Deafness in the genomics era

Deafness is the most common sensory deficit in humans, affecting 278 million people worldwide. Non-syndromic hearing loss (NSHL), hearing loss not associated with other symptoms, is the most common type of hearing loss and most NSHL in developed countries is due to a genetic cause. The inner ear is a remarkably complex organ, and as such, there are estimated to be hundreds of genes with mutations that can cause hearing loss. To date, 62 of these genes have been identified. This extreme genetic heterogeneity has made comprehensive genetic testing for deafness all but impossible due to low-throughput genetic testing methods that sequence a single gene at a time.

The human genome project was completed in 2003. Soon after, genomic technologies, including massively parallel sequencing, were developed. MPS gives the ability to sequence millions or billions of DNA base-pairs of the genome simultaneously. The goal of my thesis work was to use these newly developed genomic technologies to create a comprehensive genetic testing platform for deafness and use this platform to answer key scientific questions about genetic deafness. This platform would need to be relatively inexpensive, highly sensitive, and accurate enough for clinical diagnostics.

In order to accomplish this goal we first determined the best methods to use for this platform by comparing available methods for isolation of all exons of all genes implicated in deafness and massively parallel sequencers. We performed this pilot study on a limited number of patient samples, but were able to determine that solution-phase targeted genomic enrichment (TGE) and Illumina sequencing presented the best combination of sensitivity and cost. We decided to call this platform and diagnostic pipeline OtoSCOPE®. Also during this study we identified several weaknesses with the standard method for TGE that we sought to improve.

The next aim was to focus on these weaknesses to develop an improved protocol for TGE that was highly reproducible and efficient. We developed a new protocol and tested the limits of sequencer capacity. These findings allowed us to translate OtoSCOPE® to the clinical setting and use it to perform comprehensive genetic testing on a large number of individuals in research studies.

Finally, we used the OtoSCOPE® platform to answer crucial questions about genetic deafness that had remained unanswered due to the low-throughput genetic testing methods available previously. By screening 1,000 normal hearing individuals from 6 populations we determined the carrier frequency for non-DFNB1 recessive deafness-causing mutations to be 3.3%. Our findings will also help us to interpret variants uncovered during analysis of deafness genes in affected individuals. When we used OtoSCOPE® to screen 100 individuals with apparent genetic deafness, we were able to provide a genetic diagnosis in 45%, a large increase compared to previous gene-by-gene sequencing methods.

Because it provides a pinpointed etiological diagnosis, genetic testing with a comprehensive platform like OtoSCOPE® could provide an attractive alternative to the newborn hearing screen. In addition, this research lays the groundwork for molecular therapies to restore or reverse hearing loss that are tailored to specific genes or genetic mutations. Therefore, a molecular diagnosis with a comprehensive platform like OtoSCOPE® is integral for those affected by hearing loss.