Self-induced correction of the genetic defect in tyrosinemia type I.

A mosaic pattern of immunoreactive fumarylacetoacetase (FAH) protein was found in liver tissue in 15 of 18 tyrosinemia type I patients of various ethnic origins. One additional patient had variable levels of FAH enzyme activity in liver tissue. In four patients exhibiting mosaicism of FAH protein, analysis for the tyrosinemia-causing mutations was performed in immunonegative and immunopositive areas of liver tissue by restriction digestion analysis and direct DNA sequencing. In all four patients the immunonegative liver tissue contained the FAH mutations demonstrated in fibroblasts of the patients. In the immunopositive nodules of regenerating liver tissue one of the mutated alleles apparently had reverted to the normal genotype. This genetic correction was observed for three different tyrosinemia-causing mutations. In each case a mutant AT nucleotide pair was reverted to a normal GC pair.

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.

Demonstration of 26-hydroxylation of C27-steroids in human skin fibroblasts, and a deficiency of this activity in cerebrotendinous xanthomatosis.

26-Hydroxylation of 5 beta-cholestane-3 alpha, 7 alpha, 12 alpha-triol and other C27-steroids was demonstrated in cultured skin fibroblasts from healthy individuals. Activities in skin fibroblasts were approximately 5-10% of those previously found in human liver homogenates, and were inhibited by CO. The apparent Km was lowest for 5 beta-cholestane-3 alpha, 7 alpha, 12 alpha-triol (1.3 mumol/liter) and highest for 5-cholestene-3 beta, 7 alpha-diol (12 mumol/liter). The rate of 26-hydroxylation was highest with 7 alpha-hydroxy-4-cholesten-3-one. These characteristics are similar to those of hepatic mitochondrial C27-steroid 26-hydroxylase. In skin fibroblasts from three patients with cerebrotendinous xanthomatosis (CTX), 26-hydroxylation of C27-steroids proceeded at a rate of only 0.2-2.5% of healthy controls. No accumulation of endogenous 5 beta-cholestane-3 alpha, 7 alpha, 12 alpha-triol could be demonstrated in these cells, and the lowered formation of radioactive, 26-hydroxylated products could not be explained by dilution of the labeled exogenous substrate. The present results add strong evidence to the concept that the primary metabolic defect in CTX is a deficiency of C27-steroid 26-hydroxylase.

Lack of 3 beta-hydroxy-delta 5-C27-steroid dehydrogenase/isomerase in fibroblasts from a child with urinary excretion of 3 beta-hydroxy-delta 5-bile acids. A new inborn error of metabolism.

Cultured fibroblasts were shown to be capable of catalyzing the conversion of 7 alpha-hydroxy-cholesterol to 7 alpha-hydroxy-4-cholesten-3-one, an important reaction in bile acid synthesis. The apparent Km was approximately 7 mumol/liter and Vmax varied between 3 and 9 nmol/mg protein per h under the assay conditions used. The assay was used to investigate fibroblasts from a patient who presented with a familial giant cell hepatitis and who was found to excrete the monosulfates of 3 beta, 7 alpha-dihydroxy-5-cholenoic acid and 3 beta, 7 alpha, 12 alpha-trihydroxy-5-cholenoic acid in urine (Clayton, P. T., J. V. Leonard, A. M. Lawson, K. D. R. Setchell, S. Andersson, B. Egestad, and J. Sjövall. 1987. J. Clin. Invest. 79:1031-1038). In addition 7 alpha-hydroxy-cholesterol was found to accumulate in the circulation. Cultured fibroblasts from this boy were completely devoid of 3 beta-hydroxy-delta 5-C27-steroid dehydrogenase/isomerase activity. Fibroblasts from his parents had reduced activity, compatible with a heterozygous genotype. The results provide strong evidence for the suggestion that this patient's liver disease was caused by a primary defect in the 3 beta-hydroxy-delta 5-C27-steroid dehydrogenase/isomerase involved in bile acid biosynthesis.

Tyrosinaemia type I--de novo mutation in liver tissue suppressing an inborn splicing defect.

Many patients with tyrosinaemia type 1 have a mosaic pattern of fumarylacetoacetase (FAH) immunopositive or immunonegative nodules in liver tissue. This phenomenon has been explained by a spontaneous reversion of the mutation in one allele to a normal genotype, but only a few nodules have been examined. We now report on a Norwegian patient, compound heterozygous for the mutations IVS12g(+5)-->a and G(1009-->)A, with liver mosaicism, but with an immunopositive nodule in which both primary mutations were intact. In the immunopositive hepatocytes of this nodule, genetic analyses showed a new mutation, C(1061-->)A, 6 bp upstream of the primary mutation IVS12g(+5)-->a in the FAH gene. The splicing defect caused by the primary mutation is most likely suppressed by the new mutation due to improvement of the splicing site. In the same liver we demonstrate another nodule of regenerating immunopositive tissue due to reversion of one of the primary mutations to a normal genotype. Together with the original cells this makes a triple mosaicism of hepatocytes with one, two or three point mutations in the FAH gene.

