Genetic Contribution to Chronic Tinnitus in Patients with Meniere’s Disease and Tinnitus Extreme Phenotype (MD-TEP)

Abstract

Introduction: Tinnitus is the perception of noise in the absence of an external acoustic stimulation affecting more than15% of adult population. Severe tinnitus is observed in 1% of the general population. The genetic studies conducted in twins, adoptees and families support a significant heritability. Several studies selecting candidate genes have shown no consistent findings and the evidence to support a genetic contribution to tinnitus is weak and emphasis the need to select an appropriate tinnitus phenotype. Objectives: To identify the genes involved in the development and maintenance of severe tinnitus by selecting individuals with extreme phenotype for tinnitus using exome sequencing and gene burden analyses. The identification of potential pathways and biological processes will be carried out by gene ontology and gene-set enrichment analysis. Methods: Exomes of 59 Meniere disease patients with extreme tinnitus and 32 MD patients without severe tinnitus of Spanish ancestry as internal control were sequenced. For replication cohort, 97 Swedish patients with severe tinnitus were sequenced and a third dataset of 701 patients with genetic generalized epilepsy of European ancestry were included. Minor allele frequency of variants was compared with Non-Finnish European and Spanish reference data sets. Gene ontology and gene-set enrichment was performed to identify the pathways and biological processes. Results: We found an enrichment of rare missense variants in 24 synaptic genes in a Spanish cohort, the most significant being PRUNE2, AKAP9, SORBS1, ITGAX, ANK2, KIF20B and TSC2, when they were compared with reference datasets. This burden was replicated for ANK2 gene in a Swedish cohort with 97 tinnitus individual, and in a subset of 34 Swedish patients with severe tinnitus for ANK2, AKAP9 and TSC2 genes. In addition, the hypothesis-free data driven approach, gene burden analyses revealed ADGRV1 gene and also confirmed the excess of missense variants for ANK2 and TSC2 with tinnitus. However, these associations were not significant in the third cohort of 701 genetic generalized epilepsy individuals without tinnitus. Gene ontology (GO) and gene-set enrichment analyses showed several pathways and biological processes involved in severe tinnitus, including membrane trafficking and cytoskeletal protein binding in neurons. Conclusions: In this Thesis, the potential genes and pathways for severe tinnitus have been identified, suggesting the role of synaptic genes and axon initial branching in the development and maintenance of severe tinnitus. These results could be beneficial for future genetic studies of tinnitus.