Further evidence for loss-of-function mutations in the CEACAM16 gene causing nonsyndromic autosomal recessive hearing loss in humans

The cochlear matrisome: Importance in hearing and deafness

The extracellular matrix (ECM) consists in a complex meshwork of collagens, glycoproteins, and proteoglycans, which serves a scaffolding function and provides viscoelastic properties to the tissues. ECM acts as a biomechanical support, and actively participates in cell signaling to induce tissular changes in response to environmental forces and soluble cues. Given the remarkable complexity of the inner ear architecture, its exquisite structure-function relationship, and the importance of vibration-induced stimulation of its sensory cells, ECM is instrumental to hearing. Many factors of the matrisome are involved in cochlea development, function and maintenance, as evidenced by the variety of ECM proteins associated with hereditary deafness. This review describes the structural and functional ECM components in the auditory organ and how they are modulated over time and following injury.

A novel missense variant in CEACAM16 gene causes autosomal dominant nonsyndromic hearing loss

Aim

Autosomal dominant non‐syndromic hearing loss is a common sensorineural disorder with extremely high genetic heterogeneity. CEA antigen‐related cell adhesion molecule 16(CEACAM16)is a secreted glycoprotein encoded by the CEACAM16 gene. Mutations in CEACAM16 lead to autosomal dominant non‐syndromic hearing loss in humans, due defects in the tectorial membrane of the inner ear. Here we reported a novel missense variant in CEACAM16 gene causes autosomal dominant non‐syndromic hearing loss.

Material and methods

A four‐generation Chinese family affected by late‐onset and progressive hearing loss was enrolled in this study. The proband was analyzed by targeted next‐generation sequencing and bioinformatic analysis. And in vitro experiments were performed in overexpressed transfected HEK293T cells to investigate the pathogenesis of the mutant protein.

Results

We identified a novel missense variant in the CEACAM16 gene c.763A>G; (p.Arg255Gly) as causing autosomal dominant non‐syndromic hearing loss in the Chinese family. Using Western blot analysis, ELISA, and immunofluorescence we found increased expression level of the secreted mutant CEACAM16 protein, both intracellularly and extracellularly, compared with wild type CEACAM16 protein.

Conclusion

Our study showed that the p.Arg255Gly variant leads to increased secretion of mutant CEACAM16 protein, with potential deleterious effect to the function of the protein. Our findings expand the mutation spectrum of CEACAM16, and further the understanding CEACAM16 function and implications in disease.

Frequency and origin of the c.2090T>G p.(Leu697Trp) MYO3A variant associated with autosomal dominant hearing loss

We recently described a novel missense variant [c.2090T>G:p.(Leu697Trp)] in the MYO3A gene, found in two Brazilian families with late-onset autosomal dominant nonsyndromic hearing loss (ADNSHL). Since then, with the objective of evaluating its contribution to ADNSHL in Brazil, the variant was screened in additional 101 pedigrees with probable ADNSHL without conclusive molecular diagnosis. The variant was found in three additional families, explaining 3/101 (~3%) of cases with ADNSHL in our Brazilian pedigree collection. In order to identify the origin of the variant, 21 individuals from the five families were genotyped with a high-density SNP array (~600 K SNPs- Axiom Human Origins; ThermoFisher). The identity by descent (IBD) approach revealed that many pairs of individuals from the different families have a kinship coefficient equivalent to that of second cousins, and all share a minimum haplotype of ~607 kb which includes the c.2090T>G variant suggesting it probably arose in a common ancestor. We inferred that the mutation occurred in a chromosomal segment of European ancestry and the time since the most common ancestor was estimated in 1100 years (CI = 775-1425). This variant was also reported in a Dutch family, which shares a 87,121 bp haplotype with the Brazilian samples, suggesting that Dutch colonists may have brought it to Northeastern Brazil in the 17th century. Therefore, the present study opens new avenues to investigate this variant not only in Brazilians but also in European families with ADNSHL.

Genetic etiology of non-syndromic hearing loss in Latin America

Latin America comprises all countries from South and Central America, in addition to Mexico. It is characterized by a complex mosaic of regions with heterogeneous genetic profiles regarding the geographical origin of the ancestors and proportions of admixture between the Native American, European and African components. In the first years following the findings of the role of the GJB2/GJB6 genes in the etiology of hearing loss, most scientific investigations about the genetics of hearing loss in Latin America focused on assessing the frequencies of pathogenic variants in these genes. More recently, modern techniques allowed researchers in Latin America to make exciting contributions to the finding of new candidate genes, novel mechanisms of inheritance in previously known genes, and characterize a wide diversity of variants, many of them unique to Latin America. This review aimed to provide a general landscape of the genetic studies about non-syndromic hearing loss in Latin America and their main scientific contributions. It allows the conclusion that, although there are similar contributions of some genes, such as GJB2/GJB6, when compared to European and North American countries, Latin American populations revealed some peculiarities that indicate the need for tailored strategies of screening and diagnosis to specific geographic regions.

Accelerated Age-Related Degradation of the Tectorial Membrane in the Ceacam16βgal/βgal Null Mutant Mouse, a Model for Late-Onset Human Hereditary Deafness DFNB113

CEACAM16 is a non-collagenous protein of the tectorial membrane, an extracellular structure of the cochlea essential for normal hearing. Dominant and recessive mutations in CEACAM16 have been reported to cause postlingual and progressive forms of deafness in humans. In a previous study of young Ceacam16βgal/βgal null mutant mice on a C57Bl/6J background, the incidence of spontaneous otoacoustic emissions (SOAEs) was greatly increased relative to Ceacam16+/+ and Ceacam16+/βgal mice, but auditory brain-stem responses (ABRs) and distortion product otoacoustic emissions (DPOAEs) were near normal, indicating auditory thresholds were not significantly affected. To determine if the loss of CEACAM16 leads to hearing loss at later ages in this mouse line, cochlear structure and auditory function were examined in Ceacam16+/+, Ceacam16+/βgal and Ceacam16βgal/βgal mice at 6 and 12 months of age and compared to that previously described at 1 month. Analysis of older Ceacam16βgal/βgal mice reveals a progressive loss of matrix from the core of the tectorial membrane that is more extensive in the apical, low-frequency regions of the cochlea. In Ceacam16βgal/βgal mice at 6-7 months, the DPOAE magnitude at 2f1-f2 and the incidence of SOAEs both decrease relative to young animals. By ∼12 months, SOAEs and DPOAEs are not detected in Ceacam16βgal/βgal mice and ABR thresholds are increased by up to ∼40 dB across frequency, despite a complement of hair cells similar to that present in Ceacam16+/+ mice. Although SOAE incidence decreases with age in Ceacam16βgal/βgal mice, it increases in aging heterozygous Ceacam16+/βgal mice and is accompanied by a reduction in the accumulation of CEACAM16 in the tectorial membrane relative to controls. An apically-biased loss of matrix from the core of the tectorial membrane, similar to that observed in young Ceacam16βgal/βgal mice, is also seen in Ceacam16+/+ and Ceacam16+/βgal mice, and other strains of wild-type mice, but at much later ages. The loss of Ceacam16 therefore accelerates age-related degeneration of the tectorial membrane leading, as in humans with mutations in CEACAM16, to a late-onset progressive form of hearing loss.