Vitamin D-Dependent Rickets Type 1A (VDDR1A) — Comprehensive Research Report

1. Disease Information

Overview. Vitamin D-dependent rickets type 1A (VDDR1A), also called pseudovitamin D deficiency rickets (PDDR) or vitamin D hydroxylation-deficient rickets type 1A, is a rare autosomal recessive inborn error of vitamin D metabolism caused by biallelic loss-of-function variants in CYP27B1, the gene encoding renal 25-hydroxyvitamin D-1α-hydroxylase. The enzyme defect abolishes or severely impairs the final activating hydroxylation step that converts 25-hydroxyvitamin D (25(OH)D, calcidiol) to 1,25-dihydroxyvitamin D (1,25(OH)₂D, calcitriol), the biologically active vitamin D hormone. The resulting selective calcitriol deficiency causes intestinal calcium malabsorption, hypocalcemia, secondary hyperparathyroidism, and impaired bone mineralization (rickets in children; osteomalacia if it persisted to adulthood), despite normal-to-elevated levels of the inactive precursor 25(OH)D (OMIM #264700; PMC4489500).

Key identifiers: - OMIM: #264700 (VDDR1A phenotype); CYP27B1 gene OMIM 609506 - Orphanet: ORPHA289157 (Hypocalcemic vitamin D-dependent rickets) - ICD-10-CM: E83.32 (Hereditary vitamin D-dependent rickets, type 1 and type 2); Orphanet also cross-maps to E55.0 (Rickets, active) - GARD (NIH Genetic and Rare Diseases Information Center): "Vitamin D-dependent rickets, type 1" - Gene: CYP27B1, chromosome 12q14.1 (some sources cite 12q13.3/12q14) - MeSH/synonym set: Pseudo-vitamin D deficiency rickets; PDDR; Type I vitamin D-dependent rickets; 1α-hydroxylase deficiency; hereditary pseudo-vitamin D deficiency rickets

Synonyms/alternative names: Pseudovitamin D-deficiency rickets (PDDR), vitamin D-dependent rickets type I, 25-hydroxyvitamin D-1α-hydroxylase deficiency, hereditary vitamin D dependency, hypocalcemic vitamin D-dependent rickets.

Data source character: Understanding of VDDR1A derives almost entirely from aggregated case reports and small case series/cohorts (individual-patient literature), plus a founder-population registry (Saguenay–Lac-Saint-Jean, Quebec) and structured aggregations such as OMIM, Orphanet, and MalaCards; no large-scale EHR/claims-based epidemiologic studies exist given its rarity (PMC9671943; PMC9120640).

Sources: OMIM #264700 | PMC4489500 | ICD10Data E83.32 | MalaCards VDDR1A

2. Etiology

Disease causal factors — genetic. VDDR1A is caused exclusively by biallelic (homozygous or compound heterozygous) pathogenic loss-of-function variants in CYP27B1. As of recent surveys, ~78–100 distinct pathogenic CYP27B1 variants have been reported across >100–219 published patients from diverse ethnic groups, spanning all 9 exons; missense and nonsense substitutions predominate, alongside splice-site variants, small insertions, deletions, and duplications (PMC6398191; PLOS ONE, PMC4489500). A 2022 series of 12 Chinese children found 9 CYP27B1 variants (4 known, 5 novel: c.937G>C p.Glu313Gln, c.232delG p.Ala78Profs81, c.565G>T p.Glu189, c.1192G>A p.Gly398Ser, c.402G>A p.Trp134*), with single-nucleotide substitutions (66.7%), small deletions (22.2%), and small insertions (11.1%) (Front Pediatr 2022, PMID:36405822, PMC9671943).

Founder/recurrent variants: - c.1319_1325dupCCCACCC (p.Phe443Profs*24): a regional hotspot in southern China, found in 66.7% of a 12-patient Guangzhou cohort (45.8% of alleles) (PMC9671943). - c.262delG (p.Val88Trpfs*71): the founder variant of the Saguenay–Lac-Saint-Jean (SLSJ) region of Quebec, Canada, where every molecularly confirmed VDDR1A case traces to this single allele. - Intron-1 mutations shared by patients from a common city of origin have also suggested independent founder effects in other populations ([various case reports, search synthesis]). - p.(Ala129Thr): a recurrent partial loss-of-function ("hypomorphic") variant retaining ~50% residual enzymatic activity, associated with a distinctly milder phenotype (see Section 4/Genotype-phenotype below) (JCEM 2023, PMID:36321535).

Risk factors — genetic: Biallelic CYP27B1 pathogenic variants are both necessary and sufficient (fully penetrant with autosomal recessive inheritance); consanguinity substantially raises risk in outbred populations via increased homozygosity; population founder effects (e.g., French-Canadian SLSJ) create geographically concentrated carrier clusters.

Risk factors — environmental: No environmental exposure causes VDDR1A itself (it is a purely monogenic disorder), but vitamin D nutritional status (dietary calciferol intake, sun exposure) modulates the severity and timing of clinical presentation, since substrate (25(OH)D) availability affects how much residual/hypomorphic enzyme activity can generate 1,25(OH)₂D in partial-deficiency genotypes.

Protective factors: Hypomorphic ("leaky") CYP27B1 alleles such as p.(Ala129Thr) are protective relative to null alleles, producing later onset and milder biochemical/skeletal disease. No population-level protective environmental factor is documented; adequate vitamin D nutritional status can partially compensate for hypomorphic (but not null) alleles by providing more substrate for residual enzyme activity.

Gene-environment interactions: The clearest interaction is that ambient/dietary vitamin D sufficiency provides substrate (25(OH)D) that residual-activity hypomorphic CYP27B1 enzyme can still partially hydroxylate — meaning a child with a partial loss-of-function genotype and good vitamin D status may present later/milder than one who is also nutritionally vitamin D deficient. There is no independent environmental trigger analogous to infection or toxin exposure.

