Molecular bases of hearing loss in multi-systemic mitochondrial cytopathy

Mitochondrial Disease and Hearing Loss in Children: A Systematic Review

OBJECTIVES

Hearing loss is a clinical symptom, frequently mentioned in the context of mitochondrial disease. With no cure available for mitochondrial disease, supportive treatment of clinical symptoms like hearing loss is of the utmost importance. The aim of this study was to summarize current knowledge on hearing loss in genetically proven mitochondrial disease in children and deduce possible and necessary consequences in patient care.

METHODS

Systematic literature review, including Medline, Embase, and Cochrane library. Review protocol was established and registered prior to conduction (International prospective register of systematic reviews-PROSPERO: CRD42020165356). Conduction of this review was done in accordance with MOOSE criteria.

RESULTS

A total of 23 articles, meeting predefined criteria and providing sufficient information on 75 individuals with childhood onset hearing loss was included for analysis. Both cochlear and retro-cochlear origin of hearing loss can be identified among different types of mitochondrial disease. Analysis was hindered by inhomogeneous reporting and methodical limitations.

CONCLUSION

Overall, the findings do not allow for a general statement on hearing loss in children with mitochondrial disease. Retro-cochlear hearing loss seems to be found more often than expected. A common feature appears to be progression of hearing loss over time. However, hearing loss in these patients shows manifold characteristics. Therefore, awareness of mitochondrial disease as a possible causative background is important for otolaryngologists. Future attempts rely on standardized reporting and long-term follow-up.

LEVEL OF EVIDENCE

NA Laryngoscope, 132:2459-2472, 2022.

Mitochondrial disease in children

Mitochondrial disease presenting in childhood is characterized by clinical, biochemical and genetic complexity. Some children are affected by canonical syndromes, but the majority have nonclassical multisystemic disease presentations involving virtually any organ in the body. Each child has a unique constellation of clinical features and disease trajectory, leading to enormous challenges in diagnosis and management of these heterogeneous disorders. This review discusses the classical mitochondrial syndromes presenting most frequently in childhood and then presents an organ-based perspective including systems less frequently linked to mitochondrial disease, such as skin and hair abnormalities and immune dysfunction. An approach to diagnosis is then presented, encompassing clinical evaluation and biochemical, neuroimaging and genetic investigations, and emphasizing the problem of phenocopies. The impact of next-generation sequencing is discussed, together with the importance of functional validation of novel genetic variants never previously linked to mitochondrial disease. The review concludes with a brief discussion of currently available and emerging therapies. The field of mitochondrial medicine has made enormous strides in the last 30 years, with approaching 400 different genes across two genomes now linked to primary mitochondrial disease. However, many important questions remain unanswered, including the reasons for tissue specificity and variability of clinical presentation of individuals sharing identical gene defects, and a lack of disease-modifying therapies and biomarkers to monitor disease progression and/or response to treatment.

Association of nuclear and mitochondrial genes with audiological examinations in Iranian patients with nonaminoglycoside antibiotics-induced hearing loss

Mitochondrial DNA mutations play an important role in causing sensorineural hearing loss. The purpose of this study was to determine the association of the mitochondrial genes RNR1, MT-TL1, and ND1 as well as the nuclear genes GJB2 and GJB6 with audiological examinations in nonfamilial Iranians with cochlear implants, using polymerase chain reaction, DNA sequencing, and RNA secondary structure analysis. We found that there were no novel mutations in the mitochondrial gene 12S rRNA (MT-RNR1) in patients with and without GJB2 mutation (GJB2⁺ and GJB2⁻, respectively), but a total of six polymorphisms were found. No mutations were observed in tRNA^Leu(UUR) (MT-TL1). Furthermore, eight polymorphisms were found in the mitochondrial ND1 gene. Additionally, no mutations were observed in the nuclear GJB6 gene in patients in the GJB2⁻ and GJB2⁺ groups. The speech intelligibility rating and category of auditory perception tests were statistically assessed in patients in the GJB2⁻ and GJB2⁺ groups. The results indicated that there was a significant difference (P<0.05) between the categories of auditory perception score in the GJB2⁻ group compared to that in the GJB2⁺ group. Successful cochlear implantation was observed among individuals with GJB2 mutations (GJB2⁺) and mitochondrial polymorphisms compared to those without GJB2 mutations (GJB2⁻). In conclusion, the outcome of this study suggests that variation in the mitochondrial and nuclear genes may influence the penetrance of deafness. Therefore, further genetic and functional studies are required to help patients in making the best choice for cochlear implants.

