Hereditary tyrosinemia type 1: novel missense, nonsense and splice consensus mutations in the human fumarylacetoacetate hydrolase gene; variability of the genotype-phenotype relationship

A False-Negative Newborn Screen for Tyrosinemia Type 1-Need for Re-Evaluation of Newborn Screening with Succinylacetone

Undiagnosed and untreated tyrosinemia type 1 (TT1) individuals carry a significant risk for developing liver fibrosis, cirrhosis and hepatocellular carcinoma (HCC). Elevated succinylacetone (SA) is pathognomonic for TT1 and therefore often used as marker for TT1 newborn screening (NBS). While SA was long considered to be elevated in every TT1 patient, here we present a recent false-negative SA TT1 screen. A nine-year-old boy presented with HCC in a cirrhotic liver. Additional tests for the underlying cause unexpectedly revealed TT1. Nine years prior, the patient was screened for TT1 via SA NBS with a negative result: SA 1.08 µmol/L, NBS cut-off 1.20 µmol/L. To our knowledge, this report is the first to describe a false-negative result from the TT1 NBS using SA. False-negative TT1 NBS results may be caused by milder TT1 variants with lower SA excretion. Such patients are more likely to be missed in NBS programs and can be asymptomatic for years. Based on our case, we advise TT1 to be considered in patients with otherwise unexplained liver pathology, including fibrosis, cirrhosis and HCC, despite a previous negative TT1 NBS status. Moreover, because the NBS SA concentration of this patient fell below the Dutch cut-off value (1.20 µmol/L at that time), as well as below the range of cut-off values used in other countries (1.29-10 µmol/L), it is likely that false-negative screening results for TT1 may also be occurring internationally. This underscores the need to re-evaluate TT1 SA NBS programs.

The compound heterozygous mutations of c.607G>a and c.657delC in the FAH gene are associated with renal damage with hereditary tyrosinemia type 1 (HT1)

BACKGROUND

Hereditary tyrosinemia type 1 (HT1) is a rare inherited metabolic disease characterized by severe liver and renal dysfunction. Early identification in affected children is critical for improved treatment options and prognosis.

METHODS

In this study, we identified novel compound heterozygous mutations (NM_000137: c.657delC (p.K220Rfs*12) and c.607G>A (p.A203T)) in the fumarylacetoacetate hydrolase (FAH) gene in a family. We also characterized the clinical phenotype of the proband and verified the pathogenic effects of the mutations. Furthermore, we explored the pathogenic mechanism of renal injury through renal biopsy pathology and cell-based in vitro assays. Our study aims to verify the association between novel fumarylacetoacetate hydrolase (FAH) variants and HT1, confirm the pathogenic effects of the mutations and explore the pathogenic mechanism of renal injury.

RESULTS

We showed these FAH mutations were inherited in an autosomal recessive manner and resulted in abnormal FAH protein expression and dysfunction, leading to fumarylacetoacetate (FAA) accumulation. The proband also showed apparent renal injury, including glomerular filtration barrier dysfunction and abnormal tubular protein reabsorption.

CONCLUSIONS

These observations may provide deeper insights on disease pathogenesis and identify potential therapeutic approaches for HT1 from a genetic perspective. Similarly, we hope to provide valuable information for genetic counseling and prenatal diagnostics.

Code inside the codon: The role of synonymous mutations in regulating splicing machinery and its impact on disease

In eukaryotes, precise pre-mRNA processing, including alternative splicing, is essential to carry out the intricate protein translation process. Both point mutations (that alter the translated protein sequence) and synonymous mutations (that do not alter the translated protein sequence) are capable of affecting the splicing process. Synonymous mutations are known to affect gene expression via altering mRNA stability, mRNA secondary structure, splicing processes, and translational kinetics. In higher eukaryotes, precise splicing is regulated by three weakly conserved cis-elements, 5' and 3' splice sites and the branch site. Many other cis-acting elements (exonic/intronic splicing enhancers and silencers) and trans-acting splicing factors (serine and arginine-rich proteins and heterogeneous nuclear ribonucleoproteins) have also been found to enhance or suppress the splicing process. The appearance of synonymous mutations in cis-acting elements can alter the splicing process by changing the binding pattern of splicing factors to exonic splicing enhancers or silencer motifs. This results in exon skipping, intron retention, and various other forms of alternative splicing, eventually leading to the emergence of a wide range of diseases. The focus of this review is to elucidate the role of synonymous mutations and their impact on abnormal splicing mechanisms. Further, this study highlights the function of synonymous mutation in mediating abnormal splicing in cancer and development of X-linked, and autosomal inherited diseases.

