Microtia is a congenital anomaly, characterised by a small, abnormally shaped auricle (pinna). It is usually accompanied by a narrow, blocked or absent ear canal. Microtia can occur as the only clinical abnormality or as part of a syndrome. The estimated prevalence of microtia is 0.8–4.2 per 10 000 births, and it is more common in men. Microtia can have a genetic or environmental predisposition. Mendelian hereditary forms of microtia with an autosomal dominant or recessive mode of inheritance, and some forms due to chromosomal aberrations have been reported. Several responsible genes have been identified, most of them being homeobox genes. Mouse models have been very useful to study these genes, providing valuable information on the development of the auditory system. In this article, we review the epidemiological characteristics of microtia and the environmental causes involved. In addition, we discuss the development of the auditory system, specifically the relevant aspects of external and middle ear development. The focus of this review is to discuss the genetic aspects of microtia and associated syndromes. The clinical aspects of various disorders involving microtia are also discussed in relation to the genes that are causing them.
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The external ear consists of the auricle and the external ear canal. There is a wide range of external ear abnormalities, which are related to the size, shape, position of the ear or even the presence of preauricular pits or tags.1
Microtia (OMIM 600674, OMIM 251800) is a developmental malformation of the external ear, characterised by a small, abnormally shaped auricle. The estimated prevalence of microtia varies between 0.8 and 4.2 per 10 000 births in different populations2–7 (table 1). Microtia can occur unilaterally or bilaterally. The unilateral form is much more common, occurring in 79–93% of cases.2 3 8 In unilateral microtia the right ear is more often affected (approximately 60% of the unilateral cases).4 5 9 Individuals with unilateral microtia usually have normal hearing in the other ear. Therefore, speech and language development are usually normal, although these children are at a greater risk of delayed language development and attention deficit disorders.10 Microtia is more common in male than in female patients, with a sex ratio of 1.5,5 9 and only in a single study in China was there no difference in gender distribution.11 Microtia is associated with atresia (OMIM 607842) (absence or closing) or stenosis (narrowing) of the ear canal in 55–93% of patients.6 9 12 The general characteristics of microtia in various populations are summarised in table 1.
There are several grading systems for microtia. In the Marx classification,13 all of the features of a normal auricle are present in grade I, but the pinna is smaller than normal. In grade II, some anatomical structures are still recognisable. In the most common form, grade III (the peanut-shell type), only a rudiment of soft tissue is present.9 The extreme case where there is no external ear and auditory canal is called anotia (OMIM 600674) or microtia grade IV. The prevalence of anotia has been reported to vary between 5 and 22% of microtia cases.14 Fig 1 presents ears with grades I to IV of microtia. There is a strong correlation between the degree of microtia and the frequency and severity of middle ear dysplasia. In general, the better developed the external ear, the better developed the middle ear.15 16
In the clinical assessment of a patient with microtia, looking for associated anomalies is important as, if they are present, attributing them to a known syndrome could be crucial. Otological and audiological evaluation and possibly radiological imaging should also be considered. More than 80% of patients with microtia have aural atresia resulting in conductive hearing loss, with air-conduction hearing typically reduced by 40–65 dB, whereas bone conduction is normal in >90% of the affected ears.6 17–19 Genetic counselling should always be considered for a patient with microtia. If auricular reconstruction is indicated, a multistep earlobe reconstruction with autogenous rib cartilage can be applied in most cases.20
Development of the auditory system
The different body structures of vertebrate animals result from the development of six pharyngeal (branchial) arches (PA I to PA VI) during embryonic development. The vertebrate ear develops as a result of complex tissue interactions during embryogenesis. The outer and middle ear originate from the mesenchyme through interactions between cells at PA I and PA II and migrating neural crest cells (NCCs).21 The external ear begins to develop around the dorsal end of the first branchial cleft during the sixth week of gestation. The auricle results from the fusion of six small buds of PA I and PA II, called hillocks. The auricle is usually complete by the twelfth week. Initially, the auricles form at the base of the neck, but as the mandible develops, the auricles migrate to their normal adult location by gestational week 20.22 During the first and second month’s gestation, the external auditory meatus derives from the first branchial cleft between the mandibular and hyoid arches.
