Project description:Short tandem repeats (STRs) significantly contribute to de novo mutagenesis, driving phenotypic diversity and genetic disease. Although highly diverse, their low complexity and repetitive nature induces DNA polymerase slippage and stalling, leading to length variation and base substitutions. However, the characterisation of DNA synthesis through STR loci has been restricted to a handful of selected sequences, limiting our broader understanding of their evolutionary behaviour. In order to understand the interplay between the ability of a given STR to impair DNA synthesis and its genomic stability, we developed a high-throughput polymerase extension assay that allows monitoring the kinetics of DNA synthesis at all STR permutations in different lengths in parallel. We have used the assay to map at single-nucleotide resolution the movement of a prototypical A-family replicative DNA polymerase (T7 DNA polymerase) through the repeats over time. From this data we can infer the secondary structure adopted by a given STR from the precise manner in which it stalls polymerase and link this to slippage and point mutation during DNA synthesis.
Project description:More than 25 inherited human disorders are caused by the unstable expansion of repetitive DNA sequences termed short tandem repeats (STRs). A fundamental unresolved question is why some STRs are susceptible to pathologic expansion, whereas thousands of repeat tracts across the human genome are relatively stable. Here, we discover that nearly all disease-associated STRs (daSTRs) are located at boundaries demarcating 3D chromatin domains. We identify a subset of boundaries with markedly higher CpG island density compared to the rest of the genome. daSTRs specifically localize to ultra-high-density CpG island boundaries, suggesting they might be hotspots for epigenetic misregulation or topological disruption upon STR expansion. Fragile X Syndrome (FXS) patients exhibit severe boundary disruption in a manner that correlates with local loss of CTCF occupancy and the degree of FMR1 silencing. Our data uncover higher-order chromatin architecture as a new dimension in understanding repeat expansion disorders.
Project description:We used a two-component transgene system to study the RNA-dependent DNA methylation (RdDM) and transcriptional gene silencing (TGS) in Arabidopsis. By profiling the small RNA population in mutants defected in RdDM or RNA polymerase II-transcribed trigger for generating silencing siRNA, we investigated how repetitive loci such as tandem repeats were regulated transcriptionally through the action of RNA polymerase IV.