Cells rely on purines, which are types of molecules that make up half of the DNA and RNA building blocks, and are a key component of the chemicals that store a cell’s energy in order to perform many vital functions. The purine supply is strictly controlled by the cells, with any disruption likely to cause serious potential consequences.
In a new study published in the Proceedings of the National Academy of Sciences MIT, biological engineers have accurately measured the effects of errors in the cellular systems that control the production and breakdown of purine and discovered that defects in enzymes, which control these processes, can severely change the DNA sequences of a cell. This may provide a potential explanation as to why people with certain genetic variants of purine metabolic enzymes are at a higher risk of developing certain types of cancer.
DNA is generally made up of a sequence of four building blocks, or nucleotides, including adenine (A), guanine (G), cytosine (C) and thymine (T), which contain the genetic code. Guanine and adenine are purines, which both have a close structural relative that can replace either of them in DNA or RNA. Mutations are caused when these nucleotides, called xanthine and hypoxanthine are accidentally inserted into DNA, where they can interfere with the messenger RNA (mRNA) function that carries instructions from the DNA to the rest of the cell, as well as to the RNA molecules that translate mRNA into proteins.
Senior author, Peter Dedon, professor of biological engineering at MIT explains:
“A cell needs to control the concentrations very carefully so that it has just the right amount of building blocks when it’s synthesizing DNA. If the cell has an imbalance in the concentrations of those nucleotides, it’s going to make a mistake.”
Purines are not only a key component of ATP, which is the cell’s energy value, they also form the backbone of DNA and RNA, other molecules that regulate a cell’s energy flow, and small chemical cofactors that are necessary for the activity of thousands of cell enzymes.
In purine metabolism dozens of enzymes are implicated. Scientists have known for some time that malfunction of those enzymes causes adverse effects. For instance, losing a purine salvage enzyme that recovers purine nucleotides from degraded DNA and RNA, leads to high levels of uric acid in the blood causing gout and kidney stones, and in extreme cases, Lesch-Nyhan syndrome, a neurological disorder, whilst the loss of another salvage enzyme produces severe combined immunodeficiency.
Abnormal purine metabolism can also lead to side effects in individuals undergoing therapy with drugs classed as thiopurines, which are commonly used to treat leukemia, lymphoma, Crohn’s disease, rheumatoid arthritis, and organ transplant rejection. In some individuals these drugs can produce into toxic compounds, however, genetic testing can disclose which patients should avoid thiopurine therapy.
Dedon and his team decided to disrupt approximately half a dozen purine metabolism enzymes in E. coli and yeast in their new study. Once the enzymes were altered, they used a highly sensitive mass spectrometry technique they developed earlier to study DNA and RNA damage caused by inflammation, to measure the quantity of xanthine and hypoxanthine that was integrated into the cells’ DNA and RNA.
The authors determined that the malfunctioning enzymes were able to produce staggering increases of up to 1,000-times the amount of hypoxanthine that was incorporated into DNA and RNA in place of adenine, whereas they observed that the amount of xanthine that replaced guanine changed very little.
In light of the notable genetic variations, they discovered in human purine metabolic enzymes, the research team wants to assess the impact of those human variants on xanthine and hypoxanthine insertion into DNA in the future, in addition to studying the metabolism of the other two nucleotides found in DNA, the pyrimidines cytosine and thymine.
Written by Petra Rattue