Baumann U, Fernández-Sáiz V, Rudelius M, Lemeer S, Rad R, Knorn AM, Slawska J, Engel K, Jeremias I, Li Z, Tomiatti V, Illert AL, Targosz BS, Braun M, Perner S, Leitges M, Klapper W, Dreyling M, Miething C, Lenz G, Rosenwald A, Peschel C, Keller U, Kuster B, Bassermann F. (24-11-2014).

Nature Medicine, 2014, doi: 10.1038/nm.3740

Department: 

Research Area C

Abstract: 

We searched for genetic alterations in human B cell lymphoma that affect the ubiquitin-proteasome system. This approach identified FBXO25 within a minimal common region of frequent deletion in mantle cell lymphoma (MCL). FBXO25 encodes an orphan F-box protein that determines the substrate specificity of the SCF (SKP1-CUL1-F-box)(FBXO25) ubiquitin ligase complex. An unbiased screen uncovered the prosurvival protein HCLS1-associated protein X-1 (HAX-1) as the bona fide substrate of FBXO25 that is targeted after apoptotic stresses. Protein kinase Cδ (PRKCD) initiates this process by phosphorylating FBXO25 and HAX-1, thereby spatially directing nuclear FBXO25 to mitochondrial HAX-1. Our analyses in primary human MCL identify monoallelic loss of FBXO25 and stabilizing HAX1 phosphodegron mutations. Accordingly, FBXO25 re-expression in FBXO25-deleted MCL cells promotes cell death, whereas expression of the HAX-1 phosphodegron mutant inhibits apoptosis. In addition, knockdown of FBXO25 significantly accelerated lymphoma development in Eμ-Myc mice and in a human MCL xenotransplant model. Together we identify a PRKCD-dependent proapoptotic mechanism controlling HAX-1 stability, and we propose that FBXO25 functions as a haploinsufficient tumor suppressor and that HAX1 is a proto-oncogene in MCL.