Full sequencing of 100mer sgRNA via tandem mass spectrometry by targeted RNase H digestion with customized probes.

The use of single-guide RNA (sgRNA) for gene editing using the CRISPR Cas9 system has become a powerful technique in various fields, especially with the growing interest in such molecules as therapeutic options in the last years. An important parameter for the use of these molecules is the verification of the correct sgRNA oligonucleotide sequence. Apart from next-generation sequencing protocols, mass spectrometry (MS) has been proven as a powerful technique for this purpose. The protocol and investigations presented in this work show an optimal digestion and 100% sequence coverage of sgRNA, while top-down approaches or other ribonuclease (RNase) digestion strategies obtain a sequence coverage of up to 80-90% utilizing multiple RNases. The results in this publication were obtained by utilizing DNA-RNA hybrid GAPmer-like probes and RNase H, an enzyme which specifically hydrolyzes RNA in DNA-RNA double strands. We assessed the optimal length of the DNA segment of these hybrid probes to maximize the specificity of the RNase H digestion and to achieve complete sequence confirmation by tandem MS analysis of the resulting digestion products. Furthermore, we showed that the approach is applicable for the identification of common synthesis-related impurities, like truncations and elongations. Despite the fact that the accessibility of this approach for highly modified molecules is limited to nucleotides which are not 2'-O-methylated, the optimized sequence coverage makes it a viable method.