The Role of Mitochondrial Mutations in Sensorineural Hearing Loss

Sensorineural hearing loss is often the first symptom reported in a number of mitochondrial myopathies. Neuronal tissue is very sensitive to alterations in oxidative phosphorylation which often occur in these diseases. During an investigation of thirty three patients with non-syndromic sensorineural hearing loss by RFLP (Restriction Fragment Length Polymorphism) analysis, for known mutations associated with specific mitochondrial mutations, one patient was found to have lost the XbaI site at np 7440. This patient is part of a large maternal pedigree in which 14 members suffer from some degree of sensorineural hearing loss. The entire mitochondrial DNA sequence of this patient was sequenced revealing that the XbaI site loss was due to a homoplasmic A→G change at np 7445. The nucleotide at this position has a role on both strands: part of the stop codon of the COI (Cytochrome Oxidase subunit I) gene on the major coding strand and the extreme 3' nucleotide of tRNAser(UCN) the other. The biochemical effects of this mutation were investigated in permanent cell-lines created by EBV (Epstein Barr Virus) transformation of lymphocytes from the index case, and from a normal control. In studies of lactate/pyruvate ratios, Complex II + III activity. Complex IV activity, growth with galactose as the carbon source and in the presence of chloramphenicol there was no difference between the two cell-lines. Measurement of growth in the presence of various concentrations of streptomycin was carried out twice with inconclusive results. In addition Northern blotting experiments indicated no difference in the level or length of the COI transcript in the cell-line with the np 7445 mutation. Mitochondrial translation products were measured by 35S-labelling in the presence of emetine and no differences seen in the amount or sizes of any of the labelled proteins. The entire mitochondrial DNA of a patient with LHON (Lebers Hereditary Optic Neuropathy) and sensorineural hearing loss was sequenced and found to have the so-called "classical" LHON mutation at np 11778. In addition two patients with aminoglycoside antibiotic induced deafness were found to have a previously reported, homoplasmic A→G at np 1555 in 12SrRNA. The work in this project has established that two mitochondrial DNA mutations are found in association with sensorineural hearing loss. That at np 1555 had previously been reported; that at np 7445 was novel. Both mutations are at positions of the human mitochondrial genome which have some direct involvement in mitochondrial translation