Germline mutation: de novo mutation in reproductive lineage cells

Next-generation sequencing (NGS) has been used to determine the reference sequences of model organisms. This allows us to identify mutations by the chromosome number and sequence position where the base sequence has been altered, independent of any phenotypic alteration. Because the re-sequencing method by NGS covers all of the genome, it enables detection of the small number of spontaneous de novo germline mutations that occur in the reproductive lineage. The spontaneous mutation rate varies depending on the environment; for example, it increases when 8-oxoguanine accumulates. If the mutation rate (per replication) is greater than 1/genome size (2n), at least one mutation would generally occur in each cell division on average, producing cells with a different genome from the parent cell. Organisms with larger genomes and more divisions by cells in the reproductive lineage are expected to show higher mutation rates per generation, if the mutation rate per replication is constant among species. The accumulation of mutations that arose in the genome of ancestor cells has resulted in heterogeneity and diversity among extant species. In this sense, the ability to produce mutations in cells of the reproductive lineage can be considered as a key feature of organisms, even if mutations also present an unavoidable risk.

The Aging Mitochondria

Mitochondrial dysfunction is a central event in many pathologies and contributes as well to age-related processes. However, distinguishing between primary mitochondrial dysfunction driving aging and a secondary mitochondrial impairment resulting from other cell alterations remains challenging. Indeed, even though mitochondria undeniably play a crucial role in aging pathways at the cellular and organismal level, the original hypothesis in which mitochondrial dysfunction and production of free radicals represent the main driving force of cell degeneration has been strongly challenged. In this review, we will first describe mitochondrial dysfunctions observed in aged tissue, and how these features have been linked to mitochondrial reactive oxygen species (ROS)–mediated cell damage and mitochondrial DNA (mtDNA) mutations. We will also discuss the clues that led to consider mitochondria as the starting point in the aging process, and how recent research has showed that the mitochondria aging axis represents instead a more complex and multifactorial signaling pathway. New working hypothesis will be also presented in which mitochondria are considered at the center of a complex web of cell dysfunctions that eventually leads to cell senescence and death.

Increased Total mtDNA Copy Number Cures Male Infertility Despite Unaltered mtDNA Mutation Load

Mutations of mtDNA cause mitochondrial diseases and are implicated in age-associated diseases and aging. Pathogenic mtDNA mutations are often present in a fraction of all mtDNA copies, and it has been widely debated whether the proportion of mutant genomes or the absolute number of wild-type molecules determines if oxidative phosphorylation (OXPHOS) will be impaired. Here, we have studied the male infertility phenotype of mtDNA mutator mice and demonstrate that decreasing mtDNA copy number worsens mitochondrial aberrations of spermatocytes and spermatids in testes, whereas an increase in mtDNA copy number rescues the fertility phenotype and normalizes testes morphology as well as spermatocyte proteome changes. The restoration of testes function occurs in spite of unaltered total mtDNA mutation load. We thus demonstrate that increased copy number of mtDNA can efficiently ameliorate a severe disease phenotype caused by mtDNA mutations, which has important implications for developing future strategies for treatment of mitochondrial dysfunction.

Human stem cell aging: do mitochondrial DNA mutations have a causal role?

A decline in the replicative and regenerative capacity of adult stem cell populations is a major contributor to the aging process. Mitochondrial DNA (mtDNA) mutations clonally expand with age in human stem cell compartments including the colon, small intestine, and stomach, and result in respiratory chain deficiency. Studies in a mouse model with high levels of mtDNA mutations due to a defect in the proofreading domain of the mtDNA polymerase γ (mtDNA mutator mice) have established causal relationships between the accumulation of mtDNA point mutations, stem cell dysfunction, and premature aging. These mtDNA mutator mice have also highlighted that the consequences of mtDNA mutations upon stem cells vary depending on the tissue. In this review, we present evidence that these studies in mice are relevant to normal human stem cell aging and we explore different hypotheses to explain the tissue-specific consequences of mtDNA mutations. In addition, we emphasize the need for a comprehensive analysis of mtDNA mutations and their effects on cellular function in different aging human stem cell populations.