Profiling of length heteroplasmies in the human mitochondrial DNA control regions from blood cells in the Korean population

Spectrum of mitochondrial genome instability and implication of mitochondrial haplogroups in Korean patients with acute myeloid leukemia

Background

Mitochondrial DNA (mtDNA) mutations may regulate the progression and chemosensitivity of leukemia. Few studies regarding mitochondrial aberrations and haplogroups in acute myeloid leukemia (AML) and their clinical impacts have been reported. Therefore, we focused on the mtDNA length heteroplasmies minisatellite instability (MSI), copy number alterations, and distribution of mitochondrial haplogroups in Korean patients with AML.

Methods

This study investigated 74 adult patients with AML and 70 controls to evaluate mtDNA sequence alterations, MSI, mtDNA copy number, haplogroups, and their clinical implications. The hypervariable (HV) control regions (HV1 and HV2), tRNA^leu1gene, and cytochrome b gene of mtDNA were analyzed. Two mtDNA minisatellite markers, 16189 poly-C (¹⁶¹⁸⁴CCCCCTCCCC¹⁶¹⁹³, 5CT4C) and 303 poly-C (³⁰³CCCCCCCTCCCCC³¹⁵, 7CT5C), were used to examine the mtDNA MSI.

Results

In AML, most mtDNA sequence variants were single nucleotide substitutions, but there were no significant differences compared to those in controls. The number of mtMSI patterns increased in AML. The mean mtDNA copy number of AML patients increased approximately 9-fold compared to that of controls (P<0.0001). Haplogroup D4 was found in AML with a higher frequency compared to that in controls (31.0% vs. 15.7%, P=0.046). None of the aforementioned factors showed significant impacts on the outcomes.

Conclusion

AML cells disclosed more heterogeneous patterns with the mtMSI markers and had increased mtDNA copy numbers. These findings implicate mitochondrial genome instability in primary AML cells. Therefore, mtDNA haplogroup D4 might be associated with AML risk among Koreans.

Investigation of length heteroplasmy in mitochondrial DNA control region by massively parallel sequencing

Accurate sequencing of the control region of the mitochondrial genome is notoriously difficult due to the presence of polycytosine bases, termed C-tracts. The precise number of bases that constitute a C-tract and the bases beyond the poly cytosines may not be accurately defined when analyzing Sanger sequencing data separated by capillary electrophoresis. Massively parallel sequencing has the potential to resolve such poor definition and provides the opportunity to discover variants due to length heteroplasmy. In this study, the control region of mitochondrial genomes from 20 samples was sequenced using both standard Sanger methods with separation by capillary electrophoresis and also using massively parallel DNA sequencing technology. After comparison of the two sets of generated sequence, with the exception of the C-tracts where length heteroplasmy was observed, all sequences were concordant. Sequences of three segments 16184-16193, 303-315 and 568-573 with C-tracts in HVI, II and III can be clearly defined from the massively parallel sequencing data using the program SEQ Mapper. Multiple sequence variants were observed in the length of C-tracts longer than 7 bases. Our report illustrates the accurate designation of all the length variants leading to heteroplasmy in the control region of the mitochondrial genome that can be determined by SEQ Mapper based on data generated by massively parallel DNA sequencing.

Clinicopathological Implications of Mitochondrial Genome Alterations in Pediatric Acute Myeloid Leukemia

Background

To the best of our knowledge, the association between pediatric AML and mitochondrial aberrations has not been studied. We investigated various mitochondrial aberrations in pediatric AML and evaluated their impact on clinical outcomes.

Methods

Sequencing, mitochondrial DNA (mtDNA) copy number determination, mtDNA 4,977-bp large deletion assessments, and gene scan analyses were performed on the bone marrow mononuclear cells of 55 pediatric AML patients and on the peripheral blood mononuclear cells of 55 normal controls. Changes in the mitochondrial mass, mitochondrial membrane potential, and intracellular reactive oxygen species (ROS) levels were also examined.

