Clinical presentation of 13 children with alkaptonuria

Until now, only a few studies have focused on the early onset of symptoms of alkaptonuria (AKU) in the pediatric population. This prospective, longitudinal study is a comprehensive approach to the assessment of children with recognized AKU during childhood. The study includes data from 32 visits of 13 patients (five males, eight females; age 4-17 years) with AKU. A clinical evaluation was performed with particular attention to eye, ear, and skin pigmentation, musculoskeletal complaints, magnetic resonance imaging (MRI), and ultrasound (US) imaging abnormalities. The cognitive functioning and adaptive abilities were examined. Molecular genetic analyses were performed. The most common symptoms observed were dark urine (13/13), followed by joint pain (6/13), and dark ear wax (6/13). In 4 of 13 patients the values obtained in the KOOS-child questionnaire were below the reference values. MRI and US did not show degenerative changes in knee cartilages. One child had nephrolithiasis. Almost half of the children with AKU (5/13) presented deficits in cognitive functioning and/or adaptive abilities. The most frequent HGD variants observed in the patients were c.481G>A (p.Gly161Arg) mutation and the c.240A>T (p.His80Gln) polymorphism. The newly described allele of the HGD gene (c.948G>T, p.Val316Phe) which is potentially pathogenic was identified.

Homogentisate 1,2-dioxygenase (HGD) gene variants in young Egyptian patients with alkaptonuria

Alkaptonuria (AKU) is a rare autosomal recessive metabolic disorder caused by pathogenic variants in the homogentisate 1,2-dioxygenase (HGD) gene. This leads to a deficient HGD enzyme with the consequent accumulation of homogentisic acid (HGA) in different tissues causing complications in various organs, particularly in joints, heart valves and kidneys. The genetic basis of AKU in Egypt is completely unknown. We evaluated the clinical and genetic spectrum of six pediatric and adolescents AKU patients from four unrelated Egyptian families. All probands had a high level of HGA in urine by qualitative GC/MS before genetic confirmation by Sanger sequencing. Recruited AKU patients were four females and two males (median age 13 years). We identified four different pathogenic missense variants within HGD gene. Detected variants included a novel variant c.1079G > T;p.(Gly360Val) and three recurrent variants; c.1078G > C;p.(Gly360Arg), c.808G > A;p.(Gly270Arg) and c.473C > T;p.(Pro158Leu). All identified variants were properly segregating in the four families consistent with autosomal recessive inheritance. In this study, we reported the phenotypic and genotypic spectrum of alkaptonuria for the first time in Egypt. We further enriched the HGD-variant database with another novel pathogenic variant. The recent availability of nitisinone may promote the need for genetic confirmation at younger ages to start therapy earlier and prevent serious complications.

Biochemical and molecular confirmation of alkaptonuria in a Sumatran orangutan (Pongo abelii)

A 6-yr-old female orangutan presented with a history of dark urine that turned brown upon standing since birth. Repeated routine urinalysis and urine culture were unremarkable. Urine organic acid analysis showed elevation in homogentisic acid consistent with alkaptonuria. Sequence analysis identified a homozygous missense variant, c.1081G>A (p.Gly361Arg), of the homogentisate 1,2-dioxygenase (HGD) gene. Familial studies, molecular modeling, and comparison to human variant databases support this variant as the underlying cause of alkaptonuria in this orangutan. This is the first report of molecular confirmation of alkaptonuria in a nonhuman primate.

Breakpoints characterisation of the genomic deletions identified by MLPA in alkaptonuria patients

Until recently, mainly DNA sequencing has been used to identify variants within the gene coding for homogentisate dioxygenase (HGD, 3q13.33) that cause alkaptonuria (AKU), an autosomal recessive inborn error of metabolism of tyrosine. In order to identify possible larger genomic deletions we have developed a novel Multiplex Ligation-dependent Probe Amplification (MLPA) assay specific for this gene (HGD-MLPA) and tested it successfully in healthy controls and in patients carrying two known previously identified HGD deletions. Subsequently, we analysed 22 AKU patients in whom only one or none classical HGD variant was found by sequencing. Using HGD-MLPA and sequencing, we identified four larger deletions encompassing from 1 to 4 exons of this gene and we defined their exact breakpoints: deletion of exons 1-4 (c.1-8460_282 + 6727del), deletion of exons 5 and 6 (c.283-9199_434 + 1688del), deletion of exon 11 (c.775-1915_879 + 1293del), and deletion of exon 13 (c.1007-1709_1188 + 1121del). We suggest including MLPA in the DNA diagnostic protocols for AKU in cases where DNA sequencing does not lead to identification of both HGD variants.

