Genetic bases of endolymphatic sac hypoplasia in Meniere Disease

Abstract

Introduction - Meniere disease (MD) is a complex disease of the inner ear, characterized by wide symptomatological heterogeneity that includes sensorineural hearing loss, episodic vertigo and tinnitus, which can overlap with various comorbidities and appears as partial clinical pictures. It has a multifactorial etiology that includes an immune component, a genetic origin, and disorders related to endolymph flow. In fact, the most consistent findings in the temporal bones of patients with MD have been: i) the presence of endolymphatic hydrops, which consists of endolymph accumulation in the sensory audiovestibular organs; ii) abnormalities in the endolymphatic sac (ES) and the endolymphatic duct, being the two non-sensory organs of the inner ear that are responsible for regulating the endolymph composition and drainage, including the narrowing and collapse of their lumens and fibrosis in their epithelia, and iii) malformations of the temporal bone such as shortening and compression of the vestibular aqueduct. Hypothesis and Objectives - Recently, two endotypes of MD have been described based on the histopathological malformation of the ES. Specifically, in the hypoplastic endotype, the ES abruptly disappears at the level of the operculum, with a total absence of the extraosseous region. These patients had a family history of sensorineural hearing loss, vertigo, or MD, a tendency to present early onset, temporal bone abnormalities, a male predominance, and a higher bilateral prevalence than other MD patients. Based on these findings, the authors suggest that ES hypoplasia originates during development and may have a genetic origin. Thus, the objective of this work is to decipher the genetic basis of ES hypoplasia in patients with MD. Material and methods - A cohort of 42 patients with MD and ES hypoplasia was recruited, and exome sequencing was performed. The endotype was identified by calculating the angular trajectory of the vestibular aqueduct (ATVA) on computed tomography (CT) and magnetic resonance (MRI) images. The approach for the genetic study incorporated gene burden analysis of SNVs and InDels variants and structural variants discovery, pointing out candidate genes with higher variant enrichment in our cohort compared to the reference populations. The functional analysis highlighted the possible pathways and biological processes affected. Candidate genes were validated by immunohistochemical assays on tissue sections and western blot in the mouse inner ear. The absence of the hypoplastic endotype in the general population was addressed by ATVA analysis in a complementary study on 332 ears of otologic patients without MD. Results - Gene burden analysis pointed to 94 genes with enrichment for rare variants. Among them, the best candidates were ADAMTS18 and SDK1, whose variants were present in a large number of individuals in the cohort (35.7% and 33.3%, respectively), and showed a high number of variants (15 and 14, respectively), some of which were shared by a large number of individuals. The expression of Adamts18 has been verified in mice in the first postnatal stage, both in the sensory epithelia and the otic capsule. The functional analysis taking into account the results of the genes enriched in variants, and the genes with more represented structural variants, has highlighted certain pathways and biological processes, indicating a possible alteration in the development and function of the sensory epithelia of the inner ear, the transmission of information to higher information centers, and the generation of abnormalities in the bone matrix of the otic capsule. Conclusions - In this doctoral dissertation, the possible candidate genes and biological pathways for the development of ES hypoplasia in MD patients have been defined. The genetic and molecular characterization of this endophenotype could help to stratify different types of patients within MD, a complex and diverse pathology, improving diagnosis, prediction of evolution, and clinical decision-making.