Alkaptonuria

Mendelian MONDO:0008753 Pathograph 45 Show in embeddings browser 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.

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1
Mappings
1
Definitions
1
Inheritance
6
Pathophys.
30
Phenotypes
1
Gaps
45
Pathograph
1
Genes
1
Variants
6
Medical Actions
3
Datasets
1
Trials
1
Models
12
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. When both parents are carriers, each pregnancy has a 25% chance of an affected child, a 50% chance of an asymptomatic carrier, and a 25% chance of a child who is unaffected and not a carrier.
Autosomal recessive inheritance
Show evidence (3 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.
PMID:20301627 SUPPORT Other
"each sib of an affected individual has at conception a 25% chance of being affected, a 50% chance of being an asymptomatic carrier, and a 25% chance of being unaffected and not a carrier."
GeneReviews provides the autosomal-recessive recurrence risks used for genetic counseling.
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Discussions and Knowledge Gaps

1
Why does the Hgdaku/aku mouse reproduce systemic HGA accumulation and its pharmacodynamic response to nitisinone but fail to develop human ochronosis?
HUMAN MODEL MISMATCH OPEN alkaptonuria_hgd_mouse_ochronosis_mismatch
Attached to
pathophysiology#HGA oxidative polymerization and ochronotic pigment formation pathophysiology#Ochronotic connective tissue degeneration phenotypes#Ochronosis
The model is useful for HGA production and lowering, but absence of the defining pigment-deposition phenotype limits inference about connective-tissue degeneration and clinical disease modification. Comparative studies should identify the species-specific chemistry, matrix context, exposure duration, or protective pathways that prevent murine ochronosis.
Show evidence (1 reference)
PMID:19862842 SUPPORT Model Organism
"Hgdaku/aku mice have high levels of HGA in the urine and plasma but have no signs of ochronosis"
The cited biochemical–tissue mismatch motivates the translational knowledge gap.
⚙

Pathophysiology

6
HGD molecular function deficiency
Biallelic HGD pathogenic variants reduce homogentisate 1,2-dioxygenase activity, blocking the homogentisate step of the tyrosine degradation pathway.
HGD hgnc:4892 HUGO Gene Nomenclature Committee (hgnc) Relation: this pathophysiological event involves this gene This pathophysiological event involves HGD (hgnc:4892). hgnc:4892 is a gene from the HUGO Gene Nomenclature Committee.
L-tyrosine catabolic process GO:0006572 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves decreased L-tyrosine catabolic process (GO:0006572). GO:0006572 is a biological process from the Gene Ontology. ↓ DECREASED
homogentisate 1,2-dioxygenase activity GO:0004411 Gene Ontology (GO) Relation: this pathophysiological event involves this molecular function This pathophysiological event involves decreased homogentisate 1,2-dioxygenase activity (GO:0004411). GO:0004411 is a molecular function from the Gene Ontology. ↓ DECREASED
Downstream
Homogentisic acid accumulation 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
The HGD block causes homogentisic acid to accumulate in urine, body fluids, and tissues.
L-tyrosine catabolic process GO:0006572 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves decreased L-tyrosine catabolic process (GO:0006572). GO:0006572 is a biological process from the Gene Ontology. ↓ 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.
Nephrolithiasis Urinary tract involvement in alkaptonuria includes renal stones.
Dark urine Urinary HGA oxidizes on standing or air exposure, darkening urine.
HGA oxidative polymerization and ochronotic pigment formation Accumulated HGA undergoes oxidative coupling into ochronotic pigment polymers.
Oxidative stress in chondrocytes Chronic HGA exposure increases reactive oxygen species and mitochondrial superoxide in chondrocytes.
Prostatic calculus Prostatic calculi are a recognized urinary-tract manifestation of systemic HGA accumulation.
Prostatitis Prostatitis is reported in alkaptonuria, although its mechanistic relationship to HGA and prostatic stones is unresolved.
Hypothyroidism Hypothyroidism is an associated endocrine manifestation; its connection to the HGA pathway is not mechanistically established.
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 Human Clinical
"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.
HGA oxidative polymerization and ochronotic pigment formation
HGA slowly polymerizes at physiological pH through semiquinone-mediated oxidative coupling, producing heterogeneous, negatively charged pigment polymers with phenolic ether and biphenyl linkages.
Downstream
Ochronotic connective tissue degeneration HGA-derived pigment deposits in collagen-rich connective tissues and cartilage.
Show evidence (2 references)
PMID:41096940 SUPPORT In Vitro
"At physiological pH, HGA polymerised slowly, while alkaline catalysis accelerated pigment formation while retaining the HGA aromatic scaffold."
Direct NMR and EPR experiments define the chemistry of HGA pigment formation.
PMID:41096940 SUPPORT In Vitro
"Pigments displayed a polydisperse molecular weight range (11-50 kDa) and a strong negative charge. Solid-state NMR has revealed the presence of phenolic ether and biphenyl linkages."
NMR and electrophoretic characterization support the curated pigment structural properties.
Oxidative stress in chondrocytes
Chronic HGA exposure drives reactive oxygen species, lipid peroxidation, and mitochondrial superoxide in chondrocytes.
chondrocyte CL:0000138 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves chondrocyte (CL:0000138). CL:0000138 is a cell type from the Cell Ontology.
response to oxidative stress GO:0006979 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves increased response to oxidative stress (GO:0006979). GO:0006979 is a biological process from the Gene Ontology. ↑ INCREASED
Downstream
Autophagy-lysosomal dysfunction in chondrocytes Prolonged HGA-associated oxidative stress disrupts autophagic flux and lysosomal function.
Show evidence (1 reference)
PMID:42510554 SUPPORT In Vitro
"HGA treatment induced a time-dependent increase in oxidative stress, evidenced by elevated ROS levels, 4-HNE accumulation, and overproduction of mitochondrial superoxide."
Human chondrocyte experiments directly demonstrate HGA-induced oxidative stress.
Autophagy-lysosomal dysfunction in chondrocytes
Prolonged HGA exposure produces persistent p62 accumulation, reduced LC3/LAMP1 colocalization, altered acidic compartments, and lysosomal failure, consistent with disrupted autophagic flux.
chondrocyte CL:0000138 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves chondrocyte (CL:0000138). CL:0000138 is a cell type from the Cell Ontology.
autophagy GO:0006914 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves dysregulated autophagy (GO:0006914). GO:0006914 is a biological process from the Gene Ontology. ↕ DYSREGULATED
Downstream
Ochronotic connective tissue degeneration Collapse of chondrocyte autophagy-lysosomal homeostasis may contribute to cartilage degeneration.
Show evidence (1 reference)
PMID:42510554 SUPPORT In Vitro
"However, prolonged HGA exposure was associated with reduced LC3/LAMP1 colocalization, persistent p62 accumulation, altered acidic compartment staining, and accumulation of autophagy-related structures, supporting a dysregulation of the autophagy-lysosomal pathway."
Multiple autophagy and lysosome readouts support pathway dysfunction after prolonged HGA exposure.
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 UBERON:0002418 Uberon multi-species anatomy ontology (UBERON) Relation: this pathophysiological event occurs in this anatomical location This pathophysiological event occurs in cartilage tissue (UBERON:0002418). UBERON:0002418 is an anatomical location from the Uberon multi-species anatomy ontology. connective tissue UBERON:0002384 Uberon multi-species anatomy ontology (UBERON) Relation: this pathophysiological event occurs in this anatomical location This pathophysiological event occurs in connective tissue (UBERON:0002384). UBERON:0002384 is an anatomical location from the Uberon multi-species anatomy ontology.
Downstream
Ochronosis HGA-derived pigment deposition in connective tissue manifests as ochronosis.
Pigmentation of the sclera Ochronotic pigment can be visible as scleral pigmentation.
Abnormality of skin pigmentation Ochronotic pigment deposition produces visible skin and connective-tissue pigmentation abnormalities.
Irregular hyperpigmentation Ochronotic pigment deposition can manifest as irregular hyperpigmentation.
Oil-drop brown pigmentation of the corneal limbus Ocular ochronotic pigmentation can involve the corneal limbus.
Ochronotic osteoarthritis Ochronotic connective-tissue degeneration produces disabling axial and peripheral joint disease.
Arthritis Ochronotic arthropathy includes arthritis of axial and peripheral joints.
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.
Aortic valve stenosis Ochronotic cardiovascular involvement can progress from valve calcification to aortic stenosis.
Abnormal heart valve morphology Ochronotic cardiovascular involvement can produce structural heart-valve abnormalities.
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 Human Clinical
"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.
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Pathograph

