Complete rescue of lethal albino c14CoS mice by null mutation of 4-hydroxyphenylpyruvate dioxygenase and induction of apoptosis of hepatocytes in these mice by in vivo retrieval of the tyrosine catabolic pathway

Genetically blocking HPD via CRISPR-Cas9 protects against lethal liver injury in a pig model of tyrosinemia type I

Hereditary tyrosinemia type I (HT1) results from the loss of fumarylacetoacetate hydrolase (FAH) activity and can lead to lethal liver injury (LLI). Therapeutic options for HT1 remain limited. The FAH^−/− pig, a well-characterized animal model of HT1, represents a promising candidate for testing novel therapeutic approaches to treat this condition. Here, we report an improved single-step method to establish a biallelic (FAH^−/−) mutant porcine model using CRISPR-Cas9 and cytoplasmic microinjection. We also tested the feasibility of rescuing HT1 pigs through inactivating the 4-hydroxyphenylpyruvic acid dioxygenase (HPD) gene, which functions upstream of the pathogenic pathway, rather than by directly correcting the disease-causing gene as occurs with traditional gene therapy. Direct intracytoplasmic delivery of CRISPR-Cas9 targeting HPD before intrauterine death reprogrammed the tyrosine metabolism pathway and protected pigs against FAH deficiency-induced LLI. Characterization of the F1 generation revealed consistent liver-protective features that were germline transmissible. Furthermore, HPD ablation ameliorated oxidative stress and inflammatory responses and restored the gene profile relating to liver metabolism homeostasis. Collectively, this study not only provided a novel large animal model for exploring the pathogenesis of HT1, but also demonstrated that CRISPR-Cas9-mediated HPD ablation alleviated LLI in HT1 pigs and represents a potential therapeutic option for the treatment of HT1.

Versatile and robust genome editing with Streptococcus thermophilus CRISPR1-Cas9

Targeting definite genomic locations using CRISPR-Cas systems requires a set of enzymes with unique protospacer adjacent motif (PAM) compatibilities. To expand this repertoire, we engineered nucleases, cytosine base editors, and adenine base editors from the archetypal Streptococcus thermophilus CRISPR1-Cas9 (St1Cas9) system. We found that St1Cas9 strain variants enable targeting to five distinct A-rich PAMs and provide a structural basis for their specificities. The small size of this ortholog enables expression of the holoenzyme from a single adeno-associated viral vector for in vivo editing applications. Delivery of St1Cas9 to the neonatal liver efficiently rewired metabolic pathways, leading to phenotypic rescue in a mouse model of hereditary tyrosinemia. These robust enzymes expand and complement current editing platforms available for tailoring mammalian genomes.

Polyploid Hepatocytes Facilitate Adaptation and Regeneration to Chronic Liver Injury

The liver contains diploid and polyploid hepatocytes (tetraploid, octaploid, etc.), with polyploids comprising ≥90% of the hepatocyte population in adult mice. Polyploid hepatocytes form multipolar spindles in mitosis, which lead to chromosome gains/losses and random aneuploidy. The effect of aneuploidy on liver function is unclear, and the degree of liver aneuploidy is debated, with reports showing aneuploidy affects 5% to 60% of hepatocytes. To study relationships among liver polyploidy, aneuploidy, and adaptation, mice lacking E2f7 and E2f8 in the liver (LKO), which have a polyploidization defect, were used. Polyploids were reduced fourfold in LKO livers, and LKO hepatocytes remained predominantly diploid after extensive proliferation. Moreover, nearly all LKO hepatocytes were euploid compared with control hepatocytes, suggesting polyploid hepatocytes are required for production of aneuploid progeny. To determine whether reduced polyploidy impairs adaptation, LKO mice were bred onto a tyrosinemia background, a disease model whereby the liver can develop disease-resistant, regenerative nodules. Although tyrosinemic LKO mice were more susceptible to morbidities and death associated with tyrosinemia-induced liver failure, they developed regenerating nodules similar to control mice. Analyses revealed that nodules in the tyrosinemic livers were generated by aneuploidy and inactivating mutations. In summary, we identified new roles for polyploid hepatocytes and demonstrated that they are required for the formation of aneuploid progeny and can facilitate adaptation to chronic liver disease.

Validation of a multi-omics strategy for prioritizing personalized candidate driver genes

Significant heterogeneity between different tumors prevents the discovery of cancer driver genes, especially in a patient-specific manner. We previously prioritized five personalized candidate mutation-driver genes in a hyper-mutated hepatocellular carcinoma patient using a multi-omics strategy. However, the roles of the prioritized driver genes and patient-specific mutations in hepatocarcinogenesis are unclear. We investigated the impact of the tumor-mutated allele on structure-function relationship of the encoded protein and assessed both loss- and gain-of-function of these genes and mutations on hepatoma cell behaviors in vitro. The prioritized mutation-driver genes act as tumor suppressor genes and inhibit cell proliferation and migration. In addition, the loss-of-function effect of the patient-specific mutations promoted cell proliferation and migration. Of note, the HNF1A S247T mutation significantly reduced the HNF1A transcriptional activity for hepatocyte nuclear factor 4 alpha (HNF4A) but did not disrupt nuclear localization of HNF1A. The results provide evidence for supporting the validity of our proposed multi-omics strategy, which supplies a new avenue for prioritizing mutation-drivers towards personalized cancer therapy.

A universal system to select gene-modified hepatocytes in vivo

Many genetic and acquired liver disorders are amenable to gene and/or cell therapy. However, the efficiencies of cell engraftment and stable genetic modification are low and often subtherapeutic. In particular, targeted gene modifications from homologous recombination are rare events. These obstacles could be overcome if hepatocytes that have undergone genetic modification were to be selectively amplified or expanded. We describe a universally applicable system for in vivo selection and expansion of gene-modified hepatocytes in any genetic background. In this system, the therapeutic transgene is coexpressed with a short hairpin RNA (shRNA) that confers modified hepatocytes with resistance to drug-induced toxicity. An shRNA against the tyrosine catabolic enzyme 4-OH-phenylpyruvate dioxygenase protected hepatocytes from 4-[(2-carboxyethyl)-hydroxyphosphinyl]-3-oxobutyrate, a small-molecule inhibitor of fumarylacetoacetate hydrolase. To select for specific gene targeting events, the protective shRNA was embedded in a microRNA and inserted into a recombinant adeno-associated viral vector designed to integrate site-specifically into the highly active albumin locus. After selection of the gene-targeted cells, transgene expression increased 10- to 1000-fold, reaching supraphysiological levels of human factor 9 protein (50,000 ng/ml) in mice. This drug resistance system can be used to achieve therapeutically relevant transgene levels in hepatocytes in any setting.

