rs104894396 — GJB2 W24X

GJB2 W24X — The Ancestral South Asian Deafness Mutation

The GJB2 gene encodes connexin 26¹¹ connexin 26
A gap-junction protein that forms channels between cochlear support cells, essential for recycling potassium ions that drive sound-to-nerve signal transduction, the most common cause of hereditary non-syndromic hearing loss worldwide. While the 35delG deletion dominates European deaf populations, a different loss-of-function variant — W24X — is the principal GJB2 deafness allele across South Asia and carries the genetic signature of a single ancestral mutation that arose on the Indian subcontinent thousands of years ago.

The c.71G>A substitution creates a premature stop codon at amino acid 24 (p.Trp24Ter), terminating the connexin 26 protein at just one-tenth its normal length. The resulting truncated peptide lacks all functional domains and cannot reach the cell membrane. Individuals who inherit two copies — one from each parent — are born with complete absence of cochlear gap junction function, and severe-to-profound congenital hearing loss follows in virtually all cases.

The Mechanism

Connexin 26 proteins assemble into hexamers called connexons, which dock with connexons on adjacent cochlear support cells to form gap junction channels permeable to potassium ions and small signalling molecules. These channels are essential for at least three functions in the inner ear: recycling K⁺ from the base of hair cells back to the endolymph, propagating ATP–calcium intercellular waves during cochlear development, and supplying glucose to the sensory epithelium.

The W24X stop-gain at codon 24 produces a 23-amino-acid peptide that lacks all transmembrane and functional domains. Nonsense-mediated mRNA decay²² Nonsense-mediated mRNA decay
A cellular surveillance pathway that degrades mRNAs containing premature stop codons, preventing synthesis of truncated proteins that might exert dominant-negative effects ensures the truncated mRNA is degraded, so no connexin 26 protein reaches the membrane. The functional consequence is identical to 35delG: complete GJB2 null in homozygotes, total absence of cochlear gap junction channels, and profound sensorineural hearing loss.

The Evidence

The population genetics of W24X are anchored to South Asia. Kaushal et al. — Kerala, India³³ Kaushal et al. — Kerala, India
W24X detected in 32.5% of hearing-impaired patients; carrier frequency 3.57% in general population controls, with haplotype data confirming a founder effect. Int J Pediatr Otorhinolaryngol, 2009 established Kerala as a high-prevalence region. Azaiez et al. — British Bangladeshi families⁴⁴ Azaiez et al. — British Bangladeshi families
W24X was the most common GJB2 mutation (57% of cases); GJB2 mutations explain >25% of non-syndromic SNHL in this population. Clin Otolaryngol, 2008 demonstrated the mutation's dominance across the broader South Asian diaspora.

W24X also traces the historical migration of Roma (Gypsy) populations out of India into Europe. Álvarez et al. — Spanish Romani families⁵⁵ Álvarez et al. — Spanish Romani families
W24X accounted for 79% of DFNB1 alleles; carrier frequency ~4% in Spanish Romani. Am J Med Genet A, 2005 and Minárik et al. — Slovak Romani patients⁶⁶ Minárik et al. — Slovak Romani patients
W24X found on 23.2% of screened chromosomes; W24X/W24X homozygotes had profound hearing loss. Gen Physiol Biophys, 2003 both document the mutation's dramatic enrichment in Roma communities. A multi-country study by Bouwer et al.⁷⁷ Bouwer et al.
Average W24X carrier rate 4–5% across Roma subisolates; all share the same Indian-origin founder haplotype. Genet Test, 2007 confirmed that all W24X alleles in European Roma descend from the same South Asian ancestor.

Phenotype-severity data from the largest international GJB2 consortium confirm that biallelic truncating mutations⁸⁸ biallelic truncating mutations
Snoeckx et al., 1,531 biallelic GJB2 cases across 16 countries. Am J Hum Genet, 2005 produce significantly more severe hearing loss than non-truncating alleles (p<0.0001): among truncating homozygotes, 64% have profound loss (>90 dB HL) and 25% have severe loss (70–90 dB HL). W24X, as a complete null allele, fits squarely in the truncating category.

Cochlear implantation outcomes are consistently excellent regardless of which GJB2 null variant is present. Lustig et al.⁹⁹ Lustig et al.
No difference in speech awareness or recognition between GJB2-related and non-GJB2 CI recipients. Arch Otolaryngol Head Neck Surg, 2004 established that GJB2 etiology preserves auditory nerve integrity, making CI the gold-standard intervention for homozygotes.

Practical Actions

For carriers (one W24X allele), clinical implications are restricted to reproductive planning. Because W24X is the predominant GJB2 pathogenic allele in South Asian populations, carrier testing of a partner from a South Asian background is particularly important. Carrier couples have a 25% per-pregnancy chance of having a deaf child. Genetic counselling before conception and partner GJB2 testing allows fully informed family planning decisions; prenatal diagnosis by CVS or amniocentesis and preimplantation genetic testing (PGT-M) are available.

For homozygotes identified through newborn hearing screening, the most impactful intervention is early cochlear implantation. The 1-3-6 benchmark from the Joint Committee on Infant Hearing — hearing screening completed by one month, diagnosis confirmed by three months, early intervention started by six months — maximises speech and language outcomes. Children with GJB2-related deafness who receive early CI and intensive auditory-verbal therapy achieve spoken-language milestones at the highest rates of any deafness etiology.

Interactions

The most clinically important compound heterozygous configuration involving W24X is W24X in trans with 35delG (rs80338939) — the predominant European GJB2 deafness allele. This combination arises in mixed South Asian–European ancestry families and in any population where both alleles are polymorphic. Both alleles are complete nulls; the compound heterozygous phenotype is indistinguishable from either homozygous state and presents as severe-to-profound congenital sensorineural hearing loss.

W24X may also co-occur in trans with W77X (another truncating South Asian GJB2 allele), 235delC (the predominant East Asian allele, rs35887543), or GJB6 deletions. In South Asian individuals with a single heterozygous W24X finding and unexplained sensorineural hearing loss, full GJB2 sequencing and GJB6 deletion testing are indicated to identify a second pathogenic allele on the opposite chromosome.