Characterization and review of MTHFD1 deficiency: four new patients, cellular delineation and response to folic and folinic acid treatment.

In the folate cycle MTHFD1, encoded by MTHFD1, is a trifunctional enzyme containing 5,10-methylenetetrahydrofolate dehydrogenase, 5,10-methenyltetrahydrofolate cyclohydrolase and 10-formyltetrahydrofolate synthetase activity. To date, only one patient with MTHFD1 deficiency, presenting with hyperhomocysteinemia, megaloblastic anaemia, hemolytic uremic syndrome (HUS) and severe combined immunodeficiency, has been identified (Watkins et al J Med Genet 48:590-2, 2011). We now describe four additional patients from two different families. The second patient presented with hyperhomocysteinemia, megaloblastic anaemia, HUS, microangiopathy and retinopathy; all except the retinopathy resolved after treatment with hydroxocobalamin, betaine and folinic acid. The third patient developed megaloblastic anaemia, infection, autoimmune disease and moderate liver fibrosis but not hyperhomocysteinemia, and was successfully treated with a regime that included and was eventually reduced to folic acid. The other two, elder siblings of the third patient, died at 9 weeks of age with megaloblastic anaemia, infection and severe acidosis and had MTFHD1 deficiency diagnosed retrospectively. We identified a missense mutation (c.806C > T, p.Thr296Ile) and a splice site mutation (c.1674G > A) leading to exon skipping in the second patient, while the other three harboured a missense mutation (c.146C > T, p.Ser49Phe) and a premature stop mutation (c.673G > T, p.Glu225*), all of which were novel. Patient fibroblast studies revealed severely reduced methionine formation from [(14)C]-formate, which did not increase in cobalamin supplemented culture medium but was responsive to folic and folinic acid. These additional cases increase the clinical spectrum of this intriguing defect, provide in vitro evidence of disturbed methionine synthesis and substantiate the effectiveness of folic or folinic acid treatment.

Mitochondrial myopathy with exercise intolerance and retinal dystrophy in a sporadic patient with a G583A mutation in the mt tRNA(phe) gene.

We describe a second patient with the 583G>A mutation in the tRNA(phe) gene of mitochondrial DNA (mtDNA). This 17-year-old girl had a mitochondrial myopathy with exercise intolerance and an asymptomatic retinopathy. Muscle investigations showed occasional ragged red fibers, 30% cytochrome c oxidase (COX)-negative fibers, and reduced activities of complex I+IV in the respiratory chain. The mutation was heteroplasmic (79%) in muscle but undetectable in other tissues. Analysis of single muscle fibers revealed a significantly higher level of mutated mtDNA in COX-negative fibers. Our study indicates that the 583G>A mutation is pathogenic and expands the clinical spectrum of this mutation.

Deficiency of mitochondrial ATP synthase of nuclear genetic origin.

We present clinical and laboratory data from 14 cases with an isolated deficiency of the mitochondrial ATP synthase (7-30% of control) caused by nuclear genetic defects. A quantitative decrease of the ATP synthase complex was documented by Blue-Native electrophoresis and Western blotting and was supported by the diminished activity of oligomycin/aurovertin-sensitive ATP hydrolysis in fibroblasts (10 cases), muscle (6 of 7 cases), and liver (one case). All patients had neonatal onset and elevated plasma lactate levels. In 12 patients investigated 3-methyl-glutaconic aciduria was detected. Seven patients died, mostly within the first weeks of life and surviving patients showed psychomotor and various degrees of mental retardation. Eleven patients had hypertrophic cardiomyopathy; other clinical signs included hypotonia, hepatomegaly, facial dysmorphism and microcephaly. This phenotype markedly differs from the severe central nervous system changes of ATP synthase disorders caused by mitochondrial DNA mutations of the ATP6 gene presenting mostly as NARP and MILS.

Simultaneous A8344G heteroplasmy and mitochondrial DNA copy number quantification in myoclonus epilepsy and ragged-red fibers (MERRF) syndrome by a multiplex molecular beacon based real-time fluorescence PCR.

