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

Alkaptonuria in Russia: mutational spectrum and novel variants

Alkaptonuria is a rare genetic disease caused by mutations in HGD gene. Here we report the results of genetic and biochemical analysis of 49 Russian patients with alkaptonuria. One of the common variants c.481G > A; p.(Gly161Arg) comprising 72.4% of identified alleles was found in 45 of 49 patients in our cohort, which is probably the highest frequency of this variant worldwide. 9 novel variants were found: 6 missense, 2 splicing and 1 loss of start-codon. For missense variants we performed bioinformatic analysis, protein 3D-modeling and molecular dynamics simulations, which strongly suggest their pathogenic effect. For the rare synonymous variant c.753C > T; p.(Gly251Gly), which was found in 3 cases and predicted to activate cryptic splice site, we performed the detailed functional analysis on patient's cDNA and minigene assay and confirmed its pathogenicity.

Alkaptonuria – Many questions answered, further challenges beckon

Alkaptonuria is an iconic rare inherited inborn error of metabolism affecting the tyrosine metabolic pathway, resulting in the accumulation of homogentisic acid in the circulation, and significant excretion in urine. Dating as far back as 1500 BC in the Egyptian mummy Harwa, homogentisic acid was shown to be central to the pathophysiology of alkaptonuria through its deposition in collagenous tissues in a process termed ochronosis. Clinical manifestations occurring as a consequence of this are typically observed from the third decade of life, are lifelong and significantly affect the quality of life. In large supportive and palliative treatment measures are available to patients, including analgesia, physiotherapy and joint replacement. Studying the natural history of alkaptonuria, in a murine model and human subjects, has provided key insights into the biochemical and molecular mechanisms underlying the pathophysiology associated with the disease, and has enabled a better understanding of the common disease osteoarthritis. In the last decade, a major focus has been on an unlicensed disease-modifying therapy called nitisinone. This has been shown to be highly efficacious in reducing homogentisic acid, and it is hoped this will halt ochronosis, thus limiting the clinical complications associated with the disease. A well-documented metabolic consequence of nitisinone therapy is hypertyrosinaemia, the clinical implications of which are uncertain. Recent metabolomic studies have helped understand the wider metabolic consequences of nitisinone therapy.

Neurological Assessment and Nerve Conduction Study Findings in 22 Patients with Alkaptonuria from Jordan

Alkaptonuria is a rare metabolic disease characterised by accumulative deposition of homogentisic acid in the connective tissue of the body. This results in early degeneration of tendons, cartilages, heart valves, and other tissues. The main objective of the study is to examine the possibility of the nervous system involvement in patients with alkaptonuria The sample consists of two groups; 22 patients with AKU and 20 controls. A neurological assessment has been carried out including detailed medical history, neurological examination, and a nerve conduction study of the nerves of the dominant hand. The prevalence of any abnormality was compared between the two groups using chi square test. The mean values of the nerve conduction study were compared between the two groups using student t-test. There was a higher prevalence of low back pain, hearing problems and tinnitus, numbness and neuropathic pain in alkaptonuria patients. There was no significant difference between the two groups in other conditions such as seizures, headache, and syncope. The values of the nerve conduction study did not show significant difference between the two groups. Neurologically related symptoms in alkaptonuria mostly represent complications of the connective tissue degeneration rather than direct involvement of the nervous system. This has been supported further by the normal findings of the neurophysiology study in patients with alkaptonuria.

Genetics of alkaptonuria – an overview

Alkaptonuria (AKU) is the first described inborn error of metabolism and a classical example of rare autosomal recessive disease. AKU patients carry homozygous or compound heterozygous mutations of the gene coding for enzyme homogentisate dioxygenase (HGD) involved in metabolism of tyrosine. The metabolic block in AKU causes accumulation of homogentisic acid (HGA) that, with advancing age of the patient, leads to severe and painful ochronotic arthropathy. HGD gene was mapped to chromosome 3q13.3 and is composed of 14 exons. In about 400 patients, 142 pathogenic variants were reported that are listed in HGD mutations database (http://hgddatabase.cvtisr.sk/). In this review, we summarise different aspects of AKU genetics and impact of the HGD variants on enzyme function.

