Unravelling the Genetic Bases of Hearing Loss: Functional Characterisation of Pathogenic Variants and Novel Candidate Genes Identified by Whole-Exome Sequencing
Hearing Loss (HL) is the most common sensory disorder in humans, and more than half of cases are due to genetic factors. In 30% of cases, hereditary HL is associated with additional clinical features, and it is defined syndromic (SHL), whereas in 70% of cases HL is the only symptom, and it is considered nonsyndromic (NSHL). HL is characterised by an extreme genetic heterogeneity, with more than 150 loci currently associated and at least 100 genes identified, making extremely challenging to
... challenging to obtain a molecular diagnosis with traditional screening methods. For these reasons, whole-exome sequencing (WES) has been introduced to search for mutations and novel genes underlying the disease. In this frame, this thesis describes the study of the genetic bases of deafness in 11 HL (3 SHL and 8 NSHL) families applying WES and an accurate functional validation of the candidate mutations. This approach allowed the identification of the causative mutations in 7 out of the 11 families analysed, while for 3 NSHL families no strong candidate pathogenic variants emerged from our WES data analyses. In addition, in one NSHL family we identified a missense variant in the candidate deafness-causing gene DIAPH2, coding for a protein involved in actin filament elongation. My in-vitro studies indicate a possible functional impairment of the mutant DIAPH2 protein, which might be very relevant in hearing function. In fact, immunohistochemical studies indicate that the mouse ortholog protein Diap2 is expressed in the cochlea in the actin-rich stereocilia of the sensory outer hair cells during development. Auditory brainstem response measurements to evaluate the hearing phenotype of Diaph2 knock-out and knock-in mice were also undertaken. However, at least at the time points analysed, no hearing impairment was detected in the mouse models. Consequently, the role of DIAPH2 in HL in humans still needs to be further clarified.