Tissue distribution of 2-(2-nitro-4-trifluoromethylbenzoyl)-cyclohexane-1,3-dione (NTBC) and its effect on enzymes involved in tyrosine catabolism in the mouse

Data derived extrapolation factors (DDEFs) for rat to human interspecies extrapolation for the HPPD inhibitor mesotrione

In the risk assessment of agrochemicals, there has been a historical paucity of using data to refine the default adjustment factors, even though large datasets are available to support this. The current state of the science for addressing uncertainty regarding animal to human extrapolation (AFA) is to develop a "data-derived" adjustment factor (DDEF) to quantify such differences, if data are available. Toxicokinetic (TK) and toxicodynamic (TD) differences between species can be utilized for the DDEF, with human datasets being ideal yet rare. We identified a case for a currently registered herbicide, mesotrione, in which human TK and TD are available. This case study outlines an approach for the development of DDEFs using comparative human and animal data and based on an adverse outcome pathway (AOP) for inhibition of 4-hydroxyphenol pyruvate dioxygenase (HHPD). The calculated DDEF for rat to human extrapolation (AFA) for kinetics (AFAK = 2.5) was multiplied by the AFA for dynamics (AFAD = 0.3) resulting in a composite DDEF of ∼1 (AFA = 0.75). This reflects the AOP and available scientific evidence that humans are less sensitive than rats to the effects of HPPD inhibitors. Further analyses were conducted utilizing in vitro datasets from hepatocytes and liver cytosols and extrapolated to whole animal using in vitro to in vivo extrapolation (IVIVE) to support toxicodynamic extrapolation. The in vitro datasets resulted in the same AFAD as derived for in vivo data (AFAD = 0.3). These analyses demonstrate that a majority of the species differences are related to toxicodynamics. Future work with additional in vitro/in vivo datasets for other HPPD inhibitors and cell types will further support this result. This work demonstrates utilization of all available toxicokinetic and toxicodynamic data to replace default uncertainty factors for agrochemical human health risk assessment.

Species differences and human relevance of the toxicity of 4-hydroxyphenylpyruvate dioxygenase (HPPD) inhibitors and a new approach method in vitro for investigation

The mode of action (MoA) of the 4-hydroxyphenylpyruvate dioxygenase (HPPD) inhibitor herbicides in mammals is well described and is generally accepted to be due to a build-up of excess systemic tyrosine which is associated with the range of adverse effects reported in laboratory animals. What is less well accepted is the basis for the marked difference in the effects of HPPD inhibitors that has been observed across experimental species and humans, where some species show significant toxicities whereas in other species exposure causes few effects. The activity of the catabolic enzyme tyrosine aminotransferase (TAT) varies across species including humans and it is hypothesized that this primarily accounts for the different levels of tyrosinemia observed between species and leads to the subsequent differences in toxicity. The previously reported activities of TAT in different species showed large variation, were inconsistent, have methodological uncertainties and could lead to a reasonable challenge to the scientific basis for the species difference in response. To provide clarity, a new method was developed for the simultaneous and systematic measurement of TAT in vitro using robust methodologies in a range of mammalian species including human. The results obtained showed general correlation between high TAT activity and low in vivo toxicity when using a model based on hepatic cytosol and a very convincing correlation when using a primary hepatocyte model. These data fully support the role of TAT in explaining the species differences in toxicity. Moreover, this information should give greater confidence in selecting the most appropriate animal model (the mouse) for human health risk assessment and for key classification and labeling decision-making.

Novel therapeutics in nystagmus: what has the genetics taught us so far?

Nystagmus is a disorder characterised by uncontrolled, repetitive, to-and-fro movement of the eyes. It can occur as a seemingly isolated disorder but is most commonly the first, or most obvious, feature in a host of ophthalmic and systemic disorders. The number of underlying causes is vast, and recent improvements in the provision of genetic testing have shown that many conditions can include nystagmus as a feature, but that phenotypes overlap significantly. Therefore, an increase in the understanding of the genetic causes of nystagmus has shown that successful novel therapeutics for 'nystagmus' can target either specific underlying disorders and mechanisms (aiming to treat the underlying condition as a whole), or a final common pathway (aiming to treat the nystagmus directly).

