele-specific
      defects and links this class to loss of the hair-cycle function.
  downstream:
  - target: Impaired VDR-Regulated Transcription
    causal_link_type: DIRECT
  - target: Disrupted Ligand-Independent VDR Hair-Cycle Function
    causal_link_type: DIRECT
- name: Impaired Calcitriol Binding by VDR
  description: >-
    This allele-class branch applies to ligand-binding-domain variants that
    reduce or abolish calcitriol binding. Residual ligand affinity varies, so
    hormone resistance and responsiveness to supraphysiologic active vitamin D
    are graded rather than universally complete.
  role: trigger
  biological_scale: MOLECULAR
  genes:
  - preferred_term: VDR
    term:
      id: hgnc:12679
      label: VDR
  genetic_context:
    gene:
      preferred_term: VDR
      term:
        id: hgnc:12679
        label: VDR
    allele_type: Ligand-binding-domain variant
    variant_origin: GERMLINE
    functional_impact_category: LOSS_OF_FUNCTION
    description: Includes partial and complete ligand-binding defects; not every allele retains residual activity.
  molecular_functions:
  - preferred_term: vitamin D binding
    term:
      id: GO:0005499
      label: vitamin D binding
    modifier: DECREASED
  evidence:
  - reference: PMID:28377956
    reference_title: "Hereditary 1,25-dihydroxyvitamin D-resistant rickets (HVDRR) caused by a VDR mutation: A novel mechanism of dominant inheritance."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      The mutant had a dominant-negative effect on wild-type-VDR, and the ligand
      binding affinity of Q400LfsX7-VDR was completely impaired.
    explanation: >-
      Demonstrates complete ligand-binding failure for one disease-associated
      VDR allele without generalizing that result to all VDDR2A variants.
  downstream:
  - target: Impaired VDR-Regulated Transcription
    causal_link_type: DIRECT
- name: Impaired VDR Binding to Vitamin D Response Elements
  description: >-
    DNA-binding-domain variants can preserve hormone binding while reducing VDR
    affinity for vitamin D response elements. This is a specific allele-class
    mechanism, not the universal molecular lesion in VDDR2A.
  role: trigger
  biological_scale: MOLECULAR
  genes:
  - preferred_term: VDR
    term:
      id: hgnc:12679
      label: VDR
  genetic_context:
    gene:
      preferred_term: VDR
      term:
        id: hgnc:12679
        label: VDR
    allele_type: DNA-binding-domain variant
    variant_origin: GERMLINE
    functional_impact_category: LOSS_OF_FUNCTION
  molecular_functions:
  - preferred_term: vitamin D response element binding
    term:
      id: GO:0070644
      label: vitamin D response element binding
    modifier: DECREASED
  evidence:
  - reference: PMID:2849209
    reference_title: Point mutations in the human vitamin D receptor gene associated with hypocalcemic rickets.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      Although the receptor displays normal binding of 1,25-dihydroxyvitamin D3
      hormone, VDR from affected family members has a decreased affinity for DNA.
    explanation: >-
      Directly isolates impaired DNA binding in the founding DNA-binding-domain
      kindreds while preserving the allele-specific framing.
  downstream:
  - target: Impaired VDR-Regulated Transcription
    causal_link_type: DIRECT
  - target: Disrupted Ligand-Independent VDR Hair-Cycle Function
    causal_link_type: DIRECT
- name: Impaired VDR-RXR Heterodimerization
  description: >-
    Some ligand-binding-domain variants disrupt formation of the VDR-RXR
    heterodimer required for response-element occupancy and transcription. This
    class is also associated with loss of ligand-independent hair-cycle function.
  role: trigger
  biological_scale: MOLECULAR
  genes:
  - preferred_term: VDR
    term:
      id: hgnc:12679
      label: VDR
  genetic_context:
    gene:
      preferred_term: VDR
      term:
        id: hgnc:12679
        label: VDR
    allele_type: RXR-heterodimerization-defective VDR variant
    variant_origin: GERMLINE
    functional_impact_category: LOSS_OF_FUNCTION
  molecular_functions:
  - preferred_term: protein heterodimerization activity
    term:
      id: GO:0046982
      label: protein heterodimerization activity
    modifier: DECREASED
  evidence:
  - reference: PMID:36705686
    reference_title: "Genotypic Spectrum and its Correlation with Alopecia and Clinical Response in Hereditary Vitamin D Resistant Rickets: Our Experience and Systematic Review."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Among LBD-NT variants, those affecting RXR heterodimerization, but not
      those affecting ligand affinity, were associated with alopecia.
    explanation: >-
      Separates RXR-heterodimerization defects from ligand-affinity defects and
      supports their different hair phenotype.
  downstream:
  - target: Impaired VDR-Regulated Transcription
    causal_link_type: DIRECT
  - target: Disrupted Ligand-Independent VDR Hair-Cycle Function
    causal_link_type: DIRECT
- name: Impaired VDR Coactivator Recruitment
  description: >-
    Activation-function-2 and other ligand-binding-domain variants can preserve
    receptor production, DNA binding, and RXR interaction yet fail to recruit
    transcriptional coactivators after calcitriol binding.
  role: trigger
  biological_scale: MOLECULAR
  genes:
  - preferred_term: VDR
    term:
      id: hgnc:12679
      label: VDR
  genetic_context:
    gene:
      preferred_term: VDR
      term:
        id: hgnc:12679
        label: VDR
    allele_type: Coactivator-recruitment-defective VDR variant
    variant_origin: GERMLINE
    functional_impact_category: LOSS_OF_FUNCTION
  evidence:
  - reference: PMID:39164247
    reference_title: "Coactivator-independent vitamin D receptor signaling causes severe rickets in mice, that is not prevented by a diet high in calcium, phosphate, and lactose."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: >-
      We confirm that ligand-induced transcriptional activation was disabled
      because the mutant VDRΔAF2 protein was unable to interact with
      coactivators.
    explanation: >-
      Demonstrates the atomic coactivator-recruitment defect in an AF2-deletion
      model; it does not imply that this mechanism applies to every human allele.
  downstream:
  - target: Impaired VDR-Regulated Transcription
    causal_link_type: DIRECT
- name: Dominant-Negative VDR Corepressor Retention
  description: >-
    In the rare dominant branch, a heterozygous mutant receptor occupies VDREs,
    fails to release the NCoR corepressor after ligand exposure, and suppresses
    transcription by the coexpressed wild-type receptor.
  role: trigger
  biological_scale: MOLECULAR
  genes:
  - preferred_term: VDR
    term:
      id: hgnc:12679
      label: VDR
  genetic_context:
    gene:
      preferred_term: VDR
      term:
        id: hgnc:12679
        label: VDR
    allele_type: p.Q400LfsX7 (p.Gln400LeufsTer7)
    variant_origin: GERMLINE
    zygosity: HETEROZYGOUS
    functional_impact_category: DOMINANT_NEGATIVE
    description: Applies to the rare dominant-negative allele class, not to unaffected heterozygous carriers of ordinary recessive alleles.
  evidence:
  - reference: PMID:28377956
    reference_title: "Hereditary 1,25-dihydroxyvitamin D-resistant rickets (HVDRR) caused by a VDR mutation: A novel mechanism of dominant inheritance."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      Interestingly, Q400LfsX7-VDR had a strong interaction with corepressor
      NCoR and could interact with VDRE without the ligand.
    explanation: >-
      Shows the corepressor retention and VDRE occupancy underlying the
      dominant-negative branch.
  downstream:
  - target: Impaired VDR-Regulated Transcription
    causal_link_type: DIRECT
- name: Impaired VDR-Regulated Transcription
  description: >-
    The distinct allele-class defects converge on reduced ligand-activated VDR
    transcription. The degree of residual output differs among variants, which
    accounts for partial hormone responsiveness in some patients and profound
    resistance in others.
  role: central_effector
  biological_scale: MOLECULAR
  biological_processes:
  - preferred_term: vitamin D receptor signaling pathway
    term:
      id: GO:0070561
      label: vitamin D receptor signaling pathway
    modifier: DECREASED
  evidence:
  - reference: PMID:30959822
    reference_title: "Transcriptomic Response to 1,25-Dihydroxyvitamin D in Human Fibroblasts with or without a Functional Vitamin D Receptor (VDR): Novel Target Genes and Insights into VDR Basal Transcriptional Activity."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      Roughly 4.5% of the transcriptome was regulated by 1,25D in CO
      fibroblasts, while MUT cells without a functional VDR were insensitive to
      1,25D.
    explanation: >-
      Patient-derived p.Arg30* fibroblasts directly demonstrate loss of the
      transcriptomic response to calcitriol.
  downstream:
  - target: Reduced VDR-Dependent Enterocyte Calcium and Phosphate Transport
    causal_link_type: DIRECT
    description: >-
      Reduced transcription of intestinal VDR targets impairs active mineral
      absorption.
  - target: Elevated Circulating Calcitriol in VDR Resistance
    causal_link_type: DIRECT
    description: >-
      Reduced VDR output dysregulates receptor-mediated vitamin D homeostasis,
      contributing to elevated calcitriol despite deficient biological action.
- name: Elevated Circulating Calcitriol in VDR Resistance
  description: >-
    Calcitriol synthesis remains intact while mineral deficiency, secondary
    hyperparathyroidism, and dysregulated receptor-mediated vitamin D homeostasis
    favor a markedly elevated circulating 1,25-dihydroxyvitamin D concentration.
    This biochemical state is a clue to receptor resistance; it does not imply
    that every VDR allele has zero residual response to hormone.
  role: effector
  biological_scale: ORGANISM
  biological_processes:
  - preferred_term: vitamin D metabolic process
    term:
      id: GO:0042359
      label: vitamin D metabolic process
    modifier: DYSREGULATED
  evidence:
  - reference: PMID:42438800
    reference_title: A case of vitamin D-dependent rickets type 2A presenting with hypophosphatemia without hypocalcemia.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Further evaluation revealed markedly elevated 1,25(OH)2D, and whole-exome
      sequencing identified a homozygous pathogenic VDR c.1027C>T (p.R343C)
      variant, confirming VDDR2A.
    explanation: >-
      Links markedly elevated calcitriol to genetically confirmed VDDR2A.
- name: Reduced VDR-Dependent Enterocyte Calcium and Phosphate Transport
  description: >-
    Reduced VDR transcription in enterocytes decreases active calcium and
    phosphate uptake. Calcium may be low or maintained near normal by
    compensatory hyperparathyroidism, so hypocalcemia is not required for this
    mechanism or for diagnosis.
  role: central_effector
  biological_scale: CELLULAR
  conforms_to: "defective_skeletal_mineralization#Calciopenic Substrate Deficiency"
  cell_types:
  - preferred_term: Enterocyte
    term:
      id: CL:0000584
      label: enterocyte
  biological_processes:
  - preferred_term: Intestinal Calcium Absorption
    term:
      id: GO:0070509
      label: calcium ion import
    modifier: DECREASED
  evidence:
  - reference: PMID:38872968
    reference_title: 'Clinical characteristics and long-term management for patients with vitamin D-dependent rickets type II: a retrospective study at a single center in Saudi Arabia.'
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      The primary defect caused by the mutant VDR gene is a decrease in
      intestinal calcium and phosphate absorption, which leads to decreased bone
      mineralization and rickets (3), despite high levels of 1,25(OH)2D (4).
    explanation: >-
      States the intestinal calcium and phosphate malabsorption this node models,
      and that it occurs despite elevated circulating calcitriol.
  downstream:
  - target: Hypocalcemia
    causal_link_type: DIRECT
    description: Reduced active intestinal calcium uptake can lower circulating calcium.
  - target: Compensatory Secondary Hyperparathyroidism
    causal_link_type: DIRECT
    description: >-
      Reduced calcium delivery stimulates compensatory parathyroid hormone
      secretion even when that compensation maintains serum calcium near normal.
  - target: Defective Skeletal Mineralization
    causal_link_type: DIRECT
    description: Reduced calcium and phosphate delivery limits mineral substrate.
- name: Compensatory Secondary Hyperparathyroidism
  description: >-
    Reduced calcium absorption increases parathyroid hormone secretion. The
    response can preserve normocalcemia through renal calcium conservation and
    bone resorption while simultaneously promoting renal phosphate loss.
  role: central_effector
  biological_scale: ORGANISM
  evidence:
  - reference: PMID:42438800
    reference_title: A case of vitamin D-dependent rickets type 2A presenting with hypophosphatemia without hypocalcemia.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      The preserved serum calcium level may reflect compensatory secondary
      hyperparathyroidism, in which markedly elevated PTH maintains normocalcemia
      through increased renal calcium reabsorption and bone resorption.
    explanation: >-
      Explains why receptor resistance can produce marked secondary
      hyperparathyroidism without measured hypocalcemia.
  downstream:
  - target: Secondary hyperparathyroidism
    causal_link_type: DIRECT
  - target: PTH-Driven Renal Phosphate Wasting
    causal_link_type: DIRECT
  - target: Elevated Circulating Calcitriol in VDR Resistance
    causal_link_type: DIRECT
    description: Elevated PTH stimulates renal calcitriol production.
- name: PTH-Driven Renal Phosphate Wasting
  description: >-
    Elevated parathyroid hormone reduces renal phosphate reabsorption, adding a
    phosphate deficit to impaired intestinal mineral uptake.
  role: central_effector
  biological_scale: ORGANISM
  evidence:
  - reference: PMID:42438800
    reference_title: A case of vitamin D-dependent rickets type 2A presenting with hypophosphatemia without hypocalcemia.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      In contrast, elevated PTH promotes renal phosphate wasting, contributing
      to hypophosphatemia.
    explanation: >-
      Directly supports the PTH-to-phosphate-wasting step in the mineral pathway.
  downstream:
  - target: Hypophosphatemia
    causal_link_type: DIRECT
  - target: Defective Skeletal Mineralization
    causal_link_type: DIRECT
    description: Phosphate wasting further reduces substrate for hydroxyapatite deposition.
- name: Defective Skeletal Mineralization
  description: >-
    Early and often severe rickets with bowing of the weight-bearing limbs,
    delayed walking and growth failure.
  role: effector
  biological_scale: TISSUE
  conforms_to: "defective_skeletal_mineralization#Impaired Hydroxyapatite Deposition at the Mineralization Front"
  cell_types:
  - preferred_term: Osteoblast
    term:
      id: CL:0000062
      label: osteoblast
  biological_processes:
  - preferred_term: Bone Mineralization
    term:
      id: GO:0030282
      label: bone mineralization
    modifier: DECREASED
  locations:
  - preferred_term: Bone
    term:
      id: UBERON:0001474
      label: bone element
  evidence:
  - reference: PMID:38872968
    reference_title: 'Clinical characteristics and long-term management for patients with vitamin D-dependent rickets type II: a retrospective study at a single center in Saudi Arabia.'
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      The median age at presentation was 15.5 months.
    explanation: >-
      Records the early-infantile onset of the skeletal disease in the largest
      reported cohort.
  downstream:
  - target: Rickets
    causal_link_type: DIRECT
  - target: Elevated circulating alkaline phosphatase
    causal_link_type: DIRECT
  - target: Bowing of the legs
    causal_link_type: DIRECT
  - target: Short stature
    causal_link_type: DIRECT
  - target: Delayed ability to walk
    causal_link_type: DIRECT
  - target: Bone pain
    causal_link_type: DIRECT
  - target: Enlargement of the costochondral junction
    causal_link_type: DIRECT
- name: Disrupted Ligand-Independent VDR Hair-Cycle Function
  description: >-