Development of the middle ear requires sequential interactions between the epithelia and the underlying mesenchyme. The middle ear ossicles derive from the NCC mesenchyme. Gene-inactivation experiments have identified several genes required for the formation of different middle ear components.21 Signalling molecules, such as endothelin 1 (EDN1; OMIM 131240) and fibroblast growth factor (FGF) 8 (OMIM 600483), probably mediate epithelial–mesenchymal interactions. Other proteins, including Eya1(OMIM 601653), Prx1 (OMIM 167420), Hoxa1 (OMIM 142955), Hoxa2 (OMIM 604685), Dlx1 (OMIM 600029), Dlx2 (OMIM 126255), Dlx5 (OMIM 600028), and Gsc (OMIM 138890), are most likely involved in patterning and morphogenetic processes in the neural crest-derived mesenchyme.21
Rhombomeres are embryonic territories arising from the transient segmentation of the hindbrain.23–25 Homeobox genes express critical developmental transcription factors in embryonic development. The Hox genes are a large group of homeobox genes, and NCCs populating the second branchial arch express HoxA2 (OMIM 604685) over a prolonged period.26 In the absence of normal HoxA2, the boundary between rhombomeres 1 and 2 is lost.27–29 This result indicates that HoxA2 is a key transcription factor during development of the second branchial arch, which has a major contribution to development of the external and middle ear. The Hoxa2 knockout mouse has provided an appropriate tool to understand the mechanism of development of the auditory system, mainly the outer and middle ear.23 24 29 In these mice, transformation of some elements of the jaw, as well as of the occipital and middle ear bones, has been found. A duplicated set of skeletal elements derived from the first arch neural crest cells is also present in the Hoxa2 knockout, including ectopic incus, malleus and tympanic bones.29 The second branchial arch also forms a portion of the otic capsule. Parts of the cartilaginous otic capsule are also affected in Hoxa2 knockout mice. Hoxa2 affects the patterning of the tympanic ring and gonial bone and synergises with Hoxa1 in controlling the growth of these structures.30 In addition to Hoxa2, Hoxa1 is also important for the development of the external ear in the mouse. The Hoxa1 knockout mouse indicates that inactivation of Hoxa1 results in complete or near-complete deletion of rhombomere 5 and a severe reduction in rhombomere 4, suggesting that Hoxa1 is acting in the generation of hindbrain segments.31–33
PACT (also called Rax) (OMIM 603424) is another important protein involved in the development of auditory system, and is expressed in the pinna, middle ear and cochlea.34 To investigate the role of PACT in the development of the auditory system, the murine counterpart for the human PACT was disrupted. The Pact knockout mice were smaller than wild-type mice, exhibited hearing defects, and had a reduction in the size of the outer ear and external auditory canals. In this mouse model, the rostrum was rounded and the nose shortened. Ossicles were malformed and the middle ear space and bulla were very small.34 In contrast to the defects of the outer and middle ear, the cochlea of the Pact knockout mice was normal.
The embryological origin of the inner ear is different from that of the middle and outer ear, which largely share a common origin. The inner ear derives almost entirely from a small patch of ectodermal cells, termed the otic vesicle (otic placode), through a series of signalling interactions with the adjacent hindbrain and underlying mesenchyme. The otic placode appears on either side of the head at the level of the future hindbrain. During the fourth week, the otic vesicle differentiates into three parts: a dorsal endolymphatic duct and sac, an expanded central utricle and a ventral saccule. From the fourth to the seventh week, the utricle differentiates to form the three semicircular ducts and the ventral end of the saccule elongates and coils to form the cochlea.
