Can paternal mtDNA be inherited?

Evolutionary perspectives on the links between mitochondrial genotype and disease phenotype

BACKGROUND

Disorders of the mitochondrial respiratory chain are heterogeneous in their symptoms and underlying genetics. Simple links between candidate mutations and expression of disease phenotype typically do not exist. It thus remains unclear how the genetic variation in the mitochondrial genome contributes to the phenotypic expression of complex traits and disease phenotypes.

SCOPE OF REVIEW

I summarize the basic genetic processes known to underpin mitochondrial disease. I highlight other plausible processes, drawn from the evolutionary biological literature, whose contribution to mitochondrial disease expression remains largely empirically unexplored. I highlight recent advances to the field, and discuss common-ground and -goals shared by researchers across medical and evolutionary domains.

MAJOR CONCLUSIONS

Mitochondrial genetic variance is linked to phenotypic variance across a variety of traits (e.g. reproductive function, life expectancy) fundamental to the upkeep of good health. Evolutionary theory predicts that mitochondrial genomes are destined to accumulate male-harming (but female-friendly) mutations, and this prediction has received proof-of-principle support. Furthermore, mitochondrial effects on the phenotype are typically manifested via interactions between mitochondrial and nuclear genes. Thus, whether a mitochondrial mutation is pathogenic in effect can depend on the nuclear genotype in which is it expressed.

GENERAL SIGNIFICANCE

Many disease phenotypes associated with OXPHOS malfunction might be determined by the outcomes of mitochondrial-nuclear interactions, and by the evolutionary forces that historically shaped mitochondrial DNA (mtDNA) sequences. Concepts and results drawn from the evolutionary sciences can have broad, but currently under-utilized, applicability to the medical sciences and provide new insights into understanding the complex genetics of mitochondrial disease. This article is part of a Special Issue entitled Frontiers of Mitochondrial Research.

Emotional reactivity in mice may not be inherited but influenced by parents

Heredity is often assimilated to genetic transmission of traits. However, some traits may be socially inherited. This has been described for maternal behaviour as well as for emotional reactivity in rodents such as rats or mice. The aim of the present study was to investigate further this idea using two backcrosses between CB6 or B6C females and C57BL/6 males. Indeed, the experimental groups are genetically identical but may be exposed to very different mothering types. When adults, the offspring were subjected to rodent emotional reactivity tests such as elevated plus maze and free exploration paradigm. Results show that CB6xB6 males exhibit higher emotional reactivity than B6CxB6 and B6 males in all behavioural situations, but these effects are not seen in females. Contrarily to their offspring that show different reactivity even if sharing the same genetical background, CB6 and B6C females display quite identical emotional reactivity. A possible explanation is that emotional reactivity is induced by maternal behaviour rather than transmitted by the parents.

Mitochondrial DNA sequence heterogeneity in circulating normal human CD34 cells and granulocytes

We have reported marked mitochondrial DNA (mtDNA) sequence heterogeneity among individual CD34 clones from adult bone marrow (BM) and the age-dependent accumulation of mtDNA mutations in this mitotically active tissue. Here, we show direct evidence of clonal expansion of cells containing mtDNA mutations and that the mtDNA sequence may be easily determined by using peripheral blood (PB) as a CD34 cell source. Analysis of 594 circulating CD34 clones showed that 150 (25%) had mtDNA sequences different from the same donor's corresponding aggregate sequence. Examination of single granulocytes indicated that 103 (29%) from the same 6 individuals showed mtDNA heterogeneity, with sequences distinct from the corresponding aggregate tissue sequence and from the sequences of other single granulocytes. Circulating and BM CD34 cells showed virtually identical patterns of mtDNA heterogeneity, and the same changes were seen in progeny granulocytes as in their progenitors, indicating that blood sampling could be used in studies to determine whether mtDNA reflects an individual's cumulative or recent exposure to mutagens; as a marker of individual hematopoietic progenitors, stem cells, and their expansion; and for the detection of minimal residual disease in hematologic malignancies of CD34 cell origin.

Essential hypertension

Hypertension is a frequent, chronic, age-related disorder, which often entails debilitating cardiovascular and renal complications. Blood pressure is usually noted in combination with other cardiovascular risk factors. Diagnosis of hypertension increasingly relies on automated techniques of blood pressure measurement. The pathophysiology of essential hypertension depends on the primary or secondary inability of the kidney to excrete sodium at a normal blood pressure. The central nervous system, endocrine factors, the large arteries, and the microcirculation also have roles in the disorder. Although monogenic forms of blood pressure dysregulation exist, hypertension mostly arises as a complex quantitative trait that is affected by varying combinations of genetic and environmental factors. Non-pharmacological strategies can reduce blood pressure. Antihypertensive drug treatment diminishes the complications of hypertension. The concept that a few major genes will provide the final clue to the pathogenesis of essential hypertension is an oversimplification that contradicts the heterogeneous nature of this disorder. Further integration of genetic, molecular, clinical, and epidemiological research could disclose subsets of patients in whom specific combinations of genetic and environmental factors raise blood pressure, and might lead to more individualised treatment.

The specific mitochondrial DNA polymorphism found in Klinefelter's syndrome

Hypervariable segments of mitochondrial DNA (mtDNA) (HV1 and HV2) were analyzed in Klinefelter's syndrome and compared to normal population data. One pair of samples consisting of a Japanese mother and affected son with Klinefelter's syndrome (involved in a criminal case), and seven unrelated DNA samples from Caucasian Klinefelter males (two involved in criminal cases and five diagnosed) were collected in Japan and the United States. The diagnosis of Klinefelter's syndrome was established previously by multiplex XY-STR typing detecting two X alleles and one Y allele in the samples. Haplotype analysis of the mtDNA sequence in Klinefelter males was found to be identical, unique, and specific, as it was not found in the normal population. Astonishingly, family data exhibited that the haplotype of the mtDNA in the son was apparently different from the mother's, suggesting that the mtDNA of Klinefelter male would not be inherited from mother to son. Our data indicate that possible interaction of the sex chromosome and the mtDNA exists, and suggests that the specific mtDNA haplotype could cause the abnormal cell to fertilize and reproduce itself.

Cytoplasmic inheritance and its implications for animal biotechnology

At fertilization, the mammalian sperm transmits the haploid paternal genome. However, it also carries a variety of other factors into the oocyte that have the potential to affect embryo development. These include mRNAs left over from spermatogenesis, mitochondria with their own DNA, cytoskeletal and contractile elements, remnants of the sperm plasma membrane and, in many species, the sperm centriole. While most of these elements are eliminated, some play essential roles in early embryogenesis. In this review, I summarize the latest information on these phenomena and indicate some of the implications for animal biotechnology and, in particular, cloning.

Evolving together: the biology of symbiosis, part 2

Symbiotic trade-offs dominate the world of biology and medicine in colonist-host relationships and between separate, mutually dependent organisms of different species. Infectious and parasitic diseases can be better understood by exploring the dynamic continuum between pathogenicity and mutualism, between antagonism and cooperation-the sliding scale along which microorganisms can move in a moment's notice with a single nucleotide substitution. Organisms practicing piracy or pastoralism may be close genetic relatives. Mergers occur not only between cells but also between genomes; viruses co-opt host genes and in turn insert themselves into host genomes. Separate organisms, from ants to fungi to plants, establish symbiotic ties with each other that bind over deep time, generating much of the diversity we see in nature.