X-linked hypophosphatemia (XLH) is the most common hereditary form of rickets, caused by loss-of-function mutations in PHEX, a phosphate-regulating endopeptidase. PHEX deficiency leads to elevated FGF23 levels, causing renal phosphate wasting and impaired 1,25-dihydroxyvitamin D synthesis. This results in chronic hypophosphatemia, rickets/osteomalacia, short stature, and dental abnormalities. Burosumab (Crysvita), an anti-FGF23 antibody, is approved for treatment.
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name: X-Linked Hypophosphatemia
creation_date: '2026-02-13T00:31:42Z'
updated_date: '2026-05-06T08:10:58Z'
category: Mendelian
description: >
X-linked hypophosphatemia (XLH) is the most common hereditary form of rickets,
caused by loss-of-function mutations in PHEX, a phosphate-regulating endopeptidase.
PHEX deficiency leads to elevated FGF23 levels, causing renal phosphate wasting
and impaired 1,25-dihydroxyvitamin D synthesis. This results in chronic
hypophosphatemia, rickets/osteomalacia, short stature, and dental abnormalities.
Burosumab (Crysvita), an anti-FGF23 antibody, is approved for treatment.
disease_term:
preferred_term: X-linked dominant hypophosphatemic rickets
term:
id: MONDO:0010619
label: X-linked dominant hypophosphatemic rickets
parents:
- Metabolic Bone Disorders
inheritance:
- name: X-linked Dominant
description: >
X-linked dominant inheritance with variable expressivity. Males
are generally more severely affected than females. The PHEX gene
is located on Xp22.1.
evidence:
- reference: PMID:7550339
reference_title: "A gene (PEX) with homologies to endopeptidases is mutated in patients with X-linked hypophosphatemic rickets. The HYP Consortium."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "X-linked hypophosphatemic rickets (HYP) is a dominant disorder characterised by impaired phosphate uptake in the kidney"
explanation: "Confirms X-linked dominant inheritance pattern of XLH."
prevalence:
- population: General populations
percentage: 1 in 20,000
notes: >-
Reviews and nationwide survey data consistently place XLH around 1 in
20,000, supporting its status as the most common monogenic cause of
hypophosphatemic rickets while still remaining a rare disease.
evidence:
- reference: PMID:37321578
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "X-linked hypophosphatemia (XLH), the most common cause of hypophosphatemic rickets, affects one in every 20,000 people."
explanation: Recent review explicitly states the widely cited general-population prevalence estimate for XLH.
- reference: PMID:26135520
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "The estimated incidence of XLH was about 1 in 20,000."
explanation: Nationwide Japanese survey independently supports an XLH frequency of about 1 in 20,000.
- population: Australian and New Zealand pediatric populations
percentage: 1.33-1.60 per 100,000 children
notes: >-
Active pediatric surveillance in Australia and New Zealand produced minimum
prevalence estimates in the low per-100,000 range, with the authors noting
that true prevalence is likely somewhat higher because ascertainment was
incomplete.
evidence:
- reference: PMID:37263386
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "Estimated minimum prevalence based on these cases was 1.33 (1.04-1.66) per 100,000 and 1.60 per 100,000 (95%CI 0.97-2.58) in Australia and New Zealand respectively, with actual prevalence likely higher due to incomplete ascertainment."
explanation: Provides contemporary active-surveillance minimum prevalence estimates for pediatric XLH in Australia and New Zealand.
pathophysiology:
- name: Renal Phosphate Wasting via FGF23 Excess
description: >
Loss-of-function mutations in PHEX lead to elevated circulating FGF23
(fibroblast growth factor 23), a phosphaturic hormone produced by
osteocytes. Elevated FGF23 acts on the kidney to downregulate sodium-
phosphate cotransporters (NaPi-IIa/IIc) in the proximal tubule, causing
renal phosphate wasting. FGF23 also suppresses 1-alpha-hydroxylase
(CYP27B1), reducing conversion of 25-hydroxyvitamin D to active
1,25-dihydroxyvitamin D, further impairing phosphate absorption.
biological_processes:
- preferred_term: Phosphate Ion Homeostasis
term:
id: GO:0055062
label: phosphate ion homeostasis
modifier: ABNORMAL
- preferred_term: Vitamin D Metabolism
term:
id: GO:0042359
label: vitamin D metabolic process
modifier: DECREASED
chemical_entities:
- preferred_term: serum phosphate
term:
id: CHEBI:26020
label: phosphate
modifier: DECREASED
genes:
- preferred_term: PHEX
term:
id: hgnc:8918
label: PHEX
cell_types:
- preferred_term: Osteocyte
term:
id: CL:0000137
label: osteocyte
- preferred_term: Kidney Proximal Tubule Cell
term:
id: CL:0002306
label: epithelial cell of proximal tubule
downstream:
- target: Hypophosphatemia
description: Excess FGF23 drives renal phosphate wasting, producing the hallmark low serum phosphate phenotype.
causal_link_type: DIRECT
- target: Defective Bone Mineralization
description: Lifelong renal phosphate wasting leaves insufficient phosphate for normal skeletal mineralization.
causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
intermediate_mechanisms:
- chronic hypophosphatemia
evidence:
- reference: PMID:29947083
reference_title: "A Randomized, Double-Blind, Placebo-Controlled, Phase 3 Trial Evaluating the Efficacy of Burosumab, an Anti-FGF23 Antibody, in Adults With X-Linked Hypophosphatemia: Week 24 Primary Analysis."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "inherited loss-of-function mutations in the PHEX gene cause excess circulating levels of fibroblast growth factor 23 (FGF23), leading to lifelong renal phosphate wasting and hypophosphatemia"
explanation: "Directly describes the FGF23-mediated phosphate wasting mechanism in XLH."
- reference: PMID:7550339
reference_title: "A gene (PEX) with homologies to endopeptidases is mutated in patients with X-linked hypophosphatemic rickets. The HYP Consortium."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "impaired phosphate uptake in the kidney, which is likely to be caused by abnormal regulation of sodium phosphate cotransport in the proximal tubules"
explanation: "Early evidence linking PHEX to renal proximal tubule phosphate transport defect."
- reference: PMID:11062477
reference_title: "Autosomal dominant hypophosphataemic rickets is associated with mutations in FGF23."
supports: PARTIAL
evidence_source: HUMAN_CLINICAL
snippet: "We identified missense mutations in a gene encoding a new member of the fibroblast growth factor (FGF) family, FGF23"
explanation: "Discovery of FGF23 as the phosphaturic factor, providing the mechanistic link between PHEX and phosphate wasting."
- name: Defective Bone Mineralization
description: >
Chronic hypophosphatemia results in insufficient phosphate for
hydroxyapatite crystal formation. This causes rickets at the
growth plate in children (disorganized hypertrophic zone, widened
physis) and osteomalacia in adults (undermineralized osteoid).
The mineralization defect persists despite growth plate closure
in adults.
biological_processes:
- preferred_term: bone mineralization
term:
id: GO:0030282
label: bone mineralization
modifier: DECREASED
cell_types:
- preferred_term: Osteoblast
term:
id: CL:0000062
label: osteoblast
- preferred_term: Chondrocyte
term:
id: CL:0000138
label: chondrocyte
downstream:
- target: Rickets
description: Impaired growth-plate mineralization produces rachitic skeletal changes in children.
causal_link_type: DIRECT
- target: Osteomalacia
description: Impaired adult bone mineralization manifests as osteomalacia.
causal_link_type: DIRECT
- target: Short Stature
description: Persistent childhood rickets and growth-plate dysfunction contribute to impaired linear growth.
causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
intermediate_mechanisms:
- rickets and growth impairment
- target: Lower Limb Bowing
description: Rachitic mineralization defects cause lower-extremity bowing during weight bearing.
causal_link_type: DIRECT
- target: Pseudofractures and Fractures
description: Osteomalacia from impaired mineralization increases adult fracture and pseudofracture burden.
causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
intermediate_mechanisms:
- osteomalacia
- target: Dental Abscesses
description: Defective dentin mineralization predisposes to spontaneous dental abscesses.
causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
intermediate_mechanisms:
- defective dentin mineralization
- target: Enthesitis
description: Adult skeletal mineralization disease is associated with progressive enthesopathy through incompletely resolved joint and enthesis intermediates.
causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
- target: Craniosynostosis
description: Abnormal bone growth in pediatric XLH is associated with skull abnormalities including craniosynostosis.
causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
evidence:
- reference: PMID:29947083
reference_title: "A Randomized, Double-Blind, Placebo-Controlled, Phase 3 Trial Evaluating the Efficacy of Burosumab, an Anti-FGF23 Antibody, in Adults With X-Linked Hypophosphatemia: Week 24 Primary Analysis."
supports: PARTIAL
evidence_source: HUMAN_CLINICAL
snippet: "fractures, and pseudofractures due to osteomalacia"
explanation: "Pseudofractures and fractures reflect defective mineralization (osteomalacia) in adults with XLH."
phenotypes:
- name: Rickets
description: >
Rachitic changes at the growth plate, bowing of lower extremities
(genu varum), and metaphyseal irregularities. Typically presents
in the first two years of life when the child begins weight-bearing.
phenotype_term:
preferred_term: Rickets
term:
id: HP:0002748
label: Rickets
evidence:
- reference: PMID:31104833
reference_title: "Burosumab versus conventional therapy in children with X-linked hypophosphataemia: a randomised, active-controlled, open-label, phase 3 trial."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "X-linked hypophosphataemia in children is characterised by elevated serum concentrations of fibroblast growth factor 23 (FGF23), hypophosphataemia, rickets, lower extremity bowing, and growth impairment"
explanation: "Phase 3 trial confirms rickets and lower extremity bowing as cardinal pediatric features of XLH."
- name: Osteomalacia
description: >
Adult defective bone mineralization with undermineralized osteoid,
clinically associated with fractures and pseudofractures in XLH.
phenotype_term:
preferred_term: Osteomalacia
term:
id: HP:0002749
label: Osteomalacia
evidence:
- reference: PMID:29947083
reference_title: "A Randomized, Double-Blind, Placebo-Controlled, Phase 3 Trial Evaluating the Efficacy of Burosumab, an Anti-FGF23 Antibody, in Adults With X-Linked Hypophosphatemia: Week 24 Primary Analysis."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "fractures, and pseudofractures due to osteomalacia"
explanation: "Adult XLH trial background directly links fractures and pseudofractures to osteomalacia."
- name: Short Stature
description: >
Disproportionate short stature with relatively short lower limbs.
Adult height is typically -2 to -3 SD below mean.
phenotype_term:
preferred_term: Short stature
term:
id: HP:0004322
label: Short stature
evidence:
- reference: PMID:29947083
reference_title: "A Randomized, Double-Blind, Placebo-Controlled, Phase 3 Trial Evaluating the Efficacy of Burosumab, an Anti-FGF23 Antibody, in Adults With X-Linked Hypophosphatemia: Week 24 Primary Analysis."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "Adults with XLH present with chronic musculoskeletal pain and stiffness, short stature, lower limb deformities"
explanation: "Short stature confirmed as characteristic adult XLH feature."
- name: Lower Limb Bowing
description: >
Progressive bowing of the legs (genu varum), worse with ambulation.
May require surgical correction (osteotomy).
phenotype_term:
preferred_term: Genu varum
term:
id: HP:0002970
label: Genu varum
evidence:
- reference: PMID:29947083
reference_title: "A Randomized, Double-Blind, Placebo-Controlled, Phase 3 Trial Evaluating the Efficacy of Burosumab, an Anti-FGF23 Antibody, in Adults With X-Linked Hypophosphatemia: Week 24 Primary Analysis."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "short stature, lower limb deformities, fractures, and pseudofractures due to osteomalacia"
explanation: "Lower limb deformities confirmed as characteristic feature in adults with XLH."
- reference: PMID:29460029
reference_title: "Outcome of adult patients with X-linked hypophosphatemia caused by PHEX gene mutations."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "Orthopaedic involvement requiring surgical intervention (osteotomy) was frequent"
explanation: "Confirms that lower limb bowing frequently requires surgical correction in adult XLH patients."
- name: Pseudofractures and Fractures
description: >
Adult XLH includes fractures and pseudofractures as consequences of
osteomalacia, with fracture healing improved by burosumab in clinical trial
follow-up.
phenotype_term:
preferred_term: recurrent fractures
term:
id: HP:0002757
label: Recurrent fractures
evidence:
- reference: PMID:29947083
reference_title: "A Randomized, Double-Blind, Placebo-Controlled, Phase 3 Trial Evaluating the Efficacy of Burosumab, an Anti-FGF23 Antibody, in Adults With X-Linked Hypophosphatemia: Week 24 Primary Analysis."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "fractures, and pseudofractures due to osteomalacia"
explanation: "Adult XLH trial background explicitly lists fractures and pseudofractures due to osteomalacia."
- name: Dental Abscesses
description: >
Spontaneous dental abscesses and periapical lucencies from
defective dentin mineralization, without caries. May be the
presenting feature.
phenotype_term:
preferred_term: Tooth abscess
term:
id: HP:0030757
label: Tooth abscess
evidence:
- reference: PMID:29947083
reference_title: "A Randomized, Double-Blind, Placebo-Controlled, Phase 3 Trial Evaluating the Efficacy of Burosumab, an Anti-FGF23 Antibody, in Adults With X-Linked Hypophosphatemia: Week 24 Primary Analysis."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "accelerated osteoarthritis, dental abscesses, and enthesopathy"
explanation: "Dental abscesses confirmed as characteristic adult XLH feature."
- reference: PMID:29460029
reference_title: "Outcome of adult patients with X-linked hypophosphatemia caused by PHEX gene mutations."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "Dental disease (63%), nephrocalcinosis (42%), and hearing impairment (14%) were also common"
explanation: "63% prevalence of dental disease in adult XLH cohort."
- name: Enthesitis
description: >
Calcification and ossification of tendons, ligaments, and joint
capsules in adults. Progressive and often disabling, affecting
spine and major joints.
phenotype_term:
preferred_term: Enthesitis
term:
id: HP:0100686
label: Enthesitis
evidence:
- reference: PMID:29947083
reference_title: "A Randomized, Double-Blind, Placebo-Controlled, Phase 3 Trial Evaluating the Efficacy of Burosumab, an Anti-FGF23 Antibody, in Adults With X-Linked Hypophosphatemia: Week 24 Primary Analysis."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "dental abscesses, and enthesopathy"
explanation: "Enthesopathy confirmed as characteristic adult XLH feature."
- reference: PMID:29460029
reference_title: "Outcome of adult patients with X-linked hypophosphatemia caused by PHEX gene mutations."
supports: PARTIAL
evidence_source: HUMAN_CLINICAL
snippet: "Joint replacement and decompressive laminectomy were observed in those older than 40 years"
explanation: "Progressive enthesopathy leads to need for joint replacement and spinal surgery in older adults."
- name: Hypophosphatemia
description: >
Chronic low serum phosphate due to renal phosphate wasting.
The biochemical hallmark of the disease.
phenotype_term:
preferred_term: Hypophosphatemia
term:
id: HP:0002148
label: Hypophosphatemia
evidence:
- reference: PMID:29460029
reference_title: "Outcome of adult patients with X-linked hypophosphatemia caused by PHEX gene mutations."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "X-linked hypophosphatemia (XLH) is the most common monogenic disorder causing hypophosphatemia"
explanation: "XLH confirmed as the most common hereditary cause of hypophosphatemia."
- reference: PMID:29947083
reference_title: "A Randomized, Double-Blind, Placebo-Controlled, Phase 3 Trial Evaluating the Efficacy of Burosumab, an Anti-FGF23 Antibody, in Adults With X-Linked Hypophosphatemia: Week 24 Primary Analysis."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "leading to lifelong renal phosphate wasting and hypophosphatemia"
explanation: "Lifelong hypophosphatemia confirmed as the biochemical hallmark."
- name: Nephrocalcinosis
description: >
Renal parenchymal calcium deposition is common in adult XLH cohorts and is
clinically relevant as a complication in patients treated with conventional
phosphate and active vitamin D therapy.
phenotype_term:
preferred_term: Nephrocalcinosis
term:
id: HP:0000121
label: Nephrocalcinosis
evidence:
- reference: PMID:29460029
reference_title: "Outcome of adult patients with X-linked hypophosphatemia caused by PHEX gene mutations."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "Dental disease (63%), nephrocalcinosis (42%), and hearing impairment (14%) were also common"
explanation: "Adult XLH cohort reported nephrocalcinosis in 42% of patients."