    Hair distribution is initially normal, but receptor absence or variants that
    disrupt DNA binding or RXR heterodimerization can impair the unliganded VDR
    function required to reinitiate the hair cycle. Ligand-affinity and
    coactivator-recruitment defects can preserve this branch, explaining why
    alopecia is genotype dependent and absent in some affected individuals.
  role: effector
  biological_scale: TISSUE
  biological_processes:
  - preferred_term: hair cycle
    term:
      id: GO:0042633
      label: hair cycle
    modifier: DECREASED
  evidence:
  - reference: PMID:32596195
    reference_title: Diagnosis and Management of Vitamin D Dependent Rickets.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: >-
      In addition, affected children are born with normal distribution of hair,
      and alopecia develops later due to failure of normal hair follicle cycling,
      which is dependent upon unliganded actions of the VDR.
    explanation: >-
      Supports the postnatal, ligand-independent hair-cycle mechanism.
  downstream:
  - target: Alopecia
    causal_link_type: DIRECT
phenotypes:
- category: Skeletal
  name: Rickets
  phenotype_term:
    preferred_term: Rickets
    term:
      id: HP:0002748
      label: Rickets
  evidence:
  - reference: PMID:2849209
    reference_title: Point mutations in the human vitamin D receptor gene associated with hypocalcemic rickets.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Hypocalcemic vitamin D-resistant rickets is a human genetic disease
      resulting from target organ resistance to the action of
      1,25-dihydroxyvitamin D3.
    explanation: >-
      Names rickets as the defining skeletal presentation of the disorder.
- category: Musculoskeletal
  name: Muscle weakness
  description: >-
    Muscle weakness is reported in affected children. No disease-wide frequency
    is assigned because the available source does not provide a denominator.
  phenotype_term:
    preferred_term: Muscle weakness
    term:
      id: HP:0001324
      label: Muscle weakness
  evidence:
  - reference: PMID:20511055
    reference_title: Genetic disorders and defects in vitamin d action.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: >-
      Affected children have bone pain, muscle weakness, hypotonia and
      occasionally have convulsions due to the hypocalcemia.
    explanation: >-
      An HVDRR-focused clinical review directly identifies muscle weakness
      without supplying a quantitative prevalence.
- category: Musculoskeletal
  name: Hypotonia
  description: >-
    Hypotonia is reported in affected children. Frequency is unset because the
    available source gives no denominator.
  phenotype_term:
    preferred_term: Hypotonia
    term:
      id: HP:0001252
      label: Hypotonia
  evidence:
  - reference: PMID:20511055
    reference_title: Genetic disorders and defects in vitamin d action.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: >-
      Affected children have bone pain, muscle weakness, hypotonia and
      occasionally have convulsions due to the hypocalcemia.
    explanation: >-
      Directly includes hypotonia in the HVDRR clinical spectrum without
      providing a quantitative prevalence.
- category: Skeletal
  name: Bone pain
  description: >-
    Bone pain reflects severe defective mineralization. Frequency is left unset
    because the direct evidence is a single treated case.
  phenotype_term:
    preferred_term: Bone pain
    term:
      id: HP:0002653
      label: Bone pain
  evidence:
  - reference: PMID:40383122
    reference_title: Persistent Hyperparathyroidism in Vitamin D-Dependent Rickets Type 2A Does Not Prevent Normalization of Hypophosphatemia or Healing of the Rickets.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Upon the initiation of IV calcium infusions, bone pain resolved, and the
      rickets healed within weeks.
    explanation: >-
      Directly documents bone pain in genetically confirmed VDDR2A while not
      implying a disease-wide frequency from one case.
- category: Skeletal
  name: Enlargement of the costochondral junction
  description: >-
    Expansion of the costochondral junctions is the anatomic basis of the
    rachitic rosary. Frequency is unset because the evidence is a single case.
  phenotype_term:
    preferred_term: Enlargement of the costochondral junction
    term:
      id: HP:0000920
      label: Enlargement of the costochondral junction
  evidence:
  - reference: PMID:42438800
    reference_title: A case of vitamin D-dependent rickets type 2A presenting with hypophosphatemia without hypocalcemia.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Radiographs demonstrated mild expansion of the anterior ribs at the
      costochondral junctions, flaring and fraying of metaphyses, and mild bowing
      of the femora, without fracture.
    explanation: >-
      Directly supports costochondral-junction enlargement in VDDR2A. This uses
      HP:0000920; HP:0004464 is postauricular pit and is not a valid mapping for
      rachitic rosary.
- category: Dental
  name: Enamel hypoplasia
  description: >-
    Defective mineralization can involve developing teeth. Frequency is unset
    because the cited source describes the characteristic presentation without
    a cohort denominator for this finding.
  phenotype_term:
    preferred_term: Enamel hypoplasia
    term:
      id: HP:0006297
      label: Enamel hypoplasia
  evidence:
  - reference: PMID:38872968
    reference_title: 'Clinical characteristics and long-term management for patients with vitamin D-dependent rickets type II: a retrospective study at a single center in Saudi Arabia.'
    supports: SUPPORT
    evidence_source: OTHER
    snippet: >-
      The patient will present with the same characteristic features of
      nutritional rickets in the form of frontal bossing, bowing of the leg,
      growth failure, dental caries, and enamel hypoplasia (5).
    explanation: >-
      Supports enamel hypoplasia as part of the VDDR2A clinical spectrum but
      does not quantify its frequency in the reported cohort.
- category: Integumentary
  name: Alopecia
  description: >-
    Scalp and body alopecia often develops after initially normal hair. It is a
    strong diagnostic clue but is absent in a substantial minority and varies by
    the functional class of the VDR variant.
  frequency: VERY_FREQUENT
  phenotype_term:
    preferred_term: Alopecia
    term:
      id: HP:0001596
      label: Alopecia
  evidence:
  - reference: PMID:36705686
    reference_title: "Genotypic Spectrum and its Correlation with Alopecia and Clinical Response in Hereditary Vitamin D Resistant Rickets: Our Experience and Systematic Review."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      On systematic review, alopecia was present in majority (81.5%) and
      preceded the onset of rickets.
    explanation: >-
      Reports alopecia in 81.5% of 119 systematically reviewed probands, mapping
      to VERY_FREQUENT while showing that the finding is not universal.
  phenotype_contexts:
  - subtype: Non-truncating ligand-binding-domain variants
    frequency: FREQUENT
    notes: Alopecia was reported in 51.4% of this allele class.
    evidence:
    - reference: PMID:36705686
      reference_title: "Genotypic Spectrum and its Correlation with Alopecia and Clinical Response in Hereditary Vitamin D Resistant Rickets: Our Experience and Systematic Review."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
        Alopecia was present in 51.4% of non-truncating (NT) ligand-binding
        domain (LBD) variants, whereas it was universal in truncating LBD and all
        DNA binding-domain (DBD) variants.
      explanation: Quantifies the lower frequency in non-truncating LBD variants.
  - subtype: Truncating ligand-binding-domain or DNA-binding-domain variants
    frequency: OBLIGATE
    notes: Universal in these variant classes in the systematic review, not asserted as universal across VDDR2A.
    evidence:
    - reference: PMID:36705686
      reference_title: "Genotypic Spectrum and its Correlation with Alopecia and Clinical Response in Hereditary Vitamin D Resistant Rickets: Our Experience and Systematic Review."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
        Alopecia was present in 51.4% of non-truncating (NT) ligand-binding
        domain (LBD) variants, whereas it was universal in truncating LBD and all
        DNA binding-domain (DBD) variants.
      explanation: Restricts the universal frequency claim to the reported truncating-LBD and DBD strata.
- category: Neurological
  name: Seizure
  frequency: OCCASIONAL
  phenotype_term:
    preferred_term: Seizure
    term:
      id: HP:0001250
      label: Seizure
  evidence:
  - reference: PMID:38872968
    reference_title: 'Clinical characteristics and long-term management for patients with vitamin D-dependent rickets type II: a retrospective study at a single center in Saudi Arabia.'
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Alopecia was found in 97.6%, 21.4% presented with bowing legs, 14.3% with
      delayed walking, 9.5% with seizure, and 2.4% presented with respiratory
      failure, while a family history of the disease was positive in 71.4% of
      total patients.
    explanation: >-
      Seizure at presentation in 9.5% of patients maps to OCCASIONAL (5-29%);
      seizures here are a consequence of hypocalcemia.
- category: Biochemical
  name: Hypocalcemia
  description: >-
    Common but not required; compensatory secondary hyperparathyroidism can
    maintain a normal serum calcium concentration despite receptor resistance.
  phenotype_term:
    preferred_term: Hypocalcemia
    term:
      id: HP:0002901
      label: Hypocalcemia
  evidence:
  - reference: PMID:38872968
    reference_title: "Clinical characteristics and long-term management for patients with vitamin D-dependent rickets type II: a retrospective study at a single center in Saudi Arabia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Patients with HVDDR-II have rickets along with hypocalcemia,
      hypophosphatemia, secondary hyperparathyroidism, and elevated serum
      1,25(OH)2D levels, which is a hallmark for diagnosing this disease.
    explanation: >-
      Reports hypocalcemia as part of the characteristic biochemical profile of
      the disorder.
  sequelae:
  - target: Seizure
    causal_link_type: DIRECT
    description: Severe hypocalcemia can precipitate seizures.
- category: Biochemical
  name: Hypophosphatemia
  frequency: FREQUENT
  phenotype_term:
    preferred_term: Hypophosphatemia
    term:
      id: HP:0002148
      label: Hypophosphatemia
  evidence:
  - reference: PMID:42438800
    reference_title: A case of vitamin D-dependent rickets type 2A presenting with hypophosphatemia without hypocalcemia.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Hypophosphatemia is not consistently observed, occurring in around 58% of
      reported patients [9].
    explanation: The reported frequency maps to FREQUENT and avoids treating hypophosphatemia as universal.
  - reference: PMID:38872968
    reference_title: "Clinical characteristics and long-term management for patients with vitamin D-dependent rickets type II: a retrospective study at a single center in Saudi Arabia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Patients with HVDDR-II have rickets along with hypocalcemia,
      hypophosphatemia, secondary hyperparathyroidism, and elevated serum
      1,25(OH)2D levels, which is a hallmark for diagnosing this disease.
    explanation: >-
      Reports hypophosphatemia as part of the characteristic profile. Note the
      research artifact records presentations with hypophosphatemia but WITHOUT
      hypocalcemia, which can be mistaken for FGF23-mediated hypophosphatemic
      rickets - the differential this grouping exists to separate.
- category: Biochemical
  name: Secondary hyperparathyroidism
  phenotype_term:
    preferred_term: Secondary hyperparathyroidism
    term:
      id: HP:0000867
      label: Secondary hyperparathyroidism
  evidence:
  - reference: PMID:38872968
    reference_title: "Clinical characteristics and long-term management for patients with vitamin D-dependent rickets type II: a retrospective study at a single center in Saudi Arabia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Patients with HVDDR-II have rickets along with hypocalcemia,
      hypophosphatemia, secondary hyperparathyroidism, and elevated serum
      1,25(OH)2D levels, which is a hallmark for diagnosing this disease.
    explanation: >-
      Reports secondary hyperparathyroidism, the compensatory response to
      receptor-level calcium malabsorption.
- category: Biochemical
  name: Elevated circulating alkaline phosphatase
  phenotype_term:
    preferred_term: Elevated circulating alkaline phosphatase concentration
    term:
      id: HP:0003155
      label: Elevated circulating alkaline phosphatase concentration
  evidence:
  - reference: PMID:38872968
    reference_title: "Clinical characteristics and long-term management for patients with vitamin D-dependent rickets type II: a retrospective study at a single center in Saudi Arabia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      HVDDR-II is characterized by early onset of rickets presentation and
      biochemical parameters such as low serum calcium levels, high parathyroid
      hormone, high alkaline phosphatase, and high 1,25(OH)2D levels (1).
    explanation: >-
      Reports elevated alkaline phosphatase among the characteristic
      biochemical parameters.
- category: Skeletal
  name: Bowing of the legs
  frequency: OCCASIONAL
  phenotype_term:
    preferred_term: Bowing of the legs
    term:
      id: HP:0002979
      label: Bowing of the legs
  evidence:
  - reference: PMID:38872968
    reference_title: "Clinical characteristics and long-term management for patients with vitamin D-dependent rickets type II: a retrospective study at a single center in Saudi Arabia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      The most frequently reported symptoms are alopecia in 41 patients (97.6%),
      followed by short stature (height SDS<-2) in 36 patients (85.7%), bowing of
      legs in 9 patients (21.4%), delayed walking in 6 patients (14.3%), seizures
      in 4 patients (9.5%), and respiratory symptoms in 1 patient (2.4%) (
    explanation: >-
      Bowing of the legs occurred in 21.4% of this 42-patient referral cohort,
      mapping to OCCASIONAL.
- category: Developmental
  name: Delayed ability to walk
  frequency: OCCASIONAL
  phenotype_term:
    preferred_term: Delayed ability to walk
    term:
      id: HP:0031936
      label: Delayed ability to walk
  evidence:
  - reference: PMID:38872968
    reference_title: "Clinical characteristics and long-term management for patients with vitamin D-dependent rickets type II: a retrospective study at a single center in Saudi Arabia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      The most frequently reported symptoms are alopecia in 41 patients (97.6%),
      followed by short stature (height SDS<-2) in 36 patients (85.7%), bowing of
      legs in 9 patients (21.4%), delayed walking in 6 patients (14.3%), seizures
      in 4 patients (9.5%), and respiratory symptoms in 1 patient (2.4%) (
    explanation: >-
      Delayed walking in 14.3% of this referral cohort maps to OCCASIONAL and
      exceeds the campaign's 10% inclusion threshold.
- category: Growth
  name: Short stature
  frequency: VERY_FREQUENT
  phenotype_term:
    preferred_term: Short stature
    term:
      id: HP:0004322
      label: Short stature
  evidence:
  - reference: PMID:38872968
    reference_title: "Clinical characteristics and long-term management for patients with vitamin D-dependent rickets type II: a retrospective study at a single center in Saudi Arabia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      The most frequently reported symptoms are alopecia in 41 patients (97.6%),
      followed by short stature (height SDS<-2) in 36 patients (85.7%), bowing of
      legs in 9 patients (21.4%), delayed walking in 6 patients (14.3%), seizures
      in 4 patients (9.5%), and respiratory symptoms in 1 patient (2.4%) (
    explanation: >-
      Short stature occurred in 85.7% of this 42-patient referral cohort,
      mapping to VERY_FREQUENT; the population context is retained in the
      explanation rather than treated as a universal disease-wide estimate.
diagnosis:
- name: Biochemical and Radiographic Evaluation for VDR Resistance
  description: >-
    Evaluate a child with clinical or radiographic rickets using serum calcium,
    phosphate, alkaline phosphatase, PTH, 25-hydroxyvitamin D, and
    1,25-dihydroxyvitamin D. Markedly elevated 1,25-dihydroxyvitamin D with high
    PTH and alkaline phosphatase supports receptor resistance; hypocalcemia and
    alopecia are not required. Explicit differentials include nutritional
    rickets, the synthetic defects VDDR1A and VDDR1B, the biochemical phenocopy
    VDDR2B, and FGF23-mediated renal phosphate-wasting disorders such as
    X-linked hypophosphatemia (XLH). Normocalcemic, hypophosphatemic VDDR2A can
    initially mimic an FGF23-mediated disorder; markedly elevated
    1,25-dihydroxyvitamin D and PTH prompt evaluation for receptor resistance,
    while VDR testing distinguishes type 2A from type 2B. During follow-up,
    monitor calcium, phosphate, PTH, alkaline phosphatase, and radiographic
    healing separately because PTH can remain elevated after phosphate
    normalization and radiographic healing.
  diagnosis_term:
    preferred_term: diagnostic procedure
    term:
      id: NCIT:C18020
      label: Diagnostic Procedure
  results: Radiographic rickets with elevated PTH, alkaline phosphatase, and 1,25-dihydroxyvitamin D; phosphate is often low and calcium may be low or normal, and biochemical indices may normalize asynchronously during treatment.
  evidence:
  - reference: PMID:42438800
    reference_title: A case of vitamin D-dependent rickets type 2A presenting with hypophosphatemia without hypocalcemia.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Initial evaluation showed normocalcemia, hypophosphatemia,
      hyperparathyroidism, markedly elevated alkaline phosphatase, and
      radiographic rickets, leading to an initial diagnosis of hypophosphatemic
      rickets.
    explanation: >-
      Demonstrates the diagnostic pitfall and that a normal serum calcium does
      not exclude VDDR2A.
  - reference: PMID:38365463
    reference_title: Vitamin D deficiency or resistance and hypophosphatemia.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: >-
      Nutritional vitamin D deficiency is the leading cause of rickets and
      osteomalacia worldwide and responds well to vitamin D supplementation.
      Inherited disorders of vitamin D metabolism (vitamin D-dependent rickets,
      VDDR) account for a small proportion of calcipenic rickets/osteomalacia.
      Defective 1α hydroxylation of vitamin D, 25 hydroxylation of vitamin D,
      and vitamin D receptor result in VDDR1A, VDDR1B and VDDR2A, respectively
      whereas defective binding of vitamin D to vitamin D response element due
      to overexpression of heterogeneous nuclear ribonucleoprotein and
      accelerated vitamin D metabolism cause VDDR2B and VDDR3, respectively.
    explanation: >-
      Names nutritional rickets and VDDR1A, VDDR1B, and VDDR2B as explicit
      biochemical and mechanistic differentials.
  - reference: PMID:32596195
    reference_title: Diagnosis and Management of Vitamin D Dependent Rickets.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: >-
      Subjects who have hypophosphatemia and normal serum levels of PTH,
      calcium, and 25(OH)D are likely to have phosphopenic rickets; a normal or
      elevated tubular reabsorption of phosphorus (TRP) will be found in
      subjects with inadequate intake or absorption of phosphorus. By contrast,
      patients with XLH and other genetic or acquired causes of renal phosphate
      wasting will have reduced TRP values. Subjects with calciopenic rickets
      will have elevated serum levels of PTH, which depress the TRP and cause
      hypophosphatemia.
    explanation: >-
      Supports the explicit XLH and renal-phosphate-wasting differential and
      shows how PTH and tubular phosphate handling separate major rickets axes.
  - reference: PMID:32596195
    reference_title: Diagnosis and Management of Vitamin D Dependent Rickets.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: >-
      Today we recognize that there are multiple forms of inherited
      hypophosphatemic rickets in addition to XLH. Many of these additional forms
      share in common with XLH the presence of excess circulating levels of
      FGF23, a phosphotonin that accounts for decreased renal tubular
      reabsorption of phosphate and hypophosphatemia (9).
    explanation: >-
      Locally supports the FGF23-mediated classification of the XLH and inherited
      renal-phosphate-wasting differential.
  - reference: PMID:40383122
    reference_title: Persistent Hyperparathyroidism in Vitamin D-Dependent Rickets Type 2A Does Not Prevent Normalization of Hypophosphatemia or Healing of the Rickets.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Upon the initiation of IV calcium infusions, bone pain resolved, and the
      rickets healed within weeks. In parallel with decreasing ALP values, her
      phosphate levels normalized even though her PTH levels remained markedly
      elevated. PTH levels remained elevated for approximately 1 year after the
      normalization of S-Ca2+.
    explanation: >-
      Directly establishes asynchronous biochemical recovery and the need to
      continue PTH monitoring after phosphate and skeletal improvement.
- name: VDR Molecular Genetic Testing
  description: >-
    Sequencing of VDR by a rickets panel, exome, or genome confirms VDDR2A when
    pathogenic biallelic variants are found; segregation and functional data are
    especially important when considering the exceptional heterozygous
    dominant-negative mechanism.
  diagnosis_term:
    preferred_term: molecular genetic testing
    term:
      id: NCIT:C19770
      label: Molecular Analysis
    qualifiers:
    - predicate:
        preferred_term: has participant
        term:
          id: RO:0000057
          label: has participant
      value:
        preferred_term: VDR
        term:
          id: hgnc:12679
          label: VDR
  results: Pathogenic VDR variant(s) in a disease-consistent allelic state confirm type 2A.
  evidence:
  - reference: PMID:42438800
    reference_title: A case of vitamin D-dependent rickets type 2A presenting with hypophosphatemia without hypocalcemia.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Further evaluation revealed markedly elevated 1,25(OH)2D, and whole-exome
      sequencing identified a homozygous pathogenic VDR c.1027C>T (p.R343C)
      variant, confirming VDDR2A.
    explanation: >-
      Directly demonstrates molecular confirmation after biochemical recognition.
treatments:
- name: High-Dose Active Vitamin D Plus Oral Calcium for Residual Responders
  description: >-
    Patients with residual VDR responsiveness, particularly some
    non-truncating ligand-binding-domain variants and milder presentations
    without alopecia, may respond to supraphysiologic calcitriol or alfacalcidol
    together with oral calcium. Biochemical response, growth, and radiographic
    healing guide continued use; response cannot be inferred from alopecia alone.
  action_category: THERAPEUTIC
  treatment_term:
    preferred_term: Pharmacotherapy
    term:
      id: NCIT:C15986
      label: Pharmacotherapy
    therapeutic_agent:
    - preferred_term: calcitriol
      term:
        id: CHEBI:17823
        label: calcitriol
    - preferred_term: alfacalcidol
      term:
        id: CHEBI:31186
        label: alfacalcidol
    - preferred_term: calcium(2+)
      term:
        id: CHEBI:29108
        label: calcium(2+)
  target_mechanisms:
  - target: Impaired VDR-Regulated Transcription
    treatment_effect: MODULATES
    description: Supraphysiologic active vitamin D can increase signaling through a receptor with residual function.
    evidence:
    - reference: PMID:42438800
      reference_title: A case of vitamin D-dependent rickets type 2A presenting with hypophosphatemia without hypocalcemia.
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
        High-dose calcitriol therapy resulted in progressive biochemical
        improvement and catch-up growth.
      explanation: >-
        Genetically confirmed p.R343C VDDR2A responded clinically and
        biochemically, disproving universal calcitriol nonresponse.
    - reference: PMID:36705686
      reference_title: "Genotypic Spectrum and its Correlation with Alopecia and Clinical Response in Hereditary Vitamin D Resistant Rickets: Our Experience and Systematic Review."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
        Overall response to oral therapy was highest in LBD-NT (46.4%) as
        compared to 7.6% in LBD-truncating and 19% in DBD-NT variants.
      explanation: >-
        Quantifies genotype-stratified oral response and supports a
        response-guided rather than universal treatment claim.
  - target: Reduced VDR-Dependent Enterocyte Calcium and Phosphate Transport
    treatment_effect: BYPASSES
    description: >-
      High oral calcium concentrations increase passive calcium uptake while
      active vitamin D exploits any residual receptor activity.
    evidence:
    - reference: PMID:40219799
      reference_title: "Vitamin D dependent rickets type 2A: a case series of two siblings with novel mutation in vitamin D receptor gene responded to high dose oral calcium and calcitriol."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
        They both are being managed on high doses of oral calcium supplements and
        activated form of vitamin D.
      explanation: >-
        Documents oral calcium plus activated vitamin D management in two
        genetically characterized siblings.
- name: High-Dose Oral Calcium
  description: >-
    High-dose oral elemental calcium is an initial bypass strategy when it can
    restore normocalcemia and suppress secondary hyperparathyroidism. It may
    also permit transition away from intravenous calcium, but passive absorption
    is limited and requires close biochemical and urinary-calcium monitoring.
  action_category: THERAPEUTIC
  treatment_term:
    preferred_term: Pharmacotherapy
    term:
      id: NCIT:C15986
      label: Pharmacotherapy
    therapeutic_agent:
    - preferred_term: calcium(2+)
      term:
        id: CHEBI:29108
        label: calcium(2+)
  target_mechanisms:
  - target: Reduced VDR-Dependent Enterocyte Calcium and Phosphate Transport
    treatment_effect: BYPASSES
    description: A large luminal calcium load increases VDR-independent passive absorption.
    evidence:
    - reference: PMID:32596195
      reference_title: Diagnosis and Management of Vitamin D Dependent Rickets.
      supports: SUPPORT
      evidence_source: OTHER
      snippet: >-
        Management during the first few months of life may require only
        administration of high doses of oral calcium (5–6 g/m2 body surface of
        elemental calcium). If oral therapy is able to restore normocalcemia and
        reverse secondary hyperparathyroidism it may not be necessary to initiate
        intravenous infusions of calcium.
      explanation: >-
        Supports oral-first calcium treatment when biochemical targets respond.
- name: Intravenous Calcium for Maximal-Oral-Treatment-Refractory Disease
  description: >-
    Severe disease that fails maximal oral calcium and active vitamin D requires
    prolonged intravenous calcium to bypass intestinal absorption entirely and
    replenish skeletal calcium deficits. Central-line complications make this a
    refractory-disease strategy, not evidence that intravenous therapy is
    universally preferable.
  action_category: THERAPEUTIC
  treatment_term:
    preferred_term: Pharmacotherapy
    term:
      id: NCIT:C15986
      label: Pharmacotherapy
    therapeutic_agent:
    - preferred_term: calcium(2+)
      term:
        id: CHEBI:29108
        label: calcium(2+)
  target_mechanisms:
  - target: Reduced VDR-Dependent Enterocyte Calcium and Phosphate Transport
    treatment_effect: BYPASSES
    description: Intravenous delivery completely bypasses the receptor-dependent intestinal route.
    evidence:
    - reference: PMID:32596195
      reference_title: Diagnosis and Management of Vitamin D Dependent Rickets.
      supports: SUPPORT
      evidence_source: OTHER
      snippet: >-
        Those patients who fail to respond to maximal doses of calciferol will
        require intravenous infusions of calcium (1,000 mg elemental calcium per
        24 h infused over 12 h), which can be used even by the youngest children
        with VDDR2a.
      explanation: >-
        Restricts intravenous calcium to patients who fail maximal hormone-based
        treatment.
    - reference: PMID:36705686
      reference_title: "Genotypic Spectrum and its Correlation with Alopecia and Clinical Response in Hereditary Vitamin D Resistant Rickets: Our Experience and Systematic Review."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
        All patients had poor overall response to oral high-dose calcium and
        calcitriol and most required intravenous calcium.
      explanation: >-
        Documents escalation to intravenous calcium in an oral-refractory
        genetically confirmed cohort.
animal_models:
- name: VDR-null mouse (targeted ablation of the second zinc finger)
  species: Mouse
  genotype: Vdr knockout, second zinc finger of the DNA-binding domain deleted
  publication: PMID:9275211
  description: >-
    The reference model of this disorder, engineered to reproduce the human
    DNA-binding-domain lesion. Mice are normal at birth and develop hypocalcemia,
    hyperparathyroidism, rickets and osteomalacia on the human timeline, together
    with progressive alopecia that dietary vitamin D deficiency does not cause.
  evidence:
  - reference: PMID:9275211
    reference_title: 'Targeted ablation of the vitamin D receptor: an animal model of vitamin D-dependent rickets type II with alopecia.'
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: >-
      We have generated a mouse model of VDDR II by targeted ablation of the
      second zinc finger of the VDR DNA-binding domain.
    explanation: >-
      Establishes that the model was engineered specifically to reproduce the
      human DNA-binding-domain lesion, which is what makes it informative for
      this disorder rather than for vitamin D deficiency generally.
  modeled_mechanisms:
  - target: Defective Skeletal Mineralization
    relationship: RECAPITULATES
    fidelity: HIGH
    description: >-
      Reproduces the full mineral and skeletal sequence, and shows the growth-plate
      lesion appearing before overt rickets.
    limitations: >-
      The engineered allele removes the second zinc finger specifically; human
      disease alleles are heterogeneous across the DNA-binding and ligand-binding
      domains, so the model represents the DNA-binding class rather than the whole
      allelic spectrum.
    readouts:
    - name: Hypertrophic chondrocyte zone width in the growth plate
      target: Defective Skeletal Mineralization
      direction: INCREASED
      interpretation: >-
        Expansion of the hypertrophic zone is the growth-plate correlate of the
        mineralization defect, and it precedes radiographic rickets in this model.
      evidence:
      - reference: PMID:9275211
        reference_title: 'Targeted ablation of the vitamin D receptor: an animal model of vitamin D-dependent rickets type II with alopecia.'
        supports: SUPPORT
        evidence_source: MODEL_ORGANISM
        snippet: >-
          Rickets and osteomalacia are seen by day 35; however, as early as day
          15, there is an expansion in the zone of hypertrophic chondrocytes in
          the growth plate.
        explanation: >-
          Reports the histological measurement and its timing relative to overt
          rickets.
    evidence:
    - reference: PMID:9275211
      reference_title: 'Targeted ablation of the vitamin D receptor: an animal model of vitamin D-dependent rickets type II with alopecia.'
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: >-
        They become hypocalcemic at 21 days of age, at which time their
        parathyroid hormone (PTH) levels begin to rise.
      explanation: >-
        Establishes that the model reproduces the hypocalcemia and secondary
        hyperparathyroidism that drive the mineralization failure in patients.
  - target: Disrupted Ligand-Independent VDR Hair-Cycle Function
    relationship: RECAPITULATES
    fidelity: HIGH
    description: >-
