Significance
Neurological diseases are often thought to be irreversible, including hearing loss. In this study, we found that one type of hearing loss can be reversed as long as the treatment is delivered within a critical period early in disease progression. This result is a proof of concept that hearing loss not only can be avoided but also may be reversed. This genetic approach can be used for a wide range of diseases using existing mouse resources.
Abstract
Hearing loss is highly heterogeneous, but one common form involves a failure to maintain the local ionic environment of the sensory hair cells reflected in a reduced endocochlear potential. We used a genetic approach to ask whether this type of pathology can be reversed, using the Spns2tm1a mouse mutant known to show this defect. By activating Spns2 gene transcription at different ages after the onset of hearing loss, we found that an existing auditory impairment can be reversed to give close to normal thresholds for an auditory brainstem response (ABR), at least at low to mid stimulus frequencies. Delaying the activation of Spns2 led to less effective recovery of ABR thresholds, suggesting that there is a critical period for intervention. Early activation of Spns2 not only led to improvement in auditory function but also to protection of sensory hair cells from secondary degeneration. The genetic approach we have used to establish that this type of hearing loss is in principle reversible could be extended to many other diseases using available mouse resources.
Hearing impairment is very common in the population and can begin at any age. Over half of adults in their 70 s have a significant hearing loss. Hearing impairment isolates people from society, can be associated with depression and cognitive decline, and is a major predictor of dementia (1–4). The only remedies currently available are devices such as hearing aids and cochlear implants, but these do not restore normal function. There is a large unmet need for medical approaches to slow down or reverse hearing loss.
Treatment strategies for genetic deafness are being developed using the mouse, including gene suppression using siRNAs, antisense oligonucleotides to correct splicing, gene replacement, and gene editing to repair single base mutations (5–15). These studies usually involve introduction of the agent into the mouse inner ear soon after birth when the auditory system is immature, at a stage corresponding to around 16 to 24 wk of gestation in humans, making direct translation challenging. One report suggests that introduction of otoferlin (Otof) sequences into the mouse inner ear at later stages leads to improved hearing in Otof mutants that would otherwise show abnormal inner hair cell (IHC) synaptic function and deafness (16). We also are interested in asking whether an existing hearing impairment can be reversed because the main demand for treatments is from people who already have hearing loss, particularly progressive age-related hearing loss.
One significant pathological type underlying age-related hearing loss involves the maintenance of the endolymph, the high-potassium, low-sodium fluid that bathes the upper surface of sensory hair cells. In the cochlea, endolymph is maintained at a positive resting potential, the endocochlear potential (EP), of around +120 mV which is essential for normal hair cell sensitivity. We have used a genetic approach in Spns2 mutant mice to investigate the possibility of reversing hearing loss linked to an EP deficit.
Results
Reversal of Hearing Loss in Spns2 Mutants.
Spns2 encodes Spinster homolog 2, a sphingosine-1-phosphate (S1P) transporter. Spns2 mutant mice show a rapidly progressive hearing loss associated with a dramatic decline in EP between 2 and 3 wk after birth (17) (Fig. 1D). As EP develops to high levels at first in Spns2 mutants (Fig. 1D), we considered ways of restoring it to normal levels after the onset of hearing loss. The Spns2 mutation used is the Spns2tm1a mutation, a knockout-first, conditional-ready design that has a large DNA construct inserted into an intron that disrupts gene expression (Fig. 1A) (18). The construct is flanked by Flippase Recombination Target (FRT) sites that recombine on exposure to Flp recombinase, removing the construct and restoring gene function (17)…
