Deregulated Expression of Ubiquitin-Specific Peptidase Genes in Myeloid Leukemia

Acute myeloid leukemia (AML) is a genetically heterogeneous disease characterized by mutations in genes regulating transcription and in genes involved in signaling pathways, leading to aberrant cellular differentiation and proliferation. Transcription factors such as RUNX1, the DNA-binding subunit of the core-binding factor, are frequently rearranged in AML by chromosomal translocations. We have characterized a second case of t(7;21) (p22;q22) translocation in AML cells and confirmed that USP42 is a recurrent fusion partner of RUNX1. A 10-fold upregulation of USP42 mRNA was detected in the t(7;21) leukemic cells. The USP42 gene is a member of the ubiquitin-specific peptidase (USP) family which catalyzes the removal of an ubiquitin moiety on target proteins. Targets of USP42 are currently unknown. USPs are deubiquitinating enzymes involved in a growing number of cancers but the role of this family of proteins in leukemogenesis remains to be studied.

We have analyzed the expression profile of selected USP genes including USP7, USP14, USP16, USP28 and USP42 in myeloid leukemic samples with different morphologic and karyotypic features by real-time quantitative PCR analysis. USP7, an essential component of the p53-Mdm2 pathway, is a deubiquitinating enzyme for both Mdm2 and p53, involved in cellular proliferation and apoptosis. USP7 regulates p53 stability in a highly complex manner. Different expression profiles of USP7 have been described in cancers. A 2 to 10-fold dowregulation of USP7 was observed in 18 out of 115 (~15%) specimens of myeloid leukemia. Interestingly, we also found a 6 to 38-fold upregulation of USP7 in 9 out of 115 (~8%) leukemic specimens. Four out of five samples of chronic myeloid leukemia in blastic phase expressed high USP7 mRNA level. Involved in DNA repair and in DNA damage-induced apoptosis, USP28 was recently described as a deubiquitinating enzyme of MYC, a highly relevant gene in hematologic malignancies. In response to DNA damage, MYC levels decline following USP28 dissociation from the ubiquitin ligase FBW7. Downregulation of USP28 mRNA expression (3 to 20-fold) was detected in 6 AML specimens with different karyotypes including trisomy 8, complex and normal karyotypes. Two other USPs were also studied because of their potential relevance to hematologic cancers. USP16 regulates HOX gene expression through histone H2A deubiquitination (Joo HY, Nature 2007) and USP14 is highly expressed in myeloid and lymphoid leukemic cell lines (Ishiwata S, J.Biochem 2001). The expression level of these two USPs was not abnormal in our study, but we are currently investigating more leukemic samples to confirm these results.

The deregulation of USP7 and USP28 expression and the presence of a recurrent translocation involving USP42 in our study, suggest an important role for this family of enzymes in the pathogenesis of myeloid leukemia. Deregulated expression of genes that encode enzymes involved in the ubiquitin-proteasome pathway opens new fields of investigation, since these enzymes could potentially be targets for novel therapy in AML.