Rare A2ML1 variants confer susceptibility to otitis media

Abstract

A duplication variant within the middle ear-specific gene A2ML1 cosegregates with otitis media in an indigenous Filipino pedigree (LOD score = 7.5 at reduced penetrance) and lies within a founder haplotype that is also shared by 3 otitis-prone European-American and Hispanic-American children but is absent in non-otitis-prone children and >62,000 next-generation sequences. We identified seven additional A2ML1 variants in six otitis-prone children. Collectively, our studies support a role for A2ML1 in the pathophysiology of otitis media.

Figure 1. Segregation within the indigenous pedigree, cartoon of A2ML1 domains and molecular modeling for the A2ML1 variant

[A] Pedigree connecting 37 variant carriers who have the full spectrum of otitis media (age 3 months to 58 years, median 13 years). An individual with healed otitis media and intellectual disability (ID) is wildtype for the duplication, but her 13-year old son who has chronic otitis media but no ID and her unaffected 4-month old daughter are heterozygous. Nine variant carriers had no evidence of otitis media (median 18 years), while four individuals are wildtype and unaffected. [B] Predicted A2ML1 domains based on alpha 2-macroglobulin structure. MG, macroglobulin-like domains 1-7; BRD, bait-region domain; CUB, consists of two four-stranded antiparallel β-sheets; TED, thiol-ester binding domain; RBD, receptor-binding domain. The A2ML1 frameshift variant is expected to occur within the MG7 domain (red arrow). [C] Modeling predicts loss of the receptor-binding and thiol-ester domains due to the A2ML1 duplication.

Figure 2. A2ML1 is localized to middle ear epithelium

[A, C] Sagittal cryosections from P6 wildtype mice showing the middle ear cavity (MEC), the middle ear mucosa (MEM) and semicircular canal of the inner ear (SCC). [B, D] Higher magnification depicting boxed sections in A and C. [A-D] Confocal images of the cryosections immunostained with antibodies for DAPI (blue), A2ML1 (green in A & B), β-catenin (green in C & D) and rhodamine phalloidin (Actin, red). White arrowheads point to MEM with A2ML1 [B] and β-catenin expression [D]. DIC, differential interference contrast. Scale bar: 100 μm.