Alkaptonuria

Mendelian MONDO:0008753 Pathograph 30 Disorder of Tyrosine Metabolism Inborn Error of Metabolism

Alkaptonuria is a rare autosomal recessive disorder of tyrosine degradation caused by biallelic loss-of-function variants in HGD, which encodes homogentisate 1,2-dioxygenase. Reduced HGD activity blocks conversion of homogentisic acid to maleylacetoacetic acid, causing systemic accumulation of homogentisic acid in urine and connective tissues. Oxidation of homogentisic acid produces benzoquinone-derived, melanin-like pigment that deposits in collagen-rich tissues, producing ochronosis, darkening of urine on standing, progressive spine and large-joint osteoarthropathy, cartilage calcification, and later cardiovascular, renal, and prostatic complications. Nitisinone lowers homogentisic acid production upstream and slows clinical progression, but it does not correct HGD deficiency.

Ask OpenScientist

Ask a research question about Alkaptonuria. OpenScientist will conduct autonomous deep research using the Disorder Mechanisms Knowledge Base and PubMed literature (typically 10-30 minutes).

Submitting...

Do not include personal health information in your question. Questions and results are cached in your browser's local storage.

1
Mappings
1
Definitions
1
Inheritance
3
Pathophysiology
0
Histopathology
22
Phenotypes
30
Pathograph
1
Genes
1
Treatments
0
Subtypes
0
Differentials
0
Datasets
0
Trials
0
Models
7
References
1
Deep Research
🔗

Mappings

MONDO
MONDO:0008753 alkaptonuria
skos:exactMatch Orphanet ORPHA:56
Orphanet ORPHA:56 lists MONDO:0008753 as an exact cross-reference for alkaptonuria.
📘

Definitions

1
Orphanet alkaptonuria definition
A rare disorder of phenylalanine and tyrosine metabolism with accumulation of homogentisic acid and benzoquinone acetic acid in tissues and body fluids, causing dark urine, ochronosis, and disabling axial and peripheral joint disease.
OTHER
Show evidence (1 reference)
ORPHA:56 SUPPORT Other
"A rare disorder of phenylalanine and tyrosine metabolism characterized by the accumulation of homogentisic acid (HGA) and its oxidized product, benzoquinone acetic acid (BQA), in various tissues"
Orphanet's definition supports the disease-level biochemical and clinical characterization.
👪

Inheritance

1
Autosomal recessive HP:0000007
Alkaptonuria is caused by biallelic pathogenic HGD variants.
Autosomal recessive inheritance
Show evidence (2 references)
ORPHA:56 SUPPORT Other
"Autosomal recessive"
Orphanet records autosomal recessive inheritance for alkaptonuria.
PMID:20301627 SUPPORT Other
"Alkaptonuria is inherited in an autosomal recessive manner."
GeneReviews confirms autosomal recessive inheritance.
⚙

Pathophysiology

3
HGD molecular function deficiency
Biallelic HGD pathogenic variants reduce homogentisate 1,2-dioxygenase activity, blocking the homogentisate step of the tyrosine degradation pathway.
HGD link
L-tyrosine catabolic process link ↓ DECREASED
homogentisate 1,2-dioxygenase activity link ↓ DECREASED
Downstream
Reduced homogentisate 1,2-dioxygenase activity HGD loss of function reduces homogentisate 1,2-dioxygenase activity.
Homogentisic acid accumulation and oxidation Loss of HGD activity prevents normal homogentisic acid catabolism.
Show evidence (3 references)
ORPHA:56 SUPPORT Other
"HGD | homogentisate 1,2-dioxygenase | hgnc:4892 | Disease-causing germline mutation(s) (loss of function) in"
Orphanet identifies HGD loss of function as the disease-causing gene mechanism.
PMID:20301627 SUPPORT Other
"Alkaptonuria is caused by deficiency of homogentisate"
GeneReviews supports the initiating enzymatic block.
PMID:30737480 SUPPORT Human Clinical
"Alkaptonuria (AKU) is a rare metabolic disorder caused by a deficient enzyme in"
Large genotype cohort independently supports HGD enzyme deficiency as causal.
Homogentisic acid accumulation and oxidation
The HGD block causes homogentisic acid to accumulate in urine and tissues. Homogentisic acid undergoes oxidation to benzoquinone products that form melanin-like polymers capable of binding connective tissue components.
L-tyrosine catabolic process link ↓ DECREASED
Downstream
Elevated urinary homogentisic acid Excess HGA is excreted in urine and forms the defining urinary phenotype.
Increased urinary homogentisic acid Urinary HGA measurement is the biochemical diagnostic correlate of systemic HGA accumulation.
Aminoaciduria Orphanet retains a broad aminoaciduria HPO annotation for the urinary biochemical phenotype; the more specific elevated urinary homogentisic acid phenotype is modeled separately.
Nephrolithiasis Urinary tract involvement in alkaptonuria includes renal stones.
Dark urine Urinary HGA oxidizes on standing or air exposure, darkening urine.
Ochronotic connective tissue degeneration Oxidized HGA-derived polymers deposit in collagen-rich tissues and cartilage.
Show evidence (3 references)
ORPHA:56 SUPPORT Other
"the accumulation of homogentisic acid (HGA) and its oxidized product, benzoquinone acetic acid (BQA), in various tissues"
Orphanet directly supports HGA and BQA accumulation.
PMID:38453957 SUPPORT Other
"accumulation in body fluids and tissues leads to a multisystemic and highly"
Recent disease primer supports systemic HGA accumulation as the central biochemical mechanism.
PMID:19862842 SUPPORT Other
"AKU is caused by deficiency of homogentisic acid oxidase (HGD, EC 1.13.11.5), which leads to the accumulation of homogentisic acid (HGA)"
The article background links HGD deficiency to HGA accumulation.
Ochronotic connective tissue degeneration
Oxidized HGA-derived pigment deposits in collagen-rich connective tissues, especially cartilage. This ochronotic pigmentation is associated with visible tissue discoloration and painful axial and large-joint osteoarthropathy.
cartilage tissue link connective tissue link
Downstream
Ochronosis HGA-derived pigment deposition in connective tissue manifests as ochronosis.
Pigmentation of the sclera Ochronotic pigment can be visible as scleral pigmentation.
Ochronotic osteoarthritis Ochronotic connective-tissue degeneration produces disabling axial and peripheral joint disease.
Arthralgia Pain arises as part of severe ochronotic osteoarthropathy.
Joint stiffness Ochronotic arthropathy and cartilage degeneration produce joint stiffness.
Joint swelling Ochronotic arthropathy can manifest as joint swelling.
Joint dislocation Severe ochronotic joint disease can be associated with joint dislocation.
Back pain Axial ochronotic arthropathy involving the spine produces back pain.
Intervertebral disk calcification Spinal connective-tissue degeneration includes intervertebral disk calcification.
Cartilage calcification Ochronotic cartilage degeneration includes calcification of cartilage.
Cartilage destruction Pigment deposition and degeneration can destroy cartilage.
Tendon rupture Ochronotic involvement of collagen-rich tendon and ligament tissue increases rupture risk.
Thickened Achilles tendon Ochronotic tendon involvement can manifest as Achilles tendon thickening.
Hearing abnormality Ochronotic pigment in ear cartilage can be associated with hearing abnormalities.
Abnormality of vision Ocular ochronosis and scleral/corneal pigmentation can contribute to visual abnormalities.
Aortic valve calcification Ochronotic cardiovascular connective-tissue involvement can produce aortic valve calcification.
Mitral valve calcification Ochronotic cardiovascular connective-tissue involvement can produce mitral valve calcification.
Coronary artery calcification Later cardiovascular complications include coronary artery calcification.
Show evidence (3 references)
ORPHA:56 SUPPORT Other
"various tissues (e.g. cartilage, connective tissue) and body fluids (urine, sweat), causing urine to darken when exposed to air as well as grey-blue coloration of the sclera and ear helix (ochronosis), and a disabling joint disease"
Orphanet connects HGA tissue accumulation to ochronosis and disabling joint disease.
PMID:38453957 SUPPORT Other
"pigment in collagen-rich connective tissues), and a painful and severe form of"
Disease primer supports HGA-derived pigment deposition in connective tissue causing osteoarthropathy.
PMID:19862842 SUPPORT Other
"The clinical findings of AKU result from the reaction of homogentisic acid and its homopolymeric oxidation products, i.e., benzoquinones, with connective tissue components."
Mechanistic statement links oxidized HGA products to connective-tissue clinical findings.
⬡

