Association of mitochondrial DNA copy number with metabolic syndrome and type 2 diabetes in 14 176 individuals

BACKGROUND

Mitochondria play an important role in cellular metabolism, and their dysfunction is postulated to be involved in metabolic disturbances. Mitochondrial DNA is present in multiple copies per cell. The quantification of mitochondrial DNA copy number (mtDNA-CN) might be used to assess mitochondrial dysfunction.

OBJECTIVES

We aimed to investigate the cross-sectional association of mtDNA-CN with type 2 diabetes and the potential mediating role of metabolic syndrome.

METHODS

We examined 4812 patients from the German Chronic Kidney Disease (GCKD) study and 9364 individuals from the Cooperative Health Research in South Tyrol (CHRIS) study. MtDNA-CN was measured in whole blood using a plasmid-normalized qPCR-based assay.

RESULTS

In both studies, mtDNA-CN showed a significant correlation with most metabolic syndrome parameters: mtDNA-CN decreased with increasing number of metabolic syndrome components. Furthermore, individuals with low mtDNA-CN had significantly higher odds of metabolic syndrome (OR = 1.025; 95% CI = 1.011-1.039, P = 3.19 × 10-4 , for each decrease of 10 mtDNA copies) and type 2 diabetes (OR = 1.027; 95% CI = 1.012-1.041; P = 2.84 × 10-4 ) in a model adjusted for age, sex, smoking and kidney function in the meta-analysis of both studies. Mediation analysis revealed that the association of mtDNA-CN with type 2 diabetes was mainly mediated by waist circumference in the GCKD study (66%) and by several metabolic syndrome parameters, especially body mass index and triglycerides, in the CHRIS study (41%).

CONCLUSIONS

Our data show an inverse association of mtDNA-CN with higher risk of metabolic syndrome and type 2 diabetes. A major part of the total effect of mtDNA-CN on type 2 diabetes is mediated by obesity parameters.

Contamination detection in sequencing studies using the mitochondrial phylogeny

Within-species contamination is a major issue in sequencing studies, especially for mitochondrial studies. Contamination can be detected by analyzing the nuclear genome or by inspecting polymorphic sites in the mitochondrial genome (mtDNA). Existing methods using the nuclear genome are computationally expensive, and no appropriate tool for detecting sample contamination in large-scale mtDNA data sets is available. Here we present haplocheck, a tool that requires only the mtDNA to detect contamination in both targeted mitochondrial and whole-genome sequencing studies. Our in silico simulations and amplicon mixture experiments indicate that haplocheck detects mtDNA contamination accurately and is independent of the phylogenetic distance within a sample mixture. By applying haplocheck to The 1000 Genomes Project Consortium data, we further evaluate the application of haplocheck as a fast proxy tool for nDNA-based contamination detection using the mtDNA and identify the mitochondrial copy number within a mixture as a critical component for the overall accuracy. The haplocheck tool is available both as a command-line tool and as a cloud web service producing interactive reports that facilitates the navigation through the phylogeny of contaminated samples.

Analyzing Low-Level mtDNA Heteroplasmy-Pitfalls and Challenges from Bench to Benchmarking

Massive parallel sequencing technologies are promising a highly sensitive detection of low-level mutations, especially in mitochondrial DNA (mtDNA) studies. However, processes from DNA extraction and library construction to bioinformatic analysis include several varying tasks. Further, there is no validated recommendation for the comprehensive procedure. In this study, we examined potential pitfalls on the sequencing results based on two-person mtDNA mixtures. Therefore, we compared three DNA polymerases, six different variant callers in five mixtures between 50% and 0.5% variant allele frequencies generated with two different amplification protocols. In total, 48 samples were sequenced on Illumina MiSeq. Low-level variant calling at the 1% variant level and below was performed by comparing trimming and PCR duplicate removal as well as six different variant callers. The results indicate that sensitivity, specificity, and precision highly depend on the investigated polymerase but also vary based on the analysis tools. Our data highlight the advantage of prior standardization and validation of the individual laboratory setup with a DNA mixture model. Finally, we provide an artificial heteroplasmy benchmark dataset that can help improve somatic variant callers or pipelines, which may be of great interest for research related to cancer and aging.

