Growth factor and receptor malfunctions associated with human genetic deafness

Abstract

A variety of different signaling pathways are necessary for development and maintenance of the human auditory system. Normal hearing allows for the detection of soft sounds within the frequency range of 20 to 20,000 Hz, but more importantly to perceive the human voice frequency band of 250 to 6,000 Hertz. Loss of hearing is common, and is a clinically heterogeneous disorder that can be caused by environmental factors such as exposure to loud noise, infections and ototoxic drugs. In addition, variants of hundreds of genes have been reported to disrupt processes required for hearing. Noncoding regulatory variants and variants of additional genes necessary for hearing remain to be discovered as many individuals with inherited deafness are without a genetic diagnosis, despite the advent of whole exome sequencing. Here, we discuss in detail some of these deafness-causing variants of genes encoding a ligand or its receptor. Spotlighted in this review are three growth factor-receptor-pairs EDN3/EDNRB, HGF/MET and JAG/NOTCH, which individually are necessary for normal hearing. We also offer our perspective on unanswered questions, future challenges and potential opportunities for treatments emerging from molecular genetic and mechanistic studies of deafness due to these causes.

Graphical Abstract

1. INTRODUCTION

Impaired sound detection is a prevalent deficit of older adults as well as for 1 in 500 to 1 in 2,000 young children.^1,2 Deficits in hearing often result from dysfunction of one of the complex, delicate structures or intricate signaling pathways in the inner ear.³ Hearing loss can be conductive, sensorineural or mixed, depending on whether the defect is in the middle ear, inner ear or both, respectively (Figure 1A). The complex structure of inner ear develops from the otic placode next to the hindbrain,⁴ which invaginates to form the otic vesicle (Figure 1B). This fluid-filled cyst then undergoes organogenesis to give rise to neurons of the vestibular and cochlear ganglions as well as non-sensory and sensory epithelia of the membranous labyrinth (Figure 1B). In the mature cochlea, there are three rows of sound-amplifying outer hair cells (OHCs) and a single row of inner hair cells (IHCs) responsible for mechano-chemical transduction of sound. The apical surfaces of hair cells are topped by F-actin packed stereocila (Figure 1C).⁵

FIGURE 1.

Development and structure of the ear. (A) Structure of the human ear showing its three main parts, outer, middle and inner ear. (B) Paint-filled mouse membranous labyrinths at embryonic days 10.75 days-postcoitum to postnatal day 1 (P1). Lateral views are shown. Scale bar, 200 μm. (C) Diagram of a cross-section of the cochlea. The roof of the cochlear duct is formed by two layers of flattened cells comprising Reissner’s membrane, while the base is formed by the basilar membrane, which separate the cochlear duct (scala media) from the scala vestibuli and the scala tympani. The three rows of outer hair cells, one row of inner hair cells and different types of supporting cells and the stria vascularis are shown. IHC; Inner Hair Cells, OHC, Outer Hair Cells, IP, Inner Pillar cells, OP; Outer Pillar Cells, HC; Hensen’s Cells, CC; Claudius Cells, BC; Basal Cells, IC; Intermediate cells, MC; Marginal Cells, IBC; inner border cells.

Many pathogenic variants in genes encoding growth factors, receptors, co-receptors, adaptors and effectors have been identified in analyses of hearing loss in humans (Tables 1 and 2). Receptors and ligands, for example, LGR5 and BDNF, variants of which have not been reported to cause hearing loss in humans but have important role in development of auditory system^6,7 and ligand-gated ion channel receptors, such as P2XR2⁸ are not included in this review. Our focus is on variants in genes associated with human deafness. However, confidence in the evidence for pathogenicity of variants in the deafness causing genes differs. A ClinGen Hearing Loss Gene Curation Expert Panel has concluded that some are likely not bona fide pathogenic variants,⁹ but rather represent coincidental co-occurrence with deafness in individual families. This appears to be the case for variants in MYO1A, MYO1C, MYO1F and TSPEAR reported in deafness.^9,10

TABLE 1.

Growth factors, receptors and related proteins implicated in nonsyndromic hearing loss

