Audiological Evidence of Frequent Hereditary Mild, Moderate and Moderate-to-Severe Hearing Loss

Unusual phenotype in 35delG mutation: a case report

BACKGROUND

Mutations in the GJB2 gene, which encodes the protein connexin 26 and is involved in inner ear homeostasis, are identified in approximately 50% of patients with autosomal recessive nonsyndromic hearing loss, making it one of the primary causes of prelingual nonsyndromic hearing loss in various populations. The 35delG mutation, one of the most common mutations of the GJB2 gene, usually causes prelingual, bilateral mild to profound, nonprogressive sensorineural hearing loss.

CASE PRESENTATION

We present an unusual case of an 18-year-old Turkish female with heterozygous 35delG mutation and postlingual, profound-sloping, progressive and fluctuating unilateral sensorineural hearing loss. The phenotype is different from the usual findings.

CONCLUSIONS

The 35delG mutation causing hearing loss may not always be reflected in the phenotype as expected and therefore may have different audiologic manifestations.

Recurrent missense variant identified in two unrelated families with MPZL2-related hearing loss, expanding the variant spectrum associated with DFNB111

MPZL2-related hearing loss is a rare form of autosomal recessive hearing loss characterized by progressive, mild sloping to severe sensorineural hearing loss. Thirty-five previously reported patients had biallelic truncating variants in MPZL2, with the exception of one patient with a missense variant of uncertain significance and a truncating variant. Here, we describe the clinical characteristics and genotypes of five patients from four families with confirmed MPZL2-related hearing loss. A rare missense likely pathogenic variant [NM_005797.4(MPZL2):c.280C>T,p.(Arg94Trp)] located in exon 3 was confirmed to be in trans with a recurrent pathogenic truncating variant that segregated with hearing loss in three of the patients from two unrelated families. This is the first recurrent likely pathogenic missense variant identified in MPZL2. Apparently milder or later-onset hearing loss associated with rare missense variants in MPZL2 indicates that some missense variants in this gene may cause a milder phenotype than that resulting from homozygous or compound heterozygous truncating variants. This study, along with the identification of truncating loss of function and missense MPZL2 variants in several diverse populations, suggests that MPZL2-related hearing loss may be more common than previously appreciated and demonstrates the need for MPZL2 inclusion in hearing loss testing panels.

The congenital hearing phenotype in GJB2 in Queensland, Australia: V37I and mild hearing loss predominates

GJB2 was originally identified in severe, non-syndromic sensorineural hearing loss (SNHL), but was subsequently associated with mild and moderate SNHL. Given the increasing utilisation of genetic testing pre-conceptually, prenatally, and neonatally, it is crucial to understand genotype-phenotype correlations. This study evaluated the nature and frequency of GJB2 variants in an Australian paediatric population with varying degrees of SNHL ascertained through newborn hearing screening. Audiograms from individuals with GJB2 variants and/or a GJB6 deletion (GJB6-D13S11830) were retrospectively reviewed (n = 127). Two-thirds were biallelic (homozygous/compound heterozygous) for pathogenic/likely pathogenic variants of GJB2 and/or GJB6 (n = 80). The most frequent variant was c.109 G > A, followed by c.35delG and c.101 T > C. Compared to biallelic carriage of other GJB2 variants, c.109 G > A positive individuals (homozygous/compound heterozygous) were more likely to have mild HL at their initial and latest audiograms (p = 0.0004). Biallelic carriage of c.35delG was associated with moderately-severe or greater SNHL at both initial and latest audiograms (p = 0.007). The c.101 T > C variant presented with milder SNHL and U-shaped audiograms (p = 0.02). In this agnostically identified cohort, mild SNHL predominated in GJB2/GJB6 carriers in contrast to previous studies targeting individuals with significant loss. Consequently, c.109 G > A, associated with milder phenotypes, was the most frequent. This study provides valuable data to support prognostic confidence in genetic counselling.