Identification of Potential Inhibitors for the Treatment of Alkaptonuria Using an Integrated In Silico Computational Strategy

Alkaptonuria (AKU) is a rare genetic autosomal recessive disorder characterized by elevated serum levels of homogentisic acid (HGA). In this disease, tyrosine metabolism is interrupted because of the alterations in homogentisate dioxygenase (HGD) gene. The patient suffers from ochronosis, fractures, and tendon ruptures. To date, no medicine has been approved for the treatment of AKU. However, physiotherapy and strong painkillers are administered to help mitigate the condition. Recently, nitisinone, an FDA-approved drug for type 1 tyrosinemia, has been given to AKU patients in some countries and has shown encouraging results in reducing the disease progression. However, this drug is not the targeted treatment for AKU, and causes keratopathy. Therefore, the foremost aim of this study is the identification of potent and druggable inhibitors of AKU with no or minimal side effects by targeting 4-hydroxyphenylpyruvate dioxygenase. To achieve our goal, we have performed computational modelling using BioSolveIT suit. The library of ligands for molecular docking was acquired by fragment replacement of reference molecules by ReCore. Subsequently, the hits were screened on the basis of estimated affinities, and their pharmacokinetic properties were evaluated using SwissADME. Afterward, the interactions between target and ligands were investigated using Discovery Studio. Ultimately, compounds c and f were identified as potent inhibitors of 4-hydroxyphenylpyruvate dioxygenase.

Structure-Function Relationship of Homogentisate 1,2-dioxygenase: Understanding the Genotype-Phenotype Correlations in the Rare Genetic Disease Alkaptonuria

Alkaptonuria (AKU), a rare genetic disorder, is characterized by the accumulation of homogentisic acid (HGA) in organs, which occurs because the homogentisate 1,2-dioxygenase (HGD) enzyme is not functional due to gene variants. Over time, HGA oxidation and accumulation cause the formation of the ochronotic pigment, a deposit that provokes tissue degeneration and organ malfunction. Here, we report a comprehensive review of the variants so far reported, the structural studies on the molecular consequences of protein stability and interaction, and molecular simulations for pharmacological chaperones as protein rescuers. Moreover, evidence accumulated so far in alkaptonuria research will be re-proposed as the bases for a precision medicine approach in a rare disease.

Untargeted NMR Metabolomics Reveals Alternative Biomarkers and Pathways in Alkaptonuria

Alkaptonuria (AKU) is an ultra-rare metabolic disease caused by the accumulation of homogentisic acid (HGA), an intermediate product of phenylalanine and tyrosine degradation. AKU patients carry variants within the gene coding for homogentisate-1,2-dioxygenase (HGD), which are responsible for reducing the enzyme catalytic activity and the consequent accumulation of HGA and formation of a dark pigment called the ochronotic pigment. In individuals with alkaptonuria, ochronotic pigmentation of connective tissues occurs, leading to inflammation, degeneration, and eventually osteoarthritis. The molecular mechanisms underlying the multisystemic development of the disease severity are still not fully understood and are mostly limited to the metabolic pathway segment involving HGA. In this view, untargeted metabolomics of biofluids in metabolic diseases allows the direct investigation of molecular species involved in pathways alterations and their interplay. Here, we present the untargeted metabolomics study of AKU through the nuclear magnetic resonance of urine from a cohort of Italian patients; the study aims to unravel molecular species and mechanisms underlying the AKU metabolic disorder. Dysregulation of metabolic pathways other than the HGD route and new potential biomarkers beyond homogentisate are suggested, contributing to a more comprehensive molecular signature definition for AKU and the development of future adjuvant treatment.

Assessment of Thyroid Function in Patients With Alkaptonuria

IMPORTANCE

Alkaptonuria is an autosomal recessive disorder caused by pathogenic variants in the HGD gene. Deficiency of the HGD enzyme leads to tissue deposition of homogentisic acid (HGA), causing severe osteoarthropathies and cardiac valve degeneration. Although HGD is vital for the catabolism of tyrosine, which provides the basis for thyroid hormone synthesis, the prevalence of thyroid dysfunction in alkaptonuria is unknown.

OBJECTIVE

To assess thyroid structure and function in patients with alkaptonuria.

DESIGN, SETTING, AND PARTICIPANTS

A single-center cohort study was conducted in a tertiary referral center including patients with alkaptonuria followed up for a median of 93 (interquartile range, 48-150) months between February 1, 2000, and December 31, 2018. The alkaptonuria diagnosis was based on clinical presentation and elevated urine HGA levels. A total of 130 patients were considered for participation.

MAIN OUTCOMES AND MEASURES

Prevalence of thyroid dysfunction in adults with alkaptonuria compared with the general population. Thyrotropin and free thyroxine levels were measured by immunoassay and repeated in each patient a median of 3 (interquartile range, 2-22) times. Neck ultrasonographic scans were analyzed in a subset of participants. Logistic regression was used to test the association of thyroid dysfunction with age, sex, thyroid peroxidase (TPO) antibodies, serum tyrosine levels, and urine HGA levels.