- name: Hearing Impairment
description: >
Hearing impairment is reported in adult XLH cohorts. The direct mechanism is
not modeled here because the cached cohort abstract supports the association
but does not specify the inner-ear pathophysiology.
phenotype_term:
preferred_term: Hearing impairment
term:
id: HP:0000365
label: Hearing impairment
evidence:
- reference: PMID:29460029
reference_title: "Outcome of adult patients with X-linked hypophosphatemia caused by PHEX gene mutations."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "Dental disease (63%), nephrocalcinosis (42%), and hearing impairment (14%) were also common"
explanation: "Adult XLH cohort reported hearing impairment in 14% of patients."
- name: Craniosynostosis
description: >
Premature cranial suture fusion occurs in a subset of pediatric patients,
with systematic review evidence indicating higher prevalence in XLH than
in the general pediatric population.
phenotype_term:
preferred_term: Craniosynostosis
term:
id: HP:0001363
label: Craniosynostosis
evidence:
- reference: PMID:40220947
reference_title: "Craniosynostosis among children with X-linked hypophosphatemia: A systematic review and meta-analysis."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "The pooled prevalence of craniosynostosis among children with XLH was 22 % (95 % confidence interval (CI) 9.0 % to 44 %)"
explanation: "Systematic review and meta-analysis directly supports craniosynostosis as a pediatric XLH manifestation."
genetic:
- name: PHEX Mutations
gene_term:
preferred_term: PHEX
term:
id: hgnc:8918
label: PHEX
association: Causative
relationship_type: CAUSATIVE
variant_origin: GERMLINE
notes: >
Loss-of-function mutations in PHEX (phosphate-regulating endopeptidase
homolog, X-linked) on Xp22.1. Over 300 mutations described including
missense, nonsense, splice-site, and large deletions. The precise
mechanism by which PHEX deficiency elevates FGF23 remains incompletely
understood.
evidence:
- reference: PMID:7550339
reference_title: "A gene (PEX) with homologies to endopeptidases is mutated in patients with X-linked hypophosphatemic rickets. The HYP Consortium."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "By positional cloning, we have isolated a candidate gene from the HYP region in Xp22.1. This gene exhibits homology to a family of endopeptidase genes"
explanation: "Original cloning of the PHEX gene from Xp22.1 with endopeptidase homology."
- reference: PMID:7550339
reference_title: "A gene (PEX) with homologies to endopeptidases is mutated in patients with X-linked hypophosphatemic rickets. The HYP Consortium."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "Intragenic non-overlapping deletions from four different families and three mutations (two splice sites and one frameshift) have been detected in HYP patients"
explanation: "Early evidence of diverse mutation types in PHEX causing XLH."
- reference: PMID:29460029
reference_title: "Outcome of adult patients with X-linked hypophosphatemia caused by PHEX gene mutations."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "XLH is associated with a large number of private mutations; 37 different mutations in the PHEX gene were identified in this cohort"
explanation: "37 distinct PHEX mutations in a single cohort confirms extensive allelic heterogeneity."
- reference: PMID:11062477
reference_title: "Autosomal dominant hypophosphataemic rickets is associated with mutations in FGF23."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "Inactivating mutations of the gene PHEX, encoding a member of the neutral endopeptidase family of proteins, are responsible for XLH"
explanation: "Confirms inactivating PHEX mutations as the cause of XLH."
- reference: CGGV:assertion_7b966545-e9dc-4a75-8d6a-91b328bc4854-2025-05-15T060000.000Z
reference_title: "PHEX / X-linked dominant hypophosphatemic rickets (Definitive)"
supports: SUPPORT
evidence_source: OTHER
snippet: "PHEX | HGNC:8918 | X-linked dominant hypophosphatemic rickets | MONDO:0010619 | XL | Definitive"
explanation: ClinGen classifies the PHEX-X-linked dominant hypophosphatemic rickets gene-disease relationship as definitive with X-linked inheritance.
treatments:
- name: Burosumab (Crysvita)
description: >
Anti-FGF23 monoclonal antibody. Directly targets the pathogenic
mechanism by neutralizing excess FGF23, normalizing phosphate
homeostasis. FDA approved 2018 for pediatric and adult XLH.
Administered subcutaneously every 2-4 weeks.
treatment_term:
preferred_term: Pharmacotherapy
term:
id: NCIT:C15986
label: Pharmacotherapy
therapeutic_agent:
- preferred_term: burosumab
term:
id: NCIT:C119744
label: Burosumab
target_mechanisms:
- target: Renal Phosphate Wasting via FGF23 Excess
treatment_effect: INHIBITS
description: Burosumab binds and inhibits pathogenic FGF23 excess, reducing renal phosphate wasting and correcting hypophosphatemia.
evidence:
- reference: PMID:29947083
reference_title: "A Randomized, Double-Blind, Placebo-Controlled, Phase 3 Trial Evaluating the Efficacy of Burosumab, an Anti-FGF23 Antibody, in Adults With X-Linked Hypophosphatemia: Week 24 Primary Analysis."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "Burosumab, a fully human monoclonal antibody, binds and inhibits FGF23 to correct hypophosphatemia"
explanation: "Directly supports burosumab inhibition of the FGF23-driven phosphate-wasting mechanism."
evidence:
- reference: PMID:29947083
reference_title: "A Randomized, Double-Blind, Placebo-Controlled, Phase 3 Trial Evaluating the Efficacy of Burosumab, an Anti-FGF23 Antibody, in Adults With X-Linked Hypophosphatemia: Week 24 Primary Analysis."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "Burosumab, a fully human monoclonal antibody, binds and inhibits FGF23 to correct hypophosphatemia"
explanation: "Describes burosumab mechanism as anti-FGF23 antibody."
- reference: PMID:29947083
reference_title: "A Randomized, Double-Blind, Placebo-Controlled, Phase 3 Trial Evaluating the Efficacy of Burosumab, an Anti-FGF23 Antibody, in Adults With X-Linked Hypophosphatemia: Week 24 Primary Analysis."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "94.1% of burosumab-treated participants attained mean serum phosphate concentration above the lower limit of normal compared with 7.6% of those receiving placebo"
explanation: "Phase 3 adult trial demonstrates robust phosphate normalization with burosumab."
- reference: PMID:29947083
reference_title: "A Randomized, Double-Blind, Placebo-Controlled, Phase 3 Trial Evaluating the Efficacy of Burosumab, an Anti-FGF23 Antibody, in Adults With X-Linked Hypophosphatemia: Week 24 Primary Analysis."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "43.1% (burosumab) and 7.7% (placebo) of baseline active fractures were fully healed; the odds of healed fracture in the burosumab group was 16.8-fold greater than that in the placebo group"
explanation: "Burosumab dramatically improves fracture healing in adults with XLH."
- reference: PMID:31104833
reference_title: "Burosumab versus conventional therapy in children with X-linked hypophosphataemia: a randomised, active-controlled, open-label, phase 3 trial."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "Significantly greater clinical improvements were shown in rickets severity, growth, and biochemistries among children with X-linked hypophosphataemia treated with burosumab compared with those continuing conventional therapy"
explanation: "Phase 3 pediatric trial confirms burosumab superiority over conventional therapy for rickets, growth, and biochemistry."
- name: Conventional Therapy (Phosphate/Calcitriol)
description: >
Oral phosphate supplementation combined with active vitamin D
(calcitriol or alfacalcidol). Previously standard of care but
limited by GI side effects, secondary hyperparathyroidism, and
nephrocalcinosis risk.
treatment_term:
preferred_term: Pharmacotherapy
term:
id: NCIT:C15986
label: Pharmacotherapy
therapeutic_agent:
- preferred_term: phosphate salts
term:
id: CHEBI:26020
label: phosphate
- preferred_term: calcitriol
term:
id: CHEBI:17823
label: calcitriol
target_mechanisms:
- target: Defective Bone Mineralization
treatment_effect: MODULATES
description: Oral phosphate and active vitamin D partially modulate the downstream mineralization defect without fully correcting FGF23-driven hypophosphatemia.
evidence:
- reference: PMID:37321578
reference_title: "X-linked hypophosphatemic rickets: from diagnosis to management."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "oral phosphate salts and activated vitamin D cannot completely control chronic hypophosphatemia, leaving patients with incomplete healing and residual skeletal deformity"
explanation: "Review supports conventional therapy as partial downstream management that does not fully correct chronic hypophosphatemia."
evidence:
- reference: PMID:31104833
reference_title: "Burosumab versus conventional therapy in children with X-linked hypophosphataemia: a randomised, active-controlled, open-label, phase 3 trial."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "conventional therapy, consisting of oral phosphate and active vitamin D"
explanation: "Trial explicitly defines conventional pediatric XLH therapy as oral phosphate plus active vitamin D."