Gene	OMIM	Description; cellular location	Protein/alias	Function	HL type	Locus†
ADCY1	103072	Adenylate Cyclase 1; transmembrane	ADCY1/AC1	Generates the second messenger cAMP	Prelingual, sensorineural, bilateral, profound	DFNB44
CCDC50	611051	Coiled Coil Domain-Containing protein 50; cytoplasmic	CCDC50/YMER/C3ORF6	Inhibits downregulation of EGFR/Effector of EGF mediated signaling	Postlingual, progressive, sensorineural, mid-frequency>	DFNA44
DCDC2	605755	Double Cortin Domain-Containing protein 2; cytoplasmic	DCDC2/RU2/RU2S	Inhibits WNT signaling	Prelingual, sensorineural, profound	DFNB66
ELMOD3	615427	ELMO/ced12 Domain-containing protein 3; cytoplasmic	ELMOD3/RBED1	GTPase-activating protein (GAP) for small GTPases	Prelingual, mixed, severe to profound	DFNB88
EPS8	600206	Epidermal growth factor receptor Pathway Substrate 8; cytoplasmic	EPS8	Elongation of stereocilia by actin remodeling	Prelingual, sensorineural, bilateral, profound	DFNB102
EPS8L2	614988	Epidermal growth factor receptor Pathway Substrate 8-Like protein 2; cytoplasmic	EPS8L2	Maintenance of stereocilia, actin remodeling	Childhood onset, progressive, bilateral, sensorineural, moderate to profound	DFNB106
ESRRB	602167	EStrogen-Related Receptor, Beta; cytoplasmic	ESRRB/ESRL2/ERR2	Epithelial cell fate, development of the stria vascularis	Prelingual or postlingual, sensorineural, bilateral, severe to profound	DFNB35
GAB1	604439	GRB2-Associated Binding protein 1; cytoplasmic	GAB1	Mediator of HGF, IGF1 signaling	Prelingual, bilateral, sensorineural, profound	DFNB26
GIPC3	608792	GIPC PDZ domain-containing family, member 3; cytoplasmic	GIPC3/GAIP C-terminus-interacting protein 3	Predicted to modulate WNT signaling	Prelingual, bilateral, sensorineural, severe to profound	DFNB15/DFNB72/DFNB95
GRAP	604330	Growth factor Receptor-bound protein 2 (Grb2)-related Adaptor Protein; cytoplasmic	GRAP	Activates RAS signaling	Prelingual, bilateral, sensorineural, severe to profound	DFNB114
HGF	142409	Hepatocyte Growth Factor; secreted	HGF/SF	Incorporation of neural crest cells into the stria vascularis	Prelingual, sensorineural, bilateral, moderate to severe or severe to profound	DFNB39
IFNLR1	607404	Interferon-Lambda Receptor 1, transmembrane	IFNLR1/ IL28RA	Functions in Jak/STAT pathway	Postlingual, sensorineural, bilateral, progressive, moderate to profound	DFNA2C
ILDR1	609739	Immunoglobulin-Like Domain-containing Receptor 1; transmembrane and cytoplasmic	ILDR1	Structural role at the tricellular tight junctions; receptor role in the inner ear unknown	Prelingual, sensorineural, bilateral, moderate to profound or severe to profound	DFNB42
KITLG	184745	KIT Ligand; transmembrane and secreted	KITLG/KL/KITL/MGF/SCF/SF	Proliferation, migration from the neural crest, differentiation of melanoblasts	Prelingual, sensorineural, unilateral or asymmetric	DFNA69
LRP5	603506	Low density lipoprotein Receptor-related Protein 5; transmembrane	LRP5/LRP7/LR3	Co-receptor during WNT signaling	Sensorineural, progressive, childhood onset, bilateral, low and mid frequency	-, recessive
MET	164860	Mesenchymal EpithelialTransition factor; transmembrane	MET/HGFR	Incorporation of neural crest cells into the stria vascularis	Prelingual, sensorineural, bilateral, severe	DFNB97
NLRP3	606416	NLR family, Pyrin domain-containing 3; cytoplasmic	NLRP3/CIAS1/NALP3/PYPAF1/ AII/AVP Receptor-Like	Leads to events which end in production of cytokine IL-1β	Postlingual, bilateral, progressive	DFNA34
PDE1C	602987	Phosphodiesterase 1C; cytoplasmic	PDE1C/HCAM3	Regulates levels of second messengers cAMP and cGMP	Postlingual, progressive, bilateral, mild to profound	DFNA74
PTPRQ	603317	Protein-Tyrosine Phosphatase, Receptor-type, Q; transmembrane and cytoplasmic	PTPRQ/ PTPGMC1	Receptor role unknown, structural role as shaft connector	Sensorineural, variable onset, progressive, bilateral, mild to moderate	DFNA73
PTPRQ	603317	Protein-Tyrosine Phosphatase, Receptor-type, Q; transmembrane and cytoplasmic	PTPRQ/ PTPGMC1	Receptor role unknown, structural role as shaft connector	Sensorineural, childhood onset, progressive, bilateral, moderate to profound	DFNB84A
ROR1	602336	Receptor tyrosine kinase-like Orphan Receptor 1; transmembrane	ROR1/ NTRKR1	Innervation of auditory hair cells in response to WNT5A signaling	Prelingual, bilateral, sensironeural, profound, auditory neuropathy,	DFNB108
S1PR2	605111	Sphingosine-1-Phosphate Receptor 2; transmembrane & cytoplasmic	S1PR2/EDG5/S1P2	Development of stria vascularis	Congenital, sensorineural, bilateral, profound	DFNB68

^†DFNA, dominant hearing loss, DFNB, recessive hearing loss, OMIM, Online Mendelian Inheritance in Man (https://www.omim.org/). A comprehensive source of gene expression data in mouse and zebrafish auditory systems are available on the gEAR Portal, https://umgear.org/.

TABLE 2.

Growth factors, receptors and related proteins implicated in syndromic hearing loss

