Alternative titles; symbols
HGNC Approved Gene Symbol: MT-RNR1
SNOMEDCT: 415295002, 90828009; ICD10CM: I42.5;
Mitochondrial 12S ribosomal RNA is encoded by nucleotides 648-1601.
The mitochondrial ribosome in the cochlea is the most likely target of aminoglycoside ototoxicity (580000), since the 'natural target' of aminoglycosides is the evolutionarily related bacterial ribosome. A mitochondrial mutation responsible for antibiotic-induced ototoxicity and also causing nonsyndromic deafness was described by Prezant et al. (1993) in a large Arab Israeli kindred. Members of this kindred who had a homoplasmic 1555A-G mutation (561000.0001) had phenotypes ranging from profound hearing loss to completely normal hearing. Bu et al. (1992) and Guan et al. (1996) presented genetic and biochemical evidence for nuclear gene involvement in this family. To identify such a nuclear locus, Bykhovskaya et al. (1998) excluded 2 candidate genes through linkage analysis and sequencing, and performed a genomewide linkage search in family members who had the identical homoplasmic mitochondrial mutation but differed in their hearing status. The 2 candidate genes excluded were connexin-26 (GJB2; 121011) and a gene on chromosome 19 that encodes a ribosomal S12 protein. In a further search for candidate loci for a nuclear modifier gene for maternally inherited deafness, Bykhovskaya et al. (2000) investigated 10 multiplex Spanish and Italian families with 35 members with the 1555A-G mutation and sensorineural deafness. Parametric analysis of a genomewide screen again failed to identify significant evidence for linkage to a single autosomal locus. However, nonparametric analysis supported the role of the chromosomal region around marker D8S277, with a combined maximum allele-sharing lod score of 3.1 in Arab-Israeli/Spanish/Italian families. The authors suggested that this region on chromosome 8 contains a candidate for the first human nuclear modifier gene for an mtDNA disorder.
Bu et al. (2000) reported a 507-member Chinese family with maternally inherited nonsyndromic hearing loss (500008) in which affected members showed variable severity and age of onset. They screened 41 members for the MTRNR1 1555A-G and MTTS1 7445A-G (590080.0002) mutations and found the 1555A-G mutation in all maternal relatives, both hearing-impaired and normal hearing. In a further analysis of this 6-generation Chinese pedigree, Li et al. (2004) sequenced the complete mitochondrial genome and found cosegregation of the MTRNR1 961insC mutation (561000.0002) with the 1555A-G mutation in all maternally related members of the family, both hearing-impaired and normal hearing. Li et al. (2004) concluded that the phenotypic variability in this family suggested the involvement of nuclear modifier genes.
Zhao et al. (2004) suggested that the 1494C-T mutation (561000.0004) would form a new 1494-1555U-A basepair at the highly conserved A site of the 12S rRNA, which is in the same position as the 1494-1555C-G pair caused by the 1555A-G mutation. In lymphoblastoid cell lines derived from 4 symptomatic and 2 asymptomatic individuals carrying the 1494C-T mutation, exposure to a high concentration of paromomycin or neomycin caused a variable but significant average increase in doubling time. Furthermore, a significant decrease in the rate of total oxygen consumption was observed in the mutant cell lines.
Li et al. (2004) screened 164 children with sporadic nonsyndromic deafness for mutations in the MTRNR1 and MTTS1 genes and identified the MTRNR1 1555A-G mutation in 1 patient, for a frequency of 0.6%. They also identified a homoplasmic 961T-G transversion in MTRNR1 (561000.0005) in 5 patients. Li et al. (2004) did not detect any of the known deafness-associated mutations in MTTS1 in this population.
By performing a phylogenetic reappraisal of complete mtDNA sequences of the 12S rRNA gene in East Asian patients with aminoglycoside-induced and nonsyndromic hearing loss, Yao et al. (2006) concluded that the 1095T-C mutation (561000.0003) actually defines a basal haplotype branch of the East Asian mtDNA phylogeny, and is thus likely nonpathogenic. The authors also presented evidence disputing the pathogenicity of other reported mutations in the 12S rRNA gene associated with hearing loss.
In patients from 3 unrelated families with familial aminoglycoside-induced deafness (580000) and in a large Arab Israeli pedigree with nonsyndromic deafness (500008), Prezant et al. (1993) found a 1555A-G transition in the 12S rRNA gene (MTRNR1), a site implicated in aminoglycoside activity by analogy to the evolutionarily related bacterial ribosome.
