Hearing loss in a mouse model of Muenke syndrome

Hereditary Hearing Loss: A Systematic Review of Potential Treatments and Interventions

PURPOSE

The purpose of this study was to systematically review the research literature with regards to treatments and intervention methods for hereditary hearing loss. Our goal was to provide reference guidelines for the rational use of medication and gene-targeted therapy for patients with hereditary hearing loss and discuss the future development of research in this area.

METHOD

We searched two core databases, PubMed and Web of Science, for relevant literature relating to potential treatments and interventional methods for hereditary hearing loss. Then, we used Microsoft Excel to perform basic statistical analysis of the data, the R language to perform bibliometric analyses, and VOSviewer and CiteSpace to visualize data. In addition, we clustered and descriptively analyzed the data and identified the relative importance of each approach with regard to precise patient outcomes.

RESULTS

In this study, we followed the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) standardized screening process and identified a total of 103 research articles. The average annual growth rate of publications in this area was 12.73%. The country with the highest number of publications and citations was the United States; 80 of these publications (associated with 76.92% of funding) were supported by grants from 16 countries. Potential treatments and interventions were clustered according to the stage of research and showed that 8.74% remain in the research design stage, 59.22% are in the clinical validation stage, and 32.04% are being applied in the clinic. The main research focus in this field is cochlear implants and gene therapy.

CONCLUSIONS

Hereditary hearing loss is in a critical period of transition from preventive to therapeutic research. Gene-targeted interventions represent one of the most promising and effective treatments.

SUPPLEMENTAL MATERIAL

https://doi.org/10.23641/asha.24309193.

Cochlear Development; New Tools and Approaches

The sensory epithelium of the mammalian cochlea, the organ of Corti, is comprised of at least seven unique cell types including two functionally distinct types of mechanosensory hair cells. All of the cell types within the organ of Corti are believed to develop from a population of precursor cells referred to as prosensory cells. Results from previous studies have begun to identify the developmental processes, lineage restrictions and signaling networks that mediate the specification of many of these cell types, however, the small size of the organ and the limited number of each cell type has hampered progress. Recent technical advances, in particular relating to the ability to capture and characterize gene expression at the single cell level, have opened new avenues for understanding cellular specification in the organ of Corti. This review will cover our current understanding of cellular specification in the cochlea, discuss the most commonly used methods for single cell RNA sequencing and describe how results from a recent study using single cell sequencing provided new insights regarding cellular specification.

The clinical manifestations, molecular mechanisms and treatment of craniosynostosis

Craniosynostosis is a major congenital craniofacial disorder characterized by the premature fusion of cranial suture(s). Patients with severe craniosynostosis often have impairments in hearing, vision, intracranial pressure and/or neurocognitive functions. Craniosynostosis can result from mutations, chromosomal abnormalities or adverse environmental effects, and can occur in isolation or in association with numerous syndromes. To date, surgical correction remains the primary treatment for craniosynostosis, but it is associated with complications and with the potential for re-synostosis. There is, therefore, a strong unmet need for new therapies. Here, we provide a comprehensive review of our current understanding of craniosynostosis, including typical craniosynostosis types, their clinical manifestations, cranial suture development, and genetic and environmental causes. Based on studies from animal models, we present a framework for understanding the pathogenesis of craniosynostosis, with an emphasis on the loss of postnatal suture mesenchymal stem cells as an emerging disease-driving mechanism. We evaluate emerging treatment options and highlight the potential of mesenchymal stem cell-based suture regeneration as a therapeutic approach for craniosynostosis.

Comparative assessment of Fgf's diverse roles in inner ear development: A zebrafish perspective

Progress in understanding mechanisms of inner ear development has been remarkably rapid in recent years. The research community has benefited from the availability of several diverse model organisms, including zebrafish, chick, and mouse. The complexity of the inner ear has proven to be a challenge, and the complexity of the mammalian cochlea in particular has been the subject of intense scrutiny. Zebrafish lack a cochlea and exhibit a number of other differences from amniote species, hence they are sometimes seen as less relevant for inner ear studies. However, accumulating evidence shows that underlying cellular and molecular mechanisms are often highly conserved. As a case in point, consideration of the diverse functions of Fgf and its downstream effectors reveals many similarities between vertebrate species, allowing meaningful comparisons the can benefit the entire research community. In this review, I will discuss mechanisms by which Fgf controls key events in early otic development in zebrafish and provide direct comparisons with chick and mouse.

Inner hair cell dysfunction in Klhl18 mutant mice leads to low frequency progressive hearing loss

Age-related hearing loss in humans (presbycusis) typically involves impairment of high frequency sensitivity before becoming progressively more severe at lower frequencies. Pathologies initially affecting lower frequency regions of hearing are less common. Here we describe a progressive, predominantly low-frequency recessive hearing impairment in two mutant mouse lines carrying different mutant alleles of the Klhl18 gene: a spontaneous missense mutation (Klhl18lowf) and a targeted mutation (Klhl18tm1a(KOMP)Wtsi). Both males and females were studied, and the two mutant lines showed similar phenotypes. Threshold for auditory brainstem responses (ABR; a measure of auditory nerve and brainstem neural activity) were normal at 3 weeks old but showed progressive increases from 4 weeks onwards. In contrast, distortion product otoacoustic emission (DPOAE) sensitivity and amplitudes (a reflection of cochlear outer hair cell function) remained normal in mutants. Electrophysiological recordings from the round window of Klhl18lowf mutants at 6 weeks old revealed 1) raised compound action potential thresholds that were similar to ABR thresholds, 2) cochlear microphonic potentials that were normal compared with wildtype and heterozygous control mice and 3) summating potentials that were reduced in amplitude compared to control mice. Scanning electron microscopy showed that Klhl18lowf mutant mice had abnormally tapering of the tips of inner hair cell stereocilia in the apical half of the cochlea while their synapses appeared normal. These results suggest that Klhl18 is necessary to maintain inner hair cell stereocilia and normal inner hair cell function at low frequencies.

