Sensorineural deafness associated with recessive hypophosphataemic rickets

Involvement of Dmp1 in the Precise Regulation of Hair Bundle Formation in the Developing Cochlea

Dentin matrix protein 1 (Dmp1) is a highly phosphorylated, extracellular matrix protein that is extensively expressed in bone and teeth but also found in soft tissues, including brain and muscle. However, the functions of Dmp1 in the mice cochlea are unknown. Our study showed that Dmp1 was expressed in auditory hair cells (HCs), with the role of Dmp1 in those cells identified using Dmp1 cKD mice. Immunostaining and scanning electron microscopy of the cochlea at P1 revealed that Dmp1 deficiency in mice resulted in an abnormal stereociliary bundle morphology and the mispositioning of the kinocilium. The following experiments further demonstrated that the cell-intrinsic polarity of HCs was affected without apparent effect on the tissue planer polarity, based on the observation that the asymmetric distribution of Vangl2 was unchanged whereas the Gαi3 expression domain was enlarged and Par6b expression was slightly altered. Then, the possible molecular mechanisms of Dmp1 involvement in inner ear development were explored via RNA-seq analysis. The study suggested that the Fgf23-Klotho endocrine axis may play a novel role in the inner ear and Dmp1 may regulate the kinocilium-stereocilia interaction via Fgf23-Klotho signaling. Together, our results proved the critical role of Dmp1 in the precise regulation of hair bundle morphogenesis in the early development of HCs.

Characterization of hearing-impairment in Generalized Arterial Calcification of Infancy (GACI)

BACKGROUND AND IMPORTANCE

Hearing loss (HL) has been sporadically described, but not well characterized, in Generalized Arterial Calcification of Infancy (GACI), a rare disease in which pathological calcification typically presents in infancy.

OBJECTIVES

This study aims to describe the clinical audiologic and otologic features and potential etiology of hearing impairment in GACI and gain pathophysiological insight from a murine model of GACI.

DESIGN

Cross-sectional cohort study of individuals with GACI. Murine ossicle micromorphology of the ENPP1^asj/asj mutant compared to wild-type.

SETTING

Clinical research hospital; basic science laboratory.

PARTICIPANTS

Nineteen individuals with GACI who met clinical, biochemical, and genetic criteria for diagnosis.

MAIN OUTCOMES AND MEASURES

Clinical, biochemical, and radiologic features associated with hearing status.

RESULTS

Pure-tone thresholds could be established in 15 (n = 30 ears) of the 19 patients who underwent audiological assessments. The prevalence of HL was 50% (15/30) of ears, with conductive HL in 80% and sensorineural HL in 20%. In terms of patients with HL (n = 8), seven patients had bilateral HL and one patient had unilateral HL. Degree of HL was mild to moderate for 87% of the 15 ears with hearing loss. Of those patients with sufficient pure-tone and middle ear function data, 80% (8/10) had audiometric configurations suggestive of ossicular chain dysfunction (OCD). Recurrent episodes of otitis media (ROM) requiring pressure-equalizing tube placement were common. In patients who underwent cranial CT, 54.5% (6/11) had auricular calcification. Quantitative backscattered electron imaging (qBEI) of murine ossicles supports an OCD component of auditory dysfunction in GACI, suggesting loss of ossicular osteocytes without initiation of bone remodeling.

CONCLUSIONS AND RELEVANCE

Hearing loss is common in GACI; it is most often conductive, and mild to moderate in severity. The etiology of HL is likely multifactorial, involving dysfunction of the ossicular chain and/or recurrent otitis media. Clinically, this study highlights the importance of early audiologic and otologic evaluation in persons with GACI. Novel findings of high rates of OCD and ROM may inform management, and in cases of unclear HL etiology, dedicated temporal bone imaging should be considered.

