Abstract

Genomic double-stranded DNA (dsDNA) becomes single-stranded (ssDNA) during replication, transcription, and DNA repair. ssDNA is therefore believed to be transient, occurring in only a fraction of the genome at a given time, and variable amongst a population of cells. These transiently formed ssDNA segments can also adopt alternative, dynamic DNA conformations, such as cruciform DNA, triplexes, quadruplexes and others. To determine whether there are stable and conserved regions of ssDNA, we utilized our previously developed method S1-END-seq 1 to convert ssDNA to DNA double strand breaks (DSBs), which are then processed for high-throughput sequencing. This approach revealed two predominant dynamic DNA structures: cruciform DNA formed by expanded (TA)n repeats that accumulated uniquely in microsatellite unstable human cancer cell lines, and DNA triplexes (H-DNA) formed by homopurine/homopyrimidine (hPu/hPy) mirror repeats common across a variety of human cell lines. Triplex-forming repeats accumulated during replication, blocked DNA synthesis and were hotspots of somatic mutation. In contrast, pathologically expanded (hPu/hPy) repeats in Friedreichs ataxia patient cells formed a replication-independent and transcription-inducible DNA secondary structure. Our results identify dynamic DNA secondary structures in vivo that contribute to elevated genome instability.