Gene	OMIM	Description; cellular location	Protein/alias	Function	Hearing loss type	Locus; mode of inheritance
ACVR1	102576	ACtiVin a Receptor, type I; transmembrane	ACVR1/ ACVRLK2/ ALK2	General role as receptor for Activin, BMP or TGFB mediated SMAD signaling	Variable age of onset, conductive or sensorineural, mild to severe	Fibrodysplasia ossificans progressiva; dominant
ADGRV1	602851	ADhesion G protein-coupled Receptor V1; transmembrane	ADGRV1/ GPR98/ MASS1/ VLGR1/ KIAA0686	Receptor function unknown, component of stereocilia ankle links	Sensorineural, bilateral, progressive, moderate to severe	Usher syndrome 2C; recessive
BMP2	617877	Bone Morphogenetic Protein 2, secreted	BMP2/BMP2A	Endochondral ossification	External ear malformations, conductive	Short stature, facial dysmorphism, and skeletal anomalies with or without cardiac anomalies; dominant
BMP2	617877	Bone Morphogenetic Protein 2, secreted	BMP2/BMP2A	Endochondral ossification	Conductive	Otosclerosis
BMP4	112262	Bone Morphogenetic Protein 4; secreted	BMP4/BMP2B/BMP2B1	Specification of sensory and nonsensory regions of cochlea	Conductive	Otosclerosis
BMP4	112262	Bone Morphogenetic Protein 4; secreted	BMP4/BMP2B/BMP2B1	Specification of sensory and nonsensory regions of cochlea	Sensorineural, mild to moderate	Stickler syndrome and renal dysplasia; dominant
BMP7	112267	Bone Morphogenetic Protein 7; secreted	BMP7/OP1	Specifies tonotopic axis in developing inner ear	Sensorineural, bilateral, moderate	Ocular, brain, ear, palate, and skeletal anomalies; dominant
BRAF	164757	v-RAF murine sarcoma viral oncogene homologue B1; cytoplasmic, cell membrane, nuclear	BRAF/BRAF1/RAFB1	Regulator of MAPK/ERK signaling pathway	Outer ear malformation, sensorineural	LEOPARD syndrome 3; dominant
DVL1	601365	Dishevelled 1, cytoplasmic	DVL1/DVL	Functions downstream of WNT signaling	Outer ear malformation, conductive or sensorineural	Robinow syndrome, autosomal dominant 2; dominant
DVL3	601368	Dishevelled 3, cytoplasmic	DVL3	Functions downstream of WNT signaling	Rare ear malformation and hearing loss	Robinow syndrome, autosomal dominant 3; dominant
EDN3	131242	Endothelin 3, secreted	EDN3/ET3	Development of neural crest and its cell lineages	Sensorineural, Mild to profound, unilateral or bilateral	Waardenburg syndrome, type 4B, WS4B; dominant or recessive
EDNRA	131243	Endothelin Receptor, type A; transmembrane	EDNRA/ETA/ETRA	Development of neural crest and its cell lineages	Outer ear malformation, childhood onset, conductive, moderate	Mandibulofacial dysostosis with alopecia; dominant
EDNRB	131244	Endothelin Receptor, type B; transmembrane	EDNRB/ETB/ETBR	Development of neural crest and its cell lineages	Sensorineural, mild to profound, unilateral or bilateral	Waardenburg syndrome, type 4A, WS4A, WS1, WS2; dominant and recessive Waardenburg syndrome, type 2A, WS2A; dominant; ABCD syndrome; recessive
ESRRG	602969	EStrogen Related Receptor Gamma, nuclear	ESRRG/ERR3	Unknown	Prelingual, sensorineural, bilateral, mild to moderate	Hearing loss with mild intellectual disability; recessive
FGF3	164950	Fibroblast Growth Factor 3, secreted	FGF3/INT2	Induction of inner ear fate	Outer and inner ear anomalies, prelingual, sensorineural, profound	Congenital deafness, inner ear agenesis, microtia, microdontia; recessive
FGF10	602115	Fibroblast Growth Factor 10; secreted	FGF10	Specification of nonsensory regions of inner ear	Outer ear malformation, conductive, sensorineural or mixed hearing loss	LADD syndrome; dominant
FGFR1	136350	Fibroblast Growth Factor Receptor 1; transmembrane	FGFR1/FLT2/FLG	Early cell proliferation of cochlea, differentiation of sensory epithelium	Outer ear malformation in some, unilateral	Kallmann syndrome 2; dominant
FGFR2	176943	Fibroblast Growth Factor Receptor 2; transmembrane	FGFR2/TK14	Essential for inner ear morphogenesis	Outer ear malformations may be present, prelingual or postlingual, conductive or sensorineural or mixed, bilateral mild to severe	Apert syndrome, Crouzon syndrome CFD1; LADD syndrome; Saethre-Chotzen Syndrome, Pfeiffer syndrome type 3; Dominant
FGFR3	134934	Fibroblast Growth Factor Receptor 3; transmembrane	FGFR3	Controls fate of supporting cells in organ of Corti	Sensorineural or mixed, mild to moderate	CATSHL syndrome, LADD syndrome, Muenke syndrome, SADDAN; dominant or recessive
GDF6	601147	Growth/Differentiation Factor 6; secreted	GDF6/CDMP2	Growth of cartilage elements, SMAD signaling	Outer ear malformation, conductive or sensorineural, mild to moderate	Klippel-Feil syndrome 1; dominant multiple synostoses syndrome, dominant
GNAI3	139370	Guanine Nucleotide-binding protein, Alpha-Inhibiting activity polypeptide 3; cytoplasmic	GNAI3/ G protein	Mediator of signaling involving G protein-coupled endothelin receptor	Outer ear malformation, conductive or sensorineural, mild to moderate	Auriculocondylar syndrome 1; dominant
GPRASP2	300969	G Protein-coupled Receptor-Associated Sorting Protein 2; cytoplasmic	GPRASP2/GASP2	Turnover of G-protein-Coupled Receptors	External ear malformations, congenital or progressive, bilateral, mixed hearing loss, moderate to profound	DFNX7, Deafness X-linked 7, recessive
GPSM2	609245	G Protein Signaling Modulator 2; cytoplasmic	GPSM2/LGN/PINS	Asymmetric localization of kinocilia in postmitotic hair cells	Prelingual, sensorineural, bilateral, severe to profound	Chudley-McCullough Syndrome; recessive
IGF1	147440	Insulin-like Growth Factor I; secreted	IGF1/IGFI/ Somatomedin C	Postnatal differentiation and maturation of cochlea and central auditory neurons	Sensorineural, prelingual, bilateral, profound	Insulin-like growth factor I deficiency; recessive
IGF1R	147340	Insulin-like Growth Factor I Receptor; transmembrane	IGF1R	Postnatal differentiation and maturation of cochlea and central auditory neurons	Bilateral, profound	Insulin-like growth factor I, resistance to; dominant
JAG1	601920	JAGged 1; transmembrane	JAG1/JAGL1	Patterning of the organ of Corti, outer hair cell number	Mild to severe, mixed, mid-frequency	Deafness, congenital heart defects, and posterior embryotoxon; dominant
KIT	164920	V-KIT Hardy-Zuckerman 4 feline sarcoma viral oncogene homologue; transmembrane	KIT/SCFR	Proliferation, migration from the neural crest, differentiation of melanoblasts	Prelingual, sensorineural, profound	Piebaldism (Woolf’s syndrome); dominant
KITLG	184745	KIT Ligand; trasmembrane and secreted	KITLG/KL/KITL/MGF/SCF/SF	Proliferation, migration from the neural crest, differentiation of melanoblasts	Congenital, stable unilateral and asymmetric hearing loss	Waardenburg syndrome, type 2; dominant
LRP2	600073	Low density lipoprotein Receptor-related Protein 2; transmembrane	LRP2/ Glycoprotein 330/ megalin	Endocytic receptor, Development of ear	Outer ear malformation, sensorineural, severe to profound	Donnai-Barrow syndrome; recessive
LRP4	604270	Low density lipoprotein Receptor-related Protein 4; transmembrane	LRP4/MEGF7	Causes narrowing of foramina of the cranial nerves which leads to hearing loss	Hearing loss present, but no details provided	Sclerosteosis 2, recessive
LRP5	603506	Low density lipoprotein Receptor-related Protein 5; transmembrane	LRP5/LRP7/LR3	Co-receptor during WNT signaling	Sensorineural,	Osteosclerosis, dominant
MAP3K7	602614	Mitogen-Activated Protein Kinase kinase kinase 7; cytoplasmic	MAP3K7/ TAK1	Mediator of TGFB and BMP signaling	Outer and inner ear malformations, conductive. bilateral	Cardiospondylocarpofacial syndrome; dominant
MET	164860	Mesenchymal EpithelialTransition factor; transmembrane	MET/HGFR	Incorporation of neural crest cells into the stria vascularis	Prelingual, sensorineural, bilateral, severe to profound	Deafness with arthrogryposis and distinctive facial features; recessive
NDP	300658	Norrin cystine knot growth factor; secreted	NDP/Norrin	Vascularization of cochlea with particular importance in stria vascularis and spiral ganglion, binds to Frizzled receptors	Sensorineural, postlingual, childhood or adult onset, bilateral, progressive, mild to profound	Norrie Disease; X-linked recessive
NLRP3	606416	NLR family, Pyrin domain-containing 3; cytoplasmic	NLRP3/CIAS1/NALP3/PYPAF1/Cryopyrin	Leads to events which end in production of cytokine IL-1β	Postlingual, sensorineural, bilateral, progressive, severe	CINCA syndrome, Muckle-Wells syndrome; dominant
NOG	602991	NOGgin, mouse homologue of; secreted	NOG	Inhibits signaling by GDF6, BMP, and TGFB	Stapes ankylosis, conductive, rarely sensorineural, progressive	Multiple synostoses syndrome 1; dominant
NOTCH2	600275	Drosophila homolog of Notch, 2; transmembrane	NOTCH2	Patterning of the organ of Corti	Childhood onset, conductive, rarely sensorineural, mild to moderate	Hajdu-Cheney syndrome; dominant
NOTCH3	600276	Drosophila homolog of Notch, 3; transmembrane	NOTCH3	Patterning of the organ of Corti	Outer ear malformation, conductive or mixed, bilateral, moderate	Lateral meningocele syndrome; dominant
NSD1	606681	Nuclear receptor-binding Su-var, enhancer of zeste, and trithorax Domain protein 1; nuclear	NSD1/ARA267	unknown	Childhood onset, otitis media, conductive	Sotos syndrome 1; dominant
PTPN11	176876	Protein-Tyrosine Phosphatase, Nonreceptor-type, 11; cytoplasmic	PTPN11/PTP2C/SHP2	Mediates signaling by HGF, EGF through activation of RAS/MAPK pathways	Outer ear malformation, Childhood or adult onset, sensorineural or mixed, unilateral or bilateral, mild to profound	Noonan syndrome 1, LEOPARD syndrome 1; dominant
RAF1	164760	v-Raf-1 Rapidly Accelerated Fibrosarcoma murine leukemia viral oncogene homologue 1, cytoplasmic	RAF1/CRAF	Secondary signal transducer for receptor tyrosine kinase via RAS/MAPK pathways	Outer ear malformation, childhood or adult onset,sensorineural or mixed, unilateral or bilateral, mild to profound	Noonan Syndrome 5, LEOPARD syndrome 2; dominant
ROBO1	602430	Roundabout, drosophila, homolog of 1; transmembrane	ROBO1/SAX3	Axon guidance receptor	Sensorineural, profound	combined pituitary hormone deficiency; recessive
SEMA3E	608166	SEMAphorin 3E; secreted	SEMA3E/SEMAH/ KIAA0331	Cell migration, neural crest and otic placode induction	Outer and inner ear malformations, prelingual, mixed or sensorineural, progressive, severe or profound	CHARGE syndrome; dominant
SLITRK6	609681	SLIT- and NTRK-like family, member 6; transmembrane	SLITRK6	Survival and innervation of sensory neurons	Sensorineural or auditory neuropathy spectrum syndrome, prelingual, bilateral moderate to profound	Deafness and myopia; recessive
SMAD4	600993	Mothers Against Decapentaplegic, drosophila, homolog of, 4; cytoplasmic	SMAD4/MADH4/DPC4	Mediator of signaling of BMP and TGFB superfamily members	Outer ear malformations, early onset, mixed	Myhre syndrome; dominant
TBL1X	300196	Transducin-Beta-Like 1, X-linked	TBL1X/TBL1/EBI	Component of the nuclear corepressor (NCOR) complex	Mild hearing loss	X-linked
TBL1Y	400033	Transducin-Beta-Like 1, Y-linked	TBL1Y	Component of the nuclear corepressor NCOR complex	Sensorineural, adult onset, bilateral, mild to severe	Deafness, Y-linked 2; Y-linked
TBL1XR1	608628	Transducin-Beta-Like 1 Receptor 1; nuclear	TBL1XR1/TBLR1/IRA1/C21	General function in WNT signaling and component of the nuclear corepressor (NCOR) complex	Outer ear malformation, prelingual, unilateral or bilateral, Conductive	Pierpont syndrome; dominant
TGFB1	190180	Transforming Growth Factor, Beta-1; secreted	TGFB1, TGFB	Proliferation of mesenchymal and epithelial cell types	Conductive or sensorineural, bilateral, moderate	Camurati-Engelmann disease; dominant, Otosclerosis
TGFB1	190180	Transforming Growth Factor, Beta-1; secreted	TGFB1, TGFB	Proliferation of mesenchymal and epithelial cell types	Conductive	Otosclerosis
THRB	190160	Thyroid Hormone Receptor Beta; nuclear	THRB/ERBA2/ERBAβ	Maturation of auditory system	Prelingual, sensorineural, bilateral, severe	Thyroid hormone resistance, autosomal recessive; recessive
THRB	190160	Thyroid Hormone Receptor Beta; nuclear	THRB/ERBA2/ERBAβ	Maturation of auditory system	Sensorineural, variable degree	Thyroid hormone resistance; dominant
TNFRSF11A	603499	Tumor Necrosis Factor Receptor SuperFamily, member 11A; transmembrane	TNFRSF11A/RANK/ODFR/OSTS	Role in auricular ossification, stimulates NF-kappaB signaling	Conductive or mixed, progressive, bilateral, mild to moderate	Paget disease of bone 2, early-onset; dominant
TNFRSF11A	603499	Tumor Necrosis Factor Receptor SuperFamily, member 11A; transmembrane	TNFRSF11A/RANK/ODFR/OSTS	Role in auricular ossification, stimulates NF-kappaB signaling	Prelingual, mixed, bilateral, mild to moderate, or profound	Osteolysis, familial expansile, dominant
TNFRSF11B	602643	Tumor Necrosis Factor receptor superfamily, member 11b; secreted	TNFRSF11B/OPG/OCIF	Regulates NF-kappaB signaling as a decoy receptor ligand for TNFRS11A	Sensorineural, prelingual, bilateral, profound	Paget disease of bone 5, juvenile onset; recessive

