**3. Non-syndromic hearing loss**

Non-syndromic hearing loss, deafness without any other defects, is highly heterogeneous [33]. Genetic studies and linkage analysis have been helpful in identifying genes involved in hearing loss. Reports till 2021 indicate a total of 124 genes that have been identified for non-syndromic hearing loss. Among these, 78 genes are involved in autosomal recessive non-syndromic hearing loss while 51 genes cause autosomal dominant non-syndromic hearing loss (**Figure 1**). About 5 genes are known to cause non-syndromic deafness in an X-linked manner [34] (https://hereditaryhearingloss. org/, accessed April 2022).

### **3.1 Autosomal recessive non-syndromic hearing loss (ARNSHL)**

Worldwide data, with the prominence of Caucasian populations, indicate that *GJB2* variants account for the maximal cases of autosomal recessive non-syndromic deafness with the rate exceeding 50% of reported cases. *SLC26A4* is the second in line causative gene followed by *MYO15A*, *OTOF*, *CDH23*, and *TMC1* [35]*.*

In this section, we will shed light on the most common genes involved in autosomal recessive non-syndromic hearing loss (ARNSHL).

*DFNB1* is the first deafness locus that was mapped in 1994 [36]. *GJB2* encoding connexin26 (Cx26) was assigned to this locus during the study on three autosomal recessive non-syndromic sensorineural deaf families with nonsense variants in the gene. Connexin26 plays an important role in human ear development. The immunohistochemical staining of human cochlear cells has revealed high levels of *GJB2* expression [37]. Cx26 being a gap junction protein regulates intracellular communication and plays an important role in maintaining potassium levels in the inner ear. This potassium balance is crucial to normal auditory function [38]. Over the years, several studies have been conducted for the development of efficient therapy targeting hereditary hearing loss. A study showed that introducing normal *GJB2* gene through bacterial artificial chromosome (BAC) in *GJB2* deleted mice resulted in normal hearing and Auditory Brain Response (ABR) score [39]. Variants in *GJB2* cause approximately 16% of deafness in Iran [40]. In Pakistan, *GJB2*-related deafness frequency ranges from 6

**Figure 1.** *Frequency of various inheritance patterns for non-syndromic hearing loss.*

#### *A Short Overview on Hearing Loss and Related Auditory Defects DOI: http://dx.doi.org/10.5772/intechopen.105222*

to 7% for profound deafness [41] to 9.5% for moderate to severe hearing loss [42]. The variant c.35delG is the most common bi-allelic *GJB2* mutation worldwide with allele frequency up to 100% in European, North African, and Middle Eastern populations [43]. However, different variants are more common in other populations.

DFNB4 is caused by variants in *SLC26A4*, causing both autosomal recessive Pendred syndrome as well as non-syndromic deafness. It was first identified as a Pendred syndrome gene (PDS) using a positional cloning strategy. A year later, the gene was found to cause non-syndromic autosomal recessive deafness when a consanguineous family in southwest India was studied having a variant in the Pendred gene with no symptoms of goiter [44, 45]. Up till now 641 variants in *SLC26A4* have been reported in public databases (http://www.hgmd.cf.ac.uk/ac/search.php). *SLC26A4* encodes pendrin, an exchange of bicarbonate/chloride ions in the inner ear maintaining the homeostasis of endolymph [46]. The role of pendrin in normal hearing is elucidated by the fact that knockout mice *Slc26a4*−/− are completely deaf with vestibular dysfunction [47]. In knockout mice, reduced pH and utricular endolymphatic potential along with an increased level of Ca2+ are key factors leading to deafness. As it seems obvious, low Ca2+ concentration in human ear endolymph is crucial to the normal hearing process [48]. In a cohort of patients, single allele variants in *SLC26A4* fail to account for DFNB4 or Pendred syndrome. Digenic variants for some genes along with *SLC26A4* have been reported to be causative in such cases. *KCNJ10* an inwardly rectifying K<sup>+</sup> channel gene is important for maintaining endocochlear potential. Heterozygous variants in both *SLC26A4* and *KCNJ10* result in digenic nonsyndromic hearing loss associated with enlarged vestibular aqueduct syndrome [49]. Similarly, missense *EPHA2* variant in patients with mono-allelic *SLC26A4* variations has been reported in patients with Pendred syndrome. EPHA2 controls pendrin localization by forming a complex with it and faulty EPHA2 causes mislocalization of pendrin in the inner ear [50].