Profiling of Mitochondrial DNA Heteroplasmy in a Prospective Oral Squamous Cell Carcinoma Study

While a shift in energy metabolism is essential to cancers, the knowledge about the involvement of the mitochondrial genome in tumorigenesis and progression in oral squamous cell carcinoma (OSCC) is still very limited. In this study, we evaluated 37 OSCC tumors and the corresponding benign mucosa tissue pairs by deep sequencing of the complete mitochondrial DNA (mtDNA). After extensive quality control, we identified 287 variants, 137 in tumor and 150 in benign samples exceeding the 1% threshold. Variant heteroplasmy levels were significantly increased in cancer compared to benign tissues (p = 0.0002). Furthermore, pairwise high heteroplasmy frequency difference variants (∆HF% > 20) with potential functional impact were increased in the cancer tissues (p = 0.024). Fourteen mutations were identified in the protein-coding region, out of which thirteen were detected in cancer and only one in benign tissue. After eight years of follow-up, the risk of mortality was higher for patients who harbored at least one ∆HF% > 20 variant in mtDNA protein-coding regions relative to those with no mutations (HR = 4.6, (95%CI = 1.3-17); p = 0.019 in primary tumor carriers). Haplogroup affiliation showed an impact on survival time, which however needs confirmation in a larger study. In conclusion, we observed a significantly higher accumulation of somatic mutations in the cancer tissues associated with a worse prognosis.

Mitochondrial DNA copy number is associated with mortality and infections in a large cohort of patients with chronic kidney disease

Damage of mitochondrial DNA (mtDNA) with reduction in copy number has been proposed as a biomarker for mitochondrial dysfunction and oxidative stress. Chronic kidney disease (CKD) is associated with increased mortality and risk of cardiovascular disease, but the underlying mechanisms remain incompletely understood. Here we investigated the prognostic role of mtDNA copy number for cause-specific mortality in 4812 patients from the German Chronic Kidney Disease study, an ongoing prospective observational national cohort study of patients with CKD stage G3 and A1-3 or G1-2 with overt proteinuria (A3) at enrollment. MtDNA was quantified in whole blood using a plasmid-normalized PCR-based assay. At baseline, 1235 patients had prevalent cardiovascular disease. These patients had a significantly lower mtDNA copy number than patients without cardiovascular disease (fully-adjusted model: odds ratio 1.03, 95% confidence interval [CI] 1.01-1.05 per 10 mtDNA copies decrease). After four years of follow-up, we observed a significant inverse association between mtDNA copy number and all-cause mortality, adjusted for kidney function and cardiovascular disease risk factors (hazard ratio 1.37, 95% CI 1.09-1.73 for quartile 1 compared to quartiles 2-4). When grouped by causes of death, estimates pointed in the same direction for all causes but in a fully-adjusted model decreased copy numbers were significantly lower only in infection-related death (hazard ratio 1.82, 95% CI 1.08-3.08). A similar association was observed for hospitalizations due to infections in 644 patients (hazard ratio 1.19, 95% CI 1.00-1.42 in the fully-adjusted model). Thus, our data support a role of mitochondrial dysfunction in increased cardiovascular disease and mortality risks as well as susceptibility to infections in patients with CKD.

Validation of Next-Generation Sequencing of Entire Mitochondrial Genomes and the Diversity of Mitochondrial DNA Mutations in Oral Squamous Cell Carcinoma

BACKGROUND

Oral squamous cell carcinoma (OSCC) is mainly caused by smoking and alcohol abuse and shows a five-year survival rate of ~50%. We aimed to explore the variation of somatic mitochondrial DNA (mtDNA) mutations in primary oral tumors, recurrences and metastases.

METHODS

We performed an in-depth validation of mtDNA next-generation sequencing (NGS) on an Illumina HiSeq 2500 platform for its application to cancer tissues, with the goal to detect low-level heteroplasmies and to avoid artifacts. Therefore we genotyped the mitochondrial genome (16.6 kb) from 85 tissue samples (tumors, recurrences, resection edges, metastases and blood) collected from 28 prospectively recruited OSCC patients applying both Sanger sequencing and high-coverage NGS (~35,000 reads per base).