Assay of fumarylacetoacetate fumarylhydrolase in human liver-deficient activity in a case of hereditary tyrosinemia.

The activity of human liver fumarylacetoacetate fumarylhydrolase (EC 3.7.1.2) has been determined with fumarylacetoacetate as substrate. The Km was found to be 1.3 mu mol/l. Subcellular fractionation showed localization of the enzyme in the particle-free supernatant (cytosol). ZnCl2, CuCl2 and p-chloromercuribenzoic acid had a marked inhibitory effect on the enzyme activity, but no inhibition was observed with a number of anions and substrate analogs. Fumarylacetoacetate fumarylhydorlase activity in liver tissue from a patient with hereditary tyrosinemia was found to be less 2% of the controls. The assay is applicable to 3 mg of liver tissue which may be obtained by needle biopsy.

Concentrations of succinylacetone after homogentisate and tyrosine loading in healthy individuals with low fumarylacetoacetase activity.

Two healthy adults with low fumarylacetoacetase activity in fibroblasts and lymphocytes, one a compound heterozygote for the tyrosinaemia and the pseudodeficiency genes and the other a homozygote for the pseudodeficiency gene, produced substantial amounts of succinylacetone when given an intravenous homogentisate load. The level of metabolites correlated with the residual enzyme activity and the genotype, being higher in the compound heterozygote. This subject also showed a small increase of metabolites in urine after an oral tyrosine load. In the pseudodeficiency homozygote a depression of erythrocyte delta-aminolevulinate dehydratase activity was observed after the tyrosine load. In fasting state both individuals have erythrocyte delta-aminolevulinate dehydratase activity below the reference range, indicating a persistently raised concentration of metabolites. Thus, the pseudodeficiency state is not just an in vitro phenomenon, but results in a definite reduction of enzyme activity in vivo. We speculate that the variant gene may predispose to the development of liver disease, possibly not recognized as tyrosinaemia.

N-acetylaspartic aciduria in a child with a progressive cerebral atrophy.

Excessive excretion of N-acetylaspartic acid in urine is reported in a 6-yr-old child with extensive and progressive cerebral atrophy. The concentration in urine was 947-1,433 mumol/mmol creatinine (controls, n = 10, 5-21 mumol/mmol creatinine) and the daily excretion approximately 3-4 mmol. In cerebrospinal fluid from the patient the concentration was 611 mumol/l (controls, n = 10, not detectable, detection limit 2.3 mumol/l). The concentration of N-acetylaspartic acid in serum was 7 mumol/l. The low level in serum compared to the high urinary excretion of NAA suggests the possibility that NAA is synthesized in the kidneys in addition to the brain. This patient may cast new light on the functional role of N-acetylaspartic acid in humans.

Evidence that Alpers-Huttenlocher syndrome could be a mitochondrial disease.

We report an 11-year-old boy with a slight developmental delay and epilepsy. After he was placed on valproate, he developed hepatic failure and increasing neurologic symptoms, including epilepsia partialis continua, and died. Autopsy findings in liver and cerebrum were consistent with progressive neuronal degeneration of childhood with liver disease, also called Alpers-Huttenlocher syndrome. Ragged red fibers and cytochrome c oxidase negative fibers were present in muscle. These results suggest that Alpers-Huttenlocher syndrome, at least in some patients, is a mitochondrial disease.

Hereditary tyrosinemia type I--an overview.

Hereditary tyrosinemia type I is a metabolic disorder of autosomal recessive inheritance. The disorder is characterized by progressive liver disease and renal tubular defects with accompanying hypophosphatemic rickets. It occurs in an acute and a chronic form. Hepatocellular carcinoma is frequently encountered in the chronic form of the disorder. The primary enzyme defect is a deficiency of fumarylacetoacetase (FAH) (EC 3.7.1.2), the last enzyme in the degradation of tyrosine. The enzyme defect results in accumulation of fumaryl- and maleyl-acetoacetate which are thought to cause the cellular damage in tyrosinemia. Fumaryl- and maleyl-acetoacetate are reactive compounds and have not been identified in tyrosinemia patients. Succinylacetone, however, presumably derived from these metabolites by reduction and decarboxylation, is elevated in serum and urine from the patients. The diagnosis of tyrosinemia can be established by determination of succinylacetone in urine or serum and by assay of FAH in lymphocytes and fibroblasts. Heterozygotes for FAH can be identified by fumarylacetoacetase analysis in lymphocytes and fibroblasts. Prenatal diagnosis of tyrosinemia is possible by analysis of succinylacetone in amniotic fluid supernatant and by assay of FAH in cultured amniotic fluid cells or chorionic villus material. Genetic variants of FAH may interfere in the prenatal diagnosis of tyrosinemia by the FAH assay and in the detection of the carrier state. Immunoblotting technique has shown absence of immunoreactive protein in liver tissue from tyrosinemia patients. Liver transplantation is as yet the only definite treatment of the disorder.

