Sequence analysis of the complete mitochondrial genome in patients with Leber's hereditary optic neuropathy lacking the three most common pathogenic DNA mutations

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Abstract

Leber's hereditary optic neuropathy (LHON) is a maternally inherited disorder characterized by central vision loss in young adults. The majority of LHON cases around the world are associated with mutations in the mitochondrial genome at nucleotide positions (np) 3460, 11,778, and 14,484. Usually, these three mutations are screened in suspected LHON patients. The result is important not only in respect to the diagnosis but also as different LHON mutations lead to variations in expression, severity, and recovery of the disease. There are, however, a significant number of patients without any of these primary mutations. In these situations, genetic counselling of a patient and his family can be difficult. We sequenced the complete mitochondrial DNA (mtDNA) in 14 LHON patients with the typical clinical features but without a primary mtDNA mutation to evaluate the potential of extensive mutation screening for clinical purposes. Our results suggest to include the mutation at np 15,257 in a routine screening as well as the ND6 gene, a hot spot for LHON mutations. Screening for the secondary LHON mutations at np 4216 and np 13,708 may also help in making the diagnosis of LHON as these seem to modify the expression of LHON mutations. Although they do not allow to prove the clinical diagnosis, their presence increases the probability of LHON. Sequencing the complete mitochondrial genome can reveal novel and known rare disease causing mutations. However, considering the effort it adds little value for routine screening.

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Materials and methods

Patients. All patients included in the study showed typical clinical characteristics of LHON (Table 1). They had to fulfil the following criteria: no primary LHON mutation (np 3460, 11,778, and 14,484) found in routine screening, optic atrophy in both eyes (or, in the beginning peripapillary microangiopathy), central scotomas, vision of 0.1 or less, course of the disease of at least one year, evaluation/examination of all patients by the same clinician, and exclusion of other causes of

Results

The complete coding mitochondrial genome was analysed in 14 LHON patients (Table 2), lacking the three primary LHON mutations at np 3460, 11,778, and 14,484. Most differences relative to the reanalysed standard Cambridge sequence [6], [7] were known polymorphisms or were silent substitutions (http://www.gen.emory.edu/mitomap.html). In two patients, the rare LHON mutations 14,482 (patient #10) [9] and 14,568 (patient #1) [10], respectively, were confirmed.

The intermediate LHON mutation at np

Discussion

LHON has a strict maternal inheritance. The risk of developing the disease is inherited solely from the mother. Therefore, pathogenic mtDNA mutations are recognized as the primary cause and the predominant risk factor. But LHON is also characterized by incomplete penetrance and males are preferentially affected. So other factors seem to be required for the clinical manifestation and modification of the disease. These may include other genetic factors such as secondary mtDNA mutations, mutations

Acknowledgements

Sascha Fauser is supported by a grant from from the Federal Ministry of Education and Research (Fö. 01KS9602) and the Interdisciplinary Center of Clinical Research Tübingen (IZKF).

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