Hutchin et al. (1993) found the same mutation in 2 Japanese pedigrees and 3 Chinese pedigrees with aminoglycoside-induced deafness and in 4 of 74 sporadic deafness cases thought to be the result of exposure to aminoglycosides. The frequency of the mutation in the hearing population was less than 1 in 200. The 1555A-G mutation was inferred to create a new basepair at the terminus of the penultimate helix of the 12S RNA. Hutchin et al. (1993) proposed that this additional basepair decreases the molecular volume taken up by RNA at this site relative to the unpaired bases, thus increasing the size of the aminoglycoside binding pocket and making aminoglycoside binding tighter.
Pandya et al. (1997) ascertained 3 Mongolian pedigrees from a school for the deaf and blind, which contained multiple affected subjects with streptomycin-induced deafness in a pattern consistent with matrilineal transmission. In 2 of the 3 families they found the 1555A-G mutation in the 12S rRNA gene by restriction analysis as well as by direct sequencing. No other example of this substitution was found among 400 control samples from Mongolians with normal hearing. In countries where aminoglycosides are widely used, genetic counseling and screening of high-risk families before the use of these drugs could have a dramatic effect on the incidence of deafness. Gardner et al. (1997) reported the 1555A-G point mutation in the 12S ribosomal RNA gene in a South African family with streptomycin-induced sensorineural deafness. They came to the same conclusions as Pandya et al. (1997) regarding the usefulness of genetic counseling and screening of high-risk families in countries where aminoglycosides are widely used.
Estivill et al. (1998) studied 70 Spanish families with sensorineural deafness (36 congenital and 34 late-onset) for the mtDNA 1555A-G mutation. The mutation was found in 19 families with maternally transmitted deafness but not in the other 51 families or in 200 control subjects. In 12 families, all the patients with the 1555A-G mutation who received aminoglycosides became deaf, representing 30.3% of the deaf patients in these families. None of the deaf patients from 7 other families received aminoglycosides. Overall, only 17.7% of the patients with deafness and the 1555A-G mutation had been treated with aminoglycosides. The age at onset of deafness was lower (median age 5 years, range 1 to 52 years) in those treated with aminoglycosides than in those who did not receive aminoglycosides (median age 20 years, range 1 to 65 years). The mtDNA of these families belonged to haplotypes common in Europeans. Data indicated that the 1555A-G mutation accounts for a large proportion of the Spanish families with late-onset sensorineural deafness, that the 1555A-G mutation has an age-dependent penetrance for deafness (enhanced by treatment with aminoglycosides), and that mtDNA backgrounds probably do not play a major role in disease expression.
Abe et al. (1998) performed a phylogenetic analysis in 13 Japanese families (10 of which were from the northern part of Japan) with sensorineural hearing loss and the 1555A-G mitochondrial mutation. They used data obtained by RFLP and D-loop sequencing of mtDNA. Three families exhibited the same restriction patterns and the same sequence substitution in the D-loop; however, comparison of the 482 basepairs of the D-loop sequence with those of 62 normal Japanese subjects showed that the remaining 10 families were scattered along the phylogenetic tree. This indicated that, except for 3 families, there was no common ancestor for the families bearing the 1555A-G mutation, and that the mutation occurred multiple times in Japan.
Pandya et al. (1999) reported 6 unrelated Mongolian deaf students with cosegregation of a homoplasmic 1555A-G mutation and a homoplasmic 7444G-A mutation in the MTCO1 gene (516030.0001). Five of the individuals had a family history consistent with matrilineal transmission of hearing loss. Only 2 individuals had a definite history of aminoglycoside exposure, but all 6 had severe to profound bilateral sensorineural hearing loss detected at birth or in infancy.
Santorelli et al. (1999) described the 1555A-G mutation in a 35-year-old woman who had suffered from a restrictive cardiomyopathy from early adulthood, with a family history suggesting maternal transmission, whereas her brother and one of her daughters had transient valvular heart disease in early childhood. The daughters remained at risk for cardiomyopathy, because cardiac symptoms in the proposita did not start until she was in her early twenties and worsened considerably over the course of the next 10 years. Furthermore, both her mother and the maternal grandmother died suddenly in their thirties of cardiac failure. The 1555A-G mutation was present in heteroplasmic state, both in the patient and in her maternal relatives.