Relation between vitamin D deficiency and benign paroxysmal positional vertigo

Benign paroxysmal positional vertigo (BPPV) is the most common cause of positional vertigo. Vitamin D deficiency may be one of the causes of its development. To assess the relation between recurrent attacks BPPV and Vitamin D deficiency. A case control study in which 40 patients were clinically diagnosed as posterior canal BPPV, Serum 25(OH) D was measured at 1st visit. Patients were divided into two groups; group A (20 patients) received Vitamin D supplementation in addition to canal repositioning maneuver and group B (20 patients) treated by canal repositioning maneuver only. Follow up of all patients for 6 months, neuro-otological assessment was repeated and recurrent attacks were recorded. Serum vitamin D was repeated after 6 month. This study included 14 males and 26 females age ranged from 35 to 61 years, Average serum of 25 (OH) D at the first visit was (12.4 ± 2 ng/ml) for group A, and (12.2 ± 1.7 ng/ml) for group B, all patients had low serum level of 25(OH) D (below 20 ng/ml). Recurrent BPPV episodes, were significantly lower in group A than that of group B. There is a relation between BPPV recurrence and low serum Vitamin D.

Hypocalcaemic and hypophosphatemic rickets

Rickets refers to deficient mineralization at the growth plate and is usually associated with abnormal serum calcium and/or phosphate. There are several subtypes of rickets, including hypophosphatemic rickets (vitamin-D-resistant rickets secondary to renal phosphate wasting), vitamin D-dependent rickets (defects of vitamin D metabolism) and nutritional rickets (caused by dietary deficiency of vitamin D, and/or calcium, and/or phosphate). Most rickets manifest as bone deformities, bone pain, and impaired growth velocity. Diagnosis of rickets is established through the medical history, physical examination, biochemical tests and radiographs. It is of crucial importance to determine the cause of rickets, including the molecular characterization in case of vitamin D resistant rickets, and initiate rapidly the appropriate therapy. In this review, we describe the different causes and therapies of genetic and nutritional rickets, supported by the recent progress in genetics and development of novel molecules such as anti-FGF23 antibody.

A novel auditory ossicles membrane and the development of conductive hearing loss in Dmp1-null mice

Genetic mouse models are widely used for understanding human diseases but we know much less about the anatomical structure of the auditory ossicles in the mouse than we do about human ossicles. Furthermore, current studies have mainly focused on disease conditions such as osteomalacia and rickets in patients with hypophosphatemia rickets, although the reason that these patients develop late-onset hearing loss is unknown. In this study, we first analyzed Dmp1 lac Z knock-in auditory ossicles (in which the blue reporter is used to trace DMP1 expression in osteocytes) using X-gal staining and discovered a novel bony membrane surrounding the mouse malleus. This finding was further confirmed by 3-D micro-CT, X-ray, and alizarin red stained images. We speculate that this unique structure amplifies and facilitates sound wave transmissions in two ways: increasing the contact surface between the eardrum and malleus and accelerating the sound transmission due to its mineral content. Next, we documented a progressive deterioration in the Dmp1-null auditory ossicle structures using multiple imaging techniques. The auditory brainstem response test demonstrated a conductive hearing loss in the adult Dmp1-null mice. This finding may help to explain in part why patients with DMP1 mutations develop late-onset hearing loss, and supports the critical role of DMP1 in maintaining the integrity of the auditory ossicles and its bony membrane.

Treatment of ear and bone disease in the Phex mouse mutant with dietary supplementation

HYPOTHESIS

Phosphorus and vitamin D (calcitriol) supplementation in the Phex mouse, a murine model for endolymphatic hydrops (ELH), will improve otic capsule mineralization and secondarily ameliorate the postnatal development of ELH and sensorineural hearing loss (SNHL).

BACKGROUND

Male Phex mice have X-linked hypophosphatemic rickets (XLH), which includes osteomalacia of the otic capsule. The treatment for XLH is supplementation with phosphorus and calcitriol. The effect of this treatment has never been studied on otic capsule bone and it is unclear if improving the otic capsule bone could impact the mice's postnatal development of ELH and SNHL.