Results

mtDNA copy numbers were about two-fold higher in pediatric AML cells than in controls (P<0.0001). Furthermore, a close relationship was found between mtDNA copy number tertiles and the risk of pediatric AML. Intracellular ROS levels, mitochondrial mass, and mitochondrial membrane potentials were all elevated in pediatric AML. The frequency of the mtDNA 4,977-bp large deletion was significantly higher (P< 0.01) in pediatric AML cells, and pediatric AML patients harboring high amount of mtDNA 4,977-bp deletions showed shorter overall survival and event-free survival rates, albeit without statistical significance.

Conclusions

The present findings demonstrate an association between mitochondrial genome alterations and the risk of pediatric AML.

Investigation into length heteroplasmy in the mitochondrial DNA control region after treatment with bisulfite

We report on a method to analyze length heteroplasmy within the human mitochondrial genome in which there are polycytosine [poly(C)] stretches. These poly(C) tracts induce heteroplasmy with the resultant inherent problems of accurate sequence designations. In this study, 20 samples that exhibited length heteroplasmy due to variation in the C-tracts within hypervariable region I (HVI) were treated with bisulfite, and one or more cytosine bases in these C-tracts were converted randomly to uracil. This resulted in an accurate sequence designation for nearly all samples. The only exceptions in which the DNA sequence could still not be determined occurred when there was total conversion, or a lack of conversion, of the cytosine bases. Replicate tests on the same samples showed that individual cytosine bases were randomly converted to uracil. This simple method was useful for investigating length heteroplasmy due to 16189C and 310C transitions in the mitochondrial-DNA control region. It is valuable for medical and forensic investigations.

Peripheral blood mitochondrial DNA copy number, length heteroplasmy and breast cancer risk: a replication study

Oxidative stress has consistently been linked to breast carcinogenesis, and mitochondria play a significant role in regulating reactive oxygen species generation. In our previous study, we found that increased levels of mitochondrial DNA (mtDNA) copy number and the presence of mitochondrial length heteroplasmies in the hypervariable (HV) regions 1 and 2 (HV1 and HV2) in peripheral blood are associated with increased risk of breast cancer. In current study with 1000 breast cancer cases and 1000 healthy controls, we intended to replicate our previous findings. Overall, levels of mtDNA copy number were significantly higher in breast cancer cases than healthy controls (mean: 1.17 versus 0.94, P < 0.001). In the multivariate linear regression analysis, increased mtDNA copy number levels were associated with a 1.32-fold increased risk of breast cancer [adjusted odds ratio (OR) = 1.32, 95% confidence interval (CI) = 1.15-1.67]. Breast cancer cases were more likely to have HV1 and HV2 region length heteroplasmies than healthy controls (P < 0.001, respectively). The existence of HV1 and HV2 length heteroplasmies was associated with 2.01- and 1.63-folds increased risk of breast cancer (for HV1: OR = 2.01, 95% CI = 1.66-2.42; for HV2: OR = 1.63, 95% CI = 1.34-1.92). Additionally, joint effects among mtDNA copy number, HV1 and HV2 length heteroplasmies were observed. Our results are consistent with our previous findings and further support the roles of mtDNA copy number and mtDNA length heteroplasmies that may play in the development of breast cancer.

Mitochondrial DNA aberrations and pathophysiological implications in hematopoietic diseases, chronic inflammatory diseases, and cancers

Mitochondria are important intracellular organelles that produce energy for cellular development, differentiation, and growth. Mitochondrial DNA (mtDNA) presents a 10- to 20-fold higher susceptibility to genetic mutations owing to the lack of introns and histone proteins. The mtDNA repair system is relatively inefficient, rendering it vulnerable to reactive oxygen species (ROS) produced during ATP synthesis within the mitochondria, which can then target the mtDNA. Under conditions of chronic inflammation and excess stress, increased ROS production can overwhelm the antioxidant system, resulting in mtDNA damage. This paper reviews recent literature describing the pathophysiological implications of oxidative stress, mitochondrial dysfunction, and mitochondrial genome aberrations in aging hematopoietic stem cells, bone marrow failure syndromes, hematological malignancies, solid organ cancers, chronic inflammatory diseases, and other diseases caused by exposure to environmental hazards.

