Recent advances in management of alkaptonuria (invited review; best practice article)

Redefining a new frontier in alkaptonuria therapy with AI-driven drug candidate design via in- silico innovation

A rare metabolic condition called alkaptonuria (AKU) is caused by a decrease in homogentisate 1,2 dioxygenase (HGO) activity due to a mutation in homogentisate dioxygenase (HGD) gene. Homogentisic acid is a byproduct of the catabolism of tyrosine and phenylalanine that darkens the urine and accumulates in connective tissues which causes an agonizing arthritis. Employing the use of deep learning artificial intelligence (AI) drug design, this study aims to alleviate the current toxicity of the AKU drugs currently in use, particularly nitisinone, by utilizing the natural flavanol kaempferol molecule as a 4-hydroxyphenylpyruvate dioxygenase inhibitor. Kaempferol was employed to generate three effective de novo drug candidates targeting the enzyme 4-hydroxyphenylpyruvate dioxygenase using an AI drug design tool. We present novel AIK formulations in the present study. The AIK's (Artificial Intelligence Kaempferol) examination of drug-likeliness among the three led to its choice as a possible target. The toxicity assessment research of AIK demonstrates that it is not only safer to use than other treatments, but also more efficient. The docking of the AIGT with 4-hydroxyphenylpyruvate dioxygenase, which revealed a binding affinity of around -9.099 kcal/mol, highlights the AIK's potential as a therapeutic candidate. An innovative approach to deal with challenging circumstances is thus presented in this study by new formulations kaempferol that have been meticulously designed by AI. The results of the in vitro tests must be confirmed in vivo, even though AI-designed AIK is effective and sufficiently safe as computed.

Alkaptonuria

Alkaptonuria is a rare inborn error of metabolism caused by the deficiency of homogentisate 1,2-dioxygenase activity. The consequent homogentisic acid (HGA) accumulation in body fluids and tissues leads to a multisystemic and highly debilitating disease whose main features are dark urine, ochronosis (HGA-derived pigment in collagen-rich connective tissues), and a painful and severe form of osteoarthropathy. Other clinical manifestations are extremely variable and include kidney and prostate stones, aortic stenosis, bone fractures, and tendon, ligament and/or muscle ruptures. As an autosomal recessive disorder, alkaptonuria affects men and women equally. Debilitating symptoms appear around the third decade of life, but a proper and timely diagnosis is often delayed due to their non-specific nature and a lack of knowledge among physicians. In later stages, patients' quality of life might be seriously compromised and further complicated by comorbidities. Thus, appropriate management of alkaptonuria requires a multidisciplinary approach, and periodic clinical evaluation is advised to monitor disease progression, complications and/or comorbidities, and to enable prompt intervention. Treatment options are patient-tailored and include a combination of medications, physical therapy and surgery. Current basic and clinical research focuses on improving patient management and developing innovative therapies and implementing precision medicine strategies.

Pitfalls in the diagnosis of hematuria

Detection of hemoglobin (Hb) and red blood cells in urine (hematuria) is characterized by a large number of pitfalls. Clinicians and laboratory specialists must be aware of these pitfalls since they often lead to medical overconsumption or incorrect diagnosis. Pre-analytical issues (use of vacuum tubes or urine tubes containing preservatives) can affect test results. In routine clinical laboratories, hematuria can be assayed using either chemical (test strips) or particle-counting techniques. In cases of doubtful results, Munchausen syndrome or adulteration of the urine specimen should be excluded. Pigmenturia (caused by the presence of dyes, urinary metabolites such as porphyrins and homogentisic acid, and certain drugs in the urine) can be easily confused with hematuria. The peroxidase activity (test strip) can be positively affected by the presence of non-Hb peroxidases (e.g. myoglobin, semen peroxidases, bacterial, and vegetable peroxidases). Urinary pH, haptoglobin concentration, and urine osmolality may affect specific peroxidase activity. The implementation of expert systems may be helpful in detecting preanalytical and analytical errors in the assessment of hematuria. Correcting for dilution using osmolality, density, or conductivity may be useful for heavily concentrated or diluted urine samples.

Nitisinone Treatment Affects Biomarkers of Bone and Cartilage Remodelling in Alkaptonuria Patients

Nitisinone has been approved for treatment of alkaptonuria (AKU). Non-invasive biomarkers of joint tissue remodelling could aid in understanding the molecular changes in AKU pathogenesis and how these can be affected by treatment. Serological and urinary biomarkers of type I collagen and II collagen in AKU were investigated in patients enrolled in the randomized SONIA 2 (NCT01916382) clinical study at baseline and yearly until the end of the study (Year 4). The trajectories of the biomarkers over time were observed. After treatment with nitisinone, the biomarkers of type I collagen remodelling increased at Year 1 (19% and 40% increase in CTX-I and PRO-C1, respectively), which was potentially reflected in the higher degree of mobility seen following treatment. The biomarkers of type II collagen remodelling decreased over time in the nitisinone group: C2M showed a 9.7% decline at Year 1, and levels then remained stable over the following visits; CTX-II showed a 26% decline at Year 3 and 4 in the nitisinone-treated patients. Nitisinone treatment induced changes in biomarkers of bone and cartilage remodelling. These biomarkers can aid patient management and deepen our knowledge of the molecular mechanisms of this rare disease.

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.

Hereditary Tyrosinemia Type 1 Mice under Continuous Nitisinone Treatment Display Remnants of an Uncorrected Liver Disease Phenotype

Hereditary tyrosinemia type 1 (HT1) is a genetic disorder of the tyrosine degradation pathway (TIMD) with unmet therapeutic needs. HT1 patients are unable to fully break down the amino acid tyrosine due to a deficient fumarylacetoacetate hydrolase (FAH) enzyme and, therefore, accumulate toxic tyrosine intermediates. If left untreated, they experience hepatic failure with comorbidities involving the renal and neurological system and the development of hepatocellular carcinoma (HCC). Nitisinone (NTBC), a potent inhibitor of the 4-hydroxyphenylpyruvate dioxygenase (HPD) enzyme, rescues HT1 patients from severe illness and death. However, despite its demonstrated benefits, HT1 patients under continuous NTBC therapy are at risk to develop HCC and adverse reactions in the eye, blood and lymphatic system, the mechanism of which is poorly understood. Moreover, NTBC does not restore the enzymatic defects inflicted by the disease nor does it cure HT1. Here, the changes in molecular pathways associated to the development and progression of HT1-driven liver disease that remains uncorrected under NTBC therapy were investigated using whole transcriptome analyses on the livers of Fah- and Hgd-deficient mice under continuous NTBC therapy and after seven days of NTBC therapy discontinuation. Alkaptonuria (AKU) was used as a tyrosine-inherited metabolic disorder reference disease with non-hepatic manifestations. The differentially expressed genes were enriched in toxicological gene classes related to liver disease, liver damage, liver regeneration and liver cancer, in particular HCC. Most importantly, a set of 25 genes related to liver disease and HCC development was identified that was differentially regulated in HT1 vs. AKU mouse livers under NTBC therapy. Some of those were further modulated upon NTBC therapy discontinuation in HT1 but not in AKU livers. Altogether, our data indicate that NTBC therapy does not completely resolves HT1-driven liver disease and supports the sustained risk to develop HCC over time as different HCC markers, including Moxd1, Saa, Mt, Dbp and Cxcl1, were significantly increased under NTBC.

