Inherited mitochondrial variants are not a major cause of age-related hearing impairment in the European population

Pathology and mechanisms of cochlear aging

Presbycusis, or age-related hearing loss (ARHL), occurs in most mammals with variations in the age of onset, rate of decline, and magnitude of degeneration in the central nervous system and inner ear. The affected cochlear structures include the stria vascularis and its vasculature, spiral ligament, sensory hair cells and auditory neurons. Dysfunction of the stria vascularis results in a reduced endocochlear potential. Without this potential, the cochlear amplification provided by the electro-motility of the outer hair cells is insufficient, and a high-frequency hearing-loss results. Degeneration of the sensory cells, especially the outer hair cells also leads to hearing loss due to lack of amplification. Neuronal degeneration, another hallmark of ARHL, most likely underlies difficulties with speech discrimination, especially in noisy environments. Noise exposure is a major cause of ARHL. It is well-known to cause sensory cell degeneration, especially the outer hair cells at the high frequency end of the cochlea. Even loud, but not uncomfortable, sound levels can lead to synaptopathy and ultimately neuronal degeneration. Even in the absence of a noisy environment, aged cells degenerate. This pathology most likely results from damage to mitochondria and contributes to degenerative changes in the stria vascularis, hair cells, and neurons. The genetic underpinnings of ARHL are still unknown and most likely involve various combinations of genes. At present, the only effective strategy for reducing ARHL is prevention of noise exposure. If future strategies can improve mitochondrial activity and reduce oxidative damage in old age, these should also bring relief.

Age-related hearing loss: Unraveling the pieces

Age-related hearing loss (ARHL) is the most common cause of hearing loss in the world. The development of ARHL in each individual is multifactorial, involving both intrinsic and extrinsic factors. This review highlights several of the key findings in the ARHL literature and discusses future directions.

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Age-related hearing impairment and the triad of acquired hearing loss

Understanding underlying pathological mechanisms is prerequisite for a sensible design of protective therapies against hearing loss. The triad of age-related, noise-generated, and drug-induced hearing loss displays intriguing similarities in some cellular responses of cochlear sensory cells such as a potential involvement of reactive oxygen species (ROS) and apoptotic and necrotic cell death. On the other hand, detailed studies have revealed that molecular pathways are considerably complex and, importantly, it has become clear that pharmacological protection successful against one form of hearing loss will not necessarily protect against another. This review will summarize pathological and pathophysiological features of age-related hearing impairment (ARHI) in human and animal models and address selected aspects of the commonality (or lack thereof) of cellular responses in ARHI to drugs and noise.

Progress and prospects in human genetic research into age-related hearing impairment

Age-related hearing impairment (ARHI) is a complex, multifactorial disorder that is attributable to confounding intrinsic and extrinsic factors. The degree of impairment shows substantial variation between individuals, as is also observed in the senescence of other functions. This individual variation would seem to refute the stereotypical view that hearing deterioration with age is inevitable and may indicate that there is ample scope for preventive intervention. Genetic predisposition could account for a sizable proportion of interindividual variation. Over the past decade or so, tremendous progress has been made through research into the genetics of various forms of hearing impairment, including ARHI and our knowledge of the complex mechanisms of auditory function has increased substantially. Here, we give an overview of recent investigations aimed at identifying the genetic risk factors involved in ARHI and of what we currently know about its pathophysiology. This review is divided into the following sections: (i) genes causing monogenic hearing impairment with phenotypic similarities to ARHI; (ii) genes involved in oxidative stress, biologic stress responses, and mitochondrial dysfunction; and (iii) candidate genes for senescence, other geriatric diseases, and neurodegeneration. Progress and prospects in genetic research are discussed.

Frequency of mitochondrial mutations in non-syndromic hearing loss as well as possibly responsible variants found by whole mitochondrial genome screening

Mutations in mitochondrial DNA (mtDNA) are reported to be responsible for the pathogenesis of maternally inherited hearing loss. Complete mtDNA sequencing may detect pathogenic mutations, but whether they are indeed pathogenic can be difficult to interpret because of normal ethnic-associated haplogroup variation and other rare variations existing among control populations. In this study, we performed systemic mutational analysis of mtDNA in 394 Japanese patients with hearing loss. Two different cohorts were analyzed in this study: Cohort 1, 254 maternally inherited patients; and Cohort 2, 140 patients with various inheritance modes. After screening of the entire mtDNA genome with direct sequencing, we evaluated the frequency of previously reported mutations and the frequency and pathogenicity of the novel variants. As a result, the 'Confirmed' mitochondrial mutations were found predominantly in Cohort 1 rather than in Cohort 2 (14.6 vs 0.7%). 1555A>G (n=23) is the most common mutation, followed by the 3243A>G (n=11) mutations. On the basis of prediction analysis, we detected 10 novel homoplasmic mitochondrial variants. After further classification, the 3595A>G and 6204A>G variants were found to be new candidate mutations possibly associated with hearing loss.

The role of mitochondria in age-related hearing loss

Age-related hearing loss (ARHL), the hearing loss associated with aging, is a vital problem in present society. The severity of hearing loss is possibly associated with the degeneration of cochlear cells. Mitochondria play a key role in the energy supply, cellular redox homeostasis, signaling, and regulation of programmed cell death. In this review, we focus on the central role of mitochondria in ARHL. The mitochondrial redox imbalance and mitochondrial DNA mutation might collaboratively involve in the process of cochlear senescence in response to the aging stress. Subsequent responses, including alteration of mitochondrial biogenesis, mitophagy, apoptosis and paraptosis, participate in the aging process from different respects.

The mitochondrion: a perpetrator of acquired hearing loss

Age, drugs, and noise are major causes of acquired hearing loss. The involvement of reactive oxygen species (ROS) in hair cell death has long been discussed, but there is considerably less information available as to the mechanisms underlying ROS formation. Most cellular ROS arise in mitochondria and this review will evaluate evidence for mitochondrial pathology in general and dysfunction of the mitochondrial respiratory chain in particular in acquired hearing loss. We will discuss evidence that different pathways can lead to the generation of ROS and that oxidative stress might not necessarily be causal to all three pathologies. Finally, we will detail recent advances in exploiting knowledge of aminoglycoside-mitochondria interactions for the development of non-ototoxic antibacterials. This article is part of a Special Issue entitled "Annual Reviews 2013".