METHODS

Four cohorts were studied: 1) wild-type control, 2) Phex control, 3) Phex prevention, and 4) Phex rescue. The control groups were not given any dietary supplementation. The Phex prevention group was supplemented with phosphorus added to its drinking water and intraperitoneal calcitriol from postnatal day (P) 7-P40. The Phex rescue group was also supplemented with phosphorus and calcium but only from P20 to P40. At P40, all mice underwent auditory brainstem response (ABR) testing, serum analysis, and temporal bone histologic analysis. Primary outcome was otic capsule mineralization. Secondary outcomes were degree of SNHL and presence ELH.

RESULTS

Both treatment groups had markedly improved otic capsule mineralization with less osteoid deposition. The improved otic capsule mineralized did not prevent the development of ELH or SNHL.

CONCLUSION

Supplementation with phosphorus and calcitriol improves otic capsule bone morphology in the Phex male mouse but does not alter development of ELH or SNHL.

Ectopic Mineralization and Conductive Hearing Loss in Enpp1asj Mutant Mice, a New Model for Otitis Media and Tympanosclerosis

Otitis media (OM), inflammation of the middle ear, is a common cause of hearing loss in children and in patients with many different syndromic diseases. Studies of the human population and mouse models have revealed that OM is a multifactorial disease with many environmental and genetic contributing factors. Here, we report on otitis media-related hearing loss in asj (ages with stiffened joints) mutant mice, which bear a point mutation in the Enpp1 gene. Auditory-evoked brainstem response (ABR) measurements revealed that around 90% of the mutant mice (Enpp1^asj/asj) tested had moderate to severe hearing impairment in at least one ear. The ABR thresholds were variable and generally elevated with age. We found otitis media with effusion (OME) in all of the hearing-impaired Enpp1^asj/asj mice by anatomic and histological examinations. The volume and inflammatory cell content of the effusion varied among the asj mutant mice, but all mutants exhibited a thickened middle ear epithelium with fibrous polyps and more mucin-secreting goblet cells than controls. Other abnormalities observed in the Enpp1 mutant mice include over-ossification at the round window ridge, thickened and over-calcified stapedial artery, fusion of malleus and incus, and white patches on the inside of tympanic membrane, some of which are typical symptoms of tympanosclerosis. An excessive yellow discharge was detected in the outer ear canal of older asj mutant mice, with 100% penetrance by 5 months of age, and contributes to the progressive nature of the hearing loss. This is the first report of hearing loss and ear pathology associated with an Enpp1 mutation in mice. The Enpp1^asj mutant mouse provides a new animal model for studying tympanosclerotic otitis and otitis media with effusion, and also provides a specific model for the hearing loss recently reported to be associated with human ENPP1 mutations causing generalized arterial calcification of infancy and hypophosphatemic rickets.

Early onset hearing loss in autosomal recessive hypophosphatemic rickets caused by loss of function mutation in ENPP1

Autosomal recessive hypophosphatemic rickets 2 (ARHR2) is a rare form of renal tubular phosphate wasting disorder. Loss of function mutations of the ecto-nucleotide pyrophosphatase/pyrophosphodiesterase 1 gene (ENPP1) causes a wide spectrum of phenotypes, ranging from lethal generalized arterial calcification of infancy to hypophosphatemic rickets with hypertension. Hearing loss was not previously thought to be one of the features of the disease entities and was merely regarded as a complication rather than a part of the disease. We report two children who presented in mid to late childhood with progressive varus deformity of their legs due to hypophosphatemic rickets caused by mutations in the ENPP1 gene. Both children had evidence of progressive hearing loss requiring the use of hearing aids. This report of two unrelated infants with compound heterozygous mutations in ENPP1 and previously published cases confirms that mild to moderate hearing loss is frequently associated with ARHR2. Early onset conductive hearing loss may further distinguish the autosomal recessive ENPP1 related type from other types of hypophosphatemia.