- reference: PMID:29460029
reference_title: "Outcome of adult patients with X-linked hypophosphatemia caused by PHEX gene mutations."
supports: PARTIAL
evidence_source: HUMAN_CLINICAL
snippet: "Dental disease (63%), nephrocalcinosis (42%), and hearing impairment (14%) were also common"
explanation: "42% nephrocalcinosis rate reflects a known complication of conventional phosphate/calcitriol therapy."
- name: Orthopedic Management
description: >
Corrective osteotomies for significant lower limb deformity.
Timing typically after optimization of medical therapy and
near completion of growth.
treatment_term:
preferred_term: surgical procedure
term:
id: MAXO:0000004
label: surgical procedure
located_in:
preferred_term: leg
term:
id: UBERON:0000978
label: leg
evidence:
- reference: PMID:29460029
reference_title: "Outcome of adult patients with X-linked hypophosphatemia caused by PHEX gene mutations."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "Orthopaedic involvement requiring surgical intervention (osteotomy) was frequent. Joint replacement and decompressive laminectomy were observed in those older than 40 years"
explanation: "Confirms frequent need for osteotomy and joint replacement in adult XLH patients."
datasets: []
Disease Pathophysiology Research Report
Target Disease - Disease Name: X-Linked Hypophosphatemia (XLH) - MONDO ID: MONDO_0010619 (X-linked dominant hypophosphatemic rickets) - Category: Mendelian
Overview and current understanding XLH is an X-linked dominant, phosphate-wasting osteomalacic/rickets disorder driven by excess osteocyte-derived FGF23 consequent to loss-of-function variants in PHEX. Elevated FGF23 reduces renal phosphate reabsorption and suppresses 1,25-dihydroxyvitamin D [1,25(OH)2D] synthesis, impairing skeletal and dental mineralization from infancy through adulthood. “Loss-of-function mutations in the PHEX gene … result in upregulated FGF23 serum levels and consequent hypophosphatemia” (Orphanet J Rare Dis, 2025; URL: https://doi.org/10.1186/s13023-025-03952-5; published Oct 2025) (brandi2025xlinkedhypophosphatemiaand pages 2-4). Conventional therapy (oral phosphate plus active vitamin D) improves some features but does not correct high FGF23, whereas the FGF23-neutralizing monoclonal antibody burosumab restores phosphate balance and increases endogenous 1,25(OH)2D (Front Endocrinol, 2024; URL: https://doi.org/10.3389/fendo.2024.1414509; published Aug 2024; BDJ Open, 2024; URL: https://doi.org/10.1038/s41405-024-00223-6; published May 2024) (wang2024metaanalysisandsystematic pages 1-2, arhar2024characteristicsoforal pages 8-9).
1) Core Pathophysiology - Primary mechanisms - PHEX loss-of-function in osteocytes/odontoblasts leads to increased circulating FGF23 and accumulation of matrix mineralization inhibitors (e.g., SIBLING-derived ASARM peptides, osteopontin), producing systemic hypophosphatemia and local hypomineralization in bone and teeth (BDJ Open, 2024; URL: https://doi.org/10.1038/s41405-024-00223-6; EJPD, 2025; URL: https://doi.org/10.23804/ejpd.2025.2348) (arhar2024characteristicsoforal pages 8-9, defabianis2025xlinkedhypophosphatemiain pages 1-2). - Excess FGF23 acts on FGFR1c–Klotho in renal proximal tubule and parathyroid to: decrease phosphate reabsorption (downregulating NaPi-IIa/SLC34A1 and NaPi-IIc/SLC34A3), suppress CYP27B1 (1α-hydroxylase), and induce CYP24A1 (24-hydroxylase), lowering 1,25(OH)2D and intestinal phosphate absorption (Front Endocrinol, 2024; Orphanet J Rare Dis, 2025; URLs above) (wang2024metaanalysisandsystematic pages 1-2, brandi2025xlinkedhypophosphatemiaand pages 2-4, wang2024metaanalysisandsystematic pages 18-18). - Dysregulated pathways - Osteocyte endocrine axis: PHEX–FGF23–Klotho/FGFR1c signaling (kidney, parathyroid) (Orphanet J Rare Dis, 2025; Front Endocrinol, 2024) (brandi2025xlinkedhypophosphatemiaand pages 2-4, wang2024metaanalysisandsystematic pages 1-2, wang2024metaanalysisandsystematic pages 18-18). - Mineralization regulators: MEPE/ASARM, DMP1, ENPP1-mediated pyrophosphate (PPi) balance, and osteopontin accumulation act locally to inhibit hydroxyapatite crystal growth (BDJ Open, 2024) (arhar2024characteristicsoforal pages 8-9, arhar2024characteristicsoforal pages 10-11). - Affected cellular processes - Renal tubular phosphate transport (reduced TmP/GFR), vitamin D activation/catabolism (CYP27B1↓, CYP24A1↑), and matrix mineralization (inhibited nucleation/fusion of calcospherites) (Front Endocrinol, 2024; BDJ Open, 2024) (wang2024metaanalysisandsystematic pages 1-2, arhar2024characteristicsoforal pages 8-9).
2) Key Molecular Players - Genes/Proteins (HGNC) - PHEX (HGNC:8860): Causal gene; loss-of-function increases FGF23 and allows accumulation of ASARM peptides/osteopontin that inhibit mineralization (Orphanet J Rare Dis, 2025; BDJ Open, 2024) (brandi2025xlinkedhypophosphatemiaand pages 2-4, arhar2024characteristicsoforal pages 8-9, arhar2024characteristicsoforal pages 10-11). - FGF23 (HGNC:3689): Endocrine phosphatonin from osteocytes; reduces renal phosphate reabsorption and 1,25(OH)2D synthesis (Front Endocrinol, 2024; Orphanet J Rare Dis, 2025) (wang2024metaanalysisandsystematic pages 1-2, brandi2025xlinkedhypophosphatemiaand pages 2-4, wang2024metaanalysisandsystematic pages 18-18). - FGFR1 (HGNC:3688) and KLOTHO (HGNC:6353): Co-receptor complex confers FGF23 target specificity in kidney and parathyroid (Front Endocrinol, 2024) (wang2024metaanalysisandsystematic pages 18-18). - SLC34A1/NaPi-IIa (HGNC:11039), SLC34A3/NaPi-IIc (HGNC:11041): Proximal-tubule phosphate cotransporters downregulated by FGF23 (Front Endocrinol, 2024) (wang2024metaanalysisandsystematic pages 18-18). - CYP27B1 (HGNC:2593), CYP24A1 (HGNC:2594): Vitamin D metabolic enzymes suppressed/induced by FGF23 respectively (Front Endocrinol, 2024) (wang2024metaanalysisandsystematic pages 18-18). - MEPE (HGNC:13323), DMP1 (HGNC:2936), ENPP1 (HGNC:3352), SPP1/Osteopontin (HGNC:11255): Matrix regulators linked to impaired mineralization in XLH (BDJ Open, 2024) (arhar2024characteristicsoforal pages 8-9, arhar2024characteristicsoforal pages 10-11). - Chemical entities (ChEBI) - Phosphate (ChEBI:18367), calcitriol/1,25(OH)2D3 (ChEBI:17933/28940), and burosumab (ChEBI:132958) are central to mechanism and therapy (Front Endocrinol, 2024; BDJ Open, 2024) (wang2024metaanalysisandsystematic pages 1-2, arhar2024characteristicsoforal pages 8-9). - Cell types (CL) - Osteocytes and osteoblasts (bone); renal proximal tubule epithelial cells; parathyroid chief cells; odontoblasts/ameloblasts/cementoblasts (dental) are primary cell actors (Orphanet J Rare Dis, 2025; BDJ Open, 2024) (brandi2025xlinkedhypophosphatemiaand pages 2-4, arhar2024characteristicsoforal pages 8-9, arhar2024characteristicsoforal pages 10-11). - Anatomical locations (UBERON) - Bone, kidney (proximal tubule), parathyroid, growth plate, tooth (dentin, cementum), and entheses are key sites (Orphanet J Rare Dis, 2025; BDJ Open, 2024) (brandi2025xlinkedhypophosphatemiaand pages 2-4, arhar2024characteristicsoforal pages 8-9).
