Inhibition of Retinoic Acid Receptor Signaling by SKI in Acute Myeloid Leukemia.

Objectives: Acute myeloid leukemia (AML) with monosomy 7 or deletion of the long arm of chromosome 7 (-7/7q-) is a leukemia with a poor outcome. Microarray analysis resulted in a gene expression profile specific for this subset of AML and showed SKI, a nuclear Co-repressor protein, being strongly upregulated. SKI is described to be a transforming gene, which inhibits TGF-beta-, SMAD- and retinoic acid (RA)-signaling.

Materials, methods and patients: Gene expression profiling was performed using cDNA microarrays (detecting 4608 genes). Gene expression pattern of 20 AML (9 pts. with -7/7q- and 11 pts. with normal karyotype) patients was compared to 23 healthy bone marrow donors. Expression data of interestings genes were confirmed by real-time PCR in samples of 111 AML patients. In cell culture experiments HL60 and U937 leukemic cell lines were used to investigate the influence of the identified oncogene Ski on differentiating properties. In a reporter gene assay retinoic acid response element (RARE) was transfected in QT6 cells. U937 were retrovirally transfected with Ski wild type and Ski mutant.

Results: Microarray analysis in AML (-7/7q-), revealed SKI being upregulated in AML. The group of patients with a high SKI expression level had a poorer overall survival when compared to the low expressers (p = 0.0279). Real-time PCR analysis comparing 111 AML samples to healthy CD34+ haematopoietic stem cells revealed, that SKI levels were highest in AML with -7/7q-, moderately elevated in most other karyotypes except AML with translocations involving retinoic acid receptor alpha (RARalpha). This suggests that Ski might interfere with RARalpha function in leukemia. Using reporter gene assays we could show that expression of wild-type, but not mutated SKI blocked RA induced differentiation. SKI was down regulated in HL60 leukemia cell line when differentiation was induced by ATRA and or valproic acid.

Conclusion: Upregulation of SKI has prognostic significance in AML. Inhibition of RA signaling may be more often implicated in pathogenesis of AML than anticipated so far. The Co-repressor of histone deacetylases (HDACs) Ski might therefore become a molecular target for the induction of differentiation in the treatment of AML.