A Previously Undiagnosed Case of Alkaptonuria in an 80-Year-Old Patient: A Case Report

Alkaptonuria is a rare genetic metabolic disorder of autosomal recessive inheritance characterised by the accumulation of homogentisic acid in the body. It is diagnosed upon identification of characteristic symptoms, using various biochemical investigations, radiographic pictures, and a variety of specialised tests. Here we are discussing the case of an 80-year-old female patient with incidental findings of alkaptonuria. It is crucial to understand the fundamental diagnostic investigations that can be used in low-income nations or facilities where investigations like genetic testing, gas chromatography, and mass spectrometry are not readily available for the diagnosis of alkaptonuria.

Case report: Thoracolumbar spinal stenosis associated with alkaptonuria

Background

Alkaptonuria is a rare autosomal genetic disorder with an incidence of about 1 in 1 million per year. Spinal involvement often manifests in the later stages of the disease. However, this is the first report of the presentation of thoracolumbar spinal stenosis.

Case presentation

We report the case of a 61-year-old female patient with significant thoracolumbar stenosis symptoms. The patient had obvious kyphosis with preoperative lower extremity muscle strength grade 2/5. Symptoms and imaging signs initially suggested ankylosing spondylitis. This patient was classified into motor incomplete injury (ASIA C). However, the patient was found to have melanin deposits on the sclera and skin, and the urine was darkened at rest. CT and MRI both suggested no bone bridge connection between vertebrae, which was the key difference between ankylosing spondylitis and alkaptonuria in imaging. Most importantly, urine specimen testing and intraoperative pathology demonstrated alkaptonuria. The patient underwent spinal decompression and vertebral body fixation. Postoperative recovery was good: the patient had significantly relieved pain and could stand and walk.

Conclusion

This case is the first report of thoracolumbar spinal stenosis associated with alkaptonuria involving the spine.

Clinical development innovation in rare diseases: overcoming barriers to successful delivery of a randomised clinical trial in alkaptonuria-a mini-review

Alkaptonuria is a rare inherited disorder for which there was no disease-modifying treatment. In order to develop a successful approved therapy of AKU multiple barriers had to be overcome. These included activities before the conduct of the study including deciding on the drug therapy, the dose of the drug to be used, clarify the nature of the disease, develop outcome measures likely to yield a positive outcome, have a strategy to ensure appropriate patient participation through identification, build a consortium of investigators, obtain regulatory approval for proposed investigation plan and secure funding. Significant barriers were overcome during the conduct of the multicentre study to ensure harmonisation. Mechanisms were put in place to recruit and retain patients in the study. Barriers to patient access following completion of the study and regulatory approval were resolved.

Nutritional interventions for patients with alkaptonuria: A minireview

Alkaptonuria (AKU, OMIM, No. 203500) is a rare, slow-progressing, irreversible, multisystemic disease resulting from a deficiency of the homogentisate 1,2-dioxygenase enzyme, which leads to the accumulation of homogentisic acid (HGA) and subsequent deposition as pigment in connective tissues called ochronosis. As a result, severe arthropathy of large joints and spondyloarthropathy with frequent fractures, ligament ruptures, and osteoporosis develops in AKU patients. Since 2020, the first-time treatment with nitisinone has become available in the European Union. Nitisinone significantly reduces HGA production and arrests ochronosis in AKU patients. However, blocking of the tyrosine metabolic pathway by the drug leads to tyrosine plasma and tissue concentrations increase. The nitisinone-induced hypertyrosinemia can lead to the development of corneal keratopathy, and once it develops, the treatment needs to be interrupted. A decrease in overall protein intake reduces the risk of the keratopathy during nitisinone-induced hypertyrosinemia in AKU patients. The low-protein diet is not only poorly tolerated by patients, but over longer periods, leads to a severe muscle loss and weight gain due to increased energy intake from carbohydrates and fats. Therefore, the development of novel nutritional approaches is required to prevent the adverse events due to nitisinone-induced hypertyrosinemia and the negative impact on skeletal muscle metabolism in AKU patients.

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.

Ipsilateral hip and knee joints ochronotic arthropathy treated by total joint replacement, a case report

Background

Ochronotic arthropathy affecting the hip and knee joints is one of the musculoskeletal manifestations of alkaptonuria which results from a tyrosine amino acid metabolism disorder. The condition has no definitive management; however, end-stage joint arthritis is well treated by total joint replacement.

Case presentation

We present a case of a healthy male patient who presented with left hip and bilateral knee (more on the left side) progressive pain and limitation of motion how was diagnosed as having advanced arthritis and was treated by total hip and total knee replacement for the left hip and knee joints respectively. Intraoperatively, the tissues had blackish discoloration characterizing ochronosis. This patient was diagnosed as having ochronotic arthropathy based on the intraoperative findings.

Conclusions

Accurate diagnosis of ochronotic arthropathy could only be reached intraoperatively; furthermore, end-stage joint arthritis could be treated efficiently by total joint replacement surgery.

A robust bacterial high-throughput screening system to evaluate single nucleotide polymorphisms of human homogentisate 1,2-dioxygenase in the context of alkaptonuria

Alkaptonuria (AKU) is a rare inborn error of metabolism caused by a defective homogentisate 1,2-dioxygenase (HGD), an enzyme involved in the tyrosine degradation pathway. Loss of HGD function leads to the accumulation of homogentisic acid (HGA) in connective body tissues in a process called ochronosis, which results on the long term in an early-onset and severe osteoarthropathy. HGD's quaternary structure is known to be easily disrupted by missense mutations, which makes them an interesting target for novel treatment strategies that aim to rescue enzyme activity. However, only prediction models are available providing information on a structural basis. Therefore, an E. coli based whole-cell screening was developed to evaluate HGD missense variants in 96-well microtiter plates. The screening principle is based on HGD's ability to convert the oxidation sensitive HGA into maleylacetoacetate. More precisely, catalytic activity could be deduced from pyomelanin absorbance measurements, derived from the auto-oxidation of remaining HGA. Optimized screening conditions comprised several E. coli expression strains, varied expression temperatures and varied substrate concentrations. In addition, plate uniformity, signal variability and spatial uniformity were investigated and optimized. Finally, eight HGD missense variants were generated via site-directed mutagenesis and evaluated with the developed high-throughput screening (HTS) assay. For the HTS assay, quality parameters passed the minimum acceptance criterion for Z' values > 0.4 and single window values > 2. We found that activity percentages versus wildtype HGD were 70.37 ± 3.08% (for M368V), 68.78 ± 6.40% (for E42A), 58.15 ± 1.16% (for A122V), 69.07 ± 2.26% (for Y62C), 35.26 ± 1.90% (for G161R), 35.86 ± 1.14% (for P230S), 23.43 ± 4.63% (for G115R) and 19.57 ± 11.00% (for G361R). To conclude, a robust, simple, and cost-effective HTS system was developed to reliably evaluate and distinguish human HGD missense variants by their HGA consumption ability. This HGA quantification assay may lay the foundation for the development of novel treatment strategies for missense variants in AKU.