Hearing loss is part of the clinical picture of ENPP1 loss of function mutation

BACKGROUND

Ecto/nucleotide pyrophosphatase/phosphodiesterase-1 (ENPP1) loss-of-function mutations have been described in patients with autosomal recessive hypophosphatemic rickets (HR), in patients with generalized arterial calcification of infancy (GACI) and in several patients with both conditions. Out of more than 50 cases of homozygous or compound heterozygous ENPP1 loss-of-function mutations published so far, 1 case with labyrinthine deafness probably due to occlusion of inner ear supplying arteries and 2 cases of conductive hearing loss due to stapedovestibular calcification diagnosed in childhood have been reported.

AIMS

To report a case of ENPP1 loss-of-function novel mutation presenting with HR and very early onset and severe hearing loss.

METHODS

Case report and review of the literature.

RESULTS

We report on a patient homozygous for a novel 1-bp deletion in ENPP1 that presented with GACI evolving towards HR associated with a mixed hearing loss (both labyrinthine and conductive) diagnosed at 9 days of life that evolved towards profound labyrinthine deafness.

CONCLUSION

Hearing loss is a rare finding in patients with ENPP1 loss-of-function mutations. Interestingly, it has already been described in other affected patients, in ENPP1 knock-out mice and in other diseases of pyrophosphate metabolism. Conversely it seems to be absent in children with the X-linked form of HR.

A Genetic Murine Model of Endolymphatic Hydrops: The Phex Mouse

Animal models of endolymphatic hydrops (ELH) provide critical insight into the pathophysiology of Meniere's disease (MD). A new genetic murine model, called the Phex mouse, circumvents prior need for a time and cost-intensive surgical procedure to create ELH. The Phex mouse model of ELH, which also has X-linked hypophosphatemic rickets, creates a postnatal, spontaneous, and progressive ELH whose phenotype has a predictable decline of vestibular and hearing function reminiscent of human MD. The Phex mouse enables real-time histopathologic analysis to assess diagnostic and therapeutic interventions as well as further our understanding of ELH's adverse effects. Already the model has validated electrocochleography and cervical vestibular evoked myogenic potential as useful diagnostic tools. New data on caspase activity in apoptosis of the spiral ganglion neurons may help target future therapeutic interventions. This paper highlights the development of the Phex mouse model and highlights its role in characterizing ELH.

Therapeutic management of hypophosphatemic rickets from infancy to adulthood

In children, hypophosphatemic rickets (HR) is revealed by delayed walking, waddling gait, leg bowing, enlarged cartilages, bone pain, craniostenosis, spontaneous dental abscesses, and growth failure. If undiagnosed during childhood, patients with hypophosphatemia present with bone and/or joint pain, fractures, mineralization defects such as osteomalacia, entesopathy, severe dental anomalies, hearing loss, and fatigue. Healing rickets is the initial endpoint of treatment in children. Therapy aims at counteracting consequences of FGF23 excess, i.e. oral phosphorus supplementation with multiple daily intakes to compensate for renal phosphate wasting and active vitamin D analogs (alfacalcidol or calcitriol) to counter the 1,25-diOH-vitamin D deficiency. Corrective surgeries for residual leg bowing at the end of growth are occasionally performed. In absence of consensus regarding indications of the treatment in adults, it is generally accepted that medical treatment should be reinitiated (or maintained) in symptomatic patients to reduce pain, which may be due to bone microfractures and/or osteomalacia. In addition to the conventional treatment, optimal care of symptomatic patients requires pharmacological and non-pharmacological management of pain and joint stiffness, through appropriated rehabilitation. Much attention should be given to the dental and periodontal manifestations of HR. Besides vitamin D analogs and phosphate supplements that improve tooth mineralization, rigorous oral hygiene, active endodontic treatment of root abscesses and preventive protection of teeth surfaces are recommended. Current outcomes of this therapy are still not optimal, and therapies targeting the pathophysiology of the disease, i.e. FGF23 excess, are desirable. In this review, medical, dental, surgical, and contributions of various expertises to the treatment of HR are described, with an effort to highlight the importance of coordinated care.

