In Vitro and In Vivo Evidence That TSC-22 Is a Putative Tumor Suppressor Gene in Primary Murine and Human Leukemia.

Transforming growth factor-β-stimulated clone-22 (TSC-22) is a gene that has been shown to be silenced in brain and prostate cancer, but its function and the mechanism responsible for this silencing are unknown. We used our model of spontaneous T-natural killer (NK) acute lymphoblastic leukemia (ALL) and discovered that the TSC-22 promoter was methylated resulting in absent expression in seven of eight cases of primary NK-T ALL, but not in cells from normal mice or mice with polyclonal expansion of T and NK cells. We found that TSC-22 was undetectable or minimally expressed in mouse lymphoma cell lines YAC-1 and EL-4 and human leukemia cell lines Jurkat and RPMI 8866, but treatment with the demethylation agent 5-aza-2′-deoxycytidine restored or increased TSC-22 expression. We mapped the TSC-22 promoter and discovered a C_PG island in the proximal region and determined that its methylation was responsible for the decreased gene expression. Over-expression of TSC-22 slowed in vitro cell growth and resulted in a dramatic decrease of tumor size in vivo. Finally, TSC-22 expression was found to be absent or substantially reduced in human chronic lymphocytic leukemia and acute myeloid leukemia compared to normal human tissue. Collectively, our data indicate that TSC-22 is silenced via DNA methylation within its proximal promoter, and this silencing appears to contribute to its function as a putative tumor suppressor gene in leukemia. Silencing of TSC-22 can be reversed by 5-aza-2′-deoxycytidine, recently approved by the FDA for the treatment of myelodysplastic syndrome.