CBIO-32MUTANT IDH EXPRESSION DRIVES hTERT REACTIVATION AS PART OF THE CELLULAR TRANSFORMATION PROCESS

IDH1 mutations in lower grade glioma result in the production of 2-hydroxyglutarate (2HG), disrupted patterns of histone methylation, altered gene expression, and cellular transformation. IDH1 mutations, however, are also linked to mutations in the ATRX gene and the hTERT promoter, suggesting that IDH1 mutation contributes to events that stabilize telomeres and help drive the cellular immortalization and transformation processes. To determine if and how IDH1 mutation influences telomeric stabilization and cellular immortalization, pRb/p53-deficient normal human astrocytes (E6/E7 NHA) were lentivirally infected with constructs encoding WT (IDH1 WT) or R132H mutant IDH 1 (IDH1mut). Following selection, the cells were then monitored for 2HG levels, histone modifications, hTERT promoter mutations, ATRX and telomerase expression, and telomere length as they progressed through their lifespan. Introduction of IDH1mut, but not IDH1 WT, resulted in 2HG production and alterations in histone methylation within 20 population doublings (PD). Neither IDH1mut nor IDH1 WT expression, however, had any direct effect on telomerase expression or telomere length, and cells expressing either IDH1 WT or IDH1mut entered a telomere-induced crisis at PD 70. While cells expressing IDH1 WT failed to reactivate telomerase and escape from crisis, cells expressing IDH1mut emerged from crisis after 51 days, grew indefinitely in culture, and formed colonies in soft agar and tumors in vivo. The transformed IDH1mut cells did not exhibit the ALT phenotype or hTERT promoter mutations, but rather exhibited telomerase expression and activity, and shortened but stabilized telomeres. These results show that while IDH1 mutations do not directly contribute to ATRX gene mutation, hTERT promoter mutation, or TERT reactivation, they are permissive for the events that indirectly drive the TERT reactivation that contributes to both the immortalization and transformation of astrocytic cells.