Sydney researcher discovers new piece in cancer puzzle

A molecular geneticist working on cancer at Westmead Hospital's Children's Medical Research Institute (CMRI) has made a chance discovery that throws new light on the mechanisms that immortalise cancer cells.

Whenever normal cells divide, the telomeres progressively fray. After about 50 rounds of replication, they run out of the repetitive DNA 'caps' that prevent chromosomes unravelling, and commit suicide.

Cancerous cells have at least two ways of avoiding such a fate, and achieving immortality. They can activate the telomerase gene whose enzyme reconstructs frayed telomeres, or they can employ the ALT (Alternative Lengthening of Telomeres) mechanism, discovered by Prof Roger Reddell, director of CMRI, in 1997.

The ALT mechanism, which operates in about 10 per cent of cancerous cells, lengthens telomeres by recombination - the repetitive DNA sequences serve as templates to create new DNA sequences that insert themselves between existing telomeric DNA.

One of Reddell's colleagues, Dr Wei-Qin Jiang, has now discovered, by serendipity a way to switch off the ALT mechanism.

Jiang was trying to track the movement of doughnut-shaped structures called PML bodies that form in the nuclei of cancerous cells, which contain proteins and repetitive DNA elements normally found in telomeres. PML structures are implicated as players in telomere lengthing via the ALT mechanism.

He inserted a transgene coding for a PML-associated protein called Sp100, after attaching a green fluorescent protein (GFP) 'tag' so he could visualise its location as cells underwent replication. His aim was to over-express the Sp100-GFP gene in cancerous cells in vitro to see if their telomeres shortened as they divided.

To his great surprise, he found that the cancerous cells, instead of continuing to divide, began to die.

Wei-Qin discovered that Sp100 normally sequesters a protein called NPS1, which, with two other proteins, MRE11 and RAD50, forms complexes that are believed to be key players in ALT-lengthening of telomeres.

When Sp100 is overexpressed, along with the GFP sequence, it sequesters so much NPS1 - and possibly, its partners, MRE11 and RAD50, that it blocks ALT replication.

He confirmed his hunch by using a short-interfering RNA (siRNA) construct to knock down expression of NPS1, predicting that it would have the same disruptive effect as over-expression of Sp100. It did.

In the longer term, Wei-Qin believes therapeutics to knock down expression of NPS1 and its partners could have a role to play as a combination therapy with telomerase inhibitors, to kill cancerous cells.

But in the short term, he says that the discovery merely throws light on the workings of the ALT mechanism. Even when Sp100 is overexpressed by 100-fold in cancerous cells, they continue dividing for another 80 rounds or so, suggesting that they have a third way of avoiding programmed cell death.