Tbx1 gene expression in the PA I has a critical role in the formation of outer and middle ear. Tbx1 homozygous mutant mice have defects in middle and outer ear development. Tbx1 also plays a role in the otic vesicle, resulting in the failure of inner ear sensory organ formation in knockout mice. Tbx1 is required for sensory organ development and for suppression of neural cell fate determination in the otic vesicle.35 The human TBX1 gene (OMIM 602054) is deleted in DiGeorge syndrome.36
Environmental predisposition and microtia
Although the aetiology of microtia is poorly understood, both environmental and genetic factors have been implicated. There are many risk factors for microtia, such as gestosis, anaemia, race,4 8 high maternal or paternal age,2–4 and multiple births.3 8 In a clinical analysis of 592 patients with microtia in Japan, 28% of the patients’ mothers had a cold, imminent spontaneous abortion, gestosis or anaemia during their pregnancy.9 Maternal acute illness, such as influenza, during the first trimester of pregnancy was the major risk factor for microtia in a study from Latin America.37 Mothers with chronic type I diabetes are at significantly higher risk for having a child with microtia. Microtia children usually have normal birth weight, but low weight birth is more common than in healthy children.14 38
Certain medications such as isotretinoin, a widely used dermatological drug, or mycophenolate mofetil (MMF) taken by the mother during pregnancy have also been implicated as microtia-predisposing factors. In mothers exposed to isotretinoin, 83% of pregnancies result in spontaneous abortion or infants with serious birth defects including microtia.39 MMF is an immunosuppressive agent, prescribed after solid-organ transplantation. Patients who take MMF as immunosuppressive therapy during pregnancy are at risk for having a child with birth defects including microtia and cleft lip and palate.40
Microtia-inducing factors have also been found in experimental studies in animals. Ethane dimethane sulphonate (EDS) is an alkylating agent. All offspring of CD-1 mice exposed to EDS during the second half of gestation exhibited a bilateral, dose-related decrease in pinna size.41 Vitamin A (retinol) is necessary for cell growth and differentiation. Excess vitamin A has been associated with teratogenic effects in animals and humans. Facial clefts, micrognatia, microtia and blood vessel anomalies were seen in the offspring of pregnant rats that were administered etretinate (a synthetic retinoid).42
Genetics of microtia
Although there is strong evidence confirming the importance of environmental causes for microtia, it is believed that genetic components are also involved.43 Estimates on the percentage of familial cases among microtia cases vary widely, ranging from 3 to 34%2 5 9 12 44 (table 1). Hereditary forms of microtia with autosomal recessive or dominant modes of inheritance with variable expression and incomplete penetrance have been reported.12 14 44–46 There are also reports of familial cases with distinct clinical features including microtia, not corresponding to any known syndrome.47 Both single-gene defects and chromosomal aberrations have been reported in different microtia-associated syndromes.
Chromosomal abnormalities in microtia and aural atresia
A wide variety of chromosomal abnormalities, such as trisomy of chromosomes 13, 1848 49 and 22,50 51 as well as complex chromosomal rearrangements, microdeletions and even distinct hereditary genomic copy number variants (CNV) have been implicated.52
Chromosomal translocations affecting the 6p24 region have been associated with orofacial clefting and bilateral microtia.53 A child with a complex chromosomal rearrangement involving chromosome 20 (45,XY, psu dic (20;20)(p13;p13)) and paternal uniparental isodisomy for chromosome 20 was reported. This patient had multiple congenital abnormalities including microtia/anotia, micrencephaly, congenital heart disease, and colonic agangliosis.54
A family with an autosomal dominant syndrome characterised by microtia grade II, eye coloboma and incomplete perforation of the nasolacrimal duct was reported.52 Array comparative genome hybridisation (array-CGH) analysis showed that all affected family members had an alteration at 4p consisting of five tandem copies of an identical CNV.52 Microtia and lacrimal-duct anomalies have never been seen in patients with partial 4p deletions, duplications or translocations.55 The exact mechanism by which the CNV causes the syndrome could not be clarified.