New intragenic deletions in the Phex gene clarify X-linked hypophosphatemia-related abnormalities in mice

X-linked hypophosphatemic rickets (XLH) in humans is caused by mutation in the PHEX gene. Previously, three mutations in the mouse Phex gene have been reported: Phex(Hyp), Gy, and Phex(Ska1). Here we report analysis of two new spontaneous mutation in the mouse Phex gene, Phex(Hyp-2J) and Phex(Hyp-Duk). Phex(Hyp-2J) and Phex(Hyp-Duk) involve intragenic deletions of at least 7.3 kb containing exon 15, and 30 kb containing exons 13 and 14, respectively. Both mutations cause similar phenotypes in males, including shortened hind legs and tail, a shortened square trunk, hypophosphatemia, hypocalcemia, and rachitic bone disease. In addition, mice carrying the Phex(Hyp-Duk) mutation exhibit background-dependent variable expression of deafness, circling behavior, and cranial dysmorphology, demonstrating the influence of modifying genes on Phex-related phenotypes. Cochlear cross-sections from Phex(Hyp-2J)/Y and Phex(Hyp-Duk)/Y males reveal a thickening of the temporal bones surrounding the cochlea with the presence of a precipitate in the scala tympani. Evidence of the degeneration of the organ of Corti and spiral ganglion also are present in the hearing-impaired Phex(Hyp-Duk)/Y mice, but not in the normal-hearing Phex(Hyp-2J)/Y mice. Analysis of the phenotypes noted in Phex(Hyp-Duk)/Y and Phex(Hyp-2J)/Y males, together with those noted in Phex(Ska1)/Y and Phex(Hyp)/Y males, now allow XLH-related phenotypes to be separated from non-XLH-related phenotypes, such as those noted in Gy/Y males. Also, identification of the genetic modifiers of hearing and craniofacial dysmorphology in Phex(Hyp-Duk)/Y mice could provide insight into the phenotypic variation of XLH in humans.

Ear and kidney syndromes: molecular versus clinical approach

The association between ear and kidney anomalies is not usually due to an insult to the embryo. In recent years, many essential development control genes that coordinate the assembly and function of kidney and ear have been discovered through the generation of animal mutants and have increased our understanding of the mechanisms of human oto-renal diseases. Here, we describe ear and kidney clinical syndromes and their molecular expression.

The petrous temporal bone and deafness in X-linked hypophosphataemic osteomalacia

Radiological anatomy of the petrous bones and skull base has been studied in a group of 11 patients with X-linked hypophosphataemic osteomalacia and hearing loss, and comparison made with a control group of normal subjects. Patients with X-linked hypophosphataemic osteomalacia were found to have generalised osteosclerosis and thickening of the petrous bone, with some narrowing of the internal auditory meatus, particularly in its mid-portion. No evidence of platybasia nor of basilar impression was found in the hypophosphataemmic group. The aetiology of the hearing loss which occurs in patients with X-linked hypophosphataemic osteomalacia is discussed in relation to the radiological abnormalities described.

Audiologic findings in young patients with hypophosphatemic bone disease

Audiometric tests were conducted on 19 subjects with hypophosphatemic bone disease to investigate whether auditory impairment like that reported in affected adults occurs in young patients. No hearing loss or significant auditory findings were noted among the children or young adult patients. However, sensorineural hearing loss of cochlear origin was identified in the three oldest subjects (40 to 58 years), although a history of noise exposure in two of them could explain the observations. The results indicate that if an association exists between hypophosphatemic bone disease and hearing impairment, the auditory signs will not develop until adulthood in treated patients. Additional studies of large populations of affected adults are needed to identify the incidence and mechanism(s) of the auditory system abnormalities and to assess any effect of medical therapy for the metabolic bone disease.