Oxidative stress and mechanisms of ochronosis in alkaptonuria

Alkaptonuria (AKU) is a rare metabolic disease due to a deficient activity of the enzyme homogentisate 1,2-dioxygenase (HGD), involved in Phe and Tyr catabolism. Due to such a deficiency, AKU patients undergo accumulation of the metabolite homogentisic acid (HGA), which is prone to oxidation/polymerization reactions causing the production of a melanin-like pigment. Once the pigment is deposited onto connective tissues (mainly in joints, spine, and cardiac valves), a classical bluish-brown discoloration is imparted, leading to a phenomenon known as "ochronosis", the hallmark of AKU. A clarification of the molecular mechanisms for the production and deposition of the ochronotic pigment in AKU started only recently with a range of in vitro and ex vivo human models used for the study of HGA-induced effects. Thanks to redox-proteomic analyses, it was found that HGA could induce significant oxidation of a number of serum and chondrocyte proteins. Further investigations allowed highlighting how HGA-induced proteome alteration, lipid peroxidation, thiol depletion, and amyloid production could contribute to oxidative stress generation and protein oxidation in AKU. This review briefly summarizes the most recent findings on HGA-induced oxidative stress in AKU, helping in the clarification of the molecular mechanisms of ochronosis and potentially providing the basis for its pharmacological treatment. Future work should be undertaken in order to validate in vivo the results so far obtained in in vitro AKU models.

Twelve novel HGD gene variants identified in 99 alkaptonuria patients: focus on 'black bone disease' in Italy

Alkaptonuria (AKU) is an autosomal recessive disorder caused by mutations in homogentisate-1,2-dioxygenase (HGD) gene leading to the deficiency of HGD enzyme activity. The DevelopAKUre project is underway to test nitisinone as a specific treatment to counteract this derangement of the phenylalanine-tyrosine catabolic pathway. We analysed DNA of 40 AKU patients enrolled for SONIA1, the first study in DevelopAKUre, and of 59 other AKU patients sent to our laboratory for molecular diagnostics. We identified 12 novel DNA variants: one was identified in patients from Brazil (c.557T>A), Slovakia (c.500C>T) and France (c.440T>C), three in patients from India (c.469+6T>C, c.650-85A>G, c.158G>A), and six in patients from Italy (c.742A>G, c.614G>A, c.1057A>C, c.752G>A, c.119A>C, c.926G>T). Thus, the total number of potential AKU-causing variants found in 380 patients reported in the HGD mutation database is now 129. Using mCSM and DUET, computational approaches based on the protein 3D structure, the novel missense variants are predicted to affect the activity of the enzyme by three mechanisms: decrease of stability of individual protomers, disruption of protomer-protomer interactions or modification of residues in the region of the active site. We also present an overview of AKU in Italy, where so far about 60 AKU cases are known and DNA analysis has been reported for 34 of them. In this rather small group, 26 different HGD variants affecting function were described, indicating rather high heterogeneity. Twelve of these variants seem to be specific for Italy.

Analysis of HGD Gene Mutations in Patients with Alkaptonuria from the United Kingdom: Identification of Novel Mutations

Alkaptonuria (AKU) is a rare autosomal recessive disorder with incidence ranging from 1:100,000 to 1:250,000. The disorder is caused by a deficiency of the enzyme homogentisate 1,2-dioxygenase (HGD), which results from defects in the HGD gene. This enzyme converts homogentisic acid to maleylacetoacetate and has a major role in the catabolism of phenylalanine and tyrosine. To elucidate the mutation spectrum of the HGD gene in patients with alkaptonuria from 42 patients attending the National Alkaptonuria Centre, 14 exons of the HGD gene and the intron-exon boundaries were analysed by PCR-based sequencing. A total of 34 sequence variants was observed, confirming the genetic heterogeneity of AKU. Of these mutations, 26 were missense substitutions and four splice site mutations. There were two deletions and one duplication giving rise to frame shifts and one substitution abolishing the translation termination codon (no stop). Nine of the mutations were previously unreported novel variants. Using computational approaches based on the 3D structure, these novel mutations are predicted to affect the activity of the protein complex through destabilisation of the individual protomer structure or through disruption of protomer-protomer interactions.

