MCRI spin-off to commercialise 'plug-and-play' chromosome

There's no easy way of popping a gene into human cells to repair or replace a defective gene -- at least, not yet.

But Dr Andy Choo's research team at the Murdoch Children's Research Institute (MCRI) in Melbourne is developing such a technology: a sort of plug-and-play gene 'cassette' employing a tiny, but fully functional human artificial chromosome (HAC).

The Murdoch Institute has spun off an as-yet unlisted company, Ausgenics, and appointed former Genentech and Biota executive Dr Hugh Niall to handle commercialisation and strategic planning. Niall is also a board member.

Niall sees Choo's mini-chromosome as an ideal, safe vector for gene therapy, he also sees rich opportunities for synergies with the emerging field of stem cell science.

Ausgenics has two rivals -- Vancouver-based Canadian company Chromos, and US-based Athersys, which are both developing HACs from cut-down chromosomes. But the Ausgenics' HAC has two unique attributes: it's completely free of superfluous genes, and contains no repetitive DNA sequences around its centromere that could act as unwanted recombination sites during cell division.

"We think Andy's approach is unique, and very different to the Chronos and Athersys HACs," Niall said.

The Australian-developed HAC contains only the bare essentials: a knot-like centromere, to which attach the spindle fibres that tow newly replicated chromosomes to opposite poles of the cell during cell division, and the telomeres that 'cap' the ends of the chromosome, preventing its DNA unravelling.

The number of genes that can be inserted into the chromosomal cassette is effectively unlimited. In contrast, viral vectors such as retroviruses or adenoviruses are so tiny that they may not be able accommodate a single, large human gene such as the giant dystrophin gene involved in Duchenne's muscular dystrophy unless it is 'pruned' to fit, which may limit its function.

And viral vectors involve risks. The recent episode in which two of 10 boys in a French gene therapy trial developed leukaemia after receiving retrovirally-modified bone-marrow cells to cure severe combined immune deficiency (SCID) attested to the risks that retroviral vectors may initiate cancers by disrupting other genes in the act of inserting therapeutic genes into essentially random chromosomal locations. Retroviral vectors are also inherently unstable -- multiple copies can integrate in different chromosomes, and during genomic stress, may 'jump' to new locations, like their ancient viral cousins, retrotransposons.

Adenoviruses, another candidate vector, do not always integrate transgenes into chromosomes; their effects tend to be short-lived lasting no more than single cell cycle.

Repeated adenovirus therapy provokes an immune response against the virus, but any use immunosuppressant drugs can allow the virus to multiply out of control. The death in 2001 of US gene-therapy patient Jesse Gelsinger yellow-flagged adenoviruses as transient expression vectors. For these reasons, says Niall, gene therapy trials using retroviral and adenoviral vectors have been put on hold in some countries, including the US, and are proceeding cautiously in others - so there is a lot of interest in alternative approaches.

A human artificial chromosome sidesteps such hazards; the embedded transgene(s) are replicated naturally each time the cell divides.

Choo's team found the starting material for its mini-chromosome in a young boy diagnosed with a mild genetic disorder at the Murdoch Institute in the mid-1990s. It consists of a chromosome fragment containing a new centromere that formed spontaneously when the original chromosome fragmented.

"A human artificial chromosome has a lot going for it, and it has the capacity to do quite complex things," Niall said.

A package of cellular growth-factor genes, delivered into embryonic or adult stem cells, could reprogram them to differentiate into particular cell types -- new liver cells to repair cirrhotic livers, new heart-muscle cells to repair ailing hearts, or new bone marrow cells to treat chronic bone marrow failure and anaemia in aged people, or new neurons for brains affected by Alzheimer's disease or motor neuron disease.

In the longer term, it might even be possible to use the HAC 'cassette' to program ES cells to initiate the development of entire new organs.

"The technology is still at an early stage and will take time to develop, but it intersects with multiple fields, including stem-cell research," Niall said. "We see it growing over a number of years. But it's such a fundamentally important, generic technology, to be able to transform cells with these gene cassettes, and have them replicate in a stable manner."