Plain language summary

Novel treatments for a disorder of eye movement (nystagmus): what has the genetics taught us so far? Nystagmus is a disorder of eye movement characterised by uncontrolled, to-and-fro movements. It can occur as an isolated disorder, in conditions affecting other parts of the eye, in conditions affecting multiple other parts of the body or secondary to neurological diseases (brain diseases). In recent years, advances in genetic testing methods and increase in genetic testing in healthcare systems have provided a greater understanding of the underlying causes of nystagmus. They have highlighted the bewildering number of genetic causes that can result in what looks like a very similar eye movement disorder.In recent years, new classes of drugs have been developed for some of the causes of nystagmus, and some new drugs have been developed for other conditions which have the potential to work in certain types of nystagmus. For these reasons, genetics has taught us that identifying new possible treatments for nystagmus can either be dependent on identifying the underlying genetic cause and aiming to treat that, or aiming to treat the nystagmus per se by targeting a final common pathway. A toolkit based on specific treatments for specific conditions is more to have meaningful impact on 'nystagmus' than pursuing a panacea based on a 'one size fits all' approach.

Oxidative Stress, Glutathione Metabolism, and Liver Regeneration Pathways Are Activated in Hereditary Tyrosinemia Type 1 Mice upon Short-Term Nitisinone Discontinuation

Hereditary tyrosinemia type 1 (HT1) is an inherited condition in which the body is unable to break down the amino acid tyrosine due to mutations in the fumarylacetoacetate hydrolase (FAH) gene, coding for the final enzyme of the tyrosine degradation pathway. As a consequence, HT1 patients accumulate toxic tyrosine derivatives causing severe liver damage. Since its introduction, the drug nitisinone (NTBC) has offered a life-saving treatment that inhibits the upstream enzyme 4-hydroxyphenylpyruvate dioxygenase (HPD), thereby preventing production of downstream toxic metabolites. However, HT1 patients under NTBC therapy remain unable to degrade tyrosine. To control the disease and side-effects of the drug, HT1 patients need to take NTBC as an adjunct to a lifelong tyrosine and phenylalanine restricted diet. As a consequence of this strict therapeutic regime, drug compliance issues can arise with significant influence on patient health. In this study, we investigated the molecular impact of short-term NTBC therapy discontinuation on liver tissue of Fah-deficient mice. We found that after seven days of NTBC withdrawal, molecular pathways related to oxidative stress, glutathione metabolism, and liver regeneration were mostly affected. More specifically, NRF2-mediated oxidative stress response and several toxicological gene classes related to reactive oxygen species metabolism were significantly modulated. We observed that the expression of several key glutathione metabolism related genes including Slc7a11 and Ggt1 was highly increased after short-term NTBC therapy deprivation. This stress response was associated with the transcriptional activation of several markers of liver progenitor cells including Atf3, Cyr61, Ddr1, Epcam, Elovl7, and Glis3, indicating a concreted activation of liver regeneration early after NTBC withdrawal.

Nonhuman Primate Model of Oculocutaneous Albinism with TYR and OCA2 Mutations

Human visual acuity is anatomically determined by the retinal fovea. The ontogenetic development of the fovea can be seriously hindered by oculocutaneous albinism (OCA), which is characterized by a disorder of melanin synthesis. Although people of all ethnic backgrounds can be affected, no efficient treatments for OCA have been developed thus far, due partly to the lack of effective animal models. Rhesus macaques are genetically homologous to humans and, most importantly, exhibit structures of the macula and fovea that are similar to those of humans; thus, rhesus macaques present special advantages in the modeling and study of human macular and foveal diseases. In this study, we identified rhesus macaque models with clinical characteristics consistent with those of OCA patients according to observations of ocular behavior, fundus examination, and optical coherence tomography. Genomic sequencing revealed a biallelic p.L312I mutation in TYR and a homozygous p.S788L mutation in OCA2, both of which were further confirmed to affect melanin biosynthesis via in vitro assays. These rhesus macaque models of OCA will be useful animal resources for studying foveal development and for preclinical trials of new therapies for OCA.

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.