Pathograph

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

Phenotypes

22
Genitourinary 2
Dark urine FREQUENT Dark urine (HP:0040319)
Show evidence (2 references)
ORPHA:56 SUPPORT Other
"HP:0040319 | Dark urine | Frequent (79-30%)"
Orphanet reports dark urine as frequent.
PMID:20301627 SUPPORT Other
"features of alkaptonuria are dark urine or urine that turns dark on standing,"
GeneReviews lists dark urine as a major feature.
Nephrolithiasis FREQUENT Nephrolithiasis (HP:0000787)
Show evidence (2 references)
ORPHA:56 SUPPORT Other
"HP:0000787 | Nephrolithiasis | Frequent (79-30%)"
Orphanet reports nephrolithiasis as frequent.
PMID:20301627 SUPPORT Other
"surgical intervention for prostate stones and renal"
GeneReviews lists renal stones among other manifestations.
Metabolism 1
Joint swelling VERY_FREQUENT Joint swelling (HP:0001386)
Show evidence (1 reference)
ORPHA:56 SUPPORT Other
"HP:0001386 | Joint swelling | Very frequent (99-80%)"
Orphanet reports joint swelling as very frequent.
Musculoskeletal 3
Ochronotic osteoarthritis VERY_FREQUENT Osteoarthritis (HP:0002758)
Show evidence (2 references)
ORPHA:56 SUPPORT Other
"HP:0002758 | Osteoarthritis | Very frequent (99-80%)"
Orphanet reports osteoarthritis as very frequent.
PMID:20301627 SUPPORT Other
"the spine and larger joints."
GeneReviews lists spine and large-joint arthritis as a major feature.
Joint stiffness VERY_FREQUENT Joint stiffness (HP:0001387)
Show evidence (1 reference)
ORPHA:56 SUPPORT Other
"HP:0001387 | Joint stiffness | Very frequent (99-80%)"
Orphanet reports joint stiffness as very frequent.
Joint dislocation VERY_FREQUENT Joint dislocation (HP:0001373)
Show evidence (1 reference)
ORPHA:56 SUPPORT Other
"HP:0001373 | Joint dislocation | Very frequent (99-80%)"
Orphanet reports joint dislocation as very frequent.
Constitutional 2
Arthralgia VERY_FREQUENT Arthralgia (HP:0002829)
Show evidence (2 references)
ORPHA:56 SUPPORT Other
"HP:0002829 | Arthralgia | Very frequent (99-80%)"
Orphanet reports arthralgia as very frequent.
PMID:38453957 SUPPORT Other
"painful and severe form of"
Disease primer describes painful osteoarthropathy as a main feature.
Back pain FREQUENT Back pain (HP:0003418)
Show evidence (1 reference)
ORPHA:56 SUPPORT Other
"HP:0003418 | Back pain | Frequent (79-30%)"
Orphanet reports back pain as frequent.
Other 14
Elevated urinary homogentisic acid VERY_FREQUENT Elevated urinary homogentisic acid (HP:0033704)
Show evidence (2 references)
ORPHA:56 SUPPORT Other
"HP:0033704 | Elevated urinary homogentisic acid | Very frequent (99-80%)"
Orphanet reports elevated urinary homogentisic acid as very frequent.
PMID:20301627 SUPPORT Other
"DIAGNOSIS/TESTING: The biochemical diagnosis of alkaptonuria in a proband is"
GeneReviews identifies urinary HGA as the biochemical diagnostic marker.
Ochronosis VERY_FREQUENT Ochronosis (HP:0030764)
Show evidence (2 references)
ORPHA:56 SUPPORT Other
"HP:0030764 | Ochronosis | Very frequent (99-80%)"
Orphanet reports ochronosis as very frequent.
PMID:38453957 SUPPORT Other
"debilitating disease whose main features are dark urine, ochronosis (HGA-derived"
Disease primer identifies ochronosis as a main feature.
Intervertebral disk calcification VERY_FREQUENT Intervertebral disk calcification (HP:0005645)
Show evidence (1 reference)
ORPHA:56 SUPPORT Other
"HP:0005645 | Intervertebral disk calcification | Very frequent (99-80%)"
Orphanet reports intervertebral disk calcification as very frequent.
Cartilage calcification VERY_FREQUENT Calcification of cartilage (HP:0100593)
Show evidence (1 reference)
ORPHA:56 SUPPORT Other
"HP:0100593 | Calcification of cartilage | Very frequent (99-80%)"
Orphanet reports cartilage calcification as very frequent.
Cartilage destruction FREQUENT Cartilage destruction (HP:0100773)
Show evidence (1 reference)
ORPHA:56 SUPPORT Other
"HP:0100773 | Cartilage destruction | Frequent (79-30%)"
Orphanet reports cartilage destruction as frequent.
Tendon rupture FREQUENT Tendon rupture (HP:0100550)
Show evidence (2 references)
ORPHA:56 SUPPORT Other
"HP:0100550 | Tendon rupture | Frequent (79-30%)"
Orphanet reports tendon rupture as frequent.
PMID:38453957 SUPPORT Other
"include kidney and prostate stones, aortic stenosis, bone fractures, and tendon,"
Disease primer lists tendon, ligament, and muscle ruptures among variable manifestations.
Thickened Achilles tendon FREQUENT Thickened Achilles tendon (HP:0004690)
Show evidence (1 reference)
ORPHA:56 SUPPORT Other
"HP:0004690 | Thickened Achilles tendon | Frequent (79-30%)"
Orphanet reports thickened Achilles tendon as frequent.
Aortic valve calcification FREQUENT Aortic valve calcification (HP:0004380)
Show evidence (2 references)
ORPHA:56 SUPPORT Other
"HP:0004380 | Aortic valve calcification | Frequent (79-30%)"
Orphanet reports aortic valve calcification as frequent.
PMID:20301627 SUPPORT Other
"pigment in the sclera, ear cartilage, and skin of the hands; aortic or mitral valve calcification or regurgitation and occasionally aortic dilatation; renal"
GeneReviews supports cardiac valve calcification as a manifestation.
Coronary artery calcification VERY_FREQUENT Coronary artery calcification (HP:0001717)
Show evidence (2 references)
ORPHA:56 SUPPORT Other
"HP:0001717 | Coronary artery calcification | Very frequent (99-80%)"
Orphanet reports coronary artery calcification as very frequent.
PMID:20301627 SUPPORT Other
"CT imaging to detect coronary artery calcification."
GeneReviews surveillance guidance supports coronary artery calcification as a recognized manifestation.
Mitral valve calcification FREQUENT Mitral valve calcification (HP:0004382)
Show evidence (2 references)
ORPHA:56 SUPPORT Other
"HP:0004382 | Mitral valve calcification | Frequent (79-30%)"
Orphanet reports mitral valve calcification as frequent.
PMID:20301627 SUPPORT Other
"pigment in the sclera, ear cartilage, and skin of the hands; aortic or mitral valve calcification or regurgitation and occasionally aortic dilatation; renal"
GeneReviews lists aortic or mitral valve calcification among manifestations.
Hearing abnormality VERY_FREQUENT Hearing abnormality (HP:0000364)
Show evidence (1 reference)
ORPHA:56 SUPPORT Other
"HP:0000364 | Hearing abnormality | Very frequent (99-80%)"
Orphanet reports hearing abnormality as very frequent.
Abnormality of vision VERY_FREQUENT Abnormality of vision (HP:0000504)
Show evidence (1 reference)
ORPHA:56 SUPPORT Other
"HP:0000504 | Abnormality of vision | Very frequent (99-80%)"
Orphanet reports abnormality of vision as very frequent.
Pigmentation of the sclera FREQUENT Pigmentation of the sclera (HP:0007832)
Show evidence (2 references)
ORPHA:56 SUPPORT Other
"HP:0007832 | Pigmentation of the sclera | Frequent (79-30%)"
Orphanet reports pigmentation of the sclera as frequent.
PMID:20301627 SUPPORT Other
"pigment in the sclera, ear cartilage, and skin of the hands;"
GeneReviews supports scleral pigmentation as a manifestation.
Aminoaciduria VERY_FREQUENT Aminoaciduria (HP:0003355)
Show evidence (1 reference)
ORPHA:56 SUPPORT Other
"HP:0003355 | Aminoaciduria | Very frequent (99-80%)"
Orphanet reports aminoaciduria as very frequent.
🧬

Genetic Associations

1
HGD variants
Autosomal recessive
Show evidence (2 references)
PMID:19862842 SUPPORT Human Clinical
"AKU is due to mutations in the homogentisate"
Confirms HGD variants as the genetic cause and connects them to the biochemical block.
CGGV:assertion_5186836d-d9c6-4829-a0c9-59548460d6f2-2020-06-29T174125.541Z SUPPORT Other
"HGD | HGNC:4892 | alkaptonuria | MONDO:0008753 | AR | Definitive"
ClinGen classifies the HGD-alkaptonuria gene-disease relationship as definitive with autosomal recessive inheritance.
💊

Treatments

1
Nitisinone therapy
Action: pharmacotherapy MAXO:0000058
Agent: nitisinone ↗
Nitisinone inhibits an upstream tyrosine-degradation step to reduce HGA production. In SONIA 2, daily nitisinone markedly reduced urinary HGA and slowed clinical progression as measured by cAKUSSI.
Mechanism Target:
INHIBITS Homogentisic acid accumulation and oxidation — Nitisinone reduces HGA production upstream of the HGD block.
Show evidence (1 reference)
PMID:32822600 SUPPORT Human Clinical
"the study. u-HGA24 at 12 months was significantly decreased by 99·7% in the"
SONIA 2 directly supports HGA-lowering as the treatment mechanism.
Target Phenotypes: Elevated urinary homogentisic acid ↗ Ochronosis ↗
Show evidence (2 references)
PMID:32822600 SUPPORT Human Clinical
"INTERPRETATION: Nitisinone 10 mg daily was well tolerated and effective in"
Randomized controlled trial supports nitisinone as HGA-lowering therapy.
PMID:32822600 SUPPORT Human Clinical
"clinical signs, indicating a slower disease progression."
SONIA 2 supports clinical benefit beyond biochemical HGA reduction.
🔬

