German Cancer Research Center: Cancer
is a complex disease of the genes. The reasons why a tumor develops are
often unknown. An international team of researchers from the German
Cancer Consortium (DKTK) and the Welcome Trust Sanger Institute has now
developed a technique that can identify causes of cancer invisible to
genetic sequencing. The technique has uncovered large sets of previously
unknown pancreatic cancer genes. It is hoped that this study will boost
research into a disease that is still poorly understood and for which
five-year survival rates have stood at around 5 per cent for the past
four decades.
The technique works by introducing sections of DNA called
piggyBac transposons into the mouse genome. Transposons jump around
within the genome, reinserting themselves at random and causing a
different mutation in each cell of the mouse. This triggers cancer
development, and tracking the transposon´s fingerprints in the tumors
allows discovery of the affected cancer-causing genes. The PiggyBac tool
was engineered for the first time to allow cancer induction in
individual tissues within the mouse, and the method can now be used to
study any type of cancer.
While genome sequencing can identify
all categories of genetic alterations with high accuracy, some of these
changes are difficult to interpret. For example, hundreds or thousands
of genes are found to be transcriptionally or epigenetically
dysregulated within a cancer, meaning that they are not mutated but just
being turned on or off. Pinpointing the few cancer-causing events among
these large gene sets is extremely difficult. PiggyBac screening can
facilitate this search for the needle in the haystack because
transposons jump directly into the relevant genes. Moreover, the tool
monitors tumor development in mice and therefore researchers are also
able to see the consequences of cancerous mutations and how they help
the disease to progress.
“Recent advances in cancer genome sequencing have given extraordinary
insights into the genetic events underlying cancer. Nevertheless, we
are still far from understanding the complexity of the molecular
processes driving cancer development,” says Professor Roland Rad, from
the Technische Universität München, the German Cancer Consortium (DKTK)
and the German Cancer Research Center (DKFZ). “Unbiased genome-wide
screening in mice allows us to see cancer from a different angle and
answer biological questions that cannot be addressed with other
approaches.”
The study has identified many genes previously unknown to be involved
in pancreatic cancer, including Foxp1, which was hit by transposons at
very high frequencies in the 49 mouse tumors studied. Where Foxp1 was
induced, tumors spread from the pancreas to other organs, suggesting
that the gene drives cancer progression. This finding was confirmed when
researchers looked at human samples and found high levels of the FOXP1
gene product in cancers that had metastasized.
“PiggyBac has discovered novel players in pancreatic cancer and
additional experiments in mice allowed us to unravel the biological role
of selected genes, such as Foxp1, at an organismal level. We then still
need to confirm our findings in human cancer samples but many genes
could not have been identified in first place with the traditional
screening techniques in humans”, says Prof. Dieter Saur from the
Technische Universität München.
In a number of mice, transposons had become inserted in noncoding
regions of the genome. These insertions pinpointed enhancer areas, which
are involved in the regulation of cancer-causing genes. In addition,
similarly to humans, mice developed various subtypes of pancreatic
cancer, which not only have distinctive microscopic appearances but also
show different clinical behaviors. The study discovered molecular
processes being responsible for triggering the formation of these cancer
subtypes.
“Progress in treating pancreatic cancer has been held back because we
don’t yet understand the disease properly,” says Professor Allan
Bradley, Director Emeritus of the Sanger Institute. “PiggyBac can be
used in concert with Sleeping Beauty, another transposon tool, and
alongside whole genome sequencing to enable us to see, in great detail,
the origins of this cancer.”
Researchers will now be able to look more closely at the pancreatic
cancer genes that have been discovered by this study in the hope of
finding effective drugs for a disease that is set to be the second
leading cause of cancer death by 2030. Laboratories have also begun
using the technique to investigate cancers in other tissues.
Rad, R., Rad, L., Wang, W., Strong, A., Ponstingl, H., Bronner, I.F.,
Mayho, M., Steiger K., Weber, J., Hieber, M., et al. (2014). A
conditional piggyBac transposition system for genetic screening in mice
identifies oncogenic networks in pancreatic cancer. Nature Genetics
2014, doi:10.1038/ng.3164