Use the checkboxes to hide or show graph categories. Hover nodes for evidence and cross-linked metadata.
Pathograph: causal mechanism network for 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.
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Phenotypes

30
Cardiovascular 2
Aortic valve stenosis FREQUENT HP:0001650 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Aortic valve stenosis (HP:0001650). HP:0001650 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
ORPHA:56 SUPPORT Other
"HP:0001650 | Aortic valve stenosis | Frequent (79-30%)"
Orphanet reports aortic valve stenosis as frequent.
Abnormal heart valve morphology FREQUENT HP:0001654 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Abnormal heart valve morphology (HP:0001654). HP:0001654 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
ORPHA:56 SUPPORT Other
"HP:0001654 | Abnormal heart valve morphology | Frequent (79-30%)"
Orphanet reports abnormal heart valve morphology as frequent.
Endocrine 1
Hypothyroidism OCCASIONAL HP:0000821 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Hypothyroidism (HP:0000821). HP:0000821 is a phenotype from the Human Phenotype Ontology.
Show evidence (2 references)
ORPHA:56 SUPPORT Other
"HP:0000821 | Hypothyroidism | Occasional (29-5%)"
Orphanet reports hypothyroidism as occasional.
PMID:20301627 SUPPORT Other
"stones; prostate stones; and hypothyroidism."
GeneReviews includes hypothyroidism among other clinical manifestations.
Genitourinary 2
Dark urine FREQUENT HP:0040319 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Dark urine (HP:0040319). HP:0040319 is a phenotype from the Human Phenotype Ontology.
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 HP:0000787 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Nephrolithiasis (HP:0000787). HP:0000787 is a phenotype from the Human Phenotype Ontology.
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.
Integument 1
Abnormality of skin pigmentation VERY_FREQUENT HP:0001000 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Abnormality of skin pigmentation (HP:0001000). HP:0001000 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
ORPHA:56 SUPPORT Other
"HP:0001000 | Abnormality of skin pigmentation | Very frequent (99-80%)"
Orphanet reports abnormal skin pigmentation as very frequent.
Metabolism 1
Joint swelling VERY_FREQUENT HP:0001386 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Joint swelling (HP:0001386). HP:0001386 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
ORPHA:56 SUPPORT Other
"HP:0001386 | Joint swelling | Very frequent (99-80%)"
Orphanet reports joint swelling as very frequent.
Musculoskeletal 4
Ochronotic osteoarthritis VERY_FREQUENT HP:0002758 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Osteoarthritis (HP:0002758). HP:0002758 is a phenotype from the Human Phenotype Ontology.
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 HP:0001387 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Joint stiffness (HP:0001387). HP:0001387 is a phenotype from the Human Phenotype Ontology.
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 HP:0001373 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Joint dislocation (HP:0001373). HP:0001373 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
ORPHA:56 SUPPORT Other
"HP:0001373 | Joint dislocation | Very frequent (99-80%)"
Orphanet reports joint dislocation as very frequent.
Arthritis VERY_FREQUENT HP:0001369 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Arthritis (HP:0001369). HP:0001369 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
ORPHA:56 SUPPORT Other
"HP:0001369 | Arthritis | Very frequent (99-80%)"
Orphanet reports arthritis as very frequent.
Constitutional 2
Arthralgia VERY_FREQUENT HP:0002829 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Arthralgia (HP:0002829). HP:0002829 is a phenotype from the Human Phenotype Ontology.
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 HP:0003418 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Back pain (HP:0003418). HP:0003418 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
ORPHA:56 SUPPORT Other
"HP:0003418 | Back pain | Frequent (79-30%)"
Orphanet reports back pain as frequent.
Other 17
Elevated urinary homogentisic acid VERY_FREQUENT HP:0033704 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Elevated urinary homogentisic acid (HP:0033704). HP:0033704 is a phenotype from the Human Phenotype Ontology.
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 HP:0030764 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Ochronosis (HP:0030764). HP:0030764 is a phenotype from the Human Phenotype Ontology.
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 HP:0005645 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Intervertebral disk calcification (HP:0005645). HP:0005645 is a phenotype from the Human Phenotype Ontology.
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 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Calcification of cartilage (HP:0100593). HP:0100593 is a phenotype from the Human Phenotype Ontology.
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 HP:0100773 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Cartilage destruction (HP:0100773). HP:0100773 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
ORPHA:56 SUPPORT Other
"HP:0100773 | Cartilage destruction | Frequent (79-30%)"
Orphanet reports cartilage destruction as frequent.
Tendon rupture FREQUENT HP:0100550 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Tendon rupture (HP:0100550). HP:0100550 is a phenotype from the Human Phenotype Ontology.
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 HP:0004690 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Thickened Achilles tendon (HP:0004690). HP:0004690 is a phenotype from the Human Phenotype Ontology.
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 HP:0004380 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Aortic valve calcification (HP:0004380). HP:0004380 is a phenotype from the Human Phenotype Ontology.
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 HP:0001717 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Coronary artery calcification (HP:0001717). HP:0001717 is a phenotype from the Human Phenotype Ontology.
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 HP:0004382 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Mitral valve calcification (HP:0004382). HP:0004382 is a phenotype from the Human Phenotype Ontology.
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.
Prostatic calculus OCCASIONAL HP:0034882 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Prostatic calculus (HP:0034882). HP:0034882 is a phenotype from the Human Phenotype Ontology.
Show evidence (2 references)
ORPHA:56 SUPPORT Other
"HP:0034882 | Prostatic calculus | Occasional (29-5%)"
Orphanet reports prostatic calculus as occasional.
PMID:20301627 SUPPORT Other
"stones; prostate stones; and hypothyroidism."
GeneReviews lists prostate stones among other clinical manifestations.
Prostatitis FREQUENT HP:0000024 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Prostatitis (HP:0000024). HP:0000024 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
ORPHA:56 SUPPORT Other
"HP:0000024 | Prostatitis | Frequent (79-30%)"
Orphanet reports prostatitis as frequent; no specific mechanism is asserted.
Hearing abnormality VERY_FREQUENT HP:0000364 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Hearing abnormality (HP:0000364). HP:0000364 is a phenotype from the Human Phenotype Ontology.
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 HP:0000504 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Abnormality of vision (HP:0000504). HP:0000504 is a phenotype from the Human Phenotype Ontology.
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 HP:0007832 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Pigmentation of the sclera (HP:0007832). HP:0007832 is a phenotype from the Human Phenotype Ontology.
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.
Irregular hyperpigmentation VERY_FREQUENT HP:0007400 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Irregular hyperpigmentation (HP:0007400). HP:0007400 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
ORPHA:56 SUPPORT Other
"HP:0007400 | Irregular hyperpigmentation | Very frequent (99-80%)"
Orphanet reports irregular hyperpigmentation as very frequent.
Oil-drop brown pigmentation of the corneal limbus FREQUENT HP:6000027 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Oil-drop brown pigmentation of the corneal limbus (HP:6000027). HP:6000027 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
ORPHA:56 SUPPORT Other
"HP:6000027 | Oil-drop brown pigmentation of the corneal limbus | Frequent (79-30%)"
Orphanet reports oil-drop brown pigmentation of the corneal limbus as frequent.
🧬

Genetic Associations

1
HGD variants
Gene: HGD hgnc:4892 HUGO Gene Nomenclature Committee (hgnc) Relation: this disease-associated gene is this gene This disease-associated gene is HGD (hgnc:4892). hgnc:4892 is a gene from the HUGO Gene Nomenclature Committee.
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.
Variants (1)
Biallelic HGD pathogenic variants
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.
Show evidence (3 references)
PMID:19862842 SUPPORT Human Clinical
"missense, 13 splice site, 10 frameshift, 5 nonsense, and 1 no-stop mutation."
Mutation-spectrum study summarizes variant classes associated with alkaptonuria.
PMID:30737480 SUPPORT Human Clinical
"we identified 28 novel variants of the HGD gene,"
Large cohort expands the HGD variant spectrum and includes structural deletion alleles.
PMID:30737480 SUPPORT Human Clinical
"no difference in serum levels or absolute urinary"
Supports cautious interpretation of genotype-phenotype correlation.
🗃️

External Assertions

1
Orphanet Alkaptonuria disease record
Orphanet structured disease record ORPHA:56
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.
Show evidence (1 reference)
ORPHA:56 SUPPORT Other
"MONDO:0008753 | Exact"
Orphanet maps ORPHA:56 to the same MONDO identifier used by this entry.
💊