From phylogenetic analyses of haplotypes and detailed survey of population controls in 50 Spanish and 4 Cuban families with the 1555A-G mutation, Torroni et al. (1999) found that the 1555A-G mutation could be attributed to more than 30 independent mutational events and that it occurred on mtDNA haplogroups that are common in all European populations. This indicated that the relatively high detection rate of this mutation in Spain is not due to sampling biases or to a single major founder event. The results also supported the conclusion that mtDNA backgrounds do not play a significant role in the expression of the 1555A-G mutation. The identification of such a large number of families in Spain, relative to the few detected in other European populations (Casano et al., 1998), had prompted the study. Overall, Torroni et al. (1999) interpreted the findings as indicating that the rare detection of this mutation in other populations is most likely due to inadequacy in patient ascertainment and molecular screening. This probable lack of identification of the 1555A-G mutation in subjects affected by sensorineural hearing loss implies that their maternally related relatives are not benefiting by presymptomatic detection and information concerning their increased risk of ototoxicity due to aminoglycoside treatment.
Guan et al. (2000) studied the sensitivity to the aminoglycoside paromomycin in lymphoblastoid cell lines derived from 5 deaf individuals and 5 hearing individuals from an Arab-Israeli family carrying the 1555A-G mutation and 3 married-in controls from the same family. Exposure to a high concentration of paromomycin (2 mg/ml), which caused an 8% average increase in doubling time (DT) in the control cell lines, produced higher average DT increases (49% and 47%) in the A1555G mutation-carrying cell lines derived from symptomatic and asymptomatic individuals, respectively. The ratios of translation rates in the presence and absence of paromomycin, which reflected the effect of the drug on mitochondrial protein synthesis, were significantly decreased in the cell lines derived from symptomatic and asymptomatic individuals, compared to controls. The authors concluded that the A1555G mutation in mitochondrial 12S rRNA results in alteration of mitochondrial protein synthesis in the presence of aminoglycosides, thus reducing the overall translation rate down to and below the minimal level required for normal cellular function (40 to 50%).
Bykhovskaya et al. (2000) studied 10 multiplex Spanish and Italian families with 35 members with the 1555A-G mutation and sensorineural deafness. Nonparametric analysis supported the role of the chromosomal region around marker D8S277, with a combined maximized allele-sharing lod score of 3.1 in Arab-Israeli/Spanish/Italian families. Bykhovskaya et al. (2001) obtained 47 DNAs from members of 5 multiplex families from Spain, 1 from Italy, and 1 nuclear family from Finland with matrilineal nonsyndromic hearing loss, showing a combined lod score of 4.0 for the region containing markers D8S277, D8S561, and D8S1819. This finding represented the first identification of a modifier locus for a human mitochondrial DNA disease and supported the concept of mitochondrial DNA diseases having complex inheritance. This modifier gene would be a susceptibility gene and would probably not be sufficient to cause disease in the absence of homoplasmy for the 1555A-G mutation. Finnala and Majamaa (2003) performed fine mapping of the region around marker D8S277 in a large Finnish family with nonsyndromic sensorineural hearing loss, 3 members of which had been part of the study by Bykhovskaya et al. (2001). Haplotype comparison of 9 affected and 7 unaffected persons excluded the region around 8p23 as the site of a susceptibility locus for hearing impairment in this family.
Ostergaard et al. (2002) studied 85 Danish patients with varying degrees of hearing impairment and found 2 (2.4%) with the 1555A-G mutation. Neither had received aminoglycosides.
Malik et al. (2003) reported a large family of Balinese Indonesian origin with congenital progressive sensorineural deafness associated with the 1555A-G mutation. The pedigree showed a generally maternal inheritance pattern with some exceptions, resulting from an unusual multiple entry of the mutation into the pedigree. A complete mtDNA sequence from 3 Balinese individuals showed a relatively large number of SNPs not previously reported, and confirmed the genetic distance of Southeast Asian populations from those of Caucasians and Japanese.
Del Castillo et al. (2003) noted that in most reported cases of nonsyndromic hearing loss associated with the 1555A-G mutation, the mutation was found in homoplasmic state. In 6 Spanish families, they identified the mutation in heteroplasmic state, causing sensorineural hearing loss. The proportion of mutant copies was approximately correlated with the degree of symptoms. Patients carrying less than 20% of mutant copies were asymptomatic or had a mild hearing loss, whereas heteroplasmic patients with more than 52% of mutant copies had moderate to severe hearing loss.
