The mRNA-destabilizing proteins ZFP36L1 and ZFP36L2 are described as mediators of quiescence and play a pivotal role in hematopoietic malignancies. Both genes are mainly classified as tumor suppressor genes as they posttranscriptionally downregulate the expression of oncogenes and contribute to cellular quiescence. Here, we analyzed the role of ZFP36L1 and ZFP36L2 in chronic myeloid leukemia (CML). We found ZFP36L1 and ZFP36L2 expression to be deregulated in patients with CML. By use of in vitro models of tyrosine kinase inhibitor resistance, an increase in ZFP36L1 and ZFP36L2 expression was detected during the development of imatinib resistance. CRISPR/Cas9-derived knockout of ZFP36L1, but not of ZFP36L2, in imatinib-sensitive cells led to decreased proliferation rates in response to tyrosine kinase inhibitor treatment. This effect was also observed in untreated ZFP36L1 knockout cells, albeit to a lower extent. Genomewide gene expression analyses of ZFP36L1 knockout cells revealed differential expression of cell cycle regulators, in particular upregulation of the cell cycle inhibitor CDKN1A. In addition, the 3' untranslated region of CDKN1A was proven to be a direct target of ZFP36L1. This indicates that tumor suppressor genes can also be targeted by ZFP36L1. Hence, ZFP36L1 cannot unambiguously be regarded as a tumor suppressor gene.
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