R-loop structure: the formation and the effects on genomic stability

R-loop is a type of three-stranded nucleic acid structure that is made up of an RNA:DNA hybrid, formed due to failing separation of a nascent RNA molecule with transcripting template in transcription or by the re-annealing of RNA molecule with one of the two strands in a double stranded DNA molecule, along with the single stranded DNA, which is either the non-template strand in the transcription bubble or the RNA substituted DNA strand. Formation of R-loops can occur when transcription goes through a genomic DNA region having a tract of G bases in the non-template strand in the transcription bubble or through a type of triplet microsatellite DNA sequences that are known to be associated with certain human diseases. The negative supercoiling forces accumulated in the transcription bubble, and the misprocessing of RNA precursors, as well as the delayed utilizations and transportations of RNA molecules to cytoplasm promote R loop formation. Many studies show that cells can manage R loop formation with efficiency, and can also process the R-loops already formed in the cell, and by which, the bad effects of R-loops on DNA replication, gene mutation and homologous recombination can be regulated. In this review, we summarize the formation and the impacts of R-loops on DNA replication, mutation rates and the frequencies of homologous recombination, and also discusse the possible role of the R-loop induced DNA replication in mediating trinucleotide repeat expansions as seen with those frequently associated with human neuromuscular degenerative diseases.