RESULTS

We observed a strong correlation between Sanger sequencing and NGS in estimating the mixture ratio of heteroplasmies (r = 0.99; p<0.001). Non-synonymous heteroplasmic variants were enriched among cancerous tissues. The proportions of somatic and inherited variants in a given gene region were strongly correlated (r = 0.85; p<0.001). Half of the patients shared mutations between benign and cancerous tissue samples. Low level heteroplasmies (<10%) were more frequent in benign samples compared to tumor samples, where heteroplasmies >10% were predominant. Four out of six patients who developed a local tumor recurrence showed mutations in the recurrence that had also been observed in the primary tumor. Three out of five patients, who had tumor metastases in the lymph nodes of their necks, shared mtDNA mutations between primary tumors and lymph node metastases. The percentage of mutation heteroplasmy increased from the primary tumor to lymph node metastases.

CONCLUSIONS

We conclude that Sanger sequencing is valid for heteroplasmy quantification for heteroplasmies ≥10% and that NGS is capable of reliably detecting and quantifying heteroplasmies down to the 1%-level. The finding of shared mutations between primary tumors, recurrences and metastasis indicates a clonal origin of malignant cells in oral cancer.

WITHDRAWN: Erratum to "Evaluation of next generation mtGenome sequencing using the Ion Torrent Personal Genome Machine (PGM)" [Forensic Sci. Int.: Genet. 7 (2013) 543-549]

The Publisher regrets that this article is an accidental duplication of an article that has already been published, http://dx.doi.org/10.1016/j.fsigen.2013.09.007. The duplicate article has therefore been withdrawn. The full Elsevier Policy on Article Withdrawal can be found at http://www.elsevier.com/locate/withdrawalpolicy.

Reprint of: Evaluation of next generation mtGenome sequencing using the Ion Torrent Personal Genome Machine (PGM)

Insights into the human mitochondrial phylogeny have been primarily achieved by sequencing full mitochondrial genomes (mtGenomes). In forensic genetics (partial) mtGenome information can be used to assign haplotypes to their phylogenetic backgrounds, which may, in turn, have characteristic geographic distributions that would offer useful information in a forensic case. In addition and perhaps even more relevant in the forensic context, haplogroup-specific patterns of mutations form the basis for quality control of mtDNA sequences. The current method for establishing (partial) mtDNA haplotypes is Sanger-type sequencing (STS), which is laborious, time-consuming, and expensive. With the emergence of Next Generation Sequencing (NGS) technologies, the body of available mtDNA data can potentially be extended much more quickly and cost-efficiently. Customized chemistries, laboratory workflows and data analysis packages could support the community and increase the utility of mtDNA analysis in forensics. We have evaluated the performance of mtGenome sequencing using the Personal Genome Machine (PGM) and compared the resulting haplotypes directly with conventional Sanger-type sequencing. A total of 64mtGenomes (>1 million bases) were established that yielded high concordance with the corresponding STS haplotypes (<0.02% differences). About two-thirds of the differences were observed in or around homopolymeric sequence stretches. In addition, the sequence alignment algorithm employed to align NGS reads played a significant role in the analysis of the data and the resulting mtDNA haplotypes. Further development of alignment software would be desirable to facilitate the application of NGS in mtDNA forensic genetics.

MtDNA diversity of Ghana: a forensic and phylogeographic view

West Africa is characterized by a migration history spanning more than 150,000 years. Climate changes but also political circumstances were responsible for several early but also recent population movements that shaped the West African mitochondrial landscape. The aim of the study was to establish a Ghanaian mtDNA dataset for forensic purposes and to investigate the diversity of the Ghanaian population sample with respect to surrounding populations. We sequenced full mitochondrial control regions of 193 Akan people from Ghana and excluded two apparently close maternally related individuals due to preceding kinship testing. The remaining dataset comprising 191 sequences was applied as etalon for quasi-median network analysis and was subsequently combined with 99 additional control region sequences from surrounding West African countries. All sequences were incorporated into the EMPOP database enriching the severely underrepresented African mtDNA pool. For phylogeographic considerations, the Ghanaian haplotypes were compared to those of 19 neighboring populations comprising a total number of 6198 HVS1 haplotypes. We found extensive genetic admixture between the Ghanaian lineages and those from adjacent populations diminishing with geographical distance. The extent of genetic admixture reflects the long but also recent history of migration waves within West Africa mainly caused by changing environmental conditions. Also, evidence for potential socio-economical influences such as trade routes is provided by the occurrence of U6b and U6d sequences found in Dubai but also in Tunisia leading to the African West Coast via Mauritania and Senegal but also via Niger, Nigeria to Cameroon.

