Biochemical studies of a patient with hereditary hepatorenal tyrosinemia: evidence of glutathione deficiency

Remaining Challenges in the Treatment of Tyrosinemia from the Clinician's Viewpoint

This chapter provides a clinical perspective on the challenges that stand between current clinical practice and a cure for hepatorenal tyrosinemia (HT1). HT1 has been transformed in the last 50 years from an aggressive often undiagnosed childhood disease causing liver failure or liver cancer, with infant death in most patients, to a condition that is detectable at birth, and for which treatment with nitisinone (NTBC) and diet can prevent detectable liver or kidney abnormalities. What challenges remain? The properties of the affected metabolic pathway and the broad spectrum of severity seen in untreated patients are incompletely understood but potentially important for patients. Available treatments have potential complications, including liver transplantation (risks of surgery and of immunosuppression to prevent rejection), nitisinone and diet therapy (hypertyrosinemia, corneal opacities, nutritional imbalances and possibly developmental delay). The detection of liver cancer is imperfect and laborious. The effects of tyrosinemia during pregnancy are little-known. Although animal models of HT1 are becoming standard research tools in cell replacement and gene modification therapy, these techniques are not currently applicable to HT1 itself. Treatment adherence is variable, causing concern about long term outcome for some patients. Around the world, there are great disparities in the diagnosis and treatment of HT1. Most affected individuals are born in places where newborn screening for HT1 is not performed and where appropriate treatment is not available. We hope that this list will help to focus on some of these remaining obstacles to a cure for HT1.

Identification of Novel Mutations in FAH Gene and Prenatal Diagnosis of Tyrosinemia in Indian Family

Carrier of tyrosinemia type I was diagnosed by sequencing FAH (fumarylacetoacetate hydrolase) gene. It leads to the identification of heterozygous status for both c.648C>G (p.Ile216Met) and c.1159G>A (p.Gly387Arg) mutations in exons 8 and 13, respectively, in the parents. The experimental program PolyPhen, SIFT, and MT predicts former missense point mutation as “benign” that creates a potential donor splice site and later one as “probably damaging” which disrupts secondary structure of protein.

Rescue from neonatal death in the murine model of hereditary tyrosinemia by glutathione monoethylester and vitamin C treatment

Hereditary tyrosinemia type 1 (HT1) is a recessive disease caused by a deficiency of the enzyme fumarylacetoacetate hydrolase (FAH) that catalyzes the conversion of fumarylacetoacetate (FAA) into fumarate and acetoacetate. In mice models of HT1, FAH deficiency causes death within the first 24h after birth. Administration of 2-(2-nitro-4-trifluoro-methylbenzoyl)-1,3 cyclohexanedione (NTBC) prevents neonatal death in HT1 mice, ameliorates the HT1 phenotype but does not prevent development of hepatocellular carcinoma later on. FAA has been shown to deplete cells of glutathione by forming adducts. We tested whether a combination of a cell membrane permeable derivative of glutathione, glutathione monoethylester (GSH-MEE) and vitamin C could provide an alternative effective treatment for HT1. GSH-MEE (10 mmol/kg/j)/vitamin C (0.5 mmol/kg/j) treatment was given orally to pregnant/nursing female mice. While FAH-/- pups died in absence of treatment, all FAH-/- pups survived the critical first 24h of life when the mothers were on the GSH-MEE/vitamin C treatment and showed normal growth until postnatal day 10 (P10). However, after P10, pups showed failure to thrive, lethargy and died around P17. Thus, GSH-MEE/vitamin C supplementation could rescue the mice model of HT1 from neonatal death but it did not prevent the appearance of a HT1 phenotype in the second week after birth.

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.

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.

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.

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.