Medical Actions

6
Nitisinone therapy
Action: PharmacotherapyNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is Pharmacotherapy (NCIT:C15986). NCIT:C15986 is a clinical intervention from the NCI Thesaurus. NCIT:C15986
Agent: nitisinone CHEBI:50378 Chemical Entities of Biological Interest (CHEBI) Relation: this treatment uses this therapeutic agent This treatment uses nitisinone (CHEBI:50378). CHEBI:50378 is a therapeutic agent from Chemical Entities of Biological Interest.
Nitisinone inhibits an upstream tyrosine-degradation step to reduce HGA production. In SONIA 2, daily nitisinone markedly reduced urinary HGA and slowed composite clinical progression as measured by cAKUSSI. It increases serum tyrosine and can cause corneal keratopathy; a later arthroplasty analysis did not find a reduction in incident joint replacement.
Mechanism Target:
INHIBITS Homogentisic acid accumulation — 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 HP:0033704 Human Phenotype Ontology (HP) Relation: this treatment targets this phenotype This treatment targets Elevated urinary homogentisic acid (HP:0033704). HP:0033704 is a phenotype from the Human Phenotype Ontology. Ochronosis HP:0030764 Human Phenotype Ontology (HP) Relation: this treatment targets this phenotype This treatment targets Ochronosis (HP:0030764). HP:0030764 is a phenotype from the Human Phenotype Ontology.
Show evidence (4 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.
PMID:38846518 SUPPORT Human Clinical
"The incidence of arthroplasty was earlier and more frequent after the first JR and was not affected by nitisinone."
This cohort analysis limits the broad treatment claim because nitisinone did not reduce incident arthroplasty.
+ 1 more reference
Physical and occupational therapy
Action: physical therapyNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is physical therapy (NCIT:C15302). NCIT:C15302 is a clinical intervention from the NCI Thesaurus. Ontology label: Physical Therapy NCIT:C15302
Individualized physical and occupational therapy supports muscle strength, flexibility, mobility, and adaptation to progressive ochronotic arthropathy.
Target Phenotypes: Osteoarthritis HP:0002758 Human Phenotype Ontology (HP) Relation: this treatment targets this phenotype This treatment targets Osteoarthritis (HP:0002758). HP:0002758 is a phenotype from the Human Phenotype Ontology. Joint stiffness HP:0001387 Human Phenotype Ontology (HP) Relation: this treatment targets this phenotype This treatment targets Joint stiffness (HP:0001387). HP:0001387 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:20301627 SUPPORT Other
"physical and occupational therapy help promote optimal muscle strength and flexibility;"
GeneReviews recommends physical and occupational therapy for manifestation-directed management.
Manifestation-directed surgery
Action: surgical procedureNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is surgical procedure (NCIT:C15329). NCIT:C15329 is a clinical intervention from the NCI Thesaurus. Ontology label: Surgical Procedure NCIT:C15329
Advanced disease may require knee, hip, or shoulder replacement, removal of renal or prostate stones, or aortic valve replacement for severe stenosis.
Target Phenotypes: Osteoarthritis HP:0002758 Human Phenotype Ontology (HP) Relation: this treatment targets this phenotype This treatment targets Osteoarthritis (HP:0002758). HP:0002758 is a phenotype from the Human Phenotype Ontology. Nephrolithiasis HP:0000787 Human Phenotype Ontology (HP) Relation: this treatment targets this phenotype This treatment targets Nephrolithiasis (HP:0000787). HP:0000787 is a phenotype from the Human Phenotype Ontology. Prostatic calculus HP:0034882 Human Phenotype Ontology (HP) Relation: this treatment targets this phenotype This treatment targets Prostatic calculus (HP:0034882). HP:0034882 is a phenotype from the Human Phenotype Ontology. Aortic valve stenosis HP:0001650 Human Phenotype Ontology (HP) Relation: this treatment targets this phenotype This treatment targets Aortic valve stenosis (HP:0001650). HP:0001650 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:20301627 SUPPORT Other
"knee, hip, and shoulder replacements are options when needed; surgical intervention for prostate stones and renal stones as needed; aortic stenosis may necessitate valve replacement;"
GeneReviews lists the principal surgical interventions for advanced musculoskeletal, urinary, and valvular disease.
Thyroid hormone replacement
Action: PharmacotherapyNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is Pharmacotherapy (NCIT:C15986). NCIT:C15986 is a clinical intervention from the NCI Thesaurus. NCIT:C15986
Agent: levothyroxine CHEBI:18332 Chemical Entities of Biological Interest (CHEBI) Relation: this treatment uses this therapeutic agent This treatment uses levothyroxine, annotated with L-thyroxine (CHEBI:18332). CHEBI:18332 is a therapeutic agent from Chemical Entities of Biological Interest.
Thyroid hormone replacement is used when hypothyroidism occurs in alkaptonuria.
Target Phenotypes: Hypothyroidism HP:0000821 Human Phenotype Ontology (HP) Relation: this treatment targets this phenotype This treatment targets Hypothyroidism (HP:0000821). HP:0000821 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:20301627 SUPPORT Other
"aortic stenosis may necessitate valve replacement; thyroid hormone replacement."
GeneReviews explicitly includes thyroid hormone replacement in manifestation-directed care.
Low-protein dietary management during nitisinone therapy
Action: dietary interventionNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is dietary intervention (NCIT:C15447). NCIT:C15447 is a clinical intervention from the NCI Thesaurus. Ontology label: Dietary Intervention NCIT:C15447
A low-protein diet, sometimes supplemented with a low-tyrosine protein substitute, is used to manage nitisinone-induced tyrosinemia and reduce the risk of corneal keratopathy; it is supportive management rather than an HGD-correcting therapy.
Show evidence (2 references)
PMID:39290064 SUPPORT Human Clinical
"cGMP protein substitute is a palatable and well-tolerated option in the dietary management of AKU patients with NTBC-induced tyrosinaemia."
A prospective adult study supports a protein substitute as an option for managing nitisinone-induced tyrosinemia.
PMID:39290064 SUPPORT Human Clinical
"Adherence to dietary management of NTBC-induced tyrosinemia, a low-protein diet with or without protein substitutes, can be difficult for patients."
The study identifies low-protein dietary management, with or without a substitute, as the intervention used for treatment-induced tyrosinemia.
Joint-protective activity modification
Action: rehabilitationNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is rehabilitation (NCIT:C15315). NCIT:C15315 is a clinical intervention from the NCI Thesaurus. Ontology label: Rehabilitation NCIT:C15315
Avoiding heavy manual labor, high-impact sports, and other excessive physical stress on the spine and large joints may help limit progression of severe ochronotic arthritis.
Target Phenotypes: Osteoarthritis HP:0002758 Human Phenotype Ontology (HP) Relation: this treatment targets this phenotype This treatment targets Osteoarthritis (HP:0002758). HP:0002758 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:20301627 SUPPORT Other
"Agents/circumstances to avoid: Physical stress to the spine and large joints (including heavy manual labor or high-impact sports) to try to reduce progression of severe arthritis."
GeneReviews recommends joint-protective activity modification to try to slow severe arthritis progression.
🔬

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 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.
🔬

Diagnosis

4
Urinary homogentisic acid testing
The biochemical diagnosis is established by detecting a substantial amount of homogentisic acid in urine, typically one to eight grams per day.
laboratory procedure NCIT:C25294 NCI Thesaurus (NCIT)
Markers: Urinary homogentisic acid.
Results: Markedly elevated urinary HGA, typically 1-8 grams per day.
Show evidence (1 reference)
PMID:20301627 SUPPORT Other
"The biochemical diagnosis of alkaptonuria in a proband is based on the detection of a significant amount of HGA in the urine (usually 1 to 8 grams per day)."
GeneReviews defines the primary biochemical diagnostic criterion and typical excretion range.
HGD molecular genetic testing
Identification of biallelic pathogenic variants in HGD establishes the molecular diagnosis and enables carrier, prenatal, and preimplantation genetic testing in a family.
molecular analysis NCIT:C19770 NCI Thesaurus (NCIT)
Results: Biallelic pathogenic variants in HGD.
Show evidence (2 references)
PMID:20301627 SUPPORT Other
"The molecular diagnosis (needed to provide genetic counseling to family members) is based on identification of biallelic pathogenic variants in HGD."
GeneReviews specifies biallelic HGD variants as the molecular diagnostic criterion.
PMID:20301627 SUPPORT Other
"When both HGD pathogenic variants in the family are known, carrier testing for at-risk relatives and prenatal/preimplantation genetic testing are possible."
GeneReviews supports the family testing options described for a molecularly confirmed diagnosis.
Cardiovascular surveillance by echocardiography
From age 40 years, echocardiography is used to monitor for aortic dilatation, aortic or mitral valve calcification, and stenosis.
echocardiography NCIT:C16525 NCI Thesaurus (NCIT)
Results: Aortic dilatation, valve calcification, regurgitation, or stenosis.
Show evidence (1 reference)
PMID:20301627 SUPPORT Other
"In individuals older than age 40 years, echocardiography to detect aortic dilatation, aortic or mitral valve calcification, and stenosis."
GeneReviews specifies both the surveillance age and echocardiographic targets.
Thyroid function surveillance
Thyroid function should be assessed at diagnosis and monitored every one to two years because hypothyroidism is a treatable manifestation.
thyroid function testing NCIT:C25294 NCI Thesaurus (NCIT)
Results: Biochemical evidence of hypothyroidism.
Show evidence (1 reference)
PMID:20301627 SUPPORT Other
"Assess thyroid function at the time of initial diagnosis, and monitor every 1-2 years thereafter."
GeneReviews provides the initial and longitudinal thyroid-testing schedule.
📈

