DNA-directed RNA interference (ddRNAi)

RNA interference (RNAi) has been described as ‘the most important and exciting breakthrough of the last decade, perhaps multiple decades’ by the Nobel laureate Phillip Sharp and the ‘breakthrough of the year’ by Science magazine, 2002. RNAi is a cellular mechanism that selectively negates the expression of any gene by destroying messenger RNA, the courier that delivers instructions from a gene to the ribosomes within the cell to manufacture the proteins coded for by the gene. Proponents of the technology say that it will be able to silence the genes that cause HIV, Hepatitis and even cancer. In addition to the above, targeted inhibition of gene expression may offer the possibility of treating dominantly inherited genetic disorders. Mick Graham, from a Queensland based biotechnology company called Benitec, has developed an alternative and more efficient way of doing this called DNA-directed RNA interference (ddRNAi). This technique overcomes many of the limitations of conventional RNAi.

RNA interference (RNAi) involves introducing a long double strand of RNA into a cell, one strand of which is identical to the RNA that is being targeted to be switched off. The cell identifies the double stranded RNA as an intruder (because the cell naturally only produces single stranded RNA) and then destroys it. In the process the target RNA is destroyed, thereby switching off the function of that gene. However, this revolutionary technology has many limitations in mammalian cells, including nonspecific suppression of gene expression, as opposed to the gene-specific suppression seen in other organisms

Benitec’s ddRNAi technology involves inserting a DNA construct into a cell, triggering the production of double stranded RNA (dsRNA), which is then cleaved into small interfering RNA (siRNA) as part of the RNAi process, resulting in the destruction of the target mRNA and knocking-down or silencing the expression of the target gene. In plant research, Benitec’s DNA based technology has been used to successfully switch off the gene that causes ‘blackheart’ in pineapples. When a pineapple is put into cold storage or hit by frost, an enzyme is released that turns the centre of the pineapple black or brown. It costs the pineapple industry millions of dollars every year. After identifying the gene that causes the enzyme to be released, a DNA construct targeting that gene is fired into pineapple cells using a technique called ‘biolistics.’ These cells are then cloned and grown up to produce pineapples that have the gene that causes ‘blackheart’ disenabled. The potential applications are enormous.

Benitec has also demonstrated the ability for this technology to switch off a gene in an existing mammal. ‘Betty’ is the lab mouse who is living proof of this. Small patches of her black coat pigments were switched off using this technology. The success of this ‘proof of concept’ experiment gives Benitec a dominant intellectual property position in the field.

They have now turned their attention to human cells and have shown that human breast cancer cells can be switched off in vitro. The prospects for this technology in developing therapeutic applications for diseases including cancers, autoimmune dysfunctions and viral infections are far-reaching. Additionally, it can be used as a tool for determining gene function – if you can selectively switch off a gene you can then observe its biological function.

The fast moving biotechnology industry is dominated by giant multinational corporations; however Benitec is the world leader in RNAi and holds the dominant patent estate in this rapidly moving field. Although Benitec jointly owns the core technology with the DPI, it has exclusive worldwide rights for development and commercialisation. In addition to its medical research and therapeutic development activities, Benitec offers licenses to undertake research and development on its expanding technology platform, and tools to deliver the technology’s full capability.