Myosin VI Haploinsufficiency Reduced Hearing Ability in Mice

Targeted Next-Generation Sequencing Analysis Reveals a Novel Genetic Variant in MYO6 Gene in an Indian Family with Postlingual Nonsyndromic Hearing Loss

Background: Hereditary nonsyndromic hearing loss (NSHL) is an extremely heterogeneous disorder, both genetically and clinically. Myosin VI (MYO6) pathogenic variations have been reported to cause both prelingual and postlingual forms of NSHL. Postlingual autosomal dominant cases are often overlooked for genetic etiology in clinical setups. In this study, we used next-generation sequencing (NGS)-based targeted deafness gene panel assay to identify the cause of postlingual hearing loss in an Indian family. Methods: The proband and his father from a multigenerational Indian family affected by postlingual hearing loss were examined via targeted capture of 129 deafness genes, after excluding gap junction protein beta 2 (GJB2) pathogenic variants by Sanger sequencing. NGS data analysis and co-segregation of the candidate variants in the family were carried out. The variant effect was predicted by in silico tools and interpreted following American College of Medical Genetics and Genomics-Association for Molecular Pathology guidelines. Results: A novel heterozygous transversion c.3225T>G, p.(Tyr1075*) in MYO6 gene was identified as the disease-causing variant in this family. This stop-gained variant is predicted to form a truncated myosin VI protein, which is devoid of crucial cargo-binding domain. PCR-RFLP screening in 200 NSHL cases and 200 normal-hearing controls showed the absence of this variant indicating its de novo nature in the population. Furthermore, we reviewed MYO6 variants reported from various populations to date. Conclusions: To the best of our knowledge, this is the first family with MYO6-associated hearing loss from an Indian population. The study also highlights the importance of deafness gene panels in molecular diagnosis of GJB2-negative pedigrees, contributing to genetic counseling in the affected families.

Pathophysiology of human hearing loss associated with variants in myosins

Deleterious variants of more than one hundred genes are associated with hearing loss including MYO3A, MYO6, MYO7A and MYO15A and two conventional myosins MYH9 and MYH14. Variants of MYO7A also manifest as Usher syndrome associated with dysfunction of the retina and vestibule as well as hearing loss. While the functions of MYH9 and MYH14 in the inner ear are debated, MYO3A, MYO6, MYO7A and MYO15A are expressed in inner ear hair cells along with class-I myosin MYO1C and are essential for developing and maintaining functional stereocilia on the apical surface of hair cells. Stereocilia are large, cylindrical, actin-rich protrusions functioning as biological mechanosensors to detect sound, acceleration and posture. The rigidity of stereocilia is sustained by highly crosslinked unidirectionally-oriented F-actin, which also provides a scaffold for various proteins including unconventional myosins and their cargo. Typical myosin molecules consist of an ATPase head motor domain to transmit forces to F-actin, a neck containing IQ-motifs that bind regulatory light chains and a tail region with motifs recognizing partners. Instead of long coiled-coil domains characterizing conventional myosins, the tails of unconventional myosins have various motifs to anchor or transport proteins and phospholipids along the F-actin core of a stereocilium. For these myosins, decades of studies have elucidated their biochemical properties, interacting partners in hair cells and variants associated with hearing loss. However, less is known about how myosins traffic in a stereocilium using their motor function, and how each variant correlates with a clinical condition including the severity and onset of hearing loss, mode of inheritance and presence of symptoms other than hearing loss. Here, we cover the domain structures and functions of myosins associated with hearing loss together with advances, open questions about trafficking of myosins in stereocilia and correlations between hundreds of variants in myosins annotated in ClinVar and the corresponding deafness phenotypes.

D-Galactose and Hypoxia Induce the Early Onset of Age-Related Hearing Loss Deterioration in a Mouse Model

BACKGROUND

We previously showed that aging accelerates after 3 months of exposure to hypoxia and environmental change but not genetic modifications. Here, we aimed to simply induce early-onset age-related hearing loss within a short period based on our previous method.

METHODS

We randomly divided 16 C57BL/6 mice into four groups that were maintained under conditions of normoxia and hypoxia with or without injected D-galactose for 2 months. Deteriorated hearing, the expression of age-related factors, and oxidative stress responses were detected using the click and tone burst auditory brainstem response test, reverse transcription-polymerase chain reaction, and by measuring superoxide dismutase (SOD).

RESULTS

The group maintained under hypoxia combined with D-galactose lost hearing particularly at 24 Hz and 32 Hz at 6 weeks compared with the other groups. Aging-related factors were also significantly decreased in the hypoxia and D-galactose groups. However, SOD levels did not significantly differ among the groups.

CONCLUSION

Age-related hearing loss is an environmental disorder induced by chronic oxidative stress associated with genetic backgrounds. Our findings suggested that D-galactose and hypoxia can induce the phenotypes of age-related hearing loss and aging-associated molecules in a murine model within a short time with environmental stimulation alone.

Two Loci Contribute to Age-Related Hearing Loss Resistance in the Japanese Wild-Derived Inbred MSM/Ms Mice

An MSM/Ms strain was established using Japanese wild mice, which exhibit resistance to several phenotypes associated with aging, such as obesity, inflammation, and tumorigenesis, compared to common inbred mouse strains. MSM/Ms strain is resistant to age-related hearing loss, and their auditory abilities are sustained for long durations. The age-related hearing loss 3 (ahl3) locus contributes to age-related hearing in MSM/Ms strain. We generated ahl3 congenic strains by transferring a genomic region on chromosome 17 from MSM/Ms mice into C57BL/6J mice. Although C57BL/6J mice develop age-related hearing loss because of the ahl allele of the cadherin 23 gene, the development of middle- to high-frequency hearing loss was significantly delayed in an ahl3 congenic strain. Moreover, the novel age-related hearing loss 10 (ahl10) locus associated with age-related hearing resistance in MSM/Ms strain was mapped to chromosome 12. Although the resistance effects in ahl10 congenic strain were slightly weaker than those in ahl3 congenic strain, slow progression of age-related hearing loss was confirmed in ahl10 congenic strain despite harboring the ahl allele of cadherin 23. These results suggest that causative genes and polymorphisms of the ahl3 and ahl10 loci are important targets for the prevention and treatment of age-related hearing loss.