Malik et al. (2003) reported a high prevalence (5.3%) of the 1555A-G mutation in sensorineural deafness patients in Indonesia. This supported the need for mutation detection before the administration of aminoglycoside antibiotics in Asian populations.
Tekin et al. (2003) screened 168 patients from independent Turkish families (72 simplex and 96 multiplex) with prelingual deafness for the 1555A-G MTRNR1 mutation and the 7445A-G MTTS1 mutation (590080.0002). None of the patients had the 7445A-G mutation, but 3 probands (1.8%) had the 1555A-G mutation. All 3 had been exposed to parenteral antibiotics (possibly aminoglycosides) during the first year of life. A sister of 1 patient was also deaf. The mother, 2 sibs, and a 3-year-old niece of another patient had the 1555A-G mutations with normal hearing, suggesting mitochondrial inheritance and incomplete penetrance.
Noguchi et al. (2004) identified the 1555A-G mutation in 1 (1.6%) of 63 Japanese patients with sporadic hearing loss and in 6 (8.0%) of 75 Japanese patients with familial hearing loss. Two (33.3%) of 6 patients presenting with aminoglycoside-induced sensorineural hearing loss had the 1555A-G mutation. All but one of the patients carrying the mutation showed a high-frequency hearing loss, and audiometric studies suggested that the hearing loss was due to impairment of the cochlear hair cells.
Yuan et al. (2005) reported cosegregation of a homoplasmic 1555A-G mutation and a homoplasmic 7444G-A MTCO1 mutation in affected members of a Chinese family with aminoglycoside-induced sensorineural hearing loss. One additional family member with both mutations, who had a history of exposure to noise but not to aminoglycoside, exhibited mild hearing impairment. The dose and age at the time of drug administration seemed to be correlated with the severity of the hearing loss.
Guan et al. (2006) identified a nuclear modifier gene for 1555A-G deafness: TRMU (610230), which encodes a highly conserved mitochondrial protein related to transfer RNA (tRNA) modification. Genotyping analysis of TRMU in 613 subjects from 1 Arab Israeli kindred, 210 European (Italian and Spanish) families, and 31 Chinese pedigrees carrying the 1555A-G or the 1494C-T (561000.0004) mutation revealed a missense mutation altering an invariant amino acid residue in the evolutionarily conserved N-terminal region of the TRMU protein (A10S; 610230.0001). All 18 Arab-Israeli/Italian-Spanish matrilineal relatives carrying both the TRMU A10S and the 12S rRNA and the 1555A-G mutations exhibited prelingual profound deafness. Functional analysis showed that this mutation did not affect importation of TRMU precursors into mitochondria. However, the homozygous A10S mutation led to a marked failure in mitochondrial tRNA metabolisms, specifically reducing the steady-state levels of mitochondrial tRNA. As a consequence, these defects contribute to the impairment of mitochondrial protein synthesis. The resultant biochemical defects aggravate the mitochondrial dysfunction associated with the A1555G mutation, exceeding the threshold for expressing the deafness phenotype. These findings indicated that the mutated TRMU, acting as a modifier factor, modulates the phenotypic manifestation of the deafness-associated 12S rRNA mutations.
Using molecular dynamic simulations, Meng et al. (2017) showed that the A10S mutation introduced a ser10 dynamic electrostatic interaction with lys106 in helix-4 of the TRMU catalytic domain. Western blot analysis revealed reduced levels of TRMU in cells with the A10S mutation, and thermal shift analysis showed that the Tm value of the mutant TRMU protein was lower than wildtype. The A10S mutation also caused marked decreases in 2-thiouridine modification of U34 in tRNAs for lys (MTTK; 590060), glu (MTTE; 590025), and gln (MTTQ; 590030), while mildly increasing the aminoacylated efficiency of the tRNAs. The altered 2-thiouridine modification worsened the impairment of mitochondrial translocation associated with the MTRNR1 1555A-G mutation. Defective translation resulted in reduced activity in mitochondrial respiration chains, leading to reduction of mitochondrial ATP production and elevated production of reactive oxidative species. Thus, the A10S mutation in TRMU worsened the mitochondrial dysfunction associated with the 1555A-G mutation, exceeding the threshold for expressing the deafness phenotype.
In 4 and 16 Chinese pedigrees with aminoglycoside-induced and nonsyndromic hearing impairment, Young et al. (2005) and Dai et al. (2006), respectively, found extremely low penetrance of hearing loss, with an average of 8% for both studies. Mutational analysis showed the presence of homoplasmic 1555A-G mutations. The low penetrance in these families, particularly compared with other pedigrees, suggested that the 1555A-G mutation itself is not sufficient to produce the clinical phenotype.
