by ecancer reporter Clare Sansom

Retinoblastoma is a childhood cancer that develops in the cells of the retina at the back of the eye. Although it is the commonest inherited paediatric cancer, it is still rare, with fewer than 50 new cases diagnosed in the UK every year.

It arises when both copies of a tumour suppressor gene known as RB1 and located on human chromosome 13 are mutated. There are two forms of the disease; hereditable (in which the child inherits one defective RB1 allele and in which tumours are more likely to occur in both eyes) and sporadic.

Although the basic genetic defect in retinoblastoma has been known for decades, the overall molecular mechanisms that lead to tumour development are still unknown.

A large group of researchers from the St Jude Children’s Research Hospital –Washington University Pediatric Cancer Genome Project, led by Michael Dyer and James Downing from the St Jude Children’s Research Hospital, Memphis, Tennessee, USA, have now mapped the genetic landscape of retinoblastoma by sequencing the whole genomes of four retinoblastomas and paired samples of normal tissue from the same patients.

The transcriptomes of the four primary tumours were also sequenced.

A total of 668 validated somatic mutations and 40 structural variations were observed in the four tumour samples, but only eleven of these resulted in amino acid changes. This indicates that the retinoblastoma genome is comparatively stable, with few genetic lesions other than RB1 inactivation required for tumour development.

This was confirmed by sequencing the genome sequence of an orthoptic xenograft of one retinoblastoma, maintained for nine months in the eyes of immunocompromised mice, and showing it to be very similar to that of the primary tumour.

The researchers then sequenced the 11 genes with observed mutations leading to amino acid changes in a further 42 retinoblastoma samples, and found only one of these, the transcriptional co-repressor BCOR, to be mutated recurrently in this tumour type.

Epigenetic changes such as DNA methylation have been associated with cancer development for many years. To test the importance of epigenetic deregulation in retinoblastoma, the researchers analysed patterns DNA methylation and gene expression in samples of the primary tumours; of the xenograft; and of normal fetal retina.

A total of 104 genes, including 15 that have clear associations with cancer, were found to be epigenetically deregulated in the tumour samples.

One of the genes found to be upregulated in the retinoblastoma samples compared to normal retinal tissue was a kinase, SYK, which has already been implicated in several haematological cancers.

This increase in SYK gene expression in retinoblastomas was confirmed using real-time PCR analysis; furthermore, immunohistochemistry showed the protein SYK to be expressed in 82 of 82 retinoblastoma tumours but not in the normal retina. Knocking down SYK expression in retinoblastoma tissue using a small hairpin RNA dramatically increased tumour cell apoptosis.

The researchers then exposed retinoblastoma cells expressing high levels of SYK to two small-molecule inhibitors of this kinase, BAY61-3606 and R406, and showed them to induce tumour cell death.

The inhibitor BAY 61-3606 was also tested in vivo using the mouse xenograft model and shown to delay tumour progression. Taken together, these results suggest that SYK, which is epigenetically deregulated in retinoblastoma, is an important oncogene in this tumour type and that it may be considered a useful target for novel retinoblastoma drugs.

Reference

Zhang, J., Benavente, C.A., McEvoy, J. and 33 others (2012). A novel retinoblastoma therapy from genomic and epigenetic analyses. Nature, published online ahead of print 11 January 2012. doi:10.1038/nature10733