Co-segregation of the T1095C with the A1555G mutation of the mitochondrial 12S rRNA gene in a patient with non-syndromic hearing loss

We reported here the clinical and molecular characterization of a Chinese subject with childhood-onset hearing impairment. Clinical evaluations showed that the patient suffered from profound and non-syndromic sensorineural hearing loss with flat configurations. Sequence analysis of the mitochondrial 12S rRNA and tRNA^Ser(UCN) genes led to the identification of double deafness-associated mutations of A1555G and T1095C in the 12S rRNA gene which apparently in the homoplasmic forms. In additional, there was no other functionally significant nucleotide variants found in this subject. As previous studies have indicated that the A1555G mutation was a primary contributing factor underlying the development of deafness but not sufficient to produce clinical phenotype, the co-segregation of two mitochondrial DNA mutations raises the possibility that the T to C transition at position 1095 plays a role in the phenotypic expression of deafness-associated A1555G mutation. Actually, the T1095C mutation disrupted an evolutionarily conserved base-pair at stem-loop of helix 25 of 12S rRNA, resulting in impaired translation in mitochondrial protein synthesis and a significant reduction of cytochrome c oxidase activity. As a result, it may enhance the biochemical defect in patient carrying the A1555G mutation, thus changing the age of onset and the severity of hearing impairment.     

Extremely low penetrance of hearing loss in four Chinese families with the mitochondrial 12S rRNA A1555G mutation

Mutations in mitochondrial DNA (mtDNA) have been found to be associated with sensorineural hearing loss. We report here the clinical, genetic, and molecular characterization of four Chinese pedigrees with aminoglycoside-induced and nonsyndromic hearing impairment. Clinical evaluation revealed the variable phenotype of hearing impairment including audiometric configuration in these subjects, although these subjects share some common features: bilateral and sensorineural hearing impairment. Strikingly, these Chinese pedigrees exhibited extremely low penetrance of hearing loss (5.2%, 4.8%, 4.2%, and 13.3%, respectively, and with an average 8% penetrance). In particular, four of all five affected matrilineal relatives of these pedigrees had aminoglycoside-induced hearing loss. Sequence analysis of the complete mitochondrial genomes in these pedigrees showed the distinct sets of mtDNA polymorphism, in addition to the identical homoplasmic A1555G mutation, associated with hearing impairment in many families from different genetic backgrounds. The fact that mtDNA of those pedigrees belonged to different haplogroups R9a, N9a, D4a, and D4 suggested that the A1555G mutation occurred sporadically and multiplied through evolution of the mtDNA in China. However, there was the absence of functionally significant mutations in tRNA and rRNAs or secondary LHON mutations in these Chinese families. These data imply that the nuclear background or/and mitochondrial haplotype may not play a significant role in the phenotypic expression of the A1555G mutation in these Chinese pedigrees. However, aminoglycoside appears to be a major modifier factor for the phenotypic manifestation of the A1555G mutation in these Chinese families.     

Low incidence of GJB2, GJB6 and mitochondrial DNA mutations in North Indian patients with non-syndromic hearing impairment

Mutations at the DFNB1 locus which encode connexin 26 (CX26) and connexin 30 (CX30) proteins, respectively, are main cause for sporadic and familial non-syndromic hearing impairment (NSHI) in many populations. 342-kb deletion [del (GJB6-D13S1830)] of Cx30 gene is second most common connexin mutation. Specific mitochondrial DNA (mtDNA) mutations have been found to be associated with NSHI. In this study, we screened 210 NSHI patients for GJB2 mutations, ΔGJB6-D13S1830 deletion and three point mutations in mtDNA (A1555G, A3243G, A7445G) using PCR, DHPLC and sequencing in North Indian cohort. 35delG was found to be the most common mutation (10.9%), followed by W24X (3.8%) and W77X (1.9%) mutations. We did not observe GJB6-D13S1830 deletion and three mitochondrial point mutations in our cohort. Most of patients (50/58) carried monoallelic variations. Our results reveal different spectrum of GJB2 mutations specific to North Indian cohort, with 35delG being most prevalent. These results suggest that different types of GJB2 mutations affect autosomal recessive NSHI according to ethnic background.     

Investigation of the A1555G mutation in mitochondrial DNA (MT-RNR1) in groups of Brazilian individuals with nonsyndromic deafness and normal-hearing

BACKGROUND:

Mutations of mitochondrial DNA were described into two genes: The mitochondrially encoded 12S RNA (MT-RNR1) and the mitochondrially encoded tRNA serine^ucn (MT-TS1). The A1555G mutation in MT-RNR1 gene is a frequent cause of deafness in different countries.

