R-环是由一个RNA:DNA杂交体和一条单链状态的DNA分子共同组成的三链核酸结构。其中, RNA:DNA杂交体的形成起因于基因转录所合成的RNA分子不能与模板分开, 或RNA分子重新与一段双链DNA分子中的一条链杂交。在基因转录过程中, 当转录泡遇到富含G碱基的非模板链区或位于某些与人类疾病有关的三核苷酸卫星DNA时, 转录泡后方累积的负超螺旋可促进R环形成。同时, 新生RNA分子未被及时加工、成熟或未被快速转运到细胞质等因素也会催生R环。研究表明, 细胞拥有多种管理R环的方法, 可以有效地管理R环的形成和处理已经形成的R环, 以尽量避免R环对DNA复制、基因突变和同源重组产生不利影响。文章重点分析了R-环的形成机制及R环对DNA复制、基因突变和同源重组的影响, 并针对R-环诱导的DNA复制在某些三核苷酸重复扩增有关的神经肌肉退行性疾病发生过程中的作用进行了分析和讨论。
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.
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