The association of a particular mitochondrial DNA (mtDNA) mutation with different clinical phenotypes is a well-known feature of mitochondrial diseases. A simple genotype-phenotype correlation has not been found between mutation load and disease expression. Tissue and intercellular mosaicism as well as mtDNA copy number are thought to be responsible for the different clinical phenotypes. As disease expression of mitochondrial tRNA mutations is mostly in postmitotic tissues, studies to elucidate disease mechanisms need to be performed on patient material. Heteroplasmy quantitation and copy number estimation using small patient biopsy samples has not been reported before, mainly due to technical restrictions. In order to resolve this problem, we have developed a robust assay that utilizes Molecular Beacons to accurately quantify heteroplasmy levels and determine mtDNA copy number in small samples carrying the A8344G tRNA(Lys) mutation. It provides the methodological basis to investigate the role of heteroplasmy and mtDNA copy number in determining the clinical phenotypes.

Determination of intra- and intermolecular tritium isotope effects in the reaction of thymine 7-hydroxylase.

Different [7-3H]thymine preparations have been used to determine the inter- and intramolecular isotope effects of the 2-oxoglutarate-dependent thymine hydroxylation, catalyzed by thymine 7-hydroxylase (thymine, 2-oxoglutarate:oxygen oxidoreductase, EC 1.14.11.6). Specific activity ratios of products, viz., 3H2O and 5-hydroxymethyluracil, and remaining substrate to initial substrate have been determined. The influence on these ratios of intra- and intermolecular isotope effects at different degrees of tritium substitution has been analyzed. An intramolecular isotope effect with a kH/kT of about 6.5 has been found. No intermolecular isotope effect of TV/K could be detected when oxygen concentration was varied from 0.4 to 0.01 mM. This agrees with a mechanism in which 2-oxoglutarate is irreversibly changed before the bond-breaking in thymine takes place.

Studies on the partial reaction of thymine 7-hydroxylase in the presence of 5-fluorouracil.

The uncoupling of 2-oxoglutarate decarboxylation from hydroxylation in the reaction catalyzed by thymine 7-hydroxylase (thymine, 2-oxoglutarate:oxygen oxidoreductase (7-hydroxylating), EC 1.14.11.6) in the presence of 5-fluorouracil has been studied. In the complete reaction no external reductant is formally needed. The uncoupled reaction is almost negligible in the absence of ascorbate and the optimal ascorbate concentration is 5-times higher than in the presence of a hydroxylatable substrate. This indicates that ascorbate acts as the external reductant that is formally needed in the catalytic cycle. The complete reaction follows the steady-state kinetics of an ordered ter reactant mechanism where 2-oxoglutarate and thymine have to be bound to the enzyme before oxygen (E. Holme (1975) Biochemistry 14, 4999-5003). The uncoupled reaction follows the same kinetic pattern as the complete reaction, and in accordance with this no decarboxylation of 2-oxoglutarate occurs in the absence of a substrate analogue even at elevated oxygen tension. There is a good agreement between Kia values for 2-oxoglutarate of the two reactions, but there is at least a 6-fold increase in KO2 where a minimum value of 25% O2 in the gas phase was found for the partial reaction. The high KO2 found means that the reaction rate could increase considerably at elevated oxygen tension.

Multiple short direct repeats associated with single mtDNA deletions.

We have sequenced the breakpoints of deleted mtDNA in muscle from four children with mitochondrial myopathy and multisystem mitochondrial disorders. The deletions were 4884, 6067, 7663 and 7150 base pairs (bp) in size and affected several protein and transfer RNA genes. The sequences needed for transcription and replication of mtDNA were not affected in any case. The deletions were flanked by direct short repeats in all cases. Multiple repeats were found in case 1 and 4. Imperfect repeats were found in case 3 and 4 and this made it possible to distinguish the repeats 5' and 3' to the deletion. In both cases the 3' repeat was retained. The deletion of 7663 bp in case 3 has been reported in two other cases and may represent a second hotspot for mtDNA deletions in addition to the common deletion of 4977 bp found in one third of cases. A comparison of the breakpoint sequence of case 3 with the two other reported cases revealed that when a deletion is formed between the same repeats in different patients either the 5' or 3' repeat can be retained. This study shows that both single and multiple repeats can be associated with single mtDNA deletions and that both 5' and 3' repeated sequences can be retained. These findings are consistent with the slip-replication model for the generation of mtDNA deletions.

Inheritance and expression of mitochondrial DNA point mutations.