In 443 Spanish families and sporadic patients with hearing impairment, Ballana et al. (2006) found the 1555A-G mutation in 69 (15%) families and sporadic patients. The mutation was not fully penetrant as only 63% of individuals with the mutation had developed hearing impairment. They determined that the 1555A-G mutation is predicted to change the RNA secondary structure.
Among 24 carriers of the 1555A-G mutation from 9 Spanish families, Bravo et al. (2006) found a wide phenotypic range. Six had normal hearing, and 18 had mild to profound hearing loss most severe at high frequencies. The age at onset ranged from 1 to 20 years. Four individuals with moderate to profound hearing loss had aminoglycoside-induced deafness. Tinnitus was reported by 9 deaf and 2 hearing individuals, and 2 deaf individuals reported dizziness. All with deafness had absent otoacoustic emissions with normal auditory brainstem responses, suggesting dysfunction of the outer hair cells of the cochlea. Two normal hearing individuals had subclinical alterations of the acoustic reflexes at high frequencies. Bravo et al. (2006) stated that the findings were consistent with a model in which a defect in mitochondrial translation of ribosomes results in a decline of ATP production and an increase in reactive oxygen species, resulting in hair cell apoptosis.
Tang et al. (2007) reported 7 Han Chinese families with aminoglycoside-induced and nonsyndromic bilateral hearing loss due to the 1555A-G mutation. The penetrance of hearing loss in these pedigrees ranged from 3 to 29%, with an average of 13.6%, when aminoglycoside-induced deafness was included. When the effect of aminoglycosides was excluded, the penetrances of hearing loss ranged from 0 to 17%, with an average of 5.3%. Haplotype analysis suggested that the A1555G mutation occurred sporadically and multiplied through evolution of the mtDNA in China. Tang et al. (2007) concluded that aminoglycoside exposure appears to be a major modifier factor for the phenotypic manifestation of the A1555G mutation in these Chinese families.
Dai et al. (2008) reported a Chinese girl with onset of profound nonsyndromic hearing loss at age 6 months who had both the 1555A-G and 1095T-C (561000.0003) mutations. The authors suggested that the 2 mutations acted together to enhance the biochemical defects resulting in hearing impairment.
Since the 1555A-G mutation (561000.0001) in the MTRNR1 gene accounts for only a minority of patients with aminoglycoside ototoxicity, it was possible that other susceptibility mutations might be found in the same gene. In 35 Chinese sporadic patients with aminoglycoside ototoxicity (580000) and without the 1555A-G mutation, Bacino et al. (1995) found 3 sequence changes, but only 1 of them, an absence of a thymidine at position 961 with varying numbers of cytosines inserted, appeared likely to be a pathogenic mutation. Fischel-Ghodsian (1999) stated that analysis of 34 similar U.S. patients of varying ethnic backgrounds failed to reveal this mutation, but he and his colleagues identified an Italian family with 5 maternally related members who all became deaf after aminoglycoside treatment and who were found to have the same del/ins mutation.
In an Italian family in which 2 sisters and 3 of their children developed aminoglycoside-induced deafness, Casano et al. (1999) identified a 961delT mutation and insertion of a varying number of cytosines in the MTRNR1 gene.
Thyagarajan et al. (2000) identified a 1095T-C mutation in the 12SrRNA gene in 3 members of a large family with maternally inherited sensorineural deafness (500008). The predicted effect of the mutation is to destroy the stem-loop secondary structure, resulting in impaired translation. Respiratory chain analysis also revealed a significant decrease of COX activity, which may be a result of impaired mitochondrial translation. Thyagarajan et al. (2000) noted an association in the proband between the mutation and neuropathic symptoms, including parkinsonism, and cited previous studies that had suggested a correlation between mitochondrial COX deficiency and Parkinson disease.
Tessa et al. (2001) analyzed 80 deaf children for the presence of deafness-related mtDNA mutations. In 1 child with sudden onset of severe/profound hearing loss across all frequencies and with a medical history of aminoglycoside-induced deafness (580000) in 2 maternal relatives, they found the 1095T-C transition in the 12S rRNA gene. The mutation, which occurs at a highly conserved position of the 12S rRNA gene, was homoplasmic in the proband and less abundant in maternal relatives, and was not found in 100 haplotype-matched controls.