RESULTS

Of the 130 patients, 5 were excluded owing to thyroidectomy as the cause of hypothyroidism. The study cohort consisted of 125 patients; the median age was 45 (interquartile range, 35-51) years. Most of the patients were men (72 [57.6%]). The prevalence of primary hyperthyroidism was 0.8% (1 of 125 patients), similar to 0.5% observed in the general population (difference, 0.003; 95% CI, -0.001 to 0.04; P = .88). The prevalence of primary hypothyroidism was 16.0% (20 of 125 patients), which is significantly higher than 3.7% reported in the general population (difference, 0.12; 95% CI, 0.10-0.24; P < .001). Women were more likely to have primary hypothyroidism than men (odds ratio, 10.99; 95% CI, 3.13-38.66; P < .001). Patients with TPO antibodies had a higher likelihood of primary hypothyroidism than those without TPO antibodies (odds ratio, 7.36; 95% CI, 1.89-28.62; P = .004). There was no significant difference in the prevalence of thyroid nodules between patients in this study (29 of 49 [59.2%]) vs the general population (68%) (difference, 0.088; 95% CI, -0.44 to 0.73; P = .20) or of cancer (7% vs 5%; difference, 0.01; 95% CI, -0.01 to 0.17; P = .86).

CONCLUSIONS AND RELEVANCE

The high prevalence of primary hypothyroidism noted in patients with alkaptonuria in this study suggests that serial screening in this population should be considered and prioritized.

Presentation of 14 alkaptonuria patients from Turkey

Background Alkaptonuria (OMIM: 203500) is an inborn error of metabolism due to homogentisate 1,2-dioxygenase homogentisic acid 1,2 dioxygenase (HGD) enzyme deficiency. Due to the enzyme deficiency, homogentisic acid cannot be converted to maleylacetoacetate and it accumulates in body fluids. Increased homogentisic acid is converted to benzoquinones, the resulting benzoquinones are converted to melanin-like pigments, and these pigments are deposited in collagen - this process is called ochronosis. In patients with alkaptonuria, the urine is darkened, which is misinterpreted as hematuria, the incidences of renal stones, arthritis and cardiac valve calcification are increased, and spontaneous tendon ruptures, prostatitis and prostate stones can be encountered. The present study aimed to evaluate the HGD gene mutations in 14 patients with alkaptonuria. Methods Fourteen patients diagnosed with alkaptonuria and followed up from 1990 to 2014 were retrospectively evaluated. Their demographic, clinical and treatment-related data were retrieved from hospital files. For mutation analysis, genomic DNAs of the patients were isolated from their peripheral blood samples. Variations in the HGD gene were scanned on the HGD-mutation database (http://hgddatabase.cvtisr.sk). Results Among 14 patients, the female/male ratio was 1/1 and the median age was 9 years (range, 6-59 years). All patients were symptomatic at their first visit and the most common symptom was dark urine (71%) followed by arthralgia. Independent of the urinary homogentisic acid concentrations, patients with the presenting symptom of arthralgia were elder. Nine different mutations including p.Ser59AlafsX52, p.Gly161Arg, p.Asn219Ser, p.Gly251Asp, p.Pro274Leu, p.Arg330Ser, p.Gly372Ala, c.656_657insAATCAA and a novel mutation of p.Val316Ile were detected. All of the pediatric-age patients (n = 13) were treated with ascorbic acid at a dose of 250-1000 mg/day. Conclusions Nine different HGD gene mutations with a novel one, p.Val316Ile, were detected. The most common mutation was p.Ser59AlafsX52 for the HGD gene followed by p.Gly161Arg and p.asn219Ser, which can be considered specific to the Turkish population.

Reduced primary cilia length and altered Arl13b expression are associated with deregulated chondrocyte Hedgehog signaling in alkaptonuria

Alkaptonuria (AKU) is a rare inherited disease resulting from a deficiency of the enzyme homogentisate 1,2-dioxygenase which leads to the accumulation of homogentisic acid (HGA). AKU is characterized by severe cartilage degeneration, similar to that observed in osteoarthritis. Previous studies suggest that AKU is associated with alterations in cytoskeletal organization which could modulate primary cilia structure/function. This study investigated whether AKU is associated with changes in chondrocyte primary cilia and associated Hedgehog signaling which mediates cartilage degradation in osteoarthritis. Human articular chondrocytes were obtained from healthy and AKU donors. Additionally, healthy chondrocytes were treated with HGA to replicate AKU pathology (+HGA). Diseased cells exhibited shorter cilia with length reductions of 36% and 16% in AKU and +HGA chondrocytes respectively, when compared to healthy controls. Both AKU and +HGA chondrocytes demonstrated disruption of the usual cilia length regulation by actin contractility. Furthermore, the proportion of cilia with axoneme breaks and bulbous tips was increased in AKU chondrocytes consistent with defective regulation of ciliary trafficking. Distribution of the Hedgehog-related protein Arl13b along the ciliary axoneme was altered such that its localization was increased at the distal tip in AKU and +HGA chondrocytes. These changes in cilia structure/trafficking in AKU and +HGA chondrocytes were associated with a complete inability to activate Hedgehog signaling in response to exogenous ligand. Thus, we suggest that altered responsiveness to Hedgehog, as a consequence of cilia dysfunction, may be a contributing factor in the development of arthropathy highlighting the cilium as a novel target in AKU.