NTBC and Correction of Renal Dysfunction

Hereditary tyrosinemia type 1 (HT1) is characterized by severe progressive liver disease and renal tubular dysfunction. Kidney involvement is characterized by hypophosphatemic rickets and Fanconi syndrome. Different animal models were useful to investigate the pathophysiology of the disease and the effects of NTBC therapy on liver and kidney function. NTBC has revolutionized the prognosis of HT1 and its acute and chronic effects on renal tubular function have been proved, with normalization of tubular function within a few weeks, particularly hypophosphatemia and proteinuria. NTBC therapy is highly effective in improving renal function both at short and long-term. However, its efficacy critically depends on the age at start of treatment with normal outcome in patients diagnosed at birth by newborn screening.

CRISPR/Cas9: at the cutting edge of hepatology

Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR)/Cas9 genome engineering has revolutionised biomedical science and we are standing on the cusp of medical transformation. The therapeutic potential of this technology is tremendous, however, its translation to the clinic will be challenging. In this article, we review recent progress using this genome editing technology and explore its potential uses in studying and treating diseases of the liver. We discuss the development of new research tools and animal models as well as potential clinical applications, strategies and challenges.

Genome editing for inborn errors of metabolism: advancing towards the clinic

Inborn errors of metabolism (IEM) include many disorders for which current treatments aim to ameliorate disease manifestations, but are not curative. Advances in the field of genome editing have recently resulted in the in vivo correction of murine models of IEM. Site-specific endonucleases, such as zinc-finger nucleases and the CRISPR/Cas9 system, in combination with delivery vectors engineered to target disease tissue, have enabled correction of mutations in disease models of hemophilia B, hereditary tyrosinemia type I, ornithine transcarbamylase deficiency, and lysosomal storage disorders. These in vivo gene correction studies, as well as an overview of genome editing and future directions for the field, are reviewed and discussed herein.

Reprogramming metabolic pathways in vivo with CRISPR/Cas9 genome editing to treat hereditary tyrosinaemia

Many metabolic liver disorders are refractory to drug therapy and require orthotopic liver transplantation. Here we demonstrate a new strategy, which we call metabolic pathway reprogramming, to treat hereditary tyrosinaemia type I in mice; rather than edit the disease-causing gene, we delete a gene in a disease-associated pathway to render the phenotype benign. Using CRISPR/Cas9 in vivo, we convert hepatocytes from tyrosinaemia type I into the benign tyrosinaemia type III by deleting Hpd (hydroxyphenylpyruvate dioxigenase). Edited hepatocytes (Fah(-/-)/Hpd(-/-)) display a growth advantage over non-edited hepatocytes (Fah(-/-)/Hpd(+/+)) and, in some mice, almost completely replace them within 8 weeks. Hpd excision successfully reroutes tyrosine catabolism, leaving treated mice healthy and asymptomatic. Metabolic pathway reprogramming sidesteps potential difficulties associated with editing a critical disease-causing gene and can be explored as an option for treating other diseases.

Diagnosis and treatment of hereditary tyrosinemia in Japan

Hereditary tyrosinemia is an autosomal recessive inherited disease that manifests as three types (types I-III). We conducted a nationwide survey of this disease in Japan, and here review the results in relation to prevalence, clinical characteristics, and treatment and diagnosis. A definitive diagnosis of tyrosinemia type I is difficult to obtain based only on blood tyrosine level. Detection of succinylacetone using dried blood spots or urinary organic acid analysis, however, is useful for diagnosis. In tyrosinemia type I, dietary therapy and nitisinone (Orfandin®) are effective. Prognosis is greatly affected by the complications of liver cancer and hypophosphatemic rickets; even patients that are treated early with nitisinone may develop liver cancer. Long-term survival can be expected in type I if nitisinone therapy is effective. Prognosis in types II and III is relatively good.

Heat shock response associated with hepatocarcinogenesis in a murine model of hereditary tyrosinemia type I

Hereditary Tyrosinemia type 1 (HT1) is a metabolic liver disease caused by genetic defects of fumarylacetoacetate hydrolase (FAH), an enzyme necessary to complete the breakdown of tyrosine. The severe hepatic dysfunction caused by the lack of this enzyme is prevented by the therapeutic use of NTBC (2-[2-nitro-4-(trifluoromethyl)benzoyl] cyclohexane-1,3-dione). However despite the treatment, chronic hepatopathy and development of hepatocellular carcinoma (HCC) are still observed in some HT1 patients. Growing evidence show the important role of heat shock proteins (HSPs) in many cellular processes and their involvement in pathological diseases including cancer. Their survival-promoting effect by modulation of the apoptotic machinery is often correlated with poor prognosis and resistance to therapy in a number of cancers. Here, we sought to gain insight into the pathophysiological mechanisms associated with liver dysfunction and tumor development in a murine model of HT1. Differential gene expression patterns in livers of mice under HT1 stress, induced by drug retrieval, have shown deregulation of stress and cell death resistance genes. Among them, genes coding for HSPB and HSPA members, and for anti-apoptotic BCL-2 related mitochondrial proteins were associated with the hepatocarcinogenetic process. Our data highlight the variation of stress pathways related to HT1 hepatocarcinogenesis suggesting the role of HSPs in rendering tyrosinemia-affected liver susceptible to the development of HCC.

In vivo selection of transplanted hepatocytes by pharmacological inhibition of fumarylacetoacetate hydrolase in wild-type mice

Genetic fumarylacetoacetate hydrolase (Fah) deficiency is unique in that healthy gene-corrected hepatocytes have a strong growth advantage and can repopulate the diseased liver. Unfortunately, similar positive selection of gene-corrected cells is absent in most inborn errors of liver metabolism and it is difficult to reach the cell replacement index required for therapeutic benefit. Therefore, methods to transiently create a growth advantage for genetically modified hepatocytes in any genetic background would be advantageous. To mimic the selective pressure of Fah deficiency in normal animals, an efficient in vivo small molecule inhibitor of FAH, 4-[(2-carboxyethyl)-hydroxyphosphinyl]-3-oxobutyrate (CEHPOBA) was developed. Microarray analysis demonstrated that pharmacological inhibition of FAH produced highly similar gene expression changes to genetic deficiency. As proof of principle, hepatocytes lacking homogentisic acid dioxygenase (Hgd) and hence resistant to FAH inhibition were transplanted into sex-mismatched wild-type recipients. Time course analyses of 4-6 weeks of CEHPOBA administration after transplantation showed a linear relationship between treatment length and replacement index. Compared to controls, recipients treated with the FAH-inhibitor had 20-100-fold increases in liver repopulation. We conclude that pharmacological inhibition of FAH is a promising approach to in vivo selection of hepatocytes.

