Faculty Bibliography

Sensorineural hearing loss is one of the most common disabilities in humans. It is estimated that about 278 million people worldwide have slight to extreme hearing loss in both ears, which results in an economic loss for the country and personal loss for the individual. It is thus critical to have a deeper understanding of the causes for hearing loss to better manage and treat the affected individuals. The mouse serves as an excellent model to study and recapitulate some of these phenotypes, identify new genes which cause deafness, and to study their roles in vivo and in detail. Mutant mice have been instrumental in elucidating the function and mechanisms of the inner ear. The development and morphogenesis of the inner ear from an ectodermal layer into distinct auditory and vestibular components depends on well-coordinated gene expression and well-orchestrated signaling cascades within the otic vesicle and interactions with surrounding layers of tissues. Any disruption in these pathways can lead to hearing impairment. This review takes a look at some of the genes and their corresponding mice mutants that have shed light on the mechanism governing hearing impairment (HI) in humans.

The inner ear is one of the most complex and detailed organs in the vertebrate body and provides us with the priceless ability to hear and perceive linear and angular acceleration (hence maintain balance). The development and morphogenesis of the inner ear from an ectodermal thickening into distinct auditory and vestibular components depends upon precise temporally and spatially coordinated gene expression patterns and well orchestrated signaling cascades within the otic vesicle and upon cellular movements and interactions with surrounding tissues. Gene loss of function analysis in mice has identified homeobox genes along with other transcription and secreted factors as crucial regulators of inner ear morphogenesis and development. While otic induction seems dependent upon fibroblast growth factors, morphogenesis of the otic vesicle into the distinct vestibular and auditory components appears to be clearly dependent upon the activities of a number of homeobox transcription factors. The Pax2 paired-homeobox gene is crucial for the specification of the ventral otic vesicle derived auditory structures and the Dlx5 and Dlx6 homeobox genes play a major role in specification of the dorsally derived vestibular structures. Some Micro RNAs have also been recently identified which play a crucial role in the inner ear formation.

A tissue-specific transgenic model was employed to test the effects of intron and vector sequences on transgene expression in zebrafish after microinjection. In this model, the 2.3 kb promoter taken from the 5' upstream region of the transcription initiation site of keratin 4 (krt4) was used to drive the enhanced green fluorescence protein (EGFP) reporter gene in a transgenic vector. For assaying the strength of EGFP expression, the effects of including an intron before the EGFP coding region or using different forms of DNA, including circular plasmid, linear full-length plasmid, and the linear transgene coding region without any prokaryotic vector sequence, were tested. After microinjection, the transgene expression was analyzed using transient assays. Consequently, further comparative analysis supported by Fisher's exact test was performed based on the data generated by analyzing the strength of the transgene expression. It was shown that inclusion of an intron in the construct increases the transgene expression in a transient transgenic zebrafish assay. Furthermore, the circular plasmid containing the transgene produced the strongest EGFP expression.