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      A Critical Reassessment of the Role of Mitochondria in Tumorigenesis

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          Abstract

          Background

          Mitochondrial DNA (mtDNA) is being analyzed by an increasing number of laboratories in order to investigate its potential role as an active marker of tumorigenesis in various types of cancer. Here we question the conclusions drawn in most of these investigations, especially those published in high-rank cancer research journals, under the evidence that a significant number of these medical mtDNA studies are based on obviously flawed sequencing results.

          Methods and Findings

          In our analyses, we take a phylogenetic approach and employ thorough database searches, which together have proven successful for detecting erroneous sequences in the fields of human population genetics and forensics. Apart from conceptual problems concerning the interpretation of mtDNA variation in tumorigenesis, in most cases, blocks of seemingly somatic mutations clearly point to contamination or sample mix-up and, therefore, have nothing to do with tumorigenesis.

          Conclusion

          The role of mitochondria in tumorigenesis remains unclarified. Our findings of laboratory errors in many contributions would represent only the tip of the iceberg since most published studies do not provide the raw sequence data for inspection, thus hindering a posteriori evaluation of the results. There is no precedent for such a concatenation of errors and misconceptions affecting a whole subfield of medical research.

          Abstract

          The role of mitochondria in tumorigenesis remains unclear; in this paper Salas and colleagues raise concerns over many published studies

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          Most cited references51

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          Mitochondrial genome variation and the origin of modern humans.

          The analysis of mitochondrial DNA (mtDNA) has been a potent tool in our understanding of human evolution, owing to characteristics such as high copy number, apparent lack of recombination, high substitution rate and maternal mode of inheritance. However, almost all studies of human evolution based on mtDNA sequencing have been confined to the control region, which constitutes less than 7% of the mitochondrial genome. These studies are complicated by the extreme variation in substitution rate between sites, and the consequence of parallel mutations causing difficulties in the estimation of genetic distance and making phylogenetic inferences questionable. Most comprehensive studies of the human mitochondrial molecule have been carried out through restriction-fragment length polymorphism analysis, providing data that are ill suited to estimations of mutation rate and therefore the timing of evolutionary events. Here, to improve the information obtained from the mitochondrial molecule for studies of human evolution, we describe the global mtDNA diversity in humans based on analyses of the complete mtDNA sequence of 53 humans of diverse origins. Our mtDNA data, in comparison with those of a parallel study of the Xq13.3 region in the same individuals, provide a concurrent view on human evolution with respect to the age of modern humans.
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            Ancient DNA: do it right or not at all.

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              Somatic mutations of the mitochondrial genome in human colorectal tumours.

              Alterations of oxidative phosphorylation in tumour cells were originally believed to have a causative role in cancerous growth. More recently, mitochondria have again received attention with regards to neoplasia, largely because of their role in apoptosis and other aspects of tumour biology. The mitochondrial genome is particularly susceptible to mutations because of the high level of reactive oxygen species (ROS) generation in this organelle, coupled with a low level of DNA repair. However, no detailed analysis of mitochondrial DNA in human tumours has yet been reported. In this study, we analysed the complete mtDNA genome of ten human colorectal cancer cell lines by sequencing and found mutations in seven (70%). The majority of mutations were transitions at purines, consistent with an ROS-related derivation. The mutations were somatic, and those evaluated occurred in the primary tumour from which the cell line was derived. Most of the mutations were homoplasmic, indicating that the mutant genome was dominant at the intracellular and intercellular levels. We showed that mitochondria can rapidly become homogeneous in colorectal cancer cells using cell fusions. These findings provide the first examples of homoplasmic mutations in the mtDNA of tumour cells and have potential implications for the abnormal metabolic and apoptotic processes in cancer.
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                Author and article information

                Contributors
                Role: Academic Editor
                Journal
                PLoS Med
                pmed
                PLoS Medicine
                Public Library of Science (San Francisco, USA )
                1549-1277
                1549-1676
                November 2005
                4 October 2005
                : 2
                : 11
                : e296
                Affiliations
                [1] 1Unidade de Xenética, Instituto de Medicina Legal, Facultade de Medicina, Universidad de Santiago de Compostela, Galicia, Spain,
                [2] 2Centro Nacional de Genotipado (CeGen), Hospital Clínico Universitario, Santiago de Compostela, Galicia, Spain,
                [3] 3Key Laboratory of Cellular and Molecular Evolution, Kunming Institute of Zoology, Chinese Academy of Sciences, Kunming, Yunnan, China,
                [4] 4Department of Statistics, University of Glasgow, Glasgow, Scotland, United Kingdom,
                [5] 5Fundación Pública Galega de Medicina Xenómica (FPGMX), Hospital Clínico Universitario, Universidad de Santiago de Compostela, Galicia, Spain,
                [6] 6Department of Mathematics, University of Hamburg, Hamburg, Germany
                University of Newcastle upon Tyne United Kingdom
                Author notes
                *To whom correspondence should be addressed. E-mail: apimlase@ 123456usc.es

                Competing Interests: The authors have declared that no competing interests exist.

                Author Contributions: AS, YGY, VM, AV, AC, and HJB designed the study, analyzed the data, and contributed to writing the paper.

                Article
                10.1371/journal.pmed.0020296
                1240051
                16187796
                2c8780c2-0bc4-406c-92ab-21ac96e1089f
                Copyright: © 2005 Salas et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
                History
                : 2 May 2005
                : 25 July 2005
                Categories
                Research Article
                Cancer Biology
                Genetics/Genomics/Gene Therapy
                Oncology
                Oncology

                Medicine
                Medicine

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