4-Hydroxyphenylpyruvate Dioxygenase Inhibitors: From Chemical Biology to Agrochemicals

The development of new herbicides is receiving considerable attention to control weed biotypes resistant to current herbicides. Consequently, new enzymes are always desired as targets for herbicide discovery. 4-Hydroxyphenylpyruvate dioxygenase (HPPD, EC 1.13.11.27) is an enzyme engaged in photosynthetic activity and catalyzes the transformation of 4-hydroxyphenylpyruvic acid (HPPA) into homogentisic acid (HGA). HPPD inhibitors constitute a promising area of discovery and development of innovative herbicides with some advantages, including excellent crop selectivity, low application rates, and broad-spectrum weed control. HPPD inhibitors have been investigated for agrochemical interests, and some of them have already been commercialized as herbicides. In this review, we mainly focus on the chemical biology of HPPD, discovery of new potential inhibitors, and strategies for engineering transgenic crops resistant to current HPPD-inhibiting herbicides. The conclusion raises some relevant gaps for future research directions.

Predicting tyrosinaemia: a mathematical model of 4-hydroxyphenylpyruvate dioxygenase inhibition by nitisinone in rats

Nitisinone or 2-(2-nitro-4-trifluoromethylbenzoyl)cyclohexane-1,3-dione is a reversible inhibitor of 4-hydroxyphenylpyruvate dioxygenase (HPPD), an enzyme important in tyrosine catabolism. Today, nitisinone is successfully used to treat Hereditary Tyrosinaemia type 1, although its original expected role was as a herbicide. In laboratory animals, treatment with nitisinone leads to the elevation of plasma tyrosine (tyrosinaemia). In rats and Beagle dogs, repeat low-dose exposure to nitisinone leads to corneal opacities whilst similar studies in the mouse and Rhesus monkey showed no comparable toxicities or other treatment related findings. The differences in toxicological sensitivities have been related to the upper limit of the concentration of tyrosine that accumulates in plasma, which is driven by the amount/activity of tyrosine aminotransferase. A physiologically based, pharmacodynamics ordinary differential equation model of HPPD inhibition to bolus exposure of nitisinone in vivo is presented. Going beyond traditional approaches, asymptotic analysis is used to separate the different timescales of events involved in HPPD inhibition and tyrosinaemia. This analysis elucidates, in terms of the model parameters, a critical inhibitor concentration (at which tyrosine concentration starts to rise) and highlights the contribution of in vitro measured parameters to events in an in vivo system. Furthermore, using parameter-fitting methods, a systematically derived reduced model is shown to fit well to rat data, making explicit how the parameters are informed by such data. This model in combination with in vitro descriptors has potential as a surrogate for animal experimentation to predict tyrosinaemia, and further development can extend its application to other related medical scenarios.

The Dose Makes the Poison: Nutritional Overload Determines the Life Traits of Blood-Feeding Arthropods

Vertebrate blood composition is heavily biased towards proteins, and hemoglobin, which is a hemeprotein, is by far the most abundant protein. Typically, hematophagous insects ingest blood volumes several times their weight before the blood meal. This barbarian feast offers an abundance of nutrients, but the degradation of blood proteins generates toxic concentrations of amino acids and heme, along with unparalleled microbiota growth. Despite this challenge, hematophagous arthropods have successfully developed mechanisms that bypass the toxicity of these molecules. While these adaptations allow hematophagous arthropods to tolerate their diet, they also constitute a unique mode of operation for cell signaling, immunity, and metabolism, the study of which may offer insights into the biology of disease vectors and may lead to novel vector-specific control methods.

4-Hydroxyphenylpyruvate Dioxygenase and Its Inhibition in Plants and Animals: Small Molecules as Herbicides and Agents for the Treatment of Human Inherited Diseases

This review mainly focuses on the physiological function of 4-hydroxyphenylpyruvate dioxygenase (HPPD), as well as on the development and application of HPPD inhibitors of several structural classes. Among them, one illustrative example is represented by compounds belonging to the class of triketone compounds. They were discovered by serendipitous observations on weed growth and were developed as bleaching herbicides. Informed reasoning on nitisinone (NTBC, 14), a triketone that failed to reach the final steps of the herbicidal design and development process, allowed it to become a curative agent for type I tyrosinemia (T1T) and to enter clinical trials for alkaptonuria. These results boosted the research of new compounds able to interfere with HPPD activity to be used for the treatment of the tyrosine metabolism-related diseases.

