Researchers at the Kimmel Cancer Center at Jefferson have discovered how a powerful tumor suppressor called DACH1 works – a finding that explains why restoring its lost function in cancer cells pushes them to become normal again.
In the March 29 online early edition of the Proceedings of the National Academy of Sciences (PNAS), the researchers say that DACH1 is a potent transcription factor whose protein binds to and forms a lock, of sorts, that stops more than 1,500 human genes from being activated. Some of these genes have been previously shown to promote cancer. When DACH1 isn’t around the expression levels of these genes increase because another well-known cancer promoting protein, forkhead, slots into the unlocked genes and turns them on.
“While the study was conducted mainly in breast cancer cells, the beneficial function of DACH1 is missing in a wide variety of cancers, and restoring it might offer us a new clinical strategy for these cancers,” says Chenguang Wang, Ph.D., a co-leader of the study and an assistant professor in the Department of Stem Cell Biology and Regenerative Medicine.
The study is remarkable not only because it describes the molecular function of DACH1 – a feat that took the research team 10 years to uncover – but the scientists scanned the human genome to see what genes DACH1 interact with, says study co-leader Richard Pestell, M.D., Ph.D., Director of the Kimmel Cancer Center and Chairman of the Department of Cancer Biology.
“The studies identify a new mechanism by which this important tumor suppressor works, and by interrogating the entire human genome these studies identified all the genes that bind and are regulated by DACH1,” Pestell says. “This is a major accomplishment.” DACH1 is the human version of the so-called Dachshund (Dac) gene first discovered in Drosophila, which are small fruit flies. Flies born without this gene have shorter than normal legs – hence the Dachshund reference – and don’t develop their eyes. Because Dac is, therefore, a key gene involved in development and growth, the Jefferson researchers started looking at its role in different cancers in the 1990s. “We knew that genes that direct growth in embryonic development can be switched on or off in cancer,” says Wang.
Wang, Pestell and their laboratories discovered that DACH1 function was missing in human breast cancer, as well as in prostate and endometrial cancers. They also showed that DACH1 represses tumor initiation and progression, and when DACH1 is put back into the cancer cell lines and in animals, cancer cells revert to a normal state. “When tumor cells begin to express this gene again, it not only reverses the cancer progression, but these cells begin to excrete molecules that stop surrounding tumor cells from growing as well,” Wang says. “It is really remarkable.”
But the researchers did not know how DACH1 was functioning, and this study explains the mechanism.
The scientists solved the puzzle by first identifying the binding sequence by which DACH1’s protein latches on to genes, and then they used several approaches (computer based and laboratory cell work) to identify the genes within the human genome that DACH1 interacts with. They identified 1,606 genes that have a promoter region that matches the DACH1 protein binding site. “These genes are potentially regulated by DACH1, which means that DACH1 is a transcription factor for these genes,” Wang says.
More work established that DACH1 is a transcriptional “repressor,” meaning that it binds to genes in order to keep them from being activated. Many tumor suppressors are also transcription factors, but they work by turning genes on. “For example, p53, the best known tumor suppressor, is an activator of genes,” he says. “This is the first time to identify DACH1 as a transcription factor to represses gene expression.”
The scientists also discovered that the DACH1 binding site resembles the binding site for a protein known as FOXM1known to be over-expressed in breast cancer. FOXM1 is a member of the FOX (Forkhead box) family of more than 100 transcription factors that regulate genes involved in cell growth and proliferation.
“In this manner a fundamental connection was made between DACH1 and the forkhead family of genes that regulate metabolism and stem cells,” Pestell says.
Because DACH1 is not available to put a lock on genes involved in cancer, Forkhead proteins latch on and activate those genes, the researchers say. “FOX is over-expressed in cancer and DACH1 is down-regulated,” Wang says. “If DACH1 is made available, FOX proteins cannot bind, and a cancer cell reverts back to its normal state.
“What is most intriguing is that DACH1 seems to be more powerful than a traditional tumor suppressor gene because when tumor cells suddenly express it, surrounding cancer cells also become normal again,” he says. “If we can find a way to turn this gene back on in breast cancers, and potentially other cancer types, we may be able to reverse the cancer progression.”
Co-authors of the study include researchers from the National Institutes of Health, and Drexel University in Philadelphia. The work was funded by grants from the Susan Komen Breast Cancer Foundation, the National Institutes of Health, and the Pennsylvania Department of Health. The authors declare no conflict of interest.