rs35887622 — GJB2 M34T

GJB2 M34T — The Mild-Severity Deafness Allele That Evades Early Detection

The human cochlea relies on an extraordinary feat of ion management: within the spiral organ of Corti, potassium ions¹¹ potassium ions
K+; the primary charge carrier in cochlear mechanosensory transduction flow through hair cells during sound detection and must be continuously recycled through a network of gap junction channels before they can cause cellular toxicity. Connexin 26, encoded by GJB2²² Connexin 26, encoded by GJB2
Gap Junction Protein Beta-2; the most common cause of hereditary non-syndromic hearing loss worldwide is the principal protein of these recycling channels in the cochlear supporting cell network. The M34T variant (c.101T>C, p.Met34Thr, rs35887622) is a missense substitution in the first transmembrane domain of connexin 26 that substantially reduces channel conductance without eliminating it — creating a partial-loss-of-function allele that behaves quite differently from the severe truncating mutations that dominate the GJB2 literature.

Unlike c.35delG (rs80338939), which eliminates connexin 26 protein and causes severe-to-profound congenital deafness, M34T retains some channel activity. This subtlety has significant clinical consequences: homozygous M34T individuals typically have mild hearing loss (median pure-tone average ~30 dB), hearing loss may not be present at birth or may pass newborn screening, and onset often occurs in childhood or early adulthood. The result is a condition that is biologically meaningful but clinically easy to miss — and historically controversial because its high population frequency (approximately 1.5% carrier rate in Europeans) initially suggested it might be benign.

The Mechanism

M34T substitutes the nonpolar methionine at position 34 with the hydroxyl-bearing threonine, located within the first transmembrane helix (TM1) of connexin 26. This position is structurally critical: methionine 34 forms a hydrophobic contact with tryptophan 3³³ hydrophobic contact with tryptophan 3
W3; located in the N-terminal helix that lines the channel pore of the adjacent subunit. Molecular dynamics simulations show that the M34T substitution disrupts this hydrophobic interaction, altering the geometry of the pore funnel and causing the channel to reside primarily in a low-conductance state (approximately 13 picosiemens, versus ~120 pS for wild-type connexin 26 channels) — a roughly 90% reduction in single-channel conductance.

Importantly, M34T channels retain some residual activity rather than being completely non- functional. This distinguishes M34T from frameshift mutations and explains both the milder audiological phenotype and the reduced penetrance compared with loss-of-function alleles. Coexpression of wild-type and M34T connexin 26 in heterologous systems has also demonstrated a dominant-negative effect⁴⁴ dominant-negative effect
The mutant subunit incorporates into hexameric connexons alongside wild-type subunits, reducing the conductance of the entire channel complex, which may explain rare reports of apparent dominant inheritance in families. However, the weight of clinical and population evidence supports autosomal recessive inheritance as the operational mode in most cases.

The Evidence

The definitive classification of M34T as pathogenic came from the ClinGen Hearing Loss Variant Curation Expert Panel in 2019⁵⁵ ClinGen Hearing Loss Variant Curation Expert Panel in 2019
Shen et al., Genetics in Medicine; PMID 31160754, which reviewed functional, allelic, segregation, and population data for both M34T and the related V37I variant (rs72474224). The panel concluded that both variants are pathogenic for autosomal recessive nonsyndromic hearing loss with variable expressivity and incomplete penetrance. Despite the relatively high allele frequency in European populations (~1.5%), the evidence that M34T is significantly overrepresented in hearing loss cohorts compared to population controls outweighed the frequency concern.

Quantitative phenotyping data comes from a multicenter study of 1,531 biallelic GJB2 cases across 16 countries⁶⁶ multicenter study of 1,531 biallelic GJB2 cases across 16 countries
Snoeckx et al., PMID 16303844. M34T/M34T homozygotes had a median pure-tone average of 30 dB (mild hearing loss), while 35delG/M34T compound heterozygotes had a median of 34 dB. Both were among the three mildest genotype classes observed — far milder than the 35delG/35delG homozygotes, who had a median approaching severe-profound loss. A Polish cohort study⁷⁷ Polish cohort study
Pollak et al. 2007, PMID 17935238 estimated M34T penetrance at approximately 1/10 relative to mutations of undisputed pathogenicity, and documented significantly later onset and a progressive rather than congenital course for M34T-associated hearing loss.

The partial and progressive nature of M34T-related hearing loss means it often escapes newborn hearing screening. Standard otoacoustic emission and auditory brainstem response testing in neonates may classify an infant with biallelic M34T as "normal hearing," with measurable loss appearing only in mid-childhood or even adulthood.

Practical Implications

Biallelic M34T individuals (GG genotype) should have audiological evaluation regardless of whether they passed newborn hearing screening, since the mild loss may be subclinical at birth. Annual audiograms allow early detection of progression before communication is affected. When hearing aid candidacy is reached (typically when thresholds in the speech frequencies average 25 dB or more), early fitting prevents the cognitive burden of straining to hear in noise. The audiogram shape is typically flat or mildly downsloping, a profile that responds very well to modern digital amplification.

Noise avoidance is particularly important: cochlear K+ recycling in biallelic M34T carriers has less reserve capacity, and animal and clinical data suggest that noise-induced hearing loss and ototoxic drug effects may be amplified in GJB2-related hearing impairment. Loud occupational or recreational noise (>85 dB time-weighted average) and ototoxic antibiotics such as aminoglycosides deserve special attention.

Single heterozygous carriers (AG genotype) have normal hearing. Their significance is reproductive: one in ~35 Europeans carries a GJB2 pathogenic variant, and partner carrier testing before pregnancy can identify couples at 25% risk per pregnancy of having a biallelic child with hearing loss.

Interactions

M34T produces compound heterozygous hearing loss when inherited alongside other pathogenic GJB2 alleles on the opposite chromosome. The most common combination in European populations is M34T/35delG (rs80338939): compound heterozygotes have a median threshold around 34 dB, milder than 35delG homozygotes but generally worse than M34T homozygotes. Compound M34T heterozygosity with the Asian-dominant c.235delC allele or the Ashkenazi 167delT (rs80338942) also produces mild-to-moderate hearing loss. The clinical rule in DFNB1-spectrum hearing loss is that severity correlates with the less severe of the two alleles — since M34T is a partial loss-of-function allele, it "protects" compound heterozygotes from the severe phenotype associated with the co-inherited truncating allele.

Large deletions in the neighbouring GJB6 gene⁸⁸ neighbouring GJB6 gene
Encodes connexin 30, which forms heteromeric gap junctions with connexin 26 in cochlear supporting cells — particularly del(GJB6-D13S1830) — can also serve as a second allele in trans with M34T. A single GJB2 M34T allele in a deaf individual with no apparent second GJB2 variant should prompt testing for GJB6 regulatory deletions.