Targeted enrichment without amplification and SMRT Sequencing of repeat-expansion disease causative genomic regions
Targeted sequencing has proven to be an economical means of obtaining sequence information for one or more defined regions of a larger genome. However, most target enrichment methods are reliant upon some form of amplification. Amplification removes the epigenetic marks present in native DNA, and some genomic regions, such as those with extreme GC content and repetitive sequences, are recalcitrant to faithful amplification. Yet, a large number of genetic disorders are caused by expansions of repeat sequences. Furthermore, for some disorders, methylation status has been shown to be a key factor in the mechanism of disease. We have developed a novel, amplification-free enrichment technique that employs the CRISPR/Cas9 system for specific targeting of individual human genes. This method, in conjunction with SMRT Sequencing’s long reads, high consensus accuracy, and uniform coverage, allows the sequencing of complex genomic regions that cannot be investigated with other technologies. Using human genomic DNA samples and this strategy, we have successfully targeted the loci of a number of repeat expansion disorders (HTT, FMR1, ATXN10, C9orf72). With this data, we demonstrate the ability to isolate hundreds of individual on-target molecules and accurately sequence through long repeat stretches, regardless of the extreme GC-content, followed by accurate sequencing on a single PacBio RS II SMRT Cell or Sequel SMRT Cell 1M. The method is compatible with multiplexing of multiple targets and multiple samples in a single reaction. Furthermore, this technique also preserves native DNA molecules for sequencing, allowing for the possibility of direct detection and characterization of epigenetic signatures. We demonstrate detection of 5-mC in human promoter sequences and CpG islands.