Sources: PMC6398191 | PMC9671943 | JCEM PMID:36321535

3. Phenotypes

Onset and general pattern: Affected infants are normal at birth (maternal-fetal calcium transfer via placenta is largely 1,25(OH)₂D-independent) and become symptomatic typically between 6 months and 2 years of age (mean onset ~1.1 ± 0.4 years in one cohort; overall reported range up to ~3 years, occasionally later with hypomorphic alleles) (PMC9671943; Frontiers review PMC7860650).

Symptoms/signs (categorized):

Laboratory abnormalities (biochemical phenotype) — the core diagnostic signature: - Hypocalcemia (mean 1.57 ± 0.19 mmol/L vs. reference 2.24–2.74 mmol/L) - Hypophosphatemia (mean 0.87 ± 0.23 mmol/L vs. reference 1.29–1.94 mmol/L) — secondary to PTH-driven renal phosphate wasting - Markedly elevated alkaline phosphatase (ALP) (mean 1629 ± 673 U/L vs. reference 118–390 U/L) - Elevated PTH / secondary hyperparathyroidism (mean 57.8 ± 32.7 pmol/L vs. reference 1.2–7.1 pmol/L) - Normal to elevated 25(OH)D (mean 77.1 ± 18.4 nmol/L; distinguishes VDDR1A from nutritional deficiency and VDDR1B/CYP2R1 deficiency) - Low or inappropriately normal 1,25(OH)₂D — the diagnostic hallmark, given the accompanying secondary hyperparathyroidism and hypocalcemia that should otherwise drive 1,25(OH)₂D up (Data: PMC9671943)

Radiographic phenotype: widened metaphyses, metaphyseal cupping/fraying, generalized decreased bone density, "fuzzy" metaphyseal margins on wrist/knee radiographs — classic rachitic changes; a Rickets Severity Score (RSS) of ~9.0 ± 1.0 at diagnosis in one cohort.

Severity/progression: Symptom severity is variable and correlates with residual enzyme activity and age at diagnosis/treatment onset (earlier diagnosis correlates with better height outcome; r = −0.62, p<0.05 between HtSDS and age at diagnosis). Untreated disease is progressive; treated disease shows biochemical normalization within ~3 months but height catch-up is less reliable, especially with poor treatment adherence.

Quality of life impact: Untreated/undertreated VDDR1A produces severe skeletal deformity, short stature, impaired mobility (documented case: inability to ambulate independently after 6-year treatment lapse, requiring surgical correction of scoliosis), and dental morbidity. With early, consistent calcitriol therapy, biochemical and most radiographic parameters normalize and QoL approaches normal, though sustained height catch-up occurs in only a minority (27.3% in one long-term cohort) — underscoring the QoL cost of treatment non-adherence, frequently linked to economic/social barriers (58.3% poor long-term compliance in one cohort).

Sources: PMC9671943 (12-child cohort) | PMC7860650 | Craniofacial/dental study | GARD

4. Genetic/Molecular Information

Causal gene: CYP27B1 (HGNC:2606; OMIM 609506), chromosome 12q14.1, encoding 25-hydroxyvitamin D-1α-hydroxylase (1α-OHase), a mitochondrial cytochrome P450 enzyme expressed predominantly in the renal proximal tubule (with documented extrarenal expression, see Section 6).

Variant landscape: ~78–100+ distinct pathogenic variants reported (missense, nonsense, frameshift indels, splice-site changes) spanning all coding exons (PMC6398191; PLOS ONE PMC4489500). In one 19-family cohort: 17 variants (11 missense, 3 frameshift, 2 truncating, 1 splice-acceptor site), homozygosity in 58% (11/19) (JCEM PMID:36321535).

Variant classification/pathogenicity: Per ACMG/AMP framework as used in ClinVar — most reported CYP27B1 variants are classified pathogenic/likely pathogenic based on: (1) biochemical loss-of-function assays; (2) segregation with autosomal recessive disease; (3) absence/near-absence in population databases (gnomAD); (4) protein-truncating or highly conserved missense location.

Functional consequence categories: - Complete loss-of-function (null) alleles — nonsense, frameshift, splice-disrupting variants that eliminate enzyme activity entirely — associated with classic early-onset, severe phenotype. - Partial loss-of-function (hypomorphic) alleles — e.g., p.(Ala129Thr), retaining ~50% residual catalytic activity — associated with later-onset, milder disease (median age at diagnosis 5.0 vs. 1.2 years; serum calcium 2.26 vs. 1.85 mmol/L; PTH 4.7 vs. 7.5× ULN; ALP 759 vs. 2082 IU/L compared to other genotypes) (JCEM 2023, PMID:36321535). - Enzyme-adrenodoxin interaction-disrupting variants — e.g., R459L, which prevents CYP27B1 from forming its normal electrostatic/sulfide-bridge interaction with adrenodoxin (the mitochondrial electron-donor redox partner), dramatically reducing catalytic turnover; H441Y, which disrupts a hydrogen bond with adrenodoxin but only minimally reduces activity — illustrating a structure-function spectrum where different adrenodoxin-interface residues confer different severities (JCEM PMID unspecified in search, "Adrenodoxin interactions" paper; Biochemistry, Arg458 mouse study). - c.590G>A (p.G197D): shown to cause aberrant RNA splicing rather than a simple missense effect (Front Genet, PMC7729158).

Modifier genes: No established modifier loci distinct from CYP27B1 allelic series itself; phenotype variability is chiefly explained by allelic (residual activity) effects rather than trans-acting modifiers, though vitamin D nutritional status functions as a phenotype modifier for hypomorphic genotypes.

Population/allele frequency: Individual pathogenic CYP27B1 variants are each very rare in general population databases (gnomAD), consistent with an overall rare autosomal recessive disease (worldwide literature reports ~219 patients cumulatively). The exception is regional founder populations: SLSJ Quebec carrier frequency for c.262delG estimated at 1 in 26–29, giving a birth prevalence of ~1 in 2,358–2,916 in that region — orders of magnitude above the general-population rate.