Association between idiopathic hearing loss and mitochondrial DNA mutations: a study on 169 hearing-impaired subjects

Mutations in mitochondrial DNA (mtDNA) have been shown to be an important cause of sensorineural hearing loss (SNHL). In this study, we performed a clinical and genetic analysis of 169 hearing-impaired patients and some of their relatives suffering from idiopathic SNHL, both familial and sporadic. The analysis of four fragments of their mtDNA identified several polymorphisms, the well known pathogenic mutation, A1555G, and some novel mutations in different genes, implying changes in the aminoacidic sequence. A novel sporadic mutation in 12S rRNA (MT-RNR1), not previously reported in the literature, was found in a case of possible aminoglycoside-induced progressive deafness.

Hearing loss in children with mitochondrial disorders

OBJECTIVE

At least 1-5 children per 1000 suffer from congenital hearing loss, and 50% of these cases can be attributed to genetic causes. It has been estimated that 1% of pre-lingual hearing loss is due to mutations in mitochondrial DNA. Previous literature reports audiometric data for few patients, usually less than 20 per study. The goal of this study was to characterize the hearing loss associated with mitochondrial mutations and determine whether previously characterized patterns of hearing loss in these patients (progressive, sensorineural, high frequency losses) are found in our population as well.

METHODS

An IRB-approved retrospective chart review of the electronic medical records in the Nemours/Alfred I. dupont Hospital for Children system from January 2004 to October 2009 (a five-year period) was undertaken using ICD-9 codes 277.87 (mitochondrial disorder) and 359.89BA (mitochondrial myopathy). These 149 records were then evaluated for audiologic data, resulting in 26 charts with both a mitochondrial disorder and hearing evaluation.

RESULTS

Of 26 patients with known mitochondrial disorders and audiometric documentation, 15 (58%) had hearing loss, and 11 patients had normal hearing (42%). Ten patients had sensorineural hearing loss (38%), two patients had conductive hearing loss (7.7%), one patient had a mixed hearing loss (3.8%), and two patients had an as yet undefined hearing loss (ABR had not yet been performed at the time of this study) (7.7%).

CONCLUSION

In comparison with previous studies, generally including less than 20 patients, this is one of the largest collections of audiometric data on children with mitochondrial disorders. Unlike prior studies describing a progressive, sensorineural loss across all frequencies or mainly affecting high frequencies, the hearing loss in our patients was more variable including low frequency losses, mid-frequency losses, and conductive losses and was often not progressive or even improved. Our overall 38% rate of sensorineural hearing loss correlates well with previous case series; this study clearly justifies the use of routine audiometric screening in children with mitochondrial disorders, including use of ABR and OAEs as ASND can be seen in this population, as well as repeat testing over time to evaluate for progression.

Mitochondrial deafness

Non-syndromic deafness can be caused by mutations in both nuclear and mitochondrial genes. More than 50 nuclear genes have been shown to be involved in non-syndromic hearing loss, but mutations in mitochondrial DNA (mtDNA) might also cause hearing impairment. As mitochondria are responsible for oxidative phosphorylation, the primary energy-producing system in all eukaryotic cells, mitochondrial dysfunction has pleiotropic effects. Many mutations in mtDNA can lead to multisystem disorders, such as Kearns-Sayre syndrome, NARP, MELAS, or MERRF syndromes, the presentation of which may include hearing loss. A more specific association of mitochondrially inherited deafness and diabetes known as MIDD syndrome can be caused by a limited number of specific mitochondrial mutations. In addition, several rare mutations in the mitochondrial MTTS1 and MTRNR1 genes have been found to be responsible for non-syndromic hearing loss. The most frequent form of non-syndromic deafness is presbyacusis, affecting more than 50% of the elderly. This age-related hearing loss is a paradigm for multifactorial inheritance, involving a multitude of inherited and acquired mutations in the nuclear and mitochondrial genomes, each with a low penetrance, in complex interplay with environmental factors, such as ototoxic medication, that accumulate with age. This study reviews the different mitochondrial mutations, leading to syndromic and especially non-syndromic deafness.