A mouse model with postnatal endolymphatic hydrops and hearing loss

https://doi.org/10.1016/j.heares.2008.01.002Get rights and content

Abstract

Endolymphatic hydrops (ELH), hearing loss and neuronal degeneration occur together in a variety of clinically significant disorders, including Meniere’s disease (MD). However, the sequence of these pathological changes and their relationship to each other are not well understood. In this regard, an animal model that spontaneously develops these features postnatally would be useful for research purposes. A search for such a model led us to the PhexHyp-Duk mouse, a mutant allele of the Phex gene causing X-linked hypophosphatemic rickets. The hemizygous male (PhexHyp-Duk/Y) was previously reported to exhibit various abnormalities during adulthood, including thickening of bone, ELH and hearing loss. The reported inner-ear phenotype was suggestive of progressive pathology and spontaneous development of ELH postnatally, but not conclusive. The main focuses of this report are to further characterize the inner ear phenotype in PhexHyp-Duk/Y mice and to test the hypotheses that (a) the PhexHyp-Duk/Y mouse develops ELH and hearing loss postnatally, and (b) the development of ELH in the PhexHyp-Duk/Y mouse is associated with obstruction of the endolymphatic duct (ED) due to thickening of the surrounding bone. Auditory brainstem response (ABR) recordings at various times points and histological analysis of representative temporal bones reveal that PhexHyp-Duk/Y mice typically develop adult onset, asymmetric, progressive hearing loss closely followed by the onset of ELH. ABR and histological data show that functional degeneration precedes structural degeneration. The major degenerative correlate of hearing loss and ELH in the mutants is the primary loss of spiral ganglion cells. Further, PhexHyp-Duk/Y mice develop ELH without evidence of ED obstruction, supporting the idea that ELH can be induced by a mechanism other than the blockade of longitudinal flow of endolymphatic fluid, and occlusion of ED is not a prerequisite for the development of ELH in patients.

Introduction

Endolymphatic hydrops was first found to be associated with Meniere’s disease (MD) by Hallpike and Cairns, 1938, Yamakawa, 1938. The significance of ELH in the development of symptoms in patients with MD has been recognized by many investigators, and the histological finding of Reissner’s membrane distension is taken to be the hallmark of ELH (Arenberg et al., 1970, Belal and Ylikoski, 1980, Fraysse et al., 1980, Paparella, 1984, Schuknecht, 1976). However, ELH occurs in conditions other than Meniere’s disease, including delayed endolymphatic hydrops following trauma (Dodson et al., 2007), relapsing polychondritis (Murata et al., 2006), and some cases of acoustic neuroma (Mahmud et al., 2003). Unfortunately, current therapies directed at alleviating symptoms via presumed amelioration of ELH using diuretics and shunt surgery have been largely unsuccessful in preventing the progression of hearing deterioration in patients with ELH-related disorders. This is due, in part, to a lack of understanding of the relationship among ELH, hearing loss and neural degeneration. Animal models are necessary since appropriate human specimens are difficult to obtain.

Kimura and Schucknecht developed in 1965 a guinea pig model of ELH (Kimura and Schuknecht, 1965). ELH is achieved in this model by surgical obliteration of the endolymphatic duct (ED), and distention of Reissner’s membrane ensues days to weeks after surgery. The guinea pig model has proven to be useful in the study of ELH, particularly in relation to the human MD condition (Aran et al., 1984, Hott et al., 2003, Nadol, 1990). However, this model suffers from some important limitations. First, it requires surgery to produce the phenotype. Second, this model does not reliably produce vestibular symptoms (Horner, 1995). Lastly, and most importantly, induction of ELH by surgical obliteration of the ED is a mechanism that deviates from the spontaneous development of ELH that usually occurs in MD. An animal model that spontaneously develops ELH and hearing loss some time after birth would be a good supplement to the classic guinea pig model.

Mouse mutants have served as useful models for a wide variety of inner ear disorders. A survey of literature for a potential mouse model for hearing loss and ELH drew our attention to a mutant called PhexHyp-Duk (Lorenz-Depiereux et al., 2004). The PhexHyp-Duk mouse harbors a loss-of-function mutation in the Phex gene, and hemizygous males (PhexHyp-Duk/Y) exhibit various phenotypic features including shortened hind legs and tail, hypophosphatemia, hypocalcemia and rickets-like bone disease. In the BALB/cAnBomUrd genetic background, the PhexHyp-Duk/Y mouse develops an ear phenotype that includes variable hearing loss and compromised balance function. A cochlear duct section from a 5-month-old PhexHyp-Duk/Y mouse (only one time point was reported) showed ELH and thickening of the bone surrounding the cochlea with the presence of a precipitate in the scala tympani (Lorenz-Depiereux et al., 2004). This report led us to hypothesize that (a) PhexHyp-Duk/Y mice develop ELH and hearing loss postnatally, and (b) development of ELH in PhexHyp-Duk/Y mice is associated with obstruction of the endolymphatic duct (ED) due to thickening of the surrounding bone. To test these hypotheses, we undertook a detailed characterization of the inner ear phenotype in PhexHyp-Duk/Y mice, and the results are presented here.

Section snippets

Mice

The PhexHyp-Duk allele, a spontaneous mutation in the BALB/cAnBomUrd background (now to be shortened to BALB/cUrd), was obtained from The Jackson Laboratory (TJL). PhexHyp-Duk mutation maintained in the C57BL/6J (B6) background was also available/obtained from TJL. To obtain Phex males and control littermates, BALB/cUrd (or B6) PhexHyp-Duk carrier females (+/PhexHyp-Duk) were bred with BALB/cUrd (or B6) Phex wildtype (+/Y) mice. The Animal Care and Use Committee of CWRU approved the care and

Onset and deterioration of inner ear function

Signs of vestibular dysfunction in PhexHyp-Duk/Y mice were typically observed around P15. This time varied between affected mice from the same litter, although the balance dysfunction became obvious beyond P30 in most cases. Signs of vestibular dysfunction in PhexHyp-Duk/Y mice at 2–3 weeks suggested hearing function might be affected at a young age.

To determine the onset and progression of hearing loss in PhexHyp-Duk/Y mice, click-evoked ABR was measured starting at P21, the age of mature

Discussion

Our search for an animal model that spontaneously develops ELH, hearing loss and degeneration postnatally led us to investigate the PhexHyp-Duk mice in the BALB/cUrd and later in the B6 background. Collectively, data from both studies show that (a) PhexHyp-Duk mice typically develop adult onset, asymmetric, progressive hearing loss closely followed by the onset of ELH (as observed by distention of the Reissner’s membrane), (b) functional degeneration precedes structural degeneration, according

Acknowledgments

We would like to thank Dr. Brian McDermott and Chris Heddon for critical reading of this manuscript. This work was supported by grants to Drs. Alagramam and Megerian from Rainbow Board of Trustees, Rainbow Babies and Children’s Hospitals, University Hospitals Case Medical Center.

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