Somatic vs. germline: VDDR1A is exclusively a germline/constitutional disease; CYP27B1 has been separately studied (and largely excluded) as a candidate tumor-suppressor gene in primary and secondary/tertiary hyperparathyroidism, a distinct somatic-oncology question unrelated to VDDR1A pathogenesis (PMC2689078).

Epigenetics/chromosomal abnormalities: No epigenetic mechanism or chromosomal-scale abnormality (aneuploidy, large CNV, translocation) has been implicated in VDDR1A; it is a classic single-gene, sequence-level Mendelian disorder.

Sources: OMIM 609506 | PMC6398191 | JCEM PMID:36321535 | JCEM adrenodoxin paper | PMC7729158 | PMC2689078

5. Environmental Information

VDDR1A is a monogenic disorder with no infectious or toxin etiology. The principal environmental modulator is vitamin D nutritional status (dietary intake and sun-exposure-derived cutaneous synthesis), which determines substrate (25(OH)D) supply and thereby modulates severity/timing in patients with hypomorphic (partial-activity) CYP27B1 alleles, though it cannot compensate for null alleles. No occupational, toxin, radiation, or lifestyle risk factor beyond general vitamin D status has been documented in the literature reviewed. No infectious trigger is implicated in disease onset (as distinct from the separate observation that CYP27B1 extrarenal/immune-cell expression participates in granulomatous-disease vitamin D dysregulation, e.g., tuberculosis-associated hypercalcemia — a different clinical phenomenon from VDDR1A itself; see Section 6).

Sources: PMC7860650 | Regulation of extrarenal CYP27B1, PMID:24388948

6. Mechanism / Pathophysiology

Causal chain (initial trigger → clinical manifestation):

1. Molecular lesion: Biallelic pathogenic CYP27B1 variants → loss or severe reduction of 25-hydroxyvitamin D-1α-hydroxylase catalytic activity in renal proximal tubule mitochondria (GO molecular function: vitamin D 25-hydroxylase-derivative activity / 1,25-dihydroxyvitamin D synthesis; suggested GO:0036378 calcitriol biosynthetic process, GO:0070576 vitamin D 24-hydroxylase-adjacent metabolic pathway context).
2. Enzymatic mechanism: CYP27B1 is a mitochondrial cytochrome P450 (Type I, class I) enzyme requiring electron transfer from NADPH → ferredoxin reductase → adrenodoxin (ferredoxin) → CYP27B1 heme iron to catalyze C1α-hydroxylation of 25(OH)D₃. Mutations at the CYP27B1-adrenodoxin protein-protein interface (e.g., R459L disrupting the docking interaction, H441Y disrupting a hydrogen bond) impair electron transfer and catalytic turnover, providing a structural basis for the enzymatic loss of function ([Biochemistry, Arg458 study, PMID referenced above]; JCEM adrenodoxin interaction paper).
3. Biochemical consequence: Failure to convert 25(OH)D to 1,25(OH)₂D (calcitriol) → selective calcitriol deficiency despite normal/elevated precursor 25(OH)D.
4. Cellular/tissue consequence: Calcitriol normally binds the vitamin D receptor (VDR, a nuclear hormone receptor/transcription factor) in intestinal enterocytes to upregulate calcium transport proteins (e.g., TRPV6, calbindin-D9k) — calcitriol deficiency causes intestinal calcium malabsorption.
5. Systemic consequence: Reduced intestinal calcium absorption → hypocalcemia → compensatory secondary hyperparathyroidism (elevated PTH) → PTH-driven increased renal phosphate clearance → hypophosphatemia; combined hypocalcemia/hypophosphatemia → impaired hydroxyapatite deposition at the growth-plate mineralization front → rickets (in growing bone) with elevated alkaline phosphatase reflecting osteoblast compensatory activity.
6. End-organ/clinical manifestation: Growth-plate widening/cupping, skeletal deformity, growth retardation, hypotonia, dental enamel hypoplasia, and (in severe/untreated cases) hypocalcemic tetany/seizures.

This maps closely onto the dismech defective_skeletal_mineralization module's calciopenic arm — a calcium-deficient (as opposed to phosphopenic or mineralization-inhibitor) route converging on impaired hydroxyapatite deposition at the mineralization front.

Molecular pathway/GO term suggestions: - GO:0036378 — calcitriol biosynthetic process - GO:0042359 — vitamin D metabolic process - GO:0070257 — positive regulation of mucus secretion (not relevant) — omit - GO:0006816 — calcium ion transport (downstream, intestinal) - GO:0004497 — monooxygenase activity (CYP27B1 catalytic activity class) - GO:0005506 — iron ion binding (heme cofactor) - GO:0006874 — cellular calcium ion homeostasis (downstream systemic effect)

Cellular processes involved: Renal proximal tubular epithelial cell mitochondrial hydroxylation; intestinal enterocyte calcium transport; parathyroid chief cell PTH secretion (compensatory hyperplasia); osteoblast/osteoclast-mediated growth-plate chondro-osseous mineralization (impaired).

Protein dysfunction: Loss-of-function via (a) truncation/frameshift eliminating catalytic domain, (b) missense substitutions destabilizing heme-binding or substrate-binding pockets, (c) missense substitutions at the adrenodoxin-docking interface impairing electron transfer, (d) splice-site variants causing aberrant transcript/nonfunctional protein (e.g., c.590G>A/p.G197D causing an RNA splicing error) (PMC7729158).

Biochemical abnormalities: Enzyme deficiency (1α-hydroxylase); downstream hormonal cascade abnormalities (low calcitriol, compensatory high PTH); secondary electrolyte derangements (hypocalcemia, hypophosphatemia).