Slc26a9P2ACre : a new CRE driver to regulate gene expression in the otic placode lineage and other FGFR2b-dependent epithelia

Pan-otic CRE drivers enable gene regulation throughout the otic placode lineage, comprising the inner ear epithelium and neurons. However, intersection of extra-otic gene-of-interest expression with the CRE lineage can compromise viability and impede auditory analyses. Furthermore, extant pan-otic CREs recombine in auditory and vestibular brain nuclei, making it difficult to ascribe resulting phenotypes solely to the inner ear. We have previously identified Slc26a9 as an otic placode-specific target of the FGFR2b ligands FGF3 and FGF10. We show here that Slc26a9 is otic specific through E10.5, but is not required for hearing. We targeted P2ACre to the Slc26a9 stop codon, generating Slc26a9^P2ACre mice, and observed CRE activity throughout the otic epithelium and neurons, with little activity evident in the brain. Notably, recombination was detected in many FGFR2b ligand-dependent epithelia. We generated Fgf10 and Fgf8 conditional mutants, and activated an FGFR2b ligand trap from E17.5 to P3. In contrast to analogous mice generated with other pan-otic CREs, these were viable. Auditory thresholds were elevated in mutants, and correlated with cochlear epithelial cell losses. Thus, Slc26a9^P2ACre provides a useful complement to existing pan-otic CRE drivers, particularly for postnatal analyses.

The Warburg Effect and lactate signaling augment Fgf-MAPK to promote sensory-neural development in the otic vesicle

Recent studies indicate that many developing tissues modify glycolysis to favor lactate synthesis (Agathocleous et al., 2012; Bulusu et al., 2017; Gu et al., 2016; Oginuma et al., 2017; Sá et al., 2017; Wang et al., 2014; Zheng et al., 2016), but how this promotes development is unclear. Using forward and reverse genetics in zebrafish, we show that disrupting the glycolytic gene phosphoglycerate kinase-1 (pgk1) impairs Fgf-dependent development of hair cells and neurons in the otic vesicle and other neurons in the CNS/PNS. Fgf-MAPK signaling underperforms in pgk1- / - mutants even when Fgf is transiently overexpressed. Wild-type embryos treated with drugs that block synthesis or secretion of lactate mimic the pgk1- / - phenotype, whereas pgk1- / - mutants are rescued by treatment with exogenous lactate. Lactate treatment of wild-type embryos elevates expression of Etv5b/Erm even when Fgf signaling is blocked. However, lactate’s ability to stimulate neurogenesis is reversed by blocking MAPK. Thus, lactate raises basal levels of MAPK and Etv5b (a critical effector of the Fgf pathway), rendering cells more responsive to dynamic changes in Fgf signaling required by many developing tissues.

Language Development, Hearing Loss, and Intracranial Hypertension in Children With TWIST1-Confirmed Saethre-Chotzen Syndrome

Saethre-Chotzen syndrome (SCS) is an autosomal dominant condition defined by mutations affecting the TWIST1 gene on chromosome 7p21.1. Previous research has identified an elevated prevalence of intracranial hypertension and hearing impairment associated with this syndrome. This study aimed to investigate the influence of hearing history and presence of intracranial hypertension on language development in children with SCS.A retrospective study note analysis was performed for all patients with a confirmed TWIST1 gene abnormality who attended the Oxford Craniofacial Unit and underwent a language assessment over a 22-year period. Intracranial pressure monitoring, hearing status, and language outcomes were examined in detail.Thirty patients with genetically confirmed SCS and language assessment data were identified. Twenty-eight patients underwent surgical intervention; 10 presented with intracranial hypertension (5 prior to, and 5 after primary surgical intervention). Language data coinciding with the presentation of intracranial hypertension were available for 8 children. About 44% of children with intracranial hypertension presented with concurrent receptive and expressive language delay (n = 4/8). For both children (n = 2) with longitudinal language data available, the onset of intracranial hypertension reflected a concurrent decline in language skills. Audiometric data were available for 25 children, 80% (n = 20/25) had a history of hearing loss. About 50% of these had confirmed conductive hearing loss with middle ear effusion and the other 50% had presumed conductive hearing loss with middle ear effusion. About 100% of the children with available hearing data in our study had evidence of middle ear effusion in at least 1 ear. Results also indicated that 43% (n = 13/30) of the children presented with receptive and/or expressive language delay during childhood.Given the importance of hearing for language development and the preliminary findings of a potential decline in language skills in children during periods of intracranial hypertension, regular follow-up of hearing, language, and intracranial hypertension are indicated in children with SCS.

GSK3 regulates hair cell fate in the developing mammalian cochlea

The development of asymmetric patterns along biologically relevant axes is a hallmark of many vertebrate organs or structures. One example is the sensory epithelium of the mammalian auditory system. Two distinct types of mechanosensory hair cells (inner and outer) and at least six types of associated supporting cells are precisely and asymmetrically arrayed along the radial (medial-lateral) axis of the cochlear spiral. Immunolabeling of developing cochleae indicates differential expression of Glycogen synthase kinase 3β (GSK3β) along the same axis. To determine whether GSK3β plays a role in specification of cell fates along the medial-lateral axis, GSK3 activity was blocked pharmacologically in cochlear explants. Results indicate significant changes in both the number of hair cells and in the specification of hair cell phenotypes. The overall number of inner hair cells increased as a result of both a shift in the medial boundary between sensory and non-sensory regions of the cochlea and a change in the specification of inner and outer hair cell phenotypes. Previous studies have inhibited GSK3 as a method to examine effects of canonical Wnt signaling. However, quantification of changes in Wnt pathway target genes in GSK3-inhibited cochleae, and treatment with more specific Wnt agonists, indicated that the Wnt pathway is not activated. Instead, expression of Bmp4 in a population of GSK3β-expressing cells was shown to be down-regulated. Finally, addition of BMP4 to GSK3-inhibited cochleae achieved a partial rescue of the hair cell phenotype. These results demonstrate a role for GSK3β in the specification of cellular identities along the medial-lateral axis of the cochlea and provide evidence for a positive role for GSK3β in the expression of Bmp4.

scRNA-Seq reveals distinct stem cell populations that drive hair cell regeneration after loss of Fgf and Notch signaling

Loss of sensory hair cells leads to deafness and balance deficiencies. In contrast to mammalian hair cells, zebrafish ear and lateral line hair cells regenerate from poorly characterized support cells. Equally ill-defined is the gene regulatory network underlying the progression of support cells to differentiated hair cells. scRNA-Seq of lateral line organs uncovered five different support cell types, including quiescent and activated stem cells. Ordering of support cells along a developmental trajectory identified self-renewing cells and genes required for hair cell differentiation. scRNA-Seq analyses of fgf3 mutants, in which hair cell regeneration is increased, demonstrates that Fgf and Notch signaling inhibit proliferation of support cells in parallel by inhibiting Wnt signaling. Our scRNA-Seq analyses set the foundation for mechanistic studies of sensory organ regeneration and is crucial for identifying factors to trigger hair cell production in mammals. The data is searchable and publicly accessible via a web-based interface.