Therapeutic Modulation of RNA Splicing in Malignant and Non-Malignant Disease

RNA splicing is the enzymatic process by which non-protein coding sequences are removed from RNA to produce mature protein-coding mRNA. Splicing is thereby a major mediator of proteome diversity as well as a dynamic regulator of gene expression. Genetic alterations disrupting splicing of individual genes or altering the function of splicing factors contribute to a wide range of human genetic diseases as well as cancer. These observations have resulted in the development of therapies based on oligonucleotides that bind to RNA sequences and modulate splicing for therapeutic benefit. In parallel, small molecules that bind to splicing factors to alter their function or modify RNA processing of individual transcripts are being pursued for monogenic disorders as well as for cancer.

Presence of three mutations in the fumarylacetoacetate hydrolase gene in a patient with atypical symptoms of hereditary tyrosinemia type I

Hereditary tyrosinemia type 1 (HT1), the most severe disease of the tyrosine catabolic pathway, is caused by a deficiency of fumarylacetoacetate hydrolase (FAH). More than 90 disease-causing variants have been identified in the fah gene. We investigated the molecular defect in a patient who presented atypical symptoms for the disease. No immunoreactive FAH was found in the liver and RNA analysis by RT-PCR suggested the presence of splicing mutations. Indeed, the patient was revealed to be a compound heterozygote for IVS6-1 g- > t and two new variants, namely p.V259L and p.G398E. Using splicing minigene constructs transfected in HeLa cells, the c.775G > C variant (p.V259L) was shown to affect partially exon 9 splicing thereby allowing the production of some full-length double-mutant FAH transcripts. The p.G398E variant had a major impact on enzyme activity, which was worsened by the p.V259L variant. Surprisingly, the double mutant protein was expressed to similar level as the wild-type protein upon transfection in HeLa cells but was absent in the patient liver extract, suggesting a higher propensity to be degraded in the hepatocellular context.

Molecular Aspects of the FAH Mutations Involved in HT1 Disease

Hereditary tyrosinemia type 1 (HT1) is caused by the lack of fumarylacetoacetate hydrolase (FAH), the last enzyme of the tyrosine catabolic pathway. Up to now, around 100 mutations in the FAH gene have been associated with HT1, and despite many efforts, no clear correlation between genotype and clinical phenotype has been reported. At first, it seems that any mutation in the gene results in HT1. However, placing these mutations in their molecular context allows a better understanding of their possible effects. This chapter presents a closer look at the FAH gene and its corresponding protein in addition to provide a complete record of all the reported mutations causing HT1.

Geographical and Ethnic Distribution of Mutations of the Fumarylacetoacetate Hydrolase Gene in Hereditary Tyrosinemia Type 1

Hereditary tyrosinemia type 1 (HT1) (OMIM 276700) is a severe inherited metabolic disease affecting mainly hepatic and renal functions that leads to a fatal outcome if untreated. HT1 results from a deficiency of the last enzyme of tyrosine catabolism, fumarylacetoacetate hydrolase (FAH). Biochemical findings include elevated succinylacetone in blood and urine; elevated plasma concentrations of tyrosine, methionine and phenylalanine; and elevated tyrosine metabolites in urine. The HT1 frequency worldwide is about 1 in 100,000 individuals. In some areas, where the incidence of HT1 is noticeably higher, prevalence of characteristic mutations has been reported, and the estimated incidence of carriers of a specific mutation can be as high as 1 out of 14 adults. Because the global occurrence of HT1 is relatively low, a considerable number of cases may go unrecognized, underlining the importance to establish efficient prenatal and carrier testing to facilitate an early detection of the disease. Here we describe the 95 mutations reported so far in HT1 with special emphasis on their geographical and ethnic distributions. Such information should enable the establishment of a preferential screening process for mutations most predominant in a given region or ethnic group.

Effect of nitisinone (NTBC) treatment on the clinical course of hepatorenal tyrosinemia in Québec

BACKGROUND

Hepatorenal tyrosinemia (HT1, fumarylacetoacetate hydrolase deficiency, MIM 276700) can cause severe hepatic, renal and peripheral nerve damage. In Québec, HT1 is frequent and neonatal HT1 screening is practiced. Nitisinone (NTBC, Orfadin ®) inhibits tyrosine degradation prior to the formation of toxic metabolites like succinylacetone and has been offered to HT1 patients in Québec since 1994.

METHODS

We recorded the clinical course of 78 Québec HT1 patients born between 1984 and 2004. There were three groups: those who never received nitisinone (28 patients), those who were first treated after 1 month of age (26 patients) and those treated before 1 month (24 patients). Retrospective chart review was performed for events before 1994, when nitisinone treatment began, and prospective data collection thereafter.

