Home Search Collections Journals About Contact us My IOPscience Mitochondria and the evolutionary roots of cancer This article has been downloaded from IOPscience. Please scroll down to see the full text article. 2013 Phys. Biol. 10 026008 (http://iopscience.iop.org/1478-3975/10/2/026008) View the table of contents for this issue, or go to the journal homepage for more Download details: IP Address: 200.86.80.26 The article was downloaded on 24/03/2013 at 16:22 Please note that terms and conditions apply. OPEN ACCESS IOP PUBLISHING PHYSICAL BIOLOGY Phys. Biol. 10 (2013) 026008 (8pp) doi:10.1088/1478-3975/10/2/026008 Mitochondria and the evolutionary roots of cancer Alfonso F Davila1,3 and Pedro Zamorano2 1 SETI Institute. 189 N. Bernardo Avenue, Suite 100 Mountain View, CA 94043, USA 2 Universidad de Antofagasta, Av Angamos 601, Antofagasta, Chile E-mail: [email protected] Received 8 August 2012 Accepted for publication 14 February 2013 Published 22 March 2013 Online at stacks.iop.org/PhysBio/10/026008 Abstract Cancer disease is inherent to, and widespread among, metazoans. Yet, some of the hallmarks of cancer such as uncontrolled cell proliferation, lack of apoptosis, hypoxia, fermentative metabolism and free cell motility (metastasis) are akin to a prokaryotic lifestyle, suggesting a link between cancer disease and evolution. In this hypothesis paper, we propose that cancer cells represent a phenotypic reversion to the earliest stage of eukaryotic evolution. This reversion is triggered by the dysregulation of the mitochondria due to cumulative oxidative damage to mitochondrial and nuclear DNA. As a result, the phenotype of normal, differentiated cells gradually reverts to the phenotype of a facultative anaerobic, heterotrophic cell optimized for survival and proliferation in hypoxic environments. This phenotype matches the phenotype of the last eukaryotic common ancestor (LECA) that resulted from the endosymbiosis between an α-proteobacteria (which later became the mitochondria) and an archaebacteria. As such, the evolution of cancer within one individual can be viewed as a recapitulation of the evolution of the eukaryotic cell from fully differentiated cells to LECA. This evolutionary model of cancer is compatible with the current understanding of the disease, and explains the evolutionary basis for most of the hallmarks of cancer, as well as the link between the disease and aging. It could also open new avenues for treatment directed at reestablishing the synergy between the mitochondria and the cancerous cell. 1. Introduction trademarks of evolution. Cancer disease is widespread among animals (e.g. Scharrer and Lochead 1950, Black and Baumann Exploring the evolutionary nature of cancer might impact 1991, Nagy et al 2007), and it appears to be an evolutionary our understanding of the disease and the development of phenomenon deeply rooted in the metazoan branch of the treatments, and could also reveal new fundamental aspects of tree of life. Oncogenes have been traced all the way back the evolution of metazoans (Merlo et al 2006,Hartlet al 2010, to some of the early metazoans (Hartl et al 2010), and the Srivastava et al 2010, Davies and Lineveawer 2011). Cancer basic principles that define cancer are strikingly antagonistic cells are dysfunctional, but they are not weakened or impaired. to the basic principles that define multicellularity (Srivastava In fact, cancer cells are highly proliferating and invasive, and et al 2010). If cancer has indeed evolutionary roots, then the difficult to eradicate. Hence, while cancer might ultimately question arises: how deep are those roots? cause the death of an organism, cancer cells appear to be Mitochondrial dysregulation is one of the most common successful, self-sufficient, competent and highly adapted, the hallmarks of cancer, and it was given a central role in earlier models of cancer disease (Warburg et al 1927,Warburg 3 Author to whom any correspondence should be addressed. 1956). This role is being recognized again in more recent studies of hypoxia, oxidative stress and metabolism in tumour Content from this work may be used under the terms of the cells (Gogvadze et al 2008). Mitochondria are organelles that Creative Commons Attribution-NonCommercial-ShareAlike 3.0 licence. Any further distribution of this work must maintain attribution to evolved from the endosymbiosis of two unicellular organisms, the author(s) and the title of the work, journal citation and DOI. in one of the defining moments in the evolution of eukaryotes. 1478-3975/13/026008+08$33.00 1 © 2013 IOP Publishing Ltd Printed in the UK & the USA Phys. Biol. 10 (2013) 026008 A F Davila and P Zamorano The link between cancer, mitochondria and the evolution of been acquired before the divergence of all modern groups. eukaryotes is the basis for our hypothesis that cancer cells OXPHOS is common to all aerobic eukaryotes, and it occurs revert to a phenotype akin to the last eukaryotic common exclusively within the mitochondria. Notable exceptions are ancestor (LECA). the anaerobic eukaryotes, which contain modified versions of mitochondria—the hydrogenosomes and the mitosomes. 