A nuclear-mitochondrial DNA interaction affecting hearing impairment in mice

Abstract

The pathophysiologic pathways and clinical expression of mitochondrial DNA (mtDNA) mutations are not well understood. This is mainly the result of the heteroplasmic nature of most pathogenic mtDNA mutations and of the absence of clinically relevant animal models with mtDNA mutations. mtDNA mutations predisposing to hearing impairment in humans are generally homoplasmic, yet some individuals with these mutations have severe hearing loss, whereas their maternal relatives with the identical mtDNA mutation have normal hearing. Epidemiologic, biochemical and genetic data indicate that nuclear genes are often the main determinants of these differences in phenotype. To identify a mouse model for maternally inherited hearing loss, we screened reciprocal backcrosses of three inbred mouse strains, A/J, NOD/LtJ and SKH2/J, with age-related hearing loss (AHL). In the (A/J x CAST/Ei) x A/J backcross, mtDNA derived from the A/J strain exerted a significant detrimental effect on hearing when compared with mtDNA from the CAST/Ei strain. This effect was not seen in the (NOD/LtJ x CAST/Ei) x NOD/LtJ and (SKH2/J x CAST/Ei) x SKH2/J backcrosses. Genotyping revealed that this effect was seen only in mice homozygous for the A/J allele at the Ahl locus on mouse chromosome 10. Sequencing of the mitochondrial genome in the three inbred strains revealed a single nucleotide insertion in the tRNA-Arg gene (mt-Tr) as the probable mediator of the mitochondrial effect. This is the first mouse model with a naturally occurring mtDNA mutation affecting a clinical phenotype, and it provides an experimental model to dissect the pathophysiologic processes connecting mtDNA mutations to hearing loss.

Fig. 1

Frequency distribution of auditory-evoked brainstem response (ABR) thresholds in (A/J×CAST/Ei)×A/J backcross progeny. Hearing loss occurs earlier in NOD/LtJ and SKH2/J backcross mice than in A/J backcross mice, but otherwise their ABR threshold frequency distributions are similar. Mice were tested successively at ages 3 months (light bars) and 6 months (dark bars). The vertical dashed line indicates division between normal and elevated ABR thresholds. Backcross matings were set up using males and females from both types of reciprocal F₁ hybrid. Maternal strain origins in N2 backcross mice were noted.

Fig. 2

Evolutionary comparison and two-dimensional structure of mouse mt-Tr. a, Cloverleaf structure of mouse mt-Tr. The string of adenine residues in the D loop region is underlined. b, Sequence allignment of the tRNA-Arg from different species. Region corresponding to the adenines string in mouse is denoted by brackets.

Fig. 3

Comparison of age-related hearing loss among A/J, A/HeJ and A/WySnJ inbred strains of mice. Averages and standard errors of ABR threshold measurements (16 kHz stimulus) are shown for each strain and time point. The number of mice tested per strain varied from 4 to 11 at 8 weeks, 8 to 18 at 12 weeks, 3 to 8 at 18 weeks and 8 to 14 at 36 weeks.