OMIM, Online Mendelian Inheritance in Man

Here, we highlight three examples where there is robust data from human genetics and supporting data from animal models that both the receptor and its ligand, and other molecules in their signaling pathways, are necessary for hearing. When a ligand binds its receptor, a conformational change occurs in the receptor, initiating a signal propagated through second messengers.¹¹ Different classes of receptors can affect production of many of the same second messengers, and orchestrate development and maintenance of an organism.¹²

2. GROWTH FACTORS RELEVANT TO THE AUDITORY SYSTEM

Ear morphogenesis, cell fate and axis formation are established and guided by the concerted action of signaling molecules including retinoic acid, sonic hedgehog (SHH), various Wingless-related integration site (WNT) proteins, fibroblast growth factor (FGF) and bone morphogenetic protein (BMP) [reviewed in ¹³]. Growth factors and cytokines necessary for the development of the ear are predominantly proteins and steroids, and less commonly, lipids. A rare example of a lipid messenger is sphingosine-1-phosphate (S1P) or lysosphingolipid,¹⁴ a sphingolipid that binds to its S1PR2 receptor in the inner ear, which is necessary for hearing in human and mouse.¹⁵ In addition to the classic signaling molecules involved in hearing mentioned above, NLRP3 is part of the inflammosome complex and variants cause over-production of the cytokine interleukin (IL)-1β¹⁶ result in hearing loss.¹⁷