Congenital aural atresia occurs in approximately 66% of all patients who have a terminal deletion of 18q. The extent and nature of the chromosome 18 deletions has been studied by array-CGH. A critical region of 5 Mb that was deleted in all patients with congenital aural atresia has been identified on 18q22.3–18q23. This chromosomal region can be considered a candidate region for aural atresia.56
Isolated microtia and oculo-auriculo-vertebral spectrum
Oculo-auriculo-vertebral (OAV) spectrum is a heterogeneous disorder involving the first and second branchial arch derivatives. The OAV spectrum is broad, ranging from isolated microtia57 to Goldenhar syndrome (GS; OMIM 164210) characterised by hemifacial microsomia (HFM; OMIM 164210), ocular abnormalities and vertebral defects. Hemifacial microsomia is a congenital asymmetry of the lower face that is defined as a condition affecting primarily aural, oral and mandibular development.58 Additional characteristics, including developmental delay, cardiovascular pulmonary and gastrointestinal malformations can also be found in patients with OAV spectrum. In a Turkish population with GS, microtia was seen in 52% of patients.59
Genetics of oculo-auriculo-vertebral spectrum including hemifacial microsomia and Goldenhar syndrome
BAPX1 belongs to the NK-2 family of transcription factors, 60 and has an important role in regulating the development of the structural elements of the middle ear in mice. The BAPX1 gene (OMIM 602183) plays an essential role in craniofacial development. A strong allelic expression imbalance of BAPX1 in fibroblasts from 40% of patients with OAV syndrome was identified.61 These data suggest that epigenetic dysregulation of BAPX1 plays an important role in this syndrome in human.61
Human Goosecoid (GSC) is a homeodomain transcription factor. It plays an essential role during the process of gastrulation in early embryonic development.62 Expression of Bapx1 during mouse embryonic development at day E10 partially overlaps with Gsc expression. This overlapping expression is crucial for patterning the structural components of the middle ear.63 Mice with a homozygous disruption of gsc have multiple developmental defects affecting the lower mandible and components of the inner ear and the external auditory meatus.64 A suggestive linkage to a region on chromosome 14q32 was found by genome-wide linkage analysis in two families with features of HFM.65 The most interesting candidate gene in the linked region was GSC. No disease-causing mutation in the coding region and no possible rearrangement of the gene analysed by Southern blotting could be identified in these 2 families segregating HFM or in 120 sporadic cases of HFM.65
The mouse line 643 (also known as Hfm) represents a useful model for the hemifacial microsomia-microtia spectrum, as it presents a phenotype of ear anomalies, craniofacial features and facial asymmetry.66–68 It carries an autosomal dominant insertional mutation that results in microtia, small body size, and jaw asymmetry. There are other symptoms including structural and positional anomalies affecting the external auditory meatus, middle ear and pharyngeal structures.68
Townes–Brocks syndrome (TBS; OMIM 107480) is a rare autosomal-dominant syndrome with a combination of anal, renal, limb and ear anomalies. TBS is caused by mutations in the SALL1 gene (sal-like-1; OMIM 602218) on chromosome 16q. TBS and GS have a significant number of overlapping features, including first and second arch defects and preaxial defects of the upper limbs. The phenotypic similarities between TBS and GS suggest that they may have a common genetic aetiology.69
Around 15–60% of patients with microtia have additional abnormalities.2 8 10 14 The associated malformations are more common in the bilateral cases. The most common accompanying dysmorphic features are facial cleft, facial asymmetry, renal abnormalities, cardiac defects, microphtalmia, polydactyly and vertebral anomalies.4 14 70 For some of the microtia-associated syndromes the developmental defects are known, often to the molecular level. We have tried to classify these syndromes based on the defects or mechanisms involved and describe in brief some of the most common syndromes or syndromes with a known molecular basis.
Disorders due to abnormalities in branchial arch development
An autosomal recessive syndrome (OMIM 601536) has been found in four Saudi Arabian families and one Turkish family.71 It was characterised by congenital horizontal gaze abnormalities, delayed motor milestones, cardiac problems, autism, profound sensorineural hearing impairment and external ear defects,71 which included low-set ears and a flattened ear helix.71 72 Homozygous pathogenic variants in HOXA1 (OMIM 142955) were found in all five families studied. This indicates that a mutation in HOXA1 disrupts the inner and outer ear to a variable degree and affects facial, cardiac and brainstem structures.71 An additional three Saudi Arabian and three Native American families with HOXA1 mutations provided evidence for a broader clinical spectrum in this disorder and implied that this autosomal recessive syndrome might not be extremely rare.72 Mice homozygous for a mutation in Hox-1.6 (the homologue for human HOXA1) present hypoplastic auricles, normal auditory meatuses, a disorganised middle ear, and an underdeveloped or even absent inner ear.31 32 In addition, the mice showed defects in specific hindbrain nuclei, ganglia and bones of the skull and die at birth from anoxia.31 32
A mutation in the HOXA2 homeobox gene was identified in an Iranian family.17 This family segregated autosomal recessive bilateral microtia (Marx type II), prelingual mixed severe to profound hearing impairment, and a partial cleft palate. In one patient, the cochlea was absent unilaterally. This family shows that abnormal or lost HOXA2 function in human can cause auditory system malformations, mainly in the external and middle ear, but involvement of the inner ear is also possible.17 A knockout mouse for Hox1.11 gene (homologue for human HOXA2) was produced by deleting a large portion of Hoxa2.23 24 73 These mice were born with a wide cleft of the secondary palate. Transformation of some of the middle ear bones was another hallmark of the Hoxa2 knockout mice. They all lacked the pinna of the external ear and died within the first 24 hours, probably because of feeding difficulties due to the cleft palate.
