Presence of mutation m.14484T>C in a Chinese family with maternally inherited essential hypertension but no expression of LHON

Essential hypertension (EH, MIM 145500) is the most common cardiovascular disease and affects one-quarter of the world's adult population. Families with EH in a mode of maternal transmission have been occasionally observed in clinical settings and suggested an involvement of mitochondrial DNA (mtDNA) mutation. We aimed to characterize the role of mtDNA mutation in EH. We reported a large Han Chinese family with a maternally inherited EH and an extraordinarily high percentage of sudden death mainly in affected females. Analysis of the entire mtDNA genome of the proband identified a homoplasmic primary mutation m.14484T>C for Leber's hereditary optic neuropathy (LHON), along with several variants indicating haplogroup F1 status. Intriguingly, no maternal member in this family had LHON though they all harbored m.14484T>C. The arterial stiffness of the members carrying mutation m.14484T>C was significantly increased than that of non-maternal members without this mutation. No environmental factor (including age, sex, smoking, diabetes, hyperlipidemia) was correlated with the decreased aortic elastic properties observed in affected members. Mitochondrial respiration rate and membrane potential (ΔΨ_m) were significantly reduced in lymphoblastoid cell lines established from affected members carrying m.14484T>C when compared to control cell lines (P < 0.05). There was an elevation of reactive oxygen species and a compensatory increase of mitochondrial mass in mutant cell lines. Our results suggest that m.14484T>C causes EH under certain circumstance. This study provides a paradigm for diverse phenotypes of the primary LHON mutation and suggests for the necessity of routine cardiac evaluation in patients with the primary LHON mutation.     

Mitochondrial DNA sequence variation and haplogroup distribution in Chinese patients with LHON and m.14484T>C

Background

Leber hereditary optic neuropathy (LHON, MIM 535000) is one of the most common mitochondrial genetic disorders caused by three primary mtDNA mutations (m.3460G>A, m.11778G>A and m. 14484T>C). The clinical expression of LHON is affected by many additional factors, e.g. mtDNA background, nuclear genes, and environmental factors. Hitherto, there is no comprehensive study of Chinese LHON patients with m.14484T>C.

Methodology/Principal Findings

In this study, we analyzed the mtDNA sequence variations and haplogroup distribution pattern of the largest number of Chinese LHON patients with m.14484T>C to date. We first determined the complete mtDNA sequences in eleven LHON probands with m.14484T>C, to discern the potentially pathogenic mutations that co-segregate with m.14484T>C. We then dissected the matrilineal structure of 52 patients with m.14484T>C (including 14 from unrelated families and 38 sporadic cases) and compared it with the reported Han Chinese from general populations. Complete mtDNA sequencing showed that the eleven matrilines belonged to nine haplogroups including Y2, C4a, M8a, M10a1a, G1a1, G2a1, G2b2, D5a2a1, and D5c. We did not identify putatively pathogenic mutation that was co-segregated with m.14484T>C in these lineages based on the evolutionary analysis. Compared with the reported Han Chinese from general populations, the LHON patients with m.14484T>C had significantly higher frequency of haplogroups C, G, M10, and Y, but a lower frequency of haplogroup F. Intriguingly, we also observed a lower prevalence of F lineages in LHON subjects with m.11778G>A in our previous study, suggesting that this haplogroup may enact similar role during the onset of LHON in the presence of m.14484T>C or m.11778G>A.

Conclusions/Significance

Our current study provided a comprehensive profile regarding the mtDNA variation and background of Chinese patients with LHON and m.14484T>C. Matrilineal background might affect the expression of LHON in Chinese patients with m.14484T>C.     

Mitochondrial ND5 12338T>C variant is associated with maternally inherited hypertrophic cardiomyopathy in a Chinese pedigree

Hypertrophic cardiomyopathy is a primary disorder characterized by asymmetric thickening of the septum and left ventricular wall, which affects 1 in 500 individuals in the general population. Mutations in mitochondrial DNA have been found to be one of the most important causes of hypertrophic cardiomyopathy. Here we report the clinical, genetic and molecular characterization of a Han Chinese family with a likely maternally transmitted hypertrophic cardiomyopathy. Four (2 men/2 women) of 5 matrilineal relatives in this 3-generation family exhibited the variable severity and age at onset of 44 to 79years old. Sequence analysis of the entire mitochondrial DNA in this pedigree identified the known homoplasmic ND5 12338T>C variant. This mitochondrial DNA haplogroup belongs to the Eastern Asian F2a. The 12338T>C variant, highly evolutionarily conserved, resulted in the replacement of the translation initiating methionine with a threonine, shortening the ND5 polypeptide by 2 amino acids. The occurrence of ND5 12338T>C variant exclusively in maternal members of this Chinese family suggested that the 12338T>C variant is associated with maternally inherited hypertrophic cardiomyopathy. Our findings will provide theoretical basis for genetic counseling of maternally inherited hypertrophic cardiomyopathy.     