Analysis of the Phenotype Differences in Siblings with Alkaptonuria

Alkaptonuria (AKU) is a rare autosomal recessive disorder caused by mutations within a gene coding for homogentisate 1,2-dioxygenase (HGD). To date, 251 different variants of this gene have been reported. The metabolic disorder in AKU leads to the accumulation of homogentisic acid (HGA), resulting in ochronosis (pigmentation of the connective tissues) and severe ochronotic spondylo-arthropathy, which usually manifests in the mid-thirties. An earlier genotype−phenotype correlation study showed no differences in serum HGA levels, absolute urinary excretion of HGA, or in the clinical symptoms between patients carrying HGD variants leading to 1% or >30% residual HGD activity. Still, as reported previously, the variance of the excretion of the HGA was smaller within affected siblings that share a common genotype. The present study is the first ever to systematically analyze the baseline clinical data of 24 AKU sibling pairs/groups collected in the SONIA 2 (Suitability Of Nitisinone In Alkaptonuria 2) study to evaluate phenotypical differences between patients carrying the same HGD genetic variants. We show that even between siblings there was considerable variability in the disease severity. This indicates that some other yet unidentified genetic, biomechanical, or environmental modifying factors may contribute to accelerated pigmentation and connective tissue damage observed in some patients.

Revisiting Quantification of Phenylalanine/Tyrosine Flux in the Ochronotic Pathway during Long-Term Nitisinone Treatment of Alkaptonuria

Changes in the phenylalanine (PHE)/tyrosine (TYR) pathway metabolites before and during homogentisic acid (HGA)-lowering by nitisinone in the Suitability of Nitisinone in Alkaptonuria (AKU) 2 (SONIA 2) study enabled the magnitude of the flux in the pathway to be examined. SONIA 2 was a 48-month randomised, open-label, evaluator-blinded, parallel-group study performed in the UK, France and Slovakia recruiting patients with confirmed AKU to receive either 10 mg nitisinone or no treatment. Site visits were performed at 3 months and yearly thereafter. Results from history, photographs of eyes/ears, whole body scintigraphy, echocardiography and abdomen/pelvis ultrasonography were combined to produce the Alkaptonuria Severity Score Index (cAKUSSI). PHE, TYR, hydroxyphenylpyruvate (HPPA), hydroxyphenyllactate (HPLA) and HGA metabolites were analysed by liquid chromatography/tandem mass spectrometry in 24 h urine and serum samples collected before and during nitisinone. Serum metabolites were corrected for total body water (TBW), and the sum of 24 h urine plus total body water metabolites of PHE, TYR, HPPA, HPLA and HGA were determined. The sum of urine metabolites (PHE, TYR, HPPA, HPLA and HGA) were similar pre- and peri-nitisinone. The sum of TBW metabolites and sum TBW + URINE metabolites were significantly higher peri-nitisinone (p < 0.001 for both) compared with pre-nitisinone baseline. Significantly higher concentrations of metabolites from the tyrosine metabolic pathway were observed during treatment with nitisinone. Arguments for unmasking of the ochronotic pathway and biliary elimination of HGA are put forward.

Long-term follow-up of alkaptonuria patients: single center experience

OBJECTIVES

Alkaptonuria is a rare autosomal recessive genetic disorder resulting from the deficiency of homogentisate 1,2 dioxygenase (HGD), the third enzyme in the tyrosine degradation pathway. Homogentisic acid produced in excess oxidizes into ochronotic pigment polymer. Accumulation of this pigment in various tissues leads to systemic disease.

METHODS

Clinical, laboratory, molecular findings and treatment characteristics of 35 patients followed up in Ege University Pediatric Nutrition, and Metabolism Department with the diagnosis of alkaptonuria were evaluated retrospectively.

RESULTS

Twenty-four males (68.57%) and 11 females (31.42%) with a confirmed diagnosis of alkaptonuria from 32 different families were included in the study. We identified 11 different genetic variants; six of these were novel. c.1033C>T, c.676G>A, c.664G>A, c.731_734del, c.1009G>T, c.859_862delins ATAC were not previously reported in the literature. 24 (68.57%) patients only adhered to a low-protein diet in our study group. Seven (20%) patients initiated a low protein diet and NTBC therapy. Mean urinary HGA decreased by 88.7% with nitisinone. No statistical changes were detected in urinary HGA excretion with the low protein diet group.

CONCLUSIONS

In our study, alkaptonuria patients were diagnosed at different ages, from infancy to adulthood, and progressed with other systemic involvement in the follow-up. Since the initial period is asymptomatic, giving potentially effective treatment from an early age is under discussion. Raising disease awareness is very important in reducing disease mortality and morbidity rates.

Metabolomic studies in the inborn error of metabolism alkaptonuria reveal new biotransformations in tyrosine metabolism

Alkaptonuria (AKU) is an inherited disorder of tyrosine metabolism caused by lack of active enzyme homogentisate 1,2-dioxygenase (HGD). The primary consequence of HGD deficiency is increased circulating homogentisic acid (HGA), the main agent in the pathology of AKU disease. Here we report the first metabolomic analysis of AKU homozygous Hgd knockout (Hgd ^-/-) mice to model the wider metabolic effects of Hgd deletion and the implication for AKU in humans. Untargeted metabolic profiling was performed on urine from Hgd ^-/- AKU (n = 15) and Hgd ^+/- non-AKU control (n = 14) mice by liquid chromatography high-resolution time-of-flight mass spectrometry (Experiment 1). The metabolites showing alteration in Hgd ^-/- were further investigated in AKU mice (n = 18) and patients from the UK National AKU Centre (n = 25) at baseline and after treatment with the HGA-lowering agent nitisinone (Experiment 2). A metabolic flux experiment was carried out after administration of ¹³C-labelled HGA to Hgd ^-/-(n = 4) and Hgd ^+/-(n = 4) mice (Experiment 3) to confirm direct association with HGA. Hgd ^-/- mice showed the expected increase in HGA, together with unexpected alterations in tyrosine, purine and TCA-cycle pathways. Metabolites with the greatest abundance increases in Hgd ^-/- were HGA and previously unreported sulfate and glucuronide HGA conjugates, these were decreased in mice and patients on nitisinone and shown to be products from HGA by the ¹³C-labelled HGA tracer. Our findings reveal that increased HGA in AKU undergoes further metabolism by mainly phase II biotransformations. The data advance our understanding of overall tyrosine metabolism, demonstrating how specific metabolic conditions can elucidate hitherto undiscovered pathways in biochemistry and metabolism.

