Modulation of proteostasis by the ubiquitin-conjugating enzyme UBE2N in immunoproteasome-positive AML

Altered protein homeostasis through proteasomal degradation of ubiquitinated proteins is a hallmark of many cancers. Ubiquitination, coordinated by E1, E2, and E3 enzymes, involves up to 40 E2-conjugating enzymes in humans to specify substrate and ubiquitin linkages. In a screen for E2 dependencies in AML, UBE2N emerged as the top candidate. To investigate UBE2N's role in AML, we characterized an enzymatically-defective mouse model of UBE2N, revealing UBE2N's requirement in AML without significant impact on normal hematopoiesis. Unlike other E2s, which mediate Lysine-48 (K48) polyubiquitination and degradation of proteins, UBE2N exclusively synthesizes K63-linked chains, stabilizing or altering protein function. Proteomic analyses and a whole genome CRISPR-activation screen in pharmacologically and genetically UBE2N-inhibited AML cells unveiled a network of UBE2N-regulated oncoproteins. UBE2N inhibition reduced their protein levels, leading to increased K48-linked ubiquitination and degradation through the immunoproteasome and revealing UBE2N activity is enriched in immunoproteasome-positive AML. Furthermore, an interactome screen identified TRIM21 as the E3 ligase partnering with activated UBE2N in AML to modulate UBE2N-dependent proteostasis. In conclusion, UBE2N maintains proteostasis in AML by stabilizing oncoproteins through K63-linked ubiquitination and prevention of K48 ubiquitin-mediated degradation through the immunoproteasome. Thus, inhibition of UBE2N catalytic function suppresses leukemic cells through selective degradation of critical oncoproteins in immunoproteasome-positive AML.