Researchers at the WTCHG have discovered a key regulatory pathway in breast cancer cells that is directly associated with patient survival. The finding suggests a way to target tumours more effectively with certain drugs or even engineer them so that they become vulnerable to the effects of these drugs.
Jiannis Ragoussis, Elena Favaro and Christine Blancher at the WTCHG, collaborating with Adrian Harris and colleagues at Oxford's Weatherall Institute of Molecular Medicine, and colleagues in the United States, have published their results in PLoS one.
The Oxford groups have been working together over the past few years to look for markers for hypoxia in breast cancer. Solid tumours, including breast cancer tumours, can become restricted in their oxygen supply or 'hypoxic' as they grow, making them more aggressive and resistant to radiotherapy. Some drugs target hypoxic tumours, but identifying which tumours are hypoxic and therefore susceptible to the drugs can be difficult.
Their search has focussed on the role that microRNAs might play in identifying hypoxic tumours. microRNAs are a type of small non-coding RNA molecule that can regulate genes by interfering with the steps after transcription, ultimately resulting in a reduction in the amount of protein synthesised. The researchers previously found that hypoxia in breast cancer cells results in increased production of one specific microRNA, named miR-210. Expression of miR-210 is also strongly associated with poor patient survival.
In their recent paper, the groups describe the next step in their work - finding a target of miR-210 that is relevant for breast cancer. In work carried out at the Centre, the researchers combined bioinformatics searches for predicted targets with experimental tests, and found a likely target called ISCU (Iron Sulphur Cluster protein) that is significantly down-regulated in breast cancer cells compared to their normal counterpart.
ISCU is needed to assemble critical components of enzymes that are important for a range of functions in the cell including oxygen-dependent generation of energy (respiration). Downregulation of ISCU impairs this and other metabolic processes while promoting others, and is likely to contribute to the tumour cells' ability to survive under hypoxic conditions.
Using their collections of samples of breast cancer and another type of solid tumour, as well as samples from collaborators, the researchers went on to show an association between ISCU and patient survival. They found a striking correlation between ISCU levels and relapse-free survival in the samples - cancers with low ICSU had a worse prognosis.
miR-210 could prove useful as a prognostic tool or to direct treatment. 'We're interested in this microRNA because it's the most robust mico RNA induced in hypoxic conditions and it has got a strong association with prognosis,' says Dr Favaro. 'These microRNAs can be detected in the bloodstream, so this would be a less invasive way of testing tumours.'
Further work to determine the expression levels of miR-210 and pinpoint which genes miR-210 targets could also help to select more effective treatment regimes. Currently the group is using state of the art technologies to identify all the miR-210 targets in breast cancer cells. 'In principle, once we understand more, we could manipulate miR-210 levels or take advantage of its presence and make a tumour more vulnerable to combinations of drugs,' says Dr Ragoussis.
Breast cancer cell lines under normal (N) and reduced oxygen (02) conditions. Reduced oxygen triggers release of molecules, known as free radicals, linked to the ISCU pathway. These can be fluorescently stained (middle panel). Introducing an RNA molecule that blocks miR-210 almost completely prevents their release (right-hand panel).
Reference PMID PMC2859946
Favaro E, Ramachandran A, McCormick R, Gee H, Blancher C, Crosby M, Devlin C, Blick C, Buffa F, Li JL, Vojnovic B, Pires das Neves R, Glazer P, Iborra F, Ivan M, Ragoussis J, Harris AL. MicroRNA-210 regulates mitochondrial free radical response to hypoxia and krebs cycle in cancer cells by targeting iron sulfur cluster protein ISCU. PLoS One. 2010 Apr 26;5(4).