A novel mutation in the homogentisate 1,2 dioxygenase gene identified in Chinese Hani pediatric patients with Alkaptonuria

BACKGROUND

Alkaptonuria (AKU) is a rare tyrosine metabolism disorder caused by homogentisate 1,2-dioxygenase (HGD) mutations and homogentisic acid (HGA) accumulation. In this study, we investigated the genotype-phenotype relationship in AKU patients with a novel HGD gene mutation from a Chinese Hani family.

METHODS

Routine clinical examination and laboratory evaluation were performed, urine alkalinization test and urinary gas chromatography-mass spectrometry were used to assess HGA. Gene sequencing was utilized to study the defining features of AKU. NetGene2-2.42 and BDGP software was used to predict protein structure online. Flow cytometry and RT-PCR were used to analyze HGD proteins and HGD mRNA, respectively.

RESULTS

Two pediatric patients fulfilled diagnostic criteria for AKU with eddish-brown or black diapers and urine HGA testing. Sequencing testing revealed that all members of this family had a novel samesense mutation c.15G>A at the edge of exon 1 of the HGD. By flow cytometry, the expression of HGD protein in the pediatric patients' peripheral blood mononuclear cells was barely expressed. NetGene2-2.42 and BDGP software showed that the mutation reduced the score of the 5' splice donor site and disrupted its normal splicing, and the RT-PCR product also demonstrated that the defect in the HGD protein was due to the lack of the first exon containing the start codon ATG after the mutation.

CONCLUSIONS

The novel mutation c.15G > A in HGD is associated with the AKU phenotype. It may affect the splicing of exon 1, leading to exon skipping, which impairs the structure and function of the protein.

Alkaptonuria in Russia

Alkaptonuria is characterized by the accumulation of homogentisic acid (HGA), part of which is excreted in the urine but the excess HGA forms a dark brown ochronotic pigment that deposits in the connective tissue (ochronosis), eventually leading to early-onset severe arthropathy. We analyzed a cohort of 48 Russian AKU families by sequencing all 14 exons (including flanking intronic sequences) of the homogentisate 1,2-dioxygenase gene (HGD) and Multiplex Ligation-dependent Probe Amplification (MLPA) analysis. Nine novel likely pathogenic HGD variants were identified, which have not been reported previously in any other country. Recently, Bychkov et al. [1] reported on the variant spectrum in another cohort of 49 Russian AKU patients. Here we summarize complete data from both cohorts that include 82 Russian AKU families. Taken together, 31 different HGD variants were found in these patients, of which 14 are novel and found only in Russia. The most common variant was c.481G>A (p.(Gly161Arg)), present in almost 54% of all AKU alleles.

Comparing nitisinone 2 mg and 10 mg in the treatment of alkaptonuria-An approach using statistical modelling

BACKGROUND

Outcomes from studies employing nitisinone 10 mg and 2 mg in alkaptonuria were compared.

PATIENTS AND METHODS

Sixty-nine patients in each of the nitisinone (10 mg daily) and controls of suitability of nitisinone in alkaptonuria 2 (SONIA 2), as well as 37 and 23 in nitisinone (2 mg daily) and control cohorts at the National Alkaptonuria Centre (NAC), respectively, were followed up for 4 years. Severity of alkaptonuria (AKU) was assessed by the AKU Severity Score Index (AKUSSI). 24-h urine homogentisic acid (uHGA24), serum HGA (sHGA), serum tyrosine (sTYR) and serum nitisinone (sNIT) were also analysed at each time point. Dietetic support was used in the NAC, but not in SONIA 2. Safety outcomes were also compared. All statistical analyses were post hoc.

RESULTS

The slope of the AKUSSI was 0.55, 0.19, 0.30, and 0.06 per month in the control NAC, nitisinone NAC, control SONIA 2, and nitisinone SONIA 2 cohorts, respectively. The intersection of the slopes on the x-axis was -132, -411, -295, and - 1460 months, respectively. The control and nitisinone slope comparisons were statistically significant both in the NAC (p < 0.001) and the SONIA 2 (p < 0.001). Corneal keratopathy occurred in 3 and 10 patients in the NAC and SONIA 2, respectively.

DISCUSSION

The nitisinone 10 mg dose decreased disease progression more than the 2 mg dose although the incidence of corneal keratopathy was 14.5% and 4.9%, respectively.

CONCLUSION

Nitisinone 10 mg decreased urine and serum HGA, increased serum tyrosine, and decreased disease progression more than 2 mg. Low-protein dietetic support may be needed to mitigate tyrosinaemia following nitisinone.

HIGHLIGHTS

Nitisinone 10 mg apparently slows alkaptonuria disease progression more than 2 mg in adults.Corneal keratopathy during nitisinone therapy was more common in men.Serum nitisinone concentrations increased significantly over time.Nitisinone may inhibit cytochrome P450 self catabolism.

Musculoskeletal manifestations in Alkaptonuria: A cross-sectional study

This study aimed to determine the patient characteristics and clinical presentation of Alkaptonuria cases reported by the Biochemical Genetics Lab.An observational study was conducted at the Biochemical Genetics Lab. Alkaptonuria patients were diagnosed based on the homogentisic acid peak in urine and their demographics and clinical data collected from to 2013 to 2019. Clinical history related to joint diseases, ochronotic presentation, and urine darkening on standing was collected.During 7 years, 21 Alkaptonuria cases were reported from BGL; mean age 19.4 ± 24.5 years (range 0.2-66 years) and male to female ratio of 2:1. Of the total, only 9 were adults (mean age, 44 ± 12 years). Most adult patients had musculoskeletal involvement, with joint pain (n = 9) and ochronotic pigmentation (n = 6), whereas all patients presented with a history of urine darkening on standing (21/21 cases).The high prevalence of musculoskeletal involvement observed in patients with albuminuria is likely to be missed by physicians unless specifically tested for in such cases.

Clinical development innovation in rare diseases: lessons learned and best practices from the DevelopAKUre consortium

New opportunities have arisen for development of therapies for rare diseases with the increased focus and progress in the field. However, standardised framework integrating individual initiatives has not been formed. We present lessons learned and best practice from a collaborative success case in developing a treatment for a rare genetic disease. Our unique consortium model incorporated several of the identified developments under one project, DevelopAKUre, truly bringing together academia, industry and patient organisations in clinical drug development. We found that the equal partnership between all parties in our consortium was a key success factor creating a momentum based on a strong organisational culture where all partners had high engagement and taking ownership of the entire programme. With an agreed mutual objective, this provided synergies through connecting the strengths of the individual parties. Another key success factor was the central role of the patient organisation within the management team, and their unique study participants’ advocacy role securing the understanding and meeting the needs of the clinical study participants in real-time. This resulted in an accelerated enrolment into the clinical studies with a high retention rate allowing for delivery of the programme with significantly improved timelines. Our project was partly funded through an external EU research grant, enabling our model with equal partnership. Further attention within the community should be given to establishing a functional framework where sustainable funding and risk sharing between private and public organisations allow for our model to be replicated.