An important feature of the mitochondrial genom is the occurrence of heteroplasmy and the possibility for transmission to the offspring of various proportions of wild-type and mutated mtDNA. We have investigated the proportion of the tRNALys A8344G mutation, the tRNALeu(UUR) A3243G mutation, and the ATPase 6 T8993G mutation in patients with MERRF, MELAS, and Leigh's syndrome and their maternal relatives. The level of mutated mtDNA in the offspring of carriers of the tRNALys mutation is correlated to the level in lymphocytes in the mother and seems to be transmitted by an essentially random mechanism where only a few mtDNA copies are founders of the mitochondrial genom in the offspring and the probability that the mutation is not transmitted to the offspring is high when the mothers carriers predominantly wild-type mtDNA. However, we found age-related differences in the distribution of mutated mtDNA in carriers of the tRNALys and tRNALeu mutations, which have to be considered before levels of mutated mtDNA are used for prediction of prognosis and transmission of a disorder.

Automatic sequencing of mitochondrial tRNA genes in patients with mitochondrial encephalomyopathy.

We have investigated nine children with infantile onset of mitochondrial myopathy and two adults with myoclonus epilepsy and ragged-red fibers (MERRF) and chronic progressive external ophthalmoplegia (CPEO), respectively. These patients lacked any of the previously known pathogenic tRNA mutations. Southern blot analysis of muscle mtDNA revealed no deletions. The tRNA genes of muscle mtDNA were sequenced. Restriction enzyme analysis of PCR fragments was performed to verify the presence of the mutations identified by automatic sequencing. Several tRNA mutations were found, but they were all homoplasmic. Furthermore, the mutations were either present in controls or did not change nucleotides conserved between species. This strongly suggests that none of the tRNA mutations identified in the 11 patients with mitochondrial encephalomyopathy was pathogenic. It can thus be concluded that mitochondrial tRNA mutations and mtDNA deletions probably are an infrequent cause of mitochondrial disorders in infants. Patients with MERRF and CPEO may lack both pathogenic point mutations of tRNA genes and deletions of mtDNA.

Expression of wild-type and mutant medium-chain acyl-CoA dehydrogenase (MCAD) cDNA in eucaryotic cells.

An effective EBV-based expression system for eucaryotic cells has been developed and used for the study of the mitochondrial enzyme medium-chain acyl-CoA dehydrogenase (MCAD). 1325 bp of PCR-generated MCAD cDNA, containing the entire coding region, was placed between the SV40 early promoter and polyadenylation signals in the EBV-based vector. Both wild-type MCAD cDNA and cDNA containing the prevalent disease-causing mutation A to G at position 985 of the MCAD cDNA were tested. In transfected COS-7 cells, the steady state amount of mutant MCAD protein was consistently lower than the amount of wild-type human enzyme. The enzyme activity in extracts from cells harbouring the wild-type MCAD cDNA was dramatically higher than in the controls (harbouring the vector without the MCAD gene) while only a slightly higher activity was measured with the mutant MCAD. The mutant MCAD present behaves like wild-type MCAD with respect to solubility, subcellular location, mature protein size and tetrameric structure. In immunoblot comparisons, the MCAD protein was present in normal fibroblasts, but essentially undetectable in patient fibroblasts homozygous for the prevalent mutation. We suggest that the MCAD protein carrying this mutation has an impaired ability to form correct tetramers, leading to instability and subsequent degradation of the enzyme. This finding is discussed in relation to the results from expression of human MCAD in Escherichia coli, where preliminary results show that production of mutant MCAD leads to the formation of aggregates.

Purification and characterization of RHP (factor H) and study of its interactions with the first component of complement.

RHP has been purified from the plasma of both normal individuals and patients with rheumatoid arthritis (RA). RHP from both these sources was shown to be identical with Factor H by reaction with antisera and N-terminal amino acid sequence analysis. Factor H, from both normal and RA sera, inhibited the solubilization of immune precipitates but did not affect prevention of immune precipitation. Factor H was shown to inhibit the haemolytic activity of fluid-phase C1, but unlike C1-inhibitor, it had little effect on C1 bound to EA (EAC1). Factor H was shown to complex with intact C1, to isolated C1q and to the C1r:C1s tetramer. However, binding of factor H to C1 did not dissociate the C1 macromolecule. A C1-Factor H complex was detected in the serum and plasma from normal individuals and patients with systemic lupus erythematosus and RA. Serum levels of this complex were reduced, by EDTA-treatment of serum and by activation of complement by the classical pathway.

Complement and related clinical disorders.