Immune system involvement (extrarenal CYP27B1 biology, contextual): CYP27B1 is also expressed extrarenally in macrophages, dendritic cells, T and B lymphocytes, and keratinocytes, where it participates in intracrine/paracrine vitamin D signaling relevant to innate/adaptive immune regulation (notably in granulomatous diseases such as tuberculosis, where IFN-γ stimulates and type I interferons inhibit macrophage CYP27B1 activity). This extrarenal pathway is a distinct physiological role from the renal-endocrine axis defective in VDDR1A and is not itself part of VDDR1A pathogenesis, but is mechanistically noteworthy and occasionally causes diagnostic confusion (e.g., a VDDR1A case "mimicking pseudohypoparathyroidism in the presence of active tuberculosis," PMC11439522) (PMID:24388948; PMID:24314866).

Tissue damage mechanism: Chondro-osseous — defective mineralization of osteoid/growth-plate cartilage matrix (not classic oxidative/ischemic/fibrotic injury) due to insufficient available calcium-phosphate product at the mineralization front.

Parathyroid biology: Chronic secondary hyperparathyroidism can, in severe/prolonged untreated or undertreated cases, raise concern for autonomous (tertiary) parathyroid hyperplasia; however, CYP27B1 itself does not appear to function as a classical tumor-suppressor gene in parathyroid adenoma pathogenesis, based on molecular analyses in primary and refractory secondary/tertiary hyperparathyroidism cohorts (PMC2689078).

Advanced/omics data: No large-scale transcriptomic, proteomic, metabolomic, or single-cell/spatial datasets specific to VDDR1A patient tissue were identified in this search; mechanistic insight instead derives from biochemical enzymology (recombinant CYP27B1 kinetics, adrenodoxin-binding assays) and knockout animal models (below).

Sources: JCEM adrenodoxin paper | PMC7729158 | PMC2689078 | PMID:24388948 | PMC11439522

7. Anatomical Structures Affected

Organ level: - Primary: Skeletal system (growth plates, long bones, ribs, skull) — UBERON:0001434 (skeletal system); Kidney (site of primary enzymatic defect) — UBERON:0002113 - Secondary: Parathyroid glands (compensatory hyperplasia) — UBERON:0001132; Intestine (site of impaired calcium absorption, functional target) — UBERON:0000160; Teeth (enamel hypoplasia) — UBERON:0001091; Skull/cranial sutures (widened sutures, frontal bossing) — UBERON:0003129 - Body systems involved: Skeletal/musculoskeletal, endocrine (parathyroid-vitamin D axis), gastrointestinal (calcium absorption), and secondarily neuromuscular (hypotonia, tetany/seizures from hypocalcemia)

Tissue/cell level: - Growth plate cartilage / hypertrophic chondrocytes — affected by impaired mineralization (CL:0000058 chondrocyte) - Osteoblasts (CL:0000062) — compensatory activity reflected in elevated ALP; osteoid accumulation - Renal proximal tubule epithelial cells (CL:1001016 or CL:0002306 — kidney proximal tubule cell) — site of CYP27B1 enzymatic activity - Intestinal enterocytes (CL:0000584) — functional target of calcitriol-VDR signaling for calcium transport - Parathyroid chief cells (CL:0000426) — secondary hyperplasia/hypersecretion of PTH - Extrarenal: macrophages (CL:0000235), dendritic cells (CL:0000451), T lymphocytes (CL:0000084), B lymphocytes (CL:0000236), keratinocytes (CL:0000312) — sites of extrarenal CYP27B1 expression (not primary to VDDR1A pathology but part of the gene's broader biology)

Subcellular level: - Mitochondria (GO:0005739 mitochondrion) — CYP27B1 is a mitochondrial inner-membrane-associated cytochrome P450 enzyme - Mitochondrial matrix (site of adrenodoxin/adrenodoxin reductase electron transport chain interaction)

Localization: Bilateral/symmetric skeletal involvement (long bones, ribs, wrists, skull); no lateralization reported. Renal involvement is functional (enzymatic), not structural/anatomic (kidneys are not malformed).

Sources: synthesized from clinical descriptions across PMC9671943, PMC7860650, craniofacial/dental study

8. Temporal Development

Onset: Congenitally normal at birth; clinical onset typically 6 months to ~2 years of age (cohort mean onset 1.1 ± 0.4 years; range up to ~3 years reported); onset pattern is generally insidious/subacute (progressive failure to thrive, delayed motor milestones, evolving skeletal deformity) rather than acute, though acute hypocalcemic tetany/seizures can be a presenting event in severe null-allele cases. Milder (hypomorphic-allele) presentations can be delayed to ~5 years or discovered incidentally on family cascade screening.

Progression: Without treatment, disease is chronically progressive — worsening skeletal deformity, growth failure, and (rarely) life-threatening hypocalcemia. With calcitriol/alfacalcidol treatment, biochemical parameters (calcium, phosphate, ALP, PTH) normalize within approximately 3 months; radiographic (rachitic) improvement follows over months to a few years; height catch-up is the slowest-responding and least reliably achieved parameter, and is critically dependent on early treatment initiation and sustained adherence.

Disease course pattern: Chronic and lifelong if untreated adherence lapses — the disease is not self-limited; discontinuation of therapy (even after years of good control) leads to biologic relapse, as illustrated by a documented case of a child who stopped treatment, was lost to follow-up for 6 years, and returned with severe skeletal deformity, elevated ALP (2662 U/L) and PTH (91.8 pmol/L), and requiring surgical intervention for scoliosis.

Critical period for intervention: Diagnosis/treatment initiation timing is inversely correlated with final height outcome (r = −0.62, p<0.05 for HtSDS vs. age at diagnosis) — earlier treatment (including pre-symptomatic initiation via newborn screening in the SLSJ founder population) yields substantially better growth and skeletal outcomes, establishing early infancy/toddlerhood as the critical therapeutic window.

