PTPS-20: THE GD3 ACETYLATION PATHWAY AS A POTENTIAL THERAPEUTIC TARGET FOR PAEDIATRIC MEDULLOBLASTOMA

Medulloblastoma (MB), the most common malignant paediatric brain tumour, is associated with high mortality rates. Survivors frequently suffer from low quality-of-life as a result of aggressive treatment. Genomic studies have led to novel classification of molecularly and clinically distinct subgroups and are paving the way for novel targeted therapies. GD3, an oncofoetal ganglioside is re-expressed in some cancers of the neural crest. GD3, central to cell movement in developing brain, is internalised during maturation of residual supernumerary cells leading to cell death. When GD3 is re-expressed in cancers, the intracellular form of GD3 is acetylated (GD3^A) and is thought to protect cells from mitochondrially-mediated apoptosis. Sialate-O-acetylesterase (SIAE) is responsible for cleaving the acetyl group from GD3^A. A genome-wide transcriptional analysis of 103 MB patients revealed that this enzyme is significantly down-regulated in MB tissue compared to non-neoplastic foetal and adult brain (p > 0.02). This suggests that GD3^A may be deacetylated at low levels and thus the balance of GD3 to GD3^A favours GD3^A. Using flow cytometry to examine expression, results revealed that in MB cell lines RES256 and UW402 GD3^A expression was higher (84.5% and 74.5%) than GD3 (56.6% and 61.3%) The fluorescent intensity of GD3^A expression was 9.9 and 11.4 fold higher and GD3, 4.4 and 6.1 fold higher than negative controls. Immunocytochemical results were in agreement, providing additional evidence that these cells have a balance in favour of GD3^A. Previous studies from our group have shown that cell surface expression of GD3 correlates with invasion and in MB cell lines tested surface expression was 31.7% and 65.7% compared to GD3^A (29.5% and 11.5%). Inducible SIAE and SIAE catalytically mutated cell lines have been generated and verified in protein and esterase activity assays and will be used to evaluate this pathway as a potential therapeutic target for MB.