Mutation of the POU-domain gene Brn4/Pou3f4 affects middle-ear sound conduction in the mouse

Mutagenesis of the POU-domain gene Brn4/Pou3f4 causes defects in the cochlear duct, semicircular canal, temporal bone and stapes footplate. The footplate defect suggested a middle-ear conductive component to the hearing loss associated with this mutation. This was examined by measuring velocity transfer functions at the umbo of wild type and knockout mice during sound stimulation of the tympanic membrane. When the median umbo velocity of test frequencies in the two groups were compared, the mid-range frequencies of the knockout mice showed a statistically reliable reduction in velocity (maximum of 13 dB) and high variability among animals. These results indicated that mutation of the POU-domain gene, Brn4, changed middle-ear sound conduction when measured at the umbo. The origin of the abnormal velocity response was sought by puncturing a hole in the pars flaccida (PF), and subsequently, measuring movements at the umbo and the head of the long arm of the incus. This hole permitted us to measure velocity at the tip of the incus long arm, just above the incudostapedial joint. The comparison of umbo behavior in both groups with PF perforated showed a loss of sensitivity in the mid-range frequencies of the knockout animals. A comparison of incus velocity in the two groups also exhibited a velocity reduction in the mid-range frequencies of the knockout animals. The reduction at the incus, however, was milder than observed at the umbo. The effect of the perforation in, and variability of, the knockout incus responses may have masked a more potent mid-range frequency effect. Nevertheless, evaluation of the stapes and oval window in knockout mice showed variable pathology from ear to ear. The presence of this pathology, the mid-frequency loss in incus sensitivity and the variability in incus velocity among animals suggested that abnormal stapes behavior in Brn4 deficient mice may determine the response of the ossicles, and thus account for the abnormal mid-frequency umbo behavior seen in knockout animals.