Deficient DNA-ligase activity in the metabolic disease tyrosinemia type I

Hereditary tyrosinemia type I (HT1) is an autosomal recessive inborn error of metabolism caused by the deficiency of fumarylacetoacetate hydrolase, the last enzyme in the tyrosine catabolism pathway. This defect results in accumulation of succinylacetone (SA) that reacts with amino acids and proteins to form stable adducts via Schiff base formation, lysine being the most reactive amino acid. HT1 patients surviving beyond infancy are at considerable risk for the development of hepatocellular carcinoma, and a high level of chromosomal breakage is observed in HT1 cells, suggesting a defect in the processing of DNA. In this paper we show that the overall DNA-ligase activity is low in HT1 cells (about 20% of the normal value) and that Okazaki fragments are rejoined at a reduced rate compared with normal fibroblasts. No mutation was found by sequencing the ligase I cDNA from HT1 cells, and the level of expression of the ligase I mRNA was similar in normal and HT1 fibroblasts, suggesting the presence of a ligase inhibitor. SA was shown to inhibit in vitro the overall DNA-ligase activity present in normal cell extracts. The activity of purified T4 DNA-ligase, whose active site is also a lysine residue, was inhibited by SA in a dose-dependent manner. These results suggest that accumulation of SA reduces the overall ligase activity in HT1 cells and indicate that metabolism errors may play a role in regulating enzymatic activities involved in DNA replication and repair.

Abnormal glutathione conjugation in patients with tyrosinaemia type I

Previous studies have suggested that tyrosinaemia type I may be associated with reduced glutathione availability due to conjugation of tyrosinaemia-associated reactive intermediates with glutathione. In the present study, the glutathione/ glutathione S-transferase system of two tyrosinaemia patients and three healthy controls were characterized by administering the racemic sedative drug bromisoval, a probe drug for assessing glutathione conjugation activity in vivo. Furthermore, concentrations of glutathione and glutathione S-transferase class alpha (GSTA) isoenzymes as well as the glutathione S-transferase class mu phenotype were assessed in the blood of six tyrosinaemia patients. The excretion of bromisoval mercapturates in healthy children was comparable to that observed in healthy adults. Tyrosinaemia patients were found to have a very high urinary recovery of bromisoval mercapturates (> or = 60% of the dose compared to about 30% for healthy, age-matched children and adults), which could be attributed mainly to a higher urinary excretion of the mercapturate derived from S-bromisoval. Healthy children and adults predominantly excrete the (R)-bromisoval mercapturate. The differences in amount excreted as well as in stereoselectivity of the urinary excretion of bromisoval mercapturates in tyrosinaemia patients are possibly related to an increased activity of specific glutathione S-transferase isoenzymes. Plasma glutathione and blood cell glutathione disulphide concentrations in tyrosinaemia patients were normal. Low blood cell glutathione concentrations were in general found only in two patients with a poor clinical condition. These results indicate that, in contrast to previous suggestions, reduced glutathione availability is not a generalized problem in (stabilized) tyrosinaemia patients.

The mutagenicity of the tyrosine metabolite, fumarylacetoacetate, is enhanced by glutathione depletion

The toxicity of tyrosine metabolites has been suggested, but not proven, to play a role in the ethiopathogenesis of hepatic alterations observed in hereditary tyrosinemia type I (HT I), a metabolic disease caused by a deficiency of the last enzyme in the tyrosine catabolic pathway, fumarylacetoacetate hydrolase. One of these metabolites, fumarylacetoacetate (FAA), is mutagenic in Chinese hamster V79 cells. We report here that FAA is a powerful glutathione depletor in this cell system. Moreover, the mutagenicity of FAA (100 microM) is potentiated by depletion of cellular glutathione (12% of control levels) by pretreatment with L-buthionine-(S,R)-sulphoximine. In this case, the mutation frequency induced by FAA is 10 times higher than in untreated, control cells. This enhancement is abolished by a partial replenishment of intracellular glutathione (32% of control levels) prior to FAA treatment. Reactive oxygen species are not generated during FAA treatment of glutathione-depleted or undepleted cells. Although the mechanism(s) underlying the mutagenic activity of FAA remains to be identified, these results show that the glutathione depletion activity of FAA may play an important role in the manifestation of its mutagenicity which likely contributes to the HT I-associated liver pathologies.

