Low expression of asparagine synthetase in lymphoid blasts precludes its role in sensitivity to L-asparaginase

Childhood acute lymphoblastic leukemia (ALL) is treated with combined chemotherapy, including L-asparaginase (L-asp). Recent studies question the traditional view that the level of asparagine synthetase (ASNS), an enzyme producing the intracellular asparagine, correlates with the response to L-asp treatment. However, the importance of ASNS in response to L-asp has neither been confirmed nor refuted so far. In this study, we wanted to elucidate the effect of ASNS expression level on the sensitivity of ALL cells to L-asp treatment. We used four ALL cell lines (NALM-6, RS4;11, REH, and UOCB6) and 30 diagnostic bone marrow samples of ALL patients to study the relationship between ASNS expression and sensitivity to L-asp using MTS proliferation assay. RNA interference was used to study the effect of a range of ASNS levels on the response to L-asp treatment. Using a cell line model with a gradually knocked-down ASNS gene, we defined a cutoff level below which ASNS gene expression does not correlate with sensitivity to L-asp. Importantly, ASNS gene expression in patients' ALL blasts is below this level. We confirmed that there was no correlation between ASNS gene expression and sensitivity to L-asp in ALL blasts. In addition, we show that cells with low ASNS expression level do not respond to asparagine deprivation by upregulation of ASNS gene expression. In conclusion, the ASNS expression level does not predict sensitivity to L-asp in leukemic blasts. Moreover, cell lines with high basal expression of ASNS cannot serve as a valid model for studies on the relationship between the ASNS and L-asp cytotoxic effect.