Mouse Models of Syndromic Craniosynostosis

Craniosynostosis is a common craniofacial birth defect. This review focusses on the advances that have been achieved through studying the pathogenesis of craniosynostosis using mouse models. Classic methods of gene targeting which generate individual gene knockout models have successfully identified numerous genes required for normal development of the skull bones and sutures. However, the study of syndromic craniosynostosis has largely benefited from the production of knockin models that precisely mimic human mutations. These have allowed the detailed investigation of downstream events at the cellular and molecular level following otherwise unpredictable gain-of-function effects. This has greatly enhanced our understanding of the pathogenesis of this disease and has the potential to translate into improvement of the clinical management of this condition in the future.

FGF signaling: diverse roles during cochlear development

Mammalian inner ear comprises of six sensory organs; cochlea, utricle, saccule, and three semicircular canals. The cochlea contains sensory epithelium known as the organ of Corti which senses sound through mechanosensory hair cells. Mammalian inner ear undergoes series of morphogenesis during development beginning thickening of ectoderm nearby hindbrain. These events require tight regulation of multiple signaling cascades including FGF, Wnt, Notch and Bmp signaling. In this review, we will discuss the role of newly emerging signaling, FGF signaling, for its roles required for cochlear development. [BMB Reports 2017; 50(10): 487-495].

Achondroplasia: Development, pathogenesis, and therapy

Autosomal dominant mutations in fibroblast growth factor receptor 3 (FGFR3) cause achondroplasia (Ach), the most common form of dwarfism in humans, and related chondrodysplasia syndromes that include hypochondroplasia (Hch), severe achondroplasia with developmental delay and acanthosis nigricans (SADDAN), and thanatophoric dysplasia (TD). FGFR3 is expressed in chondrocytes and mature osteoblasts where it functions to regulate bone growth. Analysis of the mutations in FGFR3 revealed increased signaling through a combination of mechanisms that include stabilization of the receptor, enhanced dimerization, and enhanced tyrosine kinase activity. Paradoxically, increased FGFR3 signaling profoundly suppresses proliferation and maturation of growth plate chondrocytes resulting in decreased growth plate size, reduced trabecular bone volume, and resulting decreased bone elongation. In this review, we discuss the molecular mechanisms that regulate growth plate chondrocytes, the pathogenesis of Ach, and therapeutic approaches that are being evaluated to improve endochondral bone growth in people with Ach and related conditions. Developmental Dynamics 246:291-309, 2017. © 2016 Wiley Periodicals, Inc.

Genetic insights into the mechanisms of Fgf signaling

The fibroblast growth factor (Fgf) family of ligands and receptor tyrosine kinases is required throughout embryonic and postnatal development and also regulates multiple homeostatic functions in the adult. Here, Brewer et al. review the mechanisms of Fgf signaling by focusing on genetic strategies that enable in vivo analysis.

The fibroblast growth factor (Fgf) family of ligands and receptor tyrosine kinases is required throughout embryonic and postnatal development and also regulates multiple homeostatic functions in the adult. Aberrant Fgf signaling causes many congenital disorders and underlies multiple forms of cancer. Understanding the mechanisms that govern Fgf signaling is therefore important to appreciate many aspects of Fgf biology and disease. Here we review the mechanisms of Fgf signaling by focusing on genetic strategies that enable in vivo analysis. These studies support an important role for Erk1/2 as a mediator of Fgf signaling in many biological processes but have also provided strong evidence for additional signaling pathways in transmitting Fgf signaling in vivo.

A second family with CATSHL syndrome: Confirmatory report of another unique FGFR3 syndrome

Here we describe the second reported family with the CATSHL syndrome, a condition resulting from a unique mutation in the fibroblast growth factor receptor 3 (FGFR3) gene. Our family confirms the consistent and unique phenotype of this condition, and the specificity of the mutation in FGFR3. The CATSHL syndrome appears to be an autosomal dominant disorder with full penetrance. © 2016 Wiley Periodicals, Inc.

Causes and Consequences of Sensory Hair Cell Damage and Recovery in Fishes

Sensory hair cells are the mechanotransductive receptors that detect gravity, sound, and vibration in all vertebrates. Damage to these sensitive receptors often results in deficits in vestibular function and hearing. There are currently two main reasons for studying the process of hair cell loss in fishes. First, fishes, like other non-mammalian vertebrates, have the ability to regenerate hair cells that have been damaged or lost via exposure to ototoxic chemicals or acoustic overstimulation. Thus, they are used as a biomedical model to understand the process of hair cell death and regeneration and find therapeutics that treat or prevent human hearing loss. Secondly, scientists and governmental natural resource managers are concerned about the potential effects of intense anthropogenic sounds on aquatic organisms, including fishes. Dr. Arthur N. Popper and his students, postdocs and research associates have performed pioneering experiments in both of these lines of fish hearing research. This review will discuss the current knowledge regarding the causes and consequences of both lateral line and inner ear hair cell damage in teleost fishes.

Muenke syndrome: An international multicenter natural history study

Muenke syndrome is an autosomal dominant disorder characterized by coronal suture craniosynostosis, hearing loss, developmental delay, carpal, and calcaneal fusions, and behavioral differences. Reduced penetrance and variable expressivity contribute to the wide spectrum of clinical findings. Muenke syndrome constitutes the most common syndromic form of craniosynostosis, with an incidence of 1 in 30,000 births and is defined by the presence of the p.Pro250Arg mutation in FGFR3. Participants were recruited from international craniofacial surgery and genetic clinics. Affected individuals, parents, and their siblings, if available, were enrolled in the study if they had a p.Pro250Arg mutation in FGFR3. One hundred and six patients from 71 families participated in this study. In 51 informative probands, 33 cases (64.7%) were inherited. Eighty-five percent of the participants had craniosynostosis (16 of 103 did not have craniosynostosis), with 47.5% having bilateral and 28.2% with unilateral synostosis. Females and males were similarly affected with bicoronal craniosynostosis, 50% versus 44.4% (P = 0.84), respectively. Clefting was rare (1.1%). Hearing loss was identified in 70.8%, developmental delay in 66.3%, intellectual disability in 35.6%, attention deficit/hyperactivity disorder in 23.7%, and seizures in 20.2%. In patients with complete skeletal surveys (upper and lower extremity x-rays), 75% of individuals were found to have at least a single abnormal radiographical finding in addition to skull findings. This is the largest study of the natural history of Muenke syndrome, adding valuable clinical information to the care of these individuals including behavioral and cognitive impairment data, vision changes, and hearing loss.

