The molecular structure of guanine (foreground) and adenine are shown. |
Many
critical cell functions depend on a class of molecules called purines, which
form half of the building blocks of DNA and RNA, and are a major component of
the chemicals that store a cell’s energy. Cells keep tight control over their
purine supply, and any disruption of that pool can have serious consequences.
In
a new study, Massachusetts Institute of Technology (MIT) biological engineers
have precisely measured the effects of errors in systems for purine production
and breakdown. They found that defects in enzymes that control these processes
can severely alter a cell’s DNA sequences, which may explain why people who
carry certain genetic variants of purine metabolic enzymes have a higher risk
for some types of cancer.
DNA
usually consists of a sequence of four building blocks, or nucleotides: Adenine,
guanine, cytosine, and thymine (the A, G, C, and T letters that make up the
genetic code). Guanine and adenine are purines, and each has a close structural
relative that can take its place in DNA or RNA. When these nucleotides, known
as xanthine and hypoxanthine, are mistakenly inserted into DNA, they cause
mutations. They can also interfere with the function of messenger RNA (mRNA),
which carries DNA’s instructions to the rest of the cell, and the RNA molecules
that translate mRNA into proteins.
“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,” says Peter Dedon, a professor of biological engineering at MIT and
senior author of the study, which will appear in the Proceedings of the
National Academy of Sciences.
In
addition to forming the backbone of DNA and RNA, purines are also a major
component of ATP, the cell’s energy currency; other molecules that manage a
cell’s energy flow; and small chemical cofactors required for the activity of
thousands of cell enzymes.
Abnormal metabolism
Dozens of enzymes are involved in purine metabolism, and it has long been known
that malfunction of those enzymes can have adverse effects. For example, losing
a purine salvage enzyme, which recovers purine nucleotides from degraded DNA
and RNA, leads to high blood levels of uric acid, causing gout and kidney
stones—and in extreme cases, a neurological disorder called Lesch-Nyhan
syndrome. Losing another salvage enzyme produces a disease called severe
combined immunodeficiency.
Abnormal
purine metabolism can also lead to side effects for people taking a class of
drugs called thiopurines. In some people, these drugs, often used to treat
leukemia, lymphoma, Crohn’s disease, rheumatoid arthritis, and organ-transplant
rejection, can be metabolized into toxic compounds. Genetic testing can reveal
which patients should avoid thiopurine drugs.
In
the new study, Dedon and his colleagues disrupted about half a dozen purine
metabolism enzymes in E. coli and
yeast. After altering the enzymes, the researchers measured how much xanthine
and hypoxanthine was integrated into the cells’ DNA and RNA, using a highly
sensitive mass spectrometry technique they had previously developed to study
DNA and RNA damage caused by inflammation.
They
found that the malfunctioning enzymes could produce dramatic increases—up to
1,000-fold—in the amounts of hypoxanthine incorporated into DNA and RNA in
place of adenine. However, they saw very little change in the amount of
xanthine inserted in place of guanine.
Chris
Mathews, a professor emeritus of biochemistry and biophysics at Oregon State
University, says the
finding could help researchers better understand how defects in purine
metabolism produce disease. “This paper opens the door to numerous studies—for
example, looking into the biological effects resulting from the accumulation of
abnormal bases in DNA and RNA,” says Mathews, who was not involved in this
study.
Scientists
have found quite a bit of genetic variation in purine metabolic enzymes in
humans, so the research team plans to investigate the impact of those human
variants on xanthine and hypoxanthine insertion into DNA. They are also
interested in studying the metabolism of the other two nucleotides found in
DNA, cytosine and thymine, which are pyrimidines.