Juxtacrine signals mediated by NOTCH pathways establish the sensory epithelium, including hair cells and supporting cells.¹⁸ In humans, WNT¹⁹ and BMP²⁰ family members are required for development of the auditory system.²¹ Variants of WNT3, WNT4, WNT5A, WNT7A, BMP1, BMP2, BMP4, and BMP7 (OMIM#165330, 603490, 164975, 601570, 112264 112262, 112261, 112267) result in low set ears or external ear malformations including prominent or posteriorly rotated ears, and small external auditory canals. In a few individuals, rare variants of BMP2, BMP4, BMP7, and GDF6 (a BMP-class ligand) have been associated with syndromic deafness or surgically verified otosclerosis (Table 2). Variants of NOG encoding Noggin, which inhibits BMP signaling by blocking the ligand binding sites of the cognate receptors, also cause hearing loss in humans (Table 2). Although variants of the WNTs only lead to external ear malformations in humans,^22–26 targeted deletion of their frizzled receptors cause hearing loss in mouse²⁷ and are candidate genes for human deafness in individuals without a known cause. Norrin, the protein encoded by NDP, is a different molecule from WNT, is also a ligand of the frizzled receptor, FZ4. In humans, variants of NDP are associated with X-linked recessive Norrie Disease characterized by childhood onset blindness and various adult onset neurologic manifestations.²⁸ About 30% to 40% of individuals with variants of NDP have a progressive increase in hearing thresholds (Table 2).

The growth factors FGF3, FGF10, HGF, IGF1, JAG1, KITLG and TGFB1 are required for development or maintenance of hearing in humans (Table 1, Table 2). Homozygous loss of function variants of FGF3 cause ear anomalies including labyrinthine aplasia, and microtia, accompanied by prelingual, sensorineural deafness.²⁹ In mouse, FGF10 is required for development of non-sensory epithelium of the inner ear.³⁰ Double homozygous knockouts of mouse Ffg3 and Fgf10 have diminutive otic vesicles, a more severe phenotype than that exhibited by single homozygous mutants of either gene.³¹ Individuals with dominantly inherited pathogenic alleles of FGF10 exhibit LADD syndrome, which is characterized by abnormalities of teeth and distal limb segments, but only 50% of cases manifest a mixed hearing loss.³² Dominant and recessive alleles associated with deafness may have reduced penetrance and variable expressivity that can be due to modifiers in the genetic background, a supposition robustly supported by observations in mouse.^33,34 A few examples of genes with variants involved in syndromes where deafness is not a constant feature include DVL1, DVL3, FGFR1, KIT, MAP3K7, and NDP (Table 2). Genetic, environmental or stochastic factors responsible for reduced penetrance of deafness in syndromes due to variants of these genes remain to be discovered.

3. RECEPTORS RELEVANT TO AUDITORY SYSTEM

G-protein coupled receptors, receptor kinases and nuclear hormone receptors are three main classes of receptors, which respond to signaling molecules in the auditory system. Other types include enzymatic and non-enzymatic transmembrane proteins (Tables 1 and 2).

3.1. G-protein coupled Receptors

G-protein coupled receptors (GPCRs) have seven alpha-helical transmembrane domains with variable length intracellular N- and C-termini and intracellular and extracellular loops mediating interactions with protein partners and are coupled to multi-subunit trimeric G-proteins. Ligand binding to receptors activates multiple pathways (Figure 2A). Variants of the GPCR-encoding genes ADGRV1, EDNRA, EDNRB and S1PR2 are associated with hearing loss (Tables 1 and 2). Ligand binding activates GTP hydrolysis of the G-protein catalytic unit (Figure 2A). One consequence of G-protein coupled receptor signaling is the generation of cyclic AMP (cAMP) by activation of adenylate cyclase 1 (ADCY1) (Figure 2A). A nonsense variant of ADCY1 was reported to cause hearing loss in humans and zebrafish morphants of Adcy1b have a hearing loss due to hair cell dysfunction.³⁵ The inner ear morphology and hearing status of mice with a targeted disruption of Adcy1³⁶ or a spontaneous retrotransposon disruption of Adcy1 (brl)³⁷ have not been reported.

FIGURE 2.

GPCR and RTK signaling pathways. (A) G-protein Coupled Receptors (GPCR). I. The receptors are coupled to the C-terminus of heterotrimeric G-proteins. G-proteins are composed of α, β, and γ subunits. GPCRs activate signaling through multiple pathways. II. Phospholipase C, (PLC), phosphatidylinositol signaling pathway. Diacyl glycerol (DAG) and inositol 1,4,5-trisphosphate (IP3) are generated by cleavage of phospholipid phosphatidylinositol 4,5-bisphosphate (PIP2) by phospholipase C. IP₃ releases calcium ions from endoplasmic reticulum. Diacylglycrol remains bound to the plasma membrane. Both diacylglycerol and calcium ions act together activate Protein Kinase C (PKC). PKC phosphorylates multiple cytoplasmic proteins that regulate cellular activity. III. Cyclic AMP signaling pathway. Cyclic AMP is a second messenger. GPCR activates adenylate cyclase, which converts ATP to cyclic AMP (cAMP). cAMP activates protein Kinase A (PKA), which then phosphorylates different proteins and transcription factors. PKA translocates to nucleus and controls gene transcription of target genes. IP3R, inositol trisphoshphate receptor, GEF; guanidine exchange factor, P; Phosphate. (B) Receptor Tyrosine Kinase (RTK). Ligand binding causes receptor dimerization and autophosphorylation. Signaling via RTK can take place through phospholipase C pathway, left side of figure (also see Figure 2A). GRB2 with other associated proteins such as GAB1, is bound to RTK. The activated receptor phosphorylates SOS1 and other guanine nucleotide exchange factors (GEF), and members of RAS, RHO and RAF, which are tethered to membranes. Signals are further propagated through the MAPK/ERK pathway to regulate gene expression. PI3K; phosphoinositide 3- kinase, Akt; Protein Kinase B.

Cyclic GMP (cGMP) is generated from GTP by guanylyl cyclase in response to ligand binding to G-protein coupled receptors. cGMP activates cGMP-dependent protein kinases. One such kinase is PRKG1, which is expressed in sensory cells and neurons of the mouse inner ear where it protects against noise induced hearing loss.³⁸ PDE1C, which encodes phosphodiesterase 1C, and hydrolyzes cGMP and cAMP, is important for Ca⁺² homeostasis (Figure 2A). Variants of human PDE1C are associated with dominantly inherited hearing loss DFNA74 (Table 1).

