Development of transgenic mice containing an introduced stop codon on the human methylmalonyl-CoA mutase locus.

The mutation R403stop was found in an individual with mut(0) methylmalonic aciduria (MMA) which resulted from a single base change of C→T in exon 6 of the methylmalonyl-CoA mutase gene (producing a TGA stop codon). In order to accurately model the human MMA disorder we introduced this mutation onto the human methylmalonyl-CoA mutase locus of a bacterial artificial chromosome. A mouse model was developed using this construct.The transgene was found to be intact in the mouse model, with 7 copies integrated at a single site in chromosome 3. The phenotype of the hemizygous mouse was unchanged until crossed against a methylmalonyl-CoA mutase knockout mouse. Pups with no endogenous mouse methylmalonyl-CoA mutase and one copy of the transgene became ill and died within 24 hours. This severe phenotype could be partially rescued by the addition of a transgene carrying two copies of the normal human methylmalonyl-CoA mutase locus. The "humanized" mice were smaller than control litter mates and had high levels of methylmalonic acid in their blood and tissues. This new transgenic MMA stop codon model mimics (at both the phenotypic and genotypic levels) the key features of the human MMA disorder. It will allow the trialing of pharmacological and, cell and gene therapies for the treatment of MMA and other human metabolic disorders caused by stop codon mutations.

Treatment of a methylmalonyl-CoA mutase stopcodon mutation.

There are limited treatment options for the metabolic disorder methylmalonic aciduria. The disorder can be caused by nonsense mutations within the methylmalonyl-CoA mutase gene, resulting in the production of a truncated protein with little or no catalytic activity. We used a genomic reporter assay and mouse primary cell lines which carry a stop-codon mutation in the human methylmalonyl-CoA mutase gene to test the effects of gentamicin and PTC124 for stop-codon read-through potential. Fibroblast cell lines were established from methylmalonic aciduria knockout-stop codon mice. Addition of gentamicin to the culture medium caused a 1.5- to 2-fold increase in mRNA expression of the human methylmalonyl-CoA mutase gene. Without treatment the cells contained 19% of the normal levels of methylmalonyl-CoA mutase enzyme activity which increased to 32% with treatment, suggesting a functional improvement. Treatment with PTC124 increased the amount of human methylmalonyl-CoA mutase gene mRNA by 1.6±0.3-fold and a trend suggesting increased enzyme activity. The genomic reporter assay, BAC_MMA(∗)EGFP, expresses enhanced green fluorescent protein when read-through of the stop codon occurs. Using flow cytometry, RT-real-time PCR and enzyme assay, read-through was measured. Treatment with PTC124 at 20µmol/L resulted in a significant increase in enhanced green fluorescent protein, a 2-fold increase in mRNA expression and a trend to a slight increase in enzyme activity. The clinical relevance of these effects may be tested in mouse models of MMA carrying nonsense mutations in the methylmalonyl-CoA mutase gene. Pharmacological approaches have the advantage of providing a broader effect on multiple tissues, which will benefit many different disorders with similar nonsense mutations.

Fetal progenitor cell transplantation treats methylmalonic aciduria in a mouse model.

Methylmalonic aciduria is a rare disorder caused by an inborn error of organic acid metabolism. Current treatment options are limited and generally focus on disease management. We aimed to investigate the use of fetal progenitor cells to treat this disorder using a mouse model with an intermediate form of methylmalonic aciduria. Fetal liver cells were isolated from healthy fetuses at embryonic day 15-17 and intravenously transplanted into sub-lethally irradiated mice. Liver donor cell engraftment was determined by PCR. Disease correction was monitored by urine and blood methylmalonic acid concentration and weight change. Initial studies indicated that pre-transplantation sub-lethal irradiation followed by transplantation with 5 million cells were suitable. We found that a double dose of 5 million cells (1 week apart) provided a more effective treatment. Donor cell liver engraftment of up to 5% was measured. Disease correction, as defined by a decrease in blood methylmalonic acid concentration, was effected in methylmalonic acid mice transplanted with a double dose of cells and who showed donor cell liver engraftment. Mean plasma methylmalonic acid concentration decreased from 810 ± 156 (sham transplanted) to 338 ± 157 µmol/L (double dose of 5 million cells) while mean blood C3 carnitine concentration decreased from 20.5 ± 4 (sham transplanted) to 5.3 ± 1.9 µmol/L (double dose of 5 million cells). In conclusion, higher levels of engraftment may be required for greater disease correction; however these studies show promising results for cell transplantation biochemical correction of a metabolic disorder.

