Histological and Ultrastructural Characterization of Alkaptonuric Tissues

Pseudofilariasis, a Presenting Sign of Alkaptonuria

PURPOSE

To report the association of Pseudofilariasis as a presenting sign of Alkaptonuria.

METHOD

Case Report.

RESULTS

A 49-year-old Indian man was referred because of wormlike objects in his left conjunctiva. Ocular and family history was non-contributory. He had not been to India in 15 years. Past medical history revealed hypertension, hypercholesterolemia, arthritis, and a myocardial infarct. He had undergone two stents, bilateral Achilles tendon repairs and bilateral knee replacements. ROS showed longstanding back stiffness and pain. On ocular examination the vision was 20/25 in each eye and positive findings were in the left eye bulbar conjunctival which showed stationary black vermiform (filarial in appearance) foreign bodies along with 2 small corneal limbal pigmented deposits. Conjunctival biopsy showed dilated lymph channels with interstitial proteinaceous material of a light brown color consistent with Ochronotic pigment; hence diagnostic of Alkaptonuria.

CONCLUSIONS

Pseudofilariasis may be a presenting sign of Alkaptonuria and occur years before a clinical diagnosis is made. Filariasis is always involves white worms and never black. Knowing the ophthalmic signs of this rare disease may lead to an accurate diagnosis earlier thusly avoiding unnecessary tests and examinations.

Alkaptonuria: From Molecular Insights to a Dedicated Digital Platform

Alkaptonuria (AKU) is a genetic disorder that affects connective tissues of several body compartments causing cartilage degeneration, tendon calcification, heart problems, and an invalidating, early-onset form of osteoarthritis. The molecular mechanisms underlying AKU involve homogentisic acid (HGA) accumulation in cells and tissues. HGA is highly reactive, able to modify several macromolecules, and activates different pathways, mostly involved in the onset and propagation of oxidative stress and inflammation, with consequences spreading from the microscopic to the macroscopic level leading to irreversible damage. Gaining a deeper understanding of AKU molecular mechanisms may provide novel possible therapeutical approaches to counteract disease progression. In this review, we first describe inflammation and oxidative stress in AKU and discuss similarities with other more common disorders. Then, we focus on HGA reactivity and AKU molecular mechanisms. We finally describe a multi-purpose digital platform, named ApreciseKUre, created to facilitate data collection, integration, and analysis of AKU-related data.

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.

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.

Effects of Nitisinone on Oxidative and Inflammatory Markers in Alkaptonuria: Results from SONIA1 and SONIA2 Studies

Nitisinone (NTBC) was recently approved to treat alkaptonuria (AKU), but there is no information on its impact on oxidative stress and inflammation, which are observed in AKU. Therefore, serum samples collected during the clinical studies SONIA1 (40 AKU patients) and SONIA2 (138 AKU patients) were tested for Serum Amyloid A (SAA), CRP and IL-8 by ELISA; Advanced Oxidation Protein Products (AOPP) by spectrophotometry; and protein carbonyls by Western blot. Our results show that NTBC had no significant effects on the tested markers except for a slight but statistically significant effect for NTBC, but not for the combination of time and NTBC, on SAA levels in SONIA2 patients. Notably, the majority of SONIA2 patients presented with SAA > 10 mg/L, and 30 patients in the control group (43.5%) and 40 patients (58.0%) in the NTBC-treated group showed persistently elevated SAA > 10 mg/L at each visit during SONIA2. Higher serum SAA correlated with lower quality of life and higher morbidity. Despite no quantitative differences in AOPP, the preliminary analysis of protein carbonyls highlighted patterns that deserve further investigation. Overall, our results suggest that NTBC cannot control the sub-clinical inflammation due to increased SAA observed in AKU, which is also a risk factor for developing secondary amyloidosis.

Chondrocyte death involvement in osteoarthritis

Chondrocyte apoptosis is known to contribute to articular cartilage damage in osteoarthritis and is correlated to a number of cartilage disorders. Micromass cultures represent a convenient means for studying chondrocyte biology, and, in particular, their death. In this review, we focused the different kinds of chondrocyte death through a comparison between data reported in the literature. Chondrocytes show necrotic features and, occasionally, also apoptotic features, but usually undergo a new form of cell death called Chondroptosis, which occurs in a non-classical manner. Chondroptosis has some features in common with classical apoptosis, such as cell shrinkage, chromatin condensation, and involvement, not always, of caspases. The most crucial peculiarity of chondroptosis relates to the ultimate elimination of cellular remnants. Independent of phagocytosis, chondroptosis may serve to eliminate cells without inflammation in situations in which phagocytosis would be difficult. This particular death mechanism is probably due to the unusual condition chondrocytes both in vivo and in micromass culture. This review highlights on the morpho-fuctional alterations of articular cartilage and focus attention on various types of chondrocyte death involved in this degeneration. The death features have been detailed and discussed through in vitro studies based on tridimensional chondrocyte culture (micromasses culture). The study of this particular mechanism of cartilage death and the characterization of different biological and biochemical underlying mechanisms can lead to the identification of new potentially therapeutic targets in various joint diseases.

[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.

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.

Towards a Precision Medicine Approach Based on Machine Learning for Tailoring Medical Treatment in Alkaptonuria

ApreciseKUre is a multi-purpose digital platform facilitating data collection, integration and analysis for patients affected by Alkaptonuria (AKU), an ultra-rare autosomal recessive genetic disease. It includes genetic, biochemical, histopathological, clinical, therapeutic resources and quality of life scores that can be shared among registered researchers and clinicians in order to create a Precision Medicine Ecosystem (PME). The combination of machine learning application to analyse and re-interpret data available in the ApreciseKUre shows the potential direct benefits to achieve patient stratification and the consequent tailoring of care and treatments to a specific subgroup of patients. In this study, we have developed a tool able to investigate the most suitable treatment for AKU patients in accordance with their Quality of Life scores, which indicates changes in health status before/after the assumption of a specific class of drugs. This fact highlights the necessity of development of patient databases for rare diseases, like ApreciseKUre. We believe this is not limited to the study of AKU, but it represents a proof of principle study that could be applied to other rare diseases, allowing data management, analysis, and interpretation.