Accumulation of mutations over the complete mitochondrial genome in tobacco-related oral cancer from northeast India

Northeast India has one of the world's highest incidences of oral cancer and 90% of them are related to tobacco. We examined the complete mitochondrial genome to determine hot spot mutations in oral cancer. The complete mitochondrial genome was sequenced using PGM™ from 10 patients matched blood and tumour tissue. Overall, 26 somatic mutations were found of which nine mutations in d-loop and 17 mutations in the coding region. The mutations at nucleotide positions 16294, 16325 and 16463 in d-loop and 4136, 13542 and 13869 in coding region are probably an indication to be a hot spot mutation in oral cancer. The knowledge about role, patterns and timing of mitochondrial mutations may serve to be facilitating clinical applications and hot spot mutations may be helpful in assessing cancer risk in tumour.

Association of mitochondrial D-loop mutations with GSTM1 and GSTT1 polymorphisms in oral carcinoma: a case control study from northeast India

OBJECTIVES

Mitochondrial dysfunction is a hallmark of cancer cells. Tobacco consumption in various forms is one of the major risk factors for the development of oral squamous cell carcinoma which makes the mitochondrial DNA susceptible to damage by reactive oxygen species. The GSTT1 and GSTM1 members of the glutathione S-transferase multigene family are candidate carcinogen metabolizing genes. Here we determined the hot spot mutations in the D-loop region and revealing correlation if any, with clinical parameters, along with analysing the genetic polymorphism of GSTT1 and GSTM1 and its susceptibility towards oral cancer.

MATERIALS AND METHODS

To determine the hot spot mutations 25 matched tissue samples of OSCC patients with 25 control subjects were used for PCR and direct sequencing. Analysis for GSTM1 and GSTT1 gene polymorphism was done by multiplex PCR.

RESULTS

Several mutations were found within the D-loop region among which mutations at nt146, nt152 and nt196 are found to be hot spot (P<0.0001, P<0.0001 and P<0.001 respectively). A significant association was found between the numbers of D-loop mutation and GSTM1 (OR=2.03; 95% CI, 1.04-3.96, P=0.003), GSTT1 (OR=1.73; 95% CI, 1.10-2.71, P=0.0027) null genotypes respectively. We observed a significant correlation between the increased number of D-loop mutations with the advancement in tumour stage of the patients (P=0.009, r=0.48).

CONCLUSION

The association of null genotypes and mutations can be used as a possible biomarker for early detection and preventive measure of oral cancer for those habituated to tobacco consumption.

Leukocyte mitochondrial DNA alteration in systemic lupus erythematosus and its relevance to the susceptibility to lupus nephritis

The role of mitochondrial DNA (mtDNA) alterations in the pathophysiology of systemic lupus erythematosus (SLE) remains unclear. We investigated sequence variations in the D310 region and copy number change of mtDNA in 85 SLE patients and 45 normal subjects. Leukocyte DNA and RNA were extracted from leukocytes of the peripheral venous blood. The D310 sequence variations and copy number of mtDNA, and mRNA expression levels of mtDNA-encoded genes in leukocytes were determined by quantitative real-time polymerase chain reaction (Q-PCR) and PCR-based direct sequencing, respectively. We found that leukocyte mtDNA in SLE patients exhibited higher frequency of D310 heteroplasmy (69.4% vs. 48.9%, p = 0.022) and more D310 variants (2.2 vs. 1.7, p = 0.014) than those found in controls. Among normal controls and patients with low, medium or high SLE disease activity index (SLEDAI), an ever-increasing frequency of D310 heteroplasmy was observed (p = 0.021). Leukocyte mtDNA copy number tended to be low in patients of high SLEDAI group (p = 0.068), especially in those harboring mtDNA with D310 heteroplasmy (p = 0.020). Moreover, the mtDNA copy number was positively correlated with the mRNA level of mtDNA-encoded ND1 (NADH dehydrogenase subunit 1) (p = 0.041) and ATPase 6 (ATP synthase subunit 6) (p = 0.030) genes. Patients with more D310 variants were more susceptible to lupus nephritis (p = 0.035). Taken together, our findings suggest that decrease in the mtDNA copy number and increase in D310 heteroplasmy of mtDNA are related to the development and progression of SLE, and that the patients harboring more D310 variants of mtDNA are more susceptible to lupus nephritis.