Progression

1
Biochemical onset with adult multisystem complications
Homogentisic aciduria can be detected from infancy by darkening urine, while ochronosis and disabling arthropathy typically become clinically prominent in adulthood.
Show evidence (3 references)
ORPHA:56 SUPPORT Other
"Age of onset: Infancy"
Orphanet records infancy among disease onset categories.
ORPHA:56 SUPPORT Other
"Age of onset: Adult"
Orphanet records adult onset among disease onset categories.
PMID:20301627 SUPPORT Other
"Ochronosis generally occurs after age 30 years;"
GeneReviews supports delayed adult emergence of ochronosis and arthritis.
📊

Prevalence

1
Worldwide
Point Prevalence 0.1–0.9 per 100,000 1–9 per 1,000,000
Orphanet records worldwide point prevalence in the one-to-nine per million range, with higher founder-effect prevalence reported in Slovakia.
Show evidence (2 references)
ORPHA:56 SUPPORT Other
"1-9 / 1 000 000 | Worldwide | Point prevalence | EXPERT"
Orphanet reports worldwide point prevalence in the one-to-nine per million range.
PMID:12051967 SUPPORT Human Clinical
"world-wide highest incidence of AKU (1 in 19,000) was recorded"
Slovak screening data document a high-prevalence founder population.
📊

Related Datasets

3
Impact of nitisinone on the cerebrospinal fluid metabolome of a murine model of alkaptonuria metabolomics_workbench:ST002179
mouse METABOLOMICS
Located via OmicsDI, which aggregates across omics repositories; this record comes from metabolomics_workbench. Only repositories with no other discovery route in this project and with a working accession resolver are curated from OmicsDI -- GEO, ArrayExpress, PRIDE, MetaboLights and EGA hits are excluded as duplicates of dedicated passes. Matched because the disease is named in the dataset's own title ("Alkaptonuria"). Retrieved 2026-08-02.
Comprehensive biotransformation analysis of phenylalanine-tyrosine metabolism reveals alternative routes of metabolite clearance in nitisinone-treated alkaptonuria (Urine metabolomic analysis) metabolomics_workbench:ST002297
Located via OmicsDI, which aggregates across omics repositories; this record comes from metabolomics_workbench. Only repositories with no other discovery route in this project and with a working accession resolver are curated from OmicsDI -- GEO, ArrayExpress, PRIDE, MetaboLights and EGA hits are excluded as duplicates of dedicated passes. Matched because the disease is named in the dataset's own title ("Alkaptonuria"). Retrieved 2026-08-02.
Comprehensive biotransformation analysis of phenylalanine-tyrosine metabolism reveals alternative routes of metabolite clearance in nitisinone-treated alkaptonuria (Serum metabolomic analysis) metabolomics_workbench:ST002296
Located via OmicsDI, which aggregates across omics repositories; this record comes from metabolomics_workbench. Only repositories with no other discovery route in this project and with a working accession resolver are curated from OmicsDI -- GEO, ArrayExpress, PRIDE, MetaboLights and EGA hits are excluded as duplicates of dedicated passes. Matched because the disease is named in the dataset's own title ("Alkaptonuria"). Retrieved 2026-08-02.
🔬

Clinical Trials

1
NCT01916382 PHASE_III COMPLETED
SONIA 2 was the international randomized evaluator-blind, no-treatment controlled phase III study of once-daily nitisinone in adults with alkaptonuria, followed for 48 months.
Target Phenotypes: Elevated urinary homogentisic acid HP:0033704 Human Phenotype Ontology (HP) Relation: this clinical trial targets this phenotype This clinical trial targets Elevated urinary homogentisic acid (HP:0033704). HP:0033704 is a phenotype from the Human Phenotype Ontology. Ochronosis HP:0030764 Human Phenotype Ontology (HP) Relation: this clinical trial targets this phenotype This clinical trial targets Ochronosis (HP:0030764). HP:0030764 is a phenotype from the Human Phenotype Ontology.
Show evidence (2 references)
clinicaltrials:NCT01916382 SUPPORT Human Clinical
"we are now ready for this final stage of clinical development of nitisinone for AKU: a phase 3 clinical trial to prove efficacy."
ClinicalTrials.gov identifies the SONIA 2 nitisinone study as phase III clinical development for alkaptonuria.
PMID:32822600 SUPPORT Human Clinical
"SONIA 2 was a 4-year, open-label, evaluator-blind, randomised, no treatment controlled, parallel-group study"
The trial publication documents the 4-year design and randomized evaluator-blind control structure.
🐁

Animal Models

1
Homozygous Hgd c.1006+2T>A splice-site variant (Hgdaku/aku) Mus musculus
The ENU-derived Hgdaku/aku mouse has markedly elevated plasma and urinary HGA, and urinary HGA falls with nitisinone, but the model does not develop ochronosis; this limits direct extrapolation to the defining human tissue pathology.
Elevated urinary and plasma homogentisic acid Absence of ochronosis
Species
Mus musculus
Genotype
Homozygous Hgd c.1006+2T>A splice-site variant (Hgdaku/aku)
Show evidence (2 references)
PMID:19862842 SUPPORT Model Organism
"Hgdaku/aku mice have high levels of HGA in the urine and plasma but have no signs of ochronosis"
The mouse reproduces systemic HGA accumulation but not the hallmark ochronotic tissue phenotype.
PMID:19862842 SUPPORT Model Organism
"A trial of nitisinone in this mouse model showed a significant reduction of HGA excretion in the urine"
The model demonstrates pharmacodynamic lowering of urinary HGA with nitisinone.
{ }

