Gene therapy: an emerging therapy for hair cells regeneration in the cochlea

Abstract

Sensorineural hearing loss is typically caused by damage to the cochlear hair cells (HCs) due to external stimuli or because of one's genetic factors and the inability to convert sound mechanical energy into nerve impulses. Adult mammalian cochlear HCs cannot regenerate spontaneously; therefore, this type of deafness is usually considered irreversible. Studies on the developmental mechanisms of HC differentiation have revealed that nonsensory cells in the cochlea acquire the ability to differentiate into HCs after the overexpression of specific genes, such as Atoh1, which makes HC regeneration possible. Gene therapy, through in vitro selection and editing of target genes, transforms exogenous gene fragments into target cells and alters the expression of genes in target cells to activate the corresponding differentiation developmental program in target cells. This review summarizes the genes that have been associated with the growth and development of cochlear HCs in recent years and provides an overview of gene therapy approaches in the field of HC regeneration. It concludes with a discussion of the limitations of the current therapeutic approaches to facilitate the early implementation of this therapy in a clinical setting.

Keywords: Atoh1; gene therapy; hair cell regeneration; inner ear; sensorineural hearing loss.

Figure 1

Schematic diagram of HC development process. Regulatory factors Atoh1 and Wnt signaling pathways are necessary for presensory cells to differentiate into initial HCs. Atoh1 downstream targeting factors (Pouf4, Gfi1, Barhl3) and Foxg1 play essential roles in nascent HCs maturation and long-term maintenance. At the same time, Notch signal pathway can inhibit the expression of Atoh1 in presensory cells and regulate the differentiation of HCs.

Figure 2

Changes in gene expression with age in a mouse model of inner ear development. During mouse embryonic cochlea development, Atoh1 and its downstream target transcription factors Pou4f3, Gfi1, and Barhl1 were successively expressed, with Atoh1, Pou4f3, and Gfi1 decreasing in expression after birth as the cochlea matured, and Barhl1 continuing to be expressed.

Figure 3

Gene therapy promotes HCs regeneration. HCs regenerate mainly through two pathways, direct transdifferentiation of SCs or proliferative differentiation of SCs, the difference between the two is whether mitosis is performed or not. The genes related to the growth, development, and maturation of HCs are transferred to the target cells by gene vector so that the non-sensory cells in the cochlea can differentiate into HCs and realize the regeneration of HCs.

Figure 4

CRISPR/Cas9 gene editing system. The gRNA-Cas9 complex enters the cell and identifies the target gene corresponding to the PAM and shears it into a DSB. NHEJ causes the target gene shift mutation to achieve gene knock-out. HDR, on the other hand, repairs the target gene according to the exogenous template DNA and completes the gene knock-in.