Conductive Hearing Loss in the Hyp Mouse Model of X-Linked Hypophosphatemia Is Accompanied by Hypomineralization of the Auditory Ossicles

Abstract

X-linked hypophosphatemia (XLH) is a hereditary musculoskeletal disorder caused by loss-of-function mutations in the PHEX gene. In XLH, increased circulating fibroblast growth factor 23 (FGF23) levels cause renal phosphate wasting and low concentrations of 1,25-dihydroxyvitamin D, leading to an early clinical manifestation of rickets. Importantly, hearing loss is commonly observed in XLH patients. We present here data from two XLH patients with marked conductive hearing loss. To decipher the underlying pathophysiology of hearing loss in XLH, we utilized the Hyp mouse model of XLH and measured auditory brain stem responses (ABRs) and distortion product otoacoustic emissions (DPOAEs) to functionally assess hearing. As evidenced by the increased ABR/DPOAE threshold shifts in the mid-frequency range, these measurements indicated a predominantly conductive hearing loss in Hyp mice compared to wild-type (WT) mice. Therefore, we carried out an in-depth histomorphometric and scanning electron microscopic analysis of the auditory ossicles. Quantitative backscattered electron imaging (qBEI) indicated a severe hypomineralization of the ossicles in Hyp mice, evidenced by lower calcium content (CaMean) and higher void volume (ie, porosity) compared to WT mice. Histologically, voids correlated with unmineralized bone (ie, osteoid), and the osteoid volume per bone volume (OV/BV) was markedly higher in Hyp mice than WT mice. The density of osteocyte lacunae was lower in Hyp mice than in WT mice, whereas osteocyte lacunae were enlarged. Taken together, our findings highlight the importance of ossicular mineralization for hearing conduction and point toward the potential benefit of improving mineralization to prevent hearing loss in XLH.

Prevention of Hypomineralization In Auditory Ossicles of Vitamin D Receptor (Vdr) Deficient Mice

“…Interestingly, the osteoid volume in the auditory ossicles of Vdr -/- reg mice was considerably lower than in the lumbar spine, while the rescue effect appeared to be relatively attenuated. This phenomenon is likely due to a combination of rapid postnatal ossicular development with little remodeling after the first few postnatal weeks ( 10 ), and unaffected mineral homeostasis until the third week in Vdr -/- mice ( 3 ). Specifically, in the auditory ossicles, bone development is completed more rapidly than in other bones ( 10 , 22 , 23 ).…”

“…This phenomenon is likely due to a combination of rapid postnatal ossicular development with little remodeling after the first few postnatal weeks ( 10 ), and unaffected mineral homeostasis until the third week in Vdr -/- mice ( 3 ). Specifically, in the auditory ossicles, bone development is completed more rapidly than in other bones ( 10 , 22 , 23 ). The longitudinal growth is already completed after 20 days and the endochondral ossification between 6-12 weeks after birth ( 10 , 22 ).…”

“…Specifically, in the auditory ossicles, bone development is completed more rapidly than in other bones ( 10 , 22 , 23 ). The longitudinal growth is already completed after 20 days and the endochondral ossification between 6-12 weeks after birth ( 10 , 22 ). During endochondral ossification, large capillary loops lined with endothelial cells can be seen in the ossicles immediately after birth, which becomes mineralized by specific type I and type II collagen-producing auditory osteoblasts ( 24 , 25 ).…”

“…A correlation between poor bone quality and hearing ability was also shown for other mouse models. In FGF23-deficient mice suffering from hyperphosphatemia and hypercalcemia with resulting defective bone mineralization ( 36 ), a mixed conductive and sensorineural hearing loss was reported ( 11 ), whereas in Hyp mice with FGF23 overexpression and consecutive hypophosphatemia and mineralization impairment, a predominant conductive hearing loss was present ( 10 ). In Vdr -/- mice, sensorineural hearing loss associated with loss of spiral ganglion cells in the basal turn has been previously reported, however, conductive hearing loss had not been investigated in this model ( 37 ).…”

“…However, it remains unknown whether dietary or bone-targeted treatments ameliorate ossicular hypomineralization and conductive hearing loss. This question remains particularly worthy of investigation considering that in humans and mice the development and mineralization of ossicles is normally completed shortly after birth and little ossicular remodeling accompanied by high matrix mineralization has been observed across species ( 10 , 13 , 14 ). Therefore, this study aims to characterize the mineralization properties of the auditory ossicles in Vdr -/- mice, also focusing on the treatment effects by a calcium-/phosphate-rich diet.…”