Biochemical Markers

2
Increased urinary homogentisic acid (INCREASED)
Context: Urinary HGA is substantially increased and is the primary biochemical diagnostic marker.
Pathograph Readouts
Readout Of Homogentisic acid accumulation and oxidation Positive Diagnostic
Elevated urinary HGA reports the upstream HGD block and systemic HGA accumulation.
Show evidence (1 reference)
PMID:20301627 SUPPORT Other
"based on the detection of a significant amount of HGA in the urine (usually 1 to"
GeneReviews identifies urinary HGA as the diagnostic biochemical readout of alkaptonuria.
Show evidence (2 references)
PMID:20301627 SUPPORT Other
"based on the detection of a significant amount of HGA in the urine (usually 1 to"
GeneReviews provides the typical magnitude of urinary HGA excretion.
ORPHA:56 SUPPORT Other
"HP:0033704 | Elevated urinary homogentisic acid | Very frequent (99-80%)"
Orphanet supports elevated urinary HGA as the dominant biochemical phenotype.
Reduced homogentisate 1,2-dioxygenase activity (DECREASED)
Context: Reduced HGD enzyme activity is the proximal biochemical defect that impairs tyrosine catabolism.
Pathograph Readouts
Readout Of HGD molecular function deficiency Negative Diagnostic
Reduced homogentisate 1,2-dioxygenase activity directly reports HGD molecular function deficiency.
Show evidence (1 reference)
PMID:20301627 SUPPORT Other
"Alkaptonuria is caused by deficiency of homogentisate"
GeneReviews supports the enzyme activity deficit as the proximal HGD mechanism.
Show evidence (2 references)
PMID:20301627 SUPPORT Other
"Alkaptonuria is caused by deficiency of homogentisate"
GeneReviews identifies reduced HGD activity as the causal enzyme deficiency.
ORPHA:56 SUPPORT Other
"HGD | homogentisate 1,2-dioxygenase | hgnc:4892 | Disease-causing germline mutation(s) (loss of function) in"
Orphanet supports HGD loss of function as the biochemical defect.
{ }

