By Artur Smiechowski
From last night’s leftovers to pizza delivery boys flung far into the future, freezing something is generally considered a good way to preserve it. However, as we all know from ill-fated retrievals of freezer burnt Ben and Jerry’s there can be complications. While in the domestic sense these complications mean little more than wasted ice cream they can have destructive impacts in the field of medicine, where sample storage integrity is key. In research and practice, blood samples are commonly frozen to be tested at a later date, often out of convenience. Unfortunately, while the cold is a fickle beast, DNA is a fragile one, leading to issues when attempting to store blood in the long term.
A study from the University of Geneva looked into the efficacy of different methods for long and short term whole blood storage and the effects it had on blood DNA integrity. Overall, 41 whole blood samples were collected from volunteers at the university hospital. White blood cell counts were taken of each sample prior to storage. The experiment tested a combination of different storage techniques involving freezing temperatures and the addition of DNAgard blood preservative prior to freezing. Blood was stored at room temperature, -20 or -80 °C, with or without DNAgard added. Samples were then kept frozen anywhere from 24 hours to a full year before thawing.
The DNA of thawed samples was then put through a litany of tests to determine its overall quality. DNA yield was determined by comparing the amount of extracted DNA from thawed samples to a baseline of DNA extracted prior to storage. DNA yield was the best among the -80 °C samples with DNAgard added (Bulla et al., 2016). While blood stored in such a fashion retained the most DNA over a long period of time it seemed that these samples suffered some initial loss after freezing before stabilizing; suggesting some damage is caused by the freezing process itself.
The integrity of the collected DNA was tested by gel electrophoresis of the thawed samples (a process that visually separates out different sized strands of DNA with the use of electricity - the more damaged DNA is the more fragments will show up). Interestingly most samples save for those stored at room temperature showed very little variation in integrity, most staying generally intact. Strangely, the only frozen samples to exhibit structural degradation where those stored at -20 °C with the DNAgard solution, here the addition of DNAgard actually correlated with more DNA damage than just freezing, as of yet there is no explanation for this phenomenon. Methylation, a natural process that suppresses gene expression, was also tested for however no significant difference between trial and control samples where found.
Understanding the effects of different preservation techniques on blood samples can help make them more accessible in the future. This allows hospitals and clinics, which may be in a remote location or not have access to necessary equipment, to save blood samples and have them tested at a later date. By extension, blood preservation can improve medical care and the lives of people around the world. While it’s a small step as far as medical research is concerned, it is still a fundamental step forward, which can act as a basis for future research and procedures to grow from.
Not only does such a study inject some insight into the scientific knowledge base, it also raises its own questions that posit further study and illuminates the continuous nature of science. Even with DNAgard, there is some initial damage to DNA, what methods might be useful to prevent this? DNAgard actually seems to cause damage to samples at higher temperatures, what is the cause? These questions that arise in the very process of answering another just serve to show the base of human knowledge is an ever-growing circle. As it grows so too does its perimeter, showing us how little we know and how much more there is to learn.
References:
Bulla, A., Witt, B. D., Ammerlaan, W., Betsou, F., & Lescuyer, P. (2016). Blood DNA Yield but Not Integrity or Methylation Is Impacted After Long-Term Storage. Biopreservation and Biobanking,14(1), 29-38. doi:10.1089/bio.2015.0045