The novel G10680A mutation is associated with complete penetrance of the LHON/T14484C family

We report the clinical and genetic characterization of a Chinese Leber′s hereditary optic neuropathy (LHON) family with complete penetrance and high percentage of recovery. Sequence analysis of the complete mitochondrial DNA revealed the presence of heteroplasmic ND6/T14484C mutation and 27 other variants, belonging to the East-Asian haplogroup B4b′d. Of those variants, a novel homoplasmic G10680A mutation substituted a threonine for a highly conserved alanine at ND4L amino acid 71, which was not found in unaffected family members and 100 normal controls. It indicated that G10680A may play a synergistic role with the primary mutation T14484C, leading to the complete penetrance of LHON in the presenting family. In addition, the other modifier factors including nuclear background, mitochondrial haplotypes and other environmental factors should account for the phenotypic variability of visual impairment in this family.     

Unusual adult-onset Leigh syndrome presentation due to the mitochondrial m.9176T>C mutation

Leigh syndrome (LS) is an incurable, nearly always fatal, neurodegenerative, pediatric disorder that results from respiratory chain failure. The most common mitochondrial DNA (mtDNA) mutations that result in LS are m.8993T→C/G and m.9176T→C/G, which were previously found in several patients with early-onset Leigh syndrome. Here, we describe clinical and molecular features of a novel pedigree, where LS developed in two siblings. The proband was a young woman with an unusual adult-onset LS. She harbored a homoplasmic m.9176T→C mutation, based on analysis of a muscle biopsy. In contrast, the brother died at a young age. This novel case report and literature review highlights the variability of phenotypic expression of the m.9176T→C mutation.     

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.     

Maternally inherited hypertension is associated with the mitochondrial tRNA(Ile) A4295G mutation in a Chinese family

Mutations in mitochondrial DNA have been associated with cardiovascular disease. We report here the clinical, genetic, and molecular characterization of one three-generation Han Chinese family with maternally transmitted hypertension. All matrilineal relatives in this family exhibited the variable degree of hypertension at the age at onset of 36 to 56 years old. Sequence analysis of the complete mitochondrial DNA in this pedigree revealed the presence of the known hypertension-associated tRNA^Ile A4295G mutation and 33 other variants, belonging to the Asian haplogroup D4j. The A4295G mutation, which is extraordinarily conserved from bacteria to human mitochondria, is located at immediately 3′ end to the anticodon, corresponding to conventional position 37 of tRNA^Ile. The occurrence of the A4295G mutation in several genetically unrelated pedigrees affected by cardiovascular disease but the absence of 242 Chinese controls strongly indicates that this mutation is involved in the pathogenesis of cardiovascular disease. Of other variants, the tRNA^Glu A14693G and ND1 G11696A mutations were implicated to be associated with other mitochondrial disorders. The A14693G mutation, which is a highly conserved nucleoside at the TψC-loop of tRNA^Glu, has been implicated to be important for tRNA structure and function. Furthermore, the ND4 G11696A mutation was associated with Leber’s hereditary optic neuropathy. Therefore, the combination of the A4295G mutation in the tRNA^Ile gene with the ND4 G11696A mutation and tRNA^Glu A14693G mutation may contribute to the high penetrance of hypertension in this Chinese family.     

The mitochondrial ND1 T3308C mutation in a Chinese family with the secondary hypertension

Mutations in mitochondrial DNA have been associated with hypertension. We report here the clinical, genetic, and molecular characterization of one four-generation Han Chinese family with hypertension. Two matrilineal relatives in this family exhibited the variable degree of a secondary hypertension (renal hypertension) at the age-at-onset of 42 and 56 years old, respectively. Sequence analysis of the complete mitochondrial DNA in this pedigree revealed the presence of the known hypertension-associated ND1 T3308C mutation and 42 other variants, belonging to the Asian haplogroup D4h. The T3308C mutation resulted in the replacement of the first amino acid, translation-initiating methionine with a threonine in ND1. Furthermore, the ND3 T3308C mutation also locates in two nucleotides adjacent to the 3′ end of mitochondrial tRNA^Leu(UUR). Thus, this T3308C mutation caused an alteration on the processing of the H-strand polycistronic RNA precursors or the destabilization of ND1 mRNA. The occurrence of the T3308C mutation in these genetically unrelated pedigrees affected by diseases but absence of 242 Chinese controls as well as the mitochondrial dysfunctions detected in cells carrying this mutation indicate that this mutation is involved in the pathogenesis of hypertension. However, the mild biochemical defects, the lower penetrance of hypertension in this Chinese family and the presence of some control populations suggested the involvement of other modifier factors in the pathogenesis of hypertension associated with this ND1 T3308C mutation.     