Source YAML

click to show 
name: Alkaptonuria
category: Mendelian
creation_date: '2026-05-03T00:00:00Z'
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"
    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"
    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. When both
    parents are carriers, each pregnancy has a 25% chance of an affected child,
    a 50% chance of an asymptomatic carrier, and a 25% chance of a child who is
    unaffected and not a carrier.
  inheritance_term:
    preferred_term: Autosomal recessive inheritance
    term:
      id: HP:0000007
      label: Autosomal recessive inheritance
  evidence:
  - reference: ORPHA:56
    reference_title: "Alkaptonuria"
    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.
  - reference: PMID:20301627
    reference_title: "Alkaptonuria."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "each sib of an affected individual has at conception a 25% chance of being affected, a 50% chance of being an asymptomatic carrier, and a 25% chance of being unaffected and not a carrier."
    explanation: GeneReviews provides the autosomal-recessive recurrence risks used for genetic counseling.
prevalence:
- population: Worldwide
  measure_type: POINT_PREVALENCE
  prevalence_class: BAND_1_9_PER_1000000
  rate_low: 0.1
  rate_high: 0.9
  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"
    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"
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Age of onset: Infancy"
    explanation: Orphanet records infancy among disease onset categories.
  - reference: ORPHA:56
    reference_title: "Alkaptonuria"
    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
  biological_scale: MOLECULAR
  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"
    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: Homogentisic acid accumulation
    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: HUMAN_CLINICAL
      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
  biological_scale: MOLECULAR
  description: >
    The HGD block causes homogentisic acid to accumulate in urine, body fluids,
    and tissues.
  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"
    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: HUMAN_CLINICAL
    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"
      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: 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"
      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: HGA oxidative polymerization and ochronotic pigment formation
    description: Accumulated HGA undergoes oxidative coupling into ochronotic pigment polymers.
    causal_link_type: DIRECT
    evidence:
    - reference: PMID:41096940
      reference_title: "Ochronotic Deposition in Alkaptonuria: Semiquinone-Mediated Oxidative Coupling and Metabolic Drivers of Homogentisic Acid Accumulation."
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: "During the process, EPR detected a semiquinone radical intermediate, consistent with an oxidative coupling mechanism."
      explanation: Biophysical experiments identify semiquinone-mediated oxidative coupling during HGA polymerization.
  - target: Oxidative stress in chondrocytes
    description: Chronic HGA exposure increases reactive oxygen species and mitochondrial superoxide in chondrocytes.
    causal_link_type: DIRECT
    evidence:
    - reference: PMID:42510554
      reference_title: "HGA-Induced Oxidative Stress Impairs Autophagy via Lysosomal Dysfunction in Alkaptonuria."
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: "HGA treatment induced a time-dependent increase in oxidative stress, evidenced by elevated ROS levels, 4-HNE accumulation, and overproduction of mitochondrial superoxide."
      explanation: HGA-treated human chondrocytes show direct oxidative-stress readouts.
  - target: Prostatic calculus
    description: Prostatic calculi are a recognized urinary-tract manifestation of systemic HGA accumulation.
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    evidence:
    - reference: ORPHA:56
      reference_title: "Alkaptonuria"
      supports: SUPPORT
      evidence_source: OTHER
      snippet: "HP:0034882 | Prostatic calculus | Occasional (29-5%)"
      explanation: Orphanet supports the disease association and frequency, while the intermediate mechanism remains uncertain.
  - target: Prostatitis
    description: Prostatitis is reported in alkaptonuria, although its mechanistic relationship to HGA and prostatic stones is unresolved.
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    evidence:
    - reference: ORPHA:56
      reference_title: "Alkaptonuria"
      supports: SUPPORT
      evidence_source: OTHER
      snippet: "HP:0000024 | Prostatitis | Frequent (79-30%)"
      explanation: Orphanet supports the disease association and frequency but does not establish a causal intermediate.
  - target: Hypothyroidism
    description: Hypothyroidism is an associated endocrine manifestation; its connection to the HGA pathway is not mechanistically established.
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    evidence:
    - reference: PMID:20301627
      reference_title: "Alkaptonuria."
      supports: SUPPORT
      evidence_source: OTHER
      snippet: "stones; prostate stones; and hypothyroidism."
      explanation: GeneReviews identifies hypothyroidism as a manifestation but does not specify a causal mechanism.
    - reference: ORPHA:56
      reference_title: "Alkaptonuria"
      supports: SUPPORT
      evidence_source: OTHER
      snippet: "HP:0000821 | Hypothyroidism | Occasional (29-5%)"
      explanation: Orphanet independently supports an occasional disease association.
- name: HGA oxidative polymerization and ochronotic pigment formation
  biological_scale: MOLECULAR
  description: >
    HGA slowly polymerizes at physiological pH through semiquinone-mediated
    oxidative coupling, producing heterogeneous, negatively charged pigment
    polymers with phenolic ether and biphenyl linkages.
  chemical_entities:
  - preferred_term: homogentisic acid
    term:
      id: CHEBI:44747
      label: homogentisic acid
  evidence:
  - reference: PMID:41096940
    reference_title: "Ochronotic Deposition in Alkaptonuria: Semiquinone-Mediated Oxidative Coupling and Metabolic Drivers of Homogentisic Acid Accumulation."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "At physiological pH, HGA polymerised slowly, while alkaline catalysis accelerated pigment formation while retaining the HGA aromatic scaffold."
    explanation: Direct NMR and EPR experiments define the chemistry of HGA pigment formation.
  - reference: PMID:41096940
    reference_title: "Ochronotic Deposition in Alkaptonuria: Semiquinone-Mediated Oxidative Coupling and Metabolic Drivers of Homogentisic Acid Accumulation."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Pigments displayed a polydisperse molecular weight range (11-50 kDa) and a strong negative charge. Solid-state NMR has revealed the presence of phenolic ether and biphenyl linkages."
    explanation: NMR and electrophoretic characterization support the curated pigment structural properties.
  downstream:
  - target: Ochronotic connective tissue degeneration
    description: HGA-derived pigment deposits in collagen-rich connective tissues and cartilage.
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    intermediate_mechanisms:
    - ochronotic pigment binding and deposition in connective tissue
    evidence:
    - reference: PMID:19862842
      reference_title: "Mutation spectrum of homogentisic acid oxidase (HGD) in alkaptonuria."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      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: Human disease background links oxidized HGA polymers to connective-tissue injury.
- name: Oxidative stress in chondrocytes
  biological_scale: CELLULAR
  mechanism_confidence: PROVISIONAL
  description: >
    Chronic HGA exposure drives reactive oxygen species, lipid peroxidation, and
    mitochondrial superoxide in chondrocytes.
  cell_types:
  - preferred_term: chondrocyte
    term:
      id: CL:0000138
      label: chondrocyte
  biological_processes:
  - preferred_term: response to oxidative stress
    term:
      id: GO:0006979
      label: response to oxidative stress
    modifier: INCREASED
  evidence:
  - reference: PMID:42510554
    reference_title: "HGA-Induced Oxidative Stress Impairs Autophagy via Lysosomal Dysfunction in Alkaptonuria."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "HGA treatment induced a time-dependent increase in oxidative stress, evidenced by elevated ROS levels, 4-HNE accumulation, and overproduction of mitochondrial superoxide."
    explanation: Human chondrocyte experiments directly demonstrate HGA-induced oxidative stress.
  downstream:
  - target: Autophagy-lysosomal dysfunction in chondrocytes
    description: Prolonged HGA-associated oxidative stress disrupts autophagic flux and lysosomal function.
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    intermediate_mechanisms:
    - chronic HGA exposure and failure of adaptive autophagic flux
    evidence:
    - reference: PMID:42510554
      reference_title: "HGA-Induced Oxidative Stress Impairs Autophagy via Lysosomal Dysfunction in Alkaptonuria."
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: "Live-cell imaging further supported a transition from functional autophagy to lysosomal failure under chronic oxidative stress."
      explanation: Live-cell imaging links chronic oxidative stress with transition to lysosomal failure.
- name: Autophagy-lysosomal dysfunction in chondrocytes
  biological_scale: CELLULAR
  mechanism_confidence: PROVISIONAL
  description: >
    Prolonged HGA exposure produces persistent p62 accumulation, reduced
    LC3/LAMP1 colocalization, altered acidic compartments, and lysosomal failure,
    consistent with disrupted autophagic flux.
  cell_types:
  - preferred_term: chondrocyte
    term:
      id: CL:0000138
      label: chondrocyte
  biological_processes:
  - preferred_term: autophagy
    term:
      id: GO:0006914
      label: autophagy
    modifier: DYSREGULATED
  evidence:
  - reference: PMID:42510554
    reference_title: "HGA-Induced Oxidative Stress Impairs Autophagy via Lysosomal Dysfunction in Alkaptonuria."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "However, prolonged HGA exposure was associated with reduced LC3/LAMP1 colocalization, persistent p62 accumulation, altered acidic compartment staining, and accumulation of autophagy-related structures, supporting a dysregulation of the autophagy-lysosomal pathway."