FINDINGS

No hospitalizations for acute complications of HT1 occurred during 5731 months of nitisinone treatment, versus 184 during 1312 months without treatment (p<0.001). Liver transplantation was performed in 20 non-nitisinone-treated patients (71%) at a median age of 26 months, versus 7 late-treated patients (26%, p<0.001), and no early-treated patient (p<0.001). No early-treated patient has developed detectable liver disease after more than 5 years. Ten deaths occurred in non-nitisinone treated patients versus two in treated patients (p<0.01). Both of the latter deaths were from complications of transplantation unrelated to HT1. One probable nitisinone-related event occurred, transient corneal crystals with photophobia.

INTERPRETATION

Nitisinone treatment abolishes the acute complications of HT1. Some patients with established liver disease before nitisinone treatment eventually require hepatic transplantation. Patients who receive nitisinone treatment before 1 month had no detectable liver disease after more than 5 years.

Newborn Screening for Tyrosinemia Type I: Further Evidence that Succinylacetone Determination on Blood Spot Is Essential

Tyrosinemia type I is a genetic disorder characterized by accumulation in the blood and urine of the toxic metabolite succinylacetone (SUAC), not detectable in healthy samples. In many countries, newborns are screened for tyrosinemia type I using tyrosine as a primary marker. Unfortunately, tyrosine accumulation may take longer to occur and it may be not obvious when specimens are collected, in the first few days of life, as for newborn screening. In 2008, we reported changes to simultaneously measure acylcarnitines, amino acids, and SUAC during expanded newborn screening. We established the usefulness of this method after identifying a first asymptomatic newborn affected by tyrosinemia type I. Now we report a second infant with positive SUAC screening result (14.1 μmol/L, n.v. < 2) and normal tyrosine concentration (74 μmol/L; n.v. < 250). We also performed molecular analysis of FAH gene in both patients after diagnosis at newborn screening. They had consanguineous parents and were both homozygous for two known disease-causing mutations of the FAH gene. The outcome of patients detected in the MS/MS screening is significantly favorable. We also report our results of newborn screening for tyrosinemia type I before and after inclusion of SUAC as a primary marker for this disease.

Analysis of mutations that influence pre-mRNA splicing

A rapidly increasing number of human diseases are now recognized as being caused by the selection of wrong splice sites. In most cases, these changes in alternative splice site selection are due to single nucleotide exchanges in splicing regulatory elements. This chapter describes the use of bioinformatics tools to predict the influence of a mutation on alternative pre-mRNA splicing and the experimental testing of these predictions. The bioinformatic analysis determines the influence of a mutation on splicing enhancers and silencers, splice sites and RNA secondary structures. This approach generates hypotheses that are tested using splicing reporter constructs, which are then analyzed in transfection assays. We describe a recombination-based system that allows for the generation of splicing reporter constructs in the first week and their subsequent analysis in the second week.

Genomic features defining exonic variants that modulate splicing

A comparative analysis of SNPs and their exonic and intronic environments identifies the features predictive of splice affecting variants.

Background

Single point mutations at both synonymous and non-synonymous positions within exons can have severe effects on gene function through disruption of splicing. Predicting these mutations in silico purely from the genomic sequence is difficult due to an incomplete understanding of the multiple factors that may be responsible. In addition, little is known about which computational prediction approaches, such as those involving exonic splicing enhancers and exonic splicing silencers, are most informative.

Results

We assessed the features of single-nucleotide genomic variants verified to cause exon skipping and compared them to a large set of coding SNPs common in the human population, which are likely to have no effect on splicing. Our findings implicate a number of features important for their ability to discriminate splice-affecting variants, including the naturally occurring density of exonic splicing enhancers and exonic splicing silencers of the exon and intronic environment, extensive changes in the number of predicted exonic splicing enhancers and exonic splicing silencers, proximity to the splice junctions and evolutionary constraint of the region surrounding the variant. By extending this approach to additional datasets, we also identified relevant features of variants that cause increased exon inclusion and ectopic splice site activation.

Conclusions

We identified a number of features that have statistically significant representation among exonic variants that modulate splicing. These analyses highlight putative mechanisms responsible for splicing outcome and emphasize the role of features important for exon definition. We developed a web-tool, Skippy, to score coding variants for these relevant splice-modulating features.

