by ecancer reporter Clare Sansom

Cancer of the pancreas is one of the most deadly of all malignancies, with fewer than one in five patients surviving for one year and about 3% for five years after diagnosis.  

By far the most common type of this disease is pancreatic ductal adenocarcinoma (PDA), in which the tumour develops in the epithelial cells of the pancreatic duct.

Significant progress has been made recently in identifying genes that are commonly mutated in PDA; KRAS, CDKN2A, TP53 and SMAD4 have each been found to be mutated in over 50% of pancreatic adenocarcinomas studied.

A large, international group of researchers led by David Tuveson from the Cancer Research UK Cambridge Research Institute, Cambridge, UK has now identified the deubiquitinase USP9X as a fifth gene that is very commonly mutated in these tumours and as a potent tumour suppressor.

The experiments used a mouse model previously generated in Tuveson’s group in which the G12D mutation in KRAS, conditionally expressed in the developing pancreas, causes the mice to develop PDA spontaneously after a long latency period.

The researchers interbred these mice with some in which mutated DNA sequences had been introduced into their genomes using a technique known as Sleeping Beauty (SB) transposon mediated insertional mutagenesis, and monitored the offspring for early progression to pancreatic cancer. This identified a number of gene candidates for PDA development, many of which had already been implicated in human pancreatic cancer. Others were associated with pathways that are known to be associated with cancer progression.

The gene found to be most commonly associated with accelerated cancer progression, however, was the deubiquitinase USP9X, which has not previously been linked to any tumour type in humans or mouse models. This gene, which is located on the X chromosome, codes for a ubiquitin-specific protease; it was inactivated in over 50% of the tumours tested.

The researchers then used RNA interference to down-regulate USP9X expression in mouse pancreatic cancer cell lines, and found that this knock-down suppressed anoikis (a type of programmed cell death) and increased proliferation in these cells.

Tuveson and his colleagues then analysed clinical data to establish whether this gene was dysregulated in human PDA. Tumours taken from 100 Australian patients who had undergone surgery for localized PDA yielded no somatic mutations in this gene, but low expression of USP9X mRNA was shown to correlate with poor outcomes after surgery.

Tumours from two other distinct PDA patient cohorts showed correlations between low or absent USP9X mRNA and protein expression levels and both increased metastasis and poor prognosis. These results implied that the mechanism of gene suppression was epigenetic, involving changes in promoter DNA methylation rather than somatic mutations. This result was confirmed by treating human PDA cell lines with a DNA methylase inhibitor, 5-aza-29-deoxycytidine, and a HDAC inhibitor, trichostatin A; both inhibitors were found to increase USP9X mRNA and protein expression levels.

Taken together, these results indicate that epigenetic silencing of the deubiquitinase USP9X is likely to be a genetic driver in a significant subset of pancreatic ductal adeno-carcinomas, and that the development of drugs that modulate expression of this gene by preventing DNA methylation might be a promising therapeutic approach for these patients.

Results in the mouse models also suggested that USP9X cooperates both with mutated KRAS and with the ubiquitin-protein ligase ITCH in promoting carcinogenesis and that further exploration of these interactions and pathways might prove useful.  

Reference

Pérez-Mancera, P.A., Rust, A.G., van der Weyden, L. and 33 others (2012). The deubiquitinase USP9X suppresses pancreatic ductal adenocarcinoma. Nature, published online ahead of print 29 April 2012. doi:10.1038/nature11114.