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Title: Segregation of naturally occurring mitochondrial DNA variants in a mini-pig model

Author
item CAGNONE, GAEL - Mimr-Phi Institute Of Medical Research
item TSAI, TE-SHA - Mimr-Phi Institute Of Medical Research
item SRIRATTANA, KANOKWAN - Mimr-Phi Institute Of Medical Research
item ROSSELLO, FERNANDO - Monash University
item POWELL, DAVID - Monash University
item Rohrer, Gary
item CREE, LYNSEY - University Of Auckland
item TROUNCE, IAN - University Of Melbourne
item ST. JOHN, JUSTIN - Mimr-Phi Institute Of Medical Research

Submitted to: Genetics
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/17/2016
Publication Date: 3/7/2016
Publication URL: http://handle.nal.usda.gov/10113/62074
Citation: Cagnone, G., Tsai, T.-S, Srirattana, K., Rossello, F., Powell, D.R., Rohrer, G., Cree, L., Trounce, I.A., St. John, J.C. 2016. Segregation of naturally occurring mitochondrial DNA variants in a mini-pig model. Genetics. 202(3):931-944.

Interpretive Summary: The mitochondrial genome is essential to cell, tissue and organ function as it encodes key subunits of the oxidative phosphorylation enzymes that produces most of a cell’s energy. We inherit our mitochondrial from our mother. The mitochondrial DNA (mtDNA) exists as multiple copies in each cell and is prone to mutation, resulting in variant and wild type mtDNA sequences co-existing within a cell. The ratio of variant to wild-type sequences determines the variant load which, when high, can lead to mitochondrial disease. Here we show that, in a healthy colony of mini-pigs, there is prevalence of naturally occurring mtDNA variants that are maintained at low levels across generations with biases between males and females, and in specific tissues. In particular, tissues requiring more energy (namely brain, muscle, diaphragm and heart) have significantly lower levels of mtDNA variants. This reduction is positively correlated with mtDNA copy number in the tissues, suggesting selective regulation and maintenance of specific mtDNA variants. However, we find that variant load is highly variable among oocytes with variable patterns of segregation in embryos.

Technical Abstract: Within cells and tissues, the maternally inherited mitochondrial genome (mtDNA) is present in multimeric form and can harbour naturally occurring variants. Whilst high variant load can cause mitochondrial disease, naturally occurring mtDNA variants likely persist at low levels across generations of healthy populations. To determine how naturally occurring variants segregate and are transmitted and whether they remain at low levels across generations, we developed a mini-pig model, which originates from a unique maternal ancestor. Mini-pigs have a longer period of gestation than the mouse, when mtDNA is segregated, and organ sizes similar to the human. Through next generation sequencing analysis, we identified a series of low-level mtDNA variants in blood samples from the female founder and her daughters. Four variants were selected for subsequent analysis in 12 tissues from 31 animals across 3 generations by High Resolution Melting analysis. Several tissues showed significant changes in variant load across the generations, with sex-specific differences in heart and liver. Amongst the offspring, variant load was persistently reduced in high-respiratory organs (heart, brain, diaphragm and muscle), which significantly correlated with greater mtDNA copy number. In contrast to the tissues, oocytes showed increased heterogeneity in variant load, which correlated with increased mtDNA copy number during in vitro maturation. Altogether, this study presents evidence that naturally occurring mtDNA variants are segregated in a tissue-specific manner across generations. This segregation likely involves maintenance of selective mtDNA variants during organogenesis, which can be differentially regulated in oocytes and preimplantation embryos.