Molecular Mechanisms and Clinical Phenotypes of GJB2 Missense Variants

Audiological Phenotypes of Connexin Gene Mutation Patterns: A Glance at Different GJB2/GJB6 Gene Mutation Profiles

GJB2 mutations are the most common cause of autosomal-recessive non-syndromic sensorineural hearing loss (SNHL). The available evidence shows large phenotypic variability across different genotypes and allelic variants. The aim of this study was to investigate the clinical and audiological features of a cohort of subjects with different GJB2/GJB6 gene mutation profiles from a tertiary referral center in Northeastern Italy. We considered 57 patients with GJB2/GJB6 mutations presenting with congenital, non-syndromic SNHL, mainly coming from the Veneto region (Italy). The samples were screened for mutations in exons 1 and 2 of the GJB2 gene and for the GJB6 gene deletion del (GJB6-D13S1830). Free-field and air-conduction frequency-specific thresholds and the pure-tone average (PTA) were considered in the statistical analysis. Five patients (8.87%) had connexin gene mutations in simple heterozygosis, 15 (26.31%) in compound heterozygosis, 34 (59.64%) in homozygosis, and 3 (5.26%) with digenic patterns. The frequency-specific air-conduction thresholds showed significantly different mean values across the different genotypes (Roy's largest-root test, p = 0.0473). Despite the evidence already available on genetic SNHL, many new insights are to be expected. Further large-scale prospective studies including different populations are necessary to confirm these preliminary findings about the clinical and audiological features of patients with different GJB2/GJB6 gene mutation patterns.

Case report: Prenatal diagnosis in the fetus of a couple with both thalassemia and deafness genes

Background: Prenatal diagnosis and genetic counseling play an important role in preventing and controlling birth defects. No reports were found of prenatal diagnosis of couples carrying both the thalassemia and deafness genes. In this study, we presented the prenatal screening and diagnosis of a couple with both thalassemia and deafness genes, contributing to better genetic counseling. Case Report: A couple visited our hospital for a routine prenatal examination. As required by the policy in our region, they underwent screening and genetic diagnosis for thalassemia. Meanwhile, they did not accept the recommendation to test for spinal muscular atrophy and deafness genes. The female was confirmed to be a Hb Quong Sze (Hb QS) carrier (αQSα/αα, βN/βN), and the male had Hb H disease combined with β-thalassemia (--SEA/αCSα, βCDs41-42 (-TTCT)/βN). A prenatal diagnosis of the fetus revealed a Hb CS heterozygote. Subsequent complementary testing showed that the male was a double heterozygote of the GJB2 gene c.299_300delAT combined with c.109G>A, and Sanger sequencing confirmed that the female was a carrier of c.508_511dup in the GJB2. Fortunately, the chorionic villi results indicated that the fetus was only a carrier of deafness. Conclusion: Since both partners carried thalassemia and deafness genes, the couple required prenatal diagnosis for the respective mutations. Expanded carrier screening (ECS) is a more advanced technology that can detect multiple disease genes simultaneously.

Functional Consequences of Pathogenic Variants of the GJB2 Gene (Cx26) Localized in Different Cx26 Domains

One of the most common forms of genetic deafness has been predominantly associated with pathogenic variants in the GJB2 gene, encoding transmembrane protein connexin 26 (Cx26). The Cx26 molecule consists of an N-terminal domain (NT), four transmembrane domains (TM1-TM4), two extracellular loops (EL1 and EL2), a cytoplasmic loop, and a C-terminus (CT). Pathogenic variants in the GJB2 gene, resulting in amino acid substitutions scattered across the Cx26 domains, lead to a variety of clinical outcomes, including the most common non-syndromic autosomal recessive deafness (DFNB1A), autosomal dominant deafness (DFNA3A), as well as syndromic forms combining hearing loss and skin disorders. However, for rare and poorly documented variants, information on the mode of inheritance is often lacking. Numerous in vitro studies have been conducted to elucidate the functional consequences of pathogenic GJB2 variants leading to amino acid substitutions in different domains of Cx26 protein. In this work, we summarized all available data on a mode of inheritance of pathogenic GJB2 variants leading to amino acid substitutions and reviewed published information on their functional effects, with an emphasis on their localization in certain Cx26 domains.