rs111033253 — TMPRSS3 p.Ala306Thr (A306T)

TMPRSS3 p.Ala306Thr — The Most Common TMPRSS3 Pathogenic Allele, Now a Gene Therapy Target

The TMPRSS3 gene encodes a type II transmembrane serine protease¹¹ type II transmembrane serine protease
Anchored to the cell membrane with its catalytic serine protease domain facing the extracellular space; expressed in cochlear inner and outer hair cells, spiral ganglion neurons, and the stria vascularis essential for the survival of cochlear hair cells at the onset of hearing. Of the more than 87 documented TMPRSS3 pathogenic variants, the p.Ala306Thr missense change caused by c.916G>A is the single most common allele detected across all populations²² the p.Ala306Thr missense change caused by c.916G>A is the single most common allele detected across all populations
Identified in Korean, Chinese, Dutch, and German families; classified as a Korean and Chinese founder mutation; the most frequently reported mutation in published TMPRSS3 literature — making it a high-priority target for both clinical recognition and emerging gene therapy approaches.

This variant is catalogued in ClinVar (variation 46131) as Pathogenic/Likely pathogenic for autosomal recessive nonsyndromic hearing loss 8 (DFNB8), with 17 submitting laboratories providing criteria-based evidence, and no conflicting classifications.

The Mechanism

TMPRSS3 is synthesized as an inactive single-chain precursor (zymogen) that must undergo autocatalytic self-cleavage³³ autocatalytic self-cleavage
The serine protease domain cleaves itself at a specific arginine residue to release the active two-chain form; TMPRSS3 is one of the few serine proteases that activates itself rather than relying on an upstream activator to become active. This active form then contributes to epithelial sodium channel (ENaC) activation in cochlear hair cells and, through as-yet incompletely characterized pathways, supports hair cell survival.

The p.Ala306Thr substitution places a threonine — a bulkier, polar amino acid — at position 306 within the serine protease catalytic domain⁴⁴ serine protease catalytic domain
The catalytic triad of serine proteases (His, Asp, Ser) requires precise spatial arrangement for activity; Ala306 sits adjacent to the active-site residue Asp304, and any perturbation of local geometry can disrupt substrate binding and catalysis. Molecular modeling shows that wild-type Ala306 forms two salt bridges with Thr254, while mutant Thr306 forms an additional unnatural salt bridge with Val291 — a structural perturbation predicted to directly impair catalytic function. Crucially, p.Ala306Thr does not abolish TMPRSS3 autocatalytic processing the way the R216L null allele does; instead it reduces activity, making it a hypomorphic variant⁵⁵ hypomorphic variant
A hypomorph retains partial protein function, as opposed to an amorph (null). TMPRSS3 A306T knockin mice develop delayed-onset progressive hearing loss rather than the complete congenital deafness seen with biallelic null alleles, confirming residual function. The residual activity is insufficient for long-term cochlear hair cell survival but supports initial postnatal hearing — explaining the postlingual, progressive clinical phenotype.

The Evidence

Founder mutation status is established for multiple populations. In the Chinese population⁶⁶ In the Chinese population
Zheng et al. 2017, screening 151 ARNSHL families lacking GJB2/SLC26A4 mutations; TMPRSS3 accounted for 4.6% of cases; c.916G>A was found in 2% of all alleles; haplotype analysis showed linkage disequilibrium across 4/6 families supporting a common ancestor, this allele is clearly a founder mutation. In Korea⁷⁷ In Korea
Carrier rate among Korean postlingual hearing loss patients with TMPRSS3 mutations was 8.3%; p.A306T is the dominant founder allele with haplotype evidence, the same conclusion holds. It has also been independently found in Dutch and German families.

Genotype-phenotype correlations reveal a pattern with clear clinical relevance. Weegerink et al. 2011 in Dutch DFNB8/10 families⁸⁸ Weegerink et al. 2011 in Dutch DFNB8/10 families
8 families studied; the study established the principle that TMPRSS3 allele combinations predict phenotype: two severe alleles → DFNB10 (prelingual profound), one severe + one mild → DFNB8 (postlingual progressive) established p.Ala306Thr as a severe allele. When p.Ala306Thr pairs with a truncating or null allele (such as rs727503493 c.208delC frameshift or rs137853000 R216L), the result is typically prelingual profound hearing loss (DFNB10 phenotype). When it pairs with a mild missense allele, postlingual progressive loss (DFNB8 phenotype) results. This distinction has major clinical implications: DFNB8 patients retain speech for years to decades, while DFNB10 patients require intervention from infancy.