AIM:

The aim of this work was to investigate the frequency of the A1555G mutation in the MT-RNR1 gene in the mitochondrial DNA in Brazilians individuals with nonsyndromic deafness, and listeners.

MATERIALS AND METHODS:

DNA samples were submitted to polymerase chain reaction and to posterior digestion with the Hae III enzyme.

RESULTS:

Seventy eight (78) DNA samples of deaf individuals were analyzed; 75 showed normality in the region investigated, two samples (2.5%) showed the T1291C substitution, which is not related to the cause of deafness, and one sample (1.3%) showed the A1555G mutation. Among the 70 non-impaired individuals no A1555G mutation or T1291C substitution was found.

CONCLUSION:

We can affirm that A1555G mutation is not prevalent, or it must be very rare in normal-hearing subjects in the State of Paraná, the south region of Brazil. The A1555G mutation frequency (1.3%) found in individual with nonsyndromic deafness is similar to those found in other populations, with nonsyndromic deafness. Consequently, it should be examined in deafness diagnosis. The investigation of the A1555G mutation can contribute towards the determination of the nonsyndromic deafness etiology, hence, contributing to the correct genetic counseling process.     

Maternally inherited non-syndromic hearing loss associated with mitochondrial 12S rRNA A827G mutation in a Chinese family

We explored the clinical and molecular characterization of a Chinese family with non-syndromic hearing impairment. Clinical evaluations revealed a possible maternal inheritance pattern, and showed an extremely similar phenotype of hearing loss including the age of onset, severity, and audiometric configuration. Sequence analysis of the mitochondrial 12S rRNA and tRNA(Ser(UCN)) genes led to the identification of a homoplasmic A827G mutation in all maternal relatives, which was absent in other family members and 40 Chinese controls. This mutation has previously been reported sporadically in a few individuals with aminoglycoside-induced and non-syndromic hearing loss. The A827G mutation is located at the A-site of the mitochondrial 12S rRNA gene which is highly evolutionarily conserved in mammals. The occurrence of the A827G mutation in these genetically unrelated subjects strongly suggests that this mutation is involved in the pathogenesis of hearing impairment. However, incomplete penetrance of hearing loss indicates that the A827G mutation alone is not sufficient to produce clinical phenotype but requires the involvement of modifier factors for the phenotypic expression, even though aminoglycosides and GJB2 gene may not contribute to the penetrance of the A827G mutation in this Chinese family. In contrast with the variable phenotype of hearing loss associated with other mitochondrial mutations, all of the patients in our family exhibited strikingly similar clinical features. This discrepancy likely reflects the difference of genetic backgrounds between this pedigree and others.     

New polymorphic mtDNA restriction site in the 12S rRNA gene detected in Tunisian patients with non-syndromic hearing loss

The 12S rRNA gene was shown to be a hot spot for aminoglycoside-induced and non-syndromic hearing loss since several deafness-associated mtDNA mutations were identified in this gene. Among them, we distinguished the A1555G, the C1494T and the T1095C mutations and C-insertion or deletion at position 961. One hundred Tunisian patients with non-syndromic hearing loss and 100 hearing individuals were analysed in this study. A PCR-RFLP analysis with HaeIII restriction enzyme showed the presence of the A1555G mutation in the 12S rRNA gene in only one out of the 100 patients. In addition, PCR-RFLP and radioactive PCR revealed the presence of a new HaeIII polymorphic restriction site in the same gene of 12S rRNA site in 4 patients with non-syndromic hearing loss. UVIDOC-008-XD analyses showed the presence of this new polymorphic restriction site with a variable heteroplasmic rates at position +1517 of the human mitochondrial genome. On the other hand, direct sequencing of the entire mitochondrial 12S rRNA gene in the 100 patients and in 100 hearing individuals revealed the presence of the A750G and A1438G polymorphisms and the absence of the C1494T, T1095C and 961insC mutations in all the tested individuals. Sequencing of the whole mitochondrial genome in the 4 patients showing the new HaeIII polymorphic restriction site revealed only the presence of the A8860G transition in the MT-ATP6 gene and the A4769G polymorphism in the ND2 gene.     

