DNA galleries curated in liquid nitrogen preserve biological data for decades

By Monisha Ravisetti
May 20, 2021
Don't let this DNA sit out for too long. (Museum of Texas Tech University)

Don't let this DNA sit out for too long. (Museum of Texas Tech University)

Long-term tissue-storage facilities are DNA museums that freeze biological data in time, and scientists have conducted the first study to confirm how cold the freezers need to be to prevent losing this invaluable and unique history.

The common household freezer is about zero degrees Celsius; while that works for ice cream, tissue samples would start to degrade if placed inside. Scientists instead keep tissue in extremely cold temperatures that plunge into the negatives, and a study published April 28 in the Journal of Mammalogy showed what can happen when researchers do not use the iciest setting of minus 196 degrees Celsius (minus 320 degrees Fahrenheit), which was found to be the most effective.

Study author Robert D. Bradley, director of the Natural Science Research Lab at the Museum of Texas Tech University, compares the importance of these long-term tissue-storage banks, also called biobanks, to Paris' iconic Louvre museum.

"Their job is to preserve the Mona Lisa so that somebody, 300 years in the future, can go look at it," Bradley told Fastinform. "Our job is to preserve [samples] — in the best way we can — to try to anticipate what future methodologies will need, so that researchers of the future have a historic dataset to go back and look at."

Bradley relayed the example of the sudden hantavirus outbreak in the early 1990s. Out of the blue, the Four Corners region of the United States, which includes New Mexico, Colorado, Nevada and Arizona, started reporting several instances of the pulmonary disease.

Because the virus's origin wasn't clear, experts speculated whether it was a new virus; one that had originated from the Army, like a biological weapon; or a traveler from a different country, among other ideas.

"They went to our collection here at Tech and the collection at the University of New Mexico and had us go back in time, looking at rodent samples," Bradley said. "We had samples going back to the 1980s, and, sure enough, that virus was present." 

So the question was answered: Hantavirus was not something new; it had been around for at least 10 years.

In hopes of preserving biological data as meticulously as art museums curate ancient paintings for scholarly research, Bradley set out to understand exactly how cold these samples need to be kept. 

"Science has known for a long time that if you freeze [a sample] at zero degrees Celsius, yes, it'll freeze — but it will degrade over time," he said. "Some enzymes are active and some bacteria are even active at that level."

Bradley explained that as technology advanced, freezers with temperatures of minus 20 degrees Celsius (minus 4 degrees Fahrenheit) and minus 80 degrees Celsius (minus 112 degrees Fahrenheit) were introduced. Scientists adjusted accordingly and used the colder options, and when giant liquid-nitrogen freezers came about, reaching temperatures of minus 196 degrees Celsius, those too were used. However, no tests had been done to iterate how much better a liquid-nitrogen freezer was than a step-down, minus 80 degrees Celsius freezer. 

"Everybody says it's the gold standard, and that's what we ought to be doing," Bradley said. "We're really just trying to show this is the gold standard for long-term storage of biological samples."

The team conducted various tests to see how DNA in muscle and liver tissue samples held up in storage facilities from 30, 20 or 10 years or one year ago in freezers at minus 80 degrees Celsius and minus 196 degrees Celsius. The researchers found that DNA with more base pairs, or longer strands, were much better preserved in the colder liquid-nitrogen freezer.

While shorter strands were able to withstand the minus 80 degrees Celsius freezer better than longer strands, Bradley emphasized that minus 196 degrees is the absolute best to store any tissue sample over the long term while preventing degradation. 

However, there is an associated cost to consider. Bradley noted that even though liquid-nitrogen freezers are more expensive at first, minus 80 degrees Celsius freezers usually need to be replaced sooner, ultimately making them costlier than one would expect.

"The cost of one of our big liquid-nitrogen freezers will hold about as much as two of the minus 80s — maybe three," he said. "So they're more expensive, but in five years, when you have to replace one or two of those minus 80s, then suddenly the cost evens out."

Another interesting finding was that liver and muscle samples reacted differently in long-term storage, with the liver samples degrading more quickly. The researchers suggested this was because the liver has a lot of enzymatic activity. That might provoke it to degrade itself, making it prone to rapid deterioration, unlike muscles.

"Muscles are on the outside of your body — you're used to getting cuts and different things, and muscles get exposed to bacterial infections and sunburns," Bradley said. "So maybe muscles just have fewer enzymes. Maybe [they're] just a little better equipped at dealing with forces that lead to degradation."

So if a researcher has the choice to preserve only one tissue sample, Bradley recommends saving a muscle sample and putting it in liquid nitrogen — barring research that specifically calls for liver samples. 

Bradley highlighted that even though long-term storage of DNA samples takes a lot of delicate work, the information that it can offer is unparalleled. For example, he says to suppose that one day, 20 years from now, New York City starts getting reports of lead poisoning.

"You can work your way back in time and go, 'OK, in 2015, we looked at some rodents, and they had an elevated lead content. In 2010 they had a high one, but in 2005 it was zero.'"

The question would then evolve into: What happened between 2005 and 2010? Maybe there's a year that the subway was torn up, or pipes were exposed.

"I'm not saying it's absolutely going to answer your question," Bradley said. "But it does give us a dataset to go back and look at."

The study, "Temporal-dependent effects of DNA degradation on frozen tissues archived at −80°C," published April 28 in the Journal of Mammology, was authored by Taylor J. Soniat, Hendra F. Sihaloho, Richard D. Stevens, Todd D. Little, Caleb D. Phillips and Robert D. Bradley, Texas Tech University. 

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