Source YAML

click to show 
name: Alkaptonuria
category: Mendelian
creation_date: '2026-05-03T00:00:00Z'
updated_date: '2026-05-18T12:07:30Z'
synonyms:
- Hereditary ochronosis
- Homogentisic acid oxidase deficiency
- Homogentisate 1,2-dioxygenase deficiency
- AKU
description: >
  Alkaptonuria is a rare autosomal recessive disorder of tyrosine degradation
  caused by biallelic loss-of-function variants in HGD, which encodes
  homogentisate 1,2-dioxygenase. Reduced HGD activity blocks conversion of
  homogentisic acid to maleylacetoacetic acid, causing systemic accumulation of
  homogentisic acid in urine and connective tissues. Oxidation of homogentisic
  acid produces benzoquinone-derived, melanin-like pigment that deposits in
  collagen-rich tissues, producing ochronosis, darkening of urine on standing,
  progressive spine and large-joint osteoarthropathy, cartilage calcification,
  and later cardiovascular, renal, and prostatic complications. Nitisinone
  lowers homogentisic acid production upstream and slows clinical progression,
  but it does not correct HGD deficiency.
disease_term:
  preferred_term: alkaptonuria
  term:
    id: MONDO:0008753
    label: alkaptonuria
parents:
- Disorder of Tyrosine Metabolism
- Inborn Error of Metabolism
mappings:
  mondo_mappings:
  - term:
      id: MONDO:0008753
      label: alkaptonuria
    mapping_predicate: skos:exactMatch
    mapping_source: Orphanet ORPHA:56
    mapping_justification: >
      Orphanet ORPHA:56 lists MONDO:0008753 as an exact cross-reference for
      alkaptonuria.
external_assertions:
- name: Orphanet Alkaptonuria disease record
  source: Orphanet
  assertion_type: structured_disease_record
  external_id: ORPHA:56
  url: http://www.orpha.net/consor/cgi-bin/OC_Exp.php?lng=en&Expert=56
  description: >
    Orphanet's ORPHA:56 structured record for Alkaptonuria includes the exact
    MONDO cross-reference, synonyms, definition, autosomal recessive
    inheritance, epidemiology, HGD gene association, and HPO phenotype
    annotations used in this entry.
  evidence:
  - reference: ORPHA:56
    reference_title: "Alkaptonuria (Orphanet structured-database record)"
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "MONDO:0008753 | Exact"
    explanation: Orphanet maps ORPHA:56 to the same MONDO identifier used by this entry.
definitions:
- name: Orphanet alkaptonuria definition
  definition_type: OTHER
  description: >
    A rare disorder of phenylalanine and tyrosine metabolism with accumulation
    of homogentisic acid and benzoquinone acetic acid in tissues and body
    fluids, causing dark urine, ochronosis, and disabling axial and peripheral
    joint disease.
  evidence:
  - reference: ORPHA:56
    reference_title: "Alkaptonuria (Orphanet structured-database record)"
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "A rare disorder of phenylalanine and tyrosine metabolism characterized by the accumulation of homogentisic acid (HGA) and its oxidized product, benzoquinone acetic acid (BQA), in various tissues"
    explanation: Orphanet's definition supports the disease-level biochemical and clinical characterization.
inheritance:
- name: Autosomal recessive
  description: Alkaptonuria is caused by biallelic pathogenic HGD variants.
  inheritance_term:
    preferred_term: Autosomal recessive inheritance
    term:
      id: HP:0000007
      label: Autosomal recessive inheritance
  evidence:
  - reference: ORPHA:56
    reference_title: "Alkaptonuria (Orphanet structured-database record)"
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Autosomal recessive"
    explanation: Orphanet records autosomal recessive inheritance for alkaptonuria.
  - reference: PMID:20301627
    reference_title: "Alkaptonuria."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Alkaptonuria is inherited in an autosomal recessive manner."
    explanation: GeneReviews confirms autosomal recessive inheritance.
prevalence:
- population: Worldwide
  percentage: 1-9 per 1,000,000
  notes: >
    Orphanet records worldwide point prevalence in the one-to-nine per million
    range, with higher founder-effect prevalence reported in Slovakia.
  evidence:
  - reference: ORPHA:56
    reference_title: "Alkaptonuria (Orphanet structured-database record)"
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "1-9 / 1 000 000 | Worldwide | Point prevalence | EXPERT"
    explanation: Orphanet reports worldwide point prevalence in the one-to-nine per million range.
  - reference: PMID:12051967
    reference_title: "Alkaptonuria in Slovakia: thirty-two years of research on phenotype and genotype."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "world-wide highest incidence of AKU (1 in 19,000) was recorded"
    explanation: Slovak screening data document a high-prevalence founder population.
progression:
- phase: Biochemical onset with adult multisystem complications
  notes: >
    Homogentisic aciduria can be detected from infancy by darkening urine, while
    ochronosis and disabling arthropathy typically become clinically prominent
    in adulthood.
  evidence:
  - reference: ORPHA:56
    reference_title: "Alkaptonuria (Orphanet structured-database record)"
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Age of onset: Infancy"
    explanation: Orphanet records infancy among disease onset categories.
  - reference: ORPHA:56
    reference_title: "Alkaptonuria (Orphanet structured-database record)"
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Age of onset: Adult"
    explanation: Orphanet records adult onset among disease onset categories.
  - reference: PMID:20301627
    reference_title: "Alkaptonuria."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Ochronosis generally occurs after age 30 years;"
    explanation: GeneReviews supports delayed adult emergence of ochronosis and arthritis.
pathophysiology:
- name: HGD molecular function deficiency
  description: >
    Biallelic HGD pathogenic variants reduce homogentisate 1,2-dioxygenase
    activity, blocking the homogentisate step of the tyrosine degradation
    pathway.
  genes:
  - preferred_term: HGD
    term:
      id: hgnc:4892
      label: HGD
  molecular_functions:
  - preferred_term: homogentisate 1,2-dioxygenase activity
    term:
      id: GO:0004411
      label: homogentisate 1,2-dioxygenase activity
    modifier: DECREASED
  biological_processes:
  - preferred_term: L-tyrosine catabolic process
    term:
      id: GO:0006572
      label: L-tyrosine catabolic process
    modifier: DECREASED
  evidence:
  - reference: ORPHA:56
    reference_title: "Alkaptonuria (Orphanet structured-database record)"
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HGD | homogentisate 1,2-dioxygenase | hgnc:4892 | Disease-causing germline mutation(s) (loss of function) in"
    explanation: Orphanet identifies HGD loss of function as the disease-causing gene mechanism.
  - reference: PMID:20301627
    reference_title: "Alkaptonuria."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Alkaptonuria is caused by deficiency of homogentisate"
    explanation: GeneReviews supports the initiating enzymatic block.
  - reference: PMID:30737480
    reference_title: "Homogentisate 1,2-dioxygenase (HGD) gene variants, their analysis and genotype-phenotype correlations in the largest cohort of patients with AKU."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Alkaptonuria (AKU) is a rare metabolic disorder caused by a deficient enzyme in"
    explanation: Large genotype cohort independently supports HGD enzyme deficiency as causal.
  downstream:
  - target: Reduced homogentisate 1,2-dioxygenase activity
    description: HGD loss of function reduces homogentisate 1,2-dioxygenase activity.
    causal_link_type: DIRECT
    evidence:
    - reference: ORPHA:56
      reference_title: "Alkaptonuria (Orphanet structured-database record)"
      supports: SUPPORT
      evidence_source: OTHER
      snippet: "HGD | homogentisate 1,2-dioxygenase | hgnc:4892 | Disease-causing germline mutation(s) (loss of function) in"
      explanation: Orphanet links HGD loss of function to the enzyme activity defect.
  - target: Homogentisic acid accumulation and oxidation
    description: Loss of HGD activity prevents normal homogentisic acid catabolism.
    causal_link_type: DIRECT
    evidence:
    - reference: PMID:19862842
      reference_title: "Mutation spectrum of homogentisic acid oxidase (HGD) in alkaptonuria."
      supports: SUPPORT
      evidence_source: OTHER
      snippet: "AKU is caused by deficiency of homogentisic acid oxidase (HGD, EC 1.13.11.5), which leads to the accumulation of homogentisic acid (HGA)"
      explanation: The cached article text links HGD deficiency to HGA accumulation.
- name: Homogentisic acid accumulation and oxidation
  description: >
    The HGD block causes homogentisic acid to accumulate in urine and tissues.
    Homogentisic acid undergoes oxidation to benzoquinone products that form
    melanin-like polymers capable of binding connective tissue components.
  chemical_entities:
  - preferred_term: homogentisic acid
    term:
      id: CHEBI:44747
      label: homogentisic acid
    modifier: INCREASED
  biological_processes:
  - preferred_term: L-tyrosine catabolic process
    term:
      id: GO:0006572
      label: L-tyrosine catabolic process
    modifier: DECREASED
  evidence:
  - reference: ORPHA:56
    reference_title: "Alkaptonuria (Orphanet structured-database record)"
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "the accumulation of homogentisic acid (HGA) and its oxidized product, benzoquinone acetic acid (BQA), in various tissues"
    explanation: Orphanet directly supports HGA and BQA accumulation.
  - reference: PMID:38453957
    reference_title: "Alkaptonuria."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "accumulation in body fluids and tissues leads to a multisystemic and highly"
    explanation: Recent disease primer supports systemic HGA accumulation as the central biochemical mechanism.
  - reference: PMID:19862842
    reference_title: "Mutation spectrum of homogentisic acid oxidase (HGD) in alkaptonuria."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "AKU is caused by deficiency of homogentisic acid oxidase (HGD, EC 1.13.11.5), which leads to the accumulation of homogentisic acid (HGA)"
    explanation: The article background links HGD deficiency to HGA accumulation.
  downstream:
  - target: Elevated urinary homogentisic acid
    description: Excess HGA is excreted in urine and forms the defining urinary phenotype.
    causal_link_type: DIRECT
    evidence:
    - reference: ORPHA:56
      reference_title: "Alkaptonuria (Orphanet structured-database record)"
      supports: SUPPORT
      evidence_source: OTHER
      snippet: "HP:0033704 | Elevated urinary homogentisic acid | Very frequent (99-80%)"
      explanation: Orphanet reports elevated urinary HGA as a very frequent alkaptonuria phenotype.
  - target: Increased urinary homogentisic acid
    description: Urinary HGA measurement is the biochemical diagnostic correlate of systemic HGA accumulation.
    causal_link_type: DIRECT
    evidence:
    - reference: PMID:20301627
      reference_title: "Alkaptonuria."
      supports: SUPPORT
      evidence_source: OTHER
      snippet: "based on the detection of a significant amount of HGA in the urine (usually 1 to"
      explanation: GeneReviews identifies urinary HGA detection as the biochemical diagnostic marker.
  - target: Aminoaciduria
    description: >
      Orphanet retains a broad aminoaciduria HPO annotation for the urinary
      biochemical phenotype; the more specific elevated urinary homogentisic
      acid phenotype is modeled separately.
    causal_link_type: UNKNOWN
    evidence:
    - reference: ORPHA:56
      reference_title: "Alkaptonuria (Orphanet structured-database record)"
      supports: SUPPORT
      evidence_source: OTHER
      snippet: "HP:0003355 | Aminoaciduria | Very frequent (99-80%)"
      explanation: >
        Orphanet lists aminoaciduria as a very frequent broad urinary
        biochemical annotation, distinct from the separately modeled elevated
        urinary homogentisic acid phenotype.
  - target: Nephrolithiasis
    description: Urinary tract involvement in alkaptonuria includes renal stones.
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    intermediate_mechanisms:
    - urinary homogentisic acid excess and stone complications
    evidence:
    - reference: PMID:20301627
      reference_title: "Alkaptonuria."
      supports: SUPPORT
      evidence_source: OTHER
      snippet: "surgical intervention for prostate stones and renal"
      explanation: GeneReviews lists renal stones among manifestations requiring intervention.
  - target: Dark urine
    description: Urinary HGA oxidizes on standing or air exposure, darkening urine.
    causal_link_type: DIRECT
    evidence:
    - reference: ORPHA:56
      reference_title: "Alkaptonuria (Orphanet structured-database record)"