Two novel mutations in the homogentisate-1,2-dioxygenase gene identified in Chinese Han Child with Alkaptonuria

Alkaptonuria (AKU) is an autosomal recessive disorder of tyrosine metabolism, which is caused by a defect in the enzyme homogentisate 1,2-dioxygenase (HGD) with subsequent accumulation of homogentisic acid. Presently, more than 100 HGD mutations have been identified as the cause of the inborn error of metabolism across different populations worldwide. However, the HGD mutation is very rarely reported in Asia, especially China. In this study, we present mutational analyses of HGD gene in one Chinese Han child with AKU, which had been identified by gas chromatography-mass spectrometry detection of organic acids in urine samples. PCR and DNA sequencing of the entire coding region as well as exon-intron boundaries of HGD have been performed. Two novel mutations were identified in the HGD gene in this AKU case, a frameshift mutation of c.115delG in exon 3 and the splicing mutation of IVS5+3 A>C, a donor splice site of the exon 5 and exon-intron junction. The identification of these mutations in this study further expands the spectrum of known HGD gene mutations and contributes to prenatal molecular diagnosis of AKU.

Aortic valve disease as a first manifestation of Alcaptonuria in surgically treated patient. Case report

BACKGROUND

Alcaptonuria, a rare metabolic disorder (1:250 000), is usually presented with symptoms such as arthropathies of weight bearing joints.

CASE REPORT

In this case, a 65 year old woman was admitted to our hospital with severe aortic stenosis and no other symptoms that would suggest the existance of Alcaptonuria. Intraoperative findings of black discoloration of the affected valve and ascending aorta, pointed towards the diagnosis of cardiac ochronosis, what was then confirmed by a PH examination.

CONCLUSION

This case suggests that although alcaptonuria is a slow progressive disease with cardiac ochronosis as a predictable late complication, it can nevertheless be a first sign. In that case the attention should be brought to the surely affected lumbar spine and weight bearing joints, and other connective tissue.

KEY WORDS

Alcaptonuria, Aortic valve, Cardiac ochronosis, Surgery.

Alkaptonuria: a very rare metabolic disorder

Alkaptonuria (AKU) is a very rare autosomal recessive disorder of tyrosine metabolism in the liver due to deficiency of homogentisate 1,2 dioxygenase (HGD) activity, resulting in the accumulation of homogentisic acid (HGA). Circulating HGA pass into various tissues through-out the body, mainly in cartilage and connective tissues, where its oxidation products polymerize and deposit as a melanin-like pigment. Gram quantities of HGA are excreted in the urine. AKU is a progressive disease and the three main features, according the chronology of appearance, are: darkening of the urine at birth, then ochronosis (blue-dark pigmentation of the connective tissue) clinically visible at around 30 yrs in the ear and eye, and finally a severe ochronotic arthropathy at around 50 yrs with spine and large joints involvements. Cardiovascular and renal complications have been described in numerous case report studies. A treatment now is available in the form of a drug nitisinone, which decreases the production of HGA. The enzymatic defect in AKU is caused by the homozygous or compound heterozygous mutations within the HGD gene. This disease has a very low prevalence (1:100,000-250,000) in most of the ethnic groups, except Slovakia and Dominican Republic, where the incidence has shown increase up to 1:19,000. This review highlights classical and recent findings on this very rare disease.

Alkaptonuria and Pompe disease in one patient: metabolic and molecular analysis

Pompe disease is characterised by deficiency of acid α-glucosidase that results in abnormal glycogen deposition in the muscles. Alkaptonuria is caused by a defect in the enzyme homogentisate 1,2-dioxygenase with subsequent accumulation of homogentisic acid. We report the case of a 6-year-old boy diagnosed with Pompe disease and alkaptonuria. Urine organic acids and α-glucosidase were measured. Homogentisate 1,2-dioxygenase (HGO) and acid alpha-glucosidase (GAA) genes were sequenced by Sanger DNA sequencing. The level of α-glucosidase in white blood cells was markedly decreased (4 nm/mg) while the level of homogentisic acid was markedly increased (15 027 mmol/mol creatine). GAA sequencing detected two heterozygous GAA mutations (C.670C>T and C.1064T>C) while HGO sequencing revealed three polymorphisms in exons 4, 5 and 6, respectively. To the best of our knowledge, this is the first reported instance of Pompe disease and alkaptonuria occurring in the same individual.

An update on molecular genetics of Alkaptonuria (AKU)

Alkaptonuria (AKU) is an autosomal recessive disorder caused by a deficiency of homogentisate 1,2 dioxygenase (HGD) and characterized by homogentisic aciduria, ochronosis, and ochronotic arthritis. The defect is caused by mutations in the HGD gene, which maps to the human chromosome 3q21-q23. AKU shows a very low prevalence (1:100,000-250,000) in most ethnic groups, but there are countries such as Slovakia and the Dominican Republic in which the incidence of this disorder rises to as much as 1:19,000. In this work, we summarize the genetic aspects of AKU in general and the distribution of all known disease-causing mutations reported so far. We focus on special features of AKU in Slovakia, which is one of the countries with an increased incidence of this rare metabolic disorder.

[Ochronosis: report of two familial cases]

BACKGROUND

Ochronosis of alkaptonuria is a rare hereditary autosomal recessive disease in which there is an absence of homogentisic acid oxidase resulting in accumulation of homogentisic acid in tissues.