The complement system, composed of 20 plasma proteins and several membrane receptors, plays an essential role in humoral immune responses. The activation of the classical and/or alternative pathways by specific and non-specific stimuli leads to the generation of chemotactic and anaphylatoxic inflammatory mediators, the formation of the cytolytic membrane attack complex, and the formation of C3 breakdown fragments which are opsonic. There is now evidence that the membrane receptors play important immunoregulatory functions. The link between various disease states and deficiencies of complement components and membrane receptors further supports the important immunological role this system plays. This review briefly introduces the components of the system, their biological roles and diseases associated with deficiency states.

Mitochondrial DNA deletions in muscle fibers in inclusion body myositis.

Inclusion body myositis (IBM) is an autoimmune, inflammatory myopathy where morphological changes of muscle, including ragged red fibers, have indicated mitochondrial dysfunction in some muscle fibers. In this study enzyme histochemical analysis showed that cytochrome c oxidase (COX)-deficient muscle fibers were present at a frequency ranging from 0.5 to 5% of the muscle fibers in a series of 20 IBM patients. In age-matched controls, only occasional COX-deficient muscle fibers were present. Polymerase chain reaction (PCR) analysis of DNA extracted from muscle tissue of the IBM patients showed multiple mtDNA deletions. PCR analysis of isolated, single muscle fibers showed presence of mtDNA with only one type of deletion and deficiency of wild-type mtDNA in each COX-deficient muscle fiber. This finding was supported by results from in situ hybridization using different mtDNA probes on consecutive sections. A 5 kb deletion was identified in all 20 IBM patients. DNA sequencing of the breakpoint region showed that this deletion was the so-called "common deletion." Most but not all of the investigated deletion breakpoints were flanked by direct repeats. COX-deficient fibers were more frequent among fibers with positive immunostaining with antibodies directed toward a regeneration marker, the Leu-19 antigen, than in the entire fiber population. These results show that COX deficiency in muscle fiber segments in IBM is associated with deletions of mtDNA. Clonal expansion of mtDNA with deletions may take place in regenerating muscle fibers following segmental necrosis.

Autosomal dominant progressive external ophthalmoplegia: distribution of multiple mitochondrial DNA deletions.

OBJECTIVE: To relate signs and symptoms to morphologic changes and presence of multiple mitochondrial DNA (mtDNA) deletions in a patient with autosomal dominant progressive external ophthalmoplegia (adPEO) and mitochondrial myopathy. BACKGROUND: An etiologic association between the somatic multiple mtDNA deletions in adPEO and clinical manifestations other than the myopathy has so far not been demonstrated. METHODS: The authors investigated a patient with adPEO and multiorgan system manifestations including levodopa-responsive parkinsonism. She died at age 61 years of pancreatic carcinoma. Autopsy tissue specimens were investigated for morphologic alterations and occurrence of mtDNA deletions by Southern blot and long-extension PCR analyses. RESULTS: The patient had carcinoma of the pancreas with metastases to liver, lymph nodes, and bone marrow. The brain revealed slight gliosis of the gray and white matter and degeneration of the substantia nigra. The myocardium showed focal areas with loss and atrophy of myocytes and fibrosis. Analysis of mtDNA revealed multiple deletions in different regions of the brain, skeletal muscle, and myocardium. Twenty-five different mtDNA deletions were identified. Most of these were flanked by large direct-sequence repeats. Six identical deletions were found in muscle and brain. CONCLUSIONS: These findings indicate that somatic multiple mtDNA deletions are associated with degenerative tissue changes and clinical manifestations in adPEO.

Rhabdomyolysis in autosomal dominant progressive external ophthalmoplegia.

Multiple mtDNA deletions have been reported to be a cause of inherited recurrent myoglobinuria. We report a 57-year-old man with autosomal dominant progressive external ophthalmoplegia and multiple mtDNA deletions who developed acute rhabdomyolysis provoked by alcohol. A repeated alcohol intake resulted in a 57-fold increase in serum myoglobin. Patients with mitochondrial myopathy and multiple mtDNA deletions, regardless of associated phenotype and mode of inheritance, may develop rhabdomyolysis.

Threshold expression of the tRNA(Lys) A8344G mutation in single muscle fibres.