Interaction between the Ah receptor and proteins binding to the AP-1-like electrophile response element (EpRE) during murine phase II [Ah] battery gene expression

We have studied three Phase II genes in the mouse dioxin-inducible [Ah] battery: Nmo1 [encoding NAD(P)H:menadione oxidoreductase], Ahd4 (encoding the cytosolic aldehyde dehydrogenase ALDH3c), and Ugt1*06 (a UDP glucuronosyltransferase). Oxidant-induced Nmo1 gene expression in the c14CoS/c14CoS mouse appears likely to be caused by homozygous loss of the fumarylacetoacetate hydrolase (Fah) gene on Chr 7 and absence of the enzyme (FAH), which leads to increased levels of endogenous tyrosine oxidative metabolites. We show here that increases in [Ah] Phase II gene expression in the 14CoS/14CoS mouse are correlated with an AP-1-like DNA motif called the electrophile response element (EpRE), which has been found in the 5' flanking regulatory regions of all murine (Ah) Phase II genes. Aromatic hydrocarbon response element (AhREs) are responsible for dioxin-mediated upregulation of all six [Ah] battery genes, and one or more AhREs have been found in the 5' flanking regulatory regions of all of these [Ah] genes. Gel mobility shift assays, with a synthetic oligonucleotide probe corresponding to the EpRE, show that EpRE-binding proteins are more than twice as abundant in 14CoS/14CoS than in the wild-type ch/ch nuclear extracts. Competition studies of EpRE-specific binding with an excess of EpRE, mutated EpRE, AP-1, AhRE3, mutated AhRE3, and C/EBP alpha oligonucleotides suggest that several common transcriptional factors bind to the EpRE and AhRE3 motifs. Two monospecific antibodies to the Ah receptor (AHR) protein block formation of an EpRE-specific complex on gel mobility electrophoresis. These data suggest that AHR (or AHR-related protein) might be an integral part of the EpRE-binding transcriptional complex associated with the oxidative stress response. To our knowledge, this is among the first reports of the same transcription factor operating at two different response elements upstream of a single gene.

Tyrosinaemia type 1 and glutathione synthetase deficiency: two disorders with reduced hepatic thiol group concentrations and a liver 4-fumarylacetoacetate hydrolase deficiency

Thiol groups are important components of proteins and their oxidation can lead to a substantial loss of protein function. Patients with two apparently unrelated inborn errors of metabolism, tyrosinaemia type 1 and glutathione synthetase deficiency, have been reported to show reduced cell glutathione concentrations. We have found that not only glutathione but also protein thiol concentrations are reduced in the liver in tyrosinaemia type 1 patients. We also report a case of glutathione synthetase deficiency with a substantial deficiency of liver 4-fumarylacetoacetate hydrolase and provide evidence that glutathione, or some small-molecular-weight thiol, is essential for maintaining stability of this enzyme in vitro. Our results suggest that the availability of thiol groups may modify the phenotype of tyrosinaemia type 1 and that liver 4-fumarylacetoacetate hydrolase deficiency may be a secondary complicating factor in some forms of glutathione synthetase deficiency.

Lessons from lethal albino mice

Results from the analysis of mice homozygous for lethal albino deletions suggested the existence of a locus involved in the regulation of gene expression in the liver. The surprising finding that the locus encodes an enzyme active in tyrosine metabolism forces us to re-evaluate the lethal albino phenotype and advises caution in the interpretation of seemingly simple phenotypes.

Clinical features and diagnostic approach in type I tyrosinaemia in an infant with cytomegaly virus infection and bacterial sepsis

A severely ill 2-month-old female infant was admitted with meningitis and septicaemia caused by Streptococcus pneumoniae. The patient, who also had an acute cytomegalovirus (CMV) infection, revealed the typical clinical and biochemical characteristics of type I tyrosinaemia (TIT). Clinical evidence of severe hepatocellular damage was shown, but urinary succinylacetone was not detected. The diagnosis of TIT was finally confirmed by decreased activity of fumarylacetoacetase (FAA) in skin fibroblasts from the patient and both parents. Following dietary treatment and after overcoming the bacterial and viral infection, the patient's liver function improved.

Hereditary tyrosinemia type I. Self-induced correction of the fumarylacetoacetase defect

Two Norwegian patients with chronic tyrosinemia type I showed > 50% residual fumarylacetoacetase activity in liver samples obtained during liver transplantation. The enzyme characteristics of both patients were comparable with those of a normal control. Immunohistochemistry on liver sections from these patients and from three other Norwegian tyrosinemia patients revealed a mosaicism of fumarylacetoacetase immunoreactivity corresponding completely or partly to some of the regenerating nodules. This appearance of enzyme protein is presumably induced by the disease process. The mechanism involved remains unclear and could be caused by a genetic alteration, regained translation of messenger RNA, or to enhanced stability of an abnormal enzyme.