Regulators of G-protein coupled receptor signaling required for normal hearing include GPRASP2 and GPSM2 (Table 2). GPRASP2 regulates post-endocytic sorting of G-protein coupled receptors by binding to their C-termini.^39,40 GPSM2, together with its partner GNAI3, are both expressed asymmetrically at the apical surface of hair cells, and control localization of kinocilia.⁴¹ GPSM2 interacts with the α-subunits of G-proteins, (including GNAI3), and modulates their activation.⁴² Additionally, GPSM2 and GNAI3 are both normally enriched in a narrow compartment at the tips of the tallest row stereocilia.^43,44 A conditional deficiency of GPSM2 or GNAI3 in the mouse inner ear results in stereocilia that are shortened by ~40% and ~25%, respectively.⁴⁵ Consistent with these results in mice, variants of human GPRASP2 and GPSM2 have been found to cause deafness (Table 2). The accumulation of GPSM2 and GNAI3 along with WHRN and EPS8 at the tips of stereocilia requires MYO15A, a motor protein. In the absence of MYO15A, WHRN and EPS8 fail to accumulate at the stereocilia tips. These data suggest that MYO15A transports a large complex of proteins to locations where they regulate actin polymerization dynamics of both stereocilia in post-mitotic hair cells and neuronal growth cones.⁴⁵

3.1A. EDN3 and EDNRB signaling

Endothelin EDN3 is synthesized as pre-pro-endothelin and then cleaved by endothelin-converting enzyme to a 21-residue peptide.⁴⁶ Endothelins are vasoconstrictors that participate in epithelial-mesenchymal interactions and are also important for melanocyte differentiation. In the inner ear, melanocytes develop from neural crest cells, some of which migrate into the intermediate cell layer of the stria vascularis and are necessary for establishing the endocochlear potential, which drives sound transduction.⁴⁷ EDN3 binds to EDNRA or EDNRB, that transmit signals through Gαq/α11 G-protein subunits.⁴⁷ One consequence is activation of phospholipase C which hydrolyses phoshphatidylinositol into second messengers diacylglycerol (DAG) and inositol 1,4,5 triphosphate (IP3). IP3 releases Ca²⁺ from endoplasmic reticulum. DAG and Ca²⁺ together activate Protein Kinase C (PKC) (Figure 2A). PKC then activates mitogen-activated protein kinase (MAPK) pathway, phosphorylating cAMP response element binding protein (CREB).⁴⁸ CREB binds to cAMP response element (CRE) DNA sequence of MITF,⁴⁸ a transcription factor required for development of melanocytes.

Variants of either human EDNRB or EDN3 are associated with Waardenburg syndrome types 4A and 4B, respectively.^49,50 WS4 is chacterized by sensorineural hearing loss, hair, skin and eye pigmentary abnormalities and Hirschsprung’s disease (Table 2) chacterized by a deficiency of ganglion cells of the distal colon and consequently muscles in the colon fail to peristaltically move stool leading to obstructions. EDNRB also has a distinct but unknown role for normal spiral ganglion function as Ednrb knockout mice undergo degeneration of spiral ganglion neurons.⁵¹ The time course of Ednrb expression in SGNs, exactly when SGN death occurs and also which of the three SGN subtypes (1a, 1b, or 1c) are lost in Ednrb mutants remains to be studied.

3.2. Receptor Tyrosine Kinases

Receptor Tyrosine Kinases (RTK) are single-pass transmembrane proteins with extracellular immunoglobulin-like domains and other regions of variable lengths that interact with growth factors and trigger cell growth and differentiation. The intracellular region has tyrosine kinase activity and docking sites for scaffold proteins (Figure 2B). Variants of human RTKs FGFR1, FGFR2, FGFR3, IGF1, KIT, MET and ROR1 are associated with hearing loss (Table 1, Table 2). RTKs mediate signaling through RAS/RAF/MAPK/ERK pathways (Figure 2B). Regulatory proteins of RTKs are important for signaling in the inner ear including a family of mitogen-activated protein kinase kinase kinases, BRAF and RAF1 that regulate RAS/RAF/MAPK/ERK signal transduction (Figure 2B). BRAF phosphorylates MAP3K1⁵², and RAF1 is the principal component of MAPK pathway.^53,54 The importance of BRAF and RAF1 for normal hearing is demonstrated by the fact that some of their variants are associated with LEOPARD syndromes 3 and 2 and Noonan Syndrome 5, respectively (Table 2), which are separately characterized by disparate anomalies of heart, skin, genitalia or skeleton. RAS-related GTPase RIT1 regulates p38 MAPK-dependent signaling cascades⁵⁵ and variants of RIT1 (OMIM #609591) are also associated with low set ears in humans,⁵⁶ pointing to the importance of RIT1 for both the morphogenesis and function of the outer ear.

Effectors of RTK signaling participating in hearing include CCDC50, EPS8, EPS8L2 among others, variants of which are associated with hearing loss DFNA44, DFNB102 and DFNB106, respectively (Table 1). CCDC50 encodes YMER, which inhibits EGFR down-regulation ⁵⁷ and negatively regulates NF-kB signaling pathway.^58,59 In the inner ear, EGFR signaling is important for proliferation of supporting cells.⁶⁰ The epidermal growth factor receptor pathway substrate 8 (EPS8) and epidermal growth factor receptor pathway substrate 8-like protein 2 (EPS8L2) are also required for hearing.^61,62 EPS8 is part of N-methyl-d-aspartate (NMDA) receptor complex,⁶³ which controls transduction of signals from RAS to RAC. EPS8L2 acts by stimulating RAC (GTPase)-guanine nucleotide exchange factor activity of SOS1.⁶⁴ In the inner ear, EPS8 and EPS8L2 are components of an electron-dense complex of proteins at the tips of hair cell stereocilia, as visualized by transmission electron microscopy. By virtue of their actin remodeling activity, they are important for elongation and maintenance of the precise lengths of stereocilia.^64–68

3.2A. Hearing requires HGF and MET signaling

Hepatocyte growth factor (HGF) is secreted by mesenchymal cells, binds the MET receptor, and regulates epithelial cell development and motility by activating a variety of downstream signaling pathways.⁶⁹ Like many genes, human HGF and mouse Hgf encode multiple alternative transcripts, most of which have not been well studied.⁷⁰ The importance of identifying and then studying the function of each alternative transcript of a gene was recently highlighted when an alternatively spliced microexon in cytohesin 1 (CYTH1), a gene encoding a guanidine exchange factor, was shown to be necessary for spatially restricting HGF-MET signaling.⁷¹

