This is a confocal microscopy image of human fibroblasts derived from embryonic stem cells. The nuclei appear in blue, while smaller and more numerous mitochondria appear in red. [Shoukhrat Mitalipov]
This is a confocal microscopy image of human fibroblasts derived from embryonic stem cells. The nuclei appear in blue, while smaller and more numerous mitochondria appear in red. [Shoukhrat Mitalipov]

Mutations in our mitochondrial DNA tend to be inconspicuous, but they can become more prevalent as we age. They can even vary in frequency from cell to cell. Naturally, some cells will be relatively compromised because they happen to have a higher percentage of mutated mitochondrial DNA. Such cells make a poor basis for stem cell lines. They should be excluded. But how?

To answer this question, a team of scientists scrutinized skin fibroblasts, blood cells, and induced pluripotent stem cells (iPSCs) for mitochondrial genome integrity. When the scientists tested the samples for mitochondrial DNA mutations, the levels of mutations appeared low. But when the scientists sequenced the iPS cell lines, they found higher numbers of mitochondrial DNA mutations, particularly in cells from patients over 60.

The scientists were led by Shoukhrat Mitalipov, Ph.D., director of the Center for Embryonic Cell and Gene Therapy at Oregon Health & Science University, and Taosheng Huang, M.D., a medical geneticist and director of the Mitochondrial Medicine Program at Cincinnati Children's Hospital. The Mitalipov/Huang-led team also found higher percentages of mitochondria containing mutations within a cell. The higher the load of mutated mitochondrial DNA in a cell, the more compromised the cell's function.

Since each iPSC line is created from a different cell, each line may contain different types of mitochondrial DNA mutations and mutation loads. To choose the least damaged line, the authors recommend screening multiple lines per patient. “It's a good idea to check the iPS clones for mitochondrial DNA mutations and make sure you pick a good cell line,” said Dr. Huang.

This recommendation appeared April 14 in the journal Cell Stem Cell, in an article entitled, “Age-Related Accumulation of Somatic Mitochondrial DNA Mutations in Adult-Derived Human iPSCs.” This article holds that mitochondrial genome integrity is a vital readout in assessing the proficiency of patient-derived regenerative products destined for clinical applications.

“We found that pooled skin and blood mtDNA contained low heteroplasmic point mutations, but a panel of ten individual iPSC lines from each tissue or clonally expanded fibroblasts carried an elevated load of heteroplasmic or homoplasmic mutations, suggesting that somatic mutations randomly arise within individual cells but are not detectable in whole tissues,” wrote the article’s authors. “The frequency of mtDNA defects in iPSCs increased with age, and many mutations were nonsynonymous or resided in RNA coding genes and thus can lead to respiratory defects.”

Potential therapies using stem cells hold tremendous promise for treating human disease. However, defects in the mitochondria could undermine the iPS cells' ability to repair damaged tissue or organs.

“If you want to use iPS cells in a human, you must check for mutations in the mitochondrial genome,” declared Dr. Huang. “Every single cell can be different. Two cells next to each other could have different mutations or different percentages of mutations.”

Prior to the creation of a therapeutic iPS cell line, a collection of cells is taken from the patient. These cells will be tested for mutations. If the tester uses Sanger sequencing, older technology that is not as sensitive as newer next-generation sequencing, any mutation that occurs in less than 20% of the sample will go undetected. But mitochondrial DNA mutations might occur in less than 20% of mitochondria in the pooled cells. As a result, mutation rates have not been well understood. “These mitochondrial mutations are actually hidden,” explained Dr. Mitalipov.

The mitochondrial genome is relatively small, containing just 37 genes, so screening should be feasible using next generation sequencing, Dr. Mitalipov added. “It should be relatively cheap and do-able.”

Dr. Mitalipov also commented on a more general point, the implications of the current study on illuminating the mechanisms of age-related disease: “Pathogenic mutations in our mitochondrial DNA have long been thought to be a driving force in aging and age-onset diseases, though clear evidence was missing. This foundational knowledge of how cells are damaged in the natural process of aging may help to illuminate the role of mutated mitochondria in degenerative disease.”

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