Sources: PMC9671943 | PMC9120640

9. Inheritance and Population

Inheritance pattern: Autosomal recessive. Both parents are typically obligate heterozygous carriers (asymptomatic); affected individuals carry two pathogenic CYP27B1 alleles (homozygous or compound heterozygous).

Penetrance: Complete/full penetrance for biallelic null genotypes; genotype-dependent expressivity for hypomorphic alleles (see below) means "penetrance" in a strict biochemical sense is universal, but clinical severity/age-of-onset varies continuously with residual enzyme activity.

Expressivity: Variable, correlating with allele-specific residual enzymatic activity — compare classic early/severe null-allele presentation vs. milder, later-onset p.(Ala129Thr) hypomorphic phenotype (older age at diagnosis, higher calcium, lower PTH/ALP, absence of hypotonia/seizures).

Genetic anticipation: Not reported/applicable (no repeat-expansion mechanism).

Germline mosaicism: Not specifically documented in the reviewed literature for CYP27B1, though theoretically possible for any recessive Mendelian disorder; recurrence-risk counseling follows standard autosomal recessive principles (25% recurrence risk per pregnancy for carrier × carrier couples).

Founder effects: Well-documented — the Saguenay–Lac-Saint-Jean (SLSJ) region of Quebec, Canada exhibits a strong founder effect for the c.262delG (p.Val88Trpfs*71) variant, with a carrier frequency of 1 in 26–29 and a birth prevalence of ~1 in 2,358–2,916 (i.e., roughly one affected child born annually in the region) — dramatically higher than the general worldwide rate. Independent founder/common-ancestor patterns (e.g., shared intron-1 mutations among patients from a single city) have also been suggested in other populations, and a regional Chinese hotspot allele (c.1319_1325dupCCCACCC) has been reported in southern China.

Consanguinity: A significant contributor in outbred, non-founder populations — the high proportion of homozygous (as opposed to compound heterozygous) genotypes in several case series (41.7–58%) is consistent with parental consanguinity in many reported families, particularly from regions/cultures with higher consanguinity rates.

Carrier frequency: ~1 in 26–29 in the SLSJ founder population (markedly elevated); general-population carrier frequency is not well established given overall rarity but is presumably very low (<<1%) outside founder clusters.

Epidemiology (prevalence/incidence): - Overall exceedingly rare worldwide: ~219 patients reported in the cumulative literature (per one synthesis). - Denmark: prevalence among children <15 years estimated at 1/250,000. - SLSJ, Quebec: prevalence at birth estimated at 1 in 2,358 (regional founder-effect outlier). - No formal Global Burden of Disease (GBD) or large national-registry incidence estimate exists given the disease's rarity; most epidemiologic knowledge is derived from case-series aggregation and the SLSJ founder registry.

Population demographics: No strong sex predilection is reported (autosomal recessive; one cohort reported exactly 1:1 male:female ratio, 6/6). Ethnic/geographic clustering reflects founder populations (French-Canadian SLSJ; regional hotspots in southern China) and consanguineous communities (e.g., Middle Eastern, Central/South Asian populations, based on the geographic spread of case reports including Uzbekistan, Vietnam, and others cited in the literature search).

Sources: OMIM #264700 | Orphanet ORPHA289157 | PMC9120640 | PMC9671943 | Cruz Marino 2023, AJMG-A

10. Diagnostics

Core biochemical panel (LOINC-codeable): - Serum calcium (low) — LOINC 17861-6 - Serum phosphate (low) - Serum alkaline phosphatase (markedly elevated) - Serum intact PTH (markedly elevated) - 25-hydroxyvitamin D (normal or elevated — key discriminator from nutritional deficiency and VDDR1B) - 1,25-dihydroxyvitamin D (calcitriol) — low or inappropriately normal given the concurrent hypocalcemia/hyperparathyroidism that should otherwise drive it up; this is the single most discriminating biochemical test - Urinary calcium/creatinine ratio (for monitoring hypercalciuria risk during treatment)

Imaging: - Wrist and knee radiographs — widened epiphyses/metaphyses, metaphyseal cupping and fraying, generalized decreased bone density; graded via a Rickets Severity Score (RSS) - Renal ultrasound — baseline and monitoring for nephrocalcinosis (a treatment-related risk), performed at diagnosis and every 1–2 years thereafter (more frequently if hypercalciuria develops) - Skull imaging may show widened sutures, posterior flattening

Genetic testing: - Single-gene CYP27B1 sequencing (Sanger or targeted NGS) is the definitive diagnostic test given the disorder's clean genotype-phenotype relationship; a skeletal dysplasia/rickets gene panel (including CYP27B1, CYP2R1, VDR, PHEX, DMP1, FGF23, ENPP1, SLC34A3, etc.) is commonly used clinically to differentiate the vitamin D-dependent and hypophosphatemic rickets spectrum. Whole-exome sequencing is increasingly used, especially in atypical or apparently sporadic presentations. - Biochemical phenotype (low/inappropriately-normal 1,25(OH)₂D with normal/high 25(OH)D) strongly predicts CYP27B1 involvement prior to sequencing.

Newborn screening: A prospective newborn genetic screening program has been implemented in the SLSJ founder population (targeting the c.262delG founder allele), demonstrated to be safe, feasible, and efficient; pre-symptomatic identification allows calcitriol initiation before clinical manifestations develop, improving growth/skeletal outcomes (PMC9120640).

