60 61 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 2 1 / 2 2 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 2 1 / 2 2 Understanding hearing loss in osteogenesis imperfecta Related publication: de Paolis, A., Miller, B. J., Doube, M., Bodey, A. J., Rau, C., Richter, C.-P., Cardoso, L., & Carriero, A. Increased cochlear otic capsule thickness and intracortical canal porosity in the oim mousemodel of osteogenesis imperfecta. Journal of Structural Biology 213 , 107708 (2021). DOI: 10.1016/j.jsb.2021.107708 Publication keywords: Brittle bone disease; oim ; Cochlea; Otic capsule; Cortical bone; Porosity O steogenesis imperfecta (OI) is ahereditary (genetic) disease of collagendefects, commonly knownas brittle bone disease. Progressive hearing loss starts in childhood and affects 70% of people with OI. There is no cure for OI, and the current treatments for hearing loss in this population have a low success rate. A comprehensive understanding of the auditory system in OI will provide insights into the disease and guide the development of effective therapies to halt or reduce hearing loss. The mechanisms leading to progressive hearing loss in OI are currently unknown. Changes in the OI ear shape and internal bone structure may explain the hearing loss in this population and guide the development of new treatments. An international team of researchers used the Diamond Manchester Imaging Branchline (I13-2) to study the 3D ear geometry of a mouse model of OI, called oim, suffering from hearing loss. The model needed to be visualised with high-resolution topographic images (810 nm) to reveal the cochlea and otic capsule morphology and internal porosity at the tissue and cellular level to determine any difference with the healthy inner ears. They found outward growth of the otic capsule characterised by highly porous bone, with no change in the cochlea shape in the oim mouse inner ear. The disease seems to be affecting the mineralised tissue rather than the soft tissue in the inner ear. Understanding the changes in the oim ears will help develop effective treatments for hearing loss in OI. Osteogenesis imperfecta (OI or brittle bone disease) is a group of inherited genetic disorders of the connective tissues caused by mutations in the genes encoding collagen type I 1 , the major protein in the human body. As such OI affects bone, ligaments, tendons, skin, eye, ears, lungs and hearts. Clinical hallmarks of OI are extreme skeletal fragility, bone deformities, joint laxity and severe functional disabilities, such as breathing disorders and hearing loss 1-4 . Brittle bone disease affects 1 in 10,000 children, and there is currently no cure for it 1 . About 70% of people with OI are affected by progressive hearing loss, starting in childhood 2-4 . Current treatments for ameliorating hearing loss in this population rely on standard procedures with low success rate. The hearing loss in OI may affect the middle ear alone (conductive hearing loss), or the inner ear alone (sensorineural hearing loss) or both (mixed hearing loss). Studies have found that the inner ear compartment is involved inmore than half of the cases of OI hearing loss. Despite this knowledge on the auditory function in OI, little is known about the properties of ears in OI, and the mechanisms leading to the onset and progression of hearing loss. Figure 1 represents the middle and inner ear of a mouse obtained from segmented synchrotron Microtomography images taken at I13-2. In mice, as in humans, hearing starts at the pinna. Sound waves travel along the outer ear canal and vibrate the tympanic membrane, which terminates the outer ear canal and is connected to the middle ear ossicles. The middle ear ossicles, malleus, incus, and stapes, transmit the sound induced vibrations to the inner ear or cochlea, a 3D coiled structure with the appearance of a snail. Its outer shell, the otic capsule, is bone. In the cochlea, soft tissue structures including the basilar and tectorial membrane, transform acoustically induced vibrations into trains of action potentials, which are carried to the brain and perceived as sound. Inside the cochlea three “tubes” follow the coiled structure, scala vestibuli, scala media and scala tympani. From the centre of the cochlea, the modiolus, the basilar membrane, a collagen rich soft tissue structure, spans to the cochlear wall. On top of the basilar membrane sits the organ of Corti and the tectorial membrane, delicate collagen rich soft tissue structures, which transform sound induced vibrations into action potentials along the auditory nerve, information that can be read by the brain. Architecture and composition of the different compartments of the ear play a fundamental role in