Deficiency in complex IV (cytochrome c oxidase) of the respiratory chain, presenting as a leukodystrophy in two siblings with Leigh syndrome

A guide to diagnosis and treatment of Leigh syndrome

Leigh syndrome is a devastating neurodegenerative disease, typically manifesting in infancy or early childhood. However, also late-onset cases have been reported. Since its first description by Denis Archibald Leigh in 1951, it has evolved from a postmortem diagnosis, strictly defined by histopathological observations, to a clinical entity with indicative laboratory and radiological findings. Hallmarks of the disease are symmetrical lesions in the basal ganglia or brain stem on MRI, and a clinical course with rapid deterioration of cognitive and motor functions. Examinations of fresh muscle tissue or cultured fibroblasts are important tools to establish a biochemical and genetic diagnosis. Numerous causative mutations in mitochondrial and nuclear genes, encoding components of the oxidative phosphorylation system have been described in the past years. Moreover, dysfunctions in pyruvate dehydrogenase complex or coenzyme Q10 metabolism may be associated with Leigh syndrome. To date, there is no cure for affected patients, and treatment options are mostly unsatisfactory. Here, we review the most important clinical aspects of Leigh syndrome, and discuss diagnostic steps as well as treatment options.

A mutation in the gene encoding mitochondrial Mg²+ channel MRS2 results in demyelination in the rat

The rat demyelination (dmy) mutation serves as a unique model system to investigate the maintenance of myelin, because it provokes severe myelin breakdown in the central nervous system (CNS) after normal postnatal completion of myelination. Here, we report the molecular characterization of this mutation and discuss the possible pathomechanisms underlying demyelination. By positional cloning, we found that a G-to-A transition, 177 bp downstream of exon 3 of the Mrs2 (MRS2 magnesium homeostasis factor (Saccharomyces cerevisiae)) gene, generated a novel splice acceptor site which resulted in functional inactivation of the mutant allele. Transgenic rescue with wild-type Mrs2-cDNA validated our findings. Mrs2 encodes an essential component of the major Mg²⁺ influx system in mitochondria of yeast as well as human cells. We showed that the dmy/dmy rats have major mitochondrial deficits with a markedly elevated lactic acid concentration in the cerebrospinal fluid, a 60% reduction in ATP, and increased numbers of mitochondria in the swollen cytoplasm of oligodendrocytes. MRS2-GFP recombinant BAC transgenic rats showed that MRS2 was dominantly expressed in neurons rather than oligodendrocytes and was ultrastructurally observed in the inner membrane of mitochondria. Our observations led to the conclusion that dmy/dmy rats suffer from a mitochondrial disease and that the maintenance of myelin has a different mechanism from its initial production. They also established that Mg²⁺ homeostasis in CNS mitochondria is essential for the maintenance of myelin.

The myelin sheath that surrounds the axon of a neuron acts as a biological insulator. Its major function is to increase the speed at which impulses propagate along myelinated fibers in the central nervous system, as well as the peripheral nervous system. Alterations or damage affecting this structure (demyelination) result in the disruption of signals between the brain and other parts of the body. In the rat, mutations producing demyelination have been frequently identified and characterized and have contributed to a better understanding of the genetics of myelin development, physiology, and pathology. This paper reports the molecular characterization of a recessive allele responsible for the progressive disruption of myelin that was initially observed in mutant rats, previously named demyelination (dmy). This mutation generates an additional splicing acceptor site in an intron of the mitochondrial Mg²⁺ transporter gene (Mrs2), resulting in the insertion of a 83-bp genomic DNA segment into the Mrs2 transcript and complete functional inactivation of the mutant allele. We firstly defined the biological function of MRS2 in mammals and demonstrated the crucial and unexpected role of MRS2 in myelin physiology. Our findings might be helpful in the development of new therapeutic strategies for demyelinating syndromes.

SURF-1 gene mutation associated with leukoencephalopathy in a 2-year-old

Mutations in the nuclear SURF-1 gene lead directly to cytochrome-c oxidase deficiency, the most common respiratory chain defect in Leigh syndrome, a neurodegenerative mitochondrial disease involving the deep gray matter and brain stem. We describe the second documented case in the literature to have a SURF-1 mutation presenting with diffuse leukodystrophy, adding to the growing number of cases of mitochondrial syndromes presenting with white matter disease. We examine magnetic resonance imaging (MRI) findings, which suggest that high-grade cytotoxic edema on diffusion-weighted imaging may be a helpful diagnostic feature in differentiating mitochondrial leukodystrophy from other, more common leukodystrophies. We show how MRI white matter findings may progress to include the brain stem, suggesting that a leukodystrophy due to respiratory chain defects can precede more classic Leigh syndrome deep gray matter radiographic findings.