Tyrosinemia type 1 and Angelman syndrome due to paternal uniparental isodisomy 15

Uniparental isodisomy arises when an individual inherits two copies of a specific chromosome from a single parent, which can unmask a recessive mutation or cause a problem of genetic imprinting. Here we describe an exceptional case in which the patient simultaneously presents tyrosinemia type 1 and Angelman syndrome. The genetic studies showed that the patient presents paternal uniparental isodisomy of chromosome 15, with absence of the maternal homolog. As a consequence of this isodisomy, the patient is homozygous for the mutation IVS12+5G>A in the FAH gene, located in the chromosomal region 15q23-25, causing tyrosinemia type 1. The mutation was inherited from his father in double dosage, whereas the mother is not a carrier, which implies that the recurrence risk in the family is negligible. On the other hand, the lack of maternal contribution causes Angelman syndrome, a neurodevelopmental disorder associated with a loss of maternal gene expression in chromosome region 15q11-q13, and more specifically, of the UBE3A gene. This gene shows a tissue-specific imprinting, and only the maternally derived allele is expressed in certain areas of the brain. We observed through a literature review that uniparental disomy probably occurs more frequently than suspected, although it is more usually detected when the uniparental disomy implies the appearance of a disease because of the gene imprinting or by reduction to homozygosity of a recessive mutation. The conclusion is that uniparental disomy should always be considered when more than one genetic disease mapping to the same chromosome is present in a patient.

Pre-mRNA Missplicing as a Cause of Human Disease

Regulated alternative splice site selection emerges as one of the most important mechanisms to control the expression of genetic information in humans. It is therefore not surprising that a growing number of diseases are either associated with or caused by changes in alternative splicing. These diseases can be caused by mutation in regulatory sequences of the pre-mRNA or by changes in the concentration of trans-acting factors. The pathological expression of mRNA isoforms can be treated by transferring nucleic acids derivatives into cells that interfere with sequence elements on the pre-mRNA, which results in the desired splice site selection. Recently, a growing number of low molecular weight drugs have been discovered that influence splice site selection in vivo. These findings prove the principle that diseases caused by missplicing events could eventually be cured.

Defects in pre-mRNA processing as causes of and predisposition to diseases

Humans possess a surprisingly low number of genes and intensively use pre-mRNA splicing to achieve the high molecular complexity needed to sustain normal body functions and facilitate responses to altered conditions. Because hundreds of thousands of proteins are generated by 25,000 to 40,000 genes, pre-mRNA processing events are highly important for the regulation of human gene expression. Both inherited and acquired defects in pre-mRNA processing are increasingly recognized as causes of human diseases, and almost all pre-mRNA processing events are controlled by a combination of protein factors. This makes defects in these processes likely candidates for causes of diseases with complicated inheritance patterns that affect seemingly unrelated functions. The elucidation of genetic mechanisms regulating pre-mRNA processing, combined with the development of drugs targeted at consensus RNA sequences and/or corresponding proteins, can lead to novel diagnostic and therapeutic approaches.

Splicing mutations, mainly IVS6-1(G>T), account for 70% of fumarylacetoacetate hydrolase (FAH) gene alterations, including 7 novel mutations, in a survey of 29 tyrosinemia type I patients

Hereditary tyrosinemia type I (HTI) is an autosomal recessive disease characterized by a deficiency in fumarylacetoacetate hydrolase (FAH) activity. In this work, the FAH genotype was established in a group of 29 HTI patients, most of them from the Mediterranean area. We identified seven novel mutations-IVS8-1(G>A, IVS10-2(A>T), 938delC, E6/I6del26, W78X, Q328X, and G343W-and two previously described mutations-IVS6-1(G>T) and IVS12+5(G>A). Fully 92.8% of the patients were carriers of at least one splice site mutation, with IVS6-1(G>T) accounting for 58.9% of the total number of alleles. The splice mutation group of patients showed heterogeneous phenotypic patterns ranging from acute forms with severe liver malfunction to chronic forms with renal manifestations and slow progressive hepatic alterations. Qualitative FAH cDNA expression was the same in all IVS6-1(G>T) homozygous patients regardless of their clinical picture. One patient with a heterozygous combination of a nonsense (Q328X) and a frameshift (938delC) mutation showed an atypical clinical picture of hypotonia and repeated infections. Despite the high prevalence of IVS12+5(G>A) in the northwestern European population, we found only two patients with this mutation in our group.

Listening to silence and understanding nonsense: exonic mutations that affect splicing

Point mutations in the coding regions of genes are commonly assumed to exert their effects by altering single amino acids in the encoded proteins. However, there is increasing evidence that many human disease genes harbour exonic mutations that affect pre-mRNA splicing. Nonsense, missense and even translationally silent mutations can inactivate genes by inducing the splicing machinery to skip the mutant exons. Similarly, coding-region single-nucleotide polymorphisms might cause phenotypic variability by influencing splicing accuracy or efficiency. As the splicing mechanisms that depend on exonic signals are elucidated, new therapeutic approaches to treating certain genetic diseases can begin to be explored.

Alternative splicing: multiple control mechanisms and involvement in human disease

Alternative splicing is an important mechanism for controlling gene expression. It allows large proteomic complexity from a limited number of genes. An interplay of cis-acting sequences and trans-acting factors modulates the splicing of regulated exons. Here, we discuss the roles of the SR and hnRNP families of proteins in this process. We also focus on the role of the transcriptional machinery in the regulation of alternative splicing, and on those alterations of alternative splicing that lead to human disease.