Zhao et al. (2004) reported the clinical and sequence analysis of the entire mitochondrial genome in 3 Chinese subjects with aminoglycoside-induced and nonsyndromic hearing impairment. Clinical evaluation showed a variable phenotype of hearing impairment including the age of onset and audiometric configuration in these subjects. All subjects showed the 1095T-C mutation and exhibited distinct sets of mtDNA polymorphisms that may contribute to the phenotypic expression of the 1095T-C mutation.
Wang et al. (2005) identified an MTRNR1 1095T-C mutation in a 27-year-old Chinese woman with adult-onset hearing loss due to auditory neuropathy. She had no history of aminoglycoside exposure.
By performing a phylogenetic reappraisal of complete mtDNA sequences of the 12S rRNA gene in East Asian patients with aminoglycoside-induced and nonsyndromic hearing loss, Yao et al. (2006) concluded that the 1095T-C mutation actually defines a basal haplotype branch of the East Asian mtDNA phylogeny, and is thus likely nonpathogenic. The authors also presented evidence disputing the pathogenicity of other reported mutations in the 12S rRNA gene associated with hearing loss.
Dai et al. (2008) reported a Chinese girl with onset of profound nonsyndromic hearing loss at age 6 months who had both the 1555A-G (561000.0001) and 1095T-C mutations. The authors suggested that the 2 mutations acted together to enhance the biochemical defects resulting in hearing impairment.
In studies of a large Chinese family with maternally transmitted aminoglycoside-induced (580000) and nonsyndromic deafness (500008), Zhao et al. (2004) identified a novel 1494C-T mutation in the MTRNR1 gene. In the absence of aminoglycosides, some matrilineal relatives exhibited late-onset/progressive deafness, with a wide range of severity and age at onset. Notably, the average age of onset of deafness changed from 55 years in generation II to 10 years in generation IV. Clinical data showed that the administration of aminoglycosides can induce or worsen deafness in matrilineal relatives. Zhao et al. (2004) suggested that the nuclear genetic background plays a role in the development of the deafness phenotype in this family since 31% of persons carrying the 1494C-T mutation developed hearing impairment when the effect of aminoglycosides was excluded, but 51% when aminoglycoside-induced deafness was included.
Wang et al. (2006) reported a 4-generation Chinese family with aminoglycoside-induced and nonsyndromic hearing loss associated with the 1494C-T mutation. The severity of hearing loss ranged from moderate to severe, and the age at onset of hearing loss in those without aminoglycoside exposure ranged from 2 to 31 years. All displayed loss at high frequencies. Incomplete penetrance indicates that the 1494C-T mutation is not sufficient to produce the clinical phenotype but requires the involvement of modifier factors for phenotypic expression.
In 5 unrelated children with nonsyndromic sensorineural deafness (500008), Li et al. (2004) identified a homoplasmic 961T-G transversion in the MTRNR1 gene. These 5 patients exhibited distinct sets of mtDNA polymorphism in addition to the 961T-G mutation. The mutation was not found in 226 controls. Li et al. (2004) suggested that the MTRNR1 gene is a hotspot for deafness-associated mutations.
In a 3-generation family of Cuban origin with nonsyndromic sensorineural hearing impairment (500008), Ballana et al. (2006) identified a homoplasmic 1291T-C transition in the MTRNR1 gene. Severity of hearing impairment and age of onset (7 to 40 years) were variable, and the patients reported no history of aminoglycoside exposure. The mutation was not found in 100 controls. Ballana et al. (2006) determined that the mutation is predicted to change the RNA secondary structure.
In affected members of a Chinese family with nonsyndromic sensorineural hearing loss showing maternal inheritance (500008), Xing et al. (2006) identified a homoplasmic 827A-G transition in the MTRNR1 gene in a highly conserved A-site. The severe to profound hearing loss showed an early onset before age 3 years. However, there was incomplete penetrance (43.5%), suggesting that the mutation was not sufficient for development of the disorder and implying the existence of modifying factors.
Xing et al. (2006) identified 827A-G mutation in 3 affected members of a Chinese family with aminoglycoside-induced deafness (580000). Family members who carried the mutation and were not exposed to aminoglycosides did not develop hearing loss.
Chaig et al. (2008) identified a homoplasmic 827A-G mutation in 2 Argentinian sisters with aminoglycoside-induced hearing loss. Four additional maternal family members reportedly had moderate deafness without aminoglycoside exposure, but they were not examined by the authors.
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