Tyrosine Detoxification Is an Essential Trait in the Life History of Blood-Feeding Arthropods

Blood-feeding arthropods are vectors of infectious diseases such as dengue, Zika, Chagas disease, and malaria [1], and vector control is essential to limiting disease spread. Because these arthropods ingest very large amounts of blood, a protein-rich meal, huge amounts of amino acids are produced during digestion. Previous work on Rhodnius prolixus, a vector of Chagas disease, showed that, among all amino acids, only tyrosine degradation enzymes were overexpressed in the midgut compared to other tissues [2]. Here we demonstrate that tyrosine detoxification is an essential trait in the life history of blood-sucking arthropods. We found that silencing Rhodnius tyrosine aminotransferase (TAT) and 4-hydroxyphenylpyruvate dioxygenase (HPPD), the first two enzymes of the phenylalanine/tyrosine degradation pathway, caused the death of insects after a blood meal. This was confirmed by using the HPPD inhibitor mesotrione, which selectively killed hematophagous arthropods but did not affect non-hematophagous insects. In addition, mosquitoes and kissing bugs died after feeding on mice that had previously received a therapeutic effective oral dose (1 mg/kg) of nitisinone, another HPPD inhibitor used in humans for the treatment of tyrosinemia type I [3]. These findings indicate that HPPD (and TAT) can be a target for the selective control of blood-sucking disease vector populations. Because HPPD inhibitors are extensively used as herbicides and in medicine, these compounds may provide an alternative less toxic to humans and more environmentally friendly than the conventional neurotoxic insecticides that are currently used, with the ability to affect only hematophagous arthropods.

Molecular changes associated with chronic liver damage and neoplastic lesions in a murine model of hereditary tyrosinemia type 1

Hereditary tyrosinemia type 1 (HT1) is the most severe inherited metabolic disease of the tyrosine catabolic pathway, with a progressive hepatic and renal injury and a fatal outcome if untreated. Toxic metabolites accumulating in HT1 have been shown to elicit endoplasmic reticulum (ER) stress response, and to induce chromosomal instability, cell cycle arrest and apoptosis perturbation. Although many studies have concentrated on elucidating these events, the molecular pathways responsible for development of hepatocellular carcinoma (HCC) still remain unclear. In this study the fah knockout murine model (fah(-/-)) was used to investigate the cellular signaling implicated in the pathogenesis of HT1. Fah(-/-) mice were subjected to drug therapy discontinuation (Nitisinone withdrawal), and livers were analyzed at different stages of the disease. Monitoring of mice revealed an increasing degeneration of the overall physiological conditions following drug withdrawal. Histological analysis unveiled diffuse hepatocellular damage, steatosis, oval-like cells proliferation and development of liver cell adenomas. Immunoblotting results revealed a progressive and chronic activation of stress pathways related to cell survival and proliferation, including several stress regulators such as Nrf2, eIF2α, CHOP, HO-1, and some members of the MAPK signaling cascade. Impairment of stress defensive mechanisms was also shown by microarray analysis in fah(-/-) mice following prolonged therapy interruption. These results suggest that a sustained activation of stress pathways in the chronic HT1 progression might play a central role in exacerbating liver degeneration.

Old treatments for new insights and strategies: proposed management in adults and children with alkaptonuria

Alkaptonuria (AKU) is caused by deficiency of the enzyme homogentisate 1,2 dioxygenase. It results in an accumulation of homogentisate which oxidizes spontaneously to benzoquinone acetate, a highly oxidant compound, which polymerises to a melanin-like structure, in a process called ochronosis. Asymptomatic during childhood, this accumulation will lead from the second decade of life to a progressive and severe spondylo-arthopathy, associated with multisystem involvement: osteoporosis/fractures, stones (renal, prostatic, gall bladder, salivary glands), ruptures of tendons/muscle/ligaments, renal failure and aortic valve disease. The pathophysiological mechanisms of AKU remain poorly understood, but recent advances lead us to reconsider the treatment strategy in AKU patients. Besides the supporting therapies (pain killers, anti-inflammatory drugs, physiotherapy, joints replacements and others), specific therapies have been considered (anti-oxidant, low protein diet, nitisinone), but clinical studies have failed to prove efficiency on the rheumatological lesions of the disease. Here we propose a treatment strategy for children and adults with AKU, based on a review of the latest findings on AKU and lessons from other aminoacipathies, especially tyrosinemias.

