Compound heterozygosity for severe and hypomorphic mutations cause non-syndromic LHON-like optic neuropathy.

BACKGROUND

Non-syndromic hereditary optic neuropathy (HON) has been ascribed to mutations in mitochondrial fusion/fission dynamics genes, nuclear and mitochondrial DNA-encoded respiratory enzyme genes or nuclear genes of poorly known mitochondrial function. However, the disease causing gene remains unknown in many families. The objective of the present study was to identify the molecular cause of non-syndromic LHON-like disease in siblings born to non-consanguineous parents of French origin.

METHODS

We used a combination of genetic analysis (gene mapping and whole-exome sequencing) in a multiplex family of non-syndromic HON and of functional analyses in patient-derived cultured skin fibroblasts and the yeast .

RESULTS

We identified compound heterozygote disease-causing mutations (p.Tyr53Cys; p.Tyr308Cys). Studies using patient-derived cultured skin fibroblasts revealed mildly decreased NDUFS2 and complex I abundance but apparently normal respiratory chain activity. In the yeast ortholog , the mutations resulted in absence of complex I and moderate reduction in nicotinamide adenine dinucleotide-ubiquinone oxidoreductase activity, respectively.

CONCLUSIONS

Biallelism for mutations causing severe complex I deficiency has been previously reported to cause Leigh syndrome with optic neuropathy. Our results are consistent with the view that compound heterozygosity for severe and hypomorphic mutations can cause non-syndromic HON. This observation suggests a direct correlation between the severity of mutations and that of the disease and further support that there exist a genetic overlap between non-syndromic and syndromic HON due to defective mitochondrial function.