CRISPR/Cas9: targeted genome editing for the treatment of hereditary hearing loss.

Hereditary hearing loss (HHL) is a neurosensory disorder that affects every 1/500 newborns worldwide and nearly 1/3 people over the age of 65. Congenital deafness is inherited as monogenetic or polygenic disorder. The delicacy, tissue heterogeneity, deep location of the inner ear down the brainstem, and minute quantity of cells present in cochlea are the major challenges for current therapeutic approaches to cure deafness. Targeted genome editing is considered a suitable approach to treat HHL since it can target defective molecular components of auditory transduction to restore normal cochlear function. With the advent of CRISPR/Cas9 technique, targeted genome editing and biomedical research have been revolutionized. The robustness and simplicity of this technology lie in its design and delivery methods. It can directly deliver a complex of Cas9 endonuclease and single guide RNA (sgRNA) into zygote using either vector-mediated stable transfection or transient delivery of ribonucleoproteins complexes. This strategy induces DNA double strand breaks (DSBs) at target site followed by endogenous DNA repairing mechanisms of the cell. CRISPR/Cas9 has been successfully used in model animals to edit hearing genes like calcium and integrin-binding protein 2, myosin VIIA, Xin-actin binding repeat containing 2, leucine-zipper and sterile-alpha motif kinase Zak, epiphycan, transmembrane channel-like protein 1, and cadherin 23. This review discusses the utility of lipid-mediated transient delivery of Cas9/sgRNA complexes, an efficient way to restore hearing in humans, suffering from HHL. Notwithstanding, challenges like PAM requirement, HDR efficiency, off-target activity, and optimized delivery systems need to be addressed.