The pre- and post-natal diagnosis of tyrosinemia type I and the detection of the carrier state by assay of fumarylacetoacetase.

Fumarylacetoacetase has been determined in fibroblasts, lymphocytes and/or liver tissue from 46 patients affected or presumed to be affected with tyrosinemia type I and in fibroblasts or lymphocytes from 80 obligate or presumed obligate heterozygotes. Eleven patients did not have deficient enzyme activity and 11 parents did not have intermediate enzyme activities compatible with heterozygosity for tyrosinemia. In altogether 15 of the 51 families investigated the fumarylacetoacetase activity of the patient and/or the parents was not compatible with tyrosinemia in the family. Prenatal determination of fumarylacetoacetase, in cultured amniotic fluid cells or chorionic villus material, has been performed in 24 pregnancies at risk or presumed to be at risk for a child with tyrosinemia. In six cases the enzyme activity was deficient, consistent with an affected foetus. Elevation of succinylacetone was found in 32 of the 35 patients with fumarylacetoacetase deficiency when the enzyme assay was carried out. In two cases with deficient fumarylacetoacetase activity, succinylacetone was searched for but had not been found to be elevated when the enzyme defect was demonstrated. Succinylacetone, if searched for, was not elevated in any of the cases with normal fumarylacetoacetase activity.

Urinary excretion of N-acetyl amino acids in patients with some inborn errors of amino acid metabolism.

Urinary organic acid profiles of patients with Maple Syrup Urine Disease (MSUD), hereditary tyrosinemia and phenylketonuria (PKU) have been studied by means of capillary GC-MS-computer technique. In addition to the characteristic metabolites of these disorders, increased amounts of N-acetylleucine, N-acetylisoleucine and N-acetylvaline were found in MSUD-urine. Increased excretion of N-acetylphenylalanine occurred in PKU, and in tyrosinemia both the latter compound and increased N-acetyltyrosine excretion were observed. These results together with literature reports of similar studies on patients with other aminoacidopathies may indicate that most disorders which result in accumulation of one or more specific amino acids, will convert a small fraction of them into their corresponding N-acetyl derivative.

Systematic laboratory diagnosis of human metabolic disorders.

A multicomponent analytical system for diagnosis of human metabolic disorders is overviewed. After preliminary analysis of the urine with simple chemical tests and standard clinical chemistry methods, the samples undergo a variety of chromatographic separations. Paper chromatography and thin-layer chromatography determine carbohydrates and mucopolysaccharides. Amino acids are analysed by automatic ionexchange chromatography. Gas chromatography - mass spectrometry with computerized library search is used to separate and identify organic acids, and high performance liquid chromatography with computerized diodearray detector is used to analyse metabolites of nucleic acids and other non-volatile or labile constituents. The system, gradually developed during the past two decades, is routinely used to diagnose, via its detection of pathological metabolites, around 100 different metabolic disorders. The methods may also be used to monitor the efficacy of therapeutic treatment in some of the cases where this is possible.

Depletion of mitochondrial DNA copies/cell in peripheral blood mononuclear cells in HIV-1-infected treatment-naïve patients.

OBJECTIVES: Mitochondrial toxicity is believed to be the main reason for adverse effects related to nucleoside reverse transcriptase inhibitors (NRTIs). The aim of the present study was to compare mitochondrial toxicity in NRTI-treated HIV-positive patients, HIV-positive treatment-naïve patients and HIV-negative controls by comparing mitochondrial DNA (mtDNA) copies/cell in peripheral blood mononuclear cells (PBMCs) and lactate/pyruvate (L/P) ratios in the different groups. METHODS: We enrolled 60 participants in the study: 31 patients on combined antiretroviral therapy (CART), 14 HIV-positive treatment-naive patients and 15 HIV-negative controls. mtDNA (copies/cell) in peripheral blood was analysed using quantitative real-time polymerase chain reaction (PCR). Standard curves and serial dilutions of plasmid-cloned mitochondrion and retinoblastoma (RB1) PCR products with known concentrations were generated to estimate the mtDNA and nuclear DNA (nDNA) copy numbers in each sample. The L/P ratio was enzymatically and spectrophotometrically analysed in samples from individuals in a fasted, non-exercise state. Results The median mtDNA copy number was 63 copies/cell (interquartile range 33-94) in HIV-positive patients and 153 (132-283) in HIV-negative controls (P<0.001). No significant difference was seen between the HIV-positive NRTI-exposed patients and the HIV-positive treatment-naive patients. Current use of didanosine was negatively correlated with depletion of mtDNA (r=-0.36, P=0.046). HIV-positive patients also had a higher L/P ratio compared with HIV-negative controls (P=0.004). CONCLUSIONS: The number of mtDNA copies/cell in PBMCs was depleted in HIV-positive treatment-naive patients as well as in HIV-positive NRTI-exposed patients. HIV-positive patients also had a higher L/P ratio compared with HIV-negative controls, which supports this conclusion. The study suggests that neither mtDNA in PBMCs nor L/P ratio is a good marker of NRTI-associated mitochondrial toxicity.