AIM

To report a new case of alkaptonuria

CASE REPORT

A 49-year-old man had been followed for 4 years for chronic lombalgia and arthropaty of two knees. He is married to his cousin and father of 4 girls. His parents are also cousins. The clinical examination has found a cutaneuous pigmentation and a lumbar stiffness. At biological checking, creatininemia was at 190 μmol/L and there are not inflammatory indicators. The radiography have shown a discal dorsolumbar calcifications, anterior inter somatic bridges and bilateral arthritis of knees without articular chondrocalcinosis. The diagnosis of ochronosis have been suspected and confirmed by the blackness of urine and the dosage of alkaptonuria. The patient has been treated symptomatiquely. Familial investigation have revealed that his daughter suffered from the same disease with the notion of blackness of urine. She is 12 year old and she's asymptomatic on the osteoarticular level.

CONCLUSION

Alkaptonuria causes a degenerative arthropaty which can endanger functional prognosis. Early diagnosis and scanning of this innate error of metabolism by genetic study play a fundamental interest, especially for molecular and genetic advisement.

Early-onset ocular ochronosis in a girl with alkaptonuria (AKU) and a novel mutation in homogentisate 1,2-dioxygenase (HGD)

Alkaptonuria (AKU) is a disorder of phenylalanine/tyrosine metabolism due to a defect in the enzyme homogentisate 1,2-dioxygenase (HGD). This recessive disease is caused by mutations in the HGD gene. We report a 14-year-old girl who was referred after presenting black urine. Careful examination revealed ochronosis of the conjunctiva. There was no affection of the cardiac valves. Elevated excretion of homogentisic acid in urine was found. Sequence analysis of the HGD gene from genomic DNA revealed that the patient is a compound heterozygote with a previously described mutation (c.473C>T, p.Pro158Leu), and a novel one (c.821C>T, p.Pro274Leu). Her mother is heterozygous for the novel mutation, while the brother is heterozygous for the previously described mutation. In summary, we describe an alkaptonuric patient with ocular ochronosis and a novel HGD mutation, c.821C>T, p.Pro274Leu.

Alkaptonuria

Alkaptonuria is a hereditary disease resulted from accumulation of homogentisic acid within the body due to deficiency of homogentisic acid oxidase. The main clinical feature is dark brown color of urine caused by high urinary output of homogentisic acid. There are no other symptoms or signs of the disease until the fourth decade of life when ochronosis is developed. Life-long accumulation of abnormal metabolites becomes overt in form of severe spondylosis, peripheral arthropathy, tendon rupture, bone osteoporosis as well as aortic valve stenosis and skin pigmentation. The features of the disease are associated with affinity of homogentisic acid to the connective tissue and its effect on collagen structure. Only symptomatic treatment is applied in case of alkaptonuria and ochronosis.

Three-generational alkaptonuria in a non-consanguineous family

OBJECTIVE

Alkaptonuria (AKU) is a rare inborn error of metabolism of aromatic amino acids and considered to be an autosomal recessive trait caused by mutations in the homogentisate 1,2-dioxygenase (HGD) gene. A dominant pattern of inheritance has been reported but was attributed to extended consanguinity in many cases. However, we have observed a non-consanguineous family segregating AKU in a dominant manner over three generations.

RESULTS

All affected individuals presented with typical features of AKU including darkening of the urine, ochronosis, arthropathy, and elevated urinary excretion of homogentisic acid. Sequence analysis of the HGD gene from genomic DNA of two affected individuals, uncle and niece, revealed a heterozygous missense mutation (M368V) in the uncle that was not present in his niece. Microsatellite genotyping demonstrated that both were heterozygous at the HGD locus and shared one haplotype. This haplotype did not contain a detectable HGD mutation. The haplotype was also found in a healthy son of the niece, making a dominant HGD mutation unlikely. Moreover, sequencing of cDNA from lymphoblastoid cells of the niece did not reveal an HGD mRNA with a potentially dominant-negative effect.

CONCLUSION

Rare causes of the uncommon AKU inheritance in this family have to be considered, ranging from the coincidence of undetectable HGD mutations to a dominant mutation of a second, hitherto unknown AKU gene.

Coexistence of ochronosis and B 27 positive ankylosing spondylitis

We describe a 49-year-old man with coexistence of ochronosis and B27 positive ankylosing spondylitis. This is the first report documenting the simultaneous occurrence of ochronosis and B27 positive ankylosing spondylitis, with no positive familiar history for seronegative spondylarthropathies. The relations of these rheumatic diseases are discussed.

Ochronotic rheumatism in Algeria: clinical, radiological, biological and molecular studies—a case study of 14 patients in 11 families

OBJECTIVES

To confirm alkaptonuria and ochronotic arthropathy diagnosis by mutation screening of the homogentisate 1,2-dioxygenase (HGD) gene. Try to establish a genotype-phenotype correlation in the five subjects with a molecular study on HGD gene.

METHODS

We report 14 alkaptonuria cases (10 men and four women) in 11 Algerian families. Consanguineous matings were evidenced in only three families (F = 1/16). Molecular analysis was performed by sequencing genomic DNA in order to identify the mutations of the HGD gene.

