A non-rational approach to optimized targeting of CRISPR-Cas9 complexes.

In this issue of Cell Chemical Biology, Bush et al.1 report an in vitro selection method for optimizing CRISPR-Cas9 single-guide RNAs. This approach may be useful in targeting previously intractable genomic sequences. The results also provide insights into which positions in single-guide RNAs are most amenable to modification.

DNA Modifications Enabling Proximity Biotinylation.

Advances in peroxidase and biotin ligase-mediated signal amplification have enabled high-resolution subcellular mapping of endogenous RNA localization and protein-protein interactions. Application of these technologies has been limited to RNA and proteins because of the reactive groups required for biotinylation in each context. Here we report several novel methods for proximity biotinylation of exogenous oligodeoxyribonucleotides by application of well-established and convenient enzymatic tools. We describe approaches using simple and efficient conjugation chemistries to modify deoxyribonucleotides with "antennae" that react with phenoxy radicals or biotinoyl-5'-adenylate. In addition, we report chemical details of a previously undescribed adduct between tryptophan and a phenoxy radical group. These developments have potential application in the selection of exogenous nucleic acids capable of unaided entry into living cells.

Optimized quantitative PCR analysis of random DNA aptamer libraries.

The quantitative polymerase chain reaction (qPCR) with detection of duplex DNA yield by intercalator fluorescence is a common and essential technique in nucleic acid analysis. We encountered unexpected results when applying standard qPCR methods to the quantitation of random DNA libraries flanked by regions of fixed sequence, a configuration essential for in vitro selection experiments. Here we describe the results of experiments revealing why conventional qPCR methods can fail to allow automated analysis in such cases, and simple solutions to this problem. In particular we show that renaturation of PCR products containing random regions is incomplete in late PCR cycles when extension fails due to reagent depletion. Intercalator fluorescence can then be lost at standard interrogation temperatures. We show that qPCR analysis of random DNA libraries can be achieved simply by adjusting the step at which intercalator fluorescence is monitored so that the yield of annealed constant regions is detected rather than the yield of full duplex DNA products.

A suppressor of dioxygenase inhibition in a yeast model of SDH deficiency.

A fascinating class of familial paraganglioma (PGL) neuroendocrine tumors is driven by the loss of the tricarboxylic acid (TCA) cycle enzyme succinate dehydrogenase (SDH) resulting in succinate accumulation as an oncometabolite and other metabolic derangements. Here, we exploit a Saccharomyces cerevisiae yeast model of SDH loss where accumulating succinate, and possibly reactive oxygen species, poison a dioxygenase enzyme required for sulfur scavenging. Using this model, we performed a chemical suppression screen for compounds that relieve dioxygenase inhibition. After testing 1280 pharmaceutically active compounds, we identified meclofenoxate HCl and its hydrolysis product, dimethylaminoethanol (DMAE), as suppressors of dioxygenase intoxication in SDH-loss yeast cells. We show that DMAE acts to alter metabolism so as to normalize the succinate:2-ketoglutarate ratio, improving dioxygenase function. This study raises the possibility that oncometabolite effects might be therapeutically suppressed by drugs that rewire metabolism to reduce the flux of carbon into pathological metabolic pathways.