MAP3K1 function is essential for cytoarchitecture of the mouse organ of Corti and survival of auditory hair cells

MAP3K1 is a serine/threonine kinase that is activated by a diverse set of stimuli and exerts its effect through various downstream effecter molecules, including JNK, ERK1/2 and p38. In humans, mutant alleles of MAP3K1 are associated with 46,XY sex reversal. Until recently, the only phenotype observed in Map3k1^tm1Yxia mutant mice was open eyelids at birth. Here, we report that homozygous Map3k1^tm1Yxia mice have early-onset profound hearing loss accompanied by the progressive degeneration of cochlear outer hair cells. In the mouse inner ear, MAP3K1 has punctate localization at the apical surface of the supporting cells in close proximity to basal bodies. Although the cytoarchitecture, neuronal wiring and synaptic junctions in the organ of Corti are grossly preserved, Map3k1^tm1Yxia mutant mice have supernumerary functional outer hair cells (OHCs) and Deiters' cells. Loss of MAP3K1 function resulted in the downregulation of Fgfr3, Fgf8, Fgf10 and Atf3 expression in the inner ear. Fgfr3, Fgf8 and Fgf10 have a role in induction of the otic placode or in otic epithelium development in mice, and their functional deficits cause defects in cochlear morphogenesis and hearing loss. Our studies suggest that MAP3K1 has an essential role in the regulation of these key cochlear morphogenesis genes. Collectively, our data highlight the crucial role of MAP3K1 in the development and function of the mouse inner ear and hearing.

Summary:Map3k1 mutant mice exhibit early-onset profound hearing loss and supernumerary outer hair cells, along with dysregulation of the FGF signaling pathway, accentuating its function in otic epithelium development and morphogenesis.

Auditory hair cell defects as potential cause for sensorineural deafness in Wolf-Hirschhorn syndrome

WHSC1 is a histone methyltransferase (HMT) that catalyses the addition of methyl groups to lysine 36 on histone 3. In humans, WHSC1 haploinsufficiency is associated with all known cases of Wolf-Hirschhorn syndrome (WHS). The cardinal feature of WHS is a craniofacial dysmorphism, which is accompanied by sensorineural hearing loss in 15% of individuals with WHS. Here, we show that WHSC1-deficient mice display craniofacial defects that overlap with WHS, including cochlea anomalies. Although auditory hair cells are specified normally, their stereocilia hair bundles required for sound perception fail to develop the appropriate morphology. Furthermore, the orientation and cellular organisation of cochlear hair cells and their innervation are defective. These findings identify, for the first time, the likely cause of sensorineural hearing loss in individuals with WHS.

The Fibroblast Growth Factor signaling pathway

The signaling component of the mammalian Fibroblast Growth Factor (FGF) family is comprised of eighteen secreted proteins that interact with four signaling tyrosine kinase FGF receptors (FGFRs). Interaction of FGF ligands with their signaling receptors is regulated by protein or proteoglycan cofactors and by extracellular binding proteins. Activated FGFRs phosphorylate specific tyrosine residues that mediate interaction with cytosolic adaptor proteins and the RAS-MAPK, PI3K-AKT, PLCγ, and STAT intracellular signaling pathways. Four structurally related intracellular non-signaling FGFs interact with and regulate the family of voltage gated sodium channels. Members of the FGF family function in the earliest stages of embryonic development and during organogenesis to maintain progenitor cells and mediate their growth, differentiation, survival, and patterning. FGFs also have roles in adult tissues where they mediate metabolic functions, tissue repair, and regeneration, often by reactivating developmental signaling pathways. Consistent with the presence of FGFs in almost all tissues and organs, aberrant activity of the pathway is associated with developmental defects that disrupt organogenesis, impair the response to injury, and result in metabolic disorders, and cancer. For further resources related to this article, please visit the WIREs website.

Signaling regulating inner ear development: cell fate determination, patterning, morphogenesis, and defects

The membranous labyrinth of the inner ear is a highly complex organ that detects sound and balance. Developmental defects in the inner ear cause congenital hearing loss and balance disorders. The membranous labyrinth consists of three semicircular ducts, the utricle, saccule, and endolymphatic ducts, and the cochlear duct. These complex structures develop from the simple otic placode, which is established in the cranial ectoderm adjacent to the neural crest at the level of the hindbrain at the early neurula stage. During development, the otic placode invaginates to form the otic vesicle, which subsequently gives rise to neurons for the vestibulocochlear ganglion, the non-sensory and sensory epithelia of the membranous labyrinth that includes three ampullary crests, two maculae, and the organ of Corti. Combined paracrine and autocrine signals including fibroblast growth factor, Wnt, retinoic acid, hedgehog, and bone morphogenetic protein regulate fate determination, axis formation, and morphogenesis in the developing inner ear. Juxtacrine signals mediated by Notch pathways play a role in establishing the sensory epithelium, which consists of mechanosensory hair cells and supporting cells. The highly differentiated organ of Corti, which consists of uniformly oriented inner/outer hair cells and specific supporting cells, develops during fetal development. Developmental alterations/arrest causes congenital malformations in the inner ear in a spatiotemporal-restricted manner. A clearer understanding of the mechanisms underlying inner ear development is important not only for the management of patients with congenital inner ear malformations, but also for the development of regenerative therapy for impaired function.

Hearing loss in syndromic craniosynostoses: otologic manifestations and clinical findings

OBJECTIVE

This review addresses hearing loss as it occurs and has been reported in Muenke syndrome as well as six additional FGFR related craniosynostosis syndromes (Apert syndrome, Pfeiffer syndrome, Crouzon syndrome, Beare-Stevenson syndrome, Crouzon syndrome with acanthosis nigricans, and Jackson-Weiss syndrome.

DATA SOURCES

Pub-Med, Medline, Cochrane Database, Science Direct, NLM Catalog.

REVIEW METHODS

A Medline search was conducted to find all reported cases of the 7 FGFR related syndromic craniosynostosis. Special attention was paid to literature that reported hearing findings and the audiology literature.