The role of nitisinone in tyrosine pathway disorders

Nitisinone 2-(2-nitro-4-trifluoromethylbenzoyl)cyclohexane-1,3-dione (NTBC), an effective herbicide, is the licensed treatment for the human condition, hereditary tyrosinaemia type 1 (HT-1). Its mode of action interrupts tyrosine metabolism through inhibition of 4-hydroxyphenylpyruvate dioxygenase (HPPD). Nitisinone is a remarkable safe drug to use with few side effects reported. Therefore, we propose that it should be investigated as a potential treatment for other disorders of tyrosine metabolism. These include alkaptonuria (AKU), a rare disease resulting is severe, early-onset osteoarthritis. We present a case study from the disease, and attempts to use the drug both off-label and in clinical research through the DevelopAKUre consortium.

Triketone toxicity

Introduction:

Sulcotrione is a herbicidal agent belonging to the family of triketones. Sulcotrione herbicides are used for weed control in maize and flax crops. To date, no cases of human poisoning had been reported in the literature linked to different herbicidal agents in the triketone family. We report here on two cases of the voluntary ingestion of this substance in the form of the branded product MikadoTM, which were recorded by the Angers Poison Centre.

Case report:

Both cases of voluntary ingestion constituted attempted suicide, and involved two men aged 30 and 37 years. Their symptoms linked to sulcotrione were limited to vomiting, despite elevated plasma concentrations of sulcotrione. In one case, hypertyrosinemia has been demonstrated. The outcome was favourable in both patients and at follow up, no ocular disorders were observed. In the second case, hypotension and transient renal failure could be linked to the concomitant ingestion of chlorophenoxy herbicides.

Discussion:

In animal toxicity studies, sulcotrione inhibit 4-hydro-phenylpyruvate dioxygenase leading to hypertyrosinemia and corneal opacities. In both cases, no ocular disorders were observed despite hypertyrosinemia in one case. These case reports were consistent with the animal toxicology findings concerning triketones, and particularly their relative safety in mammals following acute poisoning. However it seems prudent to monitor plasma tyrosine concentrations and to screen prospectively for corneal deposits if further acute intoxication events occur.

Nitisinone improves eye and skin pigmentation defects in a mouse model of oculocutaneous albinism

Mutation of the tyrosinase gene (TYR) causes oculocutaneous albinism, type 1 (OCA1), a condition characterized by reduced skin and eye melanin pigmentation and by vision loss. The retinal pigment epithelium influences postnatal visual development. Therefore, increasing ocular pigmentation in patients with OCA1 might enhance visual function. There are 2 forms of OCA1, OCA-1A and OCA-1B. Individuals with the former lack functional tyrosinase and therefore lack melanin, while individuals with the latter produce some melanin. We hypothesized that increasing plasma tyrosine concentrations using nitisinone, an FDA-approved inhibitor of tyrosine degradation, could stabilize tyrosinase and improve pigmentation in individuals with OCA1. Here, we tested this hypothesis in mice homozygous for either the Tyrc-2J null allele or the Tyrc-h allele, which model OCA-1A and OCA-1B, respectively. Only nitisinone-treated Tyrc-h/c-h mice manifested increased pigmentation in their fur and irides and had more pigmented melanosomes. High levels of tyrosine improved the stability and enzymatic function of the Tyrc-h protein and also increased overall melanin levels in melanocytes from a human with OCA-1B. These results suggest that the use of nitisinone in OCA-1B patients could improve their pigmentation and potentially ameliorate vision loss.