Aminoglycoside-induced and non-syndromic hearing loss is associated with the G7444A mutation in the mitochondrial COI/tRNASer(UCN) genes in two Chinese families

We report here the clinical, genetic, and molecular characterization of two Chinese families with aminoglycoside induced and non-syndromic hearing impairment. Clinical and genetic evaluations revealed the variable severity and age-of-onset in hearing impairment in these families. Strikingly, there were extremely low penetrances of hearing impairment in these Chinese families. Sequence analysis of the complete mitochondrial genomes in these pedigrees showed the distinct sets of mtDNA polymorphism, in addition to the identical G7444A mutation associated with hearing loss. Indeed, the G7444A mutation in the CO1 gene and the precursor of tRNASer(UCN) gene is present in homoplasmy only in the maternal lineage of those pedigrees but not other members of these families and 164 Chinese controls. Their mitochondrial genomes belong to the Eastern Asian haplogroups C5a and D4a, respectively. In fact, the occurrence of the G7444A mutation in these several genetically unrelated subjects affected by hearing impairment strongly indicates that this mutation is involved in the pathogenesis of hearing impairment. However, there was the absence of other functionally significant mtDNA mutations in two Chinese pedigrees carrying the G7444A mutation. Therefore, nuclear modifier gene(s) or aminoglycoside(s) may play a role in the phenotypic expression of the deafness-associated G7444A mutation in these Chinese pedigrees.     

Mutation analysis of mitochondrial 12S rRNA gene in Polish patients with non-syndromic and aminoglycoside-induced hearing loss

Mutations in mitochondrial DNA have been reported as associated with non-syndromic and aminoglycoside-induced hearing loss. In the present study, we have performed mutational screening of entire 12S rRNA gene in 250 unrelated patients with non-syndromic and aminoglycoside-induced hearing loss. Twenty-one different homoplasmic sequence variants were identified, including eight common polymorphisms, one deafness-associated mutation m.1555 A>G and three putatively pathogenic variants: m.669 T>C, m.827 A>G, m.961 delT+C(n)ins. The incidence of m.1555 A>G was estimated for 3.6% (9/250); however, where aminoglycoside exposure was taken as a risk factor, the frequency was 5.5% (7/128). Substitution m.669 T>C was identified only in patients with hearing impairment and episode of aminoglycoside exposure, which may suggest that such additional risk factors must appear to induce clinical phenotype. Moreover, two 12S rRNA sequence variants: m.988 G>A and m.1453 A>G, localized at conserved sites and affected RNA secondary structure, may be new candidates for non-syndromic and aminoglycoside-induced hearing loss associated mutations.     

Co-occurrence of A1555G and G11778A in a Chinese family with high penetrance of Leber's hereditary optic neuropathy

Co-occurrence of double pathogenic mtDNA mutations with different claimed pathological roles in one mtDNA is infrequent. It is tentative to believe that each of these pathogenic mutations would have its own deleterious effect. Here we reported one three-generation Chinese family with a high penetrance of LHON (78.6%). Analysis of the complete mitochondrial genome in the proband revealed the presence of the LHON primary mutation G11778A in the NADH dehydrogenase 4 (ND4) gene and a deafness-associated mutation A1555G in the 12S rRNA gene. The other mtDNA variants in this family suggested a haplogroup status G2b. Although A1555G has long been confirmed to be a primary mutation for aminoglycoside-induced and non-syndromic hearing loss, none of the maternally related members in this family showed hearing impairment. It thus seems that the occurrence of A1555G in this family had no pathological manifestation. However, whether A1555G has a synergistic effect with G11778A and contribute to the high penetrance of LHON remained an open question. To our knowledge, this is the first report that identified the co-existence of a deafness mutation A1555G and a primary LHON mutation G11778A in one family.     

A South African family with the mitochondrial A1555G mutation on haplogroup L0d

The most common mutation associated with aminoglycoside-induced deafness is A1555G and it has been found in diverse populations worldwide. In the present study we investigated a large South African family known to harbour A1555G. A total of 97 family members were genotyped using the SNaPshot technique and 76 were found to be A1555G-positive (on haplogroup L0d) and are therefore at risk of developing irreversible hearing loss. The method worked equally well on both blood (from adults) and buccal swabs (from children). Variants in the tRNA^Ser(UCN), A10S in TRMU and 35delG in GJB2 genes were shown not to act as genetic modifiers in this family. It is important to identify mutation-positive individuals and inform them of their increased risk of developing aminoglycoside-induced deafness especially in a setting like South Africa where these drugs are still commonly used because of their efficacy and cost-effectiveness as a treatment for resistant forms of tuberculosis.     

