Shinichiro Takahashi

Fms‐like tyrosine kinase‐3 (FLT3) is a commonly mutated gene in acute myeloid leukemia (AML). The two most common mutations are the internal‐tandem duplication domain (ITD) mutation and the tyrosine kinase domain (TKD) mutation. FLT3‐ITD and FLT3‐TKD exhibit distinct protein stability, cellular localization, and intracellular signaling. To understand the underlying mechanisms, we performed proximity labeling with TurboID to identify proteins that regulate FLT3‐ITD or ‐TKD differently. We found that BRCA1/BRCA2‐containing complex subunit 36 (BRCC36), a specific K63‐linked polyubiquitin deubiquitinase, was exclusively associated with ITD, not the wild type of FLT3 and TKD. Knockdown of BRCC36 resulted in decreased signal transducers and activators of transcription 5 phosphorylation and cell proliferation in ITD cells. Consistently, treatment with thiolutin, an inhibitor of BRCC36, specifically suppressed cell proliferation and induced cell apoptosis in ITD cells. Thiolutin efficiently affected leukemia cell lines expressing FLT3‐ITD cell viability and exhibited mutual synergies with quizartinib, a standard clinical medicine for AML. Furthermore, mutation of the lysine at 609 of ITD led to significant suppression of K63 polyubiquitination and decreased its stability, suggesting that K609 is a critical site for K63 ubiquitination specifically recognized by BRCC36. These data indicate that BRCC36 is a specific regulator for FLT3‐ITD, which may shed light on developing a novel therapeutic approach for AML.