Mouse models for methylmalonic aciduria.

Methylmalonic aciduria (MMA) is a disorder of organic acid metabolism resulting from a functional defect of methylmalonyl-CoA mutase (MCM). MMA is associated with significant morbidity and mortality, thus therapies are necessary to help improve quality of life and prevent renal and neurological complications. Transgenic mice carrying an intact human MCM locus have been produced. Four separate transgenic lines were established and characterised as carrying two, four, five or six copies of the transgene in a single integration site. Transgenic mice from the 2-copy line were crossed with heterozygous knockout MCM mice to generate mice hemizygous for the human transgene on a homozygous knockout background. Partial rescue of the uniform neonatal lethality seen in homozygous knockout mice was observed. These rescued mice were significantly smaller than control littermates (mice with mouse MCM gene). Biochemically, these partial rescue mice exhibited elevated methylmalonic acid levels in urine, plasma, kidney, liver and brain tissue. Acylcarnitine analysis of blood spots revealed elevated propionylcarnitine levels. Analysis of mRNA expression confirms the human transgene is expressed at higher levels than observed for the wild type, with highest expression in the kidney followed closely by brain and liver. Partial rescue mouse fibroblast cultures had only 20% of the wild type MCM enzyme activity. It is anticipated that this humanised partial rescue mouse model of MMA will enable evaluation of long-term pathophysiological effects of elevated methylmalonic acid levels and be a valuable model for the investigation of therapeutic strategies, such as cell transplantation.

Stop codon read-through of a methylmalonic aciduria mutation.

A stop codon defect in methylmalonyl-CoA mutase (resulting in a truncated unstable protein) accounts for up to 14% of mutations identified as causes of Methylmalonic aciduria. There are currently limited treatment regimes for patients with this inherited condition. We aimed to investigate the use of stop codon read-through drugs in a genomic reporter assay cell line with a defect in the mutase gene. A single C-T base change was introduced into exon 6 of the human MUT sequence in the BAC clone RP11-463L20 resulting in an arginine residue being replaced with a TGA stop codon. An enhanced green fluorescent protein reporter gene was introduced in-frame with exon 13 of the MUT gene. The construct was transfected into HeLa cells to produce the genomic reporter assay cell line. To test the suppression of nonsense mutations, cells were incubated in the presence of different compounds for a period of 72 h then analysed by flow cytometry. Treatment of the cells with gentamicin resulted in a 1.6-fold increase in reporter protein, whilst G418 treatment resulted in no change, however the two drugs together acted synergistically to increase the production of methylmalonyl-CoA mutase 2.0-fold (confirmed by mRNA, flow cytometry and enzyme activity). Zidovudine, adefovir and cisplatin were also found to have some activity in the stop codon read-through genomic reporter assay. These results encourage further testing of compounds as well as follow up animal studies. This is the first study to demonstrate the use of stop codon read-through drugs for the potential treatment of Methylmalonic aciduria.

Gene induction for the treatment of methylmalonic aciduria.

BACKGROUND: Methylmalonic aciduria is an autosomal recessive inborn error of the propionate metabolic pathway. One form of this disorder is caused by mutations in methylmalonyl-coenzyme A mutase (MCM), resulting in reduced levels of enzyme activity. The pharmacological up-regulation of residual mutase activity is one approach to advance treatment strategies for individuals affected by this disorder. We describe the construction, characterization and use of a cellular genomic reporter assay for MCM expression that will potentially identify therapeutic pharmacological agents for methylmalonic aciduria treatment. METHODS: Homologous recombination was used to insert an enhanced green fluorescent protein (EGFP) cassette inframe before the last codon of exon 13 of the MCM gene (MUT) in a BAC clone. The construct was used to generate stable HeLa cell lines. EGFP expression was measured by flow cytometry and the real-time reverse transcriptase-polymerase chain reaction was used to quantify changes in MUT gene mRNA levels. RESULTS: The genomic reporter assay used to screen a selection of compounds. Cisplatin, zidovudine and adefovir were found to increase the levels of MCM mRNA and EGFP expression, providing support for the possible efficacy of these pharmacological compounds in treating methylmalonic aciduria. CONCLUSIONS: This assay has the potential of being used in high-throughput screening of chemical libraries for the identification of novel compounds that specifically modulate the expression of MCM.