Oculo–auricular (OA; OMIM 612109) syndrome, which has a recessive inheritance pattern, has been reported recently.74 The phenotype of this syndrome included ophthalmic anomalies and a particular aplasia of the ear lobule. The external acoustic meatus was narrow and the eardrum small. Vestibular function was normal. Imaging by CT and MRI showed normal middle and inner ears. Positional cloning led to the identification of NKX5–3, also known as HMX1 (heme oxygenase 1)75 (OMIM 142992) as the responsible gene. Mutation analysis of HMX1 showed a homozygous 26 nucleotide deletion. NKX5–3 is a homeobox transcription factor and is expressed in the external ear, lens and retina of the mouse at embryonic day 13.5.74
Branchio–otic (BO) syndrome is an autosomal dominant developmental disorder characterised by branchial cleft cysts, auricular or external auditory canal abnormalities, preauricular pits and hearing loss. Branchio–oto–renal (BOR) syndrome is diagnosed when BO is accompanied by additional malformations of the kidney or urinary tract.76 77 The major feature of BOR is hearing loss (93% of patients), which can be conductive, sensorineural or mixed.78 BO/BOR shows a prevalence of 1 in 40 000 in the general population. It is recognised as one of the most common forms of autosomal dominant syndromic hearing impairment, responsible for 2% of profound deafness in children.79
To date, four genetic loci have been mapped for BO/BOR syndrome: BOR1,80 BOR2,81 BOS282 and BOS3.83 Except for BOS2, the corresponding genes have been identified. EYA1 (OMIM 601653) was the first gene identified for BOR syndrome at the BOR1 locus.80 Mutations in the EYA1 gene are the main cause of BOR syndrome, and are found in approximately 40% of cases.77
An Eya1 mouse knockout has been bred84 Eya1 heterozygous knockout mice show renal abnormalities and a conductive hearing loss similar to BOR syndrome, whereas Eya1 homozygotes lack ears and kidneys.84
The gene SIX5 (OMIM 600963) has been cloned for the BOR2 locus. Missense mutations were identified in SIX5 in 5.2% (5/95) of the patients with BOR.81 SIX1 (OMIM 601205) is the responsible gene at the BOS3 locus.76 An experimental animal model for the Six1 gene shows that Six1 plays an essential role in early development of kidneys.85 The EYA–SIX–PAX genetic network has a critical function in the embryonic development of ear and kidney. EYA1 is part of this genetic network. The SIX1, SIX4, SIX5 and SIX6 genes are also known to play a role in this genetic complex.
Treacher Collins syndrome
Treacher Collins Syndrome (TCS; OMIM 154500) is an autosomal dominant disorder of craniofacial development. The major features of the disease include hypoplastic facial bones, microtia or severe malformation of the pinna, micrognathia and other deformities of the external and middle ears, auditory pits, hearing loss (which is conductive in 50–55% of cases) and cleft palate.86 87
Mutations in the TCOF1 gene (Treacher Collins–Franceschetti 1), have been identified as the cause of TCS in up to 78% of patients.88–91 To date, >50 mutations have been identified in the TCOF1 gene, most of them insertions or deletions.88 92 TCOF1 encodes a protein called Treacle. Expression studies of Tcof1 in the mouse embryo support a role for this gene in the development of the craniofacial complex and provide further evidence that Treacle is a phosphoprotein that may have a function in nucleolar–cytoplasmic transport. Creation of a knockout mouse model for Tcof1 resulted in heterozygous mice with severe craniofacial malformations.92 Tcof1 haploinsufficiency resulted in deficient production of mature ribosomes, leading to reduced cell proliferation and causing a phenotype similar to the characteristics found in TCS.92 Thus, Treacle is thought to be a regulator of ribosome biogenesis.