      supports: SUPPORT
      evidence_source: OTHER
      snippet: "causing urine to darken when exposed to air"
      explanation: Orphanet directly links HGA/BQA accumulation in urine to darkening on air exposure.
  - target: Ochronotic connective tissue degeneration
    description: Oxidized HGA-derived polymers deposit in collagen-rich tissues and cartilage.
    causal_link_type: DIRECT
    evidence:
    - reference: PMID:19862842
      reference_title: "Mutation spectrum of homogentisic acid oxidase (HGD) in alkaptonuria."
      supports: SUPPORT
      evidence_source: OTHER
      snippet: "The clinical findings of AKU result from the reaction of homogentisic acid and its homopolymeric oxidation products, i.e., benzoquinones, with connective tissue components."
      explanation: This supports the connective-tissue deposition mechanism downstream of oxidized HGA.
- name: Ochronotic connective tissue degeneration
  description: >
    Oxidized HGA-derived pigment deposits in collagen-rich connective tissues,
    especially cartilage. This ochronotic pigmentation is associated with
    visible tissue discoloration and painful axial and large-joint
    osteoarthropathy.
  locations:
  - preferred_term: cartilage tissue
    term:
      id: UBERON:0002418
      label: cartilage tissue
  - preferred_term: connective tissue
    term:
      id: UBERON:0002384
      label: connective tissue
  evidence:
  - reference: ORPHA:56
    reference_title: "Alkaptonuria (Orphanet structured-database record)"
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "various tissues (e.g. cartilage, connective tissue) and body fluids (urine, sweat), causing urine to darken when exposed to air as well as grey-blue coloration of the sclera and ear helix (ochronosis), and a disabling joint disease"
    explanation: Orphanet connects HGA tissue accumulation to ochronosis and disabling joint disease.
  - reference: PMID:38453957
    reference_title: "Alkaptonuria."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "pigment in collagen-rich connective tissues), and a painful and severe form of"
    explanation: Disease primer supports HGA-derived pigment deposition in connective tissue causing osteoarthropathy.
  - reference: PMID:19862842
    reference_title: "Mutation spectrum of homogentisic acid oxidase (HGD) in alkaptonuria."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "The clinical findings of AKU result from the reaction of homogentisic acid and its homopolymeric oxidation products, i.e., benzoquinones, with connective tissue components."
    explanation: Mechanistic statement links oxidized HGA products to connective-tissue clinical findings.
  downstream:
  - target: Ochronosis
    description: HGA-derived pigment deposition in connective tissue manifests as ochronosis.
    causal_link_type: DIRECT
    evidence:
    - reference: PMID:38453957
      reference_title: "Alkaptonuria."
      supports: SUPPORT
      evidence_source: OTHER
      snippet: "ochronosis (HGA-derived"
      explanation: Disease primer directly defines ochronosis as HGA-derived pigment in collagen-rich connective tissue.
  - target: Pigmentation of the sclera
    description: Ochronotic pigment can be visible as scleral pigmentation.
    causal_link_type: DIRECT
    evidence:
    - reference: ORPHA:56
      reference_title: "Alkaptonuria (Orphanet structured-database record)"
      supports: SUPPORT
      evidence_source: OTHER
      snippet: "grey-blue coloration of the sclera and ear helix (ochronosis)"
      explanation: Orphanet links ochronosis to scleral and ear-helix discoloration.
  - target: Ochronotic osteoarthritis
    description: Ochronotic connective-tissue degeneration produces disabling axial and peripheral joint disease.
    causal_link_type: DIRECT
    evidence:
    - reference: ORPHA:56
      reference_title: "Alkaptonuria (Orphanet structured-database record)"
      supports: SUPPORT
      evidence_source: OTHER
      snippet: "a disabling joint disease involving both the axial and peripheral joints (ochronotic arthropathy)"
      explanation: Orphanet directly links ochronotic tissue disease to axial and peripheral arthropathy.
    - reference: PMID:19862842
      reference_title: "Mutation spectrum of homogentisic acid oxidase (HGD) in alkaptonuria."
      supports: SUPPORT
      evidence_source: OTHER
      snippet: "patients later develop joint and spine arthritis in their thirties"
      explanation: This background passage links ochronotic tissue involvement to later joint and spine arthritis.
  - target: Arthralgia
    description: Pain arises as part of severe ochronotic osteoarthropathy.
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    intermediate_mechanisms:
    - ochronotic osteoarthropathy
    evidence:
    - reference: PMID:38453957
      reference_title: "Alkaptonuria."
      supports: SUPPORT
      evidence_source: OTHER
      snippet: "painful and severe form of"
      explanation: Disease primer links HGA-derived ochronosis to painful osteoarthropathy.
  - target: Joint stiffness
    description: Ochronotic arthropathy and cartilage degeneration produce joint stiffness.
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    intermediate_mechanisms:
    - ochronotic osteoarthropathy
    evidence:
    - reference: ORPHA:56
      reference_title: "Alkaptonuria (Orphanet structured-database record)"
      supports: SUPPORT
      evidence_source: OTHER
      snippet: "HP:0001387 | Joint stiffness | Very frequent (99-80%)"
      explanation: Orphanet lists joint stiffness as a very frequent musculoskeletal manifestation.
  - target: Joint swelling
    description: Ochronotic arthropathy can manifest as joint swelling.
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    intermediate_mechanisms:
    - ochronotic osteoarthropathy
    evidence:
    - reference: ORPHA:56
      reference_title: "Alkaptonuria (Orphanet structured-database record)"
      supports: SUPPORT
      evidence_source: OTHER
      snippet: "HP:0001386 | Joint swelling | Very frequent (99-80%)"
      explanation: Orphanet lists joint swelling as a very frequent musculoskeletal manifestation.
  - target: Joint dislocation
    description: Severe ochronotic joint disease can be associated with joint dislocation.
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    intermediate_mechanisms:
    - cartilage degeneration and ochronotic osteoarthropathy
    evidence:
    - reference: ORPHA:56
      reference_title: "Alkaptonuria (Orphanet structured-database record)"
      supports: SUPPORT
      evidence_source: OTHER
      snippet: "HP:0001373 | Joint dislocation | Very frequent (99-80%)"
      explanation: Orphanet lists joint dislocation as a very frequent musculoskeletal manifestation.
  - target: Back pain
    description: Axial ochronotic arthropathy involving the spine produces back pain.
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    intermediate_mechanisms:
    - spine ochronotic osteoarthropathy
    evidence:
    - reference: ORPHA:56
      reference_title: "Alkaptonuria (Orphanet structured-database record)"
      supports: SUPPORT
      evidence_source: OTHER
      snippet: "HP:0003418 | Back pain | Frequent (79-30%)"
      explanation: Orphanet lists back pain as a frequent axial musculoskeletal manifestation.
  - target: Intervertebral disk calcification
    description: Spinal connective-tissue degeneration includes intervertebral disk calcification.
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    intermediate_mechanisms:
    - spinal cartilage and disk degeneration
    evidence:
    - reference: ORPHA:56
      reference_title: "Alkaptonuria (Orphanet structured-database record)"
      supports: SUPPORT
      evidence_source: OTHER
      snippet: "HP:0005645 | Intervertebral disk calcification | Very frequent (99-80%)"
      explanation: Orphanet lists intervertebral disk calcification as a very frequent spinal finding.
  - target: Cartilage calcification
    description: Ochronotic cartilage degeneration includes calcification of cartilage.
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    intermediate_mechanisms:
    - cartilage ochronosis
    evidence:
    - reference: ORPHA:56
      reference_title: "Alkaptonuria (Orphanet structured-database record)"
      supports: SUPPORT
      evidence_source: OTHER
      snippet: "HP:0100593 | Calcification of cartilage | Very frequent (99-80%)"
      explanation: Orphanet lists cartilage calcification as a very frequent manifestation.
  - target: Cartilage destruction
    description: Pigment deposition and degeneration can destroy cartilage.
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    intermediate_mechanisms:
    - ochronotic cartilage degeneration
    evidence:
    - reference: ORPHA:56
      reference_title: "Alkaptonuria (Orphanet structured-database record)"
      supports: SUPPORT
      evidence_source: OTHER
      snippet: "HP:0100773 | Cartilage destruction | Frequent (79-30%)"
      explanation: Orphanet lists cartilage destruction as a frequent manifestation.
  - target: Tendon rupture
    description: Ochronotic involvement of collagen-rich tendon and ligament tissue increases rupture risk.
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    intermediate_mechanisms:
    - ochronotic tendon degeneration
    evidence:
    - reference: PMID:38453957
      reference_title: "Alkaptonuria."
      supports: SUPPORT
      evidence_source: OTHER
      snippet: "include kidney and prostate stones, aortic stenosis, bone fractures, and tendon,"
      explanation: Disease primer lists tendon, ligament, and muscle ruptures among alkaptonuria manifestations.
  - target: Thickened Achilles tendon
    description: Ochronotic tendon involvement can manifest as Achilles tendon thickening.
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    intermediate_mechanisms:
    - ochronotic tendon degeneration
    evidence:
    - reference: ORPHA:56
      reference_title: "Alkaptonuria (Orphanet structured-database record)"
      supports: SUPPORT
      evidence_source: OTHER
      snippet: "HP:0004690 | Thickened Achilles tendon | Frequent (79-30%)"
      explanation: Orphanet lists thickened Achilles tendon as a frequent tendon manifestation.
  - target: Hearing abnormality
    description: Ochronotic pigment in ear cartilage can be associated with hearing abnormalities.
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    intermediate_mechanisms:
    - ear cartilage ochronosis
    evidence:
    - reference: ORPHA:56
      reference_title: "Alkaptonuria (Orphanet structured-database record)"
      supports: SUPPORT
      evidence_source: OTHER
      snippet: "HP:0000364 | Hearing abnormality | Very frequent (99-80%)"
      explanation: Orphanet lists hearing abnormality as a very frequent manifestation.
  - target: Abnormality of vision
    description: Ocular ochronosis and scleral/corneal pigmentation can contribute to visual abnormalities.
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    intermediate_mechanisms:
    - ocular connective-tissue pigmentation
    evidence:
    - reference: ORPHA:56
      reference_title: "Alkaptonuria (Orphanet structured-database record)"
      supports: SUPPORT
      evidence_source: OTHER
      snippet: "HP:0000504 | Abnormality of vision | Very frequent (99-80%)"
      explanation: Orphanet lists abnormality of vision as a very frequent ocular manifestation.
  - target: Aortic valve calcification
    description: Ochronotic cardiovascular connective-tissue involvement can produce aortic valve calcification.
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    intermediate_mechanisms:
    - cardiovascular ochronosis and valve calcification
    evidence:
    - reference: PMID:20301627
      reference_title: "Alkaptonuria."
      supports: SUPPORT
      evidence_source: OTHER
      snippet: |-
        pigment in the sclera, ear cartilage, and skin of the hands; aortic or mitral
        valve calcification or regurgitation and occasionally aortic dilatation; renal
      explanation: GeneReviews lists aortic valve calcification among manifestations.
  - target: Mitral valve calcification
    description: Ochronotic cardiovascular connective-tissue involvement can produce mitral valve calcification.
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    intermediate_mechanisms:
    - cardiovascular ochronosis and valve calcification
    evidence:
    - reference: PMID:20301627
      reference_title: "Alkaptonuria."
      supports: SUPPORT
      evidence_source: OTHER
      snippet: |-
        pigment in the sclera, ear cartilage, and skin of the hands; aortic or mitral
        valve calcification or regurgitation and occasionally aortic dilatation; renal
      explanation: GeneReviews lists mitral valve calcification among manifestations.
  - target: Coronary artery calcification