RESULTS

Hearing loss occurs in variable percentage as a component part of all FGFR related craniosynostosis syndromes. Our literature review revealed the following incidences of hearing loss in FGFR craniosynostoses: 61% in Muenke syndrome, 80% in Apert Syndrome, 92% in Pfeiffer syndrome, 74% in Crouzon syndrome, 68% in Jackson Weiss syndrome, 4% in Beare Stevenson syndrome and 14% in Crouzon syndrome with Acanthosis Nigricans. The majority of the hearing loss is a conductive hearing loss, with the exception of Muenke syndrome where the majority of patients have a sensorineural hearing loss and Crouzon syndrome where almost half of patients have a pure or component of sensorineural hearing loss.

CONCLUSION

This manuscript presents a diagnostic and management algorithm for patients with syndromic craniosynostosis. It will aid clinicians in treating these patients and further, the recognition of a possible syndrome in patients with hearing loss who also have syndromic features.

Expanding the mutation spectrum in 182 Spanish probands with craniosynostosis: identification and characterization of novel TCF12 variants

Craniosynostosis, caused by the premature fusion of one or more of the cranial sutures, can be classified into non-syndromic or syndromic and by which sutures are affected. Clinical assignment is a difficult challenge due to the high phenotypic variability observed between syndromes. During routine diagnostics, we screened 182 Spanish craniosynostosis probands, implementing a four-tiered cascade screening of FGFR2, FGFR3, FGFR1, TWIST1 and EFNB1. A total of 43 variants, eight novel, were identified in 113 (62%) patients: 104 (92%) detected in level 1; eight (7%) in level 2 and one (1%) in level 3. We subsequently screened additional genes in the probands with no detected mutation: one duplication of the IHH regulatory region was identified in a patient with craniosynostosis Philadelphia type and five variants, four novel, were identified in the recently described TCF12, in probands with coronal or multisuture affectation. In the 19 Saethre-Chotzen syndrome (SCS) individuals in whom a variant was detected, 15 (79%) carried a TWIST1 variant, whereas four (21%) had a TCF12 variant. Thus, we propose that TCF12 screening should be included for TWIST1 negative SCS patients and in patients where the coronal suture is affected. In summary, a molecular diagnosis was obtained in a total of 119/182 patients (65%), allowing the correct craniosynostosis syndrome classification, aiding genetic counselling and in some cases provided a better planning on how and when surgical intervention should take place and, subsequently the appropriate clinical follow up.

Hearing loss in syndromic craniosynostoses: introduction and consideration of mechanisms

PURPOSE

There are a number of craniosynostosis syndromes with hearing loss-including Muenke, Apert, Pfeiffer, Crouzon, Beare-Stevenson, Crouzon with acanthosis nigricans, and Jackson-Weiss syndromes-that result from mutations in the fibroblast growth factor receptor (FGFR) genes. Studies of FGFRs and their ligands, fibroblast growth factors (FGFs), have revealed clues to the precise contribution of aberrant FGFR signaling to inner ear morphogenesis and the hearing loss encountered in craniosynostoses. The purpose of this article is to review basic studies of FGFRs with emphasis on their function and expression in the inner ear and surrounding structures.

METHOD

A Medline search was performed to find basic science articles regarding FGFR, their ligands, and their expression and relevant mouse models. Additional items searched included clinical descriptions and studies of individuals with FGFR-related craniosynostosis syndromes.

RESULTS

The FGF signaling pathway is essential for the morphogensis and proper function of the inner ear and auditory sensory epithelium.

CONCLUSION

The variable auditory phenotypes seen in individuals with Muenke syndrome may have a genetic basis, likely due to multiple interacting factors in the genetic environment or modifying factors. Further analysis and studies of mouse models of Muenke syndrome, in particular, may provide clues to the specific effects of the defining mutation in FGFR3 in the inner ear not only at birth but also into adulthood. In particular, investigations into these models may give insight into the variable expression and incomplete penetrance of this phenotype.

Fibroblast growth factor receptor 3 regulates microtubule formation and cell surface mechanical properties in the developing organ of Corti

Fibroblast Growth Factor (Fgf) signaling is involved in the exquisite cellular patterning of the developing cochlea, and is necessary for proper hearing function. Our previous data indicate that Fgf signaling disrupts actin, which impacts the surface stiffness of sensory outer hair cells (OHCs) and non-sensory supporting pillar cells (PCs) in the organ of Corti. Here, we used Atomic Force Microscopy (AFM) to measure the impact of loss of function of Fgf-receptor 3, on cytoskeletal formation and cell surface mechanical properties. We find a 50% decrease in both OHC and PC surface stiffness, and a substantial disruption in microtubule formation in PCs. Moreover, we find no change in OHC electromotility of Fgfr3-deficient mice. To further understand the regulation by Fgf-signaling on microtubule formation, we treated wild-type cochlear explants with Fgf-receptor agonist Fgf2, or antagonist SU5402, and find that both treatments lead to a significant reduction in β-Tubulin isotypes I&II. To identify downstream transcriptional targets of Fgf-signaling, we used QPCR arrays to probe 84 cytoskeletal regulators. Of the 5 genes significantly upregulated following treatment, Clasp2, Mapre2 and Mark2 impact microtubule formation. We conclude that microtubule formation is a major downstream effector of Fgf-receptor 3, and suggest this pathway impacts the formation of fluid spaces in the organ of Corti.

FGFR3 mutation causes abnormal membranous ossification in achondroplasia

FGFR3 gain-of-function mutations lead to both chondrodysplasias and craniosynostoses. Achondroplasia (ACH), the most frequent dwarfism, is due to an FGFR3-activating mutation which results in impaired endochondral ossification. The effects of the mutation on membranous ossification are unknown. Fgfr3(Y367C/+) mice mimicking ACH and craniofacial analysis of patients with ACH and FGFR3-related craniosynostoses provide an opportunity to address this issue. Studying the calvaria and skull base, we observed abnormal cartilage and premature fusion of the synchondroses leading to modifications of foramen magnum shape and size in Fgfr3(Y367C/+) mice, ACH and FGFR3-related craniosynostoses patients. Partial premature fusion of the coronal sutures and non-ossified gaps in frontal bones were also present in Fgfr3(Y367C/+) mice and ACH patients. Our data provide strong support that not only endochondral ossification but also membranous ossification is severely affected in ACH. Demonstration of the impact of FGFR3 mutations on craniofacial development should initiate novel pharmacological and surgical therapeutic approaches.

