Scientists test the accuracy of enzymes that copy DNA under weightlessness

Researchers at Queen’s University boarded a modified Falcon 20 aircraft at Ottawa airport on May 22nd, 2019 as part of a scheduled “vomit comet” flight. In this mode of flight, the plane repeatedly climbs in a steep parabola to 8 km, alternating with a freefall descent.

DNA Replication

Image Credit: Soleil Nordic/Shutterstock.com

When the plane freefalls at a rate of more than 3.3 km in 20 seconds, only gravity but no drag, lift, or thrust act on the plane. This leads to weightlessness. The aim of the researchers under such challenging conditions was to investigate whether enzymes that copy DNA are as precise under weightlessness as under earthbound conditions.

This question is more significant for future space exploration. This is because the health of astronauts will rely on the precise replication of DNA during cell division.

So-called DNA polymerases are essential enzymes that copy and repair DNA. Inevitably, they aren’t perfect: even under optimal conditions, they sometimes make mistakes. Here we show that DNA polymerases derived from the bacterium E. coli are considerably more prone to errors under microgravity, such as occurs in space. Our results thus have important implications for the health of astronauts.”

Aaron Rosenstein, Study Corresponding Author, University of Toronto

Rosenstein was a student at Queen’s University, Kingston, Canada when this study was performed. The findings of the study have been described in the open-access journal Frontiers in Cell and Developmental Biology.

Mutation rate already higher due to space radiation

Earlier studies have demonstrated that in space, DNA undergoes a higher mutation rate—for instance, substitutions of single nucleotides, deletions, crosslinks, or inversions—caused by damage from solar particles and cosmic rays.

So far, it was not known whether the natural copying mechanism of DNA is also impacted by weightless conditions in space. In case DNA polymerases turn less accurate in space, the already high rate of mutation will increase further by DNA copying, where cancer is one of the potential consequences for astronauts.

Rosenstein and Prof Virginia K. Walker, his supervisor and co-author, demonstrate in this study, for the first time, that the error rate of a DNA polymerase extracted from Escherichia coli bacteria is steadily higher under microgravity.

Rosenstein and his team members from Queen’s University achieved this result by taking part in the “Canadian Reduced Gravity Experimental Design Challenge.” They developed a semi-automatic mini-laboratory to realize a single-round replication of a 1000-nucleotide-long designed DNA fragment, at the time of the weightless stage of the parabolic flight. The findings can be generalized to space conditions.

Trial and error were used to render the micro-laboratory amendable to control by researchers who were motion-sick.

Experiencing microgravity is a unique experience. Performing even simple tasks, such as manually activating our mini-laboratory, can become difficult. We were forced to invest much effort into improving the user-friendliness of our mini-laboratory, to make it easier to operate not only in microgravity, but also in the subsequent 2G hypergravity phase of the flight once a zero-gravity parabola has been completed.”

Aaron Rosenstein, Study Corresponding Author, University of Toronto

The researchers demonstrate that the single-base substitution rate—the rate at which the nucleotide thymine (T) is coupled with a wrong nucleotide, for instance, adenosine (A), on the opposite strand of the DNA helix—was identified to be 10%–140% more than it was under earthbound conditions.

The researchers observed similar higher mutation rates for all pairwise substitutions between the A, C, G, and T nucleotides and similarly for random insertions or deletions of one to three nucleotides.

As predicted, accuracy was also based on whether the DNA polymerase keeps a “proofreading” functionality, which checks for (and if necessary eliminates) any mismatched nucleotides—a version of the enzyme the proofreading of which had been inactivated by mutations had an approximately 50% higher substitution rate.

Inaccuracy of DNA replication poses novel health risk

In summary, the researchers state that—together with the higher radiation risk in space—the DNA replication inaccuracy under microgravity could affect the health of astronauts on prolonged periods in space, for example, planned missions to the Mars and Moon.

We have shown that DNA polymerases similar to those found in mitochondria—the cell’s powerhouses – make more errors in microgravity. The combined effect of greater damage and decreased replication accuracy could lead to premature aging in astronauts. Our results show the importance of designing rotating spaceships that generate artificial gravity, to prevent these negative effects. We would love to be invited to repeat our experiments on such a ship!.”

Virginia K. Walker, Professor, Queen’s University

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