Homogentisic acid induces cytoskeleton and extracellular matrix alteration in alkaptonuric cartilage

Alkaptonuria (AKU) is an ultra-rare disease caused by the deficient activity of homogentisate 1,2-dioxygenase enzyme, leading the accumulation of homogentisic acid (HGA) in connective tissues implicating the formation of a black pigmentation called "ochronosis." Although AKU is a multisystemic disease, the most affected tissue is the articular cartilage, which during the pathology appears to be highly damaged. In this study, a model of alkaptonuric chondrocytes and cartilage was realized to investigate the role of HGA in the alteration of the extracellular matrix (ECM). The AKU tissues lost its architecture composed of collagen, proteoglycans, and all the proteins that characterize the ECM. The cause of this alteration in AKU cartilage is attributed to a degeneration of the cytoskeletal network in chondrocytes caused by the accumulation of HGA. The three cytoskeletal proteins, actin, vimentin, and tubulin, were analyzed and a modification in their amount and disposition in AKU chondrocytes model was identified. Cytoskeleton is involved in many fundamental cellular processes; therefore, the aberration in this complex network is involved in the manifestation of AKU disease.

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.

Exogenous Ochronosis as an Elastotic Disease: A Light-Microscopic Approach

BACKGROUND

Exogenous ochronosis (EO) is a deposition disease associated with application of hydroquinone-containing preparations. Characteristic ochronotic bodies (OBs) arise from endogenous connective tissues, most often reported as collagen. We highlight a significant role for elastic fibers as a precursor tissue.

OBJECTIVE

To evaluate elastic tissue pathology in EO, specifically as it relates a precursor role in ochronotic body formation.

METHODS

In this retrospective observational study, a literature review using PubMed/MEDLINE database was conducted to ascertain the most commonly ascribed precursor connective tissue. Eleven histopathologic cases of EO were identified. Patient demographics and clinical characteristics were recorded. Slides were reviewed for the presence and grade of solar elastosis (SE), the relationship of OBs to elastotic material, the presence of elastotic fibers transitioning to OBs, and positivity of bodies with Verhoeff-van Gieson elastic tissue stain.

RESULTS

Elastic fibers are uncommonly reported as the major precursor tissue of OBs. SE was uniformly present in our cases, and the majority demonstrated heavy/high-grade elastosis. Elastotic fibers transitioning to OBs were observed in all cases, and the bodies demonstrated Verhoeff-van Gieson positivity.

LIMITATIONS

Small sample size.

CONCLUSIONS

Ochronotic body formation is associated with SE, and bodies appear to arise from damaged elastic fibers.

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.

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.

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.

Ochronotic arthropathy-a rare clinical case

Ochronotic arthropathy is a rare condition found in patients with alkaptonuria that results from the accumulation of ochronotic pigment. We present the case of a 65-year-old woman who presented for medical evaluation due to hip and knee chronic pain. The physical and radiographic findings were compatible with an end-stage hip osteoarthritis and knee osteoarthritis. During total hip arthroplasty it was noticed that the articular capsule and the cartilage of the femoral head were black. In the postoperative period she was diagnosed with alkaptonuria. Later, a total knee arthroplasty was performed and once more the presence of black cartilage was noted. Alkaptonuria usually appears after age 30 and is usually asymptomatic until the involvement of the spine, hip, knee and shoulder joints. Therefore, orthopaedic surgeons must be suspicious of an atypical arthropathy in order to not be overwhelmed during surgery with the presence of darkened cartilage.

Inflammatory and oxidative stress biomarkers in alkaptonuria: data from the DevelopAKUre project

OBJECTIVE

The aim of this work was to assess baseline serum levels of established biomarkers related to inflammation and oxidative stress in samples from alkaptonuric subjects enrolled in SONIA1 (n = 40) and SONIA2 (n = 138) clinical trials (DevelopAKUre project).

METHODS

Baseline serum levels of Serum Amyloid A (SAA), IL-6, IL-1β, TNFα, CRP, cathepsin D (CATD), IL-1ra, and MMP-3 were determined through commercial ELISA assays. Chitotriosidase activity was assessed through a fluorimetric method. Advanced Oxidation Protein Products (AOPP) were determined by spectrophotometry. Thiols, S-thiolated proteins and Protein Thiolation Index (PTI) were determined by spectrophotometry and HPLC. Patients' quality of life was assessed through validated questionnaires.

RESULTS

We found that SAA serum levels were significantly increased compared to reference threshold in 57.5% and 86% of SONIA1 and SONIA2 samples, respectively. Similarly, chitotriosidase activity was above the reference threshold in half of SONIA2 samples, whereas CRP levels were increased only in a minority of samples. CATD, IL-1β, IL-6, TNFα, MMP-3, AOPP, thiols, S-thiolated protein and PTI showed no statistically significant differences from control population. We provided evidence that alkaptonuric patients presenting with significantly higher SAA, chitotriosidase activity and PTI reported more often a decreased quality of life. This suggests that worsening of symptoms in alkaptonuria (AKU) is paralleled by increased inflammation and oxidative stress, which might play a role in disease progression.

CONCLUSIONS

Monitoring of SAA may be suggested in AKU to evaluate inflammation. Though further evidence is needed, SAA, chitotriosidase activity and PTI might be proposed as disease activity markers in AKU.