A familial case of Muenke syndrome. Diverse expressivity of the FGFR3 Pro252Arg mutation--case report and review of the literature

Muenke is a fibroblast growth factor receptor 3 (FGFR-3)-associated syndrome, which was first described in late 1990 s. Muenke syndrome is an autosomal dominant disorder characterized mainly by coronal suture craniosynostosis, hearing impairment and intellectual disability. The syndrome is defined molecularly by a unique point mutation c.749C > G in exon 7 of the FGFR3 gene which results to an amino acid substitution p.Pro250Arg of the protein product. Despite the fact that the mutation rate at this nucleotide is one of the most frequently described in human genome, few Muenke familial case reports are published in current literature. We describe individuals among three generations of a Greek family who are carriers of the same mutation. Medical record and physical examination of family members present a wide spectrum of clinical manifestations. In particular, a 38-year-old woman and her father appear milder clinical findings regarding craniofacial characteristics compared to her uncle and newborn female child. This familial case illustrates the variable expressivity of Muenke syndrome in association with an identical gene mutation.

Genetic rescue of Muenke syndrome model hearing loss reveals prolonged FGF-dependent plasticity in cochlear supporting cell fates

Muenke syndrome model mice (Fgfr3^P244R/+) have hearing loss associated with a supporting cell fate transformation of two Deiters' cells to two pillar cells. Mansour et al. find that the hearing loss is not rescued by reducing the FGFR3c ligand FGF8, but reducing FGF10 is sufficient to rescue cochlear form and function. The P244R mutation caused FGFR3b and FGFR3c to acquire responsiveness to FGF10. Fgf10 heterozygosity allows a gradual reversion of fate-switched cells toward the normal phenotype.

The stereotyped arrangement of cochlear sensory and supporting cells is critical for auditory function. Our previous studies showed that Muenke syndrome model mice (Fgfr3^P244R/+) have hearing loss associated with a supporting cell fate transformation of two Deiters' cells to two pillar cells. We investigated the developmental origins of this transformation and found that two prospective Deiters' cells switch to an outer pillar cell-like fate sequentially between embryonic day 17.5 (E17.5) and postnatal day 3 (P3). Unexpectedly, the Fgfr3^P244R/+ hearing loss and supporting cell fate transformation are not rescued by genetically reducing fibroblast growth factor 8 (FGF8), the FGF receptor 3c (FGFR3c) ligand required for pillar cell differentiation. Rather, reducing FGF10, which normally activates FGFR2b or FGFR1b, is sufficient for rescue of cochlear form and function. Accordingly, we found that the P244R mutation changes the specificity of FGFR3b and FGFR3c such that both acquire responsiveness to FGF10. Moreover, Fgf10 heterozygosity does not block the Fgfr3^P244R/+ supporting cell fate transformation but instead allows a gradual reversion of fate-switched cells toward the normal phenotype between P5 and at least P14. This study indicates that Deiters' and pillar cells can reversibly switch fates in an FGF-dependent manner over a prolonged period of time. This property might be exploited for the regulation of sensory cell regeneration from support cells.

Thyroid hormone increases fibroblast growth factor receptor expression and disrupts cell mechanics in the developing organ of corti

Background

Thyroid hormones regulate growth and development. However, the molecular mechanisms by which thyroid hormone regulates cell structural development are not fully understood. The mammalian cochlea is an intriguing system to examine these mechanisms, as cellular structure plays a key role in tissue development, and thyroid hormone is required for the maturation of the cochlea in the first postnatal week.

Results

In hypothyroid conditions, we found disruptions in sensory outer hair cell morphology and fewer microtubules in non-sensory supporting pillar cells. To test the functional consequences of these cytoskeletal defects on cell mechanics, we combined atomic force microscopy with live cell imaging. Hypothyroidism stiffened outer hair cells and supporting pillar cells, but pillar cells ultimately showed reduced cell stiffness, in part from a lack of microtubules. Analyses of changes in transcription and protein phosphorylation suggest that hypothyroidism prolonged expression of fibroblast growth factor receptors, and decreased phosphorylated Cofilin.

Conclusions

These findings demonstrate that thyroid hormones may be involved in coordinating the processes that regulate cytoskeletal dynamics and suggest that manipulating thyroid hormone sensitivity might provide insight into the relationship between cytoskeletal formation and developing cell mechanical properties.

Phenotype profile of a genetic mouse model for Muenke syndrome

PURPOSE

The Muenke syndrome mutation (FGFR3 (P250R)), which was discovered 15 years ago, represents the single most common craniosynostosis mutation. Muenke syndrome is characterized by coronal suture synostosis, midface hypoplasia, subtle limb anomalies, and hearing loss. However, the spectrum of clinical presentation continues to expand. To better understand the pathophysiology of the Muenke syndrome, we present collective findings from several recent studies that have characterized a genetically equivalent mouse model for Muenke syndrome (FgfR3 (P244R)) and compare them with human phenotypes.

CONCLUSIONS

FgfR3 (P244R) mutant mice show premature fusion of facial sutures, premaxillary and/or zygomatic sutures, but rarely the coronal suture. The mice also lack the typical limb phenotype. On the other hand, the mutant mice display maxillary retrusion in association with a shortening of the anterior cranial base and a premature closure of intersphenoidal and spheno-occipital synchondroses, resembling human midface hypoplasia. In addition, sensorineural hearing loss is detected in all FgfR3 (P244R) mutant mice as in the majority of Muenke syndrome patients. It is caused by a defect in the mechanism of cell fate determination in the organ of Corti. The mice also express phenotypes that have not been previously described in humans, such as reduced cortical bone thickness, hypoplastic trabecular bone, and defective temporomandibular joint structure. Therefore, the FgfR3 (P244R) mouse provides an excellent opportunity to study disease mechanisms of some classical phenotypes of Muenke syndrome and to test novel therapeutic strategies. The mouse model can also be further explored to discover previously unreported yet potentially significant phenotypes of Muenke syndrome.