DiGeorge deletion syndrome
DiGeorge deletion syndrome (del22q11DS; OMIM 188400) is one of the most common microdeletion chromosomal disorders. In most cases, the deletion eliminates 3 Mbp of DNA encoding for approximately 30 genes.93 The clinical phenotype includes ear defects, hearing impairment, craniofacial abnormalities, thymus and parathyroid gland hypoplasia, and heart malformations.94 Most of the del22q11DS cases have middle and outer ear anomalies, and some have inner ear malformations as well.95 The ears are typically low-set, small and with abnormal folding of the pinna. Conductive hearing loss is found in most patients with DiGeorge syndrome.96 TBX1 (OMIM 602054), a member of the T-box gene family of transcription factors, is deleted in patients with del22q11DS. TBX1 is required for ear development and is expressed in multiple tissues during embryogenesis.36 Although in most cases of DiGeorge syndrome 30 genes are deleted, deletion of TBX1 is sufficient to cause most of the abnormalities seen in del22q11DS.
Patients with Nager syndrome (OMIM 154400) have micrognathia, external ear defects, external auditory canal stenosis, bilateral conductive hearing loss, cleft palate, downslanting palpebral fissures, a high nasal bridge, hypoplastic or absent thumbs, and variable lower limb and toe defects.97 Most cases of Nager syndrome are sporadic, although both autosomal recessive and dominant familial cases have also been reported.98 99
A double-mutant mouse model was developed for homeobox genes Prx1 (OMIM 167420) and Prx2 (OMIM 604675), and proved to be an appropriate animal model for studying human acrofacial dysostosis disorders such as Nager syndrome. In the double Prx1/Prx2 mutant mice, defects in the external, middle and inner ear, a lack of tympanic rings, cleft mandible and polydactyly were seen.100 Inactivation of only Prx2 in the mouse causes deficiencies in the external, middle and inner ear without any skeletal defects, but inactivation of both Prx1 and Prx2 genes also results in defects in the craniofacial, skull and vertebral structures.21 100 Because of the similarities of the observed phenotype in the Prx1/Prx2 mouse model to those of Nager syndrome, DNA sequencing of eight patients with Nager syndrome for a possible mutation in either of the PRX1 and PRX2 genes was performed, but no pathogenic variant was found.101
Fibroblast growth factor signalling defects
Fibroblast growth factor 3 deficiency
Three unrelated Turkish families with a unique autosomal recessive syndrome were identified.102 This syndrome was characterised by type I microtia, microdontia and profound congenital deafness associated with a complete absence of inner ear structures bilaterally, with normal-appearing middle ear structures. Three different homozygous mutations in the FGF3 gene (OMIM 164950) were the disease-causing variants in these families.102
Lacrimo–auriculo–dento–digital (LADD) syndrome (OMIM 149730) is an autosomal dominant disorder. Besides abnormalities in the four structures that constitute the name of the syndrome, sensorineural, conductive, or mixed hearing loss is usually also present.103 Although the major auricular feature is cup-shaped pinnas,104 these patients sometimes have bilaterally low-placed, microtic auricles,105 or hypoplastic auricles and small protruding ears without lobules.106 LADD syndrome can be caused by mutations in the genes encoding the fibroblast growth factor receptor (FGFR) genes FGFR2 (OMIM 176943), FGFR3 (OMIM 134934) or FGF10 (OMIM 602115).103 These findings are strong evidence for the crucial role of FGF signalling in outer, middle and inner ear development.
Microtia-associated syndromes with other molecular mechanisms
CHARGE syndrome (OMIM 214800) represents a non-random association of malformations including coloboma of the retina, heart defects, atresia of the auditory canal, retardation of growth or development, genital anomalies, and ear abnormalities. The incidence of CHARGE syndrome has been estimated to range from 0.1 to 1.2 per 100 000 live births.107 The highest incidence (1 per 8500 live births) was reported in Canada.108 External ear abnormalities and severe hearing loss have been found in all patients with CHARGE syndrome.109
Two genes, chromodomain helicase DNA-binding protein-7 (CHD7; OMIM 608892)109 and semaphoring-3E (SEMA3E; OMIM 608166)110 have been identified as the causative genes for CHARGE syndrome. CHD7 mutations have been found in 70% of patients with the diagnosis of CHARGE syndrome.109