3) Biological Processes (for GO annotation) - Hormone-mediated signaling: FGF23–FGFR1c–Klotho signaling (GO:0005179 ligand activity; GO:0007173 transmembrane receptor protein tyrosine kinase signaling) leading to ERK/MAPK activation in renal proximal tubule/parathyroid (Front Endocrinol, 2024) (wang2024metaanalysisandsystematic pages 18-18). - Phosphate homeostasis and transport: negative regulation of phosphate ion transmembrane transport (GO:0035435) by downregulation of SLC34A1/SLC34A3; decreased renal tubular phosphate reabsorption (reflected by low TmP/GFR) (Front Endocrinol, 2024; Orphanet J Rare Dis, 2025) (wang2024metaanalysisandsystematic pages 18-18, brandi2025xlinkedhypophosphatemiaand pages 2-4). - Vitamin D metabolic process: negative regulation of 1α-hydroxylase (CYP27B1) and positive regulation of 24-hydroxylase (CYP24A1) (GO:0038180 regulation of vitamin D biosynthetic process) (Front Endocrinol, 2024) (wang2024metaanalysisandsystematic pages 18-18). - Biomineralization: extracellular matrix organization and mineralization (GO:0030198; GO:0030282), inhibited by MEPE/ASARM, osteopontin, and altered PPi balance (BDJ Open, 2024) (arhar2024characteristicsoforal pages 8-9, arhar2024characteristicsoforal pages 10-11).
4) Cellular Components (GO) - Sites of action: osteocyte lacunar–canalicular system and bone matrix (extracellular region; GO:0005576), renal proximal tubular apical membrane (plasma membrane; GO:0005886) hosting NaPi-IIa/IIc, parathyroid gland tissues, and dental hard tissues (dentin/cementum) where PHEX and mineralization inhibitors operate (BDJ Open, 2024; Orphanet J Rare Dis, 2025) (arhar2024characteristicsoforal pages 8-9, brandi2025xlinkedhypophosphatemiaand pages 2-4).
5) Disease Progression - Sequence of events 1) Genetic trigger: PHEX loss-of-function in osteocytes/odontoblasts → overproduction of FGF23 and accumulation of ASARM/osteopontin (BDJ Open, 2024) (arhar2024characteristicsoforal pages 8-9, arhar2024characteristicsoforal pages 10-11). 2) Endocrine effects: FGF23→FGFR1c–Klotho signaling in kidney/parathyroid → phosphaturia (NaPi-IIa/IIc↓), low 1,25(OH)2D (CYP27B1↓, CYP24A1↑), disordered PTH feedback (Front Endocrinol, 2024; Orphanet J Rare Dis, 2025) (wang2024metaanalysisandsystematic pages 18-18, brandi2025xlinkedhypophosphatemiaand pages 2-4). 3) Tissue consequences: impaired mineralization in growth plates (rickets) and bone (osteomalacia); dental dentin/enamel/cementum hypomineralization → abscesses; adult enthesopathy, pseudofractures, osteoarthritis (BDJ Open, 2024; Orphanet J Rare Dis, 2025) (arhar2024characteristicsoforal pages 8-9, brandi2025xlinkedhypophosphatemiaand pages 2-4, arcidiacono2025potentialpredictorsof pages 16-20). - Phases - Childhood: limb deformities, rickets, short stature, delayed motor development; “typical childhood manifestations include limb growth retardation, abnormal walking patterns, bone pain, and rickets” (Orphanet J Rare Dis, 2025) (brandi2025xlinkedhypophosphatemiaand pages 2-4). - Adulthood: persistent osteomalacia, musculoskeletal pain, pseudofractures, enthesopathies, early osteoarthritis; complications from prior therapy (nephrocalcinosis, hyperparathyroidism) (Unknown journal, 2025) (arcidiacono2025potentialpredictorsof pages 16-20).
6) Phenotypic Manifestations (with HP terms) - Biochemical hallmark: Hypophosphatemia (HP:0002148) with decreased TmP/GFR due to FGF23 excess (Orphanet J Rare Dis, 2025; Front Endocrinol, 2024) (brandi2025xlinkedhypophosphatemiaand pages 2-4, wang2024metaanalysisandsystematic pages 1-2, wang2024metaanalysisandsystematic pages 18-18). - Skeletal: Rickets (HP:0002748) in children and Osteomalacia (HP:0002649) in adults; bowing of long bones (HP:0002970), short stature (HP:0004322), pseudofractures (HP:0002757), enthesopathy (HP:0002657) (Orphanet J Rare Dis, 2025; Unknown journal, 2025) (brandi2025xlinkedhypophosphatemiaand pages 2-4, arcidiacono2025potentialpredictorsof pages 16-20). - Dental/craniofacial: spontaneous periapical dental abscesses (HP:0001088), hypomineralized dentin, enamel cracks, aberrant cementum; craniosynostosis reported in some cohorts (BDJ Open, 2024; EJPD, 2025) (arhar2024characteristicsoforal pages 8-9, defabianis2025xlinkedhypophosphatemiain pages 1-2, arhar2024characteristicsoforal pages 10-11). - Renal/endocrine complications (often therapy-related): nephrocalcinosis (HP:0000121), secondary/tertiary hyperparathyroidism (HP:0000829) (Front Endocrinol, 2024; BDJ Open, 2024) (wang2024metaanalysisandsystematic pages 1-2, arhar2024characteristicsoforal pages 8-9).
Recent developments and latest research (prioritized 2023–2024) - Pathophysiology-focused updates - Contemporary reviews reinforce the centrality of FGF23 as the driver of renal phosphate wasting and reduced 1,25(OH)2D in XLH and emphasize standardized biochemical assessment including intact FGF23 and TmP/GFR (Orphanet J Rare Dis, 2025; Front Endocrinol, 2025 review of biochemical evaluation; URLs: https://doi.org/10.1186/s13023-025-03952-5; https://doi.org/10.3389/fendo.2025.1702656) (brandi2025xlinkedhypophosphatemiaand pages 2-4, brandi2025xlinkedhypophosphatemiaand pages 14-14). - Dental mechanistic literature in 2024 highlights that PHEX is expressed in teeth and that mineralization defects reflect “inadequate mineralisation, uneven dentin tubules, and cracks and chipping in the enamel,” with FGF23 mRNA detected in ameloblasts and odontoblasts (BDJ Open, 2024; URL: https://doi.org/10.1038/s41405-024-00223-6; published May 2024) (arhar2024characteristicsoforal pages 8-9). - Therapeutics and outcomes (2024) - Meta-analyses/systematic reviews in 2024 show burosumab improves serum phosphorus, TmP/GFR, 1,25(OH)2D, alkaline phosphatase, rickets severity scores, and functional capacity (6-minute walk test) in children; “burosumab’s superiority in managing XLH in pediatric populations” is supported, though long-term growth and quality-of-life effects require further study (Front Endocrinol, 2024; URL: https://doi.org/10.3389/fendo.2024.1414509; published Aug 2024) (wang2024metaanalysisandsystematic pages 1-2, wang2024metaanalysisandsystematic pages 18-18).
Current applications and real-world implementations - Diagnostic practice: Routine panels include serum phosphate, calcium, ALP, PTH, 25(OH)D, 1,25(OH)2D, creatinine, intact FGF23, and estimation of TmP/GFR to document renal phosphate wasting; genetic testing confirms PHEX variants (Orphanet J Rare Dis, 2025; URL above; EJPD, 2025; URL: https://doi.org/10.23804/ejpd.2025.2348) (brandi2025xlinkedhypophosphatemiaand pages 2-4, defabianis2025xlinkedhypophosphatemiain pages 1-2). - Conventional therapy: divided-dose oral phosphate plus active vitamin D analogs remains in use but may raise FGF23 and carries risks (hypercalciuria, nephrocalcinosis, hyperparathyroidism) (BDJ Open, 2024; Front Endocrinol, 2024) (arhar2024characteristicsoforal pages 8-9, wang2024metaanalysisandsystematic pages 1-2). - Targeted therapy: Burosumab (anti-FGF23 mAb) is approved; it “neutralises elevated FGF23 levels, resulting in better phosphate reabsorption in the renal tubules … increased serum phosphate levels and the synthesis of endogenous 1,25(OH)2 vitamin D” (BDJ Open, 2024; URL above) (arhar2024characteristicsoforal pages 8-9). Randomized and cohort evidence summarized in 2024 meta-analyses demonstrates consistent biochemical and radiographic improvements in pediatric XLH (Front Endocrinol, 2024) (wang2024metaanalysisandsystematic pages 1-2, wang2024metaanalysisandsystematic pages 18-18).