      Progressive alopecia develops from four weeks of age and is absent from
      dietary vitamin D deficiency, matching the human observation that alopecia
      tracks receptor loss rather than hormone deficiency.
    limitations: >-
      Alopecia is present in some but not all human kindreds, so the model
      reproduces the alopecic subset rather than the disorder uniformly.
    evidence:
    - reference: PMID:9275211
      reference_title: 'Targeted ablation of the vitamin D receptor: an animal model of vitamin D-dependent rickets type II with alopecia.'
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: >-
        In contrast to animals made vitamin D deficient by dietary means, and like
        some patients with VDDR II, these mice develop progressive alopecia from
        the age of 4 weeks.
      explanation: >-
        Contrasts receptor ablation with dietary deficiency, isolating the
        receptor-dependent, hormone-independent origin of the alopecia.
- name: Keratinocyte-targeted ligand-binding-null VDR transgene in VDR-null mice
  species: Mouse
  genotype: Vdr knockout carrying a keratinocyte-targeted VDR transgene with a hormone-binding-domain mutation abolishing ligand binding
  publication: PMID:15591533
  description: >-
    A rescue experiment that separates the two arms of the disorder. Restoring a
    receptor that cannot bind hormone, in keratinocytes only, restores normal hair
    cycling, showing that the hair-follicle function of the receptor is ligand
    independent. As a transgenic mouse rescue, it does not establish universal
    treatment nonresponse of alopecia across human VDDR2A genotypes.
  evidence:
  - reference: PMID:15591533
    reference_title: Ligand-independent actions of the vitamin D receptor maintain hair follicle homeostasis.
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: >-
      To determine which functional domains of the VDR are required for hair
      cycling, mutant VDR transgenes were targeted to the keratinocytes of VDR
      null mice.
    explanation: >-
      States the design of the transgenic rescue, which is what makes this system
      informative for the alopecia node specifically.
  modeled_mechanisms:
  - target: Disrupted Ligand-Independent VDR Hair-Cycle Function
    relationship: RESCUES
    fidelity: HIGH
    description: >-
      Keratinocyte-restricted expression of a ligand-binding-dead receptor rescues
      the hair cycle, demonstrating the mechanism this node asserts.
    limitations: >-
      A transgenic rescue rather than a disease model; it establishes sufficiency
      of the receptor protein for hair cycling but does not model any human
      genotype.
    readouts:
    - name: Hair cycling in keratinocyte-rescued VDR-null mice
      target: Disrupted Ligand-Independent VDR Hair-Cycle Function
      direction: RESTORED
      interpretation: >-
        Normal hair cycling despite an inability to bind hormone localizes the
        alopecia mechanism to a ligand-independent receptor function.
      evidence:
      - reference: PMID:15591533
        reference_title: Ligand-independent actions of the vitamin D receptor maintain hair follicle homeostasis.
        supports: SUPPORT
        evidence_source: MODEL_ORGANISM
        snippet: >-
          Keratinocyte-specific expression of a VDR transgene with a mutation in
          the hormone-binding domain that abolishes ligand binding restores normal
          hair cycling in VDR null mice
        explanation: >-
          Reports the rescue and the direction of the effect.
    evidence:
    - reference: PMID:15591533
      reference_title: Ligand-independent actions of the vitamin D receptor maintain hair follicle homeostasis.
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: >-
        These studies demonstrate that the effects of the VDR on the hair follicle
        are ligand independent and point to novel molecular and cellular actions
        of this nuclear receptor.
      explanation: >-
        States the conclusion that grounds the ligand-independent framing of this
        node.
- name: Systemic Vdr Delta-AF2 Mouse
  species: Mouse
  genotype: Systemic deletion of the Vdr activation-function-2 domain (VdrΔAF2)
  publication: PMID:39164247
  description: >-
    The mutant receptor cannot recruit coactivators and therefore cannot mount
    ligand-induced transcription, but it retains coactivator-independent
    repressive activity. Mice develop severe undermineralized short bones and
    dysmorphic growth plates. This is an allele-mechanism model rather than a
    generic proxy for all VDDR2A: its phenotype is more severe than Vdr null and
    is not corrected by the high-calcium/phosphate/lactose rescue diet that
    normalizes the null model.
  evidence:
  - reference: PMID:39164247
    reference_title: "Coactivator-independent vitamin D receptor signaling causes severe rickets in mice, that is not prevented by a diet high in calcium, phosphate, and lactose."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: >-
      Systemic VdrΔAF2 mice developed short, undermineralized bones with
      dysmorphic growth plates, a bone phenotype that was more pronounced than
      that of systemic Vdr knockout (Vdr-/-) mice.
    explanation: >-
      Defines the systemic model and its more-severe-than-null skeletal phenotype.
  modeled_mechanisms:
  - target: Impaired VDR Coactivator Recruitment
    relationship: PARTIALLY_RECAPITULATES
    fidelity: MODERATE
    description: >-
      Directly models loss of AF2-dependent coactivator recruitment while
      retaining repressive signaling.
    limitations: >-
      The engineered deletion retains AF2-independent repression and is more
      severe than Vdr absence, so its diet response must not be generalized to
      all human VDR variants or used to negate human calcium responsiveness.
    evidence:
    - reference: PMID:39164247
      reference_title: "Coactivator-independent vitamin D receptor signaling causes severe rickets in mice, that is not prevented by a diet high in calcium, phosphate, and lactose."
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: >-
        We confirm that ligand-induced transcriptional activation was disabled
        because the mutant VDRΔAF2 protein was unable to interact with
        coactivators.
      explanation: Directly validates the coactivator-recruitment defect represented by the model.
  - target: Defective Skeletal Mineralization
    relationship: PARTIALLY_RECAPITULATES
    fidelity: MODERATE
    description: Reproduces severe undermineralization and dysmorphic growth plates.
    limitations: >-
      Model-specific retained repression makes the skeletal phenotype more
      severe than Vdr null and resistant to a rescue diet; this does not predict
      universal failure of calcium treatment in patients.
    evidence:
    - reference: PMID:39164247
      reference_title: "Coactivator-independent vitamin D receptor signaling causes severe rickets in mice, that is not prevented by a diet high in calcium, phosphate, and lactose."
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: >-
        Interestingly, a rescue diet that is high in calcium, phosphate, and
        lactose, normalized this phenotype in Vdr-/-, but not in VdrΔAF2 mice.
      explanation: Captures the model-specific rescue-diet divergence and its key limitation.
experimental_models:
- name: Patient-Derived p.Arg30* VDR Dermal Fibroblasts
  experimental_model_type: PRIMARY_CELL_CULTURE
  description: >-
    Primary skin fibroblasts from one individual homozygous for the severe
    p.Arg30* VDR allele were compared with age- and sex-matched control
    fibroblasts after vehicle or 10 nM calcitriol exposure for 24 hours. The
    model directly tests a human disease genotype but is limited to one donor
    and a dermal surrogate rather than enterocytes, kidney, growth plate, or bone.
  organism:
    preferred_term: human
    term:
      id: NCBITaxon:9606
      label: Homo sapiens
  cell_types:
  - preferred_term: fibroblast
    term:
      id: CL:0000057
      label: fibroblast
  cell_source: Primary patient-derived skin fibroblasts and matched control fibroblasts
  culture_system: Two-dimensional monolayer culture exposed to vehicle or 10 nM 1,25-dihydroxyvitamin D for 24 hours
  conditions:
  - homozygous VDR p.Arg30* patient fibroblasts
  - age- and sex-matched control fibroblasts
  - vehicle or calcitriol exposure
  publication: PMID:30959822
  evidence:
  - reference: PMID:30959822
    reference_title: "Transcriptomic Response to 1,25-Dihydroxyvitamin D in Human Fibroblasts with or without a Functional Vitamin D Receptor (VDR): Novel Target Genes and Insights into VDR Basal Transcriptional Activity."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      We analyzed the transcriptomic response to 1,25D in fibroblasts bearing a
      severe homozygous hereditary vitamin D resistant rickets-related p.Arg30*
      VDR mutation (MUT) and in control fibroblasts (CO).
    explanation: Establishes the disease genotype, human primary-cell system, and matched comparator.
  modeled_mechanisms:
  - target: Impaired VDR-Regulated Transcription
    relationship: RECAPITULATES
    fidelity: MODERATE
    description: Measures loss of the calcitriol-responsive transcriptome in patient cells.
    limitations: A single patient line and a fibroblast surrogate cannot reproduce intestinal mineral transport or skeletal disease.
    evidence:
    - reference: PMID:30959822
      reference_title: "Transcriptomic Response to 1,25-Dihydroxyvitamin D in Human Fibroblasts with or without a Functional Vitamin D Receptor (VDR): Novel Target Genes and Insights into VDR Basal Transcriptional Activity."
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: >-
        Roughly 4.5% of the transcriptome was regulated by 1,25D in CO
        fibroblasts, while MUT cells without a functional VDR were insensitive to
        1,25D.
      explanation: Directly measures the impaired transcriptional response represented by the target node.
datasets:
- accession: geo:GSE127314
  title: Effects of 1,25-dihydroxyvitamin D (1,25D) on global gene expression of human fibroblasts with (CO) or without (MUT) a functional VDR
  description: >-
    Microarray profiling of primary skin fibroblasts from one homozygous VDR
    p.Arg30* VDDR2A patient and an age- and sex-matched control after vehicle or
    10 nM calcitriol for 24 hours. Fifteen quality-controlled arrays represent
    four control-vehicle, four control-calcitriol, three mutant-vehicle, and four
    mutant-calcitriol samples.
  organism:
    preferred_term: human
    term:
      id: NCBITaxon:9606
      label: Homo sapiens
  data_type: MICROARRAY
  sample_types:
  - preferred_term: primary skin fibroblast
    cell_type_term:
      preferred_term: fibroblast
      term:
        id: CL:0000057
        label: fibroblast
  sample_count: 15
  conditions:
  - homozygous VDR p.Arg30* patient fibroblasts
  - matched control fibroblasts
  - vehicle or 10 nM calcitriol for 24 hours
  platform: Affymetrix GeneChip Human Gene 2.0 ST Array
  publication: PMID:30959822
  evidence:
  - reference: PMID:30959822
    reference_title: "Transcriptomic Response to 1,25-Dihydroxyvitamin D in Human Fibroblasts with or without a Functional Vitamin D Receptor (VDR): Novel Target Genes and Insights into VDR Basal Transcriptional Activity."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      The array data were submitted to the National Center for Biotechnology
      Information Gene Expression Omnibus (http://www.ncbi.nlm.nih.gov/geo)
      under series accession no. GSE127314.
    explanation: Directly identifies the public dataset accession in the source publication.
  notes: GEO metadata and accession were independently resolved with just verify-datasets on 2026-08-20.
- accession: geo:GSE260988
  title: Coactivator-independent vitamin D receptor signaling impairs intestinal calcium transport in mice, leading to severe rickets, which is not prevented by a diet high in calcium, phosphate, and lactose.
  description: >-
    Bulk RNA sequencing of duodenum and kidney from 8-week-old female wild-type,
    Vdr-null, and systemic VdrΔAF2 mice maintained on a high-calcium,
    high-phosphate, high-lactose diet. Each tissue contributes six wild-type,
    four null, and four VdrΔAF2 samples, for 28 samples total.
  organism:
    preferred_term: mouse
    term:
      id: NCBITaxon:10090
      label: Mus musculus
  data_type: BULK_RNA_SEQ
  sample_types:
  - preferred_term: duodenum
    tissue_term:
      preferred_term: duodenum
      term:
        id: UBERON:0002114
        label: duodenum
  - preferred_term: kidney cortex
    tissue_term:
      preferred_term: kidney cortex
      term:
        id: UBERON:0001225
        label: cortex of kidney
  sample_count: 28
  conditions:
  - Vdr wild-type
  - Vdr knockout
  - systemic Vdr Delta-AF2
  - high-calcium, high-phosphate, high-lactose rescue diet
  platform: QuantSeq expression profiling on Illumina HiSeq 4000
  publication: PMID:39164247
  evidence:
  - reference: PMID:39164247
    reference_title: "Coactivator-independent vitamin D receptor signaling causes severe rickets in mice, that is not prevented by a diet high in calcium, phosphate, and lactose."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: >-
      In addition, RNA-sequencing analysis of duodenum and kidney revealed a
      decreased expression of VDR target genes in systemic VdrΔAF2 mice, which
      was not observed in Vdr-/- mice.
    explanation: Confirms the tissues, sequencing modality, and mechanistic comparison represented by the dataset.
  notes: GEO metadata and accession were independently resolved with just verify-datasets on 2026-08-20.
biochemical:
- name: 1,25-dihydroxyvitamin D (calcitriol)
  presence: Elevated
  biomarker_term:
    preferred_term: 1,25-Dihydroxyvitamin D3 Measurement
    term:
      id: NCIT:C179754
      label: 1,25-Dihydroxyvitamin D3 Measurement
  context: >-
    Markedly elevated calcitriol in a child with rickets, high PTH, and high
    alkaline phosphatase is a major clue to VDR resistance and contrasts with
    type-1 defects in calcitriol synthesis. It is not sufficient by itself to
    distinguish VDDR2A from the VDDR2B phenocopy, and interpretation must account
    for treatment and assay context; molecular VDR testing establishes type 2A.
  notes: >-
    Recorded as a biochemical marker rather than a phenotype because no HPO term
    exists for an elevated 1,25-dihydroxyvitamin D concentration. HP:0100512
    covers only a DECREASED vitamin D concentration and would state the opposite
    of the finding, so no term is bound - a candidate for an HPO new-term request.
  evidence:
  - reference: PMID:38872968
    reference_title: "Clinical characteristics and long-term management for patients with vitamin D-dependent rickets type II: a retrospective study at a single center in Saudi Arabia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Patients with HVDDR-II have rickets along with hypocalcemia,
      hypophosphatemia, secondary hyperparathyroidism, and elevated serum
      1,25(OH)2D levels, which is a hallmark for diagnosing this disease.
    explanation: >-
      Names the elevated calcitriol level explicitly as the diagnostic hallmark
      of the disorder.
- name: Parathyroid hormone
  presence: Elevated
  context: >-
    Secondary hyperparathyroidism driven by receptor-level calcium malabsorption.
  evidence:
  - reference: PMID:38872968
    reference_title: "Clinical characteristics and long-term management for patients with vitamin D-dependent rickets type II: a retrospective study at a single center in Saudi Arabia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      HVDDR-II is characterized by early onset of rickets presentation and
      biochemical parameters such as low serum calcium levels, high parathyroid
      hormone, high alkaline phosphatase, and high 1,25(OH)2D levels (1).
    explanation: >-
      Reports elevated parathyroid hormone among the characteristic biochemical
      parameters.
notes: >-
  The name is a misnomer twice over: this disorder is vitamin D-RESISTANT, not
  vitamin D-dependent, and calcitriol is high rather than low. It shares the
  "vitamin D-dependent rickets" label with type 1 only by historical convention.
  Type 2B (MONDO:0010931) has the same clinical phenotype with a structurally
  normal receptor and is curated separately.
📚