    description: Later cardiovascular complications include coronary artery calcification.
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    intermediate_mechanisms:
    - cardiovascular ochronosis and calcification
    evidence:
    - reference: PMID:20301627
      reference_title: "Alkaptonuria."
      supports: SUPPORT
      evidence_source: OTHER
      snippet: "CT imaging to detect coronary artery calcification."
      explanation: GeneReviews surveillance guidance supports coronary artery calcification as a recognized manifestation.
phenotypes:
- name: Elevated urinary homogentisic acid
  frequency: VERY_FREQUENT
  description: Large urinary HGA excretion is the defining biochemical phenotype.
  phenotype_term:
    preferred_term: Elevated urinary homogentisic acid
    term:
      id: HP:0033704
      label: Elevated urinary homogentisic acid
  evidence:
  - reference: ORPHA:56
    reference_title: "Alkaptonuria (Orphanet structured-database record)"
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0033704 | Elevated urinary homogentisic acid | Very frequent (99-80%)"
    explanation: Orphanet reports elevated urinary homogentisic acid as very frequent.
  - reference: PMID:20301627
    reference_title: "Alkaptonuria."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "DIAGNOSIS/TESTING: The biochemical diagnosis of alkaptonuria in a proband is"
    explanation: GeneReviews identifies urinary HGA as the biochemical diagnostic marker.
- name: Dark urine
  frequency: FREQUENT
  description: Urine darkens on standing or exposure to air because HGA oxidizes.
  phenotype_term:
    preferred_term: Dark urine
    term:
      id: HP:0040319
      label: Dark urine
  evidence:
  - reference: ORPHA:56
    reference_title: "Alkaptonuria (Orphanet structured-database record)"
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0040319 | Dark urine | Frequent (79-30%)"
    explanation: Orphanet reports dark urine as frequent.
  - reference: PMID:20301627
    reference_title: "Alkaptonuria."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "features of alkaptonuria are dark urine or urine that turns dark on standing,"
    explanation: GeneReviews lists dark urine as a major feature.
- name: Ochronosis
  frequency: VERY_FREQUENT
  description: Bluish-black pigmentation of connective tissues is caused by HGA-derived pigment deposition.
  phenotype_term:
    preferred_term: Ochronosis
    term:
      id: HP:0030764
      label: Ochronosis
  evidence:
  - reference: ORPHA:56
    reference_title: "Alkaptonuria (Orphanet structured-database record)"
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0030764 | Ochronosis | Very frequent (99-80%)"
    explanation: Orphanet reports ochronosis as very frequent.
  - reference: PMID:38453957
    reference_title: "Alkaptonuria."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "debilitating disease whose main features are dark urine, ochronosis (HGA-derived"
    explanation: Disease primer identifies ochronosis as a main feature.
- name: Ochronotic osteoarthritis
  frequency: VERY_FREQUENT
  description: Progressive ochronotic arthropathy affects the spine and large joints.
  phenotype_term:
    preferred_term: Osteoarthritis
    term:
      id: HP:0002758
      label: Osteoarthritis
  evidence:
  - reference: ORPHA:56
    reference_title: "Alkaptonuria (Orphanet structured-database record)"
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0002758 | Osteoarthritis | Very frequent (99-80%)"
    explanation: Orphanet reports osteoarthritis as very frequent.
  - reference: PMID:20301627
    reference_title: "Alkaptonuria."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "the spine and larger joints."
    explanation: GeneReviews lists spine and large-joint arthritis as a major feature.
- name: Arthralgia
  frequency: VERY_FREQUENT
  description: Joint pain is a major manifestation of ochronotic osteoarthropathy.
  phenotype_term:
    preferred_term: Arthralgia
    term:
      id: HP:0002829
      label: Arthralgia
  evidence:
  - reference: ORPHA:56
    reference_title: "Alkaptonuria (Orphanet structured-database record)"
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0002829 | Arthralgia | Very frequent (99-80%)"
    explanation: Orphanet reports arthralgia as very frequent.
  - reference: PMID:38453957
    reference_title: "Alkaptonuria."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "painful and severe form of"
    explanation: Disease primer describes painful osteoarthropathy as a main feature.
- name: Joint stiffness
  frequency: VERY_FREQUENT
  description: Joint stiffness is reported as a very frequent musculoskeletal manifestation in alkaptonuria.
  phenotype_term:
    preferred_term: Joint stiffness
    term:
      id: HP:0001387
      label: Joint stiffness
  evidence:
  - reference: ORPHA:56
    reference_title: "Alkaptonuria (Orphanet structured-database record)"
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0001387 | Joint stiffness | Very frequent (99-80%)"
    explanation: Orphanet reports joint stiffness as very frequent.
- name: Joint swelling
  frequency: VERY_FREQUENT
  description: Joint swelling is reported as a very frequent musculoskeletal manifestation in alkaptonuria.
  phenotype_term:
    preferred_term: Joint swelling
    term:
      id: HP:0001386
      label: Joint swelling
  evidence:
  - reference: ORPHA:56
    reference_title: "Alkaptonuria (Orphanet structured-database record)"
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0001386 | Joint swelling | Very frequent (99-80%)"
    explanation: Orphanet reports joint swelling as very frequent.
- name: Joint dislocation
  frequency: VERY_FREQUENT
  description: Joint dislocation is reported among very frequent musculoskeletal manifestations in alkaptonuria.
  phenotype_term:
    preferred_term: Joint dislocation
    term:
      id: HP:0001373
      label: Joint dislocation
  evidence:
  - reference: ORPHA:56
    reference_title: "Alkaptonuria (Orphanet structured-database record)"
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0001373 | Joint dislocation | Very frequent (99-80%)"
    explanation: Orphanet reports joint dislocation as very frequent.
- name: Back pain
  frequency: FREQUENT
  description: Back pain is reported as a frequent axial musculoskeletal manifestation in alkaptonuria.
  phenotype_term:
    preferred_term: Back pain
    term:
      id: HP:0003418
      label: Back pain
  evidence:
  - reference: ORPHA:56
    reference_title: "Alkaptonuria (Orphanet structured-database record)"
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0003418 | Back pain | Frequent (79-30%)"
    explanation: Orphanet reports back pain as frequent.
- name: Intervertebral disk calcification
  frequency: VERY_FREQUENT
  description: Intervertebral disk calcification is reported as a very frequent spinal finding in alkaptonuria.
  phenotype_term:
    preferred_term: Intervertebral disk calcification
    term:
      id: HP:0005645
      label: Intervertebral disk calcification
  evidence:
  - reference: ORPHA:56
    reference_title: "Alkaptonuria (Orphanet structured-database record)"
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0005645 | Intervertebral disk calcification | Very frequent (99-80%)"
    explanation: Orphanet reports intervertebral disk calcification as very frequent.
- name: Cartilage calcification
  frequency: VERY_FREQUENT
  description: Cartilage calcification is reported as a very frequent structural manifestation in alkaptonuria.
  phenotype_term:
    preferred_term: Calcification of cartilage
    term:
      id: HP:0100593
      label: Calcification of cartilage
  evidence:
  - reference: ORPHA:56
    reference_title: "Alkaptonuria (Orphanet structured-database record)"
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0100593 | Calcification of cartilage | Very frequent (99-80%)"
    explanation: Orphanet reports cartilage calcification as very frequent.
- name: Cartilage destruction
  frequency: FREQUENT
  description: Cartilage destruction is reported as a frequent structural manifestation in alkaptonuria.
  phenotype_term:
    preferred_term: Cartilage destruction
    term:
      id: HP:0100773
      label: Cartilage destruction
  evidence:
  - reference: ORPHA:56
    reference_title: "Alkaptonuria (Orphanet structured-database record)"
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0100773 | Cartilage destruction | Frequent (79-30%)"
    explanation: Orphanet reports cartilage destruction as frequent.
- name: Tendon rupture
  frequency: FREQUENT
  description: Tendon rupture is reported as a frequent manifestation in alkaptonuria.
  phenotype_term:
    preferred_term: Tendon rupture
    term:
      id: HP:0100550
      label: Tendon rupture
  evidence:
  - reference: ORPHA:56
    reference_title: "Alkaptonuria (Orphanet structured-database record)"
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0100550 | Tendon rupture | Frequent (79-30%)"
    explanation: Orphanet reports tendon rupture as frequent.
  - reference: PMID:38453957
    reference_title: "Alkaptonuria."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "include kidney and prostate stones, aortic stenosis, bone fractures, and tendon,"
    explanation: Disease primer lists tendon, ligament, and muscle ruptures among variable manifestations.
- name: Thickened Achilles tendon
  frequency: FREQUENT
  description: Achilles tendon thickening is reported as a frequent tendon manifestation in alkaptonuria.
  phenotype_term:
    preferred_term: Thickened Achilles tendon
    term:
      id: HP:0004690
      label: Thickened Achilles tendon
  evidence:
  - reference: ORPHA:56
    reference_title: "Alkaptonuria (Orphanet structured-database record)"
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0004690 | Thickened Achilles tendon | Frequent (79-30%)"
    explanation: Orphanet reports thickened Achilles tendon as frequent.
- name: Aortic valve calcification
  frequency: FREQUENT
  description: Cardiac valve calcification is a recognized later complication.
  phenotype_term:
    preferred_term: Aortic valve calcification
    term:
      id: HP:0004380
      label: Aortic valve calcification
  evidence:
  - reference: ORPHA:56
    reference_title: "Alkaptonuria (Orphanet structured-database record)"
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0004380 | Aortic valve calcification | Frequent (79-30%)"
    explanation: Orphanet reports aortic valve calcification as frequent.
  - reference: PMID:20301627
    reference_title: "Alkaptonuria."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: |-
      pigment in the sclera, ear cartilage, and skin of the hands; aortic or mitral
      valve calcification or regurgitation and occasionally aortic dilatation; renal
    explanation: GeneReviews supports cardiac valve calcification as a manifestation.
- name: Coronary artery calcification
  frequency: VERY_FREQUENT
  description: Coronary artery calcification is a common cardiovascular complication.
  phenotype_term:
    preferred_term: Coronary artery calcification
    term:
      id: HP:0001717
      label: Coronary artery calcification
  evidence:
  - reference: ORPHA:56
    reference_title: "Alkaptonuria (Orphanet structured-database record)"
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0001717 | Coronary artery calcification | Very frequent (99-80%)"
    explanation: Orphanet reports coronary artery calcification as very frequent.
  - reference: PMID:20301627
    reference_title: "Alkaptonuria."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "CT imaging to detect coronary artery calcification."
    explanation: GeneReviews surveillance guidance supports coronary artery calcification as a recognized manifestation.
- name: Mitral valve calcification
  frequency: FREQUENT
  description: Mitral valve calcification is part of the later cardiac-valve phenotype.
  phenotype_term:
    preferred_term: Mitral valve calcification
    term:
      id: HP:0004382
      label: Mitral valve calcification
  evidence:
  - reference: ORPHA:56
    reference_title: "Alkaptonuria (Orphanet structured-database record)"
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0004382 | Mitral valve calcification | Frequent (79-30%)"
    explanation: Orphanet reports mitral valve calcification as frequent.
  - reference: PMID:20301627
    reference_title: "Alkaptonuria."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: |-
      pigment in the sclera, ear cartilage, and skin of the hands; aortic or mitral
      valve calcification or regurgitation and occasionally aortic dilatation; renal
    explanation: GeneReviews lists aortic or mitral valve calcification among manifestations.
- name: Nephrolithiasis
  frequency: FREQUENT
  description: Renal stones can occur as part of multisystem alkaptonuria.
  phenotype_term:
    preferred_term: Nephrolithiasis
    term:
      id: HP:0000787
      label: Nephrolithiasis
  evidence:
  - reference: ORPHA:56
    reference_title: "Alkaptonuria (Orphanet structured-database record)"
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0000787 | Nephrolithiasis | Frequent (79-30%)"
    explanation: Orphanet reports nephrolithiasis as frequent.
  - reference: PMID:20301627
    reference_title: "Alkaptonuria."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "surgical intervention for prostate stones and renal"
    explanation: GeneReviews lists renal stones among other manifestations.
- name: Hearing abnormality
  frequency: VERY_FREQUENT
  description: Hearing abnormalities are frequent in the multisystem ochronotic phenotype.
  phenotype_term:
    preferred_term: Hearing abnormality
    term:
      id: HP:0000364
      label: Hearing abnormality
  evidence:
  - reference: ORPHA:56
    reference_title: "Alkaptonuria (Orphanet structured-database record)"
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0000364 | Hearing abnormality | Very frequent (99-80%)"
    explanation: Orphanet reports hearing abnormality as very frequent.
- name: Abnormality of vision
  frequency: VERY_FREQUENT
  description: Eye involvement in alkaptonuria includes visual abnormalities and ochronotic pigmentation.
  phenotype_term:
    preferred_term: Abnormality of vision
    term:
      id: HP:0000504
      label: Abnormality of vision
  evidence:
  - reference: ORPHA:56
    reference_title: "Alkaptonuria (Orphanet structured-database record)"
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0000504 | Abnormality of vision | Very frequent (99-80%)"
    explanation: Orphanet reports abnormality of vision as very frequent.
- name: Pigmentation of the sclera
  frequency: FREQUENT
  description: Ochronotic pigment can be visible in the sclera.
  phenotype_term:
    preferred_term: Pigmentation of the sclera
    term:
      id: HP:0007832
      label: Pigmentation of the sclera
  evidence:
  - reference: ORPHA:56
    reference_title: "Alkaptonuria (Orphanet structured-database record)"
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0007832 | Pigmentation of the sclera | Frequent (79-30%)"
    explanation: Orphanet reports pigmentation of the sclera as frequent.
  - reference: PMID:20301627
    reference_title: "Alkaptonuria."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "pigment in the sclera, ear cartilage, and skin of the hands;"
    explanation: GeneReviews supports scleral pigmentation as a manifestation.
- name: Aminoaciduria
  frequency: VERY_FREQUENT
  description: >
    Orphanet annotates aminoaciduria as a very frequent broad urinary
    biochemical phenotype; the canonical and more specific abnormality is
    elevated urinary homogentisic acid.
  phenotype_term:
    preferred_term: Aminoaciduria
    term:
      id: HP:0003355
      label: Aminoaciduria
  evidence:
  - reference: ORPHA:56
    reference_title: "Alkaptonuria (Orphanet structured-database record)"
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0003355 | Aminoaciduria | Very frequent (99-80%)"
    explanation: Orphanet reports aminoaciduria as very frequent.
biochemical:
- name: Increased urinary homogentisic acid
  presence: INCREASED
  context: >
    Urinary HGA is substantially increased and is the primary biochemical
    diagnostic marker.
  biomarker_term:
    preferred_term: homogentisic acid
    term:
      id: CHEBI:44747
      label: homogentisic acid
  readouts:
  - target: Homogentisic acid accumulation and oxidation
    relationship: READOUT_OF
    direction: POSITIVE
    endpoint_context: DIAGNOSTIC
    interpretation: Elevated urinary HGA reports the upstream HGD block and systemic HGA accumulation.
    evidence:
    - reference: PMID:20301627
      reference_title: "Alkaptonuria."
      supports: SUPPORT
      evidence_source: OTHER
      snippet: "based on the detection of a significant amount of HGA in the urine (usually 1 to"
      explanation: GeneReviews identifies urinary HGA as the diagnostic biochemical readout of alkaptonuria.
  evidence:
  - reference: PMID:20301627
    reference_title: "Alkaptonuria."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "based on the detection of a significant amount of HGA in the urine (usually 1 to"
    explanation: GeneReviews provides the typical magnitude of urinary HGA excretion.
  - reference: ORPHA:56
    reference_title: "Alkaptonuria (Orphanet structured-database record)"
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0033704 | Elevated urinary homogentisic acid | Very frequent (99-80%)"
    explanation: Orphanet supports elevated urinary HGA as the dominant biochemical phenotype.
- name: Reduced homogentisate 1,2-dioxygenase activity
  presence: DECREASED
  context: >
    Reduced HGD enzyme activity is the proximal biochemical defect that impairs
    tyrosine catabolism.
  readouts:
  - target: HGD molecular function deficiency
    relationship: READOUT_OF
    direction: NEGATIVE
    endpoint_context: DIAGNOSTIC
    interpretation: Reduced homogentisate 1,2-dioxygenase activity directly reports HGD molecular function deficiency.
    evidence:
    - reference: PMID:20301627
      reference_title: "Alkaptonuria."
      supports: SUPPORT
      evidence_source: OTHER
      snippet: "Alkaptonuria is caused by deficiency of homogentisate"
      explanation: GeneReviews supports the enzyme activity deficit as the proximal HGD mechanism.
  evidence:
  - reference: PMID:20301627
    reference_title: "Alkaptonuria."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Alkaptonuria is caused by deficiency of homogentisate"
    explanation: GeneReviews identifies reduced HGD activity as the causal enzyme deficiency.
  - reference: ORPHA:56
    reference_title: "Alkaptonuria (Orphanet structured-database record)"
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HGD | homogentisate 1,2-dioxygenase | hgnc:4892 | Disease-causing germline mutation(s) (loss of function) in"
    explanation: Orphanet supports HGD loss of function as the biochemical defect.
genetic:
- name: HGD variants
  gene_term:
    preferred_term: HGD
    term:
      id: hgnc:4892
      label: HGD
  inheritance:
  - name: Autosomal recessive
    evidence:
    - reference: ORPHA:56
      reference_title: "Alkaptonuria (Orphanet structured-database record)"
      supports: SUPPORT
      evidence_source: OTHER
      snippet: "Autosomal recessive"
      explanation: Orphanet reports autosomal recessive inheritance.
    - reference: PMID:20301627
      reference_title: "Alkaptonuria."
      supports: SUPPORT
      evidence_source: OTHER
      snippet: "to family members) is based on identification of biallelic pathogenic variants"
      explanation: GeneReviews supports biallelic HGD variants as the molecular diagnostic basis.
  variants:
  - name: Biallelic HGD pathogenic variants
    description: >
      Reported pathogenic variants include missense, splice-site, frameshift,
      nonsense, no-stop, and larger deletion alleles. Residual HGD activity may
      influence urinary HGA adjusted for protein intake, but genotype does not
      robustly predict clinical symptoms.
    evidence:
    - reference: PMID:19862842
      reference_title: "Mutation spectrum of homogentisic acid oxidase (HGD) in alkaptonuria."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "missense, 13 splice site, 10 frameshift, 5 nonsense, and 1 no-stop mutation."
      explanation: Mutation-spectrum study summarizes variant classes associated with alkaptonuria.
    - reference: PMID:30737480
      reference_title: "Homogentisate 1,2-dioxygenase (HGD) gene variants, their analysis and genotype-phenotype correlations in the largest cohort of patients with AKU."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "we identified 28 novel variants of the HGD gene,"
      explanation: Large cohort expands the HGD variant spectrum and includes structural deletion alleles.
    - reference: PMID:30737480
      reference_title: "Homogentisate 1,2-dioxygenase (HGD) gene variants, their analysis and genotype-phenotype correlations in the largest cohort of patients with AKU."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "no difference in serum levels or absolute urinary"
      explanation: Supports cautious interpretation of genotype-phenotype correlation.
  features: >
    HGD encodes homogentisate 1,2-dioxygenase. Biallelic pathogenic variants
    produce HGA accumulation and the alkaptonuria phenotype.
  evidence:
  - reference: PMID:19862842
    reference_title: "Mutation spectrum of homogentisic acid oxidase (HGD) in alkaptonuria."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "AKU is due to mutations in the homogentisate"
    explanation: Confirms HGD variants as the genetic cause and connects them to the biochemical block.
  - reference: CGGV:assertion_5186836d-d9c6-4829-a0c9-59548460d6f2-2020-06-29T174125.541Z
    reference_title: "HGD / alkaptonuria (Definitive)"
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HGD | HGNC:4892 | alkaptonuria | MONDO:0008753 | AR | Definitive"
    explanation: ClinGen classifies the HGD-alkaptonuria gene-disease relationship as definitive with autosomal recessive inheritance.
treatments:
- name: Nitisinone therapy
  description: >
    Nitisinone inhibits an upstream tyrosine-degradation step to reduce HGA
    production. In SONIA 2, daily nitisinone markedly reduced urinary HGA and
    slowed clinical progression as measured by cAKUSSI.
  treatment_term:
    preferred_term: pharmacotherapy
    term:
      id: MAXO:0000058
      label: pharmacotherapy
    therapeutic_agent:
    - preferred_term: nitisinone
      term:
        id: CHEBI:50378
        label: nitisinone
  target_mechanisms:
  - target: Homogentisic acid accumulation and oxidation
    treatment_effect: INHIBITS
    description: Nitisinone reduces HGA production upstream of the HGD block.
    evidence:
    - reference: PMID:32822600
      reference_title: "Efficacy and safety of once-daily nitisinone for patients with alkaptonuria (SONIA 2): an international, multicentre, open-label, randomised controlled trial."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "the study. u-HGA24 at 12 months was significantly decreased by 99·7% in the"
      explanation: SONIA 2 directly supports HGA-lowering as the treatment mechanism.
  target_phenotypes:
  - preferred_term: Elevated urinary homogentisic acid
    term:
      id: HP:0033704
      label: Elevated urinary homogentisic acid
  - preferred_term: Ochronosis
    term:
      id: HP:0030764
      label: Ochronosis
  evidence:
  - reference: PMID:32822600
    reference_title: "Efficacy and safety of once-daily nitisinone for patients with alkaptonuria (SONIA 2): an international, multicentre, open-label, randomised controlled trial."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "INTERPRETATION: Nitisinone 10 mg daily was well tolerated and effective in"
    explanation: Randomized controlled trial supports nitisinone as HGA-lowering therapy.
  - reference: PMID:32822600
    reference_title: "Efficacy and safety of once-daily nitisinone for patients with alkaptonuria (SONIA 2): an international, multicentre, open-label, randomised controlled trial."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "clinical signs, indicating a slower disease progression."
    explanation: SONIA 2 supports clinical benefit beyond biochemical HGA reduction.
references:
- reference: ORPHA:56
  title: Alkaptonuria
  found_in:
  - Alkaptonuria-deep-research-cyberian-codex.md
  findings: []
- reference: PMID:20301627
  title: Alkaptonuria.
  tags:
  - GeneReviews
  found_in:
  - Alkaptonuria-deep-research-cyberian-codex.md
  findings: []
- reference: PMID:38453957
  title: Alkaptonuria.
  found_in:
  - Alkaptonuria-deep-research-cyberian-codex.md
  findings: []
- reference: PMID:19862842
  title: Mutation spectrum of homogentisic acid oxidase (HGD) in alkaptonuria.
  found_in:
  - Alkaptonuria-deep-research-cyberian-codex.md
  findings: []
- reference: PMID:30737480
  title: "Homogentisate 1,2-dioxygenase (HGD) gene variants, their analysis and genotype-phenotype correlations in the largest cohort of patients with AKU."
  found_in:
  - Alkaptonuria-deep-research-cyberian-codex.md
  findings: []
- reference: PMID:32822600
  title: "Efficacy and safety of once-daily nitisinone for patients with alkaptonuria (SONIA 2): an international, multicentre, open-label, randomised controlled trial."
  found_in:
  - Alkaptonuria-deep-research-cyberian-codex.md
  findings: []
- reference: PMID:12051967
  title: "Alkaptonuria in Slovakia: thirty-two years of research on phenotype and genotype."
  found_in:
  - Alkaptonuria-deep-research-cyberian-codex.md
  findings: []
notes: >-
  Curation emphasizes the direct Orphanet/MONDO disease mapping and the compact
  mechanistic chain from HGD loss of function to HGA accumulation, oxidation,
  ochronotic connective-tissue deposition, and adult-onset multisystem
  complications.
📚

