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The Coronavirus lockdowns this spring disrupted many projects and students. But when life gives you lemons, make lemonade. In our lab, almost everyone took the stay-at-home orders as motivation to learn some coding. Sebastian Siegner, a Masters’ student at the time and now recently joined for a full Ph.D., had been working on proof-of-concept experiments for therapeutic base editing and prime editing. But he was frustrated with designing base editor gRNAs and newly-described prime editor pegRNAs by hand. So he used the lockdown to write PnB Designer, which is a fast and scalable helper to design your prime and base editing experiments. Check out the paper in BMC Bioinformatics.

PnB designer can be used to design base editing gRNAs and prime editing pegRNAs in both single-edit and batch mode. You can design against arbitrary DNA sequences (copy/paste your sequence as input), or you can enter genomic coordinates of your favorite gene in your favorite genome. Several species are currently supported, from human to plant. For base editors, just choose which nucleotide change you want to make and the software will take into account both the mutation and editing window to figure out the best editor to use. For prime editing, you can test all kinds of reverse transcriptase template (RTT) and primer binding site (PBS) lengths with a click of a button. The output is a table of possible gRNAs/pegRNAs, ordered by a heuristic score.

Sebastian tested PnB Designer by designing pegRNAs to model most of the human disease-associated mutations in ClinVar using prime editing. He even varied RTT and PBS length for each of these 96,000+ mutations, figuring out good parameters to keep pegRNAs at a reasonable length while still modelling ~80% of all variants.