Aneuploidy as a mechanism for stress-induced liver adaptation

Over half of the mature hepatocytes in mice and humans are aneuploid and yet retain full ability to undergo mitosis. This observation has raised the question of whether this unusual somatic genetic variation evolved as an adaptive mechanism in response to hepatic injury. According to this model, hepatotoxic insults select for hepatocytes with specific numerical chromosome abnormalities, rendering them differentially resistant to injury. To test this hypothesis, we utilized a strain of mice heterozygous for a mutation in the homogentisic acid dioxygenase (Hgd) gene located on chromosome 16. Loss of the remaining Hgd allele protects from fumarylacetoacetate hydrolase (Fah) deficiency, a genetic liver disease model. When adult mice heterozygous for Hgd and lacking Fah were exposed to chronic liver damage, injury-resistant nodules consisting of Hgd-null hepatocytes rapidly emerged. To determine whether aneuploidy played a role in this phenomenon, array comparative genomic hybridization (aCGH) and metaphase karyotyping were performed. Strikingly, loss of chromosome 16 was dramatically enriched in all mice that became completely resistant to tyrosinemia-induced hepatic injury. The frequency of chromosome 16-specific aneuploidy was approximately 50%. This result indicates that selection of a specific aneuploid karyotype can result in the adaptation of hepatocytes to chronic liver injury. The extent to which aneuploidy promotes hepatic adaptation in humans remains under investigation.

Plasma and liver proteomic analysis of 3Z-3-[(1H-pyrrol-2-yl)-methylidene]-1-(1-piperidinylmethyl)-1,3-2H-indol-2-one-induced hepatotoxicity in Wistar rats

3Z-3-[(1H-pyrrol-2-yl)-methylidene]-1-(1-piperidinylmethyl)-1,3-2H-indol-2-one (Z24), a synthetic anti-angiogenic compound, inhibits the growth and metastasis of certain tumors. Previous works have shown that Z24 induces hepatotoxicity in rodents. We examined the hepatotoxic mechanism of Z24 at the protein level and looked for potential biomarkers. We used 2-DE and MALDI-TOF/TOF MS to analyze alternatively expressed proteins in rat liver and plasma after Z24 administration. We also examined apoptosis in rat liver and measured levels of intramitochondrial ROS and NAD(P)H redox in liver cells. We found that 22 nonredundant proteins in the liver and 11 in the plasma were differentially expressed. These proteins were involved in several important metabolic pathways, including carbohydrate, lipid, amino acid, and energy metabolism, biotransformation, apoptosis, etc. Apoptosis in rat liver was confirmed with the terminal deoxynucleotidyl transferase dUTP-nick end labeling assay. In mitochondria, Z24 increased the ROS and decreased the NAD(P)H levels. Thus, inhibition of carbohydrate aerobic oxidation, fatty acid beta-oxidation, and oxidative phosphorylation is a potential mechanism of Z24-induced hepatotoxicity, resulting in mitochondrial dysfunction and apoptosis-mediated cell death. In addition, fetub protein and argininosuccinate synthase in plasma may be potential biomarkers of Z24-induced hepatotoxicity.

High volume naked DNA tail-vein injection restores liver function in Fah-knock out mice

BACKGROUND

Despite pharmaceutical treatment with NTBC (2-2-nitro-4-fluoromethylbenzoyl-1,3-cyclohexanedione), a high incidence of liver malignancies occur in humans and mice suffering from hereditary tyrosinemia type 1 (HT1) caused by mutation of the fumarylacetoacetate hydrolase (fah) gene.

METHODS

To evaluate the efficacy of a definitive treatment for HT1, we transfected fah knockout mice with naked plasmid DNA using high volume tail-vein injection. This approach was chosen to reduce the occurrence of insertional mutagenesis that is frequently observed when using other (retro-)viral vectors. To prolong gene expression, the fah gene was cloned between adeno-associated virus (AAV)-specific inverted terminal repeats (ITRs).

RESULTS

All animals treated with high volume plasmid DNA injections could be successfully weaned off NTBC and survived in the long term without any further pharmacological support. Up to 50% fah positive hepatocytes were detected in livers of naked plasmid DNA-treated animals and serum liver function tests approximated those of wild-type controls.

CONCLUSIONS

Naked plasmid DNA transfection offers a promising alternative treatment for HT1. Minimizing side-effects makes this approach especially appealing.

skn-1-Dependent and -independent regulation of aip-1 expression following metabolic stress in Caenorhabditis elegans

Maintenance of a stable, properly folded, and catalytically active proteome is a major challenge to organisms in the face of multiple internal and external stresses which damage proteins and lead to protein misfolding. Here we show that internal metabolic stress produced by reactive intermediates resulting from tyrosine degradation triggers the expression of the aip-1 gene, which is critical in responses to the environmental toxin arsenic and the clearance of unstable polyglutamine and Abeta proteins. aip-1 acts via binding to the proteosome and enhancing proteosomal function. We find that full induction of aip-1 depends on the oxidative-stress-responsive skn-1 transcription factor but significant induction still occurs without skn-1. Importantly, activation of skn-1 with wdr-23(RNAi), which dramatically induces the expression of other skn-1 target genes, produces a minimal increase in aip-1 expression. This suggests that the previously demonstrated specificity in aip-1/AIRAP induction could reflect the actions of multiple synergistic activators, such as the heat shock factor homolog hsf-1, which we also find is required for full induction. These may be triggered by proteosome dysfunction, as we find that this event links the multiple inducers of aip-1. Together, our results show that cell stress triggers aip-1 expression by both skn-1-dependent and -independent pathways.

The Caenorhabditis elegans K10C2.4 gene encodes a member of the fumarylacetoacetate hydrolase family: a Caenorhabditis elegans model of type I tyrosinemia

In eukaryotes and many bacteria, tyrosine is degraded to produce energy via a five-step tyrosine degradation pathway. Mutations affecting the tyrosine degradation pathway are also of medical importance as mutations affecting enzymes in the pathway are responsible for type I, type II, and type III tyrosinemia. The most severe of these is type I tyrosinemia, which is caused by mutations affecting the last enzyme in the pathway, fumarylacetoacetate hydrolase (FAH). So far, tyrosine degradation in the nematode Caenorhabditis elegans has not been studied; however, genes predicted to encode enzymes in this pathway have been identified in several microarray, proteomic, and RNA interference (RNAi) screens as perhaps being involved in aging and the control of protein folding. We sought to identify and characterize the genes in the worm tyrosine degradation pathway as an initial step in understanding these findings. Here we describe the characterization of the K10C2.4, which encodes a homolog of FAH. RNAi directed against K10C2.4 produces a lethal phenotype consisting of death in young adulthood, extensive damage to the intestine, impaired fertility, and activation of oxidative stress and endoplasmic stress response pathways. This phenotype is due to alterations in tyrosine metabolism as increases in dietary tyrosine enhance it, and inhibition of upstream enzymes in tyrosine degradation with RNAi or genetic mutations reduces the phenotype. We also use our model to identify genes that suppress the damage produced by K10C2.4 RNAi in a pilot genetic screen. Our results establish worms as a model for the study of type I tyrosinemia.