A novel mtDNA ND6 gene mutation associated with LHON in a Caucasian family

Leber's hereditary optic neuropathy (LHON) is a frequent cause of inherited blindness. A routine screening for common mtDNA mutations constitutes an important first in its diagnosis. However, a substantial number of LHON patients do not harbor known variants, both pointing to the genetic heterogeneity of LHON and bringing into question its genetic diagnosis. We report a familial case that exhibited typical features of LHON but lacked any of the common mutations. Genetic analysis revealed a novel pathogenic defect in the ND6 gene at 14279A that was not detected in any haplogroup-matched controls screened for it, nor has it been previously reported. This mutation causes a substantial conformational change in the secondary structure of the polypeptide matrix coil and may explain the LHON expression. Thus, it expands the spectrum of deleterious changes affecting ND6-encoding subunit and further highlights the functional significance of this gene, providing additional clues to the disease pathogenesis.     

Novel A14841G mutation is associated with high penetrance of LHON/C4171A family

We report the clinical and genetic characterization of a Chinese LHON family carrying an ND1/C4171A mutation. This family has high penetrance of visual impairment and extremely low frequency of vision recovery, which is in marked contrast to previously reported results for Korean LHON families with the same mutation. Sequence analysis of the complete mtDNA in the partially defined East Asian haplogroup N9a1 revealed the presence of 29 other variants. A novel heteroplasmic A14841G mutation, one of the variants with a serine substituted for a highly conserved asparagine at amino acid 32 of Cytochrome b (Cytb), may play a synergistic role with the C4171A mutation, leading to significantly different clinical manifestations of LHON among these families. The study further confirmed that C4171A was a rare primary LHON mutation, and the mtDNA background could also contribute to the clinical manifestation of the LHON/C4171A mutation.     

Sequence and functional analyses of mtDNA in a maternally inherited family with bipolar disorder and depression

Recent studies suggest that mutations/polymorphisms of mitochondrial DNA (mtDNA) are associated with neuropsychiatric diseases. We identified a patient with major depression and epilepsy. Some family members in the pedigree of the proband had bipolar disorder, depression, suicide, or psychotic disorder not otherwise specified. The mode of inheritance was compatible with maternal inheritance with low penetration. We assumed that the mental disorder in this family might be associated with maternally inherited mitochondrial DNA (mtDNA) mutation. We sequenced the entire mtDNA of the proband. Among the 34 base substitutions detected in the proband, two homoplasmic, nonsynonymous single substitutions of mtDNA, T3394C in MT-ND1 and A9115G in MT-ATP6, were suspected to cause functional impairment, because the former was reported to be disease-related and the latter is vary rare. To study the functional outcome of these substitutions, we examined mitochondrial membrane potential and the activity of mitochondrial ATP synthesis in the transmitochondrial cybrids, but no significant impairment was detected. The data did not support our hypothesis that these disorders in this family are caused by mtDNA mutation(s).     

Mitochondrial haplotypes may modulate the phenotypic manifestation of the LHON-associated m.14484T>C (MT-ND6) mutation in Chinese families

Mitochondrial m.14484T>C (MT-ND6) mutation has been associated with Leber's hereditary optic neuropathy. Previous investigations revealed that the m.14484T>C mutation is a primary factor underlying the development of optic neuropathy but is not sufficient to produce a clinical phenotype. However, mitochondrial haplogroups have been proposed to modulate the phenotypic manifestation of the m.14484T>C mutation. Here, we performed the clinical, genetic evaluation and complete mitochondrial genome sequence analysis of 41 Han Chinese pedigrees carrying the m.14484T>C mutation. These families exhibited a wide range of penetrances and expressivities of optic neuropathy. The average ratio between affected male/female matrilineal relatives from 41 families was 2:1. The penetrance of optic neuropathy in these Chinese pedigrees ranged from 5.6% to 100%, with the average of 23.8%. Furthermore, the age-of-onset for optic neuropathy varied from 4 to 44 years, with the average of 19.3 years. Sequence analysis of their mitochondrial genomes identified distinct sets of polymorphisms belonging to ten Eastern Asian haplogroups, indicating that the m.14484T>C mutation occurred through recurrent origins and founder events. We showed that mitochondrial haplogroups M9, M10 and N9 increased the penetrance of optic neuropathy in these Chinese families. In particular, these mitochondrial haplogroup specific variants: m.3394T>C (MT-ND1), m.14502T>C (MT-ND4) and m.14693A>G (MT-TE) enhanced the penetrance of visual loss in these Chinese families. These data provided the direct evidence that mitochondrial modifiers modulate the variable penetrance and expressivity of optic neuropathy among Chinese pedigrees carrying the m.14484T>C mutation.     