The A306T mouse model provides mechanistic confirmation. Du et al. 2023⁹⁹ Du et al. 2023
Molecular Therapy; CRISPR-Cas9 knockin of the human c.916G>A allele into CBA/CaJ mice created Tmprss3A306T/A306T homozygous animals that develop late-onset progressive HL beginning after 10.5 months, with average threshold elevation of ~26 dB by 22.5 months — exactly recapitulating the human DFNB8 pattern created and validated an A306T knockin mouse model. A single AAV2-hTMPRSS3 injection into 18.5-month-old mice (equivalent to a middle-aged adult human) restored hearing thresholds to wild-type levels, preserved outer hair cell survival, and tripled spiral ganglion neuron survival — making this the first successful gene therapy demonstration in an aged mouse model of hereditary deafness.

Cochlear implant outcomes are favorable. Colbert et al. 2024¹⁰¹⁰ Colbert et al. 2024
127 patients, 16 centers, 6 countries; mean word recognition score 76%; age at implantation is the dominant outcome predictor — each year of delay associated with measurable speech perception decrement confirmed excellent outcomes across the TMPRSS3 patient population. Individual published cases of A306T compound heterozygotes have achieved 65–91% phoneme scores with cochlear implants.

Practical Implications

For confirmed biallelic A306T carriers (TT), or compound heterozygotes carrying one A306T allele and one other pathogenic TMPRSS3 allele, the clinical priority is early audiological characterization, progressive monitoring, and timely cochlear implant referral. The progressive nature of DFNB8 means that TMPRSS3-related hearing loss often presents in childhood with high-frequency loss that progresses over years to decades — a pattern that may be initially confused with noise-induced or idiopathic progressive loss. Genetic diagnosis changes the trajectory by enabling intervention planning.

Looking forward, the AAV-TMPRSS3 gene therapy approach validated in the A306T mouse model represents a genuine therapeutic pipeline specifically targeted to this allele. Carriers with this mutation should be tracked in TMPRSS3 natural history registries that will feed into gene therapy trials.

Interactions

The dominant interaction is compound heterozygosity with other TMPRSS3 pathogenic alleles. Because p.Ala306Thr is a severe (not null) allele:

• A306T + null allele (rs137853000 R216L, rs727503493 c.208delC): Expected DFNB10 — prelingual profound deafness. The null allele contributes no function; A306T's residual activity is insufficient to rescue the phenotype to postlingual onset.
• A306T + mild missense (p.Ala426Thr, p.Ala138Glu, p.Thr248Met, or intronic variants): Expected DFNB8 — postlingual progressive hearing loss, typically onset in childhood to early adulthood, ski-slope audiogram.
• Homozygous A306T/A306T: Also causes DFNB8-type progressive HL; the mouse model Tmprss3A306T/A306T confirms delayed-onset progressive loss, not congenital profound deafness.

The severity-based prediction framework is clinically actionable: when a TMPRSS3 evaluation returns A306T on one allele, comprehensive sequencing of the second allele determines whether the patient faces DFNB8 (mild + mild = postlingual) or DFNB10 (mild + severe = prelingual profound) outcome. See also rs137853000 (R216L, complete null allele) and rs727503493 (c.208delC frameshift) for the other shipped TMPRSS3 pathogenic alleles.

A possible digenic interaction with GJB2 (connexin 26)¹¹¹¹ GJB2 (connexin 26)
GJB2 encodes the gap junction protein connexin 26, the most common cause of genetic hearing loss in most populations; initial reports suggested TMPRSS3/GJB2 digenic cases, but larger series have not confirmed this as a distinct entity has been explored but is not established. When a single TMPRSS3 pathogenic allele is found in a hearing-loss patient, comprehensive deafness gene panel testing including GJB2 should be performed before concluding the patient is a carrier rather than a compound heterozygote.