Hepatic stress in hereditary tyrosinemia type 1 (HT1) activates the AKT survival pathway in the fah-/- knockout mice model

BACKGROUND/AIMS

The AKT survival pathway is involved in a wide variety of human cancers. We investigated the implication of this pathway in hereditary tyrosinemia type 1 (HT1), a metabolic disease exhibiting hepatocellular carcinoma (HCC), despite treatment with 2-(2-nitro-4-trifluoromethylbenzoyl)-1,3-cyclohexadione (NTBC) which prevents liver damage. HT1 is an autosomal recessive disorder caused by accumulation of toxic metabolites due to a deficiency in fumarylacetoacetate hydrolase (FAH), the last enzyme in the catabolism of tyrosine.

METHODS

NTBC withdrawal in the murine fah(-/-) knockout model was used to analyze in vivo the correlation between pathophysiological, biochemical and histological features consistent with hepatocarcinogenesis and activation of the AKT survival pathway.

RESULTS

The HT1 stress initiated by NTBC discontinuation causes a progressive increase of liver and kidney pathophysiology. A stable activation of the AKT survival pathway is observed in the liver but not in kidneys of fah(-/-) mice. Hepatic survival is reinforced by inhibition of mitochondrial-mediated apoptosis through inactivation of Bad and induction of BCl-X(L) and BCl-2.

CONCLUSIONS

The chronic stress induced by liver disease in HT1 activates the AKT survival signal and inhibits intrinsic apoptosis to confer cell death resistance in vivo and favor hepatocarcinogenesis.

Animal models of tyrosinemia

Hereditary tyrosinemia I (HT I) is a genetic disorder of tyrosine metabolism characterized by progressive liver damage from infancy and by a high risk for hepatocellular carcinoma. HT I is due to mutations in the fumarylacetoacetate hydrolase (Fah) gene, which encodes the last enzyme in the tyrosine catabolic pathway. Disturbances in tyrosine metabolism lead to increased levels of succinylacetone and succinylacetoacetate. However, the mechanisms causing liver failure, cirrhosis, renal tubular dysfunction, and hepatocarcinoma are still unknown. Lethal albino deletion c14CoS mice and mice with target-disrupted Fah are models for HT I. They die in the perinatal period, although with a different phenotype from that seen in HT I in humans. In addition, 2 mouse strains that carry N-ethyl-N-nitrosourea-induced mutations in the Fah gene have been described. Mice with a splice mutation exhibit the milder features of the clinical phenotype. In mice that carry both Fah and 4-hydroxyphenylpyruvate dioxygenase gene mutations, administration of homogentisate results in rapid apoptosis of hepatocytes. Simultaneously, renal tubular epithelial cells are injured, resulting in Fanconi syndrome. These are central features of visceral injury in patients with HT I. Apoptosis of hepatocyte and renal tubular cells is prevented by the caspase inhibitors acetyl-Tyr-Val-Ala-Asp-CHO or acetyl-Asp-Glu-Val-Asp-CHO. Apoptosis of hepatocytes and renal tubular epithelial cells are central features of this disease. Alterations in gene expression found in the liver of patients with HT I are responsible for the pathogenesis of this disease, for example, acute liver failure. Therefore, gene expression analysis allows a better understanding of the specific pathogenesis. Cell fusion of hematopoietic stem cells with hepatocytes leads to liver regeneration after liver injury. This finding was possible after using the liver injury model of HT I in Fah null mice. Thus, animal models of tyrosinemia are unique and useful tools to reveal mechanisms of interest to both clinical and basic science.

Slow-onset inhibition of fumarylacetoacetate hydrolase by phosphinate mimics of the tetrahedral intermediate: kinetics, crystal structure and pharmacokinetics

FAH (fumarylacetoacetate hydrolase) catalyses the final step of tyrosine catabolism to produce fumarate and acetoacetate. HT1 (hereditary tyrosinaemia type 1) results from deficiency of this enzyme. Previously, we prepared a partial mimic of the putative tetrahedral intermediate in the reaction catalysed by FAH co-crystallized with the enzyme to reveal details of the mechanism [Bateman, Bhanumoorthy, Witte, McClard, Grompe and Timm (2001) J. Biol. Chem. 276, 15284-15291]. We have now successfully synthesized complete mimics CEHPOBA {4-[(2-carboxyethyl)-hydroxyphosphinyl]-3-oxobutyrate} and COPHPAA {3-[(3-carboxy-2-oxopropyl)hydroxyphosphinyl]acrylate}, which inhibit FAH in slow-onset tight-binding mode with K(i) values of 41 and 12 nM respectively. A high-resolution (1.35 A; 1 A=0.1 nm) crystal structure of the FAH.CEHPOBA complex was solved to reveal the affinity determinants for these compounds and to provide further insight into the mechanism of FAH catalysis. These compounds are active in vivo, and CEHPOBA demonstrated a notable dose-dependent increase in SA (succinylacetone; a metabolite seen in patients with HT1) in mouse serum after repeated injections, and, following a single injection (1 mumol/g; intraperitoneal), only a modest regain of FAH enzyme activity was detected in liver protein isolates after 24 h. These potent inhibitors provide a means to chemically phenocopy the metabolic defects of either HT1 or FAH knockout mice and promise future pharmacological utility for hepatocyte transplantation.

Gene expression profiles of homogentisate-treated Fah-/- Hpd-/-mice using DNA microarrays

Hereditary tyrosinemia I (HT I) is a genetic disorder of tyrosine metabolism caused by abnormalities of fumarylacetoacetate hydrolase. Disturbances in tyrosine metabolism lead to increased levels of succinylacetone and succinylacetoacetate. However, the mechanisms causing liver failure, cirrhosis, renal tubular dysfunction, and hepatocarcinoma are still unknown. Alterations in gene expression found in the livers of patients with HT I are responsible for the pathogenesis of this disease, for example acute liver failure. Therefore, gene expression analysis allows us to better understand its pathogenesis. We analyzed gene expressions in tyrosinemia type I model mice with liver failure using microarrays. The results were confirmed by quantitative PCR to evaluate the pathogenesis of tyrosinemia type I. We found that numerous genes, including amino acid metabolism and apoptosis related genes, were up- or down-regulated at the onset of liver failure. These findings are useful in understanding the pathogenesis of hereditary tyrosinemia.

Current strategies for the treatment of hereditary tyrosinemia type I

Hereditary tyrosinemia type I (HT-I) is the most common of the three known diseases caused by defects in tyrosine metabolism. This type of tyrosinemia is caused by a mutation in the gene coding for fumarylacetoacetate hydrolase; several mutations in this gene have been identified. The main clinical features of HT-I are caused by hepatic involvement and renal tubular dysfunction. Dietary intervention with restriction of phenylalanine and tyrosine together with supportive measures can ameliorate the symptoms, but given the high risk for hepatocellular carcinoma, a cure for these patients has so far been possible only with liver transplantation. Pharmacologic treatment with nitisinone, a peroral inhibitor of the tyrosine catabolic pathway, offers an improved means of treatment for patients with HT-I. However, longer follow-up periods are needed to establish the role of this drug in ultimately protecting patients from end-stage organ involvement and hepatocellular carcinoma. Experimental work in mice has provided some promise for the future management of tyrosinemia with gene therapy.

