LETTER TO JMG Molecular analysis of the mitochondrial 12S rRNA and tRNA genes in paediatric subjects with non- syndromic hearing loss

H earing loss is a very common congenital disorder affecting one in 1000 newborns. More than 50% of deafness cases in the paediatric population have a genetic cause with autosomal dominant, autosomal recessive, X-linked, or mitochondrial patterns of inheritance. Mutations in mitochondrial DNA (mtDNA), particularly in the 12S rRNA and tRNA genes, have been found to be one of the most important causes of sensorineural hearing loss (SNHL). 3 The homoplasmic A1555G mutation in the highly conserved decoding site of the mitochondrial 12S rRNA has been found to be associated with both aminoglycoside-induced and non-syndromic SNHL in many families of different ethnic origins. Recently, the homoplasmic C1494T mutation in the same gene has also been found to be associated with aminoglycoside-induced and non-syndromic SNHL in a large Chinese family. In addition, a C-insertion or deletion at position 961 of the 12S rRNA gene has been shown to be associated only with aminoglycoside-induced deafness. 11 Furthermore, the mitochondrial tRNA appears to be another hot spot for mutations associated with hearing impairment, as five deafness-associated mutations have been identified in the mitochondrial tRNA gene: A7445G, 13 7472insC, T7510C, T7511C, and T7512C. However, non-syndromic deafness-associated mtDNA mutations, such as the A1555G or A7445G mutation, are often not sufficient to produce the clinical phenotype since some individuals carrying these mutations have normal hearing. Thus, other factors including other mtDNA mutations/polymorphisms and/or nuclear backgrounds or environmental factors modulate the phenotypic variability and penetrance of deafness associated with these mtDNA mutations. However, little is known about the incidence of these deafness-associated mtDNA mutations. Furthermore, it is anticipated that additional deafness-associated mutations will be found in the 12S rRNA and tRNA genes. A retrospective database review of all children with SNHL and subsequent molecular analysis has been initiated in the clinical population at the Center for Hearing and Deafness Research (CHDR) at the Cincinnati Children’s Hospital Medical Center (CCHMC). In the present study, a mutational analysis of the 12S rRNA and tRNA genes was performed in 164 SNHL subjects from this clinic and 226 unaffected individuals from a comparable ethnic background. This analysis led to the identification of 17 nucleotide changes including the novel G to T transversion at position 961 (T961G) in the 12S rRNA gene in five affected subjects. To elucidate the molecular basis for the T961G mutation, we completed the mitochondrial genome sequence from these five affected subjects. To examine the role of the GJB2 gene in the phenotypic expression of the known or putative deafness-associated mtDNA mutations, we also performed mutational screening of the GJB2 gene in subjects carrying the mtDNA mutations.