Abstract
Aberrant cellular metabolism contributes significantly to the growth and proliferation of several tumour types. Identification of genes that control critical metabolic pathways is a major factor in the development of novel therapies that target metabolic defects in tumour cells. Our aim is to identify such genes in paediatric high grade glioma that are altered due to promoter hyper-methylation of cytosine residues in CpG dinucleotides. Genome wide DNA methylation profiling using Illumina infinium methylation 450K bead chip array was performed on 18 well-characterised short term cultures derived from paediatric high grade astrocytoma including 3 from diffuse intrinsic pontine glioma. Data analyses were based on beta scores of probes for each gene as measures of intensities of methylation. Genes were selected with beta scores of tumour > =0.70 and that of normal human astrocytes < =0.30. We identified that two vital genes involved in the regulation of arginine biosynthetic pathway, argininosuccinate synthetase 1(ASS1) and argininosuccinate lyase (ASL) were methylated in 9/18 (50%) cases. Hyper methylation was confirmed by methylation-specific PCR and up-regulation of gene expression following treatment with 2 µM 5-aza-2'-deoxyctidine. Down-regulation of ASS1 in hyper methylated samples was confirmed by Western blot analysis. Our findings report epigenetic deregulation of ASS1 and ASL in a subset of paediatric high grade glioma. The enzymes encoded by these genes are essential elements of urea cycle that function together in the de novo synthesis of arginine from citrulline. Tumour cells with deficient ASS1/ASL depend on external sources of arginine for survival and have been reported to be sensitive to autophagic cell death induced by arginine starvation. Therefore, further investigation may render the possibility of arginine-deprivation therapy in such sub type of paediatric high grade glioma. This therapeutic approach is of interest as tumour cells with abnormal expression of ASS1/ASL can be specifically targeted, while the normal cells can be spared.