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.

Succinylacetone Oxidation by Oxygen/Peroxynitrite: A Possible Source of Reactive Intermediates in Hereditary Tyrosinemia Type I

Hereditary tyrosinemia type I (HT1) is an inborn metabolic error characterized by hepatorenal dysfunction. Affected patients excrete large quantities of succinylacetone (SA), a tyrosine catabolite believed to be involved in the pathogenesis of HT1. A growing body of evidence relates the oxidative stress observed in metabolic disorders to free radicals generated from accumulated metabolites. In this context, oxidation of SA by peroxynitrite or cytochrome c yielding reactive intermediates and products was investigated here. Both peroxynitrite and cytochrome c were able to initiate oxygen consumption by SA, which was followed by polarimetric and chemiluminescence measurements. The light emission arises from triplet carbonyls formed by the thermolysis of dioxetane intermediates, as indicated by energy transfer experiments. EPR spin-trapping studies with 2-methyl-2-nitrosopropane revealed the intermediacy of two different carbon-centered radicals, one of them originating from cleavage of the triplet carbonyl product. The pH profiles obtained by oxygen consumption, chemiluminescence, and stopped-flow spectrophotometry point to the peroxynitrite anion as the initiator of SA aerobic oxidation. Overstoichiometric formation of organic acids based on added peroxynitrite confirms the occurrence of an oxygen-dependent chain reaction, here proposed to be initiated by one electron abstraction from the enolic form of SA. The results obtained may help shed light on the role of both SA and oxidative stress in the pathogenesis of HT1.

Chronic liver disease in murine hereditary tyrosinemia type 1 induces resistance to cell death

The murine model of hereditary tyrosinemia type 1 (HT1) was used to analyze the relationship between chronic liver disease and programmed cell death in vivo. In healthy fumarylacetoacetate hydrolase deficient mice (Fah(-/-)), protected from liver injury by the drug 2-(2- nitro-4-trifluoromethylbenzoyl)-1,3-cyclohexanedione (NTBC), the tyrosine metabolite homogentisic acid (HGA) caused rapid hepatocyte death. In contrast, all mice survived the same otherwise lethal dose of HGA if they had preexisting liver damage induced by NTBC withdrawal. Similarly, Fah(-/-) animals with liver injury were also resistant to apoptosis induced by the Fas ligand Jo-2 and to necrosis-like cell death induced by acetaminophen (APAP). Molecular studies revealed a marked up-regulation of the antiapoptotic heat shock proteins (Hsp) 27, 32, and 70 and of c-Jun in hepatocytes of stressed mice. In addition, the p38 and Jun N-terminal kinase (JNK) stress-activated kinase pathways were markedly impaired in the cell-death resistant liver. In conclusion, these results provide evidence that chronic liver disease can paradoxically result in cell death resistance in vivo. Stress-induced failure of cell death programs may lead to an accumulation of damaged cells and therefore enhance the risk for cancer as observed in HT1 and other chronic liver diseases.

Extensive changes in liver gene expression induced by hereditary tyrosinemia type I are not normalized by treatment with 2-(2-nitro-4-trifluoromethylbenzoyl)-1,3-cyclohexanedione (NTBC)

BACKGROUND

Hereditary Tyrosinemia type I, caused by deficiency of fumarylacetoacetate hydrolase (FAH), is characterized by liver and kidney damage. Administration of 2-(2-nitro-4-trifluoromethylbenzoyl)-1,3-cyclohexanedione (NTBC) corrects the tyrosinemia phenotype, but does not prevent development of hepatocellular carcinoma.

AIM

To gain insight into the pathophysiological changes associated with liver damage induced by tyrosinemia and the preventive action of NTBC on these changes.

METHODS

Differential gene expression patterns in livers of tyrosinemia-affected and healthy mice, and of tyrosinemia-affected and NTBC-treated Fah-/- mice were investigated by suppression subtractive hybridization.

RESULTS

Transcripts encoding proteins playing a role in protein turnover, growth and proliferation, RNA processing, and signal transduction were primarily induced in tyrosinemia-affected livers. Transcripts mainly contributing to the profile of suppressed genes encode proteins that are secreted by the liver, or are necessary for intermediate metabolism. NTBC treatment fails to normalize the tyrosinemia-induced alterations in expression of transcripts encoding proteins involved in protein turnover, signal transduction, and cell growth and proliferation.

CONCLUSIONS

The failure of NTBC to normalize liver gene expression of Fah-/- mice may play a role in rendering the tyrosinemia-affected liver susceptible to development of hepatocellular carcinoma under NTBC treatment.

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.

Animal models reveal pathophysiologies of tyrosinemias

The activity of the enzyme 4-hydroxyphenylpyruvic acid dioxygenase (HPD) is regulated by transcription factors. Mutations in the HPD locus are related to two known distinct diseases: hereditary tyrosinemia type 3 and hawkinsinuria. HPD-deficient mice are a good model with which to examine the biological effects of 4-hydroxyphenylpyruvic acid, which is a keto acid that causes no apparent visceral damage. In contrast, hereditary tyrosinemia type 1, a genetic disease caused by a deficiency of fumarylacetoacetate hydrolase (FAH), induces severe visceral injuries. Mice with FAH deficiency are lethal after birth; thus, efforts to elucidate the mechanisms of the disease process have been impeded. The use of Fah(-/-) Hpd(-/-) double-mutant mice has enabled studies on tyrosinemias, and essential features of visceral injury have been reveale.

A potential biomarker of kidney damage identified by proteomics: preliminary findings

4-Aminophenol (4-AP) and D-serine are established rodent nephrotoxins that selectively damage renal proximal tubules. In an attempt to understand the mechanism of action of these toxicants in greater detail, a high throughput proteomics approach was used to profile protein changes in the plasma of animals treated with these compounds. Male Fischer 344 and Alderley Park rats were treated with increasing doses of 4-AP or D-serine and plasma samples were collected over time. Control groups received either saline or the non-toxic enantiomer, L-serine. Using high throughput two-dimensional gel analysis, a number of plasma proteins showing dose- and time-dependent regulation were identified. One toxicity-associated plasma protein was identified as the cellular enzyme fumarylacetoacetate hydrolase (FAH), which is known to be required for tyrosine metabolism. The FAH gene is mutated in the human genetic disorder type I tyrosinaemia, which is associated with liver and kidney abnormalities and neurological disorders. FAH was elevated in the plasma of animals treated with 4-AP and D-serine at early time points and returned to baseline levels after 3 weeks. The protein was not elevated in the plasma of control animals or those treated with L-serine. The presence of FAH in plasma is intriguing as it is normally a cellular enzyme with no known function in plasma. It is possible that 4-AP and D-serine may work through a previously unknown mechanism in the kidney via regulation of tyrosine metabolism or FAH activity. Therefore, FAH may function in a fashion analogous to the aspartate aminotransferase (AST) and alanine aminotransferase (ALT) enzymes that are used to measure liver injury. The link between kidney toxicants and inherited tyrosinaemia also raises the possibility that FAH may be a marker of kidney toxicity in humans. These observations highlight the value of proteomics in identifying new biomarkers and providing new unprecedented insights into complex biological mechanisms.

