Epigenetic status of argininosuccinate synthetase and argininosuccinate lyase modulates autophagy and cell death in glioblastoma.

Argininedeprivation, either by nutritional starvation or exposure to ADI-PEG20, induces adaptive transcriptional upregulation of ASS1 and ASL in glioblastoma multiforme ex vivo cultures and cell lines. This adaptive transcriptional upregulation is blocked by neoplasia-specific CpG island methylation in either gene, causing arginine auxotrophyand cell death. In cells with methylated ASS1 or ASL CpG islands, ADI-PEG20 initially induces a protective autophagic response, but abrogation of this by chloroquine accelerates and potentiates cytotoxicity. Concomitant methylation in the CpG islands of both ASS1 and ASL, observed in a subset of cases, confershypersensitivity to ADI-PEG20. Cancer stem cells positive for CD133 and methylation in the ASL CpG island retain sensitivity to ADI-PEG20. Our results show for the first time that epigenetic changes occur in both of the two key genes of argininebiosynthesis in human cancer and confer sensitivity to therapeutic argininedeprivation. We demonstrate that methylation status of the CpG islands, rather than expression levels per se of the genes, predictssensitivity to argininedeprivation. Our results suggest a novel therapeutic strategy for this invariably fatal central nervous system neoplasmfor which we have identified robust biomarkers and which overcomes the limitations to conventional chemotherapy imposed by the blood/brain barrier.