Nitisinone in the Treatment of Hereditary Tyrosinaemia Type 1

Hereditary tyrosinaemia type 1 (HT-1) is a rare genetic disease caused by mutations in the gene for the enzyme fumarylacetoacetase. It usually presents with liver failure but can be manifest as chronic liver disease. Rarely, it may present with nonhepatic manifestations such as renal dysfunction, porphyria-like illness or cardiomyopathy. There is a high lifetime risk of developing hepatocellular carcinoma (HCC). Prior to the development of liver transplantation, most patients died in childhood.The clinical manifestations stem from the cytotoxicity of tyrosine metabolites accumulating proximal to the metabolic defect. Nitisinone acts on tyrosine metabolism upstream of the defect to prevent the production of these metabolites. Nitisinone is used in combination with a tyrosine- and phenylalanine-restricted diet. Nitisinone has transformed the natural history of tyrosinaemia. Liver failure is controlled in 90% of patients, those with chronic liver disease improve and nonhepatic manifestations are abolished. Nitisinone is well tolerated and has few adverse effects other than a predictable rise in plasma tyrosine levels. Nitisinone provides protection against HCC if it is started in infancy, but if commenced after the age of 2 years, a significant risk of HCC remains. Furthermore, where nitisinone is used pre-emptively, liver disease appears to be prevented, suggesting the importance of neonatal screening for tyrosinaemia where possible. Nitisinone is indicated for all children with HT-1, and liver transplantation is only indicated where nitisinone fails, or where the development of HCC is likely or suspected.

Renal proximal tubular cells acquire resistance to cell death stimuli in mice with hereditary tyrosinemia type 1

BACKGROUND

Hereditary tyrosinemia type 1 (HT1), which is associated with severe liver and kidney damage, is caused by deficiency of fumarylacetoacetate hydrolase (FAH), the last enzyme of the tyrosine breakdown cascade. HT1-associated liver and kidney failure can be prevented by blocking an enzyme upstream of FAH in the tyrosine breakdown pathway with 2-(2-nitro-4-trifluoromethylbenzoyl)-1,3-cyclohexanedione (NTBC). FAH knockout mice develop the HT1 phenotype when NTBC treatment is discontinued.

METHODS

The occurrence of cell death was investigated in kidneys of Fah(-/-) mice on and off NTBC either unchallenged or injected with 800 mg/kg of homogentisic acid (HGA), an intermediate of tyrosine breakdown.

RESULTS

No cell death could be detected in kidneys of Fah(-/-) mice on NTBC. A slight increase of cleaved caspase-3 was the only apoptosis-related feature that could be detected in kidneys of Fah(-/-) mice off NTBC. Challenge of Fah(-/-) mice on NTBC with HGA led to massive death of renal proximal tubular cells, with positive terminal deoxynucleotidyl transferase-mediated deoxyuridine diphosphate (dUDP) nick-end labeling (TUNEL) and DNA fragmentation assays, but hardly any cleavage of caspase-9 and caspase-3. Fah(-/-) mice off NTBC acquired resistance to HGA-induced renal cell death and the kidneys exhibited relatively few features of apoptosis upon challenge with HGA, with a small increase in expression of cleaved caspase-9 and caspase-3.

CONCLUSION

Kidneys of adult Fah(-/-) mice, withdrawn from NTBC for 15 days, reveal limited characteristics of apoptosis, and have acquired resistance to a caspase-9- and caspase-3-independent form of cell death provoked by HGA.

Pharmacological rescue of the 14CoS/14CoS mouse: hepatocyte apoptosis is likely caused by endogenous oxidative stress