Efficacy of Phenylalanine- and Tyrosine-Restricted Diet in Alkaptonuria Patients on Nitisinone Treatment: Case Series and Review of Literature

INTRODUCTION

Nitisinone used in alkaptonuria (AKU) can result in keratopathy due to strongly increased tyrosine levels.

METHODS

This study aimed to investigate nutritional status and changes in plasma tyrosine and phenylalanine and urinary homogentisic acid (u-HGA) levels in 8 adult AKU patients (mean age, 56.3 ± 4.7 years) who were on tyrosine/phenylalanine-restricted diet together with 2 mg/day nitisinone.

RESULTS

The treatment period was 23.4 ± 6.9 months. Daily dietary protein intake was restricted to 0.8-1.0 g/kg/day. Daily tyrosine intake was restricted to 260-450 mg/day for females and 330-550 mg/day for males. Tyrosine/phenylalanine-free amino acid supplements accounted for an average of 56.1% of daily protein intake. The following assessments were performed: anthropometric and plasma tyrosine level measurements every 2 months; ophthalmological examination every 6 months, and nutritional laboratory analyses and measurements of plasma amino acids and u-HGA once in a year. It was targeted to keep the plasma tyrosine level <500 μmol/L. The plasma tyrosine level was <100 μmol/L before the treatment in all patients and around a mean of 582.5 ± 194.8 μmol/L during the treatment. The diet was rearranged if a plasma tyrosine level of >700 μmol/L was detected. The u-HGA level before and after the 1st year of treatment was 1,429.3 ± 1,073.4 mmol/mol creatinine and 33.6 ± 9.5 mmol/mol creatinine, respectively. None of the patients developed keratopathy or experienced weight loss and protein or micronutrient deficiency.

CONCLUSION

AKU patients should receive tyrosine/phenylalanine-restricted diet for reducing plasma tyrosine level to the safe range. Tyrosine/phenylalanine-free amino acid supplements can be safely used to enhance dietary compliance. Keratopathy and nutrient deficiency should be frequently monitored.

[Severe spine lesion following alkaptonuria. Case report]

Alkaptonuria is a rare autosomal recessive disease. In these patients, melanin-like compounds as the final products of impaired metabolism of homogentisic acid are deposited mainly in connective tissue, including cartilage tissue of intervertebral discs. Similar to other degenerative spine diseases, lumbar segment is often damaged. The authors report a 67-year-old patient with alkaptonuria. Compression of cauda equina by damaged cartilage masses of intervertebral discs and spine ligaments with deposits of ochronotic pigment is described. Previously diagnosed alkaptonuria in this patient was confirmed by surgical findings (black pigmentation) and histological data.

Vitiligo, alkaptonuria, and nitisinone-A report of three families and review of the literature

Four patients, from three families, with alkaptonuria receiving 4-hydroxyphenylpyruvate dioxygenase-inhibiting nitisinone therapy, which lowers homogentisic acid and increases tyrosine, developed vitiligo. Three of the four patients were receiving nitisinone 2 mg daily, while the fourth was on 10 mg daily. All four patients were either receiving or had received transiently proton-pump inhibitors as therapy for dyspepsia. The ages of the patients were 35, 42, 40, and 67 years, respectively. Three patients were men and one was a woman. All four patients were either taking a proton-pump inhibitor or had been taking one at some point. Three of the four were of South Asian and one of Caucasian background. The three patients with South Asian background also had either a personal or family history of autoimmune disease. Distressing vitiligo, initially in an acrofacial distribution, developed unexpectedly in these four patients, before then progressing to involve other parts of the body. Potential factors in the appearance of vitiligo in this setting, including nitisinone and other drug therapy, are explored and responses to the appearance of vitiligo are discussed.

Variant Analysis of Alkaptonuria Families with Significant Founder Effect in Jordan

Background

Metabolic disorder alkaptonuria is an autosomal recessive disorder caused by mutations in the HGD gene, and a deficiency HGD enzyme activity results in an accumulation of homogentisic acid (HGA), ochronosis, and destruction of connective tissue.

Methods

We clinically evaluated 18 alkaptonuria patients (age range, 3 to 60 years) from four unrelated families. Furthermore, 11 out of 18 alkaptonuria patients and 7 unaffected members were enrolled for molecular investigations by utilizing Sanger sequencing to identify variants of the 14 exons of HGD gene.

Results

We found that the seven patients from the 4 unrelated families carried a recurrent pathogenic missense variant (c.365C>T, p. Ala122Val) in exon 6 of HGD gene. The variant was fully segregated with the disease in affected family members while the other unaffected family members were heterozygous carriers for this variant. Additionally, the clinical features were fully predicted with alkaptonuria disorder.

Conclusion

In this study, we confirmed that the most common variants in Jordanian AKU patients was c.365C>T, p. Ala122Val in exon 6 of HGD gene. Additionally, we correlated the clinical and genetic features of AKU patients at various ages (3-60 years).

Expression of tyrosine pathway enzymes in mice demonstrates that homogentisate 1,2-dioxygenase deficiency in the liver is responsible for homogentisic acid-derived ochronotic pigmentation

Alkaptonuria (AKU) is caused by homogentisate 1,2-dioxygenase (HGD) deficiency. This study aimed to determine if HGD and other enzymes related to tyrosine metabolism are associated with the location of ochronotic pigment. Liver, kidney, skin, bone, brain, eyes, spleen, intestine, lung, heart, cartilage, and muscle were harvested from 6 AKU BALB/c Hgd -/- (3 females, 3 males) and 4 male C57BL/6 wild type (WT) mice. Hgd, 4-hydroxyphenylpyruvate dioxygenase (4-Hppd), tyrosine hydroxylase (Th), and tyrosinase (Tyr) mRNA expression was investigated using qPCR. Adrenal gland and gonads from AKU Hgd tm1a -/- mice were LacZ stained, followed by qPCR analysis of Hgd mRNA. The liver had the highest expression of Hgd, followed by the kidney, with none detected in cartilage or brain. Low-level Hgd expression was observed within developing male germ cells within the testis and epididymis in Hgd tm1a -/-. 4-Hppd was most abundant in liver, with smaller amounts in kidney and low-level expression in other tissues. Th was expressed mainly in brain and Tyr was found primarily in the eyes. The tissue distribution of both Hgd and 4-Hppd suggest that ochronotic pigment in AKU mice is a consequence of enzymes within the liver, and not from enzymatic activity within ochronotic tissues. Excessive accumulation of HGA as ochronotic pigment in joints and other connective tissues originates from the circulation and therefore the extracellular fluid. The tissue distribution of both Th and Tyr suggests that these enzymes are not involved in the formation of HGA-derived ochronotic pigment.