RESULTS

Alkaptonuria was always confirmed by urinary homogentisic acid determination. Four different mutations of the HGD gene were found: an homozygous missense mutation, Serine189Isoleucine in two sisters with a mild phenotype; an homozygous splice site mutation (IVS1-1G > A) in a man with a severe phenotype (death at 61 years old from renal failure); a silent mutation, Alanine470Alanine at the heterozygous state in a man with a mild phenotype; a 'G' deletion at the position c.819 which causes a frameshift after Gly217(Gly217fs) that runs into a stop codon at c. 850. This mutation is novel and was found in heterozygosis in a woman with a mild phenotype.

CONCLUSIONS

The two homozygous mutations were associated, respectively, with a severe and a mild phenotype but no genotype-phenotype correlation could be found.

Alkaptonuria, ochronosis, and ochronotic arthropathy

OBJECTIVES

To describe the clinical presentation and course of a relatively large group of Italian adult patients screened for mutation of the homogentisate dioxygenase gene causing alkaptonuria (AKU) and ochronosis, and to review typical and atypical facets of this condition.

METHODS

We reviewed the medical records of 9 patients affected by ochronotic arthropathy who were observed in our institutions between 1979 and 2001. All patients were diagnosed as having AKU through a rapid urine test with alkali. Mutation screening was performed by single-strand conformation analysis of all homogentisate dioxygenase exons, followed by sequencing of altered conformers.

RESULTS

Our 9 cases had similar clinical features and they reflected those described in the literature: a progressive degenerative arthropathy mainly affecting axial and weight-bearing joints associated with extraarticular manifestation. Musculoskeletal symptoms began in most of our patients around the age of 30 years with back pain and stiffness: involvement of the large peripheral joints usually occurred several years after spinal changes. Ochronotic peripheral arthropathy generally was degenerative, but joint inflammation was observed in some cases; this could be attributed to an inflammatory reaction of the ochronotic shard in the synovial membrane.

CONCLUSIONS

Ochronosis is a model of arthropathy with known etiologic factors. Over time, AKU, the genetically determined metabolic defect, leads to the accumulation of pigment and the development of this crippling condition. Most of the clinical findings may be explained by inhibition of collagen crosslinks, but some require additional interpretation. For example, inflammatory features of the ochronotic joint only occur in a minority of cases, and may be attributable to ochronotic shards. Further studies are needed to establish the genotype-phenotype correlation to identify mutations that are predictive of severe disease. For this purpose, the Italian Study Group on Alkaptonuria (www.dfc.unifi.it/aku) is enrolling affected patients in an on-line database to characterize the molecular defects and their relationship to clinical data.

Ochronotic arthropathy

The authors report on 18 members of four generations of an alkaptonuric family. All three males in the third generation are clinically affected; two members of the family tree have undergone major joint surgery.

Alkaptonuria caused by compound heterozygote mutations

Alkaptonuria is a rare autosomal recessive disorder of inborn errors of metabolism. It is characterised by the deposition of "ochronotic pigment" especially in connective tissue as a result of deficieny of the "homogentisic acid oxidase" enzyme which has a role in the catabolism of tyrosine and phenylalanine. A compound heterozygote alkaptonuria patient, with manifestations in adulthood, without infantile and childhood signs is presented. The described alkaptonuria mutations are reported for the first time in the Turkish population.

Rapid detection methods for five HGO gene mutations causing alkaptonuria

Alkaptonuria (AKU) is an autosomal recessive disorder caused by the deficiency of homogentisate 1,2 dioxygenase (HGO) activity. The disease is characterized by homogentisic aciduria, ochronosis and ochronotic arthritis. AKU shows a very low prevalence (1:250 000), in most ethnic groups. Altogether 43 HGO mutations have been identified in approximately 100 patients. In Slovakia, however, the incidence of this disorder rises up to 1:19 000, and 10 different AKU mutations have been identified in this relatively small country. Here, we report detection methods developed for rapid identification of five HGO mutations. PCR primers were designed enabling detection of mutations IVS5 + 1G-->A, R58fs, and V300G by restriction digestion of amplification-created restriction sites (ACRS). Mutation G152fs is readily identified by heteroduplex analysis, and G161R by amplification refractory mutation system (ARMS) PCR.

Molecular analyses of the HGO gene mutations in Turkish alkaptonuria patients suggest that the R58fs mutation originated from central Asia and was spread throughout Europe and Anatolia by human migrations

Alkaptonuria (AKU) is a rare metabolic disorder of phenylalanine catabolism that is inherited as an autosomal recessive trait. AKU is caused by loss-of-function mutations in the homogentisate 1,2-dioxygenase (HGO) gene. The deficiency of homogentisate 1,2-dioxygenase activity causes homogentisic aciduria, ochronosis and arthritis. We present the first molecular study of the HGO gene in Turkish AKU patients. Seven unrelated AKU families from different regions in Turkey were analysed. Patients in three families were homozygous for the R58fs mutation; another three families were homozygous for the R225H mutation; and one family was homozygous for the G270R mutation. Analysis of nine intragenic HGO polymorphisms showed that the R58fs, R225H and G270R Turkish AKU mutations are associated with specific HGO haplotypes. The comparison with previously reported haplotypes associated with these mutations from other populations revealed that the R225H is a recurrent mutation in Turkey, whereas G270R most likely has a Slovak origin. Most interestingly, these analyses showed that the Turkish R58fs mutation shares an HGO haplotype with the R58fs mutation found in Finland, Slovakia and India, suggesting that R58fs is an old AKU mutation that probably originated in central Asia and spread throughout Europe and Anatolia during human migrations.