References & Deep Research

Deep Research

1

Deep research results are used as seeds for research; they do not undergo the same validation as the main records and may contain errors. How we use deep research.

Evaluations and curation notes (1)

Create: Vitamin D-Dependent Rickets Type 2A · 2026-08-19T17:29:25Z · View source

De-novo curation of the VDR (receptor resistance) member. Models the alopecia as a separate branch off the receptor node, deliberately kept OFF the mineralization conformance chain because it is not part of that mechanism and is not corrected by normalizing calcium. Adds two animal models with modeled_mechanisms links: the VDR-null mouse (RECAPITULATES both the skeletal and alopecia nodes, with a hypertrophic-zone-width readout) and the keratinocyte-targeted ligand-binding-dead VDR transgene (RESCUES the alopecia node), which is the evidence that the hair-follicle role of the receptor is ligand independent. Alopecia frequency VERY_FREQUENT is backed by a direct quantitative count (97.6% of 42 patients), and seizure OCCASIONAL by 9.5% in the same series. Deep research: just research-disorder claude_code, report at research/Vitamin_D-Dependent_Rickets_Type_2A-deep-research-claude_code.md; its reference_validation block reports 27/28 verified with needs_review true, and the single unresolved DOI was not cited. just preflight-dr against MONDO:0010186 returned PASS (VDR mentioned 82 times; report OMIM 277440 matches the MONDO xref). Evidence: PMID:2849209, PMID:38872968, PMID:9275211, PMID:15591533. Validation: linkml-validate clean; just validate-terms passed; just count-verified-snippets 18/18 verified.

Claude Code ▸
Vitamin D-Dependent Rickets Type 2A (VDDR2A) — Comprehensive Research Report
claude-haiku-4-5-20251001, claude-sonnet-5 48 citations 2026-08-19T17:20:08.788487

Vitamin D-Dependent Rickets Type 2A (VDDR2A) — Comprehensive Research Report

1. Disease Information

Overview. Vitamin D-dependent rickets type 2A (VDDR2A), also widely known as hereditary 1,25-dihydroxyvitamin D-resistant rickets (HVDRR) or hereditary vitamin D-resistant rickets, is a rare autosomal recessive Mendelian disorder caused by biallelic loss-of-function mutations in the vitamin D receptor gene (VDR), producing end-organ resistance to the active vitamin D hormone, 1,25-dihydroxyvitamin D₃ (calcitriol), rather than a defect in vitamin D synthesis itself. Unlike nutritional rickets or VDDR type 1 (a biosynthetic enzyme defect), VDDR2A patients have normal or elevated circulating 1,25(OH)₂D levels but the target tissues cannot respond to it (OMIM #277440; NORD).

Key identifiers: - OMIM: #277440 (VDDR2A); causal gene VDR, OMIM 601769 (OMIM 277440; OMIM 601769) - Gene: VDR, chromosome 12q13.11 - GTR/MedGen concept: "Vitamin D-dependent rickets, type II" and "…type II with alopecia" are indexed as related concepts (NIH GTR C3536983; NIH GTR C0342646) - Orphanet: grouped under "Hypocalcemic vitamin D-dependent rickets" (Orphanet) - Suggested MONDO* term: hereditary vitamin D-resistant rickets (VDDR2A subtype) — confirm exact MONDO CURIE against the local MONDO adapter before curation.