Tyrosinaemia type I--an update.

Tyrosinaemia type I is a recessively inherited disorder caused by a deficiency of fumarylacetoacetase (FAH), the last enzyme in tyrosine degradation. The presumed toxic agents are fumaryl- and maleylacetoacetate which are converted to succinylacetone (SA), a metabolite found in increased amounts in urine and plasma of the patients. The major clinical features are progressive liver damage and renal tubular defects with hypophosphataemic rickets. Renal tubular dysfunctions with secondary rickets may be lacking altogether, even in chronic patients. Hepatocellular carcinoma is a major cause of death in the chronic form. Diagnosis of the disorder is made by assay of SA in urine and serum and by determination of FAH in lymphocytes or fibroblasts. Prenatal diagnosis is performed by SA assay in amniotic fluid supernatant and FAH analysis in cultured amniotic fluid cells or chorionic villus material. Presence of a 'pseudodeficiency' gene for FAH prevents prenatal diagnosis by enzyme analysis in some families, and this gene also precludes identification of heterozygotes outside tyrosinaemia families. Immunoblot analyses show that acute patients and some chronic patients lack immunoreactive FAH protein. cDNA probes for FAH have been developed and several polymorphisms related to the FAH gene have been reported, which may allow prenatal diagnosis in families with complex genotypes. The gene for FAH has been mapped to chromosome 15 q23-q25. Liver transplantation is the ultimate treatment; most patients continue to excrete SA in urine after liver transplantation and therefore there is a possibility of kidney disease after transplantation.

Tyrosinaemia--treatment and outcome.

Tyrosinaemia type I is, untreated, a fatal disease: in the acute form from liver failure, in the chronic form often from hepatocellular carcinoma. Acute neurological crisis is also a cause of death. Traditionally the treatment has been with diet, but for a decade liver transplantation has been the ultimate treatment. The continuous production of the pathological metabolites in the kidneys after transplantation appears to be without significance. Introduction of the enzyme inhibitor NTBC in the treatment of tyrosinaemia has reduced the need for liver transplants. Neonatal screening may be justified as efficient treatment has become available. The complex phenotype of lethal albino mice, with severe alterations in gene expression, has been shown to be caused by fumarylacetoacetase deficiency. Prolonged hypoglycaemia in otherwise adequately treated tyrosinaemia patients may result from depressed expression of genes coding for enzymes in gluconeogenesis, as seen in the mouse model. Self-induced genetic correction in liver tissue that occurs in many tyrosinaemia patients may reduce the risk of liver failure in some patients.

Mass spectrometry in diagnosis of metabolic disorders.

Mass spectrometry is an important part of multicomponent analytical systems designed for diagnosis of metabolic disorders. In our laboratory capillary gas chromatography/mass spectrometry (GC/MS) with computerized library search is used primarily to separate and identify urinary organic acids. About 50 different diseases with increased excretion of organic acids are recognized today. Other techniques, including thin-layer chromatography, high-performance liquid chromatography with diode array detector, automatic amino acid analysis and two-dimensional electrophoresis are used to detect other compounds of diagnostic significance. The diagnostic use of GC/MS is exemplified by studies on two siblings. One died of his disease at approximately 1 year old. Both excreted 3-hydroxydicarboxylic acids (C8-C12) as identified by mass spectrometry. These metabolites are secondary to systemic carnitine deficiency. Low-fat diet normalized the clinical condition of the surviving sibling. GC/MS is now used to monitor the efficacy of dietary treatment by analysing the dicarboxylic acid excretion in this patient. Modern DNA technology is rapidly becoming increasingly important for diagnosis, particularly prenatal diagnosis, of metabolic diseases. It is suggested, however, that mass spectrometry will continue to be an important diagnostic tool for many years ahead.