*DFNB3*, a non-syndromic deafness locus maps to chromosome 17p11.2. *MYO15A* pertaining to this locus causes congenital profound deafness in humans and *shaker2* (*sh2*) phenotype with vestibular defects in mice [51–53]. MYO15A encoded by this gene is an unconventional Myosin; tail homology 4—protein 4.1, ezrin, radixin, and moesin (MyTH4-FERM) myosin [54]. MYO15A is localized at the tips of both the outer hair cells (OHCs) and the inner hair cells (IHCs) and has a developmental role in the formation of stereocilia and thus, is indispensable to the hearing process [55]. Although, identified as a gene for profound deafness, less severe cases of hearing loss due to *MYO15A* variants are well known. The severity of deafness due to variants in this gene is in accordance with the protein domain being affected [56]. Frequency of *MYO15A*-related deafness is 5.71% in the Iranian population [57] and about 7.2% in the Vietnamese population [58].

Deafness autosomal recessive non-syndromic deafness, DFNB9 is caused by *OTOF* gene encoding otoferlin protein [59]. Otoferlin is essential to human hearing as it plays a role in inner hair cell formation and exocytosis of synaptic vesicles at the auditory inner hair cell ribbon synapse [60]. Otoferlin converts low-intensity stimuli at the synapse between inner hair cells and auditory nerve fiber [61]. *OTOF* variants cause auditory neuropathy (discussed later in this chapter) manifested as severe to profound non-syndromic deafness in most individuals. Cochlear implants in patients with *OTOF-*related deafness have shown promising results [62, 63].

*CDH23* was identified in families with Usher syndrome (USH1D) mapping to *DFNB12* locus [64]. Immuno-histochemical studies on rodent models showed localization of cadherin to upper and lower tip-links. These tip-links lie near stereocilia of hair cells and gate mechanoelectrical transduction [65]. The phenotype due to mutations in *CDH23* depends upon the type of variants in the gene. Missense variants with residual protein function are thought to cause DFNB12 while homozygous nonsense, frameshift, splice site, and a few missense variants with total loss of function cause *USH1D* [66]. Cochlear implants in children aged 11–36 months with *CDH23* mutations improved their hearing, speech, and performance necessitating the need for early diagnosis and possible improvement in hearing following implants [67].

*TMC1* variants are responsible for both dominant form of deafness DFNA36 as well as non-syndromic recessive hearing loss DFNB7/B11 [68]. Most of the *TMC1* variants cause autosomal recessive non-syndromic deafness while only a few are involved in dominantly inherited hearing loss. TMC1 is a transmembrane channel protein that forms the pore of mechanosensory transduction channels (MET) in vertebrate inner ear hair cells [69]. In Pakistani population, 3.4% of autosomal recessive non-syndromic hearing loss (ARSNHL) is caused by *TMC1* variants [70]. The *TMC1* related ARSNHL is 3.1% of diagnosed cases in the western European population [71] while 4.3–8.1% in the Turkish population [72, 73].

In countries like Pakistan where consanguinity rate is high, gene variants of *HGF*, *MYO7A*, *TMPRSS3*, *CIB2*, and *CLDN14* along with the ones mentioned above also contribute to large cases of profound and moderate to severe hearing loss [74, 75]. *HGF* within *DFNB39* locus 7q21.11 encodes hepatocyte growth factor. The variants of *HGF*, responsible for autosomal-recessive, non-syndromic hearing loss are located in intron 4. Also, indels in a highly conserved 3′ untranslated region (3′UTR) affect splicing of *HGF* exons resulting in deafness [76].