Leigh and Leigh-like syndrome in children and adults

Leigh syndrome (also termed subacute, necrotizing encephalopathy) is a devastating neurodegenerative disorder, characterized by almost identical brain changes, e.g., focal, bilaterally symmetric lesions, particularly in the basal ganglia, thalamus, and brainstem, but with considerable clinical and genetic heterogeneity. Clinically, Leigh syndrome is characterized by a wide variety of abnormalities, from severe neurologic problems to a near absence of abnormalities. Most frequently the central nervous system is affected, with psychomotor retardation, seizures, nystagmus, ophthalmoparesis, optic atrophy, ataxia, dystonia, or respiratory failure. Some patients also present with peripheral nervous system involvement, including polyneuropathy or myopathy, or non-neurologic abnormalities, e.g., diabetes, short stature, hypertrichosis, cardiomyopathy, anemia, renal failure, vomiting, or diarrhea (Leigh-like syndrome). In the majority of cases, onset is in early childhood, but in a small number of cases, adults are affected. In the majority of cases, dysfunction of the respiratory chain (particularly complexes I, II, IV, or V), of coenzyme Q, or of the pyruvate dehydrogenase complex are responsible for the disease. Associated mutations affect genes of the mitochondrial or nuclear genome. Leigh syndrome and Leigh-like syndrome are the mitochondrial disorders with the largest genetic heterogeneity.

Neuroimaging of mitochondrial disease

Mitochondrial disease represents a heterogeneous group of genetic disorders that require a variety of diagnostic tests for proper determination. Neuroimaging may play a significant role in diagnosis. The various modalities of nuclear magnetic resonance imaging (MRI) allow for multiple independent detection procedures that can give important anatomical and metabolic clues for diagnosis. The non-invasive nature of neuroimaging also allows for longitudinal studies. To date, no pathonmonic correlation between specific genetic defect and neuroimaging findings have been described. However, certain neuroimaging results can give important clues that a patient may have a mitochondrial disease. Conventional MRI may show deep gray structural abnormalities or stroke-like lesions that do not respect vascular territories. Chemical techniques such as proton magnetic resonance spectroscopy (MRS) may demonstrate high levels of lactate or succinate. When found, these results are suggestive of a mitochondrial disease. MRI and MRS studies may also show non-specific findings such as delayed myelination or non-specific leukodystrophy picture. However, in the context of other biochemical, structural, and clinical findings, even non-specific findings may support further diagnostic testing for potential mitochondrial disease. Once a diagnosis has been established, these non-invasive tools can also aid in following disease progression and evaluate the effects of therapeutic interventions.

Mitochondrial Leukodystrophy: an Unusual Manifestation of Leigh's Disease. A Report of Three Cases and Review of the Literature

Leigh's disease is an inherited, progressive neurodegenerative disorder of infancy and early childhood. This metabolic disease is biochemically and genetically a heterogeneous disorder with defects involving various enzymes involved in the respiratory chain mechanism. Due to the multitude of enzyme defects known to occur in patients afflicted with Leigh's disease, this condition is known to have a variable clinical, pathological and radiological pattern of presentation. Isolated deficiency of cytochrome oxidase (COX) enzyme is one of the commonest abnormalities seen in patients afflicted by Leigh's disease. A primary white matter pattern of involvement representing a mitochondrial leukodystrophy is rare. Symmetric hyperintensities on T2WI involving the subthalamic nuclei and brainstem have been reported in patients with COX deficiency with SURF 1 mutations and are considered almost a hallmark of Leigh's disease with COX deficiency. We describe three cases of Leigh's disease with a primary white matter involvement diagnosed at our institution on the basis of clinical features, radiological appearance and laboratory findings.

White matter involvement in mitochondrial diseases

White matter involvement is recently being realized as a common finding in mitochondrial disorders. It is considered an inherent part of the classical mitochondrial syndromes which are usually associated with alterations in the mitochondrial DNA such as: Leigh disease, Kearns-Sayre syndrome, mitochondrial encephalomyopathy lactic acidosis, and stroke like episodes, mitochondrial neuro-gastro-intestinal encephalomyopathy and Leber's hereditary optic neuropathy. White matter involvement is also described in mitochondrial disorders due to mutations in the nuclear DNA which are transmitted in an autosomal pattern. MRI findings suggestive of a mitochondrial disease are: small cyst-like lesions in abnormal white matter, involvement of both cerebral and cerebellar white matter, and a combination of a leukoencephalopathy with bilateral basal ganglia lesions. The clinical manifestations may be disproportionate to the extent of white matter involvement. Other organs may frequently be involved. The onset is often in infancy with a neurodegenerative course. The finding of a leukoencephalopathy in a patient with a complex neurologic picture and multisystem involvement should prompt a thorough mitochondrial evaluation.