湖南地区人群中线粒体12S rRNA基因A1555G突变筛查

为建立快速、简便、准确筛查线粒体DNA 12S rRNA基因A1555G突变的基因检测技术平台,收集1 758例（女性808例,男性950例）正常人群样本,利用Bsm AⅠ酶切法筛查线粒体DNA A1555G突变以及通过实时荧光定量Taqman探针法和直接测序法对筛查结果进行验证,结果检测到2例A1555G阳性突变样本,其中1例为男性,1例为女性。实时荧光定量Taqman探针法与Bsm AⅠ酶切法、直接测序法检测结果完全相符,未发现假阳性和假阴性,该方法具有结果准确直观、简单省时,特异性强,敏感性高的优点,适用于对母系遗传性耳聋线粒体DNA A1555G突变的大规模筛查或氨基糖甙类抗生素应用前的预防性检测。     

A novel mitochondrial DNA missense mutation at G3421A in a family with maternally inherited diabetes and deafness

OBJECTIVE: Mutations in mtDNA are thought to be responsible for the pathogenesis of maternally inherited diabetes. Here, we report a family with maternally inherited diabetes and deafness whose members did not harbour the mtDNA A3243G mutation, the most frequent point mutation in mitochondrial diabetic patients. This study aimed to investigate a possible other mtDNA mutation and its prevalence in type 2 diabetic patients. METHODS: Height, body weight, waistline, and hip circumference were measured and serum biochemical marks determined in all members of the family. In addition, a 75 g oral glucose tolerance test and electric listening test were conducted in these members. Genomic DNA was prepared from peripheral leukocytes. Direct sequencing of PCR products was used to detect the mtDNA mutation in this family. The prevalence of mtDNA G3421A nucleotide substitutions was investigated by restriction fragment length polymorphism analysis in 1350 unrelated type 2 diabetic patients recruited by random cluster sampling from the central city area of Shanghai, China. RESULTS: (1) A new missense homoplasmic mutation of mtDNA G3421A was found in a maternally inherited diabetic family and existed neither in 1350 unrelated type 2 diabetic patients nor in 50 non-diabetic individuals. (2) The mode of mutation and diabetes transmission was typical maternal inheritance in this family. (3) All diabetic family members were found to have an onset at 35-42 years of age, accompanied by deafness of varying degrees. CONCLUSION: mtDNA G3421A (Val39Ile) found in a family with maternally inherited diabetes and deafness is a novel missense mutation. Whether this is a diabetogenic mutation and its effect on mitochondrial function needs to be further studied.     

Clinical evaluation and mitochondrial DNA sequence analysis in two Chinese families with aminoglycoside-induced and non-syndromic hearing loss

We report here the clinical, genetic, and molecular characterization of two Chinese pedigrees with aminoglycoside-induced and non-syndromic hearing impairment. Clinical evaluation revealed the variable phenotype of hearing impairment including audiometric configuration in these subjects. Penetrances of hearing loss in BJ105 and BJ106 pedigrees are 67% and 33%, respectively. In particular, three of 10 affected matrilineal relatives of BJ105 pedigree had aminoglycoside-induced hearing loss, while seven affected matrilineal relatives in BJ105 pedigree and six affected matrilineal relatives in BJ106 pedigree did not have a history of exposure to aminoglycosides. Sequence analysis of the complete mitochondrial genomes in these pedigrees showed the identical homoplasmic A1555G mutation and distinct sets of mtDNA variants belonging to haplogroups F3 and M7b. These variants showed no evolutionary conservation, implying that mitochondrial haplotype may not play a significant role in the phenotypic expression of the A1555G mutation in these Chinese pedigrees. However, aminoglycosides and nuclear backgrounds appear to be major modifier factors for the phenotypic manifestation of the A1555G mutation in these Chinese families.     

WFS1 and non-syndromic low-frequency sensorineural hearing loss: A novel mutation in a Portuguese case

Low-frequency sensorineural hearing loss (LFSNHL) is an unusual type of HL in which frequencies at 2000 Hz and below are predominantly affected. Most of the families with LFSNHL carry missense mutations in WFS1 gene, coding for wolframin.     