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 in an adolescent boy

Alkaptonuria is a rare genetic disorder resulting in abnormality of tyrosine metabolism. It is one of the Garrod's tetrad of 'inborn errors of metabolism' proposed to have Mendelian recessive inheritance. The disorder is characterised by deposition of homogentisic acid leading to ochronosis and ochronotic osteoarthropathy; however, blackish discoloration of urine is the only childhood manifestation. Other manifestations present only after third decade. A 13-year-old boy presented to paediatric nephrology clinic with blackish discolouration of urine since infancy. Examination revealed bluish black discolouration of bilateral sclera and ear cartilage; however, he had no symptoms of ochronotic osteoarthropathy. Genetic test pointed towards alkaptonuria. Currently, he is on regular follow-up and is being treated with vitamin C to delay the progression of the disease. Early diagnosis with appropriate intervention delays the onset of complications and preserves the quality of life of the patient.

Melanins as Sustainable Resources for Advanced Biotechnological Applications

Melanins are a class of biopolymers that are widespread in nature and have diverse origins, chemical compositions, and functions. Their chemical, electrical, optical, and paramagnetic properties offer opportunities for applications in materials science, particularly for medical and technical uses. This review focuses on the application of analytical techniques to study melanins in multidisciplinary contexts with a view to their use as sustainable resources for advanced biotechnological applications, and how these may facilitate the achievement of the United Nations Sustainable Development Goals.

Long-Term Outcomes of the Knee and Hip Arthroplasties in Patients with Alkaptonuria

Alkaptonuria is a rare autosomal recessive metabolic disorder. It is characterized by the accumulation of homogentisic acid in the body due to a lack of enzymes that degrade it. Over time, it results in joint degeneration and eventually leads to ochronosis. Ochronosis refers to bluish-black discoloration of connective and other tissues within the body. In this study, we present 5 distinct cases diagnosed with alkaptonuria. They have undergone 8 total joint replacement surgeries (4 hips and 4 knees) within 8 years (2010-2018). All patients had an excellent outcome over several years. The follow-up period ranged from 2 to 10 years. Although none of the presented cases had intraoperative or postoperative adverse sequelae, we must take care when dealing with patients with ochronotic arthropathy. They carry a higher risk of complications than other patients with osteoarthritis disease. These complications include fractures due to fragile bone quality, muscle or tendon rupture, joint instability, and anesthesia-related complications. Total joint arthroplasty is a valid and safe option in the management of hip and knee ochronotic arthropathy.

Nitisinone causes acquired tyrosinosis in alkaptonuria

For over two decades, nitisinone (NTBC) has been successfully used to manipulate the tyrosine degradation pathway and save the lives of many children with hereditary tyrosinaemia type 1. More recently, NTBC has been used to halt homogentisic acid accumulation in alkaptonuria (AKU) with evidence suggesting its efficacy as a disease modifying agent. NTBC-induced hypertyrosinaemia has been associated with cognitive impairment and potentially sight-threatening keratopathy. In the context of a non-lethal condition (ie, AKU), these serious risks call for an evaluation of the wider impact of NTBC on the tyrosine pathway. We hypothesised that NTBC increases the tyrosine pool size and concentrations in tissues. In AKU mice tyrosine concentrations of tissue homogenates were measured before and after treatment with NTBC. In humans, pulse injection with l-[¹³ C₉ ]tyrosine and l-[d₈ ]phenylalanine was used along with compartmental modelling to estimate the size of tyrosine pools before and after treatment with NTBC. We found that NTBC increased tyrosine concentrations in murine tissues by five to nine folds. It also significantly increased the tyrosine pool size in humans (P < .001), suggesting that NTBC increases tyrosine not just in serum but also in tissues (ie, acquired tyrosinosis). This study provides, for the first time, the experimental proof for the magnitude of NTBC-related acquired tyrosinosis which should be overcome to ensure the safe use of NTBC in AKU.

Reversal of ochronotic pigmentation in alkaptonuria following nitisinone therapy: Analysis of data from the United Kingdom National Alkaptonuria Centre

BACKGROUND

Increased homogentisic acid (HGA) causes ochronosis. Nitisinone decreases HGA. The aim was to study the effect of nitisinone on the ochronosis progression.

METHODS

Photographs of the eyes and ears were acquired from patients attending the National Alkaptonuria Centre (NAC) at V-1 (pre-baseline visit), V0 (baseline visit when 2 mg nitisinone was commenced), and yearly at V1, V2, and V3 visits. Photographs were inspected for evolution of ochronotic pigment and also scored categorically to derive eye, ear, and combined ochronosis scores. An ear cartilage biopsy was also carried out at V0 and one year after V3 (V4) and ochronotic pigment was assessed and quantitated. Visits were compared for changes in pigment. Fasting blood and 24-hour urine samples were collected for measurement of HGA.

RESULTS

There were 80 AKU patients at V0, and 52, 47, and 40 at V1, V2, and V3 in the group with variable numbers (VAR Group) respectively; 23 patients attended once before V0, in the V-1 visit. Photographs of patients show increase in eye pigment between V-1 and V0, followed by decrease post-nitisinone at V1, V2, and V3. Ear and combined ochronosis semiquantitative scoring showed an increase between V-1 and V0 (P < .01), followed by a decrease at V1, V2, and V3, in the VAR group (P < .01). Ochronotic pigment in ear biopsy between V0 and V4 showed a 19.1% decrease (P < .05).

CONCLUSIONS

Nitisinone decreases HGA and partially reverses ochronosis.

Transient pockets as mediators of gas molecules routes inside proteins: The case study of dioxygen pathway in homogentisate 1,2-dioxygenase and its implication in Alkaptonuria development

Alkaptonuria (AKU) is an ultra-rare disease caused by mutations in homogentisate 1,2-dioxygenase (HGD) enzyme, characterized by the loss of enzymatic activity and the accumulation of its substrate, homogentisic acid (HGA) in different tissues, leading to ochronosis and organ degeneration. Although the pathological effects of HGD mutations are largely studied, less is known about the structure of the enzyme, in particular the pathways for dioxygen diffusion to the active site, required for the enzymatic reaction, are still uninvestigated. In the present project, the combination of two in silico techniques, Molecular Dynamics (MD) simulation and Implicit Ligand Sampling (ILS), was used to delineate gas diffusion routes in HGD enzyme. A route from the central opening of the hexameric structure of the enzyme to the back of the active site trough the protein moiety was identified as the path for dioxygen diffusion, also overlapping with a transient pocket, which then assumes an important role in dioxygen diffusion. Along the route the sequence location of the missense variant E401Q, responsible for AKU development, was also found, suggesting such mutation to be conducive of enzymatic activity loss by altering the flow dynamics of dioxygen. Our in silico approach allowed also to delineate the route of HGA substrate to the active site, until now only supposed.