determining the auditory function. Therefore, a comprehensive understanding of the properties of the auditory system in OI would provide insights into the disease and guide the development of effective therapies to halt or reduce hearing loss. However, the small dimensions of the ear compartments and their anatomical position located internal in the cranium, limit the ability to conduct a comprehensive investigation of the ear properties in living people with OI in relation to their hearing loss. In the highlighted research we have used synchrotron X-ray Microtomography at I13-2 to investigate the morphology and porosity of the inner ear cochlea and surrounding bony otic capsule in the osteogenesis imperfecta murine model (B6C3Fe-a/aCol1a2 oim / oim or oim ), which also experiences hearing loss in its homozygotes mice 5 . Figure 1b shows an X-ray microtomographic slide of the cochlea in the transverse plane with its surrounding otic capsule after reconstruction. Changes in the cochlea and otic capsule architecture due to OI may adversely affect the inner ear auditory function and enhance our understanding of hearing loss in this disease. We collected 3D images at high-resolution (810 nm nominal resolution) of oim and WT inner ears at 8 weeks of age, using a partially-coherent, filtered, polychromatic ‘pink’ beam to analyse the soft and hard (mineralised) tissue morphology, internal porosity at the tissue and cellular level, and otic capsule tissue mineral density 1 . 3D morphology was examined and morphometric parameters of the bony otic capsule volume and thickness, and its intracortical porosity (vascular canals at the tissue level and osteocyte lacunae at the cellular level) were assessed. For the cochlea, the volume in the ducts and helicotrema, and the external spiral length were quantified. Differences between the oim andWT inner ears were evaluated for statistical significance. Our results show that the morphology of the cochlea is preserved in the oim inner ears at 8 weeks of age but their bony otic capsule is thicker and more porous, with more numerous and highly connected canals than in healthy ears (Fig. 2). These findings portray a state of compromised bone quality in the otic capsule of the oim mice that may change the mechanical properties of the inner ear and contribute to their hearing loss. Specifically, these alterations of the otic capsule architecture and porosity reduce bone conduction of sound in OI ears, and/or favour the formation of microcracks and fractures in OI ears, and/or alter ion and fluid balance in OI inner ears and impair function of their sensory hair cells. A full understanding of the changes in the oim ears will guide the development of targeted treatments to halt or reduce its hearing loss. References: 1. Carriero, A. et al. How tough is brittle bone? Investigating osteogenesis imperfecta in mouse bone. Journal of Bone and Mineral Research 29 , 1392–1401 (2014). DOI: 10.1002/jbmr.2172 2. Pedersen, U. Hearing loss in patients with osteogenesis imperfecta A Clinical and Audiological Study of 201 Patients . Scandinavian Audiology 13 , 67–74 (1984). DOI: 10.3109/01050398409043042 3. Pillion, J. P. et al . Audiological findings in osteogenesis imperfecta. Journal of the American Academy of Audiology 19 , 595–601 (2008). DOI: 10.3766/jaaa.19.8.3 4. Ting, T. H. et al. Hearing in bisphosphonate-treated children with osteogenesis imperfecta: Our experience in thirty-six young patients. Clinical Otolaryngology 37 , 229–233 (2012). DOI: 10.1111/j.1749- 4486.2012.02476.x 5. Chen, W. et al . Single-nucleotide polymorphisms in the COL1A1 regulatory regions are associated with otosclerosis. Clinical Genetics 71, 406–414 (2007). DOI: 10.1111/j.1399-0004.2007.00794.x Funding acknowledgement: This work was supported by Diamond Light Source (beamtime MT9860) and National Science Foundation CBET 1829310 (A.C.). Corresponding author: Alessandra Carriero, The City College of New York,
[email protected] Imaging andMicroscopy Group Beamline I13-2 Figure 1: ( a ) Mouse middle and inner ear as reconstructed from synchrotron X-ray Microtomographic images acquired at the I13-2 (at 1.6 um nominal resolution); ( b ) Reconstructed synchrotron X-ray Microtomographic slide at 810 nm resolution showing the cochlea ducts (dark grey), the modiolus in the centre of the cochlea and the surrounding otic capsule (grey) in a transverse plane. Figure 2: ( a , b ) Synchrotron X-ray Microtomographic slice of healthy (wild type or WT) and osteogenesis imperfecta murine model (oim) representative otic capsule (green) showing the difference in coronal cortical thickness; ( c , d ) Surface render of the canal network (red) respectively within the WT and oim cochlear otic capsule (gray).