Synonyms/alternative names: Hereditary 1,25-dihydroxyvitamin D-resistant rickets (HVDRR); vitamin D receptor deficiency rickets; hereditary hypocalcemic vitamin D-resistant rickets; pseudo-vitamin D deficiency rickets type II; hereditary vitamin D-resistant rickets with alopecia (when alopecia present).

Evidence base: Almost all published knowledge derives from individual patient case reports and small case series (often single kindreds), supplemented by two published retrospective single-center cohorts (a Saudi Arabian series and Chinese/Egyptian family series), rather than large aggregated disease-level registries — reflecting the disorder's extreme rarity. Suggested-evidence sources: HUMAN_CLINICAL (predominant), supplemented by MODEL_ORGANISM (VDR-null mouse/rat) and IN_VITRO (transfection/reporter assays of mutant VDR function).

2. Etiology

Disease causal factor: VDDR2A is caused by biallelic (typically homozygous or compound heterozygous) loss-of-function mutations in VDR on chromosome 12q13.11, encoding the nuclear vitamin D receptor. This is a purely genetic/Mendelian etiology — no environmental or infectious trigger is causal, though vitamin D intake/sunlight exposure modulates severity of the biochemical phenotype.

Genetic risk factors: - Consanguinity is a major risk factor given the autosomal recessive inheritance and the disease's concentration in consanguineous kindreds (Chinese, Egyptian, Middle Eastern, and Saudi Arabian case series predominate in the literature) (PMC4589239; PubMed 24859502). - Uniparental disomy of chromosome 12 has been documented as a distinct mechanism producing apparent homozygosity for a VDR mutation without both parents being carriers — confirmed by SNP array in at least one reported case (PLOS ONE / PMC4496068). - A dominant-negative mechanism has also been reported: a single mutant VDR allele with a constitutive corepressor (NCoR) interaction and ligand-independent VDRE binding causes dominantly inherited HVDRR, analogous to dominant-negative thyroid hormone receptor mutations (ScienceDirect / PMC5365159; PubMed 28377956). A 2025 case report also described an atypical heterozygous VDDR2A presentation (c.146+9dup) presenting with pseudoarthrosis rather than the classic recessive picture (Goldberg et al. 2025, Case Reports in Endocrinology; PMC12003035).

Environmental risk factors: None are causal, but low ambient sunlight/dietary vitamin D deficiency can exacerbate the biochemical/clinical phenotype in an already receptor-resistant patient by lowering the (already ineffective) precursor pool further, and can complicate diagnosis by mimicking nutritional rickets.

Protective factors: No specific protective genetic variant is described. Adequate dietary calcium intake mitigates (but does not cure) the phenotype, since the downstream defect is receptor unresponsiveness rather than calcium/vitamin D substrate availability — this underlies the calcium-based (rather than vitamin D-based) treatment strategy (see §12).

Gene-environment interaction: The core interaction is that the severity of clinical rickets/hypocalcemia is buffered by dietary calcium intake independent of VDR/calcitriol signaling — i.e., sufficiently high oral or IV calcium can partially bypass the VDR defect by driving passive, non-VDR-mediated paracellular intestinal calcium absorption.

3. Phenotypes

VDDR2A phenotypes fall into rickets/mineral-metabolism findings (present in essentially all patients) and alopecia (present in a majority but not universal, and correlating with mutation severity).

Phenotype Type Suggested HPO term
Rickets / rachitic bone changes Sign HP:0002748 (Rickets)
Growth retardation / short stature Sign HP:0004322 (Short stature)
Muscle weakness / hypotonia Sign HP:0001324 / HP:0001252
Bone pain Symptom HP:0002653 (Bone pain)
Bowing of long bones (genu varum) Sign HP:0002979 (Genu varum)
Widened wrists/costochondral beading (rachitic rosary) Sign HP:0000939 (Osteoporosis)-adjacent / HP:0004464 (Enlarged costochondral junctions)
Dental abnormalities / enamel hypoplasia Sign HP:0000704 (Dental caries)/HP:0006297
Hypocalcemia Lab abnormality HP:0002901 (Hypocalcemia)
Hypophosphatemia Lab abnormality HP:0002148 (Hypophosphatemia)
Secondary hyperparathyroidism Lab/sign HP:0000870 (Hyperparathyroidism)
Elevated serum alkaline phosphatase Lab abnormality HP:0003155 (Elevated alkaline phosphatase)
Elevated 1,25-dihydroxyvitamin D (distinguishing lab feature) Lab abnormality (no dedicated HPO term commonly used; describe via biochemical marker)
Alopecia totalis/partialis Sign HP:0007550/ HP:0002293 (Alopecia)
Hypocalcemic seizures / tetany Symptom HP:0032792 / HP:0001336
Milia/epidermal (cutaneous) cysts Sign HP:0001059

Onset and course: Manifestations typically present in infancy to early childhood (rickets is usually evident by the first 1–2 years of life; alopecia, when present, is often noted from birth or shortly thereafter). Severity is variable — even within families with the same genotype — and disease course is generally described as an early, severe, and treatment-resistant rachitic picture that improves with high-dose calcium repletion (see §12) but with alopecia typically not reversing.

Alopecia frequency and severity correlation: Reported figures vary by cohort, from roughly half to the majority of patients: "most patients have total alopecia in addition to rickets" (OMIM 277440) and "approximately 80% of patients with HVDRR have early-onset alopecia... the degree of alopecia is associated with the severity of the vitamin D resistance," with an unusual patchy pattern of total baldness adjacent to normal or scant hair (PMC3196847). Alopecia is a poor prognostic marker for treatment response (see §12).

Atypical presentations: VDDR2A can present with hypophosphatemia in the absence of hypocalcemia, causing initial misclassification as an FGF23-mediated hypophosphatemic rickets (JCEM Case Reports, PMC/Oxford; Karger/PMC12187100) — an important differential-diagnosis pitfall (§10).

Quality of life impact: Untreated or delayed-diagnosis disease causes significant motor disability (inability to bear weight/walk due to bone pain and deformity) that reverses substantially with calcium therapy (documented improvement from non-ambulatory to independent walking within weeks of IV calcium in case reports); alopecia carries a persistent psychosocial/cosmetic burden since it is typically treatment-refractory.

4. Genetic/Molecular Information

Causal gene: VDR (Vitamin D Receptor), OMIM *601769, chromosome 12q13.11 (GeneCards; OMIM 601769).

Variant spectrum: VDDR2A is caused by heterogeneous loss-of-function mutations distributed across the DNA-binding domain (DBD) and ligand-binding domain (LBD) of VDR, including: - Missense mutations disrupting ligand binding or DNA binding — e.g., I268T (LBD, ~5–10-fold reduced 1,25(OH)₂D₃ affinity, ~65-fold higher concentration required for equipotent transactivation) (PubMed 15308610); V26M (DBD, impairs DNA binding) (PMC2794978); R343H and R343C (recently reported, associated with alopecia and with a hypophosphatemia-predominant presentation respectively) (PMC5681508; JCEM Case Reports 2024). - Splice-site mutations — e.g., a novel splice-site mutation successfully managed with oral calcium therapy (ScienceDirect); a 2025 case with a heterozygous c.146+9dup splice-region variant with an atypical pseudoarthrosis presentation (PMC12003035). - Compound heterozygous genotypes are well documented, including in a 2008 report of compound heterozygous VDR mutations with alopecia (PubMed 19049339) and a 2025 congress report of a novel compound-heterozygous VDR mutation (Endocrine Abstracts 2025). - Uniparental disomy of chromosome 12, producing homozygosity for a maternal-only VDR mutation despite the father not being a carrier (PMC4496068). - Rare dominant-negative heterozygous mutations producing HVDRR via constitutive corepressor (NCoR) recruitment (PMC5365159).

Variant classification/functional consequence: The great majority are classified functionally as loss-of-function (complete or partial) via in-vitro transactivation/reporter assays; a minority display dominant-negative behavior. Standard ACMG/AMP pathogenicity classification should be applied per-variant via ClinVar; no single recurrent "hot-spot" variant dominates worldwide — mutations are largely private/family-specific, consistent with a rare, ethnically dispersed autosomal recessive disorder.

Allele frequency: Given the rarity and largely private nature of pathogenic VDR loss-of-function alleles, population database (gnomAD) frequencies for individual pathogenic variants are expected to be near-absent/singleton; no common founder allele with an appreciable population allele frequency has been established in the literature surveyed.

Somatic vs. germline: VDDR2A is exclusively a germline Mendelian disorder (not somatic/acquired).

Modifier genes: None firmly established; phenotypic variability (notably in alopecia severity) is attributed to the specific functional consequence of the causal VDR allele (LBD vs. DBD, partial vs. complete loss of function, dominant-negative vs. simple loss-of-function) rather than a distinct modifier locus.

Suggested GO terms for the VDR molecular function/pathway: GO:0004879 (nuclear receptor activity), GO:0070644 (vitamin D response element binding), GO:0008202 (steroid metabolic process), GO:0060348 (bone development).

5. Environmental Information

Because VDDR2A is a fully penetrant genetic receptor defect, environmental factors do not cause the disease but strongly modulate symptom severity and diagnostic presentation: - Dietary vitamin D/calcium intake and sunlight exposure affect substrate availability but cannot correct receptor unresponsiveness; low dietary calcium can precipitate more severe hypocalcemic crises (seizures/tetany) in an affected infant. - No infectious agent is implicated in VDDR2A pathogenesis. - No specific toxin/occupational exposure is implicated (this is a congenital, not acquired, receptor defect).

6. Mechanism / Pathophysiology

Molecular pathway. VDR is a member of the nuclear hormone receptor superfamily. Upon binding its ligand, 1,25-dihydroxyvitamin D₃ (calcitriol), the liganded VDR heterodimerizes with the retinoid X receptor (RXR). Helix 12 (H12) of the VDR ligand-binding domain acts as a flexible "lid" whose ligand-induced repositioning creates the activation function-2 (AF-2) hydrophobic cleft required for coactivator recruitment; RXR undergoes an allosteric "phantom ligand effect" shift toward an active conformation even without its own ligand (Oxford Mol Endocrinol; PMC3087838). The VDR-RXR heterodimer binds vitamin D response elements (VDREs) — direct repeats of the RGKTSA hexameric half-site separated by 3 nucleotides (DR3 motif) — in enhancer regions of target genes, recruiting a multiprotein complex (pioneer factors, chromatin remodelers, coactivators, Mediator complex) that docks onto RNA polymerase II to drive transcription of vitamin D target genes (e.g., intestinal calcium transporters, renal calcium-handling genes, osteoblast genes) (search synthesis of ScienceDirect VDRE overview, PMC6332450).

Causal chain — genetic lesion to clinical phenotype: 1. Biallelic (or dominant-negative) VDR mutation → loss of ligand binding (LBD mutations, e.g. I268T) or loss of DNA binding (DBD mutations, e.g. V26M) → impaired or absent VDR-RXR-VDRE transcriptional activation of calcitriol target genes. 2. End-organ (intestinal, renal, skeletal) resistance to 1,25(OH)₂D₃ → decreased active intestinal calcium (and secondarily phosphate) absorption despite normal or compensatorily elevated circulating calcitriol. 3. Hypocalcemia → secondary/compensatory hyperparathyroidism (elevated PTH) → renal phosphate wasting exacerbating hypophosphatemia; PTH-driven bone resorption contributes to elevated alkaline phosphatase. 4. Combined hypocalcemia/hypophosphatemia → defective mineralization of osteoid at growth plates and bone matrix → rickets/osteomalacia, growth retardation, bone pain/deformity, muscle weakness (mineral-dependent neuromuscular function). 5. Independently (not calcium-pathway mediated): VDR loss in hair-follicle keratinocytes disrupts a ligand-independent VDR function required for postmorphogenic anagen (growth phase) initiation of the hair cycle, causing alopecia — this occurs even though the alopecia-causing VDR function does not require 1,25(OH)₂D binding, explaining why alopecic patients are especially treatment-refractory to vitamin D/calcitriol-based therapy while their rickets can still be treated via calcium (see below and §12) (JCI 11676; Oxford Mol Endocrinol 19(4):855; PMC11720424).

Cellular processes involved: Intestinal enterocyte calcium transport (TRPV6/calbindin-mediated active transport, normally VDR-induced); osteoblast/osteoclast coupling and PTH-driven bone remodeling; parathyroid chief cell PTH secretion (normally suppressed by calcitriol-VDR signaling — loss of this suppression contributes to the hyperparathyroidism); hair-follicle keratinocyte stem-cell-driven anagen re-entry (VDR-dependent, ligand-independent, involving cooperative β-catenin/Lef1 canonical Wnt signaling that is abolished in VDR-null keratinocytes) (PNAS 0702884104).

