Implications of mitochondrial DNA mutations and mitochondrial dysfunction in tumorigenesis

Alterations in oxidative phosphorylation resulting from mitochondriai dysfunction have long been hypothesized to be involved in tumorigenesis. Mitochondria have recently been shown to play an important role in regulating both programmed cell death and cell proliferation. Furthermore, mitochondrial DNA （mtDNA） mutations have been found in various cancer cells. However, the role of these mtDNA mutations in tumorigenesis remains largely unknown. This review focuses on basic mitochondrial genetics, mtDNA mutations and consequential mitochondrial dysfunction associated with cancer. The potential molecular mechanisms, mediating the pathogenesis from mtDNA mutations and mitochondrial dysfunction to tumorigenesis are also discussed.

Mitochondrial DNA Mutations Associated with Aminoglycoside Ototoxicity

The mitochondrial 12S rRNA has been shown to be the hot spot for mutations associated with both aminoglycoside-induced and non-syndromic hearing loss. Of all the mutations, the homoplasmic A1555G and C1494T mutations at a highly conserved decoding region in the 12S rRNA have been associated with aminoglycoside-induced and non-syndromic hearing loss in many families worldwide. The A1555G or C1494T mutation is expected to form novel 1494C-G1555 or 1494U-A1555 base-pair at the highly conserved A-site of 12S rRNA. These transitions make the secondary structure of this RNA more closely resemble the corresponding region of bacterial 16S rRNA. Thus, the new U-A or G-C pair in 12S rRNA created by the C1494T or A1555G transition facilitates the binding of aminoglycosides, thereby accounting for the fact that the exposure to aminoglycosides can induce or worsen hearing loss in individuals carrying these mutations. Furthermore, the growth defect and impairment of mitochondrial translation were observed in cell lines carrying the A1555G or C1494T mutation in the presence of high concentration of aminoglycosides. In addition, nuclear modifier genes and mitochondrial haplotypes modulate the phenotypic manifestation of the A1555G and C1494T mutations. These observations provide the direct genetic and biochemical evidences that the A1555G or C1494T mutation is a pathogenic mtDNA mutation associated with aminoglycoside-induced and nonsyndromic hearing loss. Therefore, these data have been providing valuable information and technology to predict which individuals are at risk for ototoxicity, to improve the safety of aminoglycoside antibiotic therapy, and eventually to decrease the incidence of deafness.

Late-onset mitochondrial encephalomyopathy with lactic acidosis and stroke-like episodes syndrome with mitochondrial DNA 3243A>G mutation masquerading as autoimmune encephalitis:A case report

BACKGROUND Here,we present a unique case of mitochondrial encephalomyopathy with lactic acidosis and stroke-like episodes(MELAS)syndrome,which initially appeared to be autoimmune encephalitis and was ultimately confirmed as MELAS with the mitochondrial DNA 3243A>G mutation.CASE SUMMARY A 58-year-old female presented with acute-onset speech impediment and auditory hallucinations,symmetrical bitemporal lobe abnormalities,clinical and laboratory findings,and a lack of relevant prodromal history,which suggested diagnosis of autoimmune encephalitis.Further work-up,in conjunction with the patient’s medical history,family history,and lactate peak on brain lesions on magnetic resonance imaging,suggested a mitochondrial disorder.Mitochondrial genome analysis revealed the m.3243A>G variant in the MT-TL1 gene,which led to a diagnosis of MELAS syndrome.CONCLUSION This case underscores the importance of considering MELAS as a potential cause of autoimmune encephalitis even if patients are over 40 years of age,as the symptoms and signs are atypical for MELAS syndrome.

Somatic alterations in mitochondrial DNA and mitochondrial dysfunction in gastric cancer progression

Energy metabolism reprogramming was recently identified as one of the cancer hallmarks.One of the underlying mechanisms of energy metabolism reprogramming is mitochondrial dysfunction caused by mutations in nuclear genes or mitochondrial DNA(mtDNA).In the past decades,several types of somatic mtDNA alterations have been identified in gastric cancer.However,the role of these mtDNA alterations in gastric cancer progression remains unclear.In this review,we summarize recently identified somatic mtDNA alterations in gastric cancers as well as the relationship between these alterations and the clinicopathological features of gastric cancer.The causative factors and potential roles of the somatic mtDNA alterations in cancer progression are also discussed.We suggest that point mutations and mtDNA copy number decreases are the two most common mtDNA alterations that result in mitochondrial dysfunction in gastric cancers.The two primary mutation types(transition mutations and mononucleotide or dinucleotide repeat instability)imply potential causative factors.Mitochondrial dysfunction-generated reactive oxygen species may be involved in the malignant changes of gastric cancer.The search for strategies to prevent mtDNA alterations and inhibit the mitochondrial retrograde signaling will benefit the development of novel treatments for gastric cancer and other malignancies.