2. The endosymbiotic nature of the eukaryotic cell Hydrogenosomes are involved in anaerobic metabolism and produce hydrogen, acetate, carbon dioxide and ATP, whereas The endosymbiotic hypothesis states that the eukaryotic the function of mitosomes is still a matter of debate, and cell evolved from the symbiotic merging of two or more appears to be related to the biosynthesis of cytosolic Fe–S prokaryotic cells (Margulis 1970, Martin et al 2001). While proteins. These organisms were once thought to represent the exact sequence and the timing of the endosymbiotic the earliest descendents of aerobic eukaryotes. However, event are unknown, the universality of the mitochondria the realization that both hydrogenosomes and mitosomes among eukaryotes (with the exception of some anaerobic are modified mitochondria, and the distribution of anaerobic eukaryotes, see below) suggests that it was one of the early eukaryotes in all main groups of eukaryotes, but not necessarily organelles acquired by LECA. Phylogenetic reconstructions at the bottom of the eukaryotic tree (see Muller¨ et al based on mitochondrial DNA (mtDNA), and metabolic and 2012), suggest instead that these groups also evolved from a evolutionary considerations have traced the mitochondria back mitochondrion-containing ancestor common to all eukaryotes, to an ancestral α-proteobacteria (Gray 1992,Grayet al 1999, but do not necessarily represent ancient eukaryotic lineages. Martin et al 2001), while the host cell appears to have been an Hence, while still fragmented, the picture that emerges archaebacteria (e.g. Cotton and McInerney 2010). regarding the earliest eukaryotic cells is as follows: It has been estimated that the initial endosymbiotic approximately 2 Gyr ago a unicellular organism originated event leading to the mitochondria initiated about 2 Gyr ago from the symbiosis of two anaerobic cells (an α- (e.g. Sicheritz-Ponten´ et al 1998). This timing overlaps with proteobacteria endosymbiont and an archaebacteria host). The the main period of oxygenation of the Earth’s atmosphere, α-proteobacteria was capable of respiring oxygen and likely which began 2.4 billion years ago with a rapid rise in O2 levels also of fermentative metabolism (Gabaldon´ and Huynen 2003). up to 1% pO2 during the Proterozoic, and a second rise up The original metabolism of the archeabacterial host is still to 21% pO2 in the Phanerozoic, approximately 600 million not understood (Kurland and Anderson 2000; but see Martin years ago (Fike et al 2006, Canfield et al 2007). Based on and Muller¨ 1998), but it must have been replaced at an early geochemical evidence oceans must have been anoxic below the stage by the glycolytic pathway of the eubacteria. As a result, photic zone between 2.4 and 1.8 Gyr ago (Arnold et al 2004, a symbiotic organism capable of cytosolic glycolysis and Lyons and Reinhard 2009), and euxinic (anoxic and sulfidic) oxygen respiration emerged. This symbiotic organism was between 1.8 Gyr and 750 Ma (Poulton et al 2004). Hence, likely free living and motile, and occupied anoxic or hypoxic the symbiosis of the α-proteobacteria and the archaebacteria niches in the water column or in sediments. This likely was likely took place in the anoxic and sulfidic ocean, but allowed the common ancestor to modern eukaryotes (LECA), and the new organism to survive in an increasingly oxygenated subsequent evolution lead to its divergence into the modern environment, owing to the capability of the α-proteobacteria eukaryotic groups. to utilize oxygen by means of its respiratory chain. In fact, the acquisition of new metabolic pathways 3. Life after LECA by the host was one of the key benefits resulting from the endosymbiotic event. Phylogenetic reconstructions of Evidence shows that the final assimilation of the genes and enzymes involved in the metabolism of modern α-proteobacteria endosymbiont into an organelle involved a eukaryotes indicate that both the oxidative pathway and the massive reduction of its genetic code, and the assignments glycolytic pathway were inherited from the eubacteria, not of new physiological functions by the host (cf Kurland and the archaebacteria (Fothergill-Gilmore and Michels 1993, Anderson 2000). As a result, the genome of present-day mito- Muller¨ et al 2012). Key enzymes in the respiratory chain chondria little resembles that of its α-proteobacteria ancestor. of mitochondria responsible for oxidative phosphorylation Clear examples are the highly modified hydrogenosomes (OXPHOS) are codified by mtDNA, and the same respiratory and mitosomes, but even the genotype and the phenotype of chain complex is found in modern bacteria. Therefore, modern mitochondria have been substantially modified. As OXPHOS in eukaryotes is clearly a trait inherited from thoroughly explained by Kurland and Anderson (2000), the the α-proteobacteria endosymbiont. Genes and enzymes of evolution of the mitochondria from an aerobic prokaryote to the glycolytic pathway in modern eukaryotes also have an organelle is better understood as the evolution of its pro- homologues among the eubacteria, but not among the teome.