Human genetics: lessons from Quebec populations

The population of Quebec, Canada (7.3 million) contains approximately 6 million French Canadians; they are the descendants of approximately 8500 permanent French settlers who colonized Nouvelle France between 1608 and 1759. Their well-documented settlements, internal migrations, and natural increase over four centuries in relative isolation (geographic, linguistic, etc.) contain important evidence of social transmission of demographic behavior that contributed to effective family size and population structure. This history is reflected in at least 22 Mendelian diseases, occurring at unusually high prevalence in its subpopulations. Immigration of non-French persons during the past 250 years has given the Quebec population further inhomogeneity, which is apparent in allelic diversity at various loci. The histories of Quebec's subpopulations are, to a great extent, the histories of their alleles. Rare pathogenic alleles with high penetrance and associated haplotypes at 10 loci (CFTR, FAH, HBB, HEXA, LDLR, LPL, PAH, PABP2, PDDR, and SACS) are expressed in probands with cystic fibrosis, tyrosinemia, beta-thalassemia, Tay-Sachs, familial hypercholesterolemia, hyperchylomicronemia, PKU, oculopharyngeal muscular dystrophy, pseudo vitamin D deficiency rickets, and spastic ataxia of Charlevoix-Saguenay, respectively) reveal the interpopulation and intrapopulation genetic diversity of Quebec. Inbreeding does not explain the clustering and prevalence of these genetic diseases; genealogical reconstructions buttressed by molecular evidence point to founder effects and genetic drift in multiple instances. Genealogical estimates of historical meioses and analysis of linkage disequilibrium show that sectors of this young population are suitable for linkage disequilibrium mapping of rare alleles. How the population benefits from what is being learned about its structure and how its uniqueness could facilitate construction of a genomic map of linkage disequilibrium are discussed.

A missense mutation (Q279R) in the fumarylacetoacetate hydrolase gene, responsible for hereditary tyrosinemia, acts as a splicing mutation

BACKGROUND

Tyrosinemia type I, the most severe disease of the tyrosine catabolic pathway is caused by a deficiency in fumarylacetoacetate hydrolase (FAH). A patient showing few of the symptoms associated with the disease, was found to be a compound heterozygote for a splice mutation, IVS6-1g->t, and a putative missense mutation, Q279R. Analysis of FAH expression in liver sections obtained after resection for hepatocellular carcinoma revealed a mosaic pattern of expression. No FAH was found in tumor regions while a healthy region contained enzyme-expressing nodules.

RESULTS

Analysis of DNA from a FAH expressing region showed that the expression of the protein was due to correction of the Q279R mutation. RT-PCR was used to assess if Q279R RNA was produced in the liver cells and in fibroblasts from the patient. Normal mRNA was found in the liver region where the mutation had reverted while splicing intermediates were found in non-expressing regions suggesting that the Q279R mutation acted as a splicing mutation in vivo. Sequence of transcripts showed skipping of exon 8 alone or together with exon 9. Using minigenes in transfection assays, the Q279R mutation was shown to induce skipping of exon 9 when placed in a constitutive splicing environment.

CONCLUSION

These data suggest that the putative missense mutation Q279R in the FAH gene acts as a splicing mutation in vivo. Moreover FAH expression can be partially restored in certain liver cells as a result of a reversion of the Q279R mutation and expansion of the corrected cells.

Structural and functional analysis of missense mutations in fumarylacetoacetate hydrolase, the gene deficient in hereditary tyrosinemia type 1

Hereditary tyrosinemia type 1 (HT1) is an autosomal recessive disease caused by a deficiency of the enzyme involved in the last step of tyrosine degradation, fumarylacetoacetate hydrolase (FAH). Thus far, 34 mutations in the FAH gene have been reported in various HT1 patients. Site-directed mutagenesis of the FAH cDNA was used to investigate the effects of eight missense mutations found in HTI patients on the structure and activity of FAH. Mutated FAH proteins were expressed in Escherichia coli and in mammalian CV-1 cells. Mutations N16I, F62C, A134D, C193R, D233V, and W234G lead to enzymatically inactive FAH proteins. Two mutations (R341W, associated with the pseudo-deficiency phenotype, and Q279R) produced proteins with a level of activity comparable to the wild-type enzyme. The N16I, F62C, C193R, and W234G variants were enriched in an insoluble cellular fraction, suggesting that these amino acid substitutions interfere with the proper folding of the enzyme. Based on the tertiary structure of FAH, on circular dichroism data, and on solubility measurements, we propose that the studied missense mutations cause three types of structural effects on the enzyme: 1) gross structural perturbations, 2) limited conformational changes in the active site, and 3) conformational modifications with no significant effect on enzymatic activity.