In 40 large families from Pakistan segregating deafness, the phenotype was linked to markers for the DFNB39 locus on chromosome 7. Homozygosity was detected for either a non-coding, evolutionarily conserved three base pair deletion or an overlapping ten base pair deletion in the 3ʹ UTR of a short alternative splice isoform of HGF of unknown function.⁷⁰ The presence of recognizable pathogenic variants in all of other exons and conserved sequences in the DFNB39 interval was excluded by sequencing. Yet, the possibility remains that the non-coding 3bp and 10bp deletions of HGF are in linkage disequilibrium with the real deafness-causing variant in the DFNB39 interval. The pathogenicity of these noncoding variants may be addressed by knocking in the identical 3bp or 10bp deletions in mouse Hgf. Nevertheless, there is compelling evidence from mouse that a wild type HGF expression level in the inner ear is necessary for normal hearing.⁷⁰ In mouse, body-wide excessive expression of HGF from a Hgf transgene results in deafness. Additionally, a homozygous inner ear conditional knockout of Hgf causes deafness, indicating that the titer of HGF must be tightly regulated for normal hearing, and that too much or too little HGF is incompatible with normal hearing.⁷⁰ During inner ear development, HGF is required for proper incorporation of neural crest cells into the intermediate cell (middle) layer of the stria vascularis (Figure 1C) in mice.⁷² The stria maintains an endocochlear potential of +80 to +120 millivolts and a high concentration of potassium (154 millimolar) that bathes the apical surface of hair cells and which is necessary for mechano-transduction of sound by inner hair cells (Figure 1C).

The HGF receptor MET was also demonstrated genetically to be necessary for hearing. In mouse, a complete loss of MET function results in embryonic lethality and zebrafish met morphants have reduced neuromast-derived hair cells.⁷³ Zebrafish neuromasts resemble vertebrate inner ear sensory epithelia. In nine affected members of a human family, a predicted damaging missense variant p.(Phe841Val) of MET, located in the IPT4 domain of all MET isoforms, is associated with recessively inherited, nonsyndromic severe hearing loss.⁷⁴ The IPT3 and IPT4 domains constitute a high-affinity HGF binding surface of MET. A second homozygous missense variant p.(Phe1186Cys) of MET is associated with arthrogryposis and deafness in two siblings,⁷⁵ providing independent support for the conclusion that MET is required for normal hearing. It seems likely that these two damaging missense variants permit residual MET function required for embryonic development but disrupt a function of MET necessary for hearing.

The HGF-stimulated MET signaling pathway has numerous branches (Figure 2B). Variants of some of the genes encoding components of this signaling cascade are also associated with deafness in human or mouse. GAB1 is a component of a multi-subunit scaffold for various RTKs including IGF1 and MET (Figure 2B). A homozygous, hypomorphic missense variant of GAB1, p.(Gly116Glu), is associated with nonsyndromic hearing loss DFNB26 segregating in a family with eight affected individuals.⁷⁶ Unexpectedly, seven normal hearing individuals in this family were also homozygous for the p.(Gly116Glu) variant. These non-penetrant individuals (but none of the affected individuals) also carry a missense variant p.(Arg544Gln) of EEF1AKNMT (also called METTL13) ⁷⁶ at the dominant modifier locus DFNM1 of DFNB26 deafness. METTL13 (methyltransferase 13) is a predicted methyltransferase and is hypothesized to suppress GAB1-related deafness although the mechanism by which this might act is unknown. Interestingly, GAB1 and sprouty (SPRY2) interact with METTL13. SPRY2 down-regulates receptor tyrosine kinases and is also required for hearing in mouse.⁷⁷ Taken together, it is clear that HGF/MET/GAB1/SPRY2 signaling is crucial for the auditory system, although how this is mediated is not yet fully understood.

3.3. Other Enzymatic transmembrane receptors

In addition to RTK, two types of receptors expressed in the ear have intrinsic enzymatic activity, serine/threonine kinases and receptor tyrosine phosphatases. Receptor serine/ threonine kinases are single-pass transmembrane proteins with a serine/threonine kinase domain (types I, II and III). Types I and II exist as homodimers. Type III receptors act as co-receptors by presenting the ligand to the other two classes (Figure 3A). ACVR1 encodes a type I serine/threonine kinase receptor and variants cause hearing loss in some individuals with fibrodysplasia ossificans progressiva, a connective tissue disorder (Table 2).

FIGURE 3.

Other receptors with intrinsic enzymatic activity (A) Receptor serine/threonine kinase. A ligand of the TGFB superfamily or the BMP superfamily, binds first to a type II receptor, which in turn phosphorylates the type I receptor and leads to their dimerization. The activated receptor complex phosphorylates the SMAD family proteins. Phosphorylation of SMAD proteins disassociates them from the receptor complex. Bound SMADS translocate to the nucleus and form complexes with DNA regulatory proteins inhibiting or activating transcription of target genes. (B) Receptor tyrosine phosphatase. PTPRQ dephosphorylates phosphatidylinositol 3,4,5-trisphosphate (PIP3) to PIP2. (See Figure 2B for details of signaling via PIP2).

The receptor tyrosine phosphatases are a subclass of transmembrane protein tyrosine phosphatases (PTPase). PTPRQ is a type III receptor-like protein-tyrosine phosphatase (Figure 3B) that preferentially dephosphorylates and regulates levels of phosphatidylinositol 1,4,5-trisphosphate, PIP3,⁷⁸ by dephosphorylating it to PIP2, which is further dephosphorylated to IP3 (Figure 3B). Signaling through PIP3 is important for survival, proliferation, and the subcellular architecture of diverse cellular types. Several dominant and recessive variants of human PTPRQ are associated with hearing loss in human (Table 1) and mouse.⁷⁹ In the inner ear, PTPRQ contributes to hair-bundle shaft connectors, which modulate spacing of stereocilia,⁷⁹ while rootlets of stereocilia develop postnatally and function to anchor stereocilia into the actin-rich cuticular plate. It remains to be determined whether PTPRQ also functions as a receptor in the inner ear.