We investigated the distribution in skeletal muscle of mitochondrial DNA (mtDNA) with the tRNA(Lys) A8344G mutation, which is associated with myoclonus epilepsy and ragged red fibres (MERRF) syndrome. Isolated muscle fibre segments (n = 144) from six individuals of two different families carrying the mutation were studied. Two of these individuals were affected by MERRF while four had no or minor clinical symptoms. In one individual with a low overall level of mutated mtDNA (mean 18%) the variation in the proportion of mutated mtDNA between individual muscle fibres ranged from 0 to 80%. This result demonstrates that segregation of the tRNA(Lys) A8344G mutation within a tissue may lead to very marked variation of the level of mutated mtDNA between individual cells. There was a very high apparent threshold level of mutated mtDNA (95.3-97.7%) for expression of histochemical cytochrome c oxidase (COX) deficiency in individual muscle fibres. The results indicated that this apparent threshold level varied slightly between patients. Ultrastructural examination revealed that an appreciable proportion of the mitochondria in COX-positive muscle fibres lacked COX activity. Variation in intercellular and interorganellar distribution of mutated mtDNA in addition to the absolute mtDNA copy number may explain differences in clinical phenotypes in patients with high levels of the tRNA(Lys) A8344G mutation.

A novel mutation in the mitochondrial tRNA(Phe) gene associated with mitochondrial myopathy.

We report a novel heteroplasmic T-->C mutation at nt position 582 within the mitochondrial tRNA(Phe) gene of a 70-year-old woman with mitochondrial myopathy. No other family members were affected, suggesting that our patient was a sporadic case. The muscle showed frequent ragged red fibers and 43% cytochrome c oxidase deficient fibers. The mutation alters a conserved base pairing in the aminoacyl acceptor stem. The mutation load was 70% in muscle homogenate and varied from 0 to 95% in individual muscle fiber segments. Cytochrome c oxidase-negative fibers showed significantly higher levels of mutated mtDNA (>75%) than Cytochrome c oxidase-positive fibers (<55%). This mutation adds to the previously described four pathogenic mutations in the tRNA(Phe) gene.

Nontransplant treatment of tyrosinemia.

NTBC treatment has greatly improved the survival of patients with acute tyrosinemia and has reduced the need for liver transplantation during early childhood. In patients in whom treatment with NTBC was started early in life, 2 cases (1%) of HCC have occurred during the first year of treatment, but no further cases have occurred among these patients, who have been followed for up to 9 years. In patients with late start of NTBC treatment, there is a considerable risk for liver malignancy. The risk for malignancy in this group of patients must be evaluated on an individual basis, taking into account the phenotype and clinical status of the patient. Porphyric crises are not seen in patients who comply with the medication regimen. NTBC is a well-tolerated drug with few adverse effects.

Immunoalkaline phosphatase technique applied to paraffin wax embedded tissues in diagnostic renal pathology.

An indirect immunoalkaline phosphatase (IAP) technique was used to evaluate the glomerular deposition of immunoglobulins, C3, C1q and fibrinogen. In 80 renal biopsy specimens the results obtained using this technique were compared with those obtained by direct immunofluorescence to see if it could be used as a viable alternative. The IAP technique was straightforward to perform, it yielded quick results, and was highly reproducible, provided that a standardised short fixation period of two and a half hours was used. For the detection of immunoglobulin deposits, the IAP results correlated well with those of immunofluorescence. Despite poorer performance in identifying complement components and fibrinogen it could, within certain limits, provide an adequate diagnostic alternative to immunofluorescence. Each technique gave false negative results, those of immunofluorescence being related to its failure to identify mesangial deposition of IgA in two cases where its distribution seemed to be focal, and those of IAP to a failure to detect linear deposition of IgG in all three cases of anti-glomerular basement membrane disease.

Pivalic acid-induced carnitine deficiency and physical exercise in humans.

To study the effect of carnitine depletion on physical working capacity, healthy subjects were administered pivaloyl-conjugated antibiotics for 54 days. The mean carnitine concentration in serum decreased from 35.0 to 3.5 mmicromol/L, and in muscle from 10 to 4.3 micromol/g noncollagen protein (NCP). Exercise tests were performed before and after 54 days' administration of the drug. At submaximal exercise, there was a slight increase in the concentration of 3-hydroxybutyrate in serum, presumably caused by decreased fatty acid oxidation in the liver. There was also a decreased consumption of muscle glycogen, indicating decreased glycolysis in the skeletal muscle. The muscle presumably had enough energy available, since there was no significant decrease in the concentration of adenosine triphosphate (ATP) and creatine phosphate during exercise. The work at maximal oxygen uptake (VO2max) and the maximal heart rate were reduced. Since VO2max is considered dependent on heart function, carnitine depletion seemed to affect cardiac function.