Mutations in the complex I NDUFS2 gene of patients with cardiomyopathy and encephalomyopathy

Human complex I is built up and regulated by genes encoded by the mitochondrial DNA (mtDNA) as well as the nuclear DNA (nDNA). In recent years, attention mainly focused on the relation between complex I deficiency and mtDNA mutations. However, a high percentage of consanguinity and an autosomal-recessive mode of inheritance observed within our patient group as well as the absence of common mtDNA mutations make a nuclear genetic cause likely. The NDUFS2 protein is part of complex I of many pro- and eukaryotes. The nuclear gene coding for this protein is therefore an important candidate for mutational detection studies in enzymatic complex I deficient patients. Screening of patient NDUFS2 cDNA by reverse transcriptase-polymerase chain reaction (RT-PCR) in combination with direct DNA sequencing revealed three missense mutations resulting in the substitution of conserved amino acids in three families.

Point mutations in the murine fumarylacetoacetate hydrolase gene: Animal models for the human genetic disorder hereditary tyrosinemia type 1

Hereditary tyrosinemia type 1 (HT1) is a severe autosomal recessive metabolic disease associated with point mutations in the human fumarylacetoacetate hydrolase (FAH) gene that disrupt tyrosine catabolism. An acute form of HT1 results in death during the first months of life because of hepatic failure, whereas a chronic form leads to gradual development of liver disease often accompanied by renal dysfunction, childhood rickets, neurological crisis, and hepatocellular carcinoma. Mice homozygous for certain chromosome 7 deletions of the albino Tyr; c locus that also include Fah die perinatally as a result of liver dysfunction and exhibit a complex syndrome characterized by structural abnormalities and alterations in gene expression in the liver and kidney. Here we report that two independent, postnatally lethal mutations induced by N-ethyl-N-nitrosourea and mapped near Tyr are alleles of Fah. The Fah(6287SB) allele is a missense mutation in exon 6, and Fah(5961SB) is a splice mutation causing loss of exon 7, a subsequent frameshift in the resulting mRNA, and a severe reduction of Fah mRNA levels. Increased levels of the diagnostic metabolite succinylacetone in the urine of the Fah(6287SB) and Fah(5961SB) mutants indicate that these mutations cause a decrease in Fah enzymatic activity. Thus, the neonatal phenotype present in both mutants is due to a deficiency in Fah caused by a point mutation, and we propose Fah(5961SB) and Fah(6287SB) as mouse models for acute and chronic forms of human HT1, respectively.

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.

Spectrum of mutations in the fumarylacetoacetate hydrolase gene of tyrosinemia type 1 patients in northwestern Europe and Mediterranean countries

Hereditary tyrosinemia type 1 (HT1) is a rare metabolic disease caused by a deficient activity of the enzyme fumarylacetoacetase (FAH). To investigate the molecular heterogeneity of tyrosinemia, the geographic distribution and the genotype-phenotype relationship, we have analyzed the FAH genotype of 25 HT1 patients. Mutation screening was performed by PCR amplification of exons 1-14 of the FAH gene, followed by SSCP analysis and direct sequencing of the amplified exons. Fourteen different mutations were found, of which seven were novel, viz. three missense mutations (G158D, P261L, F405H), a deletion of three nucleotides causing a deletion of serine (DEL366S) and three splice site mutations: IVS2+1(g-t), IVS6-1(g-c), IVS8-1(g-c). The splice site mutations IVS6-1(g-t) and IVS12+5(g-a) were frequently found in countries around the Mediterranean and northwestern Europe, respectively. No clear correlation between the genotype and the three major HT1 subtypes could be established.

Mutations in the fumarylacetoacetate hydrolase gene causing hereditary tyrosinemia type I: overview

Tyrosinemia type I is an inborn error of metabolism caused by a deficiency in the last enzyme of the tyrosine catabolic pathway, fumarylacetoacetate hydrolase (FAH). The disease has been reported worldwide with varying incidence. Recently, there has been considerable progress in identifying mutations in the FAH gene. At present 26 mutations have been reported, all consisting of single base substitutions resulting in 16 amino acid replacements, one silent mutation causing a splicing defect, five nonsense codons, and four putative splicing defects. The location of these mutations is spread over the entire FAH gene, with a particular clustering between amino acid residues 230 and 250. The identification of these mutations in subpopulations and groups at high risk should help in the diagnosis of, and genetic counseling for, HT1. We describe all these 26 mutations reported so far and their implication in diagnosis and carrier detection.

