Mitochondrial disorders: clinical presentation and diagnostic dilemmas

The number of genes known to be involved in mitochondrial energy production and the elucidation of the function of their individual transcripts is still increasing. Although at this stage it is impossible to predict the number of human genes necessary for mitochondrial biogenesis and maintenance, the total number in humans will most probably exceed the number of mitochondrial genes found in, for example, the budding yeast, which is about 800. Without doubt we have only seen the tip of the iceberg of the clinical spectrum of mitochondrial disorders. Recent findings such as mutations in structural complex II genes in certain tumours emphasize the need to think outside the classical clinical presentation. We propose the consideration of a mitochondrial disorder in every chronic, intermittent or progressive disorder with single system or multisystem involvement, even if lactic acid is normal, and discuss such dilemmas as whether we should 'scrape the barrel' in every patient that are raised by this statement. The characterization of mitochondrial and nuclear DNA mutations in patients with enzymatically established mitochondrial defects has taught us that several of the current clinical and diagnostic assumptions have to be altered or even eliminated. The most challenging future task will be the development of new diagnostic criteria covering the expanding clinical spectrum of mitochondrial disorders.

Moderate citrullinaemia without hyperammonaemia in a child with mutated and deficient argininosuccinate synthetase

In a patient with microcephaly, feeding problems and restlessness, moderately increased serum and urine citrulline concentrations were observed. Protein and allopurinol loading did not result in additional indications for a urea cycle defect. The diagnosis of citrullinaemia was made at both the enzyme and DNA level, resulting from a novel mutation in the argininosuccinate synthetase gene. The fact that the patient has not suffered from severe deterioration, and that there were only minor abnormalities in metabolite concentrations, suggests that the argininosuccinate synthetase capacity was less affected in vivo than in vitro. In vitro nuclear magnetic resonance investigation suggested an active acetylation mechanism for citrulline. This case illustrates the importance of performing extensive biochemical and molecular investigations in order to reach a definitive diagnosis, particularly in instances of moderate citrullinaemia.

Mitochondrial disorders: a proposal for consensus diagnostic criteria in infants and children

BACKGROUND

In 1996 diagnostic criteria were published for adults with respiratory chain disorders. Modified criteria for children were also recently proposed.

OBJECTIVE

To facilitate and standardize diagnosis of respiratory chain disorders in children.

METHODS

A new classification has been developed, the Mitochondrial Disease Criteria (MDC), for the diagnosis of respiratory chain disorders in infants and children. It considers clinical, metabolic, imaging, and histopathologic features vs biochemical investigations of skeletal muscle. The criteria were applied to a group of 30 children.

RESULTS

The modified adult criteria and the MDC gave similar results, with 17 patients having a definite respiratory chain disorder. No patients reached this category using the original adult criteria.

CONCLUSIONS

The proposed Mitochondrial Disease Criteria classification allows more precise definition of clinical and metabolic items and the independent scoring of muscle biochemical investigations before combining all findings to determine the overall diagnostic certainty.

New perspectives on the assembly process of mitochondrial respiratory chain complex cytochrome c oxidase

The assembly of cytochrome c oxidase (COX) is a complicated process and requires a number of assembly factors to put all the necessary subunits in the correct position. Defects in COX assembly lead in particular to serious neuromuscular disorders. We demonstrated that COX-deficient patients can be associated with different assembly patterns. To obtain more insight in the biogenesis of COX in a living cell, we used yeast as a model organism to design a way to pulse label holo-COX with green fluorescent protein (GFP). Using blue native electrophoresis, we showed that the GFP-tagged subunit is incorporated into fully assembled COX and this GFP tagged complex still has enzymatic activity. This allows us to correlate the GFP fluorescence signal detected in vivo by microscopy with the synthesis, turnover and assembly of COX.

Adenine nucleotide translocator 1 deficiency associated with Sengers syndrome

Sengers syndrome is characterized by congenital cataracts, hypertrophic cardiomyopathy, mitochondrial myopathy, and lactic acidosis, but no abnormalities have been found with routine mitochondrial biochemical diagnostics (the determination of pyruvate oxidation rates and enzyme measurements). In immunoblot analysis, the protein content of the mitochondrial adenine nucleotide translocator 1 (ANT1) was found to be strongly reduced in the muscle tissues of two unrelated patients with Sengers syndrome. In addition, low residual adenine nucleotide translocator activity was detected upon the reconstitution of detergent-solubilized mitochondrial extracts from the patients' skeletal or heart muscle into liposomes. Sequence analysis and linkage analysis showed that ANT1 was not the primary genetic cause of Sengers syndrome. We propose that transcriptional, translational, or posttranslational events are responsible for the ANT1 deficiency associated with the syndrome.

Bcl-2 protects against apoptosis induced by antimycin A and bongkrekic acid without restoring cellular ATP levels

Several studies indicate that mitochondrial ATP production as well as ADP/ATP exchange across mitochondrial membranes are impaired during apoptosis. We investigated whether Bcl-2 could protect against cell death under conditions in which ATP metabolism is inhibited. Inhibition of ATP production using antimycin A (AA) (complex III inhibition) combined with inhibition of ADP/ATP exchange by bongkrekic acid (BA) (adenine nucleotide translocator (ANT) inhibition) induced a sharp decrease in total cellular ATP in FL5.12 parental cells (to 35% of untreated controls after 24 h of incubation). Within 24 and 48 h, 38% and 75% of the cells had died, respectively. However, in stably transfected FL5.12 Bcl-2 subclones, no cell death occurred under these experimental conditions. Similar results were obtained with Jurkat and Bcl-2 overexpressing Jurkat cells. Total cellular ATP levels were equally affected in FL5.12 Bcl-2 overexpressing cells and FL5.12 parental cells. This indicates that Bcl-2 overexpressing cells are able to survive with very low cellular ATP content. Furthermore, Bcl-2 did not protect against cell death by restoring ATP levels. This suggests that, under these conditions, Bcl-2 acts by inhibiting the signalling cascade triggered by the inhibitors that would normally lead to apoptosis.

Biochemical hallmarks of tyrosine hydroxylase deficiency

We report the biochemical hallmarks of tyrosine hydroxylase deficiency with emphasis on reliable diagnostic strategies of four new cases of an inborn error of tyrosine hydroxylase (TH). Three of our patients from different parts of the Netherlands were found homozygous for a mutation in exon 6 (G698A) of the TH gene, and one patient was found compound heterozygous for the same mutation and an additional mutation in exon 3. The first clinical symptoms of hypokinesia, rigidity of arms and legs and axial hypotonia, developed between 3 and 7 months of age. Cerebrospinal fluid investigations revealed a characteristic metabolite constellation in every case: low homovanillic acid (HVA) and 3-methoxy-4-hydroxy-phenylethyleneglycol concentrations in the presence of normal reference range 5-hydroxyindolacetic acid concentrations. Strict adherence to a standardized lumbar puncture protocol and adequate age-related reference values are essential for diagnosis of this “new” treatable neurometabolic disorder. Urinary measurements of HVA, vanillylmandelic acid, and catecholamines can lead to false-negative conclusions. All patients showed a remarkable clinical improvement on a low dose of l-dihydroxyphenylalanine/(S)-2-(3,4-dihydroxybenzyl)-2-hydrazinpropionic acid. During treatment, cerebrospinal fluid HVA, and 3-methoxy-4-hydroxy-phenylethyleneglycol increased substantially.