Mitochondria are the organelles that produce most of the chemical energy needed by the cell. Mitochondrial DNA (mtDNA) contains 16,569 nucleotides that can be mutated. Some of these mutations can lead to the development of mitochondrial diseases.
While nuclear DNA (the well-known double helix, which encodes most of the genome) is inherited from both parents, mtDNA is inherited only from the mother.
At birth, a female infant’s ovaries already contain all of the eggs a baby will ever have. During the reproductive cycle that begins at puberty, several immature eggs develop under the influence of hormones, leading to ovulation and the potential for fertilization.
The study shows for the first time that mutant mtDNA forms during the late stages of egg formation. The researchers, who conducted experiments in mice, reported that the proportion of mutated molecules increased as the eggs matured, that these mutations could impair mitochondrial function, and that they are responsible for development of disease.
The researchers found that up to 90% of mtDNA can be mutated. The existence of an upper limit is important for the understanding of how mutated and possibly pathogenic mtDNA transmission.
When wild-type and mutant mtDNA coexist in a cell (allogeneic), the effects of mutant mtDNA can be masked, facilitating transmission to offspring. “Until now, no one knew if this accumulation would happen, but our study proved it does. Now that we understand where and how it happens, we know can find a way to avoid it,” said Marcos Roberto Chiaratti, professor at the Faculty. in Genetics and Evolution at the Federal University of So Carlos (UFSCar) in the state of So Paulo, Brazil.
Chiaratti and PhD student Carolina Habermann Macabelli are among the authors of the paper. The research was supported by FAPESP through two projects (April 17,04372-0 and August 16,7868-4).
Chiaratti also received a Newton Advanced Scholarship from the UK Academy of Health Sciences. He collaborated with a team led by Patrick Francis Chinnery, the final author of the paper. Chinnery is Professor of Neurology at the University of Cambridge, and a Wellcome Trust Principal Research Fellow for the MRC Mitochondrial Biology Unit.
“The most effective treatment requires identifying mutations in the mother to prevent heredity in offspring. This was the context for our study, which aimed to verify which mutations were transmitted and analysis of the mechanisms involved, the Brazilian mitochondrial disease study Chiaratti said.
Symptoms of mitochondrial disease vary according to the mutation, the number of cells damaged, and the tissue affected. The most common include muscle weakness, loss of coordination, cognitive decline, brain degeneration, and kidney or heart failure.
Such inherited metabolic diseases can appear at any age, but the earlier the mutation is expressed, the more likely it is to lead to severe symptoms and even death. Diagnosis is difficult, often requires genetic and molecular testing, and prevalence statistics are therefore lacking.
It is estimated that diseases caused by mtDNA mutations affect at least one in every 5,000 people worldwide. However, the frequency of pathogenic mtDNA mutations is about one in 200. The m.3243A>G mutation, which causes MELAS syndrome (Mitochondrial encephalopathy, lactic acidosis, and stroke-like episodes), occurs in approximately one in 200. 80% of adults have heterotrophic disease causing mutations.
Experiment
The researchers studied transgenic mice with two types of mitochondrial genomes: wild-type, non-pathogenic, and pathogenic mutant m.5024C>T, similar to m.5650G>A, a mutant pathogens present in humans.
Analysis of 1,167 mother-child pairs found that females with low levels of m.5024C>T tended to pass on higher levels of the mutation to their offspring. However, in females with a high degree of mutation, the opposite trend was detected, indicating that pure selection was against a high degree of mutation (over 90%).
Analysis of mouse oocytes (immature eggs) at different stages of development revealed increased levels of m.5024C>T compared with wild-type mtDNA. This suggests that the mutant mtDNA is preferentially replicated during oocyte maturation, regardless of the cell cycle, as the egg does not undergo cell division until ovulation. The researchers tested several mathematical models, and the one that best explains this phenomenon points to a transcriptional advantage in favor of mutant mtDNA, and pure selection prevents mutations from reaching high levels.
First, they measured metaplasia in 42 offspring and 1,167 progeny. Next, they measured the levels of mutant mtDNA in the eggs at different stages of development and compared them with the levels of mutations in different organs at different ages.
They found evidence that the results were applicable to mice carrying another pathogenic mutation (m.3875delC tRNA) and to humans, as indicated by analysis of 236 mother-child pairs. This suggests positive selection when the mutation is passed on from mothers with low levels of metaplasia and pure selection against high levels of metaplasia (above 90%). They concluded that positive selection was the result of the mutant’s replication preference over the wild-type molecule.
“This preferential replication allows mutation levels to reach a ceiling of 90%,” said Chiaratti.
He plans to go to the UK soon to conduct new experiments. A possible next step would be to initiate a phase of drug therapy aimed at combating mtDNA levels of mutations to prevent disease transmission. “Once we understand how the accumulation of mutations that lead to mitochondrial disease occurs in the late stages of egg formation, we can produce eggs in vitro and process them, pharmacologically as well like genetics, to reduce the level of mutations, to reduce the probability that a child will develop the disease,” he said.
Source: Eurekalert