The tumor suppressor gene hCDC4 is frequently mutated in human T-cell acute lymphoblastic leukemia with functional consequences for Notch signaling

Notch signaling is of crucial importance in normal T-cell development and Notch I is frequently mutated in T-cell acute lymphoblastic leukemias (T-ALL), leading to aberrantly high Notch signaling. In this report, we determine whether T-ALL mutations occur not only in Notch1 but also in the F-box protein hCdc4 (Sel-10, Ago, or Fbxw7), a negative regulator of Notch1. We show that the hCDC4 gene is mutated in leukemic cells from more than 30% of patients with pediatric T-ALL and derived cell lines. Most hCDC4 mutations found were missense substitutions at critical arginine residues (Arg465, AX9479, and Arg 505) localized in the substrate-binding region of hCdc4. Cells inactivated for hCdc4 and T-ALL cells containing hCDC4 mutations exhibited an increased Notchi protein half-life, consistent with the proposed role of hCdc4 in ubiquitin-dependent proteolysis of Notchl. Furthermore, restoration of wild-type but not mutant hCdc4 in HCT 116 hCDC4-negative cells led to an increased Notchl ubiquitylation and decreased Notchl signaling. These results show that hCdc4 mutations interfere with normal Notchl regulation in vivo. Finally, we found that mutations in hCDC4 and NOTCH] can occur in the same cancers and that patients carrying hCDC4 and/or NOTCH] mutations have a favorable overall survival. Collectively, these data show that mutation of hCDC4 is a frequent event in T-ALL and suggest that hCDC4 mutations and gainof-function mutations in NOTCH-1 might synergize in contributing to the development of pediatric T-ALL leukemogenesis.