Clinical and molecular genetic characteristics of patients with cerebrotendinous xanthomatosis

Cerebrotendinous xanthomatosis (CTX) is a lipid storage disease caused by a deficiency of the mitochondrial enzyme 27-sterol hydroxylase (CYP 27), due to mutations in its gene. In this study we report on mutations in 58 patients with CTX out of 32 unrelated families. Eight of these were novel mutations, two of which were found together with two already known pathogenic mutations. Twelve mutations found in this patient group have been described in the literature. In the patients from 31 families, mutations were found in both alleles. In the literature, 28 mutations in 67 patients with CTX out of 44 families have been described. Pooling our patient group and the patients from the literature together, 37 different mutations in 125 patients out of 74 families were obtained. Identical mutations have been found in families from different ethnic backgrounds. In 41% of all the patients, CYP 27 gene mutations are found in the region of exons 6-8. This region encodes for adrenodoxin and haem binding sites of the protein. Of these 125 patients, a genotype-phenotype analysis was done for 79 homozygous patients harbouring 23 different mutations, out of 45 families. The patients with compound heterozygous mutations were left out of the genotype-phenotype analysis. The genotype-phenotype analysis did not reveal any correlation.

Persistent postnatal transgene expression in both muscle and liver after fetal injection of recombinant adenovirus

BACKGROUND/PURPOSE

Immune responses to both vector and transgene antigens have limited the efficacy of postnatal gene therapy. We hypothesize that the fetal period may offer immunologic and developmental advantages for successful gene therapy. In this study we examined the efficacy, persistence, and immunologic effects of recombinant adenovirus after intramuscular delivery into fetal mice.

METHODS

E1-deleted adenovirus (AdCMVlacZ) containing the beta-galactosidase marker gene was used for injection. Fetal Balb/c mice (14 to 15 days' gestation) were injected with AdCMVlacZ in 10-microL volume in either the shoulder or hindlimb musculature. Animals were killed at 18 to 20 days' gestation and up to 4 months postnatally for analysis of transgene expression and adenoviral genome persistence.

RESULTS

Fetuses were injected with doses of AdCMVlacZ from 1 x 10(8) to 2 x 10(10) viral particles (n = 80). Optimal survival rate was 83% at 18 to 20 days' gestation and 55% at 4 weeks of age using a dose of 1 x 10(9) particles. Expression of beta-galactosidasae at 18 to 20 days localized to multiple muscle groups surrounding the site of injection, as well as bone marrow stroma, liver, lung, and dorsal root ganglia. Persistent muscle and liver transgene expression was observed for as long as 16 and 8 weeks, respectively, after injection. The pattern of liver expression was confined to discrete foci of hepatocytes, which appeared to increase in size in older animals. No histological evidence of muscle or liver inflammation was observed at any time after injection. No neutralizing antibodies were observed postnatally.

CONCLUSIONS

Our results confirm that gene therapy in the fetus may be advantageous. Distribution of vector in the fetus at the site of injection is clearly broader than in the adult setting. Furthermore, the absence of immune response and persistence of transgene expression suggests that fetal exposure to foreign transgene and vector antigens may induce tolerance. Although we have not proven genomic integration, the histological appearance of transgene expression in the liver supports this conclusion. By understanding the mechanisms that underlie persistent transgene expression, fetal gene therapy may become a feasible strategy for the treatment of fatal genetic diseases.

The human nuclear-encoded acyl carrier subunit (NDUFAB1) of the mitochondrial complex I in human pathology

We present the cDNA sequence of the human mitochondrial acyl carrier protein NDUFAB1, a nuclear-encoded subunit of complex I of the mitochondrial respiratory chain. We obtained the NDUFAB1 cDNA using the cDNA sequence of the bovine mitochondrial acyl carrier protein. The human cDNA contains two putative translation initiation codons. The human NDUFAB1 protein contains a phosphopantetheine attachment site (DLGLDSLDQVEIIMAM), unique for acyl carrier proteins, and an EF-hand calcium binding domain (DIDAEKLMCPQEI). Transcripts of this gene are found in a wide range of human tissues. The highests expression levels were observed, in descending order, in adult heart, skeletal muscle and fetal heart. We subjected NDUFAB1 fibroblast cDNA of 20 patients with an isolated enzymatic complex I deficiency to mutational detection. No mutations in the NDUFAB1 open reading frame were observed. Future studies will answer whether mutations in the NDUFAB1 promoter or transcription elements are responsible for the observed complex I deficiency.