Pigmentation Chemistry and Radical-Based Collagen Degradation in Alkaptonuria and Osteoarthritic Cartilage

Alkaptonuria (AKU) is a rare disease characterized by high levels of homogentisic acid (HGA); patients suffer from tissue ochronosis: dark brown pigmentation, especially of joint cartilage, leading to severe early osteoarthropathy. No molecular mechanism links elevated HGA to ochronosis; the pigment's chemical identity is still not known, nor how it induces joint cartilage degradation. Here we give key insight on HGA-derived pigment composition and collagen disruption in AKU cartilage. Synthetic pigment and pigmented human cartilage tissue both showed hydroquinone-resembling NMR signals. EPR spectroscopy showed that the synthetic pigment contains radicals. Moreover, we observed intrastrand disruption of collagen triple helix in pigmented AKU human cartilage, and in cartilage from patients with osteoarthritis. We propose that collagen degradation can occur via transient glycyl radicals, the formation of which is enhanced in AKU due to the redox environment generated by pigmentation.

Conditional targeting in mice reveals that hepatic homogentisate 1,2-dioxygenase activity is essential in reducing circulating homogentisic acid and for effective therapy in the genetic disease alkaptonuria

Alkaptonuria is an inherited disease caused by homogentisate 1,2-dioxygenase (HGD) deficiency. Circulating homogentisic acid (HGA) is elevated and deposits in connective tissues as ochronotic pigment. In this study, we aimed to define developmental and adult HGD tissue expression and determine the location and amount of gene activity required to lower circulating HGA and rescue the alkaptonuria phenotype.

We generated an alkaptonuria mouse model using a knockout-first design for the disruption of the HGD gene. Hgd tm1a −/− mice showed elevated HGA and ochronosis in adulthood. LacZ staining driven by the endogenous HGD promoter was localised to only liver parenchymal cells and kidney proximal tubules in adulthood, commencing at E12.5 and E15.5 respectively. Following removal of the gene trap cassette to obtain a normal mouse with a floxed 6^th HGD exon, a double transgenic was then created with Mx1-Cre which conditionally deleted HGD in liver in a dose dependent manner. 20% of HGD mRNA remaining in liver did not rescue the disease, suggesting that we need more than 20% of liver HGD to correct the disease in gene therapy.

Kidney HGD activity which remained intact reduced urinary HGA, most likely by increased absorption, but did not reduce plasma HGA nor did it prevent ochronosis. In addition, downstream metabolites of exogenous ¹³C₆-HGA, were detected in heterozygous plasma, revealing that hepatocytes take up and metabolise HGA.

This novel alkaptonuria mouse model demonstrated the importance of targeting liver for therapeutic intervention, supported by our observation that hepatocytes take up and metabolise HGA.

A patient survey on the impact of alkaptonuria symptoms as perceived by the patients and their experiences of receiving diagnosis and care

BACKGROUND

Alkaptonuria (AKU) is an ultrarare and multifaceted disease characterized by the absence of functional homogentisate 1,2-dioxygenase activity, the enzyme responsible for breakdown of homogentisic acid-a tyrosine-degradation product. The presymptomatic phase of the disease makes diagnosis difficult, with many patients unidentified or diagnosed late in life.

OBJECTIVE

To date, no study has analyzed the perceived impact of different symptoms or the experiences of individuals through the patient journey in the context of AKU. This study aimed to examine patients' perceptions of AKU symptoms and their impact on quality of life as well as patients' experiences of being diagnosed and living with the disease.

METHODS

Data for this study were collected using a quantitative self-report questionnaire administered online to people with AKU.

RESULTS

Data from 45 participants indicate that symptoms with the highest impact for patients are those related to pain and ruptures, disability and inability to perform normal routines, emotional/mental health issues, and heart complications. Findings also revealed significant delays in contact with healthcare services and time to diagnosis. Furthermore, patients reported difficulty in receiving information about AKU, treatment and care, and long-term disease management support.

CONCLUSIONS

Time to diagnosis and care of AKU is significantly delayed. Symptoms of AKU with the highest impact on quality of life for patients are those related to pain and disability and the inability to perform normal routines. Bridging any gaps between patients with AKU and healthcare professionals through education could help improve patients' experiences with AKU through the patient journey.

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.

Alkaptonuria: Current Perspectives

The last 15 years have been the most fruitful in the history of research on the metabolic disorder alkaptonuria (AKU). AKU is caused by a deficiency of homogentisate dioxygenase (HGD), the enzyme involved in metabolism of tyrosine, and is characterized by the presence of dark ochronotic pigment in the connective tissue that is formed, due to high levels of circulating homogentisic acid. Almost 120 years ago, Sir Archibald Garrod used AKU to illustrate the concept of Mendelian inheritance in man. In January 2019, the phase III clinical study SONIA 2 was completed, which tested the effectiveness and safety of nitisinone in the treatment of AKU. Results were positive, and they will serve as the basis for the application for registration of nitisinone for treatment of AKU at the European Medicines Agency. Therefore, AKU might become a rare disease for which a cure will be found by 2020. We understand the natural history of the disease and the process of ochronosis much more, but at the same time there are still unanswered questions. One of them is the issue of the factors influencing the varying severity of the disease, since our recent genotype–phenotype study did not show that differences in residual homogentisic acid activity caused by the different mutations was responsible. Although nitisinone has proved to arrest the process of ochronosis, it has some unwanted effects and does not cure the disease completely. As such, enzyme replacement or gene therapy might become a new focus of AKU research, for which a novel suitable mouse model of AKU is available already. We believe that the story of AKU is also a story of effective collaboration between scientists and patients that might serve as an example for other rare diseases.

Long-Term Outcomes and Practical Considerations in the Pharmacological Management of Tyrosinemia Type 1

Tyrosinemia type 1 (TT1) is a rare metabolic disease caused by a defect in tyrosine catabolism. TT1 is clinically characterized by acute liver failure, development of hepatocellular carcinoma, renal and neurological problems, and consequently an extremely poor outcome. This review showed that the introduction of 2-(2-nitro-4-trifluoromethylbenzoyl)-1,3-cyclohexanedione (NTBC) in 1992 has revolutionized the outcome of TT1 patients, especially when started pre-clinically. If started early, NTBC can prevent liver failure, renal problems, and neurological attacks and decrease the risk for hepatocellular carcinoma. NTBC has been shown to be safe and well tolerated, although the long-term effectiveness of treatment with NTBC needs to be awaited. The high tyrosine concentrations caused by treatment with NTBC could result in ophthalmological and skin problems and requires life-long dietary restriction of tyrosine and its precursor phenylalanine, which could be strenuous to adhere to. In addition, neurocognitive problems have been reported since the introduction of NTBC, with hypothesized but as yet unproven pathophysiological mechanisms. Further research should be done to investigate the possible relationship between important clinical outcomes and blood concentrations of biochemical parameters such as phenylalanine, tyrosine, succinylacetone, and NTBC, and to develop clear guidelines for treatment and follow-up with reliable measurements. This all in order to ultimately improve the combined NTBC and dietary treatment and limit possible complications such as hepatocellular carcinoma development, neurocognitive problems, and impaired quality of life.