Differential diagnosis: | Condition | Distinguishing feature vs. VDDR1A | |---|---| | Nutritional vitamin D deficiency rickets | Low 25(OH)D (vs. normal/high in VDDR1A); responds to standard vitamin D supplementation | | VDDR1B (CYP2R1 deficiency, 25-hydroxylase) | Deficient 25(OH)D (upstream hydroxylation defect); may show gene-dosage/heterozygous partial phenotypes improving with age; can respond to calcifediol bypassing the block | | VDDR2A (VDR mutations, hereditary vitamin D-resistant rickets) | Markedly elevated 1,25(OH)₂D (end-organ resistance, not deficiency); alopecia in ~50% of cases (absent in VDDR1A); requires massive calcitriol doses or IV calcium; may show dramatic post-pubertal improvement in calcium absorption | | X-linked hypophosphatemic rickets (PHEX)/other FGF23-mediated hypophosphatemic rickets | Normal PTH and normal/low-normal calcium (vs. VDDR1A's elevated PTH and low calcium); phosphopenic rather than calciopenic mechanism | | Hypoparathyroidism/pseudohypoparathyroidism | Low or inappropriately normal PTH (hypoPTH) or PTH resistance with characteristic Albright hereditary osteodystrophy features (pseudoHP) — VDDR1A shows appropriately elevated PTH; misdiagnosis as normocalcemic primary hyperparathyroidism or pseudohypoparathyroidism has been reported due to overlapping biochemistry in atypical presentations |

Diagnostic pitfalls documented in the literature: VDDR1A has been misdiagnosed as nutritional rickets, hypophosphatemic rickets, pseudohypoparathyroidism (including in the setting of concurrent active tuberculosis, where extrarenal CYP27B1 activity in granulomas confounds vitamin D metabolite interpretation), and normocalcemic primary hyperparathyroidism — underscoring the value of genetic confirmation.

Sources: PMC11439522 | JCEM Case Reports, PMID misdiagnosis paper | PMC9120640 | Frontiers diagnosis/management review

11. Outcome/Prognosis

Survival/mortality: With timely diagnosis and appropriate calcitriol replacement, VDDR1A is not a life-shortening condition; mortality risk is essentially confined to acute, severe, untreated hypocalcemia (tetany/seizures/cardiac effects) in infancy, which is preventable with treatment. No formal survival/life-expectancy statistics were identified, consistent with the expectation of near-normal life expectancy on treatment.

Morbidity/functional outcomes: - Biochemical normalization is generally achieved within ~3 months of appropriate calcitriol/calcium therapy. - Radiographic (rachitic) improvement follows, with RSS improving significantly with good adherence (e.g., from 8.6 ± 1.0 to 3.7 ± 3.4 in one cohort, p<0.05). - Growth/height outcome is the most treatment-resistant domain: initial catch-up growth occurs in ~91% of patients in the first few years, but sustained catch-up to normal height was maintained in only ~27% at last follow-up in one long-term cohort; persistent short stature (HtSDS < −2) affected 63.6% at last visit. Biochemical normalization does not guarantee height recovery. - Renal outcomes: No nephrocalcinosis was observed in a 12-patient cohort with appropriate calcitriol dosing over a mean 6.2-year follow-up (one patient had enhanced kidney echogenicity only), suggesting that with careful monitoring the treatment-related hypercalciuria/nephrocalcinosis risk (a concern for all calcitriol-treated rachitic disorders) can be managed effectively.

Complications: Severe/relapsed disease (from treatment discontinuation) can require orthopedic surgical intervention (e.g., for severe scoliosis); dental enamel hypoplasia is a persistent complication even with adequate systemic treatment; secondary/occasionally severe hyperparathyroidism can develop with chronic undertreatment, raising a theoretical (though apparently uncommon) concern for progression toward parathyroid autonomy.

Prognostic factors: The dominant modifiable prognostic factor is age at diagnosis/treatment initiation (earlier = better height outcome) and long-term treatment adherence (the primary determinant of sustained skeletal/growth benefit — cited reasons for poor adherence include economic constraints, social problems, inadequate medical education, and adolescent psychosocial issues). Genotype (null vs. hypomorphic allele, e.g., p.(Ala129Thr)) is a non-modifiable prognostic factor correlating with baseline severity and, indirectly, with the degree of catch-up needed.

Sources: PMC9671943 | JCEM PMID:36321535

12. Treatment

Pharmacotherapy — mainstay of treatment: - Calcitriol (1,25-dihydroxyvitamin D₃) is the first-line, definitive replacement therapy, since it bypasses the defective 1α-hydroxylation step entirely. - Initial/loading dose: commonly 1–2 μg/day (some protocols 1–1.5 μg twice daily); pediatric cohort dosing reported as low as 0.25–0.5 μg/day depending on severity and body size, titrated to response. - Maintenance dose: typically 0.25–1 μg/day (ranges cited 0.3–2 μg/day), given in divided (twice-daily) doses owing to calcitriol's short biological half-life. - Calcium supplementation is co-administered, especially during the initial "hungry bone" remineralization phase (guidance cited: ~50 mg/kg/day elemental calcium in children; cohort doses of 500–1000 mg/day). - Alternative agents: 1α-hydroxyvitamin D (alfacalcidol/1α-OH-D₃) — a prodrug requiring only hepatic 25-hydroxylation (which is intact in VDDR1A) to become active — offers a longer half-life allowing once-daily dosing and is widely used, particularly in European cohorts (e.g., the 19-family JCEM genotype-phenotype cohort was treated with alfacalcidol). Eldecalcitol has shown superior osteogenic promotion versus alfacalcidol in the Cyp27b1-knockout mouse model (preclinical, not yet standard human therapy). - Therapeutic agent ontology: calcitriol (CHEBI:17823), alfacalcidol (CHEBI equivalent), calcium carbonate/citrate (elemental calcium supplementation). - NCIT treatment term suggestions: NCIT:C15986 (Pharmacotherapy) as the generic action, with therapeutic_agent bound to calcitriol/alfacalcidol.

Treatment goals: Achieve normocalcemia, maintain PTH within normal limits, avoid hypercalciuria/nephrocalcinosis, and normalize radiographic/growth parameters.