Fgf signaling regulates development and transdifferentiation of hair cells and supporting cells in the basilar papilla

The avian basilar papilla (BP) is a likely homolog of the auditory sensory epithelium of the mammalian cochlea, the organ of Corti. During mammalian development Fibroblast growth factor receptor-3 (Fgfr3) is known to regulate the differentiation of auditory mechanosensory hair cells (HCs) and supporting cells (SCs), both of which are required for sound detection. Fgfr3 is expressed in developing progenitor cells (PCs) and SCs of both the BP and the organ of Corti; however its role in BP development is unknown. Here we utilized an in vitro whole organ embryonic culture system to examine the role of Fgf signaling in the developing avian cochlea. SU5402 (an antagonist of Fgf signaling) was applied to developing BP cultures at different stages to assay the role of Fgf signaling during HC formation. Similar to the observed effects of inhibition of Fgfr3 in the mammalian cochlea, Fgfr inhibition in the developing BP increased the number of HCs that formed. This increase was not associated with increased proliferation, suggesting that inhibition of the Fgf pathway leads to the direct conversion of PCs or supporting cells into HCs, a process known as transdifferentiation. This also implies that Fgf signaling is required to prevent the conversion of PCs and SCs into HCs. The ability of Fgf signaling to inhibit transdifferentiation suggests that its down-regulation may be essential for the initial steps of HC formation, as well as for the maintenance of SC phenotypes.

Identification of novel Hoxa1 downstream targets regulating hindbrain, neural crest and inner ear development

Hox genes play a crucial role during embryonic patterning and organogenesis. Of the 39 Hox genes, Hoxa1 is the first to be expressed during embryogenesis and the only anterior Hox gene linked to a human syndrome. Hoxa1 is necessary for the proper development of the brainstem, inner ear and heart in humans and mice; however, almost nothing is known about the molecular downstream targets through which it exerts its function. To gain insight into the transcriptional network regulated by this protein, we performed microarray analysis on tissue microdissected from the prospective rhombomere 3-5 region of Hoxa1 null and wild type embryos. Due to the very early and transient expression of this gene, dissections were performed on early somite stage embryos during an eight-hour time window of development. Our array yielded a list of around 300 genes differentially expressed between the two samples. Many of the identified genes play a role in a specific developmental or cellular process. Some of the validated targets regulate early neural crest induction and specification. Interestingly, three of these genes, Zic1, Hnf1b and Foxd3, were down-regulated in the posterior hindbrain, where cardiac neural crest cells arise, which pattern the outflow tract of the heart. Other targets are necessary for early inner ear development, e.g. Pax8 and Fgfr3 or are expressed in specific hindbrain neurons regulating respiration, e.g. Lhx5. These findings allow us to propose a model where Hoxa1 acts in a genetic cascade upstream of genes controlling specific aspects of embryonic development, thereby providing insight into possible mechanisms underlying the human HoxA1-syndrome.

Audiologic findings in Pfeiffer syndrome

Hearing loss has been described in patients with certain craniosynostotic syndromes but is poorly defined in Pfeiffer syndrome (PS). Our objective was to characterize the otologic and audiologic findings in PS. The records of PS patients evaluated at our craniofacial center over a 30-year period were culled. Only patients with a confirmed diagnosis and formal audiologic examination were included. Diagnostic criteria were characteristic mutations in fibroblast growth factor receptor 1 or 2 (FGFR1, FGFR2) or, in the absence of genetic testing, typical clinical findings of PS as determined by a clinical geneticist or the most senior author. Twenty patients met the inclusion criteria, and all had hearing loss. Twenty patients had traditional audiologic testing: 14 (70%) had pure conductive loss (minor to severe), and 3 (15%) had a mixed conductive/sensorineural loss (minor to severe). Two additional patients had hearing loss by Behavioral Observational Audiometry (sound fields method). One patient with early conductive hearing loss was subsequently determined to have a pure sensorineural deficit. Nine patients (45%) had permanent hearing loss significant enough to require audiologic amplification. All patients with PS demonstrated hearing loss, although the severity and the anatomic basis (ie., neural vs conductive) were variable. Conductive hearing loss, possibly caused by structural abnormalities, was most common. Sensorineural hearing loss was less common and may be related to the effect of FGFR mutations on cranial nerve and/or inner-ear development.

Craniosynostosis

Craniosynostosis, defined as the premature fusion of the cranial sutures, presents many challenges in classification and treatment. At least 20% of cases are caused by specific single gene mutations or chromosome abnormalities. This article maps out approaches to clinical assessment of a child presenting with an unusual head shape, and illustrates how genetic analysis can contribute to diagnosis and management.

Polygenic inheritance of sensorineural hearing loss (Snhl2, -3, and -4) and organ of Corti patterning defect in the ALR/LtJ mouse strain

Progressive sensorineural hearing loss in humans is a common and debilitating impairment. Sensorineural deafness in inbred strains of mice is a similarly common and genetically diverse phenotype providing experimental models to study the underlying genetics and the biological effects of the risk factors. Here, we report that ALR/LtJ mice develop early-onset profound sensorineural hearing loss as evidenced by high-to-low frequency hearing threshold shifts, absent distortion-product otoacoustic emissions, and normal endocochlear potentials. Linkage analyses of a segregating backcross revealed three novel quantitative trait loci named sensorineural hearing loss (Snhl) -2, -3, and -4. The QTLs achieved very high LOD scores with markers on chromosome 1 (Snhl2, LOD: 12), chromosome 6 (Snhl3, LOD: 24) and chromosome 10 (Snhl4, LOD: 11). Together, they explained 90% of the phenotypic variance. While Snhl2 and Snhl3 affected hearing thresholds across a broad range of test frequencies, Snhl4 caused primarily high-frequency hearing loss. The hearing impairment is accompanied by an organ of Corti patterning defect that is characterized by the ectopic expression of supernumerary outer hair cells organized in rows along the abneural site of the sensory epithelium in the presence of unaltered planar polarity and otherwise normal cochlear duct morphology. Cloning the Snhl2, -3, and -4 genes in the ALR/LtJ mice may provide important genetic and mechanistic insights into the pathology of human progressive sensorineural deafness.