Expert opinions and analysis - Narrative review and expert opinion (Orphanet J Rare Dis, 2025) emphasizes that anti-FGF23 therapy “has revolutionized the traditional approach” to XLH/TIO management and urges standardized biochemical evaluation including intact FGF23 and TmP/GFR to guide care (URL above) (brandi2025xlinkedhypophosphatemiaand pages 2-4, brandi2025xlinkedhypophosphatemiaand pages 14-14). - Dental experts underscore that XLH dental disease is frequent and persists despite conventional therapy, reinforcing early, multidisciplinary management and consideration of burosumab’s potential oral-health benefits reported in emerging studies (BDJ Open, 2024) (arhar2024characteristicsoforal pages 8-9, arhar2024characteristicsoforal pages 10-11).
Relevant statistics and data (recent where available) - Epidemiology: XLH affects up to ~1 in 20,000 individuals and accounts for ~80% of hypophosphatemic rickets (Orphanet J Rare Dis, 2025; URL above) (brandi2025xlinkedhypophosphatemiaand pages 2-4). - Dental involvement: Reports range widely (e.g., “dental involvement in 23–67% of patients; 40–50% of children and 60–85% of adults affected”), with histology showing “reduced dentin mineralisation … cracks and chipping in the enamel” (EJPD, 2025; BDJ Open, 2024; URLs above) (defabianis2025xlinkedhypophosphatemiain pages 1-2, arhar2024characteristicsoforal pages 8-9). - Therapeutic outcomes: Pediatric burosumab meta-analysis indicates superiority over conventional therapy for improving serum phosphorus, TmP/GFR, ALP, rickets severity scores, and 6MWT; long-term height/QOL data remain limited (Front Endocrinol, 2024; URL above) (wang2024metaanalysisandsystematic pages 1-2, wang2024metaanalysisandsystematic pages 18-18).
Direct quotes (for key statements) - “Loss-of-function mutations in the PHEX gene … result in upregulated FGF23 serum levels and consequent hypophosphatemia.” (Orphanet J Rare Dis, 2025) (brandi2025xlinkedhypophosphatemiaand pages 2-4). - “Burosumab neutralises elevated FGF23 levels, resulting in better phosphate reabsorption in the renal tubules … increased serum phosphate levels and the synthesis of endogenous 1,25(OH)2 vitamin D.” (BDJ Open, 2024) (arhar2024characteristicsoforal pages 8-9). - Dental histology: “Inadequate mineralisation, uneven dentin tubules, and cracks and chipping in the enamel were observed, indicating mineralisation deviations.” (BDJ Open, 2024) (arhar2024characteristicsoforal pages 8-9).
Ontology-annotated knowledge elements - Embedded artifact with HGNC (genes/proteins), GO (processes), CL (cell types), UBERON (anatomy), HPO (phenotypes), and ChEBI (chemicals): | Category | Entity (preferred name) | Identifier (prefix:ID) | Role in XLH pathophysiology (1–2 lines) | Supporting sources | |---|---|---|---|---| | Gene/Protein | PHEX | HGNC:8860 | Loss-of-function mutations in PHEX (osteocytes/odontoblasts) lead to elevated FGF23 and accumulation of mineralization inhibitors (e.g., ASARM peptides), causing hypomineralization. | (arhar2024characteristicsoforal pages 8-9, arhar2024characteristicsoforal pages 10-11, brandi2025xlinkedhypophosphatemiaand pages 2-4) | | Gene/Protein | FGF23 | HGNC:3689 | Osteocyte-derived hormone that reduces renal phosphate reabsorption and suppresses 1,25(OH)2D synthesis; central driver of XLH hypophosphatemia. | (wang2024metaanalysisandsystematic pages 18-18, wang2024metaanalysisandsystematic pages 1-2, brandi2025xlinkedhypophosphatemiaand pages 2-4) | | Gene/Protein | FGFR1 | HGNC:3688 | FGFR1c (with Klotho) mediates FGF23 signalling in kidney/parathyroid, altering phosphate and vitamin D handling. | (wang2024metaanalysisandsystematic pages 18-18, brandi2025xlinkedhypophosphatemiaand pages 2-4) | | Gene/Protein | KLOTHO | HGNC:6353 | Co-receptor for FGF23 that confers tissue specificity (renal proximal tubule and parathyroid) for FGF23 effects. | (wang2024metaanalysisandsystematic pages 18-18, brandi2025xlinkedhypophosphatemiaand pages 2-4) | | Gene/Protein | SLC34A1 (NaPi-IIa) | HGNC:11039 | Renal proximal-tubule sodium-phosphate cotransporter downregulated by FGF23 → decreased phosphate reabsorption (low TmP/GFR). | (wang2024metaanalysisandsystematic pages 18-18, brandi2025xlinkedhypophosphatemiaand pages 2-4) | | Gene/Protein | SLC34A3 (NaPi-IIc) | HGNC:11041 | Alternate proximal-tubule phosphate transporter contributing to renal phosphate handling; mutations cause other hypophosphatemias and are relevant to renal phenotype. | (brandi2025xlinkedhypophosphatemiaand pages 2-4) | | Gene/Protein | CYP27B1 | HGNC:2593 | Encodes 1α-hydroxylase; FGF23 suppresses CYP27B1, lowering 1,25(OH)2D and reducing intestinal phosphate/calcium absorption. | (wang2024metaanalysisandsystematic pages 18-18, brandi2025xlinkedhypophosphatemiaand pages 2-4) | | Gene/Protein | CYP24A1 | HGNC:2594 | Encodes 24‑hydroxylase; FGF23 upregulates CYP24A1, increasing catabolism of 1,25(OH)2D and contributing to low calcitriol. | (wang2024metaanalysisandsystematic pages 18-18, brandi2025xlinkedhypophosphatemiaand pages 2-4) | | Gene/Protein | MEPE | HGNC:13323 | Matrix protein whose ASARM-derived peptides (when not degraded by PHEX) inhibit hydroxyapatite formation and impair mineralization. | (arhar2024characteristicsoforal pages 8-9, arhar2024characteristicsoforal pages 10-11) | | Gene/Protein | DMP1 | HGNC:2936 | Osteocyte matrix protein; loss-of-function forms cause FGF23 dysregulation in hereditary hypophosphatemias and modulate mineralization. | (arhar2024characteristicsoforal pages 8-9, arhar2024characteristicsoforal pages 10-11) | | Gene/Protein | ENPP1 | HGNC:3352 | Regulates extracellular pyrophosphate (PPi); dysregulation affects mineralization balance and can interact with FGF23-related pathways. | (arhar2024characteristicsoforal pages 8-9, arhar2024characteristicsoforal pages 10-11) | | Gene/Protein | SPP1 (Osteopontin) | HGNC:11255 | Accumulates when PHEX activity is reduced; binds mineral and can inhibit crystal growth, contributing to defective mineralization. | (arhar2024characteristicsoforal pages 8-9, arhar2024characteristicsoforal pages 10-11) | | Cell type | Osteocyte | CL:0000121 | Principal source of FGF23 and site of PHEX expression; dysfunctional osteocytes drive endocrine and local mineralization defects. | (arhar2024characteristicsoforal pages 8-9, brandi2025xlinkedhypophosphatemiaand pages 2-4) | | Cell type | Osteoblast | CL:0000062 | Bone-forming cell interacting with osteocytes and matrix SIBLING proteins; contributes to defective mineral deposition in XLH. | (arhar2024characteristicsoforal pages 8-9, arhar2024characteristicsoforal pages 10-11) | | Cell type | Chondrocyte | CL:0000138 | Growth-plate chondrocytes are affected by phosphate deficiency, causing rickets and growth-plate abnormalities in children. | (brandi2025xlinkedhypophosphatemiaand pages 2-4) | | Cell type | Renal proximal tubule epithelial cell | CL:0002306 | Primary renal target where FGF23–Klotho–FGFR1c signalling downregulates NaPi transporters, causing phosphaturia. | (wang2024metaanalysisandsystematic pages 18-18, brandi2025xlinkedhypophosphatemiaand pages 2-4) | | Cell type | Parathyroid chief cell | CL:0000772 | Parathyroid is a target of FGF23; altered FGF23/PTH interplay contributes to mineral metabolism dysregulation and secondary hyperparathyroidism risk. | (wang2024metaanalysisandsystematic pages 18-18, brandi2025xlinkedhypophosphatemiaand pages 2-4) | | Cell type | Odontoblast | CL:0000115 | Dental cell expressing PHEX and FGF23 mRNA; PHEX