Hearing loss is a common symptom that may be manifested by many etiologies and it is frequently associated to genetic problems. Genetic mutations may occur in nuclear or mitochondrial genes. Mitochondria are intracellular organelles that have their own genome (DNA); mitochondrial DNA is from exclusive maternal inheritance. Although mitochondrial DNA mutations derive from maternal inheritance, spontaneous mutations may also occur. The phenotype is the clinical expression of a mitochondrial mutation and depends on the amount of mutant mitochondrial DNA contained in the cell. This phenomenon is known as heteroplasmy. The mitochondria provide energy to the cells by releasing ATP; thus, the greater the amount of energy required by the cell, the more likely it is to be affected by mitochondrial DNA mutations. Examples of high metabolism cells are nervous system cells, muscle cells, endocrine cells, optical and auditory cells. The cochlea has great energy turnover and mitochondrial DNA mutations of the hair cells will cause sensorineural hearing loss, which is normally bilateral, symmetrical and progressive. Hearing loss secondary to mitochondrial DNA mutations comprises 0.5 to 1% of all genetic hearing losses. Based on the literature review, it may be observed that hearing loss secondary to mitochondrial DNA mutations manifest in two distinct forms: isolated hearing loss (nonsyndromic), as in aminoglycoside hypersensitivity and presbyacusis, or associated to other diseases in a syndrome, such as Kearns-Sayre syndrome and maternally inherited diabetes and deafness.
genetic disease; hearing loss; mitochondrial DNA
