Lessons from a 'natural genetic engineer'

TOWARDS BETTER GENE THERAPY: Assoc Prof Pan Shen Quan's work on Agrobacterium tumefaciens will shed more light on genetic engineering. For a long time, scientists have been fascinated about a bacterium (Agrobacterium tumefaciens) that can cause tumours in plants. But today, this bacterium is making even a greater stir in the scientific community - because what's exciting is not only how it feeds off its host in the plant kingdom, but its ability to actually transfer its DNA into virtually all kinds of cells including human cells. It is the only bacterium which can do this. With its reputation as a natural genetic engineer, the bacterium can open a window to provide us with an explicit picture of the process of genetic engineering.

A team at the Department of Biological Sciences led by Assoc Prof Pan Shen Quan has been tracking the action of this bacterium, and has made significant progress. Explained Assoc Prof Pan: "Agrobacterium tumefaciens can deliver DNA into different cells including plant, yeast, fungal and human cells. This DNA transfer represents the only known example of inter-kingdom transfer of genetic information." Hence, studying the mechanisms of the bacterium and its pathway of DNA delivery into the cell is of great significance in the development of gene therapy.

Recognised for leading edge research

Assoc Prof Pan was an inaugural winner of the Temasek Young Investigator Award. In 2002, he received the Award from the Defence Science and Technology Agency (DSTA) and NUS for his "bacterial surface display" technology to breed special proteins on cell surfaces. The technology may be used to develop novel vaccines, diagnostic devices and therapeutic treatments. One of its applications is the detection of infectious agents . The technology can speed up the identification and production of antibodies that recognise the components of infectious agents .

Assoc Prof Pan obtained his PhD from the University of Kentucky, USA and conducted postdoctoral work at the University of Washington School of Medicine, USA. He has also been awarded the National Institutes of Health Postdoctoral Fellowship (USA), China-Cornell Fellowship (USA), National Outstanding Young Scientist Research Grant ( China). "We adopt a molecular genetic approach to identify both bacterial and eukaryotic genes responsible for the transfer process. With a combination of molecular genetics, biochemistry, genomics and proteomics approaches, we determine the functions of the novel proteins encoded by the genes involved in the process. These will help illustrate the transfer process," said Assoc Prof Pan.

Previously , the team found out how the bacteria infect host plants by coordinating the expression of a variety of genes encoding different functions, and conditions which enable them to infect their hosts. To survive in their ever changing environments, the bacteria must constantly monitor external conditions and adjust their structure, physiology, and behaviour accordingly. They are able to alter their gene expression rapidly and efficiently in response to environmental signals.

After understanding the bacterium and its mechanisms, the team was ready for the next step which was to illustrate the DNA complex trafficking pathway inside the host cells before DNA integration occurs. "We have since developed the Agrobacterium-eukaryote DNA delivery system as a model system to study the fundamental process of nucleoprotein trafficking," said Assoc Prof Pan.

Their study will pave the way for more efficient human gene therapy where encoded genetic information is delivered by vectors (transporting vehicles) to target cells of the patient to restore health. Gene therapy has shown promising results for patients who face graft rejection after organ transplant, as well as for patients suffering from AIDS (Acquired Immune Deficiency Syndrome) caused by human immunodeficiency virus (HIV), with promising results.

"However, to obtain desirable therapeutic results, it is important to achieve efficient gene delivery and subsequent expression of the transferred gene (transgene). Thus, the use of a suitable gene therapy vector is extremely important," said Assoc Prof Pan.

The ability of certain viruses to integrate their genome into the host chromosome makes them good therapy vectors. However viral vector poses potential dangers such as toxicity effect in individuals with different genetic and immunological backgrounds.

A non-viral gene delivery approach is safer but has been known to be less efficient. Traditional non-viral agents have taken the form of chemical carriers such as cationic polymer and lipid. The Agrobacterium tumefaciens however, seems to be a good alternative - it can carry a large enough fragment of DNA into the cells, and it doesn't replicate in the human body to cause any problem, said Assoc Prof Pan.

His team will further conduct research aimed to understand how the DNAs transferred are trafficked within the cells. "Based on our knowledge on the transfer process, we will design and develop novel Agrobacterium-based DNA delivery systems for gene therapy. We will also develop novel gene and protein delivery systems for various organisms," said Assoc Prof Pan.