dysfunction leads to dentin hypomineralization and increased dental abscess risk. | (arhar2024characteristicsoforal pages 8-9, arhar2024characteristicsoforal pages 10-11, defabianis2025xlinkedhypophosphatemiain pages 1-2) | | Cell type | Ameloblast | CL:0002494 | Enamel-forming cell where FGF23 expression has been detected; contributes to dental phenotype alongside odontoblast defects. | (arhar2024characteristicsoforal pages 8-9, arhar2024characteristicsoforal pages 10-11) | | Cell type | Cementoblast | CL:0009011 | Cementum-forming cell implicated in abnormal cementum observed in XLH models, contributing to periodontal/dentoalveolar pathology. | (arhar2024characteristicsoforal pages 10-11) | | Anatomy | Bone | UBERON:0001474 | Primary affected tissue: impaired mineralization (rickets in children, osteomalacia in adults) due to systemic hypophosphatemia and local inhibitors. | (brandi2025xlinkedhypophosphatemiaand pages 2-4, arhar2024characteristicsoforal pages 8-9) | | Anatomy | Kidney | UBERON:0002113 | Site of phosphate wasting via proximal-tubule transporter downregulation and altered vitamin D metabolism from FGF23 action. | (wang2024metaanalysisandsystematic pages 18-18, brandi2025xlinkedhypophosphatemiaand pages 2-4) | | Anatomy | Renal proximal tubule | UBERON:0001285 | Anatomical location of NaPi transporters and Klotho expression where FGF23 exerts phosphaturic effects. | (wang2024metaanalysisandsystematic pages 18-18, brandi2025xlinkedhypophosphatemiaand pages 2-4) | | Anatomy | Growth plate | UBERON:0003616 | Phosphate-dependent cartilage mineralization occurs here; deficiency leads to widening/irregularity and rickets. | (brandi2025xlinkedhypophosphatemiaand pages 2-4) | | Anatomy | Parathyroid gland | UBERON:0001132 | Interacts with FGF23/vitamin D axis; PTH disturbances (secondary/tertiary hyperparathyroidism) are clinical concerns with therapy. | (wang2024metaanalysisandsystematic pages 18-18, arcidiacono2025potentialpredictorsof pages 16-20) | | Anatomy | Tooth | UBERON:0001091 | Dentition displays hypomineralized dentin, enamel defects, and increased periapical pathology in XLH. | (arhar2024characteristicsoforal pages 8-9, arhar2024characteristicsoforal pages 10-11, defabianis2025xlinkedhypophosphatemiain pages 1-2) | | Anatomy | Dentin | UBERON:0001750 | Site of defective mineralization (reduced dentin mineral density, abnormal tubules) linked to PHEX/ASARM effects. | (arhar2024characteristicsoforal pages 8-9, arhar2024characteristicsoforal pages 10-11) | | Anatomy | Cementum | UBERON:0001751 | Abnormal cementum reported in hyp mouse and human studies, contributing to periodontal/dental sequelae. | (arhar2024characteristicsoforal pages 10-11) | | Anatomy | Enthesis | UBERON:0002185 | Enthesopathies (calcific enthesopathy) are common adult complications related to chronic mineral imbalance and mechanical stress. | (arcidiacono2025potentialpredictorsof pages 16-20, brandi2025xlinkedhypophosphatemiaand pages 2-4) | | Phenotype | Hypophosphatemia | HPO:HP:0002148 | Biochemical hallmark caused by FGF23-driven renal phosphate wasting; central diagnostic feature (low serum phosphate, low TmP/GFR). | (wang2024metaanalysisandsystematic pages 18-18, brandi2025xlinkedhypophosphatemiaand pages 2-4) | | Phenotype | Rickets | HPO:HP:0002748 | Pediatric manifestation from impaired growth-plate mineralization leading to bowing, deformity, and delayed motor milestones. | (brandi2025xlinkedhypophosphatemiaand pages 2-4, arcidiacono2025potentialpredictorsof pages 16-20) | | Phenotype | Osteomalacia | HPO:HP:0002649 | Adult manifestation of defective bone mineralization with bone pain, fractures, and pseudofractures. | (brandi2025xlinkedhypophosphatemiaand pages 2-4, arcidiacono2025potentialpredictorsof pages 16-20) | | Phenotype | Short stature | HPO:HP:0004322 | Growth impairment from chronic phosphate deficiency and rickets in pediatric patients. | (arcidiacono2025potentialpredictorsof pages 16-20, brandi2025xlinkedhypophosphatemiaand pages 2-4) | | Phenotype | Bowing of long bone | HPO:HP:0002970 | Classic orthopedic deformity in untreated/under-treated pediatric XLH due to growth-plate pathology. | (brandi2025xlinkedhypophosphatemiaand pages 2-4) | | Phenotype | Dental abscess | HPO:HP:0001088 | Frequent spontaneous periapical infections linked to hypomineralized dentin and pulp exposures. | (arhar2024characteristicsoforal pages 8-9, arhar2024characteristicsoforal pages 10-11, defabianis2025xlinkedhypophosphatemiain pages 1-2) | | Phenotype | Craniosynostosis | HPO:HP:0001363 | Reported comorbidity in some cohorts; linked to abnormal skull growth/mineralization in XLH. | (arcidiacono2025potentialpredictorsof pages 16-20, brandi2025xlinkedhypophosphatemiaand pages 2-4) | | Phenotype | Enthesopathy | HPO:HP:0002657 | Calcific enthesopathies and joint problems arise in adults with long-standing disease. | (arcidiacono2025potentialpredictorsof pages 16-20, brandi2025xlinkedhypophosphatemiaand pages 2-4) | | Phenotype | Pseudofractures | HPO:HP:0002757 | Insufficiency fractures due to impaired bone strength from osteomalacia. | (arcidiacono2025potentialpredictorsof pages 16-20, brandi2025xlinkedhypophosphatemiaand pages 2-4) | | Phenotype | Nephrocalcinosis | HPO:HP:0000121 | Potential complication of conventional phosphate/vitamin D therapy (and monitoring concern). | (wang2024metaanalysisandsystematic pages 18-18, wang2024metaanalysisandsystematic pages 1-2) | | Phenotype | Hyperparathyroidism | HPO:HP:0000829 | Secondary/tertiary hyperparathyroidism can arise from therapy or chronic mineral dysregulation. | (wang2024metaanalysisandsystematic pages 18-18, arcidiacono2025potentialpredictorsof pages 16-20) | | Phenotype | Hearing impairment | HPO:HP:0000365 | Reported extracranial manifestation in XLH cohorts (sensorineural/conductive elements described). | (arcidiacono2025potentialpredictorsof pages 16-20) | | Chemical | Phosphate | ChEBI:18367 | Essential mineral depleted in XLH due to renal wasting; low serum phosphate drives clinical manifestations. | (wang2024metaanalysisandsystematic pages 18-18, brandi2025xlinkedhypophosphatemiaand pages 2-4) | | Chemical | 1,25-dihydroxyvitamin D3 (calcitriol) | ChEBI:28940 / ChEBI:17933 | Active vitamin D reduced in XLH (FGF23 suppresses synthesis/increases catabolism), lowering intestinal phosphate/calcium absorption. | (wang2024metaanalysisandsystematic pages 18-18, brandi2025xlinkedhypophosphatemiaand pages 2-4) | | Chemical | Calcitriol (therapeutic) | ChEBI:17933 | Used therapeutically with oral phosphate to boost mineralization but does not target FGF23 and carries risk of hypercalciuria. | (wang2024metaanalysisandsystematic pages 18-18, wang2024metaanalysisandsystematic pages 1-2) | | Chemical | Burosumab (anti-FGF23 antibody) | ChEBI:132958 | FGF23-neutralizing monoclonal antibody that restores renal phosphate reabsorption, raises serum phosphate and 1,25(OH)2D, improving rickets/osteomalacia. | (wang2024metaanalysisandsystematic pages 18-18, wang2024metaanalysisandsystematic pages 1-2, brandi2025xlinkedhypophosphatemiaand pages 2-4) |
Table: A compact mapping of key genes, cell types, anatomical sites, phenotypes, and chemicals for X-linked hypophosphatemia (XLH), linked to standard ontology identifiers and primary supporting evidence from gathered sources. This table is intended for knowledge‑base annotation and mechanistic reference (arhar2024characteristicsoforal pages 8-9, brandi2025xlinkedhypophosphatemiaand pages 2-4).