*MYO7A* mapping to 11q13.5 causes non-syndromic hearing loss both in recessive and dominant fashion and Usher syndrome (USH1B). MYO7A the unconventional myosin is required for the normal function of cochlear hair cells [77, 78].

*TMPRSS3* variants cause pre-lingual hearing impairment i.e. DFNB10 and lateonset DFNB8-associated hearing impairment. The severity of phenotype depends upon the combination of two mutant alleles. The type II transmembrane protease 3 encoded by *TMPRSS3* regulates epithelial sodium channels and potassium calciumactivated channel subfamily M alpha 1 (KCNMA1). Enac (Epithelial Amiloride Sensitive Sodium Channel) in turn controls the signaling pathway in inner ear essential to hearing. In the human ear, *TMPRSS3* variants lead to hair cell apoptosis and disruption of intracellular homeostasis [79–83].

#### **3.2 Autosomal dominant non-syndromic hearing loss**

In the case of autosomal dominant non-syndromic hearing loss, frequently reported genes in literature include *WFS1*, *KCNQ4*, *COCH*, and *GJB2* although dominant hearing loss does not account for a large number of cases as compared to autosomal recessive deafness [35].

*DFNA2* (Deafness autosomal dominant 2A) locus was assigned to cause autosomal dominant non-syndromic hearing loss (ADNSHL) by Kubisch et al. [84]. *KCNQ4* mapping to this locus encodes Potassium Voltage-Gated Channel Subfamily Q Member 4 protein expressed in OHCs in cochlea. Variants of *KCNQ4* implicated in ADNSHL disrupt the channel's ability to differentiate between K+ and Na+ ions and exert a strong dominant-negative effect on K+ currents in the inner ear [84].

Interestingly, *KCNQ4* variant has also been reported to cause hearing loss in a pseudo-dominant fashion. In a family with genetic heterogeneity, pathogenic variant c.872C > T in a homozygous state caused early-onset moderate to profound or

#### *A Short Overview on Hearing Loss and Related Auditory Defects DOI: http://dx.doi.org/10.5772/intechopen.105222*

moderate to severe deafness that progressed to profound deafness in a patient. This variant in heterozygous state caused mild to moderate hearing loss in the carrier [85]. In another study, *KCNQ4* gene variant c.1044\_1051del8 was identified to be responsible for causing autosomal recessive hearing loss with a severe phenotype [86]. *KCNQ4* variants c.211delC, c.725G > A, and c.1044\_1051del8 induce cell death in heterologous expression systems in a dominant manner [87]. Recent studies propose possible contribution of *KCNQ4* to age-related deafness as well [88].

*COCH* mapping to 14q12 was described to cause DFNA9 with vestibular dysfunction in three unrelated families [89]. In DFNA9, pathogenic variants of *COCH* lead to the accumulation of acellular deposits in the inner ear due to gain of function of mutant cochlin. Cochlin protein is the major component of interossicular joints and tympanic membrane of middle ear [90]. For many years, it was thought to be a gene implicated only for autosomal dominant hearing loss, when in 2018 a homozygous nonsense variant in *COCH* was identified to cause congenital prelingual recessive deafness DFNB110 [91].

*WSF1* was identified as a gene for Wolfram syndrome; an autosomal recessive disorder, by a positional cloning approach [92]. In 2001, Bespalova et al. defined families associated with autosomal dominant non-syndromic low-frequency sensorineural hearing loss (NSLFHL), DFNA6/14/38 having variants in *WSF1* gene [93].

*GJB2* has already been described for autosomal recessive non-syndromic hearing loss. The dominant mode of inheritance for *GJB2* was proposed in deaf families with palmoplantar keratoderma [37, 94]. The role of *GJB2* for autosomal dominant deafness 3A (DFNA3) was defined in a study on *GJB2* variants in the Austrian population [95].

#### **3.3 X-linked non-syndromic hearing loss**

The prevalence of X-linked non-syndromic deafness is 1–3%. As males are hemizygous for X-chromosome, they are predominantly affected by X-linked deafness [11].