Frequent occurrence of mitochondrial DNA mutations in Barrett's metaplasia without the presence of dysplasia

Background

Barrett's esophagus (BE) is one of the most common premalignant lesions and can progress to esophageal adenocarcinoma (EA). The numerous molecular events may play a role in the neoplastic transformation of Barrett’s mucosa such as the change of DNA ploidy, p53 mutation and alteration of adhesion molecules. However, the molecular mechanism of the progression of BE to EA remains unclear and most studies of mitochondrial DNA (mtDNA) mutations in BE have performed on BE with the presence of dysplasia.

Methods/Findings

Thus, the current study is to investigate new molecular events (Barrett’s esophageal tissue-specific-mtDNA alterations/instabilities) in mitochondrial genome and causative factors for their alterations using the corresponding adjacent normal mucosal tissue (NT) and tissue (BT) from 34 patients having Barrett’s metaplasia without the presence of dysplasia. Eighteen patients (53%) exhibited mtDNA mutations which were not found in adjacent NT. mtDNA copy number was about 3 times higher in BT than in adjacent NT. The activity of the mitochondrial respiratory chain enzyme complexes in tissues from Barrett’s metaplasia without the presence of dysplasia was impaired. Reactive oxygen species (ROS) level in BT was significantly higher than those in corresponding samples.

Conclusion/Significance

High ROS level in BT may contribute to the development of mtDNA mutations, which may play a crucial role in disease progression and tumorigenesis in BE.

High-frequency minisatellite instability of the mitochondrial genome in colorectal cancer tissue associated with clinicopathological values

Most studies of mitochondrial DNA (mtDNA) mutations in colorectal cancer have used case-control and case-database comparisons without searching their clinical relevance. This study was to investigate colorectal cancer tissue-specific mtDNA mutations from 54 matched colorectal cancer and adjacent normal tissues and then to evaluate their clinical values. This study focused on analyzing control region including mtDNA minisatellites and coding regions. Cancer tissue-specific mtDNA mutations were found in over half of the patients (59%). The patterns of mtDNA mutations were substitution only (13%), mtDNA minisatellite instability (mtMSI) (20%) and both mutations combined (26%). mtMSI in colorectal cancer was mainly occurred in the 303 polyC (35%) and 16184 poly C (19%) minisatellite. mtDNA copy number and hydrogen peroxide level were significantly increased in colorectal cancer tissue. The amount of mtDNA large deletions was significantly decreased in colorectal cancer tissue compared with those from matched normal mucosa (p = 0.03). The activity of the mitochondrial respiratory chain enzyme complexes I, II and III in colorectal cancer tissues was impaired. mtDNA haplogroup B4 might be closely associated with colorectal cancer risk. The patient group harboring cancer tissue-specific mtDNA mutations showed larger tumor sizes (p = 0.005) and more advanced TNM stages (p = 0.002). Thus, mtDNA mutations in colorectal cancer might be implicated in risk factors that induce poor outcomes and tumorigenesis.

Analysis of fluorescence in situ hybridization, mtDNA quantification, and mtDNA sequence for the detection of early bladder cancer

We designed this study to test the sensitivities of cytology, the nuclear matrix protein 22 (NMP22) assay, and fluorescence in situ hybridization (FISH) in the early detection of urothelial carcinoma, and to identify mtDNA alterations in urinary epithelial cells. We collected 41 urine samples and 26 corresponding peripheral blood samples from patients with clinically suspected urothelial carcinoma. The FISH and NMP22 assays detected 92.1% of the cancers, and cytology detected 60.5%. In the low-grade group, NMP22 and FISH analyses were more sensitive than cytology, but in the high-grade group, all three methods showed approximately 90% sensitivity. Overall, the FISH and NMP22, or FISH and cytology assays combined detected 97.4% of cancers, while cytology with NMP22 detected 92.1%. In the low-grade group, the sensitivity of the three methods combined was above 80%, but in high-grade group, the combined sensitivity was approximately 100%. In the mtDNA control region, we detected characteristic heteroplasmic mtDNA substitution mutations in 1 patient and a mtDNA length heteroplasmic mutation in 303 polyC or 16184 poly C in 20 patients. Overall, urothelial carcinoma-specific mtDNA mutations were observed in 20 of the 26 patients (76.9%). The average mtDNA copy numbers in urine samples and corresponding peripheral blood samples (83.45 +/- 60.36 and 39.0 +/- 24.38, respectively) (mean +/- standard deviation [SD]) differed significantly (P < 0.001). The mtDNA copy numbers in the urine samples from patients with high-grade and low-grade tumors (81.83 +/- 67.78 and 86.49 +/- 46.69, respectively) did not differ significantly (P = 0.589). In conclusion, the FISH assay showed the highest sensitivity for detecting low-grade urothelial carcinoma, and mtDNA copy numbers in urine samples were higher than those in the corresponding peripheral blood samples. The frequency of mtDNA mutations in the D-loop region in patients with cancer was approximately 80% in our study. This report further supports the significance of genetic alteration in urothelial carcinoma and the clinical utility of the FISH, mtDNA quantitation polymerase chain reaction, mtDNA sequencing, and capillary electrophoresis for this purpose.