Mechanism of action and outcomes of burosumab - Mechanism: Anti-FGF23 monoclonal antibody that restores renal phosphate handling (NaPi-IIa/IIc expression indirectly via FGF23 blockade) and increases endogenous 1,25(OH)2D by relieving CYP27B1 suppression/CYP24A1 induction (BDJ Open, 2024; Front Endocrinol, 2024) (arhar2024characteristicsoforal pages 8-9, wang2024metaanalysisandsystematic pages 1-2, wang2024metaanalysisandsystematic pages 18-18). - Outcomes: In children, improved serum phosphate, TmP/GFR, ALP, rickets severity scores, and functional capacity; favorable safety compared with conventional therapy, with ongoing assessment of long-term growth and QOL (Front Endocrinol, 2024) (wang2024metaanalysisandsystematic pages 1-2, wang2024metaanalysisandsystematic pages 18-18).
Evidence items (with PMIDs/DOIs/URLs/dates) - Brandi ML et al. X-linked hypophosphatemia and tumor-induced osteomalacia: narrative review/expert opinion. Orphanet J Rare Dis. Published Oct 2025. DOI: 10.1186/s13023-025-03952-5. URL: https://doi.org/10.1186/s13023-025-03952-5 (brandi2025xlinkedhypophosphatemiaand pages 2-4, brandi2025xlinkedhypophosphatemiaand pages 14-14). - Wang K et al. Meta-analysis and systematic review: burosumab in children with XLH. Front Endocrinol. Published Aug 2024. DOI: 10.3389/fendo.2024.1414509. URL: https://doi.org/10.3389/fendo.2024.1414509 (wang2024metaanalysisandsystematic pages 1-2, wang2024metaanalysisandsystematic pages 18-18). - Arhar A et al. Characteristics of oral health of patients with XLH. BDJ Open. Published May 2024. DOI: 10.1038/s41405-024-00223-6. URL: https://doi.org/10.1038/s41405-024-00223-6 (arhar2024characteristicsoforal pages 8-9, arhar2024characteristicsoforal pages 10-11). - Defabianis P et al. XLH in childhood: dental involvement, diagnosis, and treatment. Eur J Paediatr Dent. Published Dec 2025. DOI: 10.23804/ejpd.2025.2348. URL: https://doi.org/10.23804/ejpd.2025.2348 (defabianis2025xlinkedhypophosphatemiain pages 1-2). - Arcidiacono GP. Potential predictors of response to burosumab in adults with XLH. 2025. (arcidiacono2025potentialpredictorsof pages 16-20).
Limitations - Some 2025 sources are narrative/expert reviews. Where randomized and meta-analytic evidence is summarized, we prioritized 2024 systematic reviews/meta-analyses. Long-term outcomes (adult skeletal structure, enthesopathy reversal, dental organ regeneration) require further longitudinal data (Front Endocrinol, 2024) (wang2024metaanalysisandsystematic pages 1-2).
Pathophysiology description (knowledge-base ready) - XLH results from PHEX (HGNC:8860) loss-of-function in osteocytes/odontoblasts, elevating FGF23 (HGNC:3689), which activates FGFR1 (HGNC:3688) with KLOTHO (HGNC:6353) in renal proximal tubule (UBERON:0001285) and parathyroid (UBERON:0001132), reducing NaPi-IIa (SLC34A1; HGNC:11039)/NaPi-IIc (SLC34A3; HGNC:11041) activity and suppressing CYP27B1 (HGNC:2593) while inducing CYP24A1 (HGNC:2594). Systemic effects are hypophosphatemia (HP:0002148) and low/normal 1,25(OH)2D, with local matrix inhibition by MEPE/ASARM (HGNC:13323) and osteopontin/SPP1 (HGNC:11255), and contributions from DMP1 (HGNC:2936) and ENPP1 (HGNC:3352) to PPi balance, leading to rickets (HP:0002748) and osteomalacia (HP:0002649), dental abscesses (HP:0001088), and adult enthesopathy (HP:0002657) (Orphanet J Rare Dis, 2025; Front Endocrinol, 2024; BDJ Open, 2024) (brandi2025xlinkedhypophosphatemiaand pages 2-4, wang2024metaanalysisandsystematic pages 1-2, wang2024metaanalysisandsystematic pages 18-18, arhar2024characteristicsoforal pages 8-9, arhar2024characteristicsoforal pages 10-11).
References
(brandi2025xlinkedhypophosphatemiaand pages 2-4): Maria Luisa Brandi, Cristina Eller Vainicher, Danilo Fintini, Andrea Giusti, Andrea Magnolato, Salvatore Minisola, and Sandro Giannini. X-linked hypophosphatemia and tumor-induced osteomalacia: a narrative review and expert opinion on the diagnostic and therapeutic challenges in the era of burosumab. Orphanet Journal of Rare Diseases, Oct 2025. URL: https://doi.org/10.1186/s13023-025-03952-5, doi:10.1186/s13023-025-03952-5. This article has 0 citations and is from a peer-reviewed journal.
(wang2024metaanalysisandsystematic pages 1-2): Kangning Wang, Runze Zhang, Ziyi Chen, Yi Bai, and Qing He. Meta-analysis and systematic review: burosumab as a promising treatment for children with x-linked hypophosphatemia. Frontiers in Endocrinology, Aug 2024. URL: https://doi.org/10.3389/fendo.2024.1414509, doi:10.3389/fendo.2024.1414509. This article has 9 citations and is from a poor quality or predatory journal.
(arhar2024characteristicsoforal pages 8-9): Ana Arhar, Alenka Pavlič, and Luka Hočevar. Characteristics of oral health of patients with x-linked hypophosphatemia: case reports and literature review. BDJ Open, May 2024. URL: https://doi.org/10.1038/s41405-024-00223-6, doi:10.1038/s41405-024-00223-6. This article has 5 citations and is from a peer-reviewed journal.
(defabianis2025xlinkedhypophosphatemiain pages 1-2): P. Defabianis, N. Bocca, and R. Ninivaggi. X-linked hypophosphatemia in childhood: dental involvement, diagnosis, and treatment. European journal of paediatric dentistry, pages 1, Dec 2025. URL: https://doi.org/10.23804/ejpd.2025.2348, doi:10.23804/ejpd.2025.2348. This article has 0 citations and is from a peer-reviewed journal.
(wang2024metaanalysisandsystematic pages 18-18): Kangning Wang, Runze Zhang, Ziyi Chen, Yi Bai, and Qing He. Meta-analysis and systematic review: burosumab as a promising treatment for children with x-linked hypophosphatemia. Frontiers in Endocrinology, Aug 2024. URL: https://doi.org/10.3389/fendo.2024.1414509, doi:10.3389/fendo.2024.1414509. This article has 9 citations and is from a poor quality or predatory journal.
(arhar2024characteristicsoforal pages 10-11): Ana Arhar, Alenka Pavlič, and Luka Hočevar. Characteristics of oral health of patients with x-linked hypophosphatemia: case reports and literature review. BDJ Open, May 2024. URL: https://doi.org/10.1038/s41405-024-00223-6, doi:10.1038/s41405-024-00223-6. This article has 5 citations and is from a peer-reviewed journal.
(arcidiacono2025potentialpredictorsof pages 16-20): GP Arcidiacono. Potential predictors of response to burosumab treatment in adult patients with x-linked hypophosphatemia. Unknown journal, 2025.
(brandi2025xlinkedhypophosphatemiaand pages 14-14): Maria Luisa Brandi, Cristina Eller Vainicher, Danilo Fintini, Andrea Giusti, Andrea Magnolato, Salvatore Minisola, and Sandro Giannini. X-linked hypophosphatemia and tumor-induced osteomalacia: a narrative review and expert opinion on the diagnostic and therapeutic challenges in the era of burosumab. Orphanet Journal of Rare Diseases, Oct 2025. URL: https://doi.org/10.1186/s13023-025-03952-5, doi:10.1186/s13023-025-03952-5. This article has 0 citations and is from a peer-reviewed journal.