Mutation Tool Designed to Identify New Cancer Genes

The system called PiggyBac has already been used by the investigators to identify unique candidate cancer-causing genes.

This new development of the PiggyBac system makes it a powerful addition to the arsenal of genetic techniques available to researchers for picking apart the genetic causes of cancer. It will complement developments in genomics and genetics of cancer, by providing biologic validation to human mutations identified by cancer genome sequencing.

The PiggyBac process involves shipping payloads of genetic material, called transposons, around the genome using an engine known as a transposase. The team has integrated the PiggyBac system into the mouse genome, where the transposons can hop from gene to gene, from chromosome to chromosome, disrupting, or altering the activity of the genes where they land.

"Far from being destructive, this process is empowering our search for genes underlying cancer,” said Prof. Allan Bradley, from the Wellcome Trust Sanger Institute (Hixton, Cambridge, UK) and senior author on the study, which was published October 14, 2010, in the journal Science. "Some genes, when disrupted, will push cells along the road to tumor development. When we look at the tumors that develop in our mice, we can search for the molecular fingerprint of the transposons in the genome; this allows us to identify the disrupted genes that are the cause. However, what is extraordinary about this new model is its adaptability--with PiggyBac, we can look at specific organs, we can switch genes on and switch genes off, we can look for cancer genes across the whole genome. It's the organism version of whole-genome study.”

The researchers searched for novel cancer genes in 63 mouse blood cancers. The system opened new doors in the genome: when the researchers inspected 72 well-defined locations at which their transposon had entered the genome, they found that an amazing two-fifths of these genetic sites had never been detected before. "What is extraordinary about this new model is its adaptability--with PiggyBac, we can look at specific organs, we can switch genes on and switch genes off, we can look for cancer genes across the whole genome,” remarked Prof. Bradley.

"As well as highlighting the potential of this system to get at genetic regions previously beyond reach, the new genes that we have already identified using PiggyBac open exciting new avenues for future studies,” said Dr. Roland Rad, from the Wellcome Trust Sanger Institute and first author on the paper. "For instance, we found that one of the genes, called Spic, was disrupted in nine distinct myeloid leukemia tumors in our mice. An event of this frequency merits study in human cancer and, when we take into account recent studies that have found this gene has a role in the development of white blood cells, we can be even more optimistic about the potential of this finding.”

Other genes identified include Hdac7, which is known to participate in the creation of white blood cells in the thymus but has not been studied in the context of blood cancers; and Bcl9, a gene whose human equivalent is thought to be involved in leukemia.

Researchers can now look in detail at the genetic equivalents in the human genome and try to determine what role their new genes play. One of the challenges of cancer genetics is that genomes in cancer cells can be devastated by hundreds or even thousands of mutations. By looking at cancers modeled in the mouse, investigators can begin to understand--at a biologic level, which, among the thousands of mutations present, is the cause.

Before transposons, scientists frequently used other methods, such as viruses, to cause mutations and generate tumors. Although these have had success in identifying genetic perpetrators in cancers of the blood and breast, they have not been effective in other cancer types. It is only in recent years that researchers have been able to activate transposons to mutate genomes of higher organisms, such as mice--starting with a model called Sleeping Beauty. PiggyBac has many advantages over Sleeping Beauty and considerably extends the toolkit available to researchers. But the two systems can also complement one another.

"These transposons have particular preferences, particular ways of working,” stated Dr. Pentao Liu, from the Wellcome Trust Sanger Institute and an author on the article. "While Sleeping Beauty transposons slot into the genome most comfortably according to one pattern, PiggyBac follows another. So, naturally, one system will find genes that another might not. What is really exciting is that we have been able to incorporate both systems into our mouse lines so that they can be used together. By optimizing PiggyBac in this way and by sharing these tools with researchers worldwide, we can hope to drive new discovery in cancer research.”

The researchers have developed three types of transposons, which can be moved around the genome to achieve different effects. Some will find genes involved in blood cancers, some in solid tumors, and some can find genes in both. They have also developed innovative methods that let researchers activate the transposon only in the specific organ they are examining--be it lung, liver, pancreas, or any other tissue in the mouse.

With the PiggyBac model now working to identify genes, the scientists will extend its reach-- looking for additional genes underlying a whole range of cancers in different organs of the mouse.

The centers predicating on the project are the Wellcome Trust Sanger Institute, Wellcome Trust Genome Campus, and the Instituto de Medicina Oncológica y Molecular de Asturias (IMOMA; Oviedo, Spain).


Related Links:
Wellcome Trust Sanger Institute
Instituto de Medicina Oncológica y Molecular de Asturias