Biochemical abnormalities: Hypocalcemia, hypophosphatemia (variable — can be absent in some presentations), elevated alkaline phosphatase, elevated PTH, and the biochemically distinguishing feature of markedly elevated (not low) circulating 1,25(OH)₂D, which differentiates VDDR2A from VDDR1A (CYP27B1 biosynthetic defect, where 1,25(OH)₂D is low).

Suggested GO/biological process terms: GO:0070508 (cholesterol import)-adjacent calcium pathway terms are less relevant; more directly: GO:0006816 (calcium ion transport), GO:0070508 n/a, GO:0060349 (bone morphogenesis), GO:0042633 (hair cycle), GO:0022416 (chaeta development)-adjacent is not applicable to humans — use GO:0042633 (hair cycle) and GO:0030855 (epithelial cell differentiation) for the alopecia arm.

Suggested CL (cell type) terms: CL:0000584 (enterocyte), CL:0000062 (osteoblast), CL:0000092 (osteoclast), CL:0000446 (parathyroid chief cell), CL:0000312 (keratinocyte), CL:0002337 (hair follicle stem cell / bulge keratinocyte).

Model system / omics data: The literature surveyed is predominantly clinical case reports and targeted functional (reporter-gene transactivation) assays of specific VDR mutants; no large-scale transcriptomic/proteomic/single-cell dataset specific to human VDDR2A patient tissue was identified in this search. Model-organism transcriptomic characterization exists for VDR-null mice (see §15).

7. Anatomical Structures Affected

Organ level: - Primary: Skeletal system (long bones, growth plates, ribs — costochondral junctions), skin/hair follicles. - Secondary: Parathyroid glands (secondary hyperparathyroidism), kidney (secondary renal phosphate wasting via PTH), skeletal muscle (hypotonia/weakness), teeth (enamel defects). - Body systems: Musculoskeletal, endocrine, integumentary; less so cardiovascular/neurological (hypocalcemic seizures reflect a neuromuscular/CNS excitability consequence of the mineral disturbance rather than primary CNS pathology).

Tissue/cell level: Growth-plate chondrocytes and osteoid-forming osteoblasts (impaired mineralization); intestinal enterocytes (impaired active transcellular calcium absorption); renal tubular epithelium (secondary phosphate handling); hair follicle keratinocytes, specifically the bulge stem cell niche, which forms normally in VDR-null models but fails to regenerate the lower hair follicle without VDR (PNAS 0702884104).

Subcellular level: VDR is a nuclear receptor; the defect is localized to nuclear transcriptional machinery (GO Cellular Component: GO:0005634 nucleus; GO:0090575 RNA polymerase II transcription regulator complex) rather than membrane, mitochondrial, or lysosomal compartments.

Localization/laterality: Systemic/bilateral, symmetric — skeletal changes affect long bones bilaterally (e.g., bilateral genu varum), and alopecia, when present, is typically diffuse/total rather than a focal or lateralized process.

Suggested UBERON terms: UBERON:0002481 (bone tissue), UBERON:0002049 (vasculature)-not primary, UBERON:0000014 (zone of skin), UBERON:0002073 (hair follicle), UBERON:0001103 (diaphragm)-n/a; more precisely UBERON:0002365 (exocrine gland)-n/a — best fits: UBERON:0001474 (bone element), UBERON:0002073 (hair follicle), UBERON:0001737 (parathyroid gland), UBERON:0002113 (kidney), UBERON:0001911 (mammary gland)-n/a.

8. Temporal Development

Onset: Congenital genetic lesion with clinical onset in infancy to early childhood — most reported cases present with rachitic signs (bowing, growth failure) and/or alopecia within the first 1–2 years of life; alopecia, when it occurs, is frequently apparent from birth or the first months.

Onset pattern: Insidious/progressive for the skeletal phenotype (worsening over months if undiagnosed), occasionally punctuated by acute hypocalcemic events (seizures/tetany) that can be the presenting acute event prompting diagnosis.

Progression: Without treatment, rachitic bone disease progresses with worsening deformity, growth retardation, and motor disability. With calcium-based treatment (see §12), the skeletal phenotype is generally reversible/healable — radiographic and biochemical normalization documented over roughly 8 weeks to 6 months depending on severity/delay to diagnosis (Frontiers, Saudi Arabia cohort; case reports above). Alopecia, in contrast, is typically stable/persistent and largely treatment-refractory even when the skeletal disease resolves.

Disease course pattern: Chronic but medically manageable — not classically relapsing-remitting, though inadequate treatment adherence can allow biochemical relapse. A 2025 Hormone Research in Paediatrics report specifically notes that persistent hyperparathyroidism can remain despite normalization of hypophosphatemia and radiographic healing of rickets, indicating dissociation between different biochemical/clinical axes during treatment response (Karger 2025; PMC12187100).

Critical periods: Early diagnosis and initiation of high-dose calcium therapy is critical to prevent long-term skeletal deformity and to shorten time-to-healing; delayed diagnosis is explicitly associated with longer treatment courses (up to 6 months of IV calcium in severe/delayed cases per the Saudi cohort).

9. Inheritance and Population

Epidemiology: VDDR2A/HVDRR is an ultra-rare disorder without a well-established global prevalence figure specific to the 2A subtype. The broader category of hypocalcemic vitamin D-dependent rickets (VDDR overall, types 1 and 2) has been estimated in Denmark at approximately 1 in 250,000 children under age 15 (search synthesis, cf. Springer VDDR review); no dedicated Orphanet prevalence class specific to VDDR2A was retrieved in this search (VDDR is grouped under "Hypocalcemic vitamin D-dependent rickets" in Orphanet without a granular 2A-specific number). Given the low observed patient counts in the literature (dominated by single-family/single-case reports across Chinese, Egyptian, Saudi, and other largely consanguineous populations), a prevalence_class of ULTRA_RARE or the Orphanet numeric band BELOW_1_IN_1000000 is a reasonable placeholder pending confirmation from an authoritative source.

Inheritance pattern: Autosomal recessive in the great majority of reported cases (biallelic VDR mutations); rare dominant-negative heterozygous cases have been documented via a distinct molecular mechanism (constitutive corepressor recruitment) (PMC5365159), and at least one atypical single-heterozygous-variant presentation has been reported clinically (PMC12003035) — curators should model both an AR Inheritance block and, where the evidence supports it, a distinct dominant-negative note rather than conflating the two mechanisms.

Penetrance: Effectively complete for the biochemical/skeletal phenotype in biallelic loss-of-function carriers; variable expressivity for alopecia (ranging from absent to total), correlating with the severity/type of the underlying functional VDR defect (ligand-binding vs. DNA-binding vs. dominant-negative).

Genetic anticipation: Not reported/applicable (VDR loss-of-function is not a repeat-expansion disorder).

Germline mosaicism: Not specifically documented in the sources reviewed, though theoretically possible for any autosomal recessive condition; not a prominent feature of the VDDR2A literature.

Founder effects / consanguinity: Strongly relevant — many reported kindreds are from consanguineous populations (Chinese, Egyptian, Middle Eastern/Saudi), and uniparental disomy has been documented as an alternative route to apparent homozygosity without biparental transmission (PMC4496068).

Carrier frequency: Not established/reported in the literature surveyed, consistent with the largely private nature of pathogenic VDR alleles.

Population demographics: No strong sex predilection is reported (autosomal, not X-linked); case series span pediatric populations globally, with concentration of published cohorts in the Middle East/North Africa and East Asia, likely reflecting both true prevalence (consanguinity rates) and reporting/ascertainment patterns.

10. Diagnostics

Core laboratory pattern: Hypocalcemia, hypophosphatemia (though may be absent — see atypical presentations), elevated alkaline phosphatase, elevated PTH (secondary hyperparathyroidism), and the key discriminating test: markedly elevated (not low) serum 1,25-dihydroxyvitamin D, which distinguishes VDDR2A from VDDR1A (CYP27B1 defect, low 1,25(OH)₂D) and from nutritional vitamin D deficiency (low 25-OH-D, low/normal 1,25(OH)₂D).

Imaging: Plain radiographs showing classic rachitic changes — metaphyseal widening/fraying/cupping, growth plate widening, long-bone bowing (genu varum), demineralization — used both for diagnosis and to monitor healing response to treatment.

Genetic testing: VDR sequence analysis (single-gene sequencing or inclusion in a rickets/metabolic bone disease gene panel, or exome sequencing) is the definitive diagnostic step, especially important because hypophosphatemia-predominant presentations can be clinically indistinguishable from FGF23-mediated hypophosphatemic rickets without genetic confirmation (Karger 2025). Whole-exome sequencing has been used successfully to identify novel homozygous pathogenic VDR variants (e.g., R343C) in atypical presentations (JCEM Case Reports 2024).

Differential diagnosis: - VDDR1A (CYP27B1 biosynthetic defect): low 1,25(OH)₂D (vs. elevated in VDDR2A), elevated PTH in both. - VDDR1B (CYP2R1 defect — 25-hydroxylase deficiency). - VDDR2B (normal VDR gene/protein; end-organ resistance instead caused by a nuclear ribonucleoprotein that interferes with VDR-DNA interaction — phenotypically similar to 2A but with an intact receptor gene) (OMIM 277440). - Nutritional (vitamin D deficiency) rickets — distinguished by low 25-OH-D. - FGF23-mediated hereditary hypophosphatemic rickets (e.g., XLH/PHEX) — an important pitfall because VDDR2A can present with isolated hypophosphatemia without hypocalcemia, mimicking this category; genetic testing is required to discriminate (JCEM Case Reports 2024; PMC12187100).

Screening: No population newborn-screening program targets VDDR2A specifically, given its rarity; diagnosis relies on clinical suspicion (rickets ± alopecia with markedly elevated 1,25(OH)₂D) followed by targeted or exome genetic testing. Cascade/carrier testing within affected consanguineous families is appropriate once a familial pathogenic variant is identified.

11. Outcome/Prognosis

Mortality: Not associated with increased mortality when appropriately diagnosed and treated with calcium repletion; severe untreated hypocalcemia can produce life-threatening seizures/tetany acutely, but chronic mortality data specific to VDDR2A were not identified in this literature review — the disorder is generally considered manageable rather than lethal.

Morbidity/functional outcome: With prompt, adequately dosed calcium therapy, rickets is fully radiographically and biochemically reversible — documented cases show return to normal ambulation within weeks and complete radiographic healing within ~3 months to 6 months depending on delay-to-diagnosis and severity. Persistent secondary hyperparathyroidism can, however, remain elevated even after phosphate/rickets normalization, per a 2025 Hormone Research in Paediatrics report (Karger 2025) — an important point for long-term monitoring guidance.

Alopecia prognosis: Generally persistent/refractory to treatment — patients with alopecia tend to have more severe underlying receptor dysfunction and respond less well to vitamin D metabolite therapy overall, often requiring the more aggressive calcium-infusion regimens (PMC3196847). Some case reports document partial or full resolution of alopecia with conventional-dose 1α-hydroxycholecalciferol plus oral calcium in milder genotypes (PubMed 21118628), indicating genotype-dependent variability rather than a uniformly fixed outcome.

Prognostic factors: Presence/severity of alopecia (marker of more severe VDR dysfunction and poorer response to vitamin D-based therapy), timeliness of diagnosis (delayed diagnosis associated with longer treatment courses), and specific mutation type (ligand-binding-domain vs. DNA-binding-domain vs. dominant-negative mutations differ in residual receptor function and thus treatment responsiveness).

12. Treatment

First-line strategy — bypass the receptor defect with calcium, not vitamin D metabolites. Because the fundamental lesion is receptor unresponsiveness rather than ligand deficiency, simply increasing vitamin D or calcitriol dosing is often ineffective; the mainstay of therapy is high-dose calcium supplementation, sufficient to drive passive (non-VDR-dependent, paracellular) intestinal calcium absorption:

  • Mild-to-moderate cases: managed with high-dose oral calcitriol plus supplemental oral calcium (case-based review; PubMed 7833085 — oral calcium treatment in VDDR type II).
  • Severe/alopecic/treatment-refractory cases: require intravenous (IV) calcium infusion, shown to be more potent/effective than oral therapy for rapid clinical, radiological, and biochemical improvement. Reported protocols include daily calcium gluconate infusion (e.g., 250 mg elemental calcium every 8 hours, several days per week) continued until calcium/phosphate normalize — typically averaging 8 weeks, up to 6 months in severe/delayed-diagnosis cases (Frontiers, Saudi Arabia cohort). Documented outcomes include return to independent ambulation within ~2 weeks and complete radiographic healing by ~3 months.
  • An adult-onset case documented favorable changes in bone metabolic parameters following oral calcium supplementation (PubMed 28367941).

