GJB2, the primary gene responsible for DFNB1, the most prevalent non-syndromic hearing loss (NSHL), has variants that account for over 50% of all prelingual hearing loss (HL). Mice are the main model for congenital hearing loss (CHL) research, but they have delayed auditory maturation postnatally, and unconditional Gjb2 knockout in mice causes embryonic lethality. Pigs have similar inner-ear anatomy to humans and, like humans, have matured auditory function and fully differentiated cochlea at birth. Currently, there is no GJB2 unconditional knockout animal model for GJB2-related CHLs research, and whether unconditional GJB2 deletion causes embryonic lethality in pigs or if GJB2-deficient pigs can recapitulate typical clinical pathological characteristics remains unclear. In this study, we employed CRISPR/Cas9 to establish the first unconditional GJB2 knockout pig model. The mutant GJB2 alleles in the founder pig were stably germline-transmitted to subsequent generations. Homozygous GJB2 knockout pigs exhibited no embryonic lethality and showed profound hearing loss, cochlear hair cell depletion and impaired Organ of Corti's development. This GJB2 unconditional knockout pig model has not been reported before and demonstrates GJB2 mutation pathological characteristics consistent with clinical patients, validating its potential in investigating the pathogenic mechanisms and therapeutic interventions of GJB2-deficient CHLs.

Heimler syndrome is a rare autosomal recessive disorder at the mild end of the peroxisomal biogenesis disorders (PBDs), characterized by sensorineural hearing loss, amelogenesis imperfecta, and retinal dystrophy. Nail abnormalities affect a minority. We present a 67-year-old woman diagnosed with non-syndromic retinitis pigmentosa in her fifties, who was later found to carry compound heterozygous variants in the PEX6 gene. Her medical history included prelingual hearing loss, early tooth decay, and brittle nails that were deemed unrelated for decades. Hearing loss and vision loss remained relatively stable up to last consultation. Our literature review includes 46 published Heimler syndrome cases with confirmed molecular diagnoses. Retinal dystrophy, predominantly of the rod-cone type with pigment clumping, was present in 89% of reported cases, with macular edema noted in 40%. Serum peroxisomal metabolite abnormalities seem to correlate with worse neurodevelopmental outcomes. There are no clear genotype-phenotype correlations, although residual peroxisomal function due to the presence of at least one missense, leaky splice site, or stable truncated allele explains the relatively mild phenotype for PBD. We underscore the importance to include syndromic disorders in the differential diagnosis of retinal dystrophy in older adults.

Non-syndromic sensorineural hearing loss (NSHL) significantly affects quality of life and is often associated with the MT-RNR1 A1555G variant. This meta-analysis investigated the global prevalence of the A1555G variant, considering factors such as age of onset and aminoglycoside exposure. A systematic review of 97 studies published between 2000 and the present included 31,013 participants. The overall prevalence of the A1555G variant was 3.37 %, with higher rates in East Asia. Subgroup analysis revealed variant frequencies of 7.24 % in postlingual deafness cases and 1.45 % in prelingual cases. Familial cases and those with aminoglycoside exposure showed significantly higher prevalence rates (9.2 % vs. 1.9 %). These findings underscore the variant's critical role in NSHL etiology and the necessity of incorporating genetic screening into clinical practices, especially for patients with aminoglycoside exposure.

Hearing loss exhibits significant clinical and genetic heterogeneity. More than 50% of Hearing loss cases have a genetic etiology. In terms of genetics,, hearing loss can be classified as either syndromic or non-syndromic. It has been demonstrated that over 100 genes and 1,000 associated mutations are involved in hearing loss that can be inherited through autosomal recessive, autosomal dominant, X-linked, or mitochondrial mechanisms This vast genetic heterogeneity has posed a significant challenge for genetic researchers in identifying the specific mutated gene in affected individuals from diverse ethnic backgrounds. However, recent advancements in next-generation sequencing technologies, particularly whole-exome sequencing (WES), have facilitated the identification of mutated genes in individuals with deafness. The primary objective of this study was to employ whole-exome sequencing (WES) to ascertain the genetic underpinnings of non-syndromic hearing loss in a Kurdish consanguineous family and to examine the associated clinical manifestations of the identified genetic mutation. A cohort of fifteen affected (fourteen with prelingual and one with postlingual hearing loss) and fifteen unaffected individuals from a Kurdish family was enrolled in this study. A comprehensive evaluation was conducted, encompassing meticulous physical examinations and audiometric assessments, to ascertain the presence of hearing impairment among the affected participants. Genomic DNA was extracted from blood samples and subjected to whole-exome sequencing. Subsequent variant identification and annotation were conducted to identify potential pathogenic mutations. To corroborate the finding of whole-exome sequencing (WES), a polymerase chain reaction (PCR) was performed on the flanking region encompassing the identified variant. Subsequent Sanger sequencing of the PCR product verified the presence of the WES-derived variant. The variant was than investigated in additional affected families through Sanger sequencing and restriction fragment length polymorphism (RFLP)-PCR analysis. A thorough analysis of whole-exome sequencing data led to the identification of a pathogenic c.1180 C > T variant (NM_198159.3) in the MITF gene, which is likely to be a causative factor for non-syndromic hearing loss in this family. This particular nucleotide substitution leads to the formation of a premature stop codon at amino acid position 394 (p. Arg394Ter, NP_937802.1) of the MITF protein. It is predicted that this will result in a truncated and potentially non-functional protein product. The identified pathogenic variant was detected in a heterozygous state in 13 of the affected individuals, which is consistent with an autosomal dominant inheritance pattern. However, the pathogenic variant was also detected in a homozygous state in 2 individuals. Also, in examining the clinical manifestations of this mutation, no notable differences were observed between homozygous and heterozygous individuals. The c.1180C>T variant in MITF (NM_198159.3), previously reported in ClinVar (Variation ID: 995923) as pathogenic for Waardenburg syndrome type 2A, was identified. Unlike prior reports associating this variant with a broad spectrum of symptoms, including pigmentation abnormalities, our study found it to be linked solely to hearing loss in this population. Notably, no differences in clinical manifestations were observed between homozygous and heterozygous individuals, suggesting population-specific factors may influence the phenotypic expression of this variant.

Methionine sulfoxide reductases (MSRs) are enzymes responsible for catalyzing the reduction of methionine sulfoxides. We previously demonstrated that variants in human MSRB3, an MSR family member, are associated with profound autosomal recessive prelingual non-syndromic deafness, DFNB74. To better understand the role of MSRB3 in the auditory pathway, we generated complete Msrb3 gene knockout mice. The Msrb3-deficient mice showed profound deafness by postnatal day 16, which was accompanied by morphological abnormalities including altered stereocilia bundle shape and cuticular plate degeneration, followed by hair cell apoptotic death. Although the absence of MSRB3 primarily affected the actin cytoskeleton, rootlets were present, and the localization of major F-actin stereocilia-core proteins was unaltered. Biochemical assays demonstrated that wild-type MSRB3, but not MSRB3 harboring p.Cys89Gly, the same variant reported for DFNB74, can repolymerize oxidized actin. Consistent with these results, we observed a decreased ratio of reduced/total actin in the inner ears of Msrb3 knockout mice. These data suggest a protective role for MSRB3 in the maintenance and maturation of stereocilia and hair cells, a conserved mechanism aimed at maintaining actin redox dynamics in these sensory cells.