    explanation: Multiple autophagy and lysosome readouts support pathway dysfunction after prolonged HGA exposure.
  downstream:
  - target: Ochronotic connective tissue degeneration
    description: Collapse of chondrocyte autophagy-lysosomal homeostasis may contribute to cartilage degeneration.
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    intermediate_mechanisms:
    - chondrocyte degeneration and cartilage damage
    evidence:
    - reference: PMID:42510554
      reference_title: "HGA-Induced Oxidative Stress Impairs Autophagy via Lysosomal Dysfunction in Alkaptonuria."
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: "The progressive collapse of these adaptive mechanisms may contribute to chondrocyte degeneration and to the pathogenesis of cartilage damage in AKU."
      explanation: The study proposes, but does not prove in vivo, that autophagy-lysosomal collapse contributes to cartilage damage.
- name: Ochronotic connective tissue degeneration
  biological_scale: TISSUE
  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"
    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: HUMAN_CLINICAL
    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"
      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: Abnormality of skin pigmentation
    description: Ochronotic pigment deposition produces visible skin and connective-tissue pigmentation abnormalities.
    causal_link_type: DIRECT
    evidence:
    - reference: ORPHA:56
      reference_title: "Alkaptonuria"
      supports: SUPPORT
      evidence_source: OTHER
      snippet: "HP:0001000 | Abnormality of skin pigmentation | Very frequent (99-80%)"
      explanation: Orphanet lists abnormal skin pigmentation as a very frequent manifestation.
  - target: Irregular hyperpigmentation
    description: Ochronotic pigment deposition can manifest as irregular hyperpigmentation.
    causal_link_type: DIRECT
    evidence:
    - reference: ORPHA:56
      reference_title: "Alkaptonuria"
      supports: SUPPORT
      evidence_source: OTHER
      snippet: "HP:0007400 | Irregular hyperpigmentation | Very frequent (99-80%)"
      explanation: Orphanet lists irregular hyperpigmentation as a very frequent manifestation.
  - target: Oil-drop brown pigmentation of the corneal limbus
    description: Ocular ochronotic pigmentation can involve the corneal limbus.
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    intermediate_mechanisms:
    - ocular connective-tissue pigmentation
    evidence:
    - reference: ORPHA:56
      reference_title: "Alkaptonuria"
      supports: SUPPORT
      evidence_source: OTHER
      snippet: "HP:6000027 | Oil-drop brown pigmentation of the corneal limbus | Frequent (79-30%)"
      explanation: Orphanet lists oil-drop brown pigmentation of the corneal limbus as frequent.
  - 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"
      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: HUMAN_CLINICAL
      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: Arthritis
    description: Ochronotic arthropathy includes arthritis of axial and peripheral joints.
    causal_link_type: DIRECT
    evidence:
    - reference: ORPHA:56
      reference_title: "Alkaptonuria"
      supports: SUPPORT
      evidence_source: OTHER
      snippet: "HP:0001369 | Arthritis | Very frequent (99-80%)"
      explanation: Orphanet lists arthritis as a very frequent manifestation.
  - 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"
      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"
      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"
      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"
      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"
      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"
      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"
      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"
      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"
      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"
      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: Aortic valve stenosis
    description: Ochronotic cardiovascular involvement can progress from valve calcification to aortic stenosis.
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    intermediate_mechanisms:
    - aortic valve calcification
    evidence:
    - reference: ORPHA:56
      reference_title: "Alkaptonuria"
      supports: SUPPORT
      evidence_source: OTHER
      snippet: "HP:0001650 | Aortic valve stenosis | Frequent (79-30%)"
      explanation: Orphanet lists aortic valve stenosis as frequent.
  - target: Abnormal heart valve morphology
    description: Ochronotic cardiovascular involvement can produce structural heart-valve abnormalities.
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    intermediate_mechanisms:
    - cardiovascular ochronosis and valve calcification
    evidence:
    - reference: ORPHA:56
      reference_title: "Alkaptonuria"
      supports: SUPPORT
      evidence_source: OTHER
      snippet: "HP:0001654 | Abnormal heart valve morphology | Frequent (79-30%)"
      explanation: Orphanet lists abnormal heart valve morphology as frequent.
  - 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"
    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"
    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"
    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"
    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"
    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"
    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"
    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"
    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"
    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"
    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"
    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"
    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"
    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"
    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"
    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"
    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"
    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"
    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: Prostatic calculus
  frequency: OCCASIONAL
  description: Prostate stones are an occasional genitourinary manifestation of alkaptonuria.
  phenotype_term:
    preferred_term: Prostatic calculus
    term:
      id: HP:0034882
      label: Prostatic calculus
  evidence:
  - reference: ORPHA:56
    reference_title: "Alkaptonuria"
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0034882 | Prostatic calculus | Occasional (29-5%)"
    explanation: Orphanet reports prostatic calculus as occasional.
  - reference: PMID:20301627
    reference_title: "Alkaptonuria."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "stones; prostate stones; and hypothyroidism."
    explanation: GeneReviews lists prostate stones among other clinical manifestations.
- name: Prostatitis
  frequency: FREQUENT
  description: Prostatitis is a frequent structured Orphanet annotation for alkaptonuria.
  phenotype_term:
    preferred_term: Prostatitis
    term:
      id: HP:0000024
      label: Prostatitis
  evidence:
  - reference: ORPHA:56
    reference_title: "Alkaptonuria"
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0000024 | Prostatitis | Frequent (79-30%)"
    explanation: Orphanet reports prostatitis as frequent; no specific mechanism is asserted.
- name: Hypothyroidism
  frequency: OCCASIONAL
  description: Hypothyroidism is an occasional, treatable endocrine manifestation requiring surveillance.
  phenotype_term:
    preferred_term: Hypothyroidism
    term:
      id: HP:0000821
      label: Hypothyroidism
  evidence:
  - reference: ORPHA:56
    reference_title: "Alkaptonuria"
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0000821 | Hypothyroidism | Occasional (29-5%)"
    explanation: Orphanet reports hypothyroidism as occasional.
  - reference: PMID:20301627
    reference_title: "Alkaptonuria."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "stones; prostate stones; and hypothyroidism."
    explanation: GeneReviews includes hypothyroidism among other clinical 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"
    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"
    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"
    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: Abnormality of skin pigmentation
  frequency: VERY_FREQUENT
  description: >
    Ochronotic pigment deposition produces visible skin and connective-tissue
    pigmentation abnormalities.
  phenotype_term:
    preferred_term: Abnormality of skin pigmentation
    term:
      id: HP:0001000
      label: Abnormality of skin pigmentation
  evidence:
  - reference: ORPHA:56
    reference_title: "Alkaptonuria"
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0001000 | Abnormality of skin pigmentation | Very frequent (99-80%)"
    explanation: Orphanet reports abnormal skin pigmentation as very frequent.
- name: Irregular hyperpigmentation
  frequency: VERY_FREQUENT
  description: >
    Irregular hyperpigmentation is a very frequent pigmentation manifestation in
    Orphanet's alkaptonuria phenotype table.
  phenotype_term:
    preferred_term: Irregular hyperpigmentation
    term:
      id: HP:0007400
      label: Irregular hyperpigmentation
  evidence:
  - reference: ORPHA:56
    reference_title: "Alkaptonuria"
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0007400 | Irregular hyperpigmentation | Very frequent (99-80%)"
    explanation: Orphanet reports irregular hyperpigmentation as very frequent.
- name: Oil-drop brown pigmentation of the corneal limbus
  frequency: FREQUENT
  description: >
    Ocular ochronosis can manifest as oil-drop brown pigmentation of the
    corneal limbus.
  phenotype_term:
    preferred_term: Oil-drop brown pigmentation of the corneal limbus
    term:
      id: HP:6000027
      label: Oil-drop brown pigmentation of the corneal limbus
  evidence:
  - reference: ORPHA:56
    reference_title: "Alkaptonuria"
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:6000027 | Oil-drop brown pigmentation of the corneal limbus | Frequent (79-30%)"
    explanation: Orphanet reports oil-drop brown pigmentation of the corneal limbus as frequent.
- name: Arthritis
  frequency: VERY_FREQUENT
  description: >
    Ochronotic arthropathy includes arthritis of axial and peripheral joints.
  phenotype_term:
    preferred_term: Arthritis
    term:
      id: HP:0001369
      label: Arthritis
  evidence:
  - reference: ORPHA:56
    reference_title: "Alkaptonuria"
