Oxidative lesions modulate G-quadruplex stability and structure in the human BCL2 promoter

Misregulation of BCL2 expression has been observed with many diseases and is associated with cellular exposure to reactive oxygen species. A region upstream of the P1 promoter in the human BCL2 gene plays a major role in regulating transcription. This G/C-rich region is highly polymorphic and capable of forming G-quadruplex structures. Herein we report that an oxidative event simulated with an 8-oxo-7,8-dihydroguanine ((oxo)G) substitution within a long G-tract results in a reduction of structural polymorphism. Surprisingly, (oxo)G within a 25-nt construct boosts thermal stability of the resulting G-quadruplex. This is achieved by distinct hydrogen bonding properties of (oxo)G, which facilitate formation of an antiparallel basket-type G-quadruplex with a three G-quartet core and a G·(oxo)G·C base triad. While (oxo)G has previously been considered detrimental for G-quadruplex formation, its stabilizing effect within a promoter described in this study suggests a potential novel regulatory role of oxidative stress in general and specifically in BCL2 gene transcription.