References & Deep Research

References

7
ORPHA:56
Alkaptonuria
No top-level findings curated for this source.
PMID:20301627
Alkaptonuria.
No top-level findings curated for this source.
PMID:38453957
Alkaptonuria.
No top-level findings curated for this source.
PMID:19862842
Mutation spectrum of homogentisic acid oxidase (HGD) in alkaptonuria.
No top-level findings curated for this source.
PMID:30737480
Homogentisate 1,2-dioxygenase (HGD) gene variants, their analysis and genotype-phenotype correlations in the largest cohort of patients with AKU.
No top-level findings curated for this source.
PMID:32822600
Efficacy and safety of once-daily nitisinone for patients with alkaptonuria (SONIA 2): an international, multicentre, open-label, randomised controlled trial.
No top-level findings curated for this source.
PMID:12051967
Alkaptonuria in Slovakia: thirty-two years of research on phenotype and genotype.
No top-level findings curated for this source.

Deep Research

1
Cyberian Codex ▸
Evidence Basis
codex-local-synthesis 7 citations 2026-05-03T12:11:43Z

Evidence Basis

This local Codex synthesis uses the generated Orphanet structured record for ORPHA:56 and the PubMed caches integrated into the YAML. Falcon and OpenAI live provider attempts both timed out without artifacts, so the curated YAML is based on local review of the deterministic evidence caches listed below.

Core Disease Mechanism

  • Alkaptonuria maps directly to MONDO:0008753 and ORPHA:56.
  • Orphanet lists HGD as the disease-causing gene and records autosomal recessive inheritance.
  • HGD encodes homogentisate 1,2-dioxygenase, which normally converts homogentisic acid to maleylacetoacetic acid in tyrosine degradation.
  • Biallelic HGD pathogenic variants reduce this enzyme activity, causing accumulation of homogentisic acid in urine, body fluids, and tissues.
  • Oxidation of homogentisic acid generates benzoquinone-derived products that form melanin-like polymers and bind connective tissue components.
  • Ochronotic pigment deposition in collagen-rich tissues, especially cartilage, explains the characteristic ochronosis, cartilage and disk calcification, and progressive spine and large-joint osteoarthropathy.

Clinical Interpretation

  • Urinary homogentisic acid elevation is the highest-confidence biochemical phenotype and supports diagnosis.
  • Dark urine can appear early because homogentisic acid oxidizes on standing, whereas ochronosis and arthritis usually become prominent in adulthood.
  • The phenotype is multisystemic in later disease, with recognized cardiac valve calcification, renal or prostatic stones, and tendon or ligament involvement.
  • HGD variant classes are heterogeneous. Published cohorts support HGD as the causal gene but show limited genotype-to-clinical-phenotype prediction.

Treatment-Relevant Mechanism

  • Nitisinone acts upstream of HGA formation in tyrosine degradation and reduces urinary HGA. SONIA 2 showed a 99.7 percent reduction in urinary HGA at 12 months and slower clinical progression by cAKUSSI over 48 months.
  • This treatment targets HGA accumulation and downstream ochronosis, but it does not restore HGD enzymatic function; the YAML therefore models it as inhibiting the HGA accumulation node rather than correcting the genetic defect.

YAML Integration Notes

  • The pathophysiology chain is intentionally compact: HGD molecular-function deficiency, HGA accumulation and oxidation, and ochronotic connective-tissue degeneration.
  • Phenotypes are anchored primarily to Orphanet frequency annotations and reinforced with GeneReviews or recent review evidence where available.
  • Genetics are represented as biallelic HGD pathogenic variants with cautious genotype-phenotype interpretation.

Citation Inventory

  • ORPHA:56
  • PMID:20301627
  • PMID:38453957
  • PMID:19862842
  • PMID:30737480
  • PMID:32822600
  • PMID:12051967