Stressors to the auditory system : sound, xenobiotics, aging... Genetic aspects of individual susceptibility to these stressors

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The first lecture (February 6, 2014) focused on auditory system stressors and recent advances concerning the mechanisms involved in susceptibility to aminoglycoside-induced deafness. Alongside long-known auditory system stressors such as noise, aminoglycosides and cisplatin, opiates and opioids have recently come to the fore.

The mechanisms of cisplatin ototoxicity caught our attention. They involve, in sensory cells, a copper transporter, CTR1, and an organic cation transporter, OCT2 (Thomas et al., 2013). Cisplatin toxicity involves the auditory mechanoelectric transduction channel, although it is not known whether or not this dependence relates to cisplatin passing through this channel. Genetic susceptibility to cisplatin-related hearing loss has been associated with two genes encoding methyltransferases, thiopurine-S-methyltransferase (TPMT) and catechol-O-methyltransferase (COMT), involved in drug metabolism (Weinshilboum, 2006; Ross et al., 2009). However, the results of this pharmacogenomic study have not yet been reproduced.

Since 1992, aminoglycoside-induced hearing loss has been associated with mutations in the mitochondrial DNA that codes for the RNA of the small subunit of the mitochondrial ribosome, 12S ribosomal RNA (Prezant et al., 1992). The most frequent mutation, A1555G, is present in the homoplasmic state in around one in five hundred individuals. It is associated with various mitochondrial haplotypes. As a rule, it has no clinical translation in young children; when hearing loss occurs, it is often asymmetrical. In the Shanghai region of China, where aminoglycoside use is thought to account for 25% of hearing impairment in young people, this mutation is found in a quarter of these patients. It is also responsible for hearing loss in some individuals who are not known to have taken aminoglycosides. From the point of view of the evolution of species, mitochondria originated in bacteria, whose sensitivity to aminoglycosides in the translation of messenger RNA into proteins has long been known and understood. Aminoglycosides cause translation errors by binding to the 16S RNA of the small subunit of the bacterial ribosome. As a result, the A1555G mutation was quickly identified as being located at the aminoglycoside binding site, close to the transfer RNA (tRNA) binding site. Aminoglycoside-induced deafness is explained by the translation errors these antibiotics cause within the proteins encoded by the mitochondrial genome, i.e. 13 proteins, most of which are involved in oxidative phosphorylation. In 2002, a new mitochondrial transcription factor, TFB, was discovered (McCulloch et al., 2002), which shares with the transcription factor encoded by the bacterial KsgA gene a methyltransferase activity (using S-adenosyl-methionine as methyl donor) acting on the translation of messenger RNAs into proteins. Both factors are therefore involved in transcription and translation. TFB methylates two evolutionarily highly conserved adenines located on mitochondrial 12S ribosomal RNA in its 3' region, close to the A1555G mutation. KsgA does the same. It methylates the same two adenines on bacterial 16S ribosomal RNA. Mutations that inactivate the KsgA gene, however, protect bacteria from the effects of aminoglycosides. Two TFB factors, TFB1 and TFB2, are present in mammals. The human TFB1 factor is essentially a methyltransferase, and therefore mainly controls translation. The genomic region carrying the TFB1 gene acts as a modifier locus for the phenotypic expression of the A1555G mutation in patients. Finally, some studies indicate that the A1555G mutation leads to hypermethylation of the two adenines targeted by TFB1. All in all, the following chain of reactions seems to emerge: the mitochondrial A1555G mutation, which causes hypermethylation of the two adenines of mitochondrial 12S ribosomal RNA, affects mitochondrial protein biosynthesis.

Alongside the effect of aminoglycosides on protein translation in mitochondria, an effect exacerbated by the presence of the A1555G mutation, an effect viareactive oxygen species (ROS) has recently been reported (Raimundo et al., 2012). Alteration of mitochondrial proteins leads to an increase in ROS production, followed by activation of the proapoptotic factor E2F1 and the Rb-E2F1 proapoptotic signalling pathway, leading to cell apoptosis.

What remains to be understood is how, in this scheme, E2F1 perceives the increase in ROS. AMP kinase phosphorylates the Rb protein, which regulates the activity of the E2F1 factor. AMP kinase is itself activated by ROS-dependent phosphorylation. The proposed sequence of events, from A1555G mutation to 12S RNA hypermethylation, ROS production, and finally AMP kinase phosphorylation activating E2F1, is consistent with results obtained in transgenic mice overexpressing TFB1. These mice show increased protein expression of E2F1 in the cochlea. Its restriction to the cochlea could explain the limitation of the effect of the A1555G mutation to this organ. It should be noted that in recent years, the cochlear entry of aminoglycosides has been clarified using gentamicin coupled to a fluorescent compound, injected intraperitoneally.