CHD7 has a critical role in the development of the central nervous system, inner ear and neural crest of pharyngeal arches. At the end of the first trimester, CHD7 is expressed in dorsal root ganglia, neural retina, cranial nerves, pituitary gland, and auditory tissues.111 The CHD7 protein is involved in ATP-dependent unwinding of DNA or RNA duplexes and histone deacetylation.112 It plays an important role in chromatin remodelling during early development. Semaphorin proteins are involved in a variety of cellular processes, including axon guidance and cell migration. The mouse homologue of SEMA3E is strongly expressed in the epithelium of semicircular duct walls during embryonic development.110 In a patient with CHARGE syndrome presenting a de novo balanced translocation involving chromosomes 2 and 7, SEMA3E was identified within 200 kb of the translocation breakpoint on 7q21.11. Sequencing of additional patients for mutations in SEMA3E found a de novo mutation in an unrelated patient.110
Walker–Warburg syndrome (WWS; OMIM 236670) is an autosomal recessive developmental disorder characterised by congenital complex brain and eye abnormalities. WWS is sometimes associated with ear abnormalities. Typical findings include hydrocephalus, retinal dysplasia and cerebellar dysgenesis. The additional phenotypic spectrum of WWS includes microtia, absent auditory canal, a dysmorphic face with low-set malformed ears, preauricular tags, glaucoma and congenital muscular dystrophy.113
To date, five genes (POMT1114 (OMIM 607423), POMT2115 (OMIM 607439), FKTN (Fukutin)116 (OMIM 607440), FKRP117 (OMIM 606596) and LARGE118 (OMIM 603590)) have been identified in the pathogenesis of WWS. POMT1 encodes the protein O-mannosyl transferase 1 (POMT1), and the POMT2 gene encodes POMT2. As patients with WWS have defective O-glycosylation of alpha-dystroglycan (alpha-DG),115 as shown by immunohistochemistry of muscle tissue, it is logical that these two proteins with O-mannosyltransferase activity would be involved in the pathogenesis of WWS.119 Mutation analysis of POMT1 in familial WWS, as well as in several unrelated isolated cases, has identified pathogenic variants in 7–20% of patients with WWS.114 120–122 Mutations in POMT2 have been found in 17.6% (3/17) of the families with WWS.115 123 Mutations in FKTN, FKRP, and LARGE genes have also been shown to cause WWS.68 116–118
Klippel–Feil syndrome (KFS; OMIM 118100) is a congenital spinal malformation characterised by the failure in segmentation of two or more cervical vertebrae. Patients with KFS usually have a short neck, limited neck movement, cleft palate, scoliosis, pre-auricular appendages and upper eyelid coloboma.124 125 Several otological abnormalities have also been described in KFS, including external ear malformation, ossicular chain abnormalities and structural abnormalities of the inner ear. Hearing impairment is a major feature of KFS and may be sensorineural, conductive or mixed. No characteristic audiometric profile was noted.126
Although most cases of KFS are sporadic, both autosomal dominant and autosomal recessive inheritance have been reported.127 128 The GDF6 gene (growth/differentiation factor 6; OMIM 601147) was already identified for a spectrum of ocular developmental anomalies,129 but recently mutations in this gene have been reported in familial and sporadic cases of KFS.130
Meier–Gorlin syndrome (MGS; OMIM 224690) is an autosomal recessive disorder with microtia, absent patellae, craniofacial anomalies, and delayed skeletal development.131 It has been suggested that this syndrome is the human equivalent of the short ear mouse model, which carries a mutation in the bone morphogenetic protein 5 (Bmp5) gene.132 Human BMP5 is a member of a subfamily of the bone morphogenetic proteins and a member of the transforming growth factor-beta superfamily of regulatory molecules. It is expressed in condensing mesenchyme, which gives rise to external ear cartilage.133 However, no BMP5 mutation in human has been reported.
Microtia is a developmental disorder with an aetiology that can be both environmental and genetic. Several genes that are important in development of the auditory system can be involved, but hard evidence for this involvement is available for only some of the genes. Identification of additional microtia-causing genes will reveal novel information on the molecular mechanisms involved in human craniofacial and ear development. Understanding the genetics of microtia-related syndromes provides tools for appropriate genetic counselling and molecular genetic testing can be helpful to confirm an exact diagnosis. In addition, the creation of mouse models with targeted mutations has provided experimental possibilities to determine the function of these genes in the development of the auditory system. The final goal of all this research is to have a complete cellular and molecular insight into the development of the ear and an understanding of the genetics and pathogenesis of different microtia-associated disorders.
We thank Dr T Klockars for his very helpful comments, and Dr A Sadeghi for help in preparing this manuscript.
Funding: This work was supported by the European Commission (FP6 Integrated project EuroHear LSHG-CT-20054–512063) and the Flemish FWO (grant G.0138.07).
Competing interests: None declared.
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