Experiment
The C57BL/6J (B6) strain is a well-known mouse model of late-onset age related hearing loss and develops severe hearing loss with respect to stimuli at mid frequencies (1-10 kHz) at 9-15 months of age, stimuli at high frequencies (10-20 kHz) at 4-10 months of age, and stimuli at ultra-sonic frequencies (<20 kHz) at 2-6 months of age. Contrastingly, the DBA/2J (D2) strain shows early-onset progressive hearing loss (PHL). The hearing loss of D2 mice rapidly progresses to the low frequencies from the ultra-sonic frequencies and to profound levels at approximately 8 months of age.
In the current study hearing abilities were measured in B6, D2, F1 and N2 mice using an auditory brain stem response (ABR) workstation to test the mice for ABR thresholds. The thresholds from both ears in all mice were measured using a tone tip stimulus at 8, 16 and 32 kHz. Data for the ABR thresholds from the F1 and N2 populations suggested that the QTL responsible for hearing loss in D2 mice differed according to sound frequency.
((DBA/2Jcl x C57BL/6Jcl) F1 x DBA/2Jcl) backcrossed progeny were used to map the susceptibility loci in D2 mice linked to early onset age related hearing loss. DNA samples from 90 N2 mice were genotyped using 103 polymorphic microsatellite markers located throughout the mouse genome, based on size variation between the PCR products from D2 and B6 mice. Linkage mapping was performed by QTL interval mapping using the R/qtl program of the R Statistical package to perform single-QTL genome scans by Haley-Knott regression using a step size of 1.0 cM. The logarithm of odds (LOD) scores for significance were calculated by 1,000 permutations testing using.
QTL linkage analysis revealed a QTL on Chromosome 11 with significant LOD scores of 5.01 at 8kHz and 8.84 at 16kHz. The authors state this locus (D11Mit103) was in the same position as the previously mapped QTL Ahl8, indicating a strong effect of the homozygous Fscn2Ahl8 allele on hearing loss in D2 mice. Most N2 mice exhibited elevated ABR thresholds with the homozygous D2 allele at D11Mit103.
QTL peaks were revealed on Chromosome 5 with significant LOD scores from 2.80-3.91 at 12 markers located approximately within 52.7-126.1 Mb by analysis using ABR thresholds at 16 kHz in D2 mice, Table 1:
QTL Ahl10 (age related hearing loss, early onset 10) mapped to Chr 5 peaking at marker D5Mit233 (57.0 Mb) with a LOD score of 3.91.
QTL Ahl11 (age related hearing loss, early onset 11) mapped to Chr 5 peaking at marker D5Mit300 with a LOD score of 3.71.
QTL Ahl12 (age related hearing loss, early onset 12) mapped to Chr 5 peaking at marker D5Mit255 with a LOD score of 2.80.
QTL Ahl13 (age related hearing loss, early onset 13) mapped to Chr 5 peaking at marker D5Mit336 with a LOD score of 3.00.
QTL Ahl14 (age related hearing loss, early onset 14) mapped to Chr 5 peaking at marker D5Mit113 with a LOD score of 3.52.
QTL Ahl15 (age related hearing loss, early onset 15) mapped to Chr 5 peaking at marker D5Mit309 with a LOD score of 3.34.
QTL Ahl16 (age related hearing loss, early onset 16) mapped to Chr 5 peaking at marker D5Mit7 (93.3 Mb) with a LOD score of 3.78.
QTL Ahl17 (age related hearing loss, early onset 17) mapped to Chr 5 peaking at marker D5Mit155 with a LOD score of 3.58.
QTL Ahl18 (age related hearing loss, early onset 18) mapped to Chr 5 peaking at marker D5Mit10 with a LOD score of 3.50.
QTL Ahl19 (age related hearing loss, early onset 19) mapped to Chr 5 peaking at marker D5Mit24 with a LOD score of 2.98.
QTL Ahl20 (age related hearing loss, early onset 20) mapped to Chr 5 peaking at marker D5Mit95 with a LOD score of 3.46.
QTL Ahl21 (age related hearing loss, early onset 21) mapped to Chr 5 peaking at marker D5Mit214 (126.1 Mb) with a LOD score of 3.89.
These QTL were primarily detected as two comparatively sharp curves at peak LOD scores of markers D5Mit233 and D5Mit214 and a gently sloping curve at peak LOD score of marker D5Mit7. Results suggested that the DD homozygous alleles contributed to hearing loss at the Chr 5 loci.
Genes on Chromosome 5 with known relationships with the development and maintenance of stereocilia were screened as candidates that could be responsible for hearing loss in D2 mice. Gene Fscn1 was a strong functional candidate gene because it is a paralogous gene of Fsnc2 and is abundant in stereocillia bundles. Actb was also a functional candidate gene. Actb-flox Foxg1-cre mice display age related degeneration of stereocilia similar to the short stereocillia phenotypes in D2 mice.
Analysis Tools