Hereditary tyrosinaemia (HT) type 1A

Two infants, aged 5 and 6 months, with the chronic form of HT Type 1 A were studied with sonography and CT. The imaging findings mirrored the pathological process. The sonographic findings included marked hepatomegaly, the parenchyma being of increased echogenicity due to fibrosis and fatty infiltration, and containing multiple hypoechoic regenerating nodules of varying sizes. There was marked nephromegaly, with uniform thickening of the renal cortices. Apart from the density measurement of the hepatic nodules and the perfusion status of the liver and kidney, no additional information was added by CT imaging. The diagnosis of HT Type 1 A should be excluded in an infant presented with the described typical sonographic features.

Chemistry and biology of heme. Effect of metal salts, organometals, and metalloporphyrins on heme synthesis and catabolism, with special reference to clinical implications and interactions with cytochrome P-450

Although free porphyrins occur in nature in small quantities, no known function has been assigned to them. In contrast, heme and cobalamin, which are Fe and Co chelates of porphyrins or porphyrin derivatives, respectively, carry out crucial biological functions. Heme is the prosthetic group for a number of hemoproteins. These include myoglobin and hemoglobin, which carry out oxygen binding or transport; mitochondrial cytochromes aa3, b, c, and c3, which are important in transferring electrons; microsomal cytochrome P-450, which catalyzes mixed-function oxidations; catalase, which decomposes H2O2; peroxidase, which activates H2O2; and tryptophan pyrrolase, which catalyzes the oxidation of tryptophan. Recently, heme has also been shown to be the prosthetic group of prostaglandin and peroxide synthetase and indoleamine dioxygenase. The elegant studies of the biochemical pathway for the formation of heme demonstrated the arrangement in the porphyrin macrocycle of the carbon and nitrogen atoms originating from the eight glycine and the succinic acid molecule that are the precursors of porphyrins. There are eight enzymes involved in the synthesis of heme. The first and last three of these enzymes are localized in mitochondria, while the intermediate enzymes are localized in cytosol. The catalytic site of HMOX recognizes metalloporphyrins with central metal atoms other than iron; it favors some of these metalloporphyrins over heme as a potential substrate, sometimes by a large factor, permitting the synthetic heme analogue to serve as a potent competitive inhibitor of HMOX reaction. Since these synthetic metalloporphyrins do not bind molecular oxygen, they are not metabolically degraded by ring rupture and do not add to the body pool of bile pigment. One possible consequence of this competitive inhibition of heme degradation is suppression of bile pigment formation to such a degree that excessive plasma levels of bilirubin may be diminished. The studies of Drummond and Kappas (1981) and later studies in rats, mice, monkeys, and man, and also our studies have proved the latter phenomenon. The compound does not appear to affect the metabolic disposition of preformed bilirubin but inhibits biliary bilirubin excretion derived from the metabolism of endogenous or exogenous heme. Whether some of the effect of Sn-PP on naturally occurring or experimentally induced jaundice in animals reflects diversion of heme to nonheme to oxygenase-dependent pathways of heme metabolism, or whether a pathway which is normally latent becomes activated concurrent with HMOX inhibition is not known.(ABSTRACT TRUNCATED AT 400 WORDS)

Type 1 hereditary tyrosinemia. Evidence for molecular heterogeneity and identification of a causal mutation in a French Canadian patient

