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Maternally inherited hearing impairment
1Department of Medical Genetics, University of Antwerp, Belgium. gv-camp@uia.ac.be
Abstract:
Mitochondria are intracellular organelles responsible for the majority of a cell's energy production. They have their own small maternally inherited genome which, when mutated, can give rise to a large spectrum of diseases. The phenotype most commonly includes neurological and muscular symptoms, although hearing impairment is an additional feature in some mitochondrial syndromes. Often, syndromic mutations affect only a fraction of all mitochondrial DNA molecules, a condition referred to as heteroplasmy. It is believed that the degree of heteroplasmy in different tissues contributes to the phenotypic heterogeneity that is a hallmark of these syndromes. Five homoplasmic mutations leading to nonsyndromic hearing impairment have been reported (1555A-->G, 7445A-->G, 7472insC, 7510T-->C, 7511T-->C). The 1555A-->G is in the 12S rRNA gene, and in some populations, appears to be a frequent cause of hearing impairment. Carriers of the mutation are abnormally sensitive to aminoglycoside-induced ototoxicity even at 'appropriate' drug levels; in addition, even without aminoglycoside exposure, these persons can develop hearing impairment. The other four nonsyndromic mutations are located in the tRNA(Ser(UCN)) gene. In addition to hearing impairment, with two of these mutations (7445A-->G, 7472insC), other symptoms can be present in some patients. However, why these five mutations preferentially affect the inner ear, despite the crucial role of mitochondria in nearly all cells of the body, is unknown.
Insights
Mitochondrial DNA mutations cause a range of diseases, including hearing loss. Five specific mutations are linked to nonsyndromic hearing impairment, but the reason for inner ear targeting remains unclear.
Area of Science:
- Genetics
- Cell Biology
- Otolaryngology
Background:
- Mitochondria generate cellular energy and possess their own genome.
- Mutations in mitochondrial DNA (mtDNA) can lead to diverse diseases, often affecting neurological and muscular systems.
- Hearing impairment is a known feature of some mitochondrial disorders, and mtDNA mutations are implicated.
Purpose of the Study:
- To investigate the role of specific mitochondrial DNA mutations in nonsyndromic hearing impairment.
- To understand the genetic basis of hearing loss associated with mitochondrial dysfunction.
- To explore the reasons behind the preferential impact of certain mtDNA mutations on the inner ear.
Main Methods:
- Analysis of reported homoplasmic mutations in mitochondrial DNA associated with hearing loss.
- Review of genetic databases and literature for mutations in the 12S rRNA and tRNA(Ser(UCN)) genes.
- Examination of clinical phenotypes associated with identified mtDNA mutations.
Main Results:
- Five homoplasmic mtDNA mutations (1555A-->G, 7445A-->G, 7472insC, 7510T-->C, 7511T-->C) are linked to nonsyndromic hearing impairment.
- The 1555A-->G mutation in the 12S rRNA gene is a frequent cause of hearing loss in some populations and confers sensitivity to aminoglycoside ototoxicity.
- Four other mutations in the tRNA(Ser(UCN)) gene also cause hearing impairment, with some associated with additional symptoms.
Conclusions:
- Specific mitochondrial DNA mutations are significant contributors to nonsyndromic hearing impairment.
- The 1555A-->G mutation highlights the link between mtDNA, aminoglycoside sensitivity, and hearing loss.
- The underlying mechanism by which these mtDNA mutations selectively affect the inner ear requires further investigation.