Correction of copper metabolism is not sustained long term in Wilson's disease mice post bone marrow transplantation.

PURPOSE: Alternative cell sources have been sought for the treatment of liver diseases, since liver cells are in short supply for cell transplantation. Although bone marrow-derived cells have been investigated as an alternative cell source, few studies have demonstrated long-term disease correction. Here we examined bone marrow stem cell transplantation into the toxic milk (tx) mouse model for Wilson's disease, a mild liver disease characterized by hepatic copper accumulation. The aim of this study was to determine whether bone marrow cells engrafted in the liver could sustain correction of abnormal copper metabolism in the tx mouse. METHODS: Bone marrow cells were isolated from congenic normal mice, transduced to express enhanced green fluorescent protein, sorted for stem cell (Sca-1) and lineage cell (Lin) surface markers, and then transplanted into sub-lethally irradiated normal or tx mice. Analysis for donor cell activity and distribution was undertaken 5 and 9 months post-transplant to allow for sufficient time to repopulate the liver and demonstrate disease correction. RESULTS: Donor bone marrow cells engrafted in both normal and tx mouse liver within 5 months. Significantly reduced liver copper was found in tx mice with donor cell liver engraftment at 5 months post-transplant compared to controls, demonstrating partial correction of abnormal copper metabolism in the short term. However, disease correction was not maintained 9 months post-transplantation. Lin(-)Sca-1(+) cells were more likely to partially correct disease than Lin(+)Sca-1(-) cells in the short term. CONCLUSION: These results demonstrate that bone marrow transplants cannot maintain disease correction in a mouse model of mild hepatic damage, although initial copper metabolism correction was observed.

Heterozygous tx mice have an increased sensitivity to copper loading: implications for Wilson's disease carriers.

Wilson's disease carriers constitute 1% of the human population. It is unknown whether Wilson's disease carriers are at increased susceptibility to copper overload when exposed to chronically high levels of ingested copper. This study investigated the effect of chronic excess copper in drinking water on the heterozygous form of the Wilson's disease mouse model--the toxic milk (tx) mouse. Mice were provided with drinking water containing 300 mg/l copper for 4-7, 8-11, 12-15 or 16-20 months. At the completion of the study liver, spleen, kidney and brain tissue were analyzed by atomic absorption spectroscopy to determine copper concentration. Plasma ceruloplasmin oxidase activity and liver histology were also assessed. Chronic copper loading resulted in significantly increased liver copper in both tx heterozygous and tx homozygous mice, while wild type mice were resistant to the effects of copper loading. Copper loading effects were greatest in tx homozygous mice, with increased extrahepatic copper deposition in spleen and kidney - an effect absent in heterozygote and wild type mice. Although liver histology in homozygous mice was markedly abnormal, no histological differences were noted between heterozygous and wild type mice with copper loading. Tx heterozygous mice have a reduced ability to excrete excess copper, indicating that half of the normal liver Atp7b copper transporter activity is insufficient to deal with large copper intakes. Our results suggest that Wilson's disease carriers in the human population may be at increased risk of copper loading if chronically exposed to elevated copper in food or drinking water.

Chronological changes in tissue copper, zinc and iron in the toxic milk mouse and effects of copper loading.