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.

Ochronotic pigmentation is caused by homogentisic acid and is the key event in alkaptonuria leading to the destructive consequences of the disease-A review

Ochronosis is the process in alkaptonuria (AKU) that causes all the debilitating morbidity. The process involves selective deposition of homogentisic acid (HGA)-derived pigment in tissues altering the properties of these tissues, leading to their failure. Some tissues like cartilage are more easily affected by ochronosis while others such as the liver and brain are unaffected for reasons that are still not understood. In vitro and mouse models of ochronosis have confirmed the dose relationships between HGA and ochronosis and also their modulation by p-hydroxyphenylpyruvate dioxygenase inhibition. Ochronosis cannot be fully reversed and is a key factor in influencing treatment decisions. Earlier detection of ochronosis preferably by noninvasive means is desirable. A cause-effect relationship between HGA and ochronosis is discussed. The similarity in AKU and familial hypercholesterolaemia is explored and lessons learnt. More research is needed to more fully understand the crucial nature of ochronosis.

Interactive alkaptonuria database: investigating clinical data to improve patient care in a rare disease

Alkaptonuria (AKU) is an ultrarare autosomal recessive disorder (MIM 203500) that is caused byby a complex set of mutations in homogentisate 1,2-dioxygenasegene and consequent accumulation of homogentisic acid (HGA), causing a significant protein oxidation. A secondary form of amyloidosis was identified in AKU and related to high circulating serum amyloid A (SAA) levels, which are linked with inflammation and oxidative stress and might contribute to disease progression and patients' poor quality of life. Recently, we reported that inflammatory markers (SAA and chitotriosidase) and oxidative stress markers (protein thiolation index) might be disease activity markers in AKU. Thanks to an international network, we collected genotypic, phenotypic, and clinical data from more than 200 patients with AKU. These data are currently stored in our AKU database, named ApreciseKUre. In this work, we developed an algorithm able to make predictions about the oxidative status trend of each patient with AKU based on 55 predictors, namely circulating HGA, body mass index, total cholesterol, SAA, and chitotriosidase. Our general aim is to integrate the data of apparently heterogeneous patients with AKUAKU by using specific bioinformatics tools, in order to identify pivotal mechanisms involved in AKU for a preventive, predictive, and personalized medicine approach to AKU.-Cicaloni, V., Spiga, O., Dimitri, G. M., Maiocchi, R., Millucci, L., Giustarini, D., Bernardini, G., Bernini, A., Marzocchi, B., Braconi, D., Santucci, A. Interactive alkaptonuria database: investigating clinical data to improve patient care in a rare disease.

[Black knee-ochronotic alterations in alkaptonuria]

This article presents the case of a 53-year-old male patient born in Sri Lanka, who presented to the outpatient unit with the suspicion of empyema of the knee joint. Within the framework of knee arthroscopy, the diagnosis of ochronosis was made and later confirmed by histopathological biopsy. The alkaptonuria is caused by a homogentisate 1,2-dioxygenase deficiency and leads to an accumulation of homogentisic acid, a degradation product of tyrosine. This leads to the characteristic appearance of ochronosis with bluish-black deposits in the tissue (e.g. in connective tissue, sclera and ear cartilage) and a black coloration of the urine.

Quantification of the flux of tyrosine pathway metabolites during nitisinone treatment of Alkaptonuria

Nitisinone decreases homogentisic acid (HGA) in Alkaptonuria (AKU) by inhibiting the tyrosine metabolic pathway in humans. The effect of different daily doses of nitisinone on circulating and 24 h urinary excretion of phenylalanine (PA), tyrosine (TYR), hydroxyphenylpyruvate (HPPA), hydroxyphenyllactate (HPLA) and HGA in patients with AKU was studied over a four week period. Forty AKU patients, randomised into five groups of eight patients, received doses of 1, 2, 4 or 8 mg of nitisinone daily, or no drug (control). Metabolites were analysed by tandem mass spectrometry in 24 h urine and serum samples collected before and after nitisinone. Serum metabolites were corrected for total body water and the sum of 24 hr urine plus total body water metabolites of PA, TYR, HPPA, HPLA and HGA were determined. Body weight and urine urea were used to check on stability of diet and metabolism over the 4 weeks of study. The sum of quantities of urine metabolites (PA, TYR, HPPA, HPLA and HGA) were similar pre- and post-nitisinone. The sum of total body water metabolites were significantly higher post-nitisinone (p < 0.0001) at all doses. Similarly, combined 24 hr urine:total body water ratios for all analytes were significantly higher post-nitisinone, compared with pre-nitisinone baseline for all doses (p = 0.0002 – p < 0.0001). Significantly higher concentrations of metabolites from the tyrosine metabolic pathway were observed in a dose dependant manner following treatment with nitisinone and we speculate that, for the first time, experimental evidence of the metabolite pool that would otherwise be directed towards pigment formation, has been unmasked.

Efficacy of low dose nitisinone in the management of alkaptonuria

AIM

To study the efficacy of low dosage of nitisinone in alkaptonuria.

BACKGROUND

Alkaptonuria (AKU) is a rare genetic disease which induces deposition of homogentisic acid (HGA) in connective inducing premature arthritis, lithiasis, cardiac valve disease, fractures, muscle and tendon ruptures and osteopenia. Recent studies showed that nitisinone decreases HGA and is a beneficial therapy in AKU. This treatment induces an increase in tyrosine levels which can induces adverse effects as keratopathy.

METHODS

We described the evolution HGA excretion and tyrosine evolution in 3 AKU patients treated by very low dosage of nitisinone with regards to their daily protein intakes. We also described the first pregnancy in an AKU patient treated by nitisinone.

RESULTS

We found mild clinical signs of alkaptonuria on vertebra MRI in two young adults and homogentisate deposition in teeth of a 5 years old girl. Very low dose of nitisinone (10% of present recommended dose: 0.2 mg/day) allowed to decrease homogentisic acid by >90% without increasing tyrosine levels above 500 μmol/ in these three patients.

INTERPRETATIONS

The analysis of the follow-up data shows that, in our three patients, a low-dosage of nitisinone is sufficient to decrease urinary HGA without increasing plasma tyrosine levels above the threshold of 500 μmol/L.