Daniel Rabier

Clinical aspects of mitochondrial disorders

SummaryMitochondrial disorders have long been regarded as neuromuscular diseases only. In fact, owing to the ubiquitous nature of the oxidative phosphorylation, a broad spectrum of clinical features should be expected in mitochondrial disorders. Here, we present eight puzzling observations which give support to the view that a disorder of oxidative phosphorylation can give rise to any symptom in any organ or tissue with any apparent mode of inheritance. Consequently, we suggest giving consideration to the diagnosis of a mitochondrial disorder when dealing with an unexplained association of symptoms, with an early onset and a rapidly progressive course involving seemingly unrelated organs. Determination of lactate/pyruvate and ketone body molar ratios in plasma can help to select patients at risk for this condition.

Alpha-ketoglutarate dehydrogenase deficiency presenting as congenital lactic acidosis

We report an inborn error of the tricarboxylic acid cycle, alpha-ketoglutarate dehydrogenase deficiency, in three siblings with hypotonia, metabolic acidosis, and hyperlactatemia immediately after birth. Neurologic deterioration resulted in death at about 30 months of age. We propose low molar ratios of ketone bodies in plasma of neonates with congenital lactic acidosis as an indication of dysfunction of the tricarboxylic acid cycle.

Liver cytochrome c oxidase deficiency in a case of neonatal-onset hepatic failure

In the last few years, inborn errors of oxidative phosphorylation have been recognized as possible causes of hepatic failure in infancy and respiratory enzyme deficiencies have been described in several tissues of affected individuals. Here, we report on cytochrome c oxidase deficiency in the liver but not in the skeletal muscle of a 5-monthold girl who presented hepatic failure in early infancy. Persistent hyperlactataemia (>4 mM, normal <2.4) with high lactate/pyruvate (L/P) molar ratios in plasma, and their further elevation in the post-absorptive period were suggestive of an inborn error of oxidative phosphorylation. However, no mutation in the coding sequences of the liver-specific subunits of cytochrome c oxidase (VIa and VIIa) has been detected and no major rearrangement or depletion of the mitochondrial DNA has been observed. Based on this observation we suggest that inborn errors of oxidative phosphorylation be considered in the diagnosis of severe hepatocellular dysfunction of unknown origin, especially when an abnormal oxidation-reduction status is found in the plasma and even if normal respiratory enzyme activities are found in peripheral tissues. The finding of normal respiratory enzyme activities in skeletal muscle, circulating lymphocytes or cultured skin fibroblasts does not rule out this diagnosis. Instead, the negativity of these tests should prompt one to carry out the specific enzyme assays in the tissue which expresses the disease, namely the liver.

Cytochrome c oxidase deficiency presenting as recurrent neonatal myoglobinuria

Markedly reduced cytochrome c oxidase (COX) activity was found in cultured skin fibroblasts of an infant with recurrent episodes of acute myoglobinuria, hypertonia, muscle stiffness and elevated plasma levels of sarcoplasmic enzymes (creatine kinase 96950 U/l, normal below 150) since the age of 3 weeks (COX activity: 36 nmol/min/mg protein; normal 65-440; COX/succinate cytochrome c reductase ratio: 1.4, normal 3.0 +/- 0.4). The expression of the disease in cultured fibroblasts allowed us to carry out a prenatal diagnosis during the next pregnancy. Hitherto, mitochondrial respiratory chain deficiency has not been established as a cause of recurrent myoglobinuria in childhood. Since most cases of myoglobinurias remain poorly understood, we suggest giving consideration to respiratory chain deficiency in elucidating the origin of unexplained recurrent myoglobinuria in childhood, especially when seemingly unrelated symptoms are present.

Carrier detection in a partially dominant X-linked disease: ornithine transcarbamylase deficiency

SummaryOrnithine transcarbamylase (OTC) deficiency is an X-linked disease responsible for lethal neonatal hyperammonemia in males. Partial OTC deficiency also occurs in females and can be responsible for life-threatening hyperammonemic comas in heterozygotes (15%). Increased orotic acid excretion occurs in both symptomatic and asymptomatic carriers, especially under protein loading tests. The disease is therefore partially dominant with neonatal lethality in the hemizygous male; the fraction of new mutations has previously been estimated to be low in males (point estimation = 0, upper bound of the confidence interval = 0.16) and 57% in females. Genetic counseling in this disease is difficult because it is not clear whether a negative protein loading test rules out carrier status. In an attempt to determine how reliable the test is for carrier detection, we investigated ten obligate carriers for orotic acid excretion; considering all data available, we concluded that the test is rarely negative in obligate carriers (8%). Consequently, a negative test in a mother decreases the minimum risk of being a carrier from 84% a priori to 30% if she had an affected son and from 43% a priori to 5% if she had a heterozygous daughter. Finally, the diagnosis of a new mutation in the germ cells of the maternal grandfather in one particular family could be ascertained by extensive DNA analysis.

Fatal hyperammonemia resulting from a C-to-T mutation at a MspI site of the ornithine transcarbamylase gene.

SummaryOrnithine transcarbamylase (OTC) deficiency is the most common inborn error of the urea cycle in humans and is responsible for lethal neonatal hyperammonemia in males. Partial OTC deficiency also occurs in females and can be responsible for life-threatening hyperammonemic comas in heterozygotes. The cosegregation of the trait with a 5.8-kb abnormal MspI fragment in an affected family led us to hypothesize that this unexpected migration pattern was related to the mutation event in this particular family. Using polymerase chain reaction amplification of the specific mRNA derived from a post-mortem biopsy of the liver, we found that the MspI site located in the seventh exon of the gene was abolished and we finally identified a C-to-T transition at codon 225 of the cDNA, changing a proline to a leucine in the protein. Subsequent digestion of amplified exon 7 using the restriction enzyme MspI allowed direct screening for the mutant genotype during the next pregnancy. The present study supports the view that direct detection of the mutant genotype using either Southern blotting or digestion of amplified exons of the gene can contribute to genetic counselling in non-informative families. Finally, since MspI digestions are routinely performed for restriction fragment length polymorphism-based family studies in OTC deficiency, we suggest that the possible presence of the 5.8-kb abnormal fragment should be investigated on Southern blots of affected individuals.

Persistent hypocitrullinaemia as a marker for mtDNA NARP T 8993 G mutation

hypocitrullinaemia (\10 kmol/L) is one of the main biochemical characterSevere istics of two clinical situations : the disorders of mitochondrial urea-cycle enzymes (N-acetylglutamate synthase, carbamoylphosphate synthetase I (CPS-I) and ornithine carbamoyltransferase (OCT)) and the deÐciency of pyrroline-5-carboxylate synthetase and Kamoun Hypocitrullinaemia was also associated with (Rabier 1995). Pearson syndrome et al Here we report a new situation in which (Ribes 1993). hypocitrullinaemia was observed in 5 patients with mt ATP6 NARP T 8993 G mutation. The NARP 8993 mutation impairs the function of the subunit a or ATPase 6 of the Fo segment of the complex V. This ATPase 6 polypeptide is involved in proton translocation through the inner mitochondrial membrane.