T14484C and T14502C in the mitochondrial ND6 gene are associated with Leber's hereditary optic neuropathy in a Chinese family

Leber's hereditary optic neuropathy (LHON) is a maternally inherited ocular disease which has been associated with three primary mitochondrial DNA mutations: G3640A, G11778A, and T14484C. In this study, we clinically characterized a Chinese family with complete penetrance of LHON. The patients in the family presented with variable clinical features. By direct DNA sequence analysis, we identified both T14484C mutation and a nearby T to C variant at nucleotide 14502 of mitochondria DNA. The T14502C variant altered I58 to V of the protein ND6, which was present in all patients of the family, but not in four unaffected family members and 200 normal controls. The co-existence of both T14484C mutation and T14502C substitution in all patients from the same LHON family suggests that T14502C may play a synergistic role with the primary mutation T14484C. The two variants together may account for the complete penetrance and absence of marked gender bias and visual recovery in the Chinese LHON family although we cannot exclude the possibility of simultaneous involvement of additional mitochondrial variant(s).     

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.     

Maternally inherited aminoglycoside-induced and nonsyndromic deafness is associated with the novel C1494T mutation in the mitochondrial 12S rRNA gene in a large Chinese family

We report here the characterization of a large Chinese family with maternally transmitted aminoglycoside-induced and nonsyndromic deafness. In the absence of aminoglycosides, some matrilineal relatives in this family exhibited late-onset/progressive deafness, with a wide range of severity and age at onset. Notably, the average age at onset of deafness has changed from 55 years (generation II) to 10 years (generation IV). Clinical data reveal that the administration of aminoglycosides can induce or worsen deafness in matrilineal relatives. The age at the time of drug administration appears to be correlated with the severity of hearing loss experienced by affected individuals. Sequence analysis of mitochondrial DNA in this pedigree identified a homoplasmic C-to-T transition at position 1494 (C1494T) in the 12S rRNA gene. The C1494T mutation is expected to form a novel U1494-1555A base pair, which is in the same position as the C1494-1555G pair created by the deafness-associated A1555G mutation, at the highly conserved A site of 12S rRNA. Exposure to a high concentration of paromomycin or neomycin caused a variable but significant average increase in doubling time in lymphoblastoid cell lines derived from four symptomatic and two asymptomatic individuals in this family carrying the C1494T mutation when compared to four control cell lines. Furthermore, a significant decrease in the rate of total oxygen consumption was observed in the mutant cell lines. Thus, our data strongly support the idea that the A site of mitochondrial 12S rRNA is the primary target for aminoglycoside-induced deafness. These results also strongly suggest that the nuclear background plays a role in the aminoglycoside ototoxicity and in the development of the deafness phenotype associated with the C1494T mutation in the mitochondrial 12S rRNA gene.     

Maternally inherited nonsyndromic hearing loss is associated with the T7511C mutation in the mitochondrial tRNASerUCN gene in a Japanese family

We report here the characterization of a Japanese family with maternally transmitted nonsyndromic hearing loss. Fourteen of 21 matrilineal relatives in this family exhibited early or late-onset/progressive but noncongenital hearing impairment with a wide range of severity, ranging from severe to normal hearing. The age-of-onset varies from 3 to 30 years. Sequence analysis of the complete mitochondrial genome in one matrilineal relative of this family revealed the presence of T7511C mutation and other variants. However, the levels of heteroplasmy of T7511C mutation did not correlate with the severity and age-of-onset of hearing loss in this family. Furthermore, none of other mtDNA variants are evolutionarily conserved and implicated to have significantly functional consequence. The absence of the ND1 T3308C and tRNA(Ala) T5655C mutations in this Japanese family but the presence of these mtDNA mutations in an African family with a high penetrance seems to account for different penetrance between two pedigrees. Incomplete penetrance in this family indicates the involvement of modulatory factors in the phenotypic expression of hearing impairment associated with the T7511C mutation. Here, two known variants G79A and G109A in the GJB2 gene were identified in the hearing-impaired and normal hearing matrilineal relatives of this Japanese family. However, the lack of correlation in the severity and age-of-onset in hearing impairment with homozygous or heterozygous G79A or G109A or combination of both variants in the GJB2 gene in those subjects with hearing impairment and normal hearing indicates that those variants of GJB2 gene may not be a modifier of the phenotypic effects of the T7511C mutation in those subjects. Thus, the phenotypic variability in this family is due to the involvement of other modifier factor(s).