Alkaptonuria

Alkaptonuria (AKU) is a rare disorder of autosomal recessive inheritance. It is caused by a mutation in a gene that results in the accumulation of homogentisic acid (HGA). Characteristically, the excess HGA means sufferers pass dark urine, which upon standing turns black. This is a feature present from birth. Over time patients develop other manifestations of AKU, due to deposition of HGA in collagenous tissues, namely ochronosis and ochronotic osteoarthropathy.   Although this condition does not reduce life expectancy, it significantly affects quality of life. The natural history of this condition is becoming better understood, despite gaps in knowledge. Clinical assessment of the condition has also improved along with the development of a potentially disease-modifying therapy. Furthermore, recent developments in AKU research have led to new understanding of the disease, and further study of the AKU arthropathy has the potential to influence therapy in the management of osteoarthritis.

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.

Alkaptonuria: leading to the treasure in exceptions

The brilliant geneticist, William Bateson, a formidable English experimentalist, was the first to recognize the nature of the "inborn" in Archibald Garrod's errors of metabolism. Bateson's advice to young scientists: "Treasure your exceptions!" summarizes much of the vigorous empiricism associated with the study of rare disorders.The first inborn error of metabolism to be so recognized was alkaptonuria, and it is only recently that a proper understanding of this condition as a disease, rather than a biochemical curiosity, has emerged. Abnormal excretion of the reactive tyrosine metabolite, homogentisic acid, not only provides a tangible biomarker of alkaptonuria, but also a focus for detailed mechanistic understanding.Currently, there is no proven treatment for alkaptonuria but emergence of orphan drug legislation internationally has promoted the licensing of nitisinone (Orfadin™) for an equally rare disorder of tyrosine metabolism - hereditary tyrosinaemia type 1. Nitisinone, a triketone competitive inhibitor of a proximal step leading to the formation of homogentisic acid, has potent therapeutic effects in hereditary tyrosinemia and rapidly ameliorates the primary biochemical abnormality in patients with alkaptonuria.Here, we discuss the context in which nitisinone should be further explored for the treatment of alkaptonuria. This exceptional disease is a paradigm case, which opens up unusual opportunities for basic and applied research. In modern times, it also shows how the conflation of orphan drug legislation and the emerging power and commitment of patient organizations can synergize effectively to advance basic research and therapeutic development in ultra-orphan diseases.

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.

Identification of 11 Novel Homogentisate 1,2 Dioxygenase Variants in Alkaptonuria Patients and Establishment of a Novel LOVD-Based HGD Mutation Database

Enzymatic loss in alkaptonuria (AKU), an autosomal recessive disorder, is caused by mutations in the homogentisate 1,2 dioxygenase (HGD) gene, which decrease or completely inactivate the function of the HGD protein to metabolize homogentisic acid (HGA). AKU shows a very low prevalence (1:100,000-250,000) in most ethnic groups, but there are countries with much higher incidence, such as Slovakia and the Dominican Republic. In this work, we report 11 novel HGD mutations identified during analysis of 36 AKU patients and 41 family members from 27 families originating from 9 different countries, mainly from Slovakia and France. In Slovak patients, we identified two additional mutations, thus a total number of HGD mutations identified in this small country is 12. In order to record AKU-causing mutations and variants of the HGD gene, we have created a HGD mutation database that is open for future submissions and is available online ( http://hgddatabase.cvtisr.sk/ ). It is founded on the Leiden Open (source) Variation Database (LOVD) system and includes data from the original AKU database ( http://www.alkaptonuria.cib.csic.es ) and also all so far reported variants and AKU patients. Where available, HGD-haplotypes associated with the mutations are also presented. Currently, this database contains 148 unique variants, of which 115 are reported pathogenic mutations. It provides a valuable tool for information exchange in AKU research and care fields and certainly presents a useful data source for genotype-phenotype correlations and also for future clinical trials.

