The mutant genes metabolize folic acid and apparently pumps higher-than-normal amounts of the vitamin into DNA production, according to the study published in the journal Proceedings of the National Academy of Sciences.
Acute lymphocytic leukemia, which destroys bone marrow, is often fatal and accounts for 10 percent of adult leukemia.
Researchers found that people with two mutant genes were more than five times less likely to develop the leukemia. Even those with only one of the mutant genes were three to four times less likely to get the cancer, the study found.
The study also suggests that even people who do not have the mutant genes can reduce their risk of cancer by getting enough folic acid in their diet, said Martyn Smith, a toxicology professor at the University of California, Berkeley, who led the study.
"What it tells us is that folate is important in preventing this type of leukemia," said Smith. "In the absence of this genetic protection, it's important to get our folate."
Acute lymphocytic leukemia is most common form of childhood leukemia. The study released Monday does not address the mutant genes' possible effect on children. Another study is underway to investigate that issue.
An estimated one-third of adults have both mutant genes, which help produce the enzyme 5,10-
thylenetetrahydrofolate reductase, or MTHFR.
Earlier studies have shown that people with one of the mutant genes have half the colorectal cancer risk of people without it.
About 300 participants in the study had either acute lymphocytic leukemia or acute myeloid leukemia. Nearly 500 were healthy. Researchers working in England found no correlation between mutant genes and development of acute myeloid leukemia.
Folic acid, one of the chemicals in the vitamin B complex, is found in liver, oranges and green vegetables such as spinach. People with the mutant genes who don't get enough folic acid may not receive the same protective benefits, Smith said.
About 28,000 adults and 5,000 children in the United States are diagnosed with acute lymphocytic leukemia annually.