Type 1 hereditary tyrosinemia (HT1) is a metabolic disorder caused by a deficiency of fumarylacetoacetate hydrolase (FAH). Using a full-length FAH cDNA and specific antibodies, we investigated liver specimens from seven unrelated HT1 patients (six of French Canadian and one of Scandinavian origin). The expression of FAH in livers of these individuals was analyzed at several molecular levels including mRNA, immunoreactive material (IRM), and enzymatic activity. Four phenotypic variants were differentiated by these assays: (i) presence of FAH mRNA without any IRM or enzymatic activity, (ii) decreased FAH mRNA, IRM, and enzymatic activity, (iii) moderately decreased FAH mRNA and IRM with severely reduced enzymatic activity, and (iv) undetectable FAH mRNA, IRM, and enzymatic activity. These various molecular phenotypes suggest that this disorder may be caused by a variety of FAH mutations. Interestingly, we found no apparent relationship between the clinical and the molecular phenotypes, except that patients with absent IRM and enzymatic activity tend to have higher levels of serum alpha-fetoprotein and an earlier clinical onset. To further analyze the molecular basis of HT1, the FAH cDNA of a patient designated as variant A was amplified and sequenced. An A-to-T transversion, which substitutes asparagine16 with isoleucine (N16I), was identified. This patient was heterozygous as shown by direct sequencing of the amplified region and hybridization with allele-specific oligonucleotide probes. The N16I allele originates from the father and the second allele appears not to be expressed in the liver of the proband. CV-1 cells transfected with the mutant cDNA produced FAH mRNA, but no protein or hydrolytic activity, as predicted by the "A" phenotype of the patient. This is the first demonstration of heterogeneity in the expression of FAH at the levels of protein, mRNA, and enzymatic activity in the livers of HT1 patients and is the first identification of a causal mutationin this disease.

Deficiency of an enzyme of tyrosine metabolism underlies altered gene expression in newborn liver of lethal albino mice

Mice homozygous for albino deletions encompassing the locus alf/hsdr-1 die shortly after birth. Lethality is thought to be the consequence of hypoglycemia, which results from the failure to activate hormone-dependent genes in liver and kidney encoding enzymes important for gluconeogenesis. Within the region in which alf/hsdr-1 has been defined by physical mapping, we identified the gene encoding fumarylacetoacetate hydrolase (FAH), an enzyme of tyrosine metabolism. Lack of FAH activity should lead to accumulation of toxic tyrosine metabolites. In man, genetically determined FAH deficiency is the primary defect in tyrosinemia type I, a fatal liver disease of infants. Northern blot and in situ hybridization analysis of mouse tissues showed that the cell types that normally express FAH correspond to those that exhibit a phenotype in alf/hsdr-1 deletion mice. Moreover, we could mimic aspects of the alf/hsdr-1 deletion phenotype in vitro by treating primary hepatocyte cultures with an intermediate of tyrosine metabolism. These findings strongly suggest that alf/hsdr-1 encodes FAH and that absence of FAH is responsible for neonatal lethality in albino deletion mice. Mechanisms by which this metabolic defect might bring about alterations in gene expression characteristic of the alf/hsdr-1 deletion phenotype are discussed.

The physiological consequences of glutathione variations

The major low molecular weight thiol inside cells, the tripeptide glutathione (GSH), is of importance for protection of the cell against oxidative challenge, for thiol homeostasis required to guarantee basic functions, and for defence mechanisms against xenobiotics. Since the pathophysiological significance of a perturbed GSH status in human disease is less clear, this review evaluates the consequences of in vivo variations of GSH. Owing to intracellular GSH concentrations above 2 mM depletion of GSH as such has little metabolic consequences unless an additional stress is superimposed. The kinetic properties of GSH-dependent enzymes imply that loss of up to 90% of intracellular GSH may still be compatible with cellular integrity. Mitochondrial GSH, which accounts for about 10% of total cellular GSH, may define the threshold beyond that toxicity commences. Thus, in cases of severe GSH-depletion a substitution of GSH as a therapeutic measure seems justified. Such a severe depletion of GSH has been described for some diseases such as liver dysfunction, AIDS or pulmonary fibrosis.

Neurologic crises in hereditary tyrosinemia

Hereditary tyrosinemia results from an inborn error in the final step of tyrosine metabolism. The disease is known to cause acute and chronic liver failure, renal Fanconi's syndrome, and hepatocellular carcinoma. Neurologic manifestations have been reported but not emphasized as a common problem. In this paper, we describe neurologic crises that occurred among children identified as having tyrosinemia on neonatal screening since 1970. Of the 48 children with tyrosinemia, 20 (42 percent) had neurologic crises that began at a mean age of one year and led to 104 hospital admissions. These abrupt episodes of peripheral neuropathy were characterized by severe pain with extensor hypertonia (in 75 percent), vomiting or paralytic ileus (69 percent), muscle weakness (29 percent), and self-mutilation (8 percent). Eight children required mechanical ventilation because of paralysis, and 14 of the 20 children have died. Between crises, most survivors regained normal function. We found no reliable biochemical marker for the crises (those we evaluated included blood levels of tyrosine, succinylacetone, and hepatic aminotransferases). Urinary excretion of delta-aminolevulinic acid, a neurotoxic intermediate of porphyrin biosynthesis, was elevated during crises but also during the asymptomatic periods. Electrophysiologic studies in seven patients and neuromuscular biopsies in three patients showed axonal degeneration and secondary demyelination. We conclude that episodes of acute, severe peripheral neuropathy are common in hereditary tyrosinemia and resemble the crises of the neuropathic porphyrias.