Maleylacetoacetate isomerase (MAAI/GSTZ)-deficient mice reveal a glutathione-dependent nonenzymatic bypass in tyrosine catabolism

In mammals, the catabolic pathway of phenylalanine and tyrosine is found in liver (hepatocytes) and kidney (proximal tubular cells). There are well-described human diseases associated with deficiencies of all enzymes in this pathway except for maleylacetoacetate isomerase (MAAI), which converts maleylacetoacetate (MAA) to fumarylacetoacetate (FAA). MAAI is also known as glutathione transferase zeta (GSTZ1). Here, we describe the phenotype of mice with a targeted deletion of the MAAI (GSTZ1) gene. MAAI-deficient mice accumulated FAA and succinylacetone in urine but appeared otherwise healthy. This observation suggested that either accumulating MAA is not toxic or an alternate pathway for MAA metabolism exists. A complete redundancy of MAAI could be ruled out because substrate overload of the tyrosine catabolic pathway (administration of homogentisic acid, phenylalanine, or tyrosine) resulted in renal and hepatic damage. However, evidence for a partial bypass of MAAI activity was also found. Mice doubly mutant for MAAI and fumarylacetoacetate hydrolase (FAH) died rapidly on a normal diet, indicating that MAA could be isomerized to FAA in the absence of MAAI. Double mutants showed predominant renal injury, indicating that this organ is the primary target for the accumulated compound(s) resulting from MAAI deficiency. A glutathione-mediated isomerization of MAA to FAA independent of MAAI enzyme was demonstrated in vitro. This nonenzymatic bypass is likely responsible for the lack of a phenotype in nonstressed MAAI mutant mice.

Tyrosinaemia type I and apoptosis of hepatocytes and renal tubular cells

Hereditary tyrosinaemia type I (HT 1) (McKusick 276700) is caused by a deficiency of fumarylacetoacetate hydrolase (FAH) activity, the last enzyme in the tyrosine catabolic pathway. Homozygous disruption of the gene encoding FAH in mice (Fah) causes neonatal lethality (i.e. lethal Albino deletion c14CoS mice), which limits the use of this animal as a model for HT I. We developed a new mouse model that carries two genetic defects, Fah and 4-hydroxyphenylpyruvate dioxygenase (Hpd). The double mutant Fah -/- Hpd -/- mice grew normally without evidence of liver and renal disease, showing a phenotype similar to Hpd -/- mice. Complete blockage of the tyrosine catabolic pathway at the, step of HPD prevents development of clinical phenotypes. Administration of homogentisate resulted in rapid apoptosis of hepatocytes and renal tubular epithelial cells, a central feature of visceral injury in patients with HT I. Simultaneously, renal tubular function was impaired, resulting in Fanconi syndrome. Apoptosis of hepatocyte and renal tubular cells is prevented by the caspase inhibitors YVAD or DEVD. However, these inhibitors do not prevent the release of cytochrome c or the development of renal tubular dysfunction. Apoptosis of hepatocytes and of renal tubular epithelial cells are characteristic features of this disease and the apoptotic signal in this disease seems to be initiated by fumarylacetoacetate.

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.

Mechanistic inferences from the crystal structure of fumarylacetoacetate hydrolase with a bound phosphorus-based inhibitor

Fumarylacetoacetate hydrolase (FAH) catalyzes the hydrolytic cleavage of a carbon-carbon bond in fumarylacetoacetate to yield fumarate and acetoacetate as the final step of Phe and Tyr degradation. This unusual reaction is an essential human metabolic function, with loss of FAH activity causing the fatal metabolic disease hereditary tyrosinemia type I (HT1). An enzymatic mechanism involving a catalytic metal ion, a Glu/His catalytic dyad, and a charged oxyanion hole was previously proposed based on recently determined FAH crystal structures. Here we report the development and characterization of an FAH inhibitor, 4-(hydroxymethylphosphinoyl)-3-oxo-butanoic acid (HMPOBA), that competes with the physiological substrate with a K(i) of 85 microM. The crystal structure of FAH complexed with HMPOBA refined at 1.3-A resolution reveals the molecular basis for the competitive inhibition, supports the proposed formation of a tetrahedral alkoxy transition state intermediate during the FAH catalyzed reaction, and reveals a Mg(2+) bound in the enzyme's active site. The analysis of FAH structures corresponding to different catalytic states reveals significant active site side-chain motions that may also be related to catalytic function. Thus, these results advance the understanding of an essential catabolic reaction associated with a fatal metabolic disease and provide insight into the structure-based development of FAH inhibitors.

Hereditary tyrosinaemia type I: from basics to progress in treatment

Hereditary tyrosinaemia type I is the most common of the diseases caused by defects in tyrosine metabolism. The underlying genetic defect is a mutation in the gene for fumarylacetate hydrolase (FAH), and more than 30 different mutations in this gene have been identified. The main clinical consequences of this defect include hepatic involvement, with a high risk for liver cancer, and renal tubular dysfunction. Restriction of phenylalanine and tyrosine from the diet along with supportive measures can ameliorate the symptoms, but cure has so far been possible only with liver transplantation. Recent discovery of a pharmacological treatment with a peroral inhibitor of tyrosine catabolic pathway, 2-(2-nitro-4-trifluoromethylbenzoyl)-1,3-cyclohexanedione (NTBC), offers a new promising tool for the treatment of patients with hereditary tyrosinaemia type I. Mouse models of FAH deficiency have been successfully used in experimental gene therapy, and these studies indicate that future management of tyrosinaemia with a gene therapeutic approach may become feasible.

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

Administration of a single oral dose of 2-(2-nitro-4-trifluoromethyl-benzoyl)-cyclohexane-1,3-dione (NTBC) to mice increases the concentration of tyrosine in the plasma and aqueous humour. The tyrosinaemia is both time and dose-dependent with a single dose of 30 micromol NTBC/kg (10 mg/kg) producing maximal concentrations of tyrosine in plasma of about 1200 nmol/ml and in aqueous humour of about 2200 nmol/ml at 16 h after dosing. Analysis of the key hepatic enzymes involved in tyrosine catabolism, following a single dose of 30 micromol NTBC/kg, showed that 4-hydroxyphenylpyruvate dioxygenase (HPPD) was markedly inhibited soon after dosing and that the activity recovered very slowly. In response to the tyrosinaemia, the activity of hepatic tyrosine aminotransferase (TAT) was induced about two-fold, while the activity of hepatic homogentisic acid oxidase (HGO) was reduced at 4 and 5 days after dosing. Daily oral administration of NTBC at doses up to 480 micromol NTBC/kg (160mg/kg/day) to mice produced a maximal tyrosinaemia of about 600-700nmol/ml plasma, showing some adaptation relative to a single dose. Unlike the rat, no treatment-related corneal lesions of the eye were seen at any dose levels up to 6 weeks. Administration of a single oral dose of [14C]-NTBC at 30 micromol/kg led to selective retention of radiolabel in the liver and to a lesser extent the kidneys. Our studies show that NTBC is a potent inhibitor of mouse liver HPPD, which following repeat exposure produces a marked and persistent tyrosinaemia, which does not result in ocular toxicity.