Loss of function variants in *PRPS1* encoding phosphoribosyl pyrophosphate (PRPP) synthetase 1 enzyme was assigned to non-syndromic X-linked sensorineural deafness, DFNX1, in a Chinese family [96]. Another gene for X-linked non-syndromic deafness, *POU3F4* was defined by De kok et al. Nonsense mutations in *SMPX*, c.109G > T in a German family and c.175G > T in a Spanish family were assigned to *DFNX4* locus for X-linked non-syndromic deafness [97]. *AIFM1* pathogenic variants are involved in familial and sporadic cases of X-linked recessive auditory neuropathy spectrum disorder [98]. Single mutations in *COL4A6* were linked to a genetic disorder when a pathogenic variant c.1771G > A was found to cause X-linked non-syndromic hearing loss DFNX6 with cochlear malformation in a Hungarian family [99].

### **4. Auditory neuropathy**

Auditory Neuropathy was first defined by Arnold and his colleagues in 1996 while working on individuals with hearing loss. These individuals had normal outer hair cells in the cochlea, and preserved otoacoustic emissions while the ABRs were either absent or severely abnormal due to malfunction in eight cranial nerves. Auditory neuropathy may occur alone or as part of generalized neuropathic process [100].

Auditory neuropathy is divided into two categories according to the cause of neuropathy. In auditory neuropathy type I (AN type I) demyelination and axonal loss of auditory nerve is the predominant cause, while auditory neuropathy type II

(AN type II) occurs due to lesions in eight cranial nerves either at inner hair cells (IHCs) or synapses between IHCs and auditory nerve dendrites or at both [100–103].

Auditory neuropathy can be inherited as either non-syndromic or with accompanying clinical features as a syndrome. Syndromic auditory neuropathy can be due to dominant syndromes like Charcot-Marie-Tooth and Leber's Hereditary Optic Neuropathy (LHON) or recessive syndromes like Fredreich's Ataxia [104]. Mitochondrially inherited case of auditory neuropathy was reported by Deltenre et al. [105].

Non-syndromic auditory neuropathy can be dominant, recessive, or X-linked. AUNA1 was the first locus to be studied for autosomal dominant auditory neuropathy [106]. Pathogenic variants in *OTOF* cause non-syndromic recessive auditory neuropathy (NSRAN) [107]. Deafness due to *OTOF* gene variants is manifested in two ways:


Some studies suggest variants in *GJB2* can cause non-syndromic recessive ANSD [109, 110]. X-linked pattern for NSRAN was identified when it was reported that *AUNX1* gene variant is responsible for causing auditory neuropathy and progressive peripheral sensory neuropathy in X- linked manner [111].

Since the discovery of auditory neuropathy scientists has been trying to pinpoint the underlying reason. A study has shown that more than 40% of cases of ANSD are due to hereditary neurological disorders [112]. Although majority of cases of ANSD are sporadic in nature, familial cases have also been reported [113].

The frequency of auditory neuropathy among patients with hearing loss has remained underestimated. In a recent study on Saudi Arabian children diagnosed with NSHL (non-syndromic hearing loss), 9.85% were identified to have ANSD [114]. Thus, the disorder is more prevalent than it was once thought. Other than genetic cause, one of the prominent reasons of auditory neuropathy is bilirubin toxicity, as it damages the auditory nerve and brainstem auditory nuclei [115].

### **5. Age-related hearing loss (ARHL)**

Age-related hearing loss (ARHL) which is also known as presbycusis, is defined as a progressive, bilateral, and symmetrical sensorineural hearing loss which is mostly observed at high sound frequencies. It is the most common sensory deficit occurring in individuals over the age of 75, severely affecting their communication, cognitive abilities, and social activities [116]. ARHL is the third most prevalent health condition in the world, affecting older adults after heart disease and arthritis [117]. In estimation by the World Health Organization (WHO) 580 million people worldwide over the age of 65 are experiencing hearing loss. It is anticipated that by the next decade over one billion people over the age of 60 will be affected by ARHL (http://www.who.int/en/).