Natural history of alkaptonuria

BACKGROUND

Alkaptonuria, caused by mutations in the HGO gene and a deficiency of homogentisate 1,2-dioxygenase, results in an accumulation of homogentisic acid (HGA), ochronosis, and destruction of connective tissue. There is no effective therapy for this disorder, although nitisinone inhibits the enzyme that produces HGA. We performed a study to delineate the natural history of alkaptonuria.

METHODS

We evaluated 58 patients with alkaptonuria (age range, 4 to 80 years), using clinical, radiographic, biochemical, and molecular methods. A radiographic scoring system was devised to assess the severity of spinal and joint damage. Two patients were treated with nitisinone for 10 and 9 days, respectively.

RESULTS

Life-table analyses showed that joint replacement was performed at a mean age of 55 years and that renal stones developed at 64 years, cardiac-valve involvement at 54 years, and coronary-artery calcification at 59 years. Linear regression analysis indicated that the radiographic score for the severity of disease began increasing after the age of 30 years, with a more rapid increase in men than in women. Twenty-three new HGO mutations were identified. In a 51-year-old woman, urinary HGA excretion fell from 2.9 to 0.13 g per day after a 10-day course of nitisinone (7 days at a dose of 0.7 mg per day and 3 days at 2.8 mg per day). In a 59-year-old woman, urinary HGA fell from 6.4 g to 1.7 g per day after nine days of treatment with nitisinone (0.7 mg per day). Plasma tyrosine levels in these patients rose from approximately 1.1 mg per deciliter (60 micromol per liter) in both to approximately 12.8 mg per deciliter (700 micromol per liter) and 23.6 mg per deciliter (1300 micromol per liter), respectively, with no clinical signs or symptoms.

CONCLUSIONS

The reported data on the natural history of alkaptonuria provide a basis for the evaluation of long-term therapies. Although nitisinone can reduce HGA production in humans with homogentisate 1,2-dioxygenase deficiency, the long-term safety and efficacy of this treatment require further evaluation.

Exacerbation of the ochronosis of alkaptonuria due to renal insufficiency and improvement after renal transplantation

In alkaptonuria, homogentisate 1,2-dioxygenase deficiency causes tissue accumulation of homogentisic acid (HGA), followed by signs and symptoms of ochronosis. These include massive urinary excretion of HGA, arthritis and joint destruction, pigmentation of cartilage and connective tissue, and cardiac valve deterioration. We describe a 46-year-old man with alkaptonuria and diabetic renal failure whose plasma HGA concentration was twice that of any other alkaptonuria patient, and whose ochronosis progressed much more rapidly than that of his two alkaptonuric siblings. After renal transplantation, the plasma HGA normalized, and the daily urinary excretion of HGA decreased by 2-3g. This case illustrates the critical role of renal tubular secretion in eliminating HGA from the body, and suggests that renal transplantation in a uremic patient not only restores HGA excretion, but may also provide homogentisate 1,2-dioxygenase activity for the metabolism of HGA.

Aortic valve stenosis in alkaptonuric ochronosis

Alkaptonuria is a rare genetic disorder of tyrosine metabolism in which a bluish pigment accumulates in the connective tissues throughout the body, and causes degenerative changes. The most common clinical manifestation of ochronosis is arthropathy. Heart valves may also be affected, though cardiac involvement is rare. The present patient has cardiac ochronosis, and has several family members diagnosed with ochronosis and aortic valve stenosis.

