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To accomplish a diagnosis in patients with a rare unclassified disorder is difficult. In this study, we used magnetic resonance imaging pattern recognition analysis to identify patients with the same novel heritable disorder. Whole-exome sequencing was performed to discover the mutated gene. We identified seven patients sharing a previously undescribed magnetic resonance imaging pattern, characterized by initial swelling with T2 hyperintensity of the basal nuclei, thalami, cerebral white matter and cortex, pons and midbrain, followed by rarefaction or cystic degeneration of the white matter and, eventually, by progressive cerebral, cerebellar and brainstem atrophy. All patients developed a severe encephalopathy with rapid deterioration of neurological functions a few weeks after birth, followed by respiratory failure and death. Lactate was elevated in body fluids and on magnetic resonance spectroscopy in most patients. Whole-exome sequencing in a single patient revealed two predicted pathogenic, heterozygous missense mutations in the SLC19A3 gene, encoding the second thiamine transporter. Additional predicted pathogenic mutations and deletions were detected by Sanger sequencing in all six other patients. Pathology of brain tissue of two patients demonstrated severe cerebral atrophy and microscopic brain lesions similar to Leigh's syndrome. Although the localization of SLC19A3 expression in brain was similar in the two investigated patients compared to age-matched control subjects, the intensity of the immunoreactivity was increased. Previously published patients with SLC19A3 mutations have a milder clinical phenotype, no laboratory evidence of mitochondrial dysfunction and more limited lesions on magnetic resonance imaging. In some, cerebral atrophy has been reported. The identification of this new, severe, lethal phenotype characterized by subtotal brain degeneration broadens the phenotypic spectrum of SLC19A3 mutations. Recognition of the associated magnetic resonance imaging pattern allows a fast diagnosis in affected infants.

Leigh syndrome is an early onset, often fatal progressive neurodegenerative disorder caused by mutations in the mitochondrial or nuclear DNA. Until now, mutations in more than 35 genes have been reported to cause Leigh syndrome, indicating an extreme genetic heterogeneity for this disorder, but still only explaining part of the cases. The possibility of whole exome sequencing enables not only mutation detection in known candidate genes, but also the identification of new genes associated with Leigh syndrome in small families and isolated cases. Exome sequencing was combined with homozygosity mapping to identify the genetic defect in a Moroccan family with fatal Leigh syndrome in early childhood and specific magnetic resonance imaging abnormalities in the brain. We detected a homozygous nonsense mutation (c.20C>A; p.Ser7Ter) in the thiamine transporter SLC19A3. In vivo overexpression of wild-type SLC19A3 showed an increased thiamine uptake, whereas overexpression of mutant SLC19A3 did not, confirming that the mutation results in an absent or non-functional protein. Seventeen additional patients with Leigh syndrome were screened for mutations in SLC19A3 using conventional Sanger sequencing. Two unrelated patients, both from Moroccan origin and one from consanguineous parents, were homozygous for the same p.Ser7Ter mutation. One of these patients showed the same MRI abnormalities as the patients from the first family. Strikingly, patients receiving thiamine had an improved life-expectancy. One patient in the third family deteriorated upon interruption of the thiamine treatment and recovered after reinitiating. Although unrelated, all patients came from the province Al Hoceima in Northern Morocco. Based on the recombination events the mutation was estimated to have occurred 1250-1750 years ago. Our data shows that SLC19A3 is a new candidate for mutation screening in patients with Leigh syndrome, who might benefit from high doses of thiamine and/or biotin. Especially, Moroccan patients with Leigh syndrome should be tested for the c.20C>A founder mutation in SLC19A3.

In a search for pathogenic factors that might play roles in the selective vulnerability of brain regions to the lesions of Leigh's disease, archival material from 20 cases of this condition, dying between 1975 and 1992 and aged from 4 days to 11.75 years at death, have been examined. Attention was paid to the topography of the lesions, their nature and timing in the evolution of the disease, the clinico-pathological correlations and the ages of the subjects at onset and at death. The following observations would appear to be explicable in terms of the present understanding that impairment of cellular energy generation is known to be defective in some, and probably all, cases. (i) The characteristic lesion of this disease is symmetrical vasculonecrotic damage affecting several brainstem centres, the topography of which is variable and may partly depend upon the age of the individual. (ii) Early features of this lesion are indistinguishable from a small partial infarction and progress similarly. The size of the damaged area is generally related to the size of the region affected. There is no haemorrhagic component and haemosiderin is not at any time found, unlike the lesions of Wernicke's disease. (iii) The process is episodic and total tissue damage is thus cumulative. More than one episode of damage may be seen in a region, changes of clearly different ages being often present together. (iv) In some regions the lesions appear to be age dependent, e.g. inferior olivary nuclei, and may be related to behavioral development and neuronal activity. Other regions show damage at any age, e.g. substantia nigra. (v) Myelin and sometimes axon loss in optic pathways is usually central, the periphery being spared. This occurred in more than half the cases and may represent a partial infarct-like change. (vi) The characteristic dorsal spinal column degeneration is always associated with focal necrosis of central grey and white matter; this also resembles a partial infarction with secondary ascending degeneration. (vii) Massive myelin loss in the centra semiovalia occurred in one-third of the cases, with or without cavitation, often in association with spongy myelin changes elsewhere. A mild general spongy change in myelin alone occurred in two cases. The massive lesions are focal, infarct-like and analogous to Binswanger's disease. (viii) Selective neuronal loss, common in some mitochondrial disorders, is not a major feature of Leigh's disease.(ABSTRACT TRUNCATED AT 400 WORDS)

Sural nerves were examined in 3 childhood cases of Leigh's disease (from 2 families), each with electrophysiological documentation of peripheral neuropathy. Postmortem confirmation of Leigh's disease was made in 2 cases; the third had characteristic CT scan appearances. Myelinated fibre densities were within normal limits, with no evidence of degeneration or regeneration. The myelin sheaths, however, appeared thin and this was confirmed quantitatively. In teased fibres there was no demyelination in 2 cases but some in 1 case. The findings suggest that there is hypomyelination of peripheral nerves leading eventually to demyelination.