The Gy mutation: another cause of X-linked hypophosphatemia in mouse

An X-linked dominant mutation (gyro, gene symbol Gy) in the laboratory mouse causes hypophosphatemia, rickets/osteomalacia, circling behavior, inner ear abnormalities, and sterility in males and a milder phenotype in females. Gy maps closely (crossover value 0.4-0.8%) to another X-linked gene (Hyp) that also causes hypophosphatemia in the mouse. Gy and Hyp genes have similar quantitative expression in serum phosphorus values, renal excretion of phosphate, and impairment of Na+/phosphate cotransport by renal brush-border membrane vesicles. These findings indicate that independent translation products of two X-linked genes serve phosphate transport in mouse kidney and thereby control phosphate content of extracellular fluid. The Gy translation product, unlike the Hyp product, is also expressed in the inner ear. These findings have implications for our understanding of the human counterpart known as "X-linked hypophosphatemia."

Electrocochleographic changes in the hearing loss associated with X-linked hypophosphataemic osteomalacia

Transtympanic electrocochleography was carried out on 13 patients (21 ears) suffering from X-linked hypophosphataemic osteomalacia, and known to have cochlear hearing loss. The majority of ears exhibited a cochlear pattern of cochleogram with a recruiting input/output function and a markedly biphasic action potential. In addition 14 ears yielded a greatly enhanced negative summating potential, suggesting the presence of endolymphatic hydrops. It is proposed that the hearing loss so frequently associated with this condition is in part due to endolymphatic hydrops.

Vitamin D deficiency and otosclerosis

A prospective study of 47 patients with otosclerosis was undertaken to investigate the possible etiologic role of vitamin D undernutrition. The population comprised 27 women and 20 men, with a mean age of 46.4 years (range 21 to 79). The disease was bilateral in 43 patients, and cochlear involvement was present in 84.4%. The mean duration of symptoms was 17.1 years. Vitamin D status was evaluated by measuring the plasma 25-hydroxy vitamin D3 (25-OHD), which is the main storage metabolite. Abnormally low 25-OHD levels were found in 10 patients (21.7%) and borderline low levels in another two. Raised serum alkaline phosphatase levels were present in 32.6%, calcium in 6.5%, and inorganic phosphate in 4.3%. Calcium and vitamin D replacement therapy resulted in significant hearing improvement in 3 of 16 patients; these data support a causal correlation. Vitamin D deficiency is probably a factor in the etiology of some cases of otosclerosis and is important, since the deafness resulting from cochlear involvement may be reversible.

Vitamin D deficiency--a new cause of cochlear deafness

Ten patients are reported with bilateral cochlear deafness which was associated with vitamin D deficiency. The features of these cases are discussed following an overview of the clinical aspects and diagnosis of vitamin D deficiency. The most likely pathogenesis is localized demineralization of the cochlea resulting in secondary morphological changes. Replacement therapy resulted in unilateral hearing improvement in two of the four patients in whom the response to treatment could be assessed. This suggests a previously unrecognized causal correlation between vitamin D deficiency and cochlear deafness. Impaired vitamin D activity may be important in the aetiology of otosclerosis, presbyacusis and the deafness associated with chronic renal failure. Vitamin D deficiency should be considered in the differential diagnosis of unexplained bilateral cochlear deafness. It is important, as this 'new' metabolic type of sensorineural deafness may be reversible, and may also lead to the diagnosis of early osteomalacia before more serious generalized skeletal symptoms can occur.

Spinal canal stenosis in adult with hypophosphataemic vitamin D-resistant rickets

A case is described of a 55-year-old male with hypophosphataemic vitamin D-resistant rickets, who presented with long-standing back pain and paraesthesiae in both legs considered to be due to osteoid encroachment on an already narrow spinal canal. Marked symptomatic improvement has followed extensive lumbar laminectomy.