The coexistence of mitochondrial ND6 T14484C and 12S rRNA A1555G mutations in a Chinese family with Leber's hereditary optic neuropathy and hearing loss

We report here the clinical, genetic and molecular characterization of one three-generation Han Chinese family with Leber's hereditary optic neuropathy (LHON) and hearing loss. Four of 14 matrilineal relatives exhibited the moderate central vision loss at the average age of 12.5 years. Of these, one subject exhibited both LHON and mild hearing impairment. Sequence analysis of the complete mitochondrial genomes in the pedigree showed the presence of homoplasmic LHON-associated ND6 T14484C mutation, deafness-associated 12S rRNA A1555 mutation and 47 other variants belonging to Eastern Asian haplogroup H2. None of other mitochondrial variants was evolutionarily conserved and functional significance. Therefore, the coexistence of the A1555G mutation and T14484C mutations in this Chinese family indicate that the A1555G mutation may play a synergistic role in the phenotypic manifestation of LHON associated ND6 T14484C mutation. However, the incomplete penetrance of vision and hearing loss suggests the involvement of nuclear modifier genes and environmental factors in the phenotypic expression of these mtDNA mutations.     

Clinical phenotype, prognosis and mitochondrial DNA mutation load in mitochondrial encephalomyopathies

We studied 42 individuals, including 8 patients with either complete or partial syndrome of mitochondrial myopathy, encephalopathy, lactic acidosis and stroke-like episodes (MELAS), 8 patients with either complete or partial syndrome of myoclonic epilepsy with ragged-red fibers (MERRF) and 26 maternal family members who carried either the A3243G or A8344G mutation of mitochondrial DNA (mtDNA). Clinical manifestations and prognosis were followed up in the patients harboring the A3243G or A8344G mutation. The relationship between clinical features and proportions of mutant mtDNAs in muscle biopsies, blood cells and/or hair follicles was studied. In the 8 regularly followed patients with the A3243G mutation, 4 died within 1 month to 7 years due to status epilepticus and/or recurrent stroke-like episodes. Two patients developed marked mental deterioration and 2 remained stationary. All of the patients harboring the A8344G mutation were stable or deteriorated slightly, except for 1 patient who died due to brain herniation after putaminal hemorrhage. The A3243G and A8344G mtDNA mutations were heteroplasmic in the muscle biopsies, blood cells and hair follicles of both the probands and their maternal family members. The mean proportion of A3243G mutant mtDNA in the muscle biopsies of the patients with MELAS syndrome (68.5 ± 21.3%, range 33–92%) was significantly higher than that of the asymptomatic family members (37.1 ± 12.6%, range 0–51%). The average proportions of A8344G mutant mtDNA in the muscle biopsies (90.1 ± 3.9%, range 89–95%) and hair follicles (93.9 ± 6.4%, range 84–99%) of the patients with MERRF syndrome were also significantly higher than those of the asymptomatic family members (muscle: 40.3 ± 39.5%, range 1–80%; hair follicles: 51.0 ± 44.5%, range 0.1–82%). We concluded that measurement of the proportion of mutant mtDNA in muscle biopsies may provide useful information in the identification of symptomatic patients with mitochondrial encephalomyopathies. For patients with the A3243G mutation, the prognosis was related to status epilepticus and the number of recurrent stroke-like episodes and was much worse than for patients with the A8344G mutation of mtDNA, who had stable or slowly deteriorating clinical courses.     

Only male matrilineal relatives with Leber's hereditary optic neuropathy in a large Chinese family carrying the mitochondrial DNA G11778A mutation

We report here the characterization of a five-generation large Chinese family with Leber's hereditary optic neuropathy (LHON). Very strikingly, six affected individuals of 38 matrilineal relatives (17 females/21 males) are exclusively males in this Chinese family. These matrilineal relatives in this family exhibited late-onset/progressive visual impairment with a wide range of severity, ranging from blindness to normal vision. The age of onset in visual impairment varies from 17 to 30 years. Sequence analysis of the complete mitochondrial genome in this pedigree revealed the presence of the G11778A mutation in ND4 gene and 29 other variants. This mitochondrial genome belongs to the Southern Chinese haplogroup B5b. We showed that the G11778A mutation is present at near homoplasmy in matrilineal relatives of this Chinese family but not in 164 Chinese controls. Incomplete penetrance of LHON in this family indicates the involvement of modulatory factors in the phenotypic expression of visual dysfunction associated with the G11778A mutation. However, none of other mtDNA variants are evolutionarily conserved and implicated to have significantly functional consequence. Thus, nuclear modifier gene(s) or environmental factor(s) seem to account for the penetrance and phenotypic variability of LHON in this Chinese family carrying the G11778A mutation.     