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0001369 | Arthritis | Very frequent (99-80%)"
    explanation: Orphanet reports arthritis as very frequent.
- name: Aortic valve stenosis
  frequency: FREQUENT
  description: >
    Aortic valve stenosis is a frequent cardiovascular manifestation in
    Orphanet's alkaptonuria phenotype table.
  phenotype_term:
    preferred_term: Aortic valve stenosis
    term:
      id: HP:0001650
      label: Aortic valve stenosis
  evidence:
  - reference: ORPHA:56
    reference_title: "Alkaptonuria"
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0001650 | Aortic valve stenosis | Frequent (79-30%)"
    explanation: Orphanet reports aortic valve stenosis as frequent.
- name: Abnormal heart valve morphology
  frequency: FREQUENT
  description: >
    Ochronotic cardiovascular involvement can produce structural heart-valve
    abnormalities.
  phenotype_term:
    preferred_term: Abnormal heart valve morphology
    term:
      id: HP:0001654
      label: Abnormal heart valve morphology
  evidence:
  - reference: ORPHA:56
    reference_title: "Alkaptonuria"
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "HP:0001654 | Abnormal heart valve morphology | Frequent (79-30%)"
    explanation: Orphanet reports abnormal heart valve morphology as 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
    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"
    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"
    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"
      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.
diagnosis:
- name: Urinary homogentisic acid testing
  description: >
    The biochemical diagnosis is established by detecting a substantial amount
    of homogentisic acid in urine, typically one to eight grams per day.
  diagnosis_term:
    preferred_term: laboratory procedure
    term:
      id: NCIT:C25294
      label: Laboratory Procedure
  markers: Urinary homogentisic acid.
  results: Markedly elevated urinary HGA, typically 1-8 grams per day.
  evidence:
  - reference: PMID:20301627
    reference_title: "Alkaptonuria."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "The biochemical diagnosis of alkaptonuria in a proband is based on the detection of a significant amount of HGA in the urine (usually 1 to 8 grams per day)."
    explanation: GeneReviews defines the primary biochemical diagnostic criterion and typical excretion range.
- name: HGD molecular genetic testing
  description: >
    Identification of biallelic pathogenic variants in HGD establishes the
    molecular diagnosis and enables carrier, prenatal, and preimplantation
    genetic testing in a family.
  diagnosis_term:
    preferred_term: molecular analysis
    term:
      id: NCIT:C19770
      label: Molecular Analysis
  results: Biallelic pathogenic variants in HGD.
  evidence:
  - reference: PMID:20301627
    reference_title: "Alkaptonuria."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "The molecular diagnosis (needed to provide genetic counseling to family members) is based on identification of biallelic pathogenic variants in HGD."
    explanation: GeneReviews specifies biallelic HGD variants as the molecular diagnostic criterion.
  - reference: PMID:20301627
    reference_title: "Alkaptonuria."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "When both HGD pathogenic variants in the family are known, carrier testing for at-risk relatives and prenatal/preimplantation genetic testing are possible."
    explanation: GeneReviews supports the family testing options described for a molecularly confirmed diagnosis.
- name: Cardiovascular surveillance by echocardiography
  description: >
    From age 40 years, echocardiography is used to monitor for aortic dilatation,
    aortic or mitral valve calcification, and stenosis.
  diagnosis_term:
    preferred_term: echocardiography
    term:
      id: NCIT:C16525
      label: Echocardiography Test
  results: Aortic dilatation, valve calcification, regurgitation, or stenosis.
  evidence:
  - reference: PMID:20301627
    reference_title: "Alkaptonuria."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "In individuals older than age 40 years, echocardiography to detect aortic dilatation, aortic or mitral valve calcification, and stenosis."
    explanation: GeneReviews specifies both the surveillance age and echocardiographic targets.
- name: Thyroid function surveillance
  description: >
    Thyroid function should be assessed at diagnosis and monitored every one to
    two years because hypothyroidism is a treatable manifestation.
  diagnosis_term:
    preferred_term: thyroid function testing
    term:
      id: NCIT:C25294
      label: Laboratory Procedure
  results: Biochemical evidence of hypothyroidism.
  evidence:
  - reference: PMID:20301627
    reference_title: "Alkaptonuria."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Assess thyroid function at the time of initial diagnosis, and monitor every 1-2 years thereafter."
    explanation: GeneReviews provides the initial and longitudinal thyroid-testing schedule.
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 composite clinical progression as measured by cAKUSSI. It increases
    serum tyrosine and can cause corneal keratopathy; a later arthroplasty
    analysis did not find a reduction in incident joint replacement.
  therapeutic_modality: SMALL_MOLECULE
  treatment_term:
    preferred_term: Pharmacotherapy
    term:
      id: NCIT:C15986
      label: Pharmacotherapy
    therapeutic_agent:
    - preferred_term: nitisinone
      term:
        id: CHEBI:50378
        label: nitisinone
  target_mechanisms:
  - target: Homogentisic acid accumulation
    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.
  - reference: PMID:38846518
    reference_title: "Joint replacement risk is markedly increased in alkaptonuria (AKU) in those with prior arthroplasty."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The incidence of arthroplasty was earlier and more frequent after the first JR and was not affected by nitisinone."
    explanation: This cohort analysis limits the broad treatment claim because nitisinone did not reduce incident arthroplasty.
  - reference: PMID:39290064
    reference_title: "Evaluation of a casein glycomacropeptide-based protein substitute, in the dietary management of NTBC-induced tyrosinaemia in patients with alkaptonuria: A prospective open-label study."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "NTBC treatment of alkaptonuria (AKU) leads to increased blood tyrosine levels, causing skin issues and potentially sight-threatening corneal keratopathy."
    explanation: Prospective dietary-management research documents clinically important nitisinone-induced tyrosinemia and keratopathy risk.
- name: Physical and occupational therapy
  description: >
    Individualized physical and occupational therapy supports muscle strength,
    flexibility, mobility, and adaptation to progressive ochronotic arthropathy.
  therapeutic_modality: BEHAVIORAL
  treatment_term:
    preferred_term: physical therapy
    term:
      id: NCIT:C15302
      label: Physical Therapy
  target_phenotypes:
  - preferred_term: Osteoarthritis
    term:
      id: HP:0002758
      label: Osteoarthritis
  - preferred_term: Joint stiffness
    term:
      id: HP:0001387
      label: Joint stiffness
  evidence:
  - reference: PMID:20301627
    reference_title: "Alkaptonuria."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "physical and occupational therapy help promote optimal muscle strength and flexibility;"
    explanation: GeneReviews recommends physical and occupational therapy for manifestation-directed management.
- name: Manifestation-directed surgery
  description: >
    Advanced disease may require knee, hip, or shoulder replacement, removal of
    renal or prostate stones, or aortic valve replacement for severe stenosis.
  therapeutic_modality: SURGERY
  treatment_term:
    preferred_term: surgical procedure
    term:
      id: NCIT:C15329
      label: Surgical Procedure
  target_phenotypes:
  - preferred_term: Osteoarthritis
    term:
      id: HP:0002758
      label: Osteoarthritis
  - preferred_term: Nephrolithiasis
    term:
      id: HP:0000787
      label: Nephrolithiasis
  - preferred_term: Prostatic calculus
    term:
      id: HP:0034882
      label: Prostatic calculus
  - preferred_term: Aortic valve stenosis
    term:
      id: HP:0001650
      label: Aortic valve stenosis
  evidence:
  - reference: PMID:20301627
    reference_title: "Alkaptonuria."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "knee, hip, and shoulder replacements are options when needed; surgical intervention for prostate stones and renal stones as needed; aortic stenosis may necessitate valve replacement;"
    explanation: GeneReviews lists the principal surgical interventions for advanced musculoskeletal, urinary, and valvular disease.
- name: Thyroid hormone replacement
  description: Thyroid hormone replacement is used when hypothyroidism occurs in alkaptonuria.
  therapeutic_modality: SMALL_MOLECULE
  treatment_term:
    preferred_term: Pharmacotherapy
    term:
      id: NCIT:C15986
      label: Pharmacotherapy
    therapeutic_agent:
    - preferred_term: levothyroxine
      term:
        id: CHEBI:18332
        label: L-thyroxine
  target_phenotypes:
  - preferred_term: Hypothyroidism
    term:
      id: HP:0000821
      label: Hypothyroidism
  evidence:
  - reference: PMID:20301627
    reference_title: "Alkaptonuria."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "aortic stenosis may necessitate valve replacement; thyroid hormone replacement."
    explanation: GeneReviews explicitly includes thyroid hormone replacement in manifestation-directed care.
- name: Low-protein dietary management during nitisinone therapy
  description: >
    A low-protein diet, sometimes supplemented with a low-tyrosine protein
    substitute, is used to manage nitisinone-induced tyrosinemia and reduce the
    risk of corneal keratopathy; it is supportive management rather than an
    HGD-correcting therapy.
  therapeutic_modality: BEHAVIORAL
  treatment_term:
    preferred_term: dietary intervention
    term:
      id: NCIT:C15447
      label: Dietary Intervention
  evidence:
  - reference: PMID:39290064
    reference_title: "Evaluation of a casein glycomacropeptide-based protein substitute, in the dietary management of NTBC-induced tyrosinaemia in patients with alkaptonuria: A prospective open-label study."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "cGMP protein substitute is a palatable and well-tolerated option in the dietary management of AKU patients with NTBC-induced tyrosinaemia."
