Alternative Splicing of Tcfl5 Fine-tunes Notch1 Transcriptional Output and Cell Fate Decisions in T-cell Acute Lymphoblastic Leukemias

Aberrant NOTCH1 signaling drives T-cell acute lymphoblastic leukemia (T-ALL). TCFL5 is highly expressed in T-ALL clinical samples, producing four isoforms with distinct expression patterns and risk associations. In leukemic cells, TCFL5 isoforms play opposing roles: the CHA isoform promotes proliferation, while TCFL5_E6 and TCFL5_E8 suppress tumor growth. Mechanistically, TCFL5 acts as a transcriptional effector and feedback regulator of NOTCH1, modulating its transcriptional output by cobinding promoters of key target genes like BCL2 and HES1. NOTCH1 reciprocally regulates TCFL5 isoform expression, forming a feedback loop that governs leukemic cell fate. Disrupting this loop—via pan-TCFL5 knockout or isoform-specific modulation—alters proliferation and survival in T-ALL models. These findings identify TCFL5 as a critical node in the NOTCH1 signaling network and underscore the therapeutic potential of targeting TCFL5 isoform dynamics to modulate oncogenic signaling in T-ALL.