Voltage-dependent anion channel involved in the mitochondrial calcium cycle of cell lines carrying the mitochondrial DNA A4263G mutation

In this study, we investigated the effects of the voltage-dependent anion channel (VDAC) on the mitochondrial calcium cycle in cell lines carrying the mitochondrial DNA A4263G mutation. We established lymphoblastoid cell lines from three symptomatic individuals and one asymptomatic individual from the large Chinese Han family carrying the A4263G mutation; these were compared with three control cell lines. The mitochondrial Ca²⁺ concentration and membrane potential were detected by loading cells with Rhod-2 and JC-1, respectively. Confocal imagines showed the average Rhod-2 and JC-1 fluorescence levels of individuals carrying the tRNA^Ile A4263G mutation were lower than those of the control group (P < 0.05). The baseline Rhod-2 fluorescence in the control group increased after exposure to atractyloside (an opener of the adenine nucleotide translocator, P < 0.05), but no significant change was detected in the cell line harboring the A4263G mutation (P > 0.05). The baseline JC-1 fluorescence in both the mutated and control cell lines decreased after subsequent exposure to atractyloside (P < 0.05), whereas this effect of atractyloside was inhibited by Cyclosporin A (CsA, a VDAC blocker). We conclude that the mitochondrial VDAC is involved in both the increase of mitochondrial permeability to Ca²⁺ and the decrease of mitochondrial membrane potential in cell lines carrying the mtDNA A4263G mutation.     

Coexistence of mitochondrial 12S rRNA C1494T and CO1/tRNA(Ser(UCN)) G7444A mutations in two Han Chinese pedigrees with aminoglycoside-induced and non-syndromic hearing loss

Mutations in mitochondrial DNA are one of the important causes of hearing loss. We report here the clinical, genetic, and molecular characterization of two Han Chinese pedigrees with maternally transmitted aminoglycoside-induced and nonsyndromic bilateral hearing loss. Clinical evaluation revealed the wide range of severity, age-at-onset, and audiometric configuration of hearing impairment in matrilineal relatives in these families. The penetrances of hearing loss in these pedigrees were 20% and 18%, when aminoglycoside-induced deafness was included. When the effect of aminoglycosides was excluded, the penetrances of hearing loss in these seven pedigrees were 10% and 15%. Sequence analysis of the complete mitochondrial genomes in these pedigrees showed the presence of the deafness-associated 12S rRNA C1494T and CO1/tRNA(Ser(UCN)) G7444A mutations. Their distinct sets of mtDNA polymorphism belonged to Eastern Asian haplogroup C4a1, while other previously identified six Chinese mitochondrial genomes harboring the C1494T mutation belong to haplogroups D5a2, D, R, and F1, respectively. This suggested that the C1494T or G7444A mutation occurred sporadically and multiplied through evolution of the mitochondrial DNA (mtDNA). The absence of functionally significant mutations in tRNA and rRNAs or secondary LHON mutations in their mtDNA suggest that these mtDNA haplogroup-specific variants may not play an important role in the phenotypic expression of the 12S rRNA C1494T and CO1/tRNA(Ser(UCN)) G7444A mutations in those Chinese families. However, aminoglycosides and other nuclear modifier genes play a modifying role in the phenotypic manifestation of the C1494T mutation in these Chinese families.     

Mutational analysis of the mitochondrial 12S rRNA and tRNASer(UCN) genes in Tunisian patients with nonsyndromic hearing loss

We explored the mitochondrial 12S rRNA and the tRNASer(UCN) genes in 100 Tunisian families affected with NSHL and in 100 control individuals. We identified the mitochondrial A1555G mutation in one out of these 100 families and not in the 100 control individuals. Members of this family harbouring the A1555G mutation showed phenotypic heterogeneity which could be explained by an eventual nuclear-mitochondrial interaction. So, we have screened three nuclear genes: GJB2, GJB3, and GJB6 but we have not found correlation between the phenotypic heterogeneity and variants detected in these genes. We explored also the entire mitochondrial 12S rRNA and the tRNASer(UCN) genes. We detected five novel polymorphisms: T742C, T794A, A813G, C868T, and C954T, and 12 known polymorphisms in the mitochondrial 12S rRNA gene. None of the 100 families or the 100 controls were found to carry mutations in the tRNASer(UCN) gene. We report here the first mutational screening of the mitochondrial 12S rRNA and the tRNASer(UCN) genes in the Tunisian population which describes the second family harbouring the A1555G mutation in Africa and reveals novel polymorphisms in the mitochondrial 12S rRNA gene.