Alkaptonuria in Slovakia: thirty-two years of research on phenotype and genotype

Research on alkaptonuria (AKU; OMIM # 230500) in Slovakia started in 1968 by the Research Laboratory (later on the Institute) for Clinical Genetics at Martin. Its first stage was focused on clinical, biochemical, genetic and epidemiologic questions and on the reasons for the high prevalence of AKU in Slovakia. Based on a screening programme of now over 611,000 inhabitants (509,000 newborns) the world-wide highest incidence of AKU (1 in 19,000) was recorded, and a total of 208 patients (110 children) were registered. Extensive genealogical studies (sometimes over two centuries) resulted in the fusion of several "unrelated" nuclear families into larger pedigrees and enabled tracing most AKU ancestors to their original geographic localities, predominantly in remote mountain areas. A likely founder effect was detected among the shepherd population of the so-called Valachian colonization that resulted in a high degree of inbreeding and persisting genetic isolation. These epidemiologic data formed the basis for molecular studies in collaboration with the Würzburg group. The AKU locus was mapped to human chromosome 3q2 by orthology to the mouse locus aku. Following the cloning of the homogentisate-1,2 dioxygenase (HGD) genes from human and mouse, nine different mutations were identified in 21 AKU index patients. These include 4 missense, 2 splice-site, 2 single-base insertion and 1 deletion mutation. The most frequent mutations among the 42 AKU chromosomes of the index cases are c.648G > A (Gly161Arg; 42.9%), and c.1278insC (Pro370fs; 19.1%). To date, the genotypes of 29 patients and of 74 gene carriers from 21 families have been established. The highest prevalence and allelic heterogeneity were observed in the Kysuce district with five different mutations. Molecular epidemiology studies by haplotyping were carried out to uncover the original geographic localities of all AKU index chromosomes. This strongly suggests that several founders have contributed to the HGD gene mutation pool. While there is no straightforward explanation for the clustering of independent mutations, the genetic isolation in the past is likely to be responsible for the high prevalence of AKU in Slovakia.

A fungal perspective on human inborn errors of metabolism: alkaptonuria and beyond

Crucial for the establishment and development of biochemical genetics as a self-standing discipline was Beadle and Tatum's choice of Neurospora crassa as experimental organism some 60 years ago. Although Garrod's insights on biochemical genetics and his astonishingly modern concepts of biochemical individuality and susceptibility to disease had been ignored by their contemporaries, Beadle acknowledged on several occasions how close Garrod had come to the "one-gene-one-enzyme" hypothesis. In an unexpected turn of events, several genes involved in human inborn errors of metabolism, including the gene for Garrod's favorite disease, alkaptonuria, have been characterized by exploitation of the experimental advantages of another mold, Aspergillus nidulans, which shares with N. crassa the experimental advantages that prompted pioneers of biochemical genetics to use them: rapid growth, facile genetic manipulation, and an environment (the composition of the growth medium) that can be manipulated à la carte.

High frequency of alkaptonuria in Slovakia: evidence for the appearance of multiple mutations in HGO involving different mutational hot spots

Alkaptonuria (AKU) is an autosomal recessive disorder caused by the deficiency of homogentisate 1,2 dioxygenase (HGO) activity. AKU shows a very low prevalence (1:100,000-250,000) in most ethnic groups. One notable exception is in Slovakia, where the incidence of AKU rises to 1:19,000. This high incidence is difficult to explain by a classical founder effect, because as many as 10 different AKU mutations have been identified in this relatively small country. We have determined the allelic associations of 11 HGO intragenic polymorphisms for 44 AKU chromosomes from 20 Slovak pedigrees. These data were compared to the HGO haplotype data available in our laboratory for >80 AKU chromosomes from different European and non-European countries. The results show that common European AKU chromosomes have had only a marginal contribution to the Slovak AKU gene pool. Six of the ten Slovak AKU mutations, including the prevalent G152fs, G161R, G270R, and P370fs mutations, most likely originated in Slovakia. Data available for 17 Slovak AKU pedigrees indicate that most of the AKU chromosomes have their origins in a single very small region in the Carpathian mountains, in the northwestern part of the country. Since all six Slovak AKU mutations are associated with HGO mutational hot spots, we suggest that an increased mutation rate at the HGO gene is responsible for the clustering of AKU mutations in such a small geographical region.

Structural and functional analysis of mutations in alkaptonuria

Alkaptonuria (AKU), the prototypic inborn error of metabolism, was the first human disease to be interpreted as a Mendelian trait by Garrod and Bateson at the beginning of last century. AKU results from impaired function of homogentisate dioxygenase (HGO), an enzyme required for the catabolism of phenylalanine and tyrosine. With the novel 7 AKU and 22 fungal mutations reported here, a total of 84 mutations impairing this enzyme have been found in the HGO gene from humans and model organisms. Forty-three of these mutations result in single amino acid substitutions. This mutational information is analysed here in the context of the HGO structure and function using kinetic assays performed using purified AKU mutant enzymes and the crystal structure of human HGO. HGO is a topologically complex structure which assembles as a functional hexamer arranged as a dimer of trimers. We show how the intricate pattern of intra- and inter-subunit interactions and the extensive surfaces required for subunit folding and association of this oligomeric enzyme can be inactivated at multiple levels by single-residue substitutions. This explains, in part, the predominance of missense mutations (67%) in AKU.

Crystal structure of human homogentisate dioxygenase

Homogentisate dioxygenase (HGO) cleaves the aromatic ring during the metabolic degradation of Phe and Tyr. HGO deficiency causes alkaptonuria (AKU), the first human disease shown to be inherited as a recessive Mendelian trait. Crystal structures of apo-HGO and HGO containing an iron ion have been determined at 1.9 and 2.3 A resolution, respectively. The HGO protomer, which contains a 280-residue N-terminal domain and a 140-residue C-terminal domain, associates as a hexamer arranged as a dimer of trimers. The active site iron ion is coordinated near the interface between subunits in the HGO trimer by a Glu and two His side chains. HGO represents a new structural class of dioxygenases. The largest group of AKU associated missense mutations affect residues located in regions of contact between subunits.