Pharmacotherapy — vitamin D analogs: High-dose active vitamin D metabolites (calcitriol, 1α-hydroxycholecalciferol/alfacalcidol) are used adjunctively, particularly in milder genotypes, and can improve both rickets and (in some reports) alopecia (PubMed 21118628). Suggested NCIT term: NCIT:C15986 (Pharmacotherapy) with therapeutic_agent bound to CHEBI calcitriol (CHEBI:17823) or alfacalcidol.

Alopecia-directed/experimental approaches: - Because alopecia in VDR-null models arises from a ligand-independent VDR function in keratinocytes, systemic calcitriol/calcium therapy does not reliably restore hair growth; topical calcipotriol (a vitamin D analog with reduced calcemic activity, used for its VDR-binding/epidermal-differentiation effects independent of systemic calcium metabolism) has been explored for alopecia areata and is mechanistically relevant here, though robust VDDR2A-specific efficacy data are limited (Ann Dermatol / calcipotriol review). - Gene-therapy proof-of-concept: a VDR-expressing adenoviral vector has been used experimentally to treat alopecia in a rat model of type II rickets, restoring hair-follicle VDR expression (PMC10613246) — MODEL_ORGANISM evidence, not yet a human therapy. - Note: seocalcitol (EB1089) and related low-calcemic vitamin D analogs are primarily investigated as anticancer agents (pancreatic, hepatocellular, CLL) rather than as VDDR2A treatments specifically — relevant mainly as background on structure-activity of VDR ligands, not a direct VDDR2A therapeutic (BJC EB1089 pancreatic cancer trial).

Surgical/orthopedic: Corrective orthopedic surgery may be required for severe or fixed long-bone deformity in cases with delayed diagnosis/treatment, though this is supportive rather than disease-modifying (suggested NCIT C16186, Orthopedic Surgical Procedure).

Supportive care: Physical therapy/rehabilitation (NCIT:C15302) to address motor delay from rachitic myopathy/deformity during and after biochemical treatment.

Monitoring: Serial calcium, phosphate, alkaline phosphatase, and PTH; per the 2025 Karger report, PTH may remain persistently elevated despite normalization of phosphate and radiographic healing, so PTH cannot be used alone as a marker of complete treatment response (Karger 2025).

Experimental/clinical trials: No VDDR2A-specific registered interventional trials were identified in this search; management is derived from case-report-level and small single-center retrospective evidence, not randomized trial data.

13. Prevention

Primary prevention: Not applicable in the classic sense (this is a fully penetrant Mendelian genetic disorder) — the relevant "primary prevention" tool is genetic counseling and carrier/prenatal testing in consanguineous families or families with a known proband, given the autosomal recessive (occasionally dominant-negative) inheritance.

Secondary prevention (early detection): Prompt clinical suspicion (rickets with elevated, not low, 1,25(OH)₂D; ± alopecia) and early genetic confirmation are the practical "secondary prevention" tools that shorten time-to-treatment and reduce the risk of severe/fixed skeletal deformity.

Tertiary prevention: Adequate ongoing calcium supplementation and monitoring to prevent recurrence of hypocalcemic crises and to protect against long-term skeletal deformity and growth impairment; monitoring for persistent hyperparathyroidism as a residual complication even after apparent rickets healing.

Genetic counseling: Recommended for parents of an affected child (autosomal recessive recurrence risk ~25% per pregnancy for the typical biallelic form) and for extended family members in consanguineous kindreds; prenatal or preimplantation testing is technically feasible once the familial pathogenic VDR variant(s) are known, though this was not specifically documented as routine practice in the sources reviewed.

Public health/behavioral: General population-level vitamin D/calcium sufficiency programs (e.g., sunlight exposure guidance, dietary fortification) address nutritional rickets but have no bearing on the genetic VDR-resistance mechanism underlying VDDR2A specifically.

14. Other Species / Natural Disease

No naturally occurring VDDR2A/HVDRR case in a non-human companion-animal or wildlife species (i.e., a spontaneous VDR-null phenotype analogous to human disease) was identified in this search; the available cross-species data are from engineered rodent models (see §15) rather than naturally occurring veterinary disease. Curators should check OMIA (Online Mendelian Inheritance in Animals) directly if a naturally occurring animal correlate is needed for the entry, as this was not surfaced by the searches performed here.

15. Model Organisms

VDR knockout (Vdr⁻/⁻) mouse: The primary and best-characterized animal model. Targeted ablation of VDR produces a mouse model that recapitulates the core human VDDR2A phenotype: mice are normal at birth but develop growth retardation, hypocalcemia, hyperparathyroidism, rickets, osteomalacia, and alopecia — closely mirroring human VDDR2A with alopecia (PNAS 94(18):9831; JCI 11676; PubMed 22903507 — physiological insights review). - Fidelity: High for the skeletal/mineral-metabolism phenotype (RECAPITULATES) and for alopecia (RECAPITULATES) — the mouse model directly established that alopecia results from defective anagen (hair growth phase) initiation and demonstrated the ligand-independent role of VDR in hair-follicle keratinocytes (a keratinocyte-specific VDR transgene with a ligand-binding-abolishing mutation still restores normal hair cycling in VDR-null mice) (Oxford Mol Endocrinol 19(4):855; PNAS 0702884104). - Limitation: Because the alopecia-preventing VDR function is ligand-independent, this model demonstrates that vitamin D/calcitriol-based pharmacotherapy is mechanistically unlikely to reverse alopecia — a translational insight directly informing human treatment expectations (§12).

Humanized VDR mouse models: A "humanized" mouse model of HVDRR without alopecia has been engineered, allowing dissociation of the rachitic/mineral phenotype from the alopecia phenotype and enabling study of genotype-phenotype correlation for specific human VDR mutations (PubMed 25147982; Endocrinology 155(11):4137).

Rat model: A rat model of "type II rickets" with alopecia has been used as a preclinical platform for VDR gene-therapy proof-of-concept (adenoviral VDR-expressing vector) targeting the alopecia phenotype specifically (PMC10613246).

Applications: These rodent models have been used to (1) establish the causal relationship between VDR loss and both the mineral/skeletal phenotype and alopecia, (2) dissect ligand-dependent (mineral metabolism) versus ligand-independent (hair cycle) VDR functions, (3) test genotype-specific functional consequences of human VDR mutations via "humanized" knock-in approaches, and (4) pilot gene-replacement therapeutic strategies for the alopecia arm of the disease.

Suggested NCBITaxon terms: NCBITaxon:10090 (Mus musculus), NCBITaxon:10116 (Rattus norvegicus).

Summary of Key PMIDs/Sources for Curation

Topic Citation
OMIM disease/gene entries OMIM #277440 (VDDR2A); OMIM *601769 (VDR)
Core clinical/molecular review PMID (via PMC4589239) — Chinese HVDRR mutation series
Alopecia mechanism/frequency PMC3196847
I268T ligand-binding mutant functional study PubMed 15308610
V26M DNA-binding-domain mutant PMC2794978
Dominant-negative HVDRR mechanism PubMed 28377956 / PMC5365159
Uniparental disomy mechanism PMC4496068
Atypical hypophosphatemia-only presentation, R343C JCEM Case Reports 2024 (academic.oup.com/jcemcr)
Persistent hyperparathyroidism despite healing Karger 2025, PMC12187100
Heterozygous VDDR2A with pseudoarthrosis PMC12003035 (2025)
VDR knockout mouse — original model PNAS 94(18):9831 (1997)
Ligand-independent VDR/alopecia mechanism JCI 11676; Mol Endocrinol 19(4):855; PNAS 0702884104
Humanized mouse model without alopecia PubMed 25147982
Gene-therapy rat model PMC10613246
VDR-RXR-VDRE transcriptional mechanism Mol Endocrinol 17(11):2320; PMC3087838; PMC6332450

Note on evidence-source classification for curation: Case reports/series (majority of citations above) = HUMAN_CLINICAL; VDR-null mouse/rat and humanized-mouse studies = MODEL_ORGANISM; in-vitro transactivation/reporter-gene assays of specific VDR mutants = IN_VITRO. No COMPUTATIONAL-only evidence was identified as primary support for any major claim in this report.

Sources: - Entry - #277440 - VITAMIN D-DEPENDENT RICKETS, TYPE 2A; VDDR2A - OMIM - Entry - *601769 - VITAMIN D RECEPTOR; VDR - OMIM - vitamin D-dependent rickets, type 2A - NORD - Heterozygous Hereditary Vitamin D-Dependent Rickets Type 2A (VDDR2A) in a Patient Presenting With Pseudoarthrosis - PMC - Novel Vitamin D Receptor Mutations in Hereditary Vitamin D Resistant Rickets in Chinese - PMC - The Role of Vitamin D Receptor Mutations in the Development of Alopecia - PMC - Compound heterozygous mutations in the vitamin D receptor in a patient with hereditary 1,25-dihydroxyvitamin D-resistant rickets with alopecia - PubMed - Hereditary 1,25-dihydroxyvitamin D-resistant rickets with alopecia resulting from a novel missense mutation in the DNA-binding domain of the vitamin D receptor - PMC - Hereditary vitamin D resistant rickets: novel splice site mutation, oral calcium therapy - ScienceDirect - Functional Analysis of VDR Gene Mutation R343H - PMC - Hereditary 1,25-dihydroxyvitamin D-resistant rickets with alopecia in four Egyptian families - PubMed - Case of vitamin D–dependent rickets type 2A presenting with hypophosphatemia without hypocalcemia - JCEM Case Reports - Persistent Hyperparathyroidism in Vitamin D-Dependent Rickets Type 2A - PMC - Persistent Hyperparathyroidism in Vitamin D-Dependent Rickets Type 2A - Karger - Clinical characteristics and long-term management for VDDR type II - Saudi Arabia cohort - Frontiers - Oral calcium treatment in vitamin D-dependent rickets type II - PubMed - Changes in bone metabolic parameters following oral calcium supplementation - PubMed - Hereditary 1,25-dihydroxyvitamin D-resistant rickets (HVDRR) caused by a VDR mutation: novel mechanism of dominant inheritance - PMC - Hereditary 1,25-dihydroxyvitamin D resistant rickets due to a mutation causing multiple defects in VDR function - PubMed - Detection of Hereditary 1,25-Hydroxyvitamin D-Resistant Rickets Caused by Uniparental Disomy of Chromosome 12 - PMC - VDR gene - GeneCards - Vitamin D Dependent Rickets 2A With Alopecia: Three Cases With Novel Genetic Variants - PubMed - Heterozygous Hereditary Vitamin D‐Dependent Rickets Type 2A (VDDR2A) in a Patient Presenting With Pseudoarthrosis - Wiley - Novel VDR gene mutation in a VDDR2A compound heterozygote - Endocrine Abstracts - Vitamin D dependent rickets type 2A in a 1-year-old girl - Endocrine Abstracts - JCI - Metabolic and cellular analysis of alopecia in vitamin D receptor knockout mice - A humanized mouse model of hereditary 1,25-dihydroxyvitamin D-resistant rickets without alopecia - PubMed - Humanized Mouse Model of HVDRR Without Alopecia - Oxford Academic - Physiological insights from the vitamin D receptor knockout mouse - PubMed - Targeted ablation of the vitamin D receptor: An animal model of VDDR type II with alopecia - PNAS - Gene therapy for alopecia in type II rickets model rats using VDR-expressing adenovirus vector - PMC - The role of vitamin D receptor signaling in hair follicle health and alopecia - PMC - Vitamin D receptor is essential for normal keratinocyte stem cell function - PNAS - Ligand-Independent Actions of the Vitamin D Receptor Maintain Hair Follicle Homeostasis - Oxford Academic - Ligand-Independent Vitamin D Receptor Actions Essential for Keratinocyte Homeostasis - PMC - Retinoid X Receptor Is a Nonsilent Major Contributor to VDR-Mediated Transcriptional Activation - Oxford Academic - DNA binding alters coactivator interaction surfaces of the intact VDR–RXR complex - PMC - Relationship of Structure and Function of DNA-Binding Domain in Vitamin D Receptor - PMC - Vitamin D Hydroxylation-Deficient Rickets (VDDR) – Types 1A, 1B, 2A, 2B - Springer - Orphanet: Hypocalcemic vitamin D-dependent rickets - Vitamin D-dependent rickets, type 2 - NIH GTR - Vitamin D-dependent rickets type II with alopecia - NIH GTR - A phase II trial of seocalcitol (EB1089) in inoperable pancreatic cancer - British Journal of Cancer - Alopecia in vitamin D-dependent rickets type II responding to 1α-hydroxycholecalciferol - PubMed

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