Novel mitochondrial tRNA Leu(CUN) transition and D4Z4 partial deletion in a patient with a facioscapulohumeral phenotype.

Point mutations in mtDNA-encoded tRNA genes frequently cause isolated myopathies but rarely cause the facioscapulohumeral phenotype. We report on a patient affected with chronic progressive weakness of facioscapulohumeral/peroneal muscles whose muscle biopsy showed a mitochondrial myopathy. mtDNA direct sequencing and RFLP analysis revealed a heteroplasmic transition T12313C which disrupts a conserved site in the T Psi C stem of the tRNA(Leu(CUN)) gene and fulfills the accepted criteria of pathogenicity. A partial deletion of the nuclear DNA D4Z4 region with residual repeat sizes of 25 kb was also found in the patient and in her mother. This is the first reported case of mitochondrial myopathy/facioscapulohumeral muscular dystrophy (FSHD) "double trouble".