Monitoring protocol: Serum calcium, phosphorus, PTH, alkaline phosphatase, creatinine, and vitamin D metabolites every 3–6 months; 24-hour urinary calcium or spot calcium:creatinine ratio; renal ultrasound every 1–2 years (more frequently if hypercalciuria present); annual wrist/knee radiographs during active growth.

Duration: Lifelong — "treatment must be continued indefinitely"; discontinuation reliably leads to biochemical and skeletal relapse (documented case of a 6-year treatment lapse producing severe deformity requiring surgery).

Surgical/interventional care: Orthopedic surgical correction may be required for severe, established skeletal deformities (e.g., scoliosis) in patients with delayed diagnosis or prolonged non-adherence.

Supportive/rehabilitative care: Nutritional counseling to support adequate dietary calcium/vitamin D intake; physical therapy may be used adjunctively for motor delay/deformity-related functional limitation (NCIT:C15302, Physical Therapy), though this is not emphasized as a primary modality in the literature reviewed.

Genetic counseling: Recommended for families given autosomal recessive inheritance and (in founder populations) availability of targeted carrier/newborn screening; NCIT:C15240 (Genetic Counseling).

Experimental/investigational therapies: No gene therapy, RNA-based therapy, or novel targeted biologic specific to VDDR1A was identified in this search — enzyme-replacement is achieved pragmatically via direct hormone (calcitriol) replacement rather than protein or gene-based correction, which is feasible precisely because the deficient product (calcitriol) itself is an inexpensive, orally bioavailable small molecule.

Treatment response/outcomes: Excellent biochemical and radiographic response rates with adherent therapy; the principal "failure mode" is non-adherence rather than pharmacologic non-response — reinforcing that VDDR1A is, mechanistically, a highly treatable condition once diagnosed.

Sources: PMC9671943 | Frontiers diagnosis/management review | JCEM PMID:36321535 | Calcitriol treatment, ScienceDirect | Eldecalcitol vs alfacalcidol mouse study, PMC6169848

13. Prevention

Primary prevention: Not applicable in the classic sense (this is a genetic, not an acquired/exposure-based disease), but pre-symptomatic treatment initiation via newborn/carrier screening in founder populations effectively prevents the clinical manifestations of the disease from ever developing — the SLSJ program demonstrates that daily calcitriol initiated before symptom onset can prevent the phenotype entirely.

Secondary prevention/screening: - Targeted newborn genetic screening for the founder c.262delG variant in the SLSJ Quebec population — shown safe, feasible, and effective at enabling pre-symptomatic treatment. - Carrier screening / cascade family testing is recommended in founder populations and in families with an index case, given the high carrier frequency locally (1/26–29 in SLSJ) and the straightforward single-gene test. - General population newborn screening for VDDR1A is not standard practice outside founder/high-prevalence populations, given the disease's overall rarity.

Tertiary prevention: Once diagnosed, ongoing monitoring (per Section 12) prevents complications (nephrocalcinosis, severe deformity, growth failure) and enables early detection/correction of relapse from non-adherence.

Genetic counseling: Central to prevention strategy in affected families and in founder communities — informing reproductive risk (25% recurrence for carrier couples) and enabling prenatal or preimplantation genetic testing where desired.

Public health interventions: No population-wide public health intervention (e.g., water fortification, mass supplementation) is applicable, since VDDR1A is refractory to standard vitamin D supplementation (the defect is downstream of 25(OH)D availability) — this is an important practical distinction from nutritional rickets prevention programs.

Prophylaxis: N/A beyond the therapeutic calcitriol regimen itself, which functions simultaneously as treatment and (when started pre-symptomatically) as prevention of phenotypic expression.

Sources: PMC9120640

14. Other Species / Natural Disease

Naturally occurring VDDR1A in companion animals — a well-characterized comparative model: - Dogs (Canis lupus familiaris, NCBI Taxon:9615): Naturally occurring CYP27B1-mutation vitamin D-dependent rickets type IA has been documented in pugs and Saint Bernards, catalogued in OMIA:000837-9615 (OMIA). A 2023 study identified a stop-gain mutation (chr10:2182971G>T) in CYP27B1 in affected pugs, causing premature truncation at codon 87 (loss of ~83% of the protein), producing a clinical phenotype "indistinguishable" from nutritional vitamin D deficiency and life-threatening if untreated in young pugs (J Vet Intern Med 2023, PMID:37293695; PMC10365047). - Breed relevance (VBO): Pug and Saint Bernard breeds are specifically documented; this represents genuine veterinary clinical importance (a naturally arising, breed-associated inherited disease) rather than only a laboratory-induced model.

Comparative biology: The canine phenotype recapitulates the core human biochemical and skeletal signature (hypocalcemia, secondary hyperparathyroidism, rachitic bone disease), supporting deep evolutionary conservation of the CYP27B1-vitamin D endocrine axis across mammals. No zoonotic or transmission relevance applies — this is a purely genetic, non-communicable disease in both species.

No non-mammalian natural disease models identified in this search (the vitamin D endocrine/calcitriol-VDR axis is a vertebrate, largely mammalian/avian physiological system, but naturally occurring CYP27B1-deficiency disease was not found reported outside canines in the literature surveyed).