Identification of alkaptonuria in the general population: a United Kingdom experience describing the challenges, possible solutions and persistent barriers

Progress in research into rare diseases is challenging. This paper discusses strategies to identify individuals with the rare genetic disease alkaptonuria (AKU) within the general population. Strategies used included a questionnaire survey of general practitioners, a dedicated website and patient network contact, targeted family screening and medical conference targeting. Primary care physicians of the UK were targeted by a postal survey that involved mailing 11,151 UK GPs; the response rate was 18.2%. We have identified 75 patients in the UK with AKU by the following means: postal survey (23), targeted family screening (11), patient networks and the website (41). Targeting medical conferences (AKU, rare diseases, rheumatology, clinical biochemistry, orthopaedics, general practitioners) did not lead to new identification in the UK but helped identify overseas cases. We are now aware of 626 patients worldwide including newly identified non-UK people with AKU in the following areas: Slovakia (208), the rest of Europe (including Turkey) (79), North America (including USA and Canada) (110), and the rest of the world (154). A mechanism for identifying individuals with AKU in the general population-not just in the UK but worldwide-has been established. Knowledge of patients with AKU, both in the UK and outside, is often confined to establishing their location in a particular GP practice or association with a particular medical professional. Mere identification, however, does not always lead to full engagement for epidemiological research purposes or targeting treatment since further barriers exist.

Novel mutations in the homogentisate 1,2 dioxygenase gene identified in Jordanian patients with alkaptonuria

This study was conducted to identify mutations in the homogentisate 1,2 dioxygenase gene (HGD) in alkaptonuria patients among Jordanian population. Blood samples were collected from four alkaptonuria patients, four carriers, and two healthy volunteers. DNA was isolated from peripheral blood. All 14 exons of the HGD gene were amplified using the polymerase chain reaction (PCR) technique. The PCR products were then purified and analyzed by sequencing. Five mutations were identified in our samples. Four of them were novel C1273A, T1046G, 551-552insG, T533G and had not been previously reported, and one mutation T847C has been described before. The types of mutations identified were two missense mutations, one splice site mutation, one frameshift mutation, and one polymorphism. We present the first molecular study of the HGD gene in Jordanian alkaptonuria patients. This study provides valuable information about the molecular basis of alkaptonuria in Jordanian population.

Mutation spectrum of homogentisic acid oxidase (HGD) in alkaptonuria

Alkaptonuria (AKU) is a rare autosomal recessive metabolic disorder, characterized by accumulation of homogentisic acid, leading to darkened urine, pigmentation of connective tissue (ochronosis), joint and spine arthritis, and destruction of cardiac valves. AKU is due to mutations in the homogentisate dioxygenase gene (HGD) that converts homogentisic acid to maleylacetoacetic acid in the tyrosine catabolic pathway. Here we report a comprehensive mutation analysis of 93 patients enrolled in our study, as well as an extensive update of all previously published HGD mutations associated with AKU. Within our patient cohort, we identified 52 HGD variants, of which 22 were novel. This yields a total of 91 identified HGD variations associated with AKU to date, including 62 missense, 13 splice site, 10 frameshift, 5 nonsense, and 1 no-stop mutation. Most HGD variants reside in exons 3, 6, 8, and 13. We assessed the potential effect of all missense variations on protein function, using five bioinformatic tools specifically designed for interpretation of missense variants (SIFT, POLYPHEN, PANTHER, PMUT, and SNAP). We also analyzed the potential effect of splice-site variants using two different tools (BDGP and NetGene2). This study provides valuable resources for molecular analysis of alkaptonuria and expands our knowledge of the molecular basis of this disease.

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.

Alkaptonuria diagnosed in a 4-month-old baby girl: a case report

The mother of a four month old female baby attended in the well baby clinic with the complaint of black staining of the diaper after few minutes of urination. The baby was born of a non consanguineous marriage, healthy and breast fed. Mother noticed that stain first at the age of two and half month. The urine when kept in a test tube for two hours turned black. Laboratory examination of urine revealed increased concentration of homogentisic acid. The patient was diagnosed as alkaptonuria.

Premature arthritis in an elderly woman

We, in this study, report a 72-year-old woman presenting with premature osteoarthritis of the spine, the hips, the knees and the shoulders symptomatic since the age of 50 years. The initial presentation of backache led to a mistaken diagnosis of ankylosing spondylitis. Subsequent manifestation of characteristic discoloration of her sclera and overnight dark urine led to the correct diagnosis of ochronosis.

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.