Base excision repair within structure-forming repeat sequences and its impact on cancer and other diseases.

Oxidative DNA damage is a major driver of genome instability and human disease. Among the various types of oxidative DNA base lesions, 8-oxo-7,8-dihydroguanine (8oxoG) is particularly prevalent due to guanine's low oxidation potential and the abundance of guanine-rich (G-rich) sequences across the genome. Structure-forming repeat sequences, which are commonly G-rich, can adopt alternative DNA secondary structures that further expose nucleobases to oxidative damage. The base excision repair (BER) pathway is primarily responsible for the repair of 8oxoG lesions; however, the complex topologies and dynamic conformations formed by these repeat sequences present challenges for complete repair. Inefficient BER within these structures can lead to DNA strand breaks, mutations, and large chromosomal rearrangements, all of which are associated with human disease. Notably, structure-forming repeat sequences are often enriched at regulatory genomic regions, where BER can directly influence processes such as replication and transcription. This review summarizes current insights into BER activity within oxidatively damaged structure-forming repeat sequences and highlights how repair efficiency within these sequences impacts genome stability and disease.