CD44 is a marker for the outer pillar cells in the early postnatal mouse inner ear

Cluster of differentiation antigens (CD proteins) are classically used as immune cell markers. However, their expression within the inner ear is still largely undefined. In this study, we explored the possibility that specific CD proteins might be useful for defining inner ear cell populations. mRNA expression profiling of microdissected auditory and vestibular sensory epithelia revealed 107 CD genes as expressed in the early postnatal mouse inner ear. The expression of 68 CD genes was validated with real-time RT-PCR using RNA extracted from microdissected sensory epithelia of cochleae, utricles, saccules, and cristae of newborn mice. Specifically, CD44 was identified as preferentially expressed in the auditory sensory epithelium. Immunohistochemistry revealed that within the early postnatal organ of Corti, the expression of CD44 is restricted to outer pillar cells. In order to confirm and expand this finding, we characterized the expression of CD44 in two different strains of mice with loss- and gain-of-function mutations in Fgfr3 which encodes a receptor for FGF8 that is essential for pillar cell development. We found that the expression of CD44 is abolished from the immature pillar cells in homozygous Fgfr3 knockout mice. In contrast, both the outer pillar cells and the aberrant Deiters' cells in the Fgfr3 ( P244R/ ) (+) mice express CD44. The deafness phenotype segregating in DFNB51 families maps to a linkage interval that includes CD44. To study the potential role of CD44 in hearing, we characterized the auditory system of CD44 knockout mice and sequenced the entire open reading frame of CD44 of affected members of DFNB51 families. Our results suggest that CD44 does not underlie the deafness phenotype of the DFNB51 families. Finally, our study reveals multiple potential new cell type-specific markers in the mouse inner ear and identifies a new marker for outer pillar cells.

Expression patterns of FGF receptors in the developing mammalian cochlea

Many studies have shown the importance of the fibroblast growth factor (FGF) family of factors in the development of the mammalian cochlea. There are four fibroblast growth factor receptors (FGFR1-4) and all four are expressed in the cochlea during development. While there are examples in the literature of expression patterns of some of the receptors at specific stages of cochlear development there has been no systematic study. We have assembled a full analysis of the patterns of receptor expression during cochlear development for all four Fgfrs using in situ hybridization. We have analyzed the expression patterns from embryonic day 13.5 through postnatal ages. We find that Fgfr1, 2, and 3 are expressed in the epithelium of the cochlear duct and Fgfr4 is limited in its expression to the mesenchyme surrounding the duct. We compare the receptor expression pattern to markers of the sensory domain (p27kip1) and the early hair cells (math1).

Regulation of cell fate and patterning in the developing mammalian cochlea

PURPOSE OF REVIEW

A significant proportion of hearing loss and deafness is caused by defects in the structure or function of cells within the organ of Corti. Identification of the molecular factors that regulate the development of this structure should provide valuable insights regarding inner ear formation and the signaling pathways that underlie congenital auditory deficits. In addition, targeted modulation of these same factors could be developed as therapies for hair cell regeneration.

RECENT FINDINGS

Results from experiments using transgenic and mutant mice, as well as in-vitro techniques, have identified genes and signaling pathways that are required to either specify unique auditory cell types, such as hair cells or supporting cells, or to generate the highly ordered cellular pattern that is characteristic for the organ of Corti. In particular, the hedgehog and fibroblast growth factor signaling pathways modulate the formation of the progenitor cells that will give rise to the organ of Corti. SRY-box containing gene 2, a transcription factor that is required for the formation of the cochlear progenitor cell population, has paradoxically been shown to also act as an inhibitor of hair cell development. Finally, the motor protein myosin II regulates extension of the organ of Corti and the alignment of hair cells and supporting cells into ordered rows.

SUMMARY

A better understanding of the signaling pathways that direct different aspects of cochlear development, such as specific of cell fates or cellular patterning, offers the potential to identify new pathways or molecules that could be targeted for therapeutic interventions.

Skeletal analysis of the Fgfr3(P244R) mouse, a genetic model for the Muenke craniosynostosis syndrome

Muenke syndrome, defined by heterozygosity for a Pro250Arg substitution in fibroblast growth factor receptor 3 (FGFR3), is the most common genetic cause of craniosynostosis in humans. We have used gene targeting to introduce the Muenke syndrome mutation (equivalent to P244R) into the murine Fgfr3 gene. A rounded skull and shortened snout (often skewed) with dental malocclusion was observed in a minority of heterozygotes and many homozygotes. Development of this incompletely penetrant skull phenotype was dependent on genetic background and sex, with males more often affected. However, these cranial abnormalities were rarely attributable to craniosynostosis, which was only present in 2/364 mutants; more commonly, we found fusion of the premaxillary and/or zygomatic sutures. We also found decreased cortical thickness and bone mineral densities in long bones. We conclude that although both cranial and long bone development is variably affected by the murine Fgfr3(P244R) mutation, coronal craniosynostosis is not reliably reproduced.

Quo vadis, hair cell regeneration?

Hearing loss is a global health problem with profound socioeconomic impact. We contend that acquired hearing loss is mainly a modern disorder caused by man-made noise and modern drugs, among other causes. These factors, combined with increasing lifespan, have exposed a deficit in cochlear self-regeneration that was irrelevant for most of mammalian evolution. Nevertheless, the mammalian cochlea has evolved from phylogenetically older structures, which do have the capacity for self-repair. Moreover, nonmammalian vertebrates can regenerate auditory hair cells that restore sensory function. We will offer a critical perspective on recent advances in stem cell biology, gene therapy, cell cycle regulation and pharmacotherapeutics to define and validate regenerative medical interventions for mammalian hair cell loss. Although these advances are promising, we are only beginning to fully appreciate the complexity of the many challenges that lie ahead.

Activating Fgfr3 Y367C mutation causes hearing loss and inner ear defect in a mouse model of chondrodysplasia

Fibroblast growth factor receptor 3 (FGFR3) is a key regulator of skeletal development and activating mutations in FGFR3 cause skeletal dysplasias, including hypochondroplasia, achondroplasia and thanatophoric dysplasia. The introduction of the Y367C mutation corresponding to the human Y373C thanatophoric dysplasia type I (TDI) mutation into the mouse genome, resulted in dwarfism with a skeletal phenotype remarkably similar to that of human chondrodysplasia. To investigate the role of the activating Fgfr3 Y367C mutation in auditory function, the middle and inner ear of the heterozygous mutant Fgfr3(Y367C/+) mice were examined. The mutant Fgfr3(Y367C/+) mice exhibit fully penetrant deafness with a significantly elevated auditory brainstem response threshold for all frequencies tested. The inner ear defect is mainly associated with an increased number of pillar cells or modified supporting cells in the organ of Corti. Hearing loss in the Fgfr3(Y367C/+) mouse model demonstrates the crucial role of Fgfr3 in the development of the inner ear and provides novel insight on the biological consequences of FGFR3 mutations in chondrodysplasia.