Infantile leukoencephalopathy owing to mitochondrial enzyme dysfunction

Mitochondrial disease is classically associated with deep gray-matter lesions. When white matter is involved, the lesions are typically subcortical and overshadowed by more significant disease in the gray matter. We report six infants in five families who developed neurodegenerative diseases characterized primarily by abnormalities in deep white-matter structures such as the periventricular region, internal capsule, and corpus callosum. Five patients had impairments of mitochondrial enzymes, including a pre-electron transport chain defect and defects in respiratory chain complexes I, III, and IV (cytochrome-c oxidase). One patient, the sibling of one of the others, was diagnosed clinically with complex III deficiency. These six patients, along with others in the literature, appear to represent a distinct syndrome of mitochondrial infantile leukoencephalopathy. Our observations suggest that infants with leukoencephalopathies, especially leukodystrophies, who do not have one of the more common causes of white-matter disease should be evaluated for mitochondrial dysfunction.

Cerebral white matter disease in children may be caused by mitochondrial respiratory chain deficiency

Several mitochondrial diseases are known to occasionally involve the cerebral white matter, namely Leigh syndrome, Kearns-Sayre syndrome, and MELAS syndrome, but in these cases the major finding is alteration in the basal ganglia and brainstem. Here we report on severe diffuse white matter involvement and respiratory chain enzyme deficiency or mitochondrial DNA rearrangement in 5 unrelated families. It is interesting that white matter lesions were the only abnormal neuroradiologic feature in 3 of the 5 families, and multiple small cyst-like white matter lesions were found in 2 of 5 probands. Respiratory chain deficiency should be considered in the diagnosis of severe white matter involvement in childhood.

Diffuse leukodystrophy in an infant with cytochrome-c oxidase deficiency

A 25-month-old boy, born to consanguineous parents, had progressive spastic tetraplegia, and increased signal of the white matter on cerebral T2-weighted magnetic resonance imaging indicative of diffuse leukodystrophy. Elevated blood and cerebrospinal fluid lactate levels pointed to a respiratory chain defect. Cytochrome-c oxidase deficiency was demonstrated in cultured skin fibroblasts and skeletal muscle. This report extends the phenotype of COX deficiency in infancy. Systematic study of blood and CSF lactate should be carried out in every infant with leukodystrophy.

Clinical and radiologic improvements in mitochondrial encephalomyelopathy following sodium dichloroacetate therapy

We administered sodium dichloroacetate (DCA), which reduces the circulating lactate and pyruvate concentrations by stimulating the activity of the pyruvate dehydrogenase complex (PDHC), to three children with mitochondrial encephalomyelopathy. Significant clinical, biochemical and radiologic improvements were obtained following DCA therapy (approximately 30 mg/kg per day, divided into three doses). All three patients had non-pyruvate dehydrogenase complex (PDHC) deficiencies: two exhibited Leigh syndrome (complex I deficiency and unknown etiology), and one abnormal myelination (multienzyme deficiency), demonstrated on magnetic resonance imaging (MRI). The lactic and pyruvic acid concentrations in serum and cerebrospinal fluid (CSF) were decreased significantly by the oral DCA treatment. The lactic acid peak on MR spectroscopy also markedly decreased in parallel with the CSF level. In addition, the brain lesions observed on MRI were improved in all patients. No exacerbation was observed in any of the patients, who have been followed-up more than 21 months following the DCA therapy. These results suggest that DCA therapy should be considered in all patients with a mitochondria-related enzyme deficiency.

Magnetic resonance imaging in lactic acidosis

Mitochondrial defects, defects in gluconeogenesis, and biotin-responsive multiple carboxylase deficiency are disorders characterized by primary lactic acidosis. In this review, characteristic findings in magnetic resonance imaging (MRI) of the brain, as related to histopathological abnormalities, are described for the different disorders and the diagnostic value of the MRI findings is discussed. Inborn errors of metabolism with primary lactic acidosis should be considered in particular when MRI shows lesions similar to or reminiscent of effects of focal or generalized hypoxia-ischaemia, or when MRI shows signs of chronic neurodegeneration, but rarely in cases with predominantly white-matter changes.