Iron mobilization by succinylacetone methyl ester in rats. A model study for hereditary tyrosinemia and porphyrias characterized by 5-aminolevulinic acid overload

Accumulation of 5-aminolevulinic acid (ALA) is an event characteristic of porphyrias that may contribute to their pathological manifestations. To investigate effects of ALA independent of porphyrin accumulation we treated rats with the methyl ester of succinylacetone, an inhibitor of 5-aminolevulinic acid dehydratase that accumulates in the porphyric-like syndrome hereditary tyrosinemia. Acute 2-day treatment of fasted rats with succinylacetone methyl ester (SAME) promoted a 27% increase in plasma ALA. This increase in plasma ALA was accompanied by augmentation of the level of total nonheme iron in liver (37%) and brain (20%). Mobilization of iron was also indicated by 49% increase in plasma iron and a 77% increase in plasma transferrin saturation. Liver responded with a mild (12%) increase in ferritin. Under these acute conditions, some indications of oxidative stress were evident: a 15% increase in liver reactive protein carbonyls, and a 42% increase in brain subcellular membrane TBARS. Brain also showed a 44% increase in CuZnSOD activity, consistent with observations in treatment with ALA. Overall, the data indicate that SAME promotes ALA-driven changes in iron metabolism that could lead to increased production of free radicals. The findings support other evidence that accumulation of ALA in porphyrias and hereditary tyrosinemia may induce iron-dependent biological damage that contributes to neuropathy and hepatoma.

A mouse model of renal tubular injury of tyrosinemia type 1: development of de Toni Fanconi syndrome and apoptosis of renal tubular cells in Fah/Hpd double mutant mice

Hereditary tyrosinemia type 1 (HT1) (McKusick 276700), a severe autosomal recessive disorder of tyrosine metabolism, is caused by mutations in the fumarylacetoacetate hydrolase gene Fah (EC 3.7.1.2), which encodes the last enzyme in the tyrosine catabolic pathway. HT1 is characterized by severe progressive liver disease and renal tubular dysfunction. Homozygous disruption of the gene encoding Fah in mice causes neonatal lethality (e.g., lethal Albino deletion c14CoS mice), an event that limits use of this animal as a model for HT1. A new mouse model was developed with two genetic defects, Fah and 4-hydroxyphenylpyruvate dioxygenase (Hpd). The Fah-/- Hpd-/- mice grew normally without evidence of liver and renal disease, and the phenotype is similar to that in Fah+/+ Hpd-/- mice. The renal tubular cells of Fah-/- Hpd-/- mice, particularly proximal tubular cells, underwent rapid apoptosis when homogentisate, the intermediate metabolite between HPD and FAH, was administered to the Fah-/- Hpd-/- mice. Simultaneously, renal tubular function was impaired and Fanconi syndrome occurred. Apoptotic death of renal tubular cells, but not renal dysfunction, was prevented by pretreatment of the animals with YVAD, a specific inhibitor of caspases. In the homogentisate-treated Fah-/- Hpd-/- mice, massive amounts of succinylacetone were excreted into the urine, regardless of treatment with inhibitors. It is suggested that apoptotic death of renal tubular cells, as induced by administration of homogentisate to Fah-/- Hpd-/- mice, was caused by an intrinsic process, and that renal apoptosis and tubular dysfunctions in tubular cells occurred through different pathways. These observations shed light on the pathogenesis of renal tubular injury in subjects with FAH deficiency. These Fah-/- Hpd-/- mice can serve as a model in experiments related to renal tubular damage.

Role of the aromatic hydrocarbon receptor and [Ah] gene battery in the oxidative stress response, cell cycle control, and apoptosis

The chronology and history of characterizing the aromatic hydrocarbon [Ah] battery is reviewed. This battery represents the Ah receptor (AHR)-mediated control of at least six, and probably many more, dioxin-inducible genes; two cytochrome P450 genes-P450 1A1 and 1A2 (Cypla1, Cypla2-and four non-P450 genes, have experimentally been documented to be members of this battery. Metabolism of endogenous and exogenous substrates by perhaps every P450 enzyme, but certainly CYP1A1 and CYP1A2 (which are located, in part, in the mitochondrion), have been shown to cause reactive oxygenated metabolite (ROM)-mediated oxidative stress. Oxidative stress activates genes via the electrophile response element (EPRE) DNA motif, whereas dioxin (acutely) activates genes via the AHR-mediated aromatic hydrocarbon response element (AHRE) DNA motif. In contrast to dioxin, AHR ligands that are readily metabolized to ROMs (e.g. benzo[a]pyrene, beta-naphthoflavone) activate genes via both AHREs and the EPRE. The importance of the AHR in cell cycle regulation and apoptosis has just begun to be realized. Current evidence suggests that the CYP1A1 and CYP1A2 enzymes might control the level of the putative endogenous ligand of the AHR, but that CYPA1/1A2 metabolism generates ROM-mediated oxidative stress which can be ameliorated by the four non-P450 EPRE-driven genes in the [Ah] battery. Oxidative stress is a major signal in precipitating apoptosis; however, the precise mechanism, or molecule, which determines the cell's decision between apoptosis and continuation with the cell cycle, remains to be elucidated. The total action of AHR and the [Ah] battery genes therefore represents a pivotal upstream event in the apoptosis cascade, providing an intricate balance between promoting and preventing ROM-mediated oxidative stress. These proposed endogenous functions of the AHR and [Ah] enzymes are, of course, in addition to the frequently described functions of "metabolic potentiation" and "detoxification" of various foreign chemicals.