Alterations of length heteroplasmy in mitochondrial DNA under various amplification conditions

There are several areas within mitochondrial DNA that show length heteroplasmy. If the heteroplasmy pattern is unique and consistent for each person, it may be used to support an interpretation of exclusion in identity testing. We investigated whether the length heteroplasmy pattern would be consistent under different amplification conditions. We also determined whether various amplification parameters would affect the homopolymeric cytosine stretches (C-stretch) in HV1. Monoclonal samples tended to be heteroplasmic after amplification. After several repetitions, C-stretch patterns of all samples were inconsistent even under the same amplification conditions. Increased PCR cycles and high template concentrations resulted in a more frequent heteroplasmic tendency. These amplification parameters seem to have little effect if samples are not long enough in C-stretch or total length of the segment from nt 16180 to nt 16193. It is suggested that the pattern of length heteroplasmy cannot be used as an additional polymorphic marker.

Alteration of mitochondrial DNA sequence and copy number in nasal polyp tissue

This study was designed to investigate the possibility that mtDNA mutations might arise in inflammatory or chronically damaged nasal polyp tissue from 23 patients. Thirteen patients (57%) displayed nasal polyp tissue-specific mtDNA mutations in the hypervariable segment of the control region and cytochrome b gene, which were not found in the corresponding blood cells and/or adjacent normal tissue. Nasal polyp tissue-specific length heteroplasmic mutations were also detected in nucleotide position (np) 303-315 homopolymeric poly C track (39%), np 514-523 CA repeats (17%) and np 16184-16193 poly C track (30%). The average mtDNA copy number was about three times higher in nasal polyp tissue than in the corresponding peripheral blood cells and adjacent non-polyp tissues. The level of reactive oxygen species (ROS) was significantly higher in the nasal polyp tissues compared to those from the corresponding samples. High level of ROS in nasal polyp tissue may contribute to development of mtDNA mutations, which may play a crucial role in the vicious cycle of pathophysiology of nasal polyps.

Differences in tissue distribution of HV2 length heteroplasmy in mitochondrial DNA between mothers and children

Sequence analysis of HV2 in mitochondrial DNA has been performed as a tool for forensic identification, in addition to that of HV1. HV2 contains length heteroplasmy, which shows high variability within an individual or in maternal relatives. In this study, we used cloning analysis and PCR direct sequencing to compare, between mothers and their children, HV2 length heteroplasmic profiles in different tissues. For two mother-child pairs, different types of variant distribution were observed by cloning analysis. In pair 1, length heteroplasmic patterns in most tissues were similar (predominantly 9 and 10Cs variants), but different length heteroplasmic levels, with shifts in predominant genotype, were observed for some hairs in both mother and child. In pair 2, genotype distribution was similar for all tissues, with a predominant 8Cs genotype, but varying in the proportion of minor component. The proportion of one minor length variant (9Cs) in blood from the child was significantly higher than that from the mother, but the proportions of minor components (7 and/or 9Cs) in other tissue samples decreased from mother to child. Moreover, we could confirm that sequence types of PCR products were reflected by the distribution of length variants, which were observed especially in high proportion, in cloning analysis. Our results reveal variable changes in length heteroplasmic level in various tissues between generations. Variability between tissues, especially among hairs, within an individual would result in complicated differences in genotype distribution between maternal generations, and correlate with longer length of Cs for predominant variants.