Whereas ch/ch wild-type mice and ch/14CoS heterozygotes are viable, 14CoS/14CoS mice homozygous for a 3800 kb deletion on chromosome 7 die during the first day postpartum. Death is caused by disruption of the fumarylacetoacetate hydrolase (Fah) gene; absence of FAH, final enzyme in the tyrosine catabolism pathway, leads to accumulation of reactive electrophilic intermediates. In this study, we kept 14CoS/14CoS mice alive for 60 d with oral 2-(2-nitro-4-trifluoromethyl-benzyol)-1,3-cyclohexanedione (NTBC), an inhibitor of p-hydroxyphenylpyruvate dioxygenase, second enzyme in the tyrosine catabolic pathway. The 70% of NTBC-treated 14CoS/14CoS mice that survived 60 d showed poor growth and developed corneal opacities, compared with ch/14CoS littermates; NTBC-rescued Fah(-/-) knockout mice did not show growth retardation or ocular toxicity. NTBC-rescued 14CoS/14CoS mice also exhibited a striking oxidative stress response in liver and kidney, as measured by lower GSH levels and mRNA induction of four genes: glutamate cysteine ligase catalytic (Gclc) and modifier (Gclm) subunits, NAD(P)H:quinone oxidoreductase (Nqo1), and heme oxygenase-1 (Hmox1). Withdrawal of NTBC for 24-48 h from rescued adult 14CoS/14CoS mice resulted in severe apoptosis of the liver, detected histologically and by cytochrome c release from the mitochondria, increased caspase 3-like activity, and further decreases in GSH content. In kidney, proximal tubular epithelial cells were abnormal. Human hereditary tyrosinemia type I (HT1), caused by mutations in the FAH gene, is an autosomal recessive disorder in which the patient usually dies of liver fibrosis and cirrhosis during early childhood; NTBC treatment is known to prolong HT1 children's lives-although liver fibrosis, cirrhosis, hepatocarcinoma, and corneal opacities sometimes occur. The mouse data in the present study are consistent with the possibility that endogenous oxidative stress-induced apoptosis may be the underlying cause of liver pathology seen in NTBC-treated HT1 patients.

Ophthalmic follow-up of patients with tyrosinaemia type I on NTBC

NTBC has revolutionized the management of tyrosinaemia type I, although animal experiments have shown that long-term administration may produce corneal opacities analogous to those in tyrosinaemia type II. We have assessed the prevalence of ocular side-effects in 11 tyrosinaemia type I patients on NTBC attending the Birmingham Children's Hospital. Despite high plasma tyrosine concentrations in some patients, they did not experience symptoms or signs of ocular toxicity.

Long-term therapy with NTBC and tyrosine-restricted diet in a murine model of hereditary tyrosinemia type I

In human patients with hereditary tyrosinemia type I (HT1) a combination therapy of 2-(2-nitro-4-trifluoromethylbenzoyl)-1,3 cyclohexane dione (NTBC) and dietary restriction of phenylalanine and tyrosine is currently widely used. We previously reported that the use of NTBC in a murine model of HT1 abolished acute liver failure but did not prevent the development of hepatocellular carcinoma (HCC) in the setting of nonrestricted protein intake. Here we present the results obtained with higher doses of NTBC plus dietary tyrosine restriction on long-term follow up (>2 years). Liver function tests and succinylacetone levels were completely corrected with this regimen and cancer-free survival was improved when compared to historical controls. However, while no HT1 animals had HCC at age 13 months, the incidence was 2/16 (13%) at age 18 months and 1/6 (17%) after 24 months. Thus, even the most stringent therapy could not prevent the emergence of HCC in the mouse model of HT1, even when initiated prenatally.

Tyrosinemia: a review

Hypertyrosinemia encompasses several entities, of which tyrosinemia type I (or hepatorenal tyrosinemia, HT1) results in the most extensive clinical and pathological manifestations involving mainly the liver, kidney, and peripheral nerves. The clinical findings range from a severe hepatopathy of early infancy to chronic liver disease and rickets in the older child; gradual refinements in the diagnosis and medical management of this disorder have greatly altered its natural course, mirroring recent advances in the field of metabolic diseases in the past quarter century. Hepatorenal tyrosinemia is the inborn error with the highest incidence of progression to hepatocellular carcinoma, likely due to profound mutagenic effects and influences on the cell cycle by accumulated metabolites. The appropriate follow-up of patients with cirrhosis, the proper timing of liver transplantation in the prevention of carcinoma, and the long-term evolution of chronic renal disease remain important unresolved issues. The introduction of a new pharmacologic agent, NTBC, holds the hope of significantly alleviating some of the burdens of this disease. Mouse models of this disease have permitted the exploration of newer treatment modalities, such as gene therapy by viral vectors, including ex vivo and in utero methods. Finally, recent observations on spontaneous genetic reversion of the mutation in HT1 livers challenge conventional concepts in human genetics.