Accumulation of mutations over the entire mitochondrial genome of breast cancer cells obtained by tissue microdissection

The occurrence of heteroplasmy and mixtures is technically challenging for the analysis of mitochondrial DNA. More than that, observed mutations need to be carefully interpreted in the light of the phylogeny as mitochondrial DNA is a uniparental marker reflecting human evolution. Earlier attempts to explain the role of mtDNA in cancerous tissues led to substantial confusion in medical genetics mainly due to the presentation of low sequence data quality and misinterpretation of mutations representing a particular haplogroup background rather than being cancer-specific. The focus of this study is to characterize the extent and level of mutations in breast cancer samples obtained by tissue microdissection by application of an evaluated full mtDNA genome sequencing protocol. We amplified and sequenced the complete mitochondrial genomes of microdissected breast cancer cells of 15 patients and compared the results to those obtained from paired non-cancerous breast tissue derived from the same patients. We observed differences in the heteroplasmic states of substitutions between cancerous and normal cells, one of which was affecting a position that has been previously reported in lung cancer and another one that has been identified in 16 epithelial ovarian tumors, possibly indicating functional relevance. In the coding region, we found full transitions in two cancerous mitochondrial genomes and 12 heteroplasmic substitutions as compared to the non-cancerous breast cells. We identified somatic mutations over the entire mtDNA of human breast cancer cells potentially impairing the mitochondrial OXPHOS system.

Accumulation of mutations over the entire mitochondrial genome of breast cancer cells obtained by tissue microdissection

The occurrence of heteroplasmy and mixtures is technically challenging for the analysis of mitochondrial DNA. More than that, observed mutations need to be carefully interpreted in the light of the phylogeny as mitochondrial DNA is a uniparental marker reflecting human evolution. Earlier attempts to explain the role of mtDNA in cancerous tissues led to substantial confusion in medical genetics mainly due to the presentation of low sequence data quality and misinterpretation of mutations representing a particular haplogroup background rather than being cancer-specific. The focus of this study is to characterize the extent and level of mutations in breast cancer samples obtained by tissue microdissection by application of an evaluated full mtDNA genome sequencing protocol. We amplified and sequenced the complete mitochondrial genomes of microdissected breast cancer cells of 15 patients and compared the results to those obtained from paired non-cancerous breast tissue derived from the same patients. We observed differences in the heteroplasmic states of substitutions between cancerous and normal cells, one of which was affecting a position that has been previously reported in lung cancer and another one that has been identified in 16 epithelial ovarian tumors, possibly indicating functional relevance. In the coding region, we found full transitions in two cancerous mitochondrial genomes and 12 heteroplasmic substitutions as compared to the non-cancerous breast cells. We identified somatic mutations over the entire mtDNA of human breast cancer cells potentially impairing the mitochondrial OXPHOS system.

Sequencing strategy for the whole mitochondrial genome resulting in high quality sequences

Background

It has been demonstrated that a reliable and fail-safe sequencing strategy is mandatory for high-quality analysis of mitochondrial (mt) DNA, as the sequencing and base-calling process is prone to error. Here, we present a high quality, reliable and easy handling manual procedure for the sequencing of full mt genomes that is also appropriate for laboratories where fully automated processes are not available.

Results

We amplified whole mitochondrial genomes as two overlapping PCR-fragments comprising each about 8500 bases in length. We developed a set of 96 primers that can be applied to a (manual) 96 well-based technology, which resulted in at least double strand sequence coverage of the entire coding region (codR).

Conclusion

This elaborated sequencing strategy is straightforward and allows for an unambiguous sequence analysis and interpretation including sometimes challenging phenomena such as point and length heteroplasmy that are relevant for the investigation of forensic and clinical samples.