Disruption of the β1L Isoform of GABP Reverses Glioblastoma Replicative Immortality in a TERT Promoter Mutation-Dependent Manner

Abstract

TERT promoter mutations reactivate telomerase, allowing for indefinite telomere maintenance and enabling cellular immortalization. These mutations specifically recruit the multimeric ETS factor GABP, which can form two functionally independent transcription factor species: a dimer or a tetramer. We show that genetic disruption of GABPβ1L (β1L), a tetramer-forming isoform of GABP that is dispensable for normal development, results in TERT silencing in a TERT promoter mutation-dependent manner. Reducing TERT expression by disrupting β1L culminates in telomere loss and cell death exclusively in TERT promoter mutant cells. Orthotopic xenografting of β1L-reduced, TERT promoter mutant glioblastoma cells rendered lower tumor burden and longer overall survival in mice. These results highlight the critical role of GABPβ1L in enabling immortality in TERT promoter mutant glioblastoma.

Cancer-specific loss ofTERTactivation sensitizes glioblastoma to DNA damage

“…Surprisingly, we also find that reduced GABPB1L combined with TMZ treatment dramatically potentiates anti-tumor effects, emphasizing the clinical potential of GABPB1L inhibition in vivo. Targeting of TERT through the long isoform of GABPB1 (GABPB1L) specifically is particularly promising given that both GABPA and total GABPB1 (the short and long isoform together) are required for normal murine development, while the GABPB1L isoform is dispensable 9,28,29 .…”

“…GABPB1S functions as a heterodimer with GABPA (GABPA 1 B 1 ), while GABPB1L forms a heterotetramer (GABPA 2 B 2 ) due to its unique terminal exon that contains a leucine zipper-like domain 30,31 . The necessity for two proximal GABP binding sites in TERTp mutant cells for TERT reactivation is suggestive of the recruitment of a GABPB1L-containing heterotetramer 9,26,32 (Fig. 1A).…”

“…1A). TERT re-expression is regulated in part by GABPB1L 9 , but presence of GABPB1L at the mutant TERT promoter has not been tested. To explore this, we purified the human GABPA-B1L heterodimer to directly assay TERT promoter DNA binding in vitro using an electrophoretic mobility shift assay (EMSA) (Fig.…”

“…GABP transcription factors are obligate multimers that consist of the DNA-binding GABPA subunit and a transactivating GABPB subunit. Reduced function of the long isoform of GABPB1 (GABPB1L) via indel mutations has previously been linked to TERT downregulation and long-term telomere attrition 9 . This may provide a cancer-specific way to target TERT, particularly given that GABPB1L is dispensable for normal murine development, while GABPA and total GABPB1 are not 28,29 .…”

TERTgene: its function and dysregulation in cancer

“…The likely ETS factor responsible for increased expression has been identified as GABPA,45 which is the only one of the 26 members of the ETS family that forms multimeric complexes when driving gene expression. A recent study examining glioblastoma cell lines found GABPβ1L to be the critical binding partner of GABPA in driving TERT expression in the presence of a TERT promoter mutation 19. Another group has suggested that the C228T and C250T mutations are functionally distinct: the former leading to GABPA recruitment; the latter leading to the generation of both an ETS site and a functional p52 site requiring ETS1/2 as the culpable ETS factor 46…”