Regulating nucleotides synthesis can promote response to immunotherapy

Aberrant expression of urea cycle enzymes occurs in multiple types of cancer. We have previously demonstrated that the urea cycle enzyme ASS1 is downregulated in different tumor types to increase the availability of its substrate aspartate for nucleotide synthesis, supporting cell proliferation. However, in several prevalent types of cancer ASS1 is overexpressed for a yet unknown metabolic benefit that associates with poor patients’ prognosis. Outside the liver, ASS1 is essential for arginine and nitric oxide (NO) generation, we thus hypothesized that cancer cells with high ASS1 levels might be arginine/ NO dependent.

We now find that ASS1 expression in cancer is regulated together with c-MYC transcriptional signature in response to nutrient deficiency and correlates with an increased gluconeogenesis flux. We further demonstrate that under glucose deprivation, ASS1 contributes to cancer survival in vitro and in vivo, by generating NO required for activating PC and PEPCK via S-nitrosylation. The increase in ASS1 expression generates a nucleotide imbalance favouring purines, leading to transversion mutations manifested at the DNA, RNA and protein level. This metabolic alteration leads to presentation of less immunogenic neo-antigens and to decreased response to immune checkpoint inhibitors independent of mutational load. Reversing the nucleotide imbalance to favour pyrimidines leads to better response to immunotherapy.

Our data firmly establish that beyond mutational load and tumor heterogeneity, purine/pyrimidine bias is a strong determinant of the response to immunotherapy. Importantly, our findings suggest that we can metabolically manipulate tumor mutations to improve patients’ response to immunotherapy.