Cyclin B-dependent kinase and caspase-1 activation precedes mitochondrial dysfunction in fumarylacetoacetate-induced apoptosis

Hereditary tyrosinemia type I is the most severe metabolic disease of the tyrosine catabolic pathway mainly affecting the liver. It is caused by deficiency of fumarylacetoacetate hydrolase, which prevents degradation of the toxic metabolite fumarylacetoacetate (FAA). We report here that FAA induces common effects (i.e., cell cycle arrest and apoptosis) in both human (HepG2) and rodent (Chinese hamster V79) cells, effects that seem to be temporally related. Both the antiproliferative and apoptosis-inducing activities of FAA are dose dependent and enhanced by glutathione (GSH) depletion with L-buthionine-(S,R)-sulfoximine (BSO). Short treatment (2 h) with 35 microM FAA/+BSO or 100 microM FAA/-BSO induced a transient cell cycle arrest at the G2/M transition (20% and 37%, respectively) 24 h post-treatment. In cells treated with 100 microM FAA/-BSO, an inactivation, followed by a rapid over-induction of cyclin B-dependent kinase occurred, which peaked 24 h post-treatment. Maximum levels of caspase-1 and caspase-3 activation were detected at 3 h and 32 h, respectively, whereas release of mitochondrial cytochrome c was maximal at 24-32 h post-treatment. The G2/M peak declined 24 h later, concomitantly with the appearance of a sub-G1, apoptotic population showing typical nucleosomal-sized DNA fragmentation and reduced mitochondrial transmembrane potential (Deltapsi(m)). These events were prevented by the general caspase inhibitor z-VAD-fmk, whereas G2/M arrest and subsequent apoptosis were abolished by GSH-monoethylester or N-acetylcysteine. Other tyrosine metabolites, maleylacetoacetate and succinylacetone, had no antiproliferative effects and induced only very low levels of apoptosis. These results suggest a modulator role of GSH in FAA-induced cell cycle disturbance and apoptosis where activation of cyclin B-dependent kinase and caspase-1 are early events preceding mitochondrial cytochrome c release, caspase-3 activation, and Deltapsi(m) loss. -Jorquera, R., Tanguay, R. M. Cyclin B-dependent kinase and caspase-1 activation precedes mitochondrial dysfunction in fumarylacetoacetate-induced apoptosis.

In vivo suppressor mutations correct a murine model of hereditary tyrosinemia type I

Hereditary tyrosinemia type I and alkaptonuria are disorders of tyrosine catabolism caused by deficiency of fumarylacetoacetate hydrolase (FAH) and homogentisic acid dioxygenase (HGD), respectively. Tyrosinemia is a severe childhood disease that affects the liver and kidneys, but alkaptonuria is a more benign adult disorder in comparison. Because HGD is upstream of FAH in the tyrosine pathway, mice doubly mutant in both enzymes were found to be protected from the liver and renal damage of tyrosinemia as hypothesized. Mice mutant at the tyrosinemic locus but heterozygous for alkaptonuria spontaneously developed clonal nodules of functionally normal hepatocytes that were able to rescue the livers of some mice with this genotype. This phenotypic rescue was a result of an inactivating mutation of the wild-type homogentisic acid dioxygenase gene, thus presenting an example of an in vivo suppressor mutation in a mammalian model.

Crystal structure and mechanism of a carbon-carbon bond hydrolase

BACKGROUND

Fumarylacetoacetate hydrolase (FAH) catalyzes the final step of tyrosine and phenylalanine catabolism, the hydrolytic cleavage of a carbon-carbon bond in fumarylacetoacetate, to yield fumarate and acetoacetate. FAH has no known sequence homologs and functions by an unknown mechanism. Carbon-carbon hydrolysis reactions are essential for the human metabolism of aromatic amino acids. FAH deficiency causes the fatal metabolic disease hereditary tyrosinemia type I. Carbon-carbon bond hydrolysis is also important in the microbial metabolism of aromatic compounds as part of the global carbon cycle.

RESULTS

The FAH crystal structure has been determined by rapid, automated analysis of multiwavelength anomalous diffraction data. The FAH polypeptide folds into a 120-residue N-terminal domain and a 300-residue C-terminal domain. The C-terminal domain defines an unusual beta-strand topology and a novel 'mixed beta-sandwich roll' structure. The structure of FAH complexed with its physiological products was also determined. This structure reveals fumarate binding near the entrance to the active site and acetoacetate binding to an octahedrally coordinated calcium ion located in close proximity to a Glu-His dyad.

CONCLUSIONS

FAH represents the first structure of a hydrolase that acts specifically on carbon-carbon bonds. FAH also defines a new class of metalloenzymes characterized by a unique alpha/beta fold. A mechanism involving a Glu-His-water catalytic triad is suggested based on structural observations, sequence conservation and mutational analysis. The histidine imidazole group is proposed to function as a general base. The Ca(2+) is proposed to function in binding substrate, activating the nucleophile and stabilizing a carbanion leaving group. An oxyanion hole formed from sidechains is proposed to stabilize a tetrahedral alkoxide transition state. The proton transferred to the carbanion leaving group is proposed to originate from a lysine sidechain. The results also reveal the molecular basis for mutations causing the hereditary tyrosinemia type 1.

Hepatocyte injury in tyrosinemia type 1 is induced by fumarylacetoacetate and is inhibited by caspase inhibitors

Tyrosinemia type 1, caused by mutations in the fumarylacetoacetate hydrolase gene (Fah), is characterized by severe liver injury. We earlier developed a tyrosinemic mouse model with two genetic defects, Fah and 4-hydroxyphenylpyruvate dioxygenase (Hpd) deficiencies. Apoptosis of hepatocytes was induced and an acute onset of liver failure occurred after administration of homogentisic acid (HGA), the intermediate metabolite between the enzymes HPD and FAH. Cytochrome c was released from mitochondria prior to liver failure in the Fah-/- Hpd-/- double-mutant mice after the administration of HGA. In a cell-free system, the addition of fumarylacetoacetate induced the release of cytochrome c from the mitochondria. We also found that caspase inhibitors were highly effective in preventing the liver failure induced by HGA in the double-mutant mice. Therefore, fumarylacetoacetate apparently induces the release of cytochrome c, which in turn triggers activation of the caspase cascade in hepatocytes of subjects with hereditary tyrosinemia type 1.

Tyrosinaemia type I and NTBC (2-(2-nitro-4-trifluoromethylbenzoyl)-1,3-cyclohexanedione)

In tyrosinaemia type I (McKusick 276700), fatal liver disease results either because of liver failure during infancy or early childhood or because of development of hepatocellular carcinoma during childhood or adolescence. This is caused by toxic metabolites which accumulate because of deficiency of fumarylacetoacetase, the last enzyme in the tyrosine catabolic pathway. NTBC is a potent inhibitor of 4-hydroxyphenylpyruvate dioxygenase and has been shown to efficiently prevent tyrosine degradation, and production of succinylacetone, in patients with tyrosinaemia. Since the first trial of NTBC treatment for tyrosinaemia type I in 1991, over 220 patients have been treated by the drug using a protocol which includes regular follow-up with reports of clinical and laboratory investigations to the study centre in Gothenburg, where additional analysis of critical variables is done on regularly collected samples. The course of the disease in patients with acute tyrosinaemia has changed dramatically. Only 10% of the patients have not clinically responded to NTBC treatment. In half of these patients, successful liver transplantation has been performed which has further reduced the mortality rate during infancy to 5%. The international NTBC study has now been going for 5 years and data have emerged that indicate a decreased risk for early development of hepatocellular carcinoma in patients who started treatment at an early age. There are now 101 patients aged 2-8 years who have started NTBC treatment before 2 years of age, and no cancer has developed after 2 years of age among these patients. However, there is no safe age with respect to occurrence of liver cancer, which has been recognized at diagnosis at 1 year of age in one patient and after a few months of treatment in an infant who was given NTBC at 5 months of age.