A Mitochondrial DNA A8701G Mutation Associated with Maternally Inherited Hypertension and Dilated Cardiomyopathy in a Chinese Pedigree of a Consanguineous Marriage

Background： Cardiovascular diseases, including dilated cardiomyopathy （DCM） and hypertension, are the leading cause of death worldwide.The role of mitochondrial DNA （mtDNA） in the pathogenesis of these diseases has not been completely clarified.In this study, we evaluate whether A8701G mutation is associated with maternally inherited hypertension and DCM in a Chinese pedigree of a consanguineous marriage.Methods： Fourteen subjects in a three-generation Han Chinese family with hypertension and DCM, in which consanguineous marriage was present in the parental generation, were interviewed.We divided all the family members into case （7 maternal members） and control group （7 nonmaternal members） for comparison.Clinical evaluations and sequence analysis ofmtDNA were obtained from all participants.Frequency differences between maternal and nonmaternal members were tested to locate the disease-associated mutations.Results： The majority of the family members presented with a maternal inheritance of hypertension and DCM.Sequence analysis of mtDNA in this pedigree identified eight mtDNA mutations.Among the mutations identified, there was only one significant mutation： A8701G （P =0.005）, which is a homoplasmic mitochondrial missense mutation in all the matrilineal relatives.There was no clear evidence for any synergistic effects between A8701G and other mutations.Conclusions： A8701G mutation may act as an inherited risk factor for the matrilineal transmission of hypertension and DCM in conj unction with genetic disorders caused by consanguineous marriage.

Point mutations of muscle mitochondrial DNA from patients with mitochondrial encephalomyopathies

Objective To study the relation between point mutations at nt3243 and nt8344 of muscle mitochondrial DNA from patients with mitochondrial encephalomyopathies and phenotypes. Methods DNA was extracted from muscle specimens from 5 patients with mitochondrial encephalomyopathies and amplified by PCR method, using corresponding oligonucleotide primers. DNA fragments were digested with restriction enzymes BglⅠ and ApaⅠ, then the digested DNA fragments were analyzed with an electrophoresis method.Results The point mutation at nt3243 of mtDNA was found in 2 patients, one with mitochondrial encephalomyopathy, lactic acidosis and stroke-like episodes (MELAS) and another with myoclonic epilepsy with ragged red fibers (MERRF). The point mutation at nt8344 was found in 2 patients with MERRF, including the one with point mutation at nt3243.Conclusion The point mutation of DNA at nt3243 correlated with MELAS and nt8344 correlated with MERRF. In addition, the detection of point mutations at both nt3243 and nt8344 in a patient with MERRF shows the association of mutation with diversity in clinical manifestations of mitochondrial encephalomyopathies.

Relationship between mutations of mitochondrial DNA ND1 gene and type 2 diabetes

Background Recent studies have indicated that many mutations in mitochondrial (mt)DNA NDI gene region are related to diabetes mellitus. In this study we explored the relationship between various mtDNA ND1 gene mutations and type 2 diabetes mellitus (DM) among Chinese. Methods Using PCR restriction fragment length polymorphism (PCR-RFLP) analysis and gene sequencing, 4 spots of mtDNA (nt3243, nt3316, nt3394, nt3426) were screened in 478 diabetics and 430 non-diabetic subjects.Results In diabetic group, there were 13 carriers (2.72%)of 3316 G→A mutation,12 (2.51%) of 3394 T→C mutation and 2 (0.42%) of 3426A→G mutation. In controls, only 3394 T→C mutation was observed in 2 subjects (0.47%). There was significant difference in the frequency of 3316 and 3394 mutation between two groups (P<0.05, respectively). More subjects with mitochondrial DNA ND1 gene mutations had DM family history and greater tendency of maternal inheritance when compared to those patients without mutation in diabetic group(P<0.01). A 3426 mutation diabetic pedigree was studied, and we found 12 maternal members in the family had the same mutation. Conclusion mtDNA ND1 gene mutations at nt3316 (G→A), nt3394 (T→C) and 3426 (A→G) might contribute to the pathogenesis of DM with other genetic factors and environment factors.