Researchers at the Johns Hopkins Kimmel Cancer Center have discovered that loss of a gene already implicated in human cancers also leads to age- and gender-linked cancers in mice. The findings, reported in the February issue of Nature Genetics, validate the gene's dysfunction as an early step in development of cancer in people, the scientists say.
Screening tumor samples for genes with too many methyl groups attached, a few years ago a Johns Hopkins graduate student, Michele Makos-Wales, found hypermethylated-in-cancer 1, known as Hic1. Methyl groups are one of many types of additions to the DNA sequence that help control gene expression.
"Most genes linked to cancer were first identified from mutations in families, and subsequently some of the same genes were found to be hypermethylated, but Hic1 was identified solely because of extra methylation in cancer cells," says Stephen Baylin, professor in the cancer biology division. "Now we've shown directly that loss of the gene's function leads to cancers in mice."
The finding also provides a powerful message to other scientists studying genes linked to cancer through "epigenetic" changes like abnormal methylation. While genes whose cancer-causing abilities are traced to mutations in the DNA sequence have long been studied in "knockout" mice, this is the first evidence that knockout mice also can reveal the cancerous effects of epigenetic problems, Baylin says.
Since excess methylation turns the gene off, former graduate student Mark Carter and the team tried to measure its impact on cancer by completely knocking out Hic1 in mice, but the mice died before birth. So they switched to studies of mice with only one copy of the gene.
"For a long time, nothing happened," says postdoctoral fellow WenYong Chen, who led the research team. In fact, not until the mice were more than 70 weeks old--"pretty old for a mouse," he says--did excessive cancers begin to appear. By 90 weeks of age, 16.4 percent of the experimental mice had developed tumors, while no normal mice had. By 100 weeks, experimental mice had almost three times the rate of tumors as normal mice (34.2 percent vs. 14.3 percent of normal mice).
Oddly, the researchers say, the kind of cancer that popped up depended on the mouse's sex. Female mice tended to develop lymphomas and sarcomas, while male mice developed more epithelial cancers.
Chen also found that each cancer lacked any sign of Hic1 expression. Furthermore, loss of the existing copy's function was due solely to hypermethylation of one of the gene's two "promoters"--strips of DNA that allow the gene to be read. Which promoter was silenced generally depended on the cancer type and hence gender, he adds.
The scientists don't know yet why excessive Hic1 methylation in mice is linked to gender or age, or why hypermethylation at one promoter would result in a different type of cancer, but there are ongoing experiments to unravel the gene's mechanism, Baylin says.
While Hic1 hypermethylation in mice mirrors the range of human cancers better than the individual loss of most tumor-suppressor genes, it is not known whether the gender effects seen in mice are present in human cancer.
Authors on the study are Chen, Baylin, Xiaobei Zeng, Ray-Whay Chiu Yen, Neil Watkins, James Herman, Craig Morrell and Joseph Mankowski, all of Johns Hopkins; Mark Carter, now with the National Institute on Aging; and Manel Esteller, the National Center of Cancer Investigation, Madrid, Spain.