In the current study, they have achieved the first miRNA sequencing of otic precursors in human specimens. Using HTG miRNA Whole Transcriptome assays, we examined miRNA expression in the cochleovestibular ganglion (CVG), neural crest (NC), and otic vesicle (OV) from paraffin embedded (FFPE) human specimens in the Carnegie developmental stages. Even performed gene set enrichment analyses to determine differentially regulated pathways that are relevant to CVG development in humans.
They found that in human embryonic tissues, there are different patterns of miRNA expression in the CVG, NC and OV. They further identified transcription factors that are differentially targeted in the CVG compared to the other tissues from stages. These findings not only provide insight into the mechanisms governing the development of the human inner ear, but also identify potential signaling pathways for promoting regeneration of the spiral ganglion and other components of the inner ear.
MicroRNAs (miRNA) are a class of endogenously expressed small non-coding RNAs that function in RNA silencing and post-transcriptional regulation of gene expression. The human genome encodes more than 1000 miRNAs that may target as many as 60% of human protein-encoding genes. Further, the standard use in the United States and elsewhere of suction curettage, which generally renders embryonic structures unidentifiable, now makes it difficult even to obtain human tissue for study.
Identification of miRNA-targeted genes is one of the most significant elements for understanding the function of a miRNA in the human inner ear development. Among our predicted miR-183 family targeted genes that are related to human inner ear (CVG) development, it is worthwhile to mention TBX1 (T-box domain 1), as a previous study has confirmed tbx-1 as one of target genes for the conserved miR-183 family in a mouse model. Tbx1 was required for morphogenesis and growth of the murine otocyst.
In humans, TBX1 is a critical gene in DiGeorge syndrome, with patients presenting phenotypes including inner ear abnormalities, sensorineural hearing loss and vestibular loss. Therefore, we speculate that the transcriptional regulation of TBX1 in human may be influenced by the miR-183 family, providing a crucial function in developmental signaling pathways in the human inner ear.
Differences between tissue types for a given time point could similarly be influenced by individual-specific variation. The small sample size also limits the statistical power to detect differential miRNA expression. While our findings are suitable for hypothesis generation, they should be critically considered on a case-by-case basis, bearing these limitations in mind.
In future studies, some addition to providing insight into the mechanisms governing the development of the human inner ear, our findings also identify potential signaling pathways for promoting regeneration of human spiral ganglions and other components of the inner ear using pluripotent stem cells.