The toxic milk (tx) mouse is a rodent model for Wilson disease, an inherited disorder of copper overload. Here we assessed the effect of copper accumulation in the tx mouse on zinc and iron metabolism. Copper, zinc and iron concentrations were determined in the liver, kidney, spleen and brain of control and copper-loaded animals by atomic absorption spectroscopy. Copper concentration increased dramatically in the liver, and was also significantly higher in the spleen, kidney and brain of control tx mice in the first few months of life compared with normal DL mice. Hepatic zinc was increased with age in the tx mouse, but zinc concentrations in the other organs were normal. Liver and kidney iron concentrations were significantly lower at birth in tx mice, but increased quickly to be comparable with control mice by 2 months of age. Iron concentration in the spleen was significantly higher in tx mice, but was lower in 5 day old tx pups. Copper-loading studies showed that normal DL mice ingesting 300 mg/l copper in their diet for 3 months maintained normal liver, kidney and brain copper, zinc and iron levels. Copper-loading of tx mice did not increase the already high liver copper concentrations, but spleen and brain copper concentrations were increased. Despite a significant elevation of copper in the brain of the copper-loaded tx mice no behavioural changes were observed. The livers of copper-loaded tx mice had a lower zinc concentration than control tx mice, whilst the kidney had double the concentration of iron suggesting that there was increased erythrocyte hemolysis in the copper-loaded mutants.

Liver cell transplantation leads to repopulation and functional correction in a mouse model of Wilson's disease.

BACKGROUND AND AIM: The toxic milk (tx) mouse is a non-fatal animal model for the metabolic liver disorder, Wilson's disease. The tx mouse has a mutated gene for a copper-transporting protein, causing early copper accumulation in the liver and late accumulation in other tissues. The present study investigated the efficacy of liver cell transplantation (LCT) to correct the tx mouse phenotype. METHODS: Congenic hepatocytes were isolated and intrasplenically transplanted into 3-4-month-old tx mice, which were then placed on various copper-loaded diets to examine its influence on repopulation by transplanted cells. The control animals were age-matched untransplanted tx mice. Liver repopulation was determined by comparisons of restriction fragment length polymorphism ratios (DNA and mRNA), and copper levels were measured by atomic absorption spectroscopy. RESULTS: Repopulation in recipient tx mice was detected in 11 of 25 animals (44%) at 4 months after LCT. Dietary copper loading (whether given before or after LCT, or both) provided no growth advantage for donor cells, with similar repopulation incidences in all copper treatment groups. Overall, liver copper levels were significantly lower in repopulated animals (538 +/- 68 microg/g, n = 11) compared to non-repopulated animals (866 +/- 62 microg/g, n = 14) and untreated controls (910 +/- 103 microg/g, n = 6; P < 0.05). This effect was also seen in the kidney and spleen. Brain copper levels remained unchanged. CONCLUSION: Transplanted liver cells can proliferate and correct a non-fatal metabolic liver disease, with some restoration of hepatic copper homeostasis after 4 months leading to reduced copper levels in the liver and extrahepatic tissues, but not in the brain.

The potential of bone marrow stem cells to correct liver dysfunction in a mouse model of Wilson's disease.

Metabolic liver diseases are excellent targets for correction using novel stem cell, hepatocyte, and gene therapies. In this study, the use of bone marrow stem cell transplantation to correct liver disease in the toxic milk (tx) mouse, a murine model for Wilson's disease, was evaluated. Preconditioning with sublethal irradiation, dietary copper loading, and the influence of cell transplantation sites were assessed. Recipient tx mice were sublethally irradiated (4 Gy) prior to transplantation with bone marrow stem cells harvested from normal congenic (DL) littermates. Of 46 transplanted tx mice, 11 demonstrated genotypic repopulation in the liver. Sublethal irradiation was found to be essential for donor cell engraftment and liver repopulation. Dietary copper loading did not improve cell engraftment and repopulation results. Both intravenously and intrasplenically transplanted cells produced similar repopulation successes. Direct evidence of functionality and disease correction following liver repopulation was observed in the 11 mice where liver copper levels were significantly reduced when compared with mice with no liver repopulation. The reversal of copper loading with bone marrow cells is similar to the level of correction seen when normal congenic liver cells are used. Transplantation of bone marrow cells partially corrects the metabolic phenotype in a mouse model for Wilson's disease.