A gain‐of‐function mutation in microRNA 142 is sufficient to cause the development of T‐cell leukemia in mice

MicroRNAs (miRNAs) play a crucial role in regulating gene expression. MicroRNA expression levels fluctuate, and point mutations and methylation occur in cancer cells; however, to date, there have been no reports of carcinogenic point mutations in miRNAs. MicroRNA 142 (miR‐142) is frequently mutated in patients with follicular lymphoma, diffuse large B‐cell lymphoma, chronic lymphocytic leukemia (CLL), and acute myeloid leukemia/myelodysplastic syndrome (AML/MDS). To understand the role of miR‐142 mutation in blood cancers, the CRISPR‐Cas9 system was utilized to successfully generate miR‐142‐55A>G mutant knock‐in (Ki) mice, simulating the most frequent mutation in patients with miR‐142 mutated AML/MDS. Bone marrow cells from miR‐142 mutant heterozygous Ki mice were transplanted, and we found that the miR‐142 mutant/wild‐type cells were sufficient for the development of CD8(+) T‐cell leukemia in mice post‐transplantation. RNA‐sequencing analysis in hematopoietic stem/progenitor cells and CD8(+) T‐cells revealed that miR‐142‐Ki/+ cells had increased expression of the mTORC1 activator, a potential target of wild‐type miR‐142‐3p. Notably, the expression of genes involved in apoptosis, differentiation, and the inhibition of the Akt–mTOR pathway was suppressed in miR‐142‐55A>G heterozygous cells, indicating that these genes are repressed by the mutant miR‐142‐3p. Thus, in addition to the loss of function due to the halving of wild‐type miR‐142‐3p alleles, mutated miR‐142‐3p gained the function to suppress the expression of distinct target genes, sufficient to cause leukemogenesis in mice.