3.4. Non-enzymatic Receptors

Transmembrane receptors with no known intrinsic enzymatic activity include ILDR1, NOTCH, TNFRS11A and SLITRK6 (Figure 4A–4D). ILDR1 is a receptor having an immunoglobulin-like extracellular domain (Figure 4A). In zebrafish, ildr1b morphants exhibit hearing loss and have a significantly reduced expression of fgf3, fgf10, and fgfr1, disrupting migration of the lateral line primordium.⁸⁰ In mouse, Ildr1 is expressed in the small intestine and regulates fat-stimulated cholecystokinin secretion.⁸¹ Variants of human ILDR1 are associated with nonsyndromic deafness DFNB42 inherited as a recessive trait (Table 1), a phenotype that is recapitulated in mouse homozygous for a deletion of Ildr1.^82,83 It remains to be determined if deaf human subjects with biallelic pathogenic variants of ILDR1 alleles have an additional clinically relevant phenotype involving a disruption of cholecystokinin secretion. In the mouse inner ear, ILDR1 is necessary for retention of marvel domain-containing protein 2 (MARVELD2)-originally named tricellulin encoded by TRIC- at the tricellular tight junctions.⁸³ Human deafness-causing equivalent knock-in variants of mouse Marveld2 are also deaf.⁸⁴ It is not known if there is a ligand for ILDR1 in the inner ear. Perhaps in the sensory epithelium of the inner ear, ILDR1 just functions as a structural protein at the tricellular junction between three epithelial cells.

FIGURE 4.

Non-enzymatic receptors (A) ILDR1 has an extracellular immunoglobin like domain and intracellular cysteine and arginine regions. Both of these are separated by a dileucine motif. Signaling through this receptor in the inner ear remains to be elucidated. (B) NOTCH receptors bind to transmembrane ligands such as JAG1. As a result of binding, NOTCH is proteolytically cleaved by γ-secretase releasing the intracellular domain of the protein (NICD), which translocates to the nucleus forming complexes with other proteins and regulate transcription of target genes. (C) SLITRK6 is a transmembrane receptor with leucine rich repeats in its extracellular domain. The mechanism of signaling through SLITRK6 is unknown. (D) TRADD and TRAFF are adapters of TNRFS receptors and participate in downstream signaling. Binding of TNF to its receptors activates either MAPK8/JNK or NF-kB (NFKB1), which is sequestered in the cytoplasm by IKB. The activation of NFKB1 by receptor binding activates a kinase (IKK) which phosphorylates IKB at specific serine residues. IKB is then ubiquitinated and degraded by the proteasome. Free NFKB1 translocates to the nucleus and regulates transcription. The second signaling pathway involves MAPK8/JNK. MAPK8 phosphorylates a number of transcription factors which modulate transcription of specific genes. (E) Ligand binding to nuclear hormone receptors cause dimerization and translocation to the nucleus. Receptor-DNA binding controls transcription of targeted genes involved in a variety of cellular activities including ion transport and proliferation. IKB; nuclear factor of kappa light polypeptide gene enhancer in B-cells inhibitor, alpha, IKK; IKB Kinase.

3.4A. JAG/NOTCH signaling

Signaling through NOTCH receptors determines cell fates during development.⁸⁵ NOTCH receptors interact with membrane bound ligands JAG and DLL that are present on adjacent cells (Figure 4B). Signaling induced by binding of NOTCH or JAG results in cleavage of the NOTCH receptor, which then translocates to the nucleus and alters transcription of target genes, some of which are important for hearing, such as HES1 (Figure 4B).^86,87 Variants of the human genes encoding JAG1, and NOTCH2 and NOTCH3 receptors cause syndromes involving hearing loss (Table 2). In the cochlea of a Jag1 p.(Gly289Asp) heterozygous mouse, referred to as headturner, the number of outer hair cells is reduced by 33% with a slight increase in the number of inner hair cells. However, these mice are not deaf.⁸⁸ In contrast, mice with a deletion of Jag1 or Notch2 limited to neural crest cells have malformed stapes and incus, and have a mild to moderate hearing loss.⁸⁹

3.5. Nuclear Hormone Receptors

Nuclear hormone receptors are cytoplasmic proteins. Binding to ligand activates the receptors, which then translocate to the nucleus where they regulate target gene transcription (Figure 4E). Nuclear hormone receptors required for hearing in human include ESRRB, ESRRG and THRB (Table 1, Table 2). Studies of conditional knockout mice have shown that ESRRB controls expression of many genes encoding transporters and ion channels, such as Atp1b2, Kcnq1, Kcne1, which are all important for inner ear function,⁹⁰ specifically in strial marginal cells. One consequence of the absence of ESRRB in mouse is a partial transformation of strial marginal cells into epithelial cells.⁹⁰

Repressors of nuclear hormone receptors also play a role in audition. TBL1X and TBL1Y are members of WD40 repeat-containing protein family and act as repressors of nuclear hormone receptors. They are important for ear development and hearing.^91,92 TBL1X and TBL1Y are components of nuclear receptor co-repressor (NCOR) complex which is required for Tri-iodothyronine (T3)-regulated gene expression.⁹³ This regulation is important since thyroid hormone T3 and its receptor THRB play a role in cochlear development by controlling expression of genes necessary for hearing, which include KCNQ4 and SLC26A5.⁹⁴

3.6. CO-RECEPTORS

Heparan sulfate proteoglycans are required for dimerization of some receptor tyrosine kinases including FGFR and EGFR.⁹⁵ In addition, there are many proteins, which function as co-receptors. For example, the low-density lipoprotein (LDL) receptor-related proteins serve as co-receptors to frizzled proteins during canonical WNT signaling and their variants are associated with hearing loss.^96–98 LRP5/6 is a WNT co-receptor in vertebrates,⁹⁹ and Lrp5 knockdown in zebrafish with morpholinos reduces the number of supporting cells and hair cells.⁹⁸ Additionally, variants of human LRP5 cause either nonsyndromic hearing loss (Table 1) or sensorineural hearing loss in patients with dominantly inherited osteosclerosis (Table 2).

4. CONCLUSION

The development of the auditory system requires multiple precisely orchestrated biochemical events. Genetic studies of hundreds of genes necessary for audition have uncovered several pathways important for normal hearing. Based initially upon studies of animal model, growth factors are now being explored for treatment of noise and ototoxic induced deafness and age related hearing loss,^100–109 and some of these are currently in clinical trials.^110,111 For example, topical application of IGF1 to the middle ear was used to treat sudden sensorineural hearing loss in humans, and some treated individuals showed improvements of 10 dB to 30 dB for tested frequencies as compared to controls.^103,112 Hair cell regeneration in response to application of growth factors and by inhibition of specific signaling pathways is also being explored.¹¹³ For example, an inhibitor of NOTCH signaling is in phase II clinical trial for treatment of sensorineural moderate to severe hearing loss.²¹ As signaling pathways are involved with differentiation, proliferation, maintenance and regeneration processes, greater understanding of the in vivo ligands, receptors, protein partners and the modulation of their expression may provide opportunities to rebuild a properly patterned and functional adult human inner ear.