    explanation: A prospective adult study supports a protein substitute as an option for managing nitisinone-induced tyrosinemia.
  - reference: PMID:39290064
    reference_title: "Evaluation of a casein glycomacropeptide-based protein substitute, in the dietary management of NTBC-induced tyrosinaemia in patients with alkaptonuria: A prospective open-label study."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Adherence to dietary management of NTBC-induced tyrosinemia, a low-protein diet with or without protein substitutes, can be difficult for patients."
    explanation: The study identifies low-protein dietary management, with or without a substitute, as the intervention used for treatment-induced tyrosinemia.
- name: Joint-protective activity modification
  description: >
    Avoiding heavy manual labor, high-impact sports, and other excessive physical
    stress on the spine and large joints may help limit progression of severe
    ochronotic arthritis.
  therapeutic_modality: BEHAVIORAL
  treatment_term:
    preferred_term: rehabilitation
    term:
      id: NCIT:C15315
      label: Rehabilitation
  target_phenotypes:
  - preferred_term: Osteoarthritis
    term:
      id: HP:0002758
      label: Osteoarthritis
  evidence:
  - reference: PMID:20301627
    reference_title: "Alkaptonuria."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Agents/circumstances to avoid: Physical stress to the spine and large joints (including heavy manual labor or high-impact sports) to try to reduce progression of severe arthritis."
    explanation: GeneReviews recommends joint-protective activity modification to try to slow severe arthritis progression.
animal_models:
- species: Mus musculus
  genotype: Homozygous Hgd c.1006+2T>A splice-site variant (Hgdaku/aku)
  description: >
    The ENU-derived Hgdaku/aku mouse has markedly elevated plasma and urinary HGA,
    and urinary HGA falls with nitisinone, but the model does not develop ochronosis;
    this limits direct extrapolation to the defining human tissue pathology.
  associated_phenotypes:
  - Elevated urinary and plasma homogentisic acid
  - Absence of ochronosis
  evidence:
  - reference: PMID:19862842
    reference_title: "Mutation spectrum of homogentisic acid oxidase (HGD) in alkaptonuria."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "Hgdaku/aku mice have high levels of HGA in the urine and plasma but have no signs of ochronosis"
    explanation: The mouse reproduces systemic HGA accumulation but not the hallmark ochronotic tissue phenotype.
  - reference: PMID:19862842
    reference_title: "Mutation spectrum of homogentisic acid oxidase (HGD) in alkaptonuria."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "A trial of nitisinone in this mouse model showed a significant reduction of HGA excretion in the urine"
    explanation: The model demonstrates pharmacodynamic lowering of urinary HGA with nitisinone.
discussions:
- discussion_id: alkaptonuria_hgd_mouse_ochronosis_mismatch
  kind: HUMAN_MODEL_MISMATCH
  status: OPEN
  prompt: >
    Why does the Hgdaku/aku mouse reproduce systemic HGA accumulation and its
    pharmacodynamic response to nitisinone but fail to develop human ochronosis?
  rationale: >
    The model is useful for HGA production and lowering, but absence of the
    defining pigment-deposition phenotype limits inference about
    connective-tissue degeneration and clinical disease modification.
    Comparative studies should identify the species-specific chemistry, matrix context, exposure
    duration, or protective pathways that prevent murine ochronosis.
  attaches_to:
  - pathophysiology#HGA oxidative polymerization and ochronotic pigment formation
  - pathophysiology#Ochronotic connective tissue degeneration
  - phenotypes#Ochronosis
  evidence:
  - reference: PMID:19862842
    reference_title: "Mutation spectrum of homogentisic acid oxidase (HGD) in alkaptonuria."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "Hgdaku/aku mice have high levels of HGA in the urine and plasma but have no signs of ochronosis"
    explanation: The cited biochemical–tissue mismatch motivates the translational knowledge gap.
clinical_trials:
- name: NCT01916382
  phase: PHASE_III
  status: COMPLETED
  description: >
    SONIA 2 was the international randomized evaluator-blind, no-treatment
    controlled phase III study of once-daily nitisinone in adults with
    alkaptonuria, followed for 48 months.
  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: clinicaltrials:NCT01916382
    reference_title: "An International, Multicentre, Randomised, Evaluator-blind, No-treatment Controlled, Parallel-group Study to Assess the Efficacy and Safety of Once Daily Nitisinone in Patients With Alkaptonuria After 12 Months of Treatment, Followed by an Additional 36 Month Treatment Period."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "we are now ready for this final stage of clinical development of nitisinone for AKU: a phase 3 clinical trial to prove efficacy."
    explanation: ClinicalTrials.gov identifies the SONIA 2 nitisinone study as phase III clinical development for alkaptonuria.
  - 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: "SONIA 2 was a 4-year, open-label, evaluator-blind, randomised, no treatment controlled, parallel-group study"
    explanation: The trial publication documents the 4-year design and randomized evaluator-blind control structure.
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: []
- reference: PMID:41096940
  title: "Ochronotic Deposition in Alkaptonuria: Semiquinone-Mediated Oxidative Coupling and Metabolic Drivers of Homogentisic Acid Accumulation."
  findings: []
- reference: PMID:42510554
  title: "HGA-Induced Oxidative Stress Impairs Autophagy via Lysosomal Dysfunction in Alkaptonuria."
  findings: []
- reference: PMID:38846518
  title: "Joint replacement risk is markedly increased in alkaptonuria (AKU) in those with prior arthroplasty."
  findings: []
- reference: PMID:39290064
  title: "Evaluation of a casein glycomacropeptide-based protein substitute, in the dietary management of NTBC-induced tyrosinaemia in patients with alkaptonuria: A prospective open-label study."
  findings: []
- reference: clinicaltrials:NCT01916382
  title: "An International, Multicentre, Randomised, Evaluator-blind, No-treatment Controlled, Parallel-group Study to Assess the Efficacy and Safety of Once Daily Nitisinone in Patients With Alkaptonuria After 12 Months of Treatment, Followed by an Additional 36 Month Treatment Period."
  findings: []
notes: >-
  Publication review retained the direct Orphanet/MONDO mapping while separating
  HGA accumulation from oxidative pigment polymerization and adding provisional
  chondrocyte oxidative-stress and autophagy-lysosomal mechanisms supported by
  2025-2026 experiments. Treatment evidence distinguishes composite SONIA 2
  benefit from the absence of an observed arthroplasty reduction and from
  nitisinone-induced tyrosinemia management. The broad Orphanet terms
  HP:0000366 (abnormality of the nose) and HP:0001597 (abnormality of the nail)
  were reviewed but not modeled: neither source specifies a morphology or
  mechanism, and adding umbrella phenotypes would reduce computability rather
  than refine the already modeled ochronotic pigmentation spectrum.
datasets:
- accession: metabolomics_workbench:ST002179
  title: Impact of nitisinone on the cerebrospinal fluid metabolome of a murine model of alkaptonuria
  organism:
    preferred_term: mouse
    term:
      id: NCBITaxon:10090
      label: Mus musculus
  data_type: METABOLOMICS
  notes: Located via OmicsDI, which aggregates across omics repositories; this record comes from metabolomics_workbench. Only repositories with no other discovery route in this project and with a working accession resolver are curated from OmicsDI -- GEO, ArrayExpress, PRIDE, MetaboLights and EGA hits are excluded as duplicates of dedicated passes. Matched because the disease is named in the dataset's own title ("Alkaptonuria"). Retrieved 2026-08-02.
- accession: metabolomics_workbench:ST002297
  title: Comprehensive biotransformation analysis of phenylalanine-tyrosine metabolism reveals alternative routes of metabolite clearance in nitisinone-treated alkaptonuria (Urine metabolomic analysis)
  notes: Located via OmicsDI, which aggregates across omics repositories; this record comes from metabolomics_workbench. Only repositories with no other discovery route in this project and with a working accession resolver are curated from OmicsDI -- GEO, ArrayExpress, PRIDE, MetaboLights and EGA hits are excluded as duplicates of dedicated passes. Matched because the disease is named in the dataset's own title ("Alkaptonuria"). Retrieved 2026-08-02.
- accession: metabolomics_workbench:ST002296
  title: Comprehensive biotransformation analysis of phenylalanine-tyrosine metabolism reveals alternative routes of metabolite clearance in nitisinone-treated alkaptonuria (Serum metabolomic analysis)
  notes: Located via OmicsDI, which aggregates across omics repositories; this record comes from metabolomics_workbench. Only repositories with no other discovery route in this project and with a working accession resolver are curated from OmicsDI -- GEO, ArrayExpress, PRIDE, MetaboLights and EGA hits are excluded as duplicates of dedicated passes. Matched because the disease is named in the dataset's own title ("Alkaptonuria"). Retrieved 2026-08-02.
📚

References & Deep Research

References

12
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.
PMID:41096940
Ochronotic Deposition in Alkaptonuria: Semiquinone-Mediated Oxidative Coupling and Metabolic Drivers of Homogentisic Acid Accumulation.
No top-level findings curated for this source.
PMID:42510554
HGA-Induced Oxidative Stress Impairs Autophagy via Lysosomal Dysfunction in Alkaptonuria.
No top-level findings curated for this source.
PMID:38846518
Joint replacement risk is markedly increased in alkaptonuria (AKU) in those with prior arthroplasty.
No top-level findings curated for this source.
PMID:39290064
Evaluation of a casein glycomacropeptide-based protein substitute, in the dietary management of NTBC-induced tyrosinaemia in patients with alkaptonuria: A prospective open-label study.
No top-level findings curated for this source.
clinicaltrials:NCT01916382
An International, Multicentre, Randomised, Evaluator-blind, No-treatment Controlled, Parallel-group Study to Assess the Efficacy and Safety of Once Daily Nitisinone in Patients With Alkaptonuria After 12 Months of Treatment, Followed by an Additional 36 Month Treatment Period.
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