Sources: OMIA:000837-9615 | J Vet Intern Med, PMID:37293695 | PMC10365047

15. Model Organisms

Mouse (Mus musculus) — Cyp27b1 knockout: - Model type: Targeted gene-knockout (constitutive), the principal genetic animal model of VDDR1A/pseudo-vitamin-D-deficiency rickets. - Phenotype recapitulation: Cyp27b1-knockout mice develop hypocalcemia, hypophosphatemia, secondary hyperparathyroidism, and short, deformed bones with dysmorphic growth plates — closely mirroring the human biochemical and skeletal phenotype. After weaning, mice show marked hypocalcemia and high PTH with decreased growth, osteodystrophy (bone hypocalcification), and growth-plate cartilage hypertrophy. - Rescue experiments: Treatment with exogenous 1,25-dihydroxyvitamin D₃ (calcitriol) rescues the pseudo-vitamin-D-deficiency-rickets phenotype in Cyp27b1-deficient mice — confirmed via biochemical, histomorphometric, and biomechanical analyses (PMID:12674324) — directly validating calcitriol replacement as mechanistically corrective, mirroring the human standard-of-care. - Comparative model characteristics: The mineral/skeletal phenotype of Cyp27b1-KO mice is more severe than that of Vdr-KO mice (VDDR2A model) and, notably, cannot be fully rescued by a "rescue diet" high in calcium, phosphate, and lactose — a diet strategy that does substantially normalize the Vdr-KO phenotype — highlighting a biologically meaningful difference between the ligand-deficiency (VDDR1A) and receptor-resistance (VDDR2A) mechanisms even though both converge on impaired VDR signaling output (Bone Research 2024). - Model limitations: As with all knockout models, developmental compensation and species-specific differences in mineral handling/diet (rodent vs. human) limit direct translational fidelity, particularly regarding the precise growth/height catch-up dynamics seen in human patients on treatment. - Applications: Studying calcitriol-VDR axis physiology, comparing therapeutic vitamin D analogs (e.g., eldecalcitol vs. alfacalcidol — eldecalcitol showed superior osteogenic promotion in Cyp27b1-KO mice, PMC6169848), and dissecting genetic vs. dietary rescue strategies.

Rat (Rattus norvegicus): Novel genetically modified rat models (including Vdr-KO rats) have been generated to further probe molecular mechanisms of vitamin D action; Vdr-KO rats notably show alopecia (as in human VDDR2A) — a feature that, by contrast, is absent in CYP27B1-deficient animals (and in human VDDR1A), reinforcing alopecia as a VDR-signaling/receptor-pathway-specific (not ligand-deficiency) feature (Sci Rep, PMC7105495).

Naturally occurring large-animal model: The pug/Saint Bernard dog model (Section 14) functions additionally as a valuable spontaneous (non-engineered) genetic model, complementing the engineered mouse knockout, particularly for large-animal/companion-animal translational and veterinary-clinical study.

Cellular/in vitro models: Recombinant CYP27B1 expression systems (e.g., in bacterial or mammalian expression systems) have been used extensively for structure-function mutagenesis studies (e.g., Arg458/Arg459 adrenodoxin-interaction mutants) to dissect enzyme kinetics and electron-transfer partner interactions at the molecular level, though these are biochemical rather than whole-cell disease models.

Model databases: MGI (Mouse Genome Informatics) for Cyp27b1 mouse alleles; OMIA for the canine model; no zebrafish, Drosophila, C. elegans, or yeast VDDR1A-specific model was identified (unsurprising, given the mammalian-specific renal-endocrine vitamin D axis).

Sources: Bone Research 2024, Nature | PMID:12674324 (rescue study) | PMC6169848 (eldecalcitol study) | PMC7105495 (rat models) | OMIA:000837-9615

Summary Ontology Term Suggestions

(Per dismech convention, all suggested ontology terms should be independently verified via OAK — runoak -i sqlite:obo:<ontology> info <CURIE> -O obo — before use in curation, to guard against label mismatch or hallucination.)

Master Source List

• OMIM #264700 — VDDR1A
• OMIM 609506 — CYP27B1
• OMIM #277440 — VDDR2A
• Orphanet ORPHA289157
• GARD — Vitamin D-dependent rickets, type 1
• ICD-10-CM E83.32
• Sørensen KV et al. — PLOS ONE / PMC4489500, Novel CYP27B1 Gene Mutations
• Peng et al., Front Pediatr 2022, PMID:36405822 — 12-child Chinese cohort, PMC9671943
• Molin/Kaufmann et al. — Genetic and clinical characteristics of VDDR1A patients, PMC6398191
• Benefits of Newborn Screening for VDDR1A in a Founder Population, PMC9120640
• Fauquier/Colin et al., JCEM 2023, PMID:36321535 — CYP27B1 p.(Ala129Thr) Genotype-Phenotype, JCEM
• Kim CJ et al., JCEM — CYP27B1 Adrenodoxin Interaction Mutations
• Molecular Analysis: c.590G>A p.G197D Splicing Error, PMC7729158
• CYP27B1 as candidate tumor suppressor in hyperparathyroidism, PMC2689078
• Regulation of extrarenal CYP27B1-hydroxylase, PMID:24388948
• Murine CD8+ T cells express vitamin D 1α-hydroxylase, PMID:24314866
• VDDR1A Mimicking Pseudohypoparathyroidism with Active TB, PMC11439522
• Craniofacial and dental characteristics of VDDR1A, Clin Oral Investig
• Diagnosis and Management of Vitamin D Dependent Rickets, Front Pediatr
• Coactivator-independent VDR signaling causes severe rickets in mice, Bone Research 2024
• Rescue of pseudo-VDDR phenotype in Cyp27b1-KO mice, PMID:12674324
• Eldecalcitol vs alfacalcidol in Cyp27b1-KO mice, PMC6169848
• Generation of genetically modified rats for vitamin D action, PMC7105495
• Mutations in CYP27B1 cause VDDR in pugs, J Vet Intern Med, PMID:37293695 / PMC10365047
• OMIA:000837-9615 — Vitamin D-deficiency rickets, type IA in dog
• Cruz Marino et al. 2023, AJMG-A — SLSJ founder population autosomal recessive diseases
• MalaCards — VDDR1A

Reference Validation

Checked with linkml-reference-validator 0.2.1.

Quotes not found in the cited source

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• PMC:PMC11439522 (abstract only): "mimicking pseudohypoparathyroidism in the presence of active tuberculosis,"
• closest text in source: "This report describes a case of vitamin D-dependent rickets type 1A, mimicking pseudohypoparathyroidism owing to presence of concurrent illness like extrapulmonary tuberculosis."