Hereditary tyrosinemias (type I): a new vista on tyrosine toxicity and cancer

Review of the literature of the past 40 years on tyrosine and its toxicity shows that no direct link between this aromatic amino acid and carcinogenesis has been well established. Ten years ago, studies of tyrosine toxicity in mice suggested the formation of an epoxide adduction product presumably derived from tyrosine by way of the liver microsomal detoxification system. Another study showed an increased frequency of hepatomas following long-term treatment with para-hydroxyphenyllactic acid, a tyrosine derivative occurring in the absence of p-hydroxyphenylpyruvate oxydase activity. Recently, studies on hereditary tyrosinemias (Type I) have indicated that the primary enzyme defect in these diseases is a deficiency of liver and renal fumarylacetoacetase. This results in an accumulation of natural alkylating derivatives of homogentisic acid such as maleyl- and fumarylacetoacetate in liver. Adduction of these compounds by glutathione is demonstrated by the presence of the mercapturic acid S-2-fumaryl-acetone-N-acetylcysteine in urine of patients. This adduct is also present in the urine of a number of heterozygote carriers after oral loads consisting of small quantities of homogentisic acid. In this report, we present the results of preliminary animal studies on the biochemical nature of the toxic effects of these tyrosine derivatives in these diseases along with preliminary data on the influence of fumarylacetone on protein synthesis in cultured eucaryotic cells. Fumarylacetone reacts as a natural alkylating agent and may, along with maleylacetoacetate, be responsible for the high incidence of late-onset hepatoma in the clinical chronic forms of hereditary tyrosinemias.

Hereditary tyrosinemia. Formation of succinylacetone-amino acid adducts

Succinylacetone (SA) (4,6-dioxoheptanoic acid) is an abnormal metabolite produced in patients with hereditary tyrosinemia as a consequence of an inherited deficiency of fumaryl acetoacetate hydrolase activity. Patients with this disease are associated with a number of abnormalities, including aminoaciduria, proteinuria, liver failure, commonly hepatoma, and decreased GSH concentration in the liver. In the course of our studies of tyrosinemia, we found that the urine of patients with this disorder contains material(s) that absorbs light at 315 nm. We investigated the nature of the 315 nm material in detail. SA was found to react with amino acids and protein nonenzymatically, to form stable adducts at physiological temperature and pH. All SA adducts with amino acids and/or proteins exhibited an absorption peak at 315 nm. Although all amino acids reacted with SA, the most reactive amino acid was lysine (Lys), followed, in order, by glycine, methionine, phenylalanine, serine, alanine, and glutamine. SA-adducts were unstable at pH below 6, while they were made considerably more stable after reduction with NaBH4, suggesting that SA forms an adduct via Schiff base formation. High-performance liquid chromatography (HPLC) analysis of urines from patients with tyrosinemia revealed the existence of SA-glycine, SA-methionine, SA-tyrosine, and SA-phenylalanine. After digestion of urines with proteinase K, three more HPLC peaks appeared, which all corresponded to SA-Lys adducts. TLC analysis of SA-Lys showed that SA-Lys could form as many as seven different adducts. No SA-adduct peaks were observed in HPLC in urines from normal subjects, patients with other forms of aminoaciduria, or patients with the nephrotic syndrome. In addition to amino acids and proteins, SA reacted with reduced glutathione (GSH) and formed a stable adduct. These findings suggest that SA adduct formation with amino acids, GSH, and proteins is a significant process occurring in tyrosinemia, and may account for certain of the pathologic findings in this hereditary disorder.