The human NADH: ubiquinone oxidoreductase NDUFS5 (15 kDa) subunit: cDNA cloning, chromosomal localization, tissue distribution and the absence of mutations in isolated complex I-deficient patients

We have cloned the cDNA of the NDUFS5 subunit (15 kDa) of the human mitochondrial respiratory chain complex NADH: ubiquinone oxidoreductase (complex I). The open reading frame consists of 321 base-pairs, coding for 106 amino acids, with a calculated molecular mass of 12.5 kDa. There is an 81.0% identity with the bovine equivalent on cDNA level and 74.5% identity on amino acid basis. PCR analysis of rodent-human somatic cell hybrids revealed that the human NDUFS5 gene maps to chromosome 1. The NDUFS5 mRNA is expressed ubiquitously in human tissues, with a relative higher expression in human heart, skeletal muscle, liver, kidney and fetal heart. A mutation detection study of twenty isolated enzymatic complex I-deficient patients revealed no mutations, nor polymorphisms.

Genotyping of a case of tyrosinaemia type I with normal level of succinylacetone in amniotic fluid

Tyrosinaemia type I is caused by a deficiency of fumarylacetoacetate hydrolase and mainly affects the liver. This disease is characterized by the presence of a high level of succinylacetone. This metabolite has been used for prenatal diagnosis from amniotic fluid samples. One case with a normal level of succinylacetone in amniotic fluid has recently been described (Grenier et al., 1996). Here, we report that this patient is a compound heterozygote for two known mutations: E364X and IVS6-1g-->t. The low level of succinylacetone cannot be explained by these mutations.

cDNA of eight nuclear encoded subunits of NADH:ubiquinone oxidoreductase: human complex I cDNA characterization completed

NADH:ubiquinone oxidoreductase (complex I) is an extremely complicated multiprotein complex located in the inner mitochondrial membrane. Its main function is the transport of electrons from NADH to ubiquinone, which is accompanied by translocation of protons from the mitochondrial matrix to the intermembrane space. Human complex I appears to consist of 41 subunits of which 34 are encoded by nDNA. Here we report the cDNA sequences of the hitherto uncharacterized 8 nuclear encoded subunits, all located within the hydrophobic protein (HP) fraction of complex I. Now all currently known 41 proteins of human NADH:ubiquinone oxidoreductase have been characterized and reported in literature, which enables more complete mutational analysis studies of isolated complex I-deficient patients.

The first nuclear-encoded complex I mutation in a patient with Leigh syndrome

Nicotinamide adenine dinucleotide (NADH):ubiquinone oxidoreductase (complex I) is the largest multiprotein enzyme complex of the respiratory chain. The nuclear-encoded NDUFS8 (TYKY) subunit of complex I is highly conserved among eukaryotes and prokaryotes and contains two 4Fe4S ferredoxin consensus patterns, which have long been thought to provide the binding site for the iron-sulfur cluster N-2. The NDUFS8 cDNA contains an open reading frame of 633 bp, coding for 210 amino acids. Cycle sequencing of amplified NDUFS8 cDNA of 20 patients with isolated enzymatic complex I deficiency revealed two compound heterozygous transitions in a patient with neuropathologically proven Leigh syndrome. The first mutation was a C236T (P79L), and the second mutation was a G305A (R102H). Both mutations were absent in 70 control alleles and cosegregated within the family. A progressive clinical phenotype proceeding to death in the first months of life was expressed in the patient. In the 19 other patients with enzymatic complex I deficiency, no mutations were found in the NDUFS8 cDNA. This article describes the first molecular genetic link between a nuclear-encoded subunit of complex I and Leigh syndrome.

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.

cDNA sequence and chromosomal localization of the remaining three human nuclear encoded iron sulphur protein (IP) subunits of complex I: the human IP fraction is completed

NADH:ubiquinone oxidoreductase (complex I) of the mitochondrial respiratory chain can be fragmented in a flavoprotein (FP), iron-sulfur protein (IP), and hydrophobic protein (HP) subfraction. The IP subfraction is hypothesized to be significant, since it contains important prosthetic groups highly conserved among species. We cloned the cDNA of three remaining human NADH:ubiquinone oxidoreductase subunits of this IP fraction: the NDUFS2 (49 kDa), NDUFS3 (30 kDa), and NDUFS6 (13 kDa) subunits. All presented cDNAs include the complete open reading frame (ORF), which consist of 1392, 795, and 375 base pairs, coding for 463, 264, and 124 amino acids, respectively. The latter show 96, 90, and 83% homology with the corresponding bovine translation products. The 3' untranslated regions (UTR) are complete in all three cDNAs. Polymerase chain reaction performed with DNA isolated from somatic human-rodent cell hybrids containing defined human chromosomes as template gave a human-specific signal which mapped the NDUFS2 and NDUFS3 subunits to chromosomes 1 and 11, respectively. In the case of the NDUFS6 subunit a pseudogene may be present since signals were seen in the lanes containing chromosomes 5 and 6. The NDUFS2 contains a highly conserved protein kinase C phosphorylation site and the NDUFS3 subunit contains a highly conserved casein kinase II phosphorylation site which make them strong candidates for future mutation detection studies in enzymatic complex I-deficient patients.