Audiologic and genetic features of the A3243G mtDNA mutation

Mitochondrial Diabetes is Associated with tRNALeu(UUR) A3243G and ND6 T14502C Mutations

BACKGROUND

Mutations in mitochondrial DNA (mtDNA) are associated with type 2 diabetes mellitus (T2DM). In particular, m.A3243G is the most common T2DM-related mtDNA mutation in many families worldwide. However, the clinical features and pathophysiology of m.A3243G-induced T2DM are largely undefined.

METHODS

Two pedigrees with maternally inherited T2DM were underwent clinical, molecular and biochemical assessments. The mtDNA genes were PCR amplified and sequenced. Mitochondrial adenosine triphosphate (ATP) and reactive oxygen species (ROS) were measured in polymononuclear leukocytes derived from three patients with both the m.A3243G and m.T14502C mutations, three patients with only the m.A3243G mutation and three controls without these mutations. Moreover, GJB2, GJB3 and GJB6 mutations were screened by PCR-Sanger sequencing.

RESULTS

Members of the two pedigrees manifestated variable clinical phenotypes including diabetes and hearing and vision impairments. The age at onset of T2DM varied from 31 to 66 years, with an average of 41 years. Mutational analysis of mitochondrial genomes indicated the presence of the m.A3243G mutation in both pedigrees. Matrilineal relatives in one of the pedigrees harbored the coexisting of m.A3243G and m.T14502C mutations. Remarkably, the m.T14502C mutation, which causes the substitution of a conserved isoleucine for valine at position 58 in ND6 mRNA, may affect the mitochondrial respiratory chain functions. Biochemical analysis revealed that cell lines bearing both the m.A3243G and m.T14502C mutations exhibited greater reductions in ATP levels and increased ROS production compared with those carrying only the m.A3243G mutation. However, we did not find any mutations in the GJB2, GJB3 and GJB6 genes.

CONCLUSION

Our study indicated that mitochondrial diabetes is associated with the tRNA^Leu(UUR) A3243G and ND6 T14502C mutations.

Audiological and Vestibular Findings in Subjects with MELAS Syndrome

OBJECTIVES

The mitochondrial DNA (mtDNA) point mutation m.3243A>G is known to express the following two syndromes among others: maternally inherited diabetes and deafness (MIDD) and mitochondrial encephalopathy, lactic acidosis, and stroke-like episodes (MELAS). Sensorineural hearing loss (SNHL) is the most frequent symptom in individuals harboring the m.3243A>G mutation. However, dysfunction of the vestibular organs has been scarcely examined. Therefore, the present study aimed to study the impact of the m.3243A>G mutation on the inner ear.

MATERIALS AND METHODS

A total of 8 subjects harboring the blood-verified m.3243A>G mutation underwent thorough audiological and vestibular examinations, including tone and speech audiometry, video head impulse test (vHIT), ocular and cervical vestibular-evoked myogenic potential (oVEMP and cVEMP), and full otoneurological examination. The subjects also answered a Dizziness Handicap Inventory (DHI) questionnaire.

RESULTS

SNHL was identified in all the 8 subjects, with a mean pure-tone average-4 (PTA-4) of 59 dB. Speech discrimination score (n=7) ranged from 24% to 100% (mean 74%), and vHIT (n=42) detected pathology in nine lateral semicircular canals (SCCs), five posterior SCCs, and one anterior SCC, whereas three measurements were inconclusive. All oVEMPs (n=14 ears) were absent, nine cVEMPs were absent, and two were inconclusive. Based on the DHI scores, 6 subjects reported none to mild dizziness, 1 reported moderate, and 1 reported severe dizziness.

CONCLUSION

Our study population had pathological findings from every audiological and vestibular end organs. The results indicated that the pathological findings originated from within the end organs themselves and not within the superior and inferior vestibular or cochlear nerve.

Progression of Peripheral Vestibular Dysfunctions in Patients With a Mitochondrial A3243G Mutation

OBJECTIVE

To evaluate the progression of peripheral vestibular dysfunction in patients with an A-to-G point mutation at nucleotide pair 3243 in mitochondrial DNA (A3243G mutation).

STUDY DESIGN

Retrospective patient series.

SETTING

Tertiary referral center.

PATIENTS

Six unrelated patients with an A3243G mutation (four mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes; and two maternally inherited diabetes and deafness; five females and one male; average age 41 ± 11.3 yr old), who underwent repeated vestibular examinations, were included (average interval between the first and second examinations: 5.7 ± 2.1 yr).

INTERVENTION

Diagnostic.

MAIN OUTCOME MEASURE

Results of caloric testing and cervical vestibular evoked myogenic potential testing in response to air-conducted sound were analyzed.

RESULTS

All the patients except one, who was already completely deaf in both ears at the first examination, showed progression of hearing loss (average 4.0 ± 6.3 dB per year). Five of the six patients had vestibular symptoms at the first examination. The other patient developed dizziness later. Caloric responses were abnormal in four patients at the first examination and in five patients at the second examination. Cervical vestibular evoked myogenic potential responses were bilaterally absent in three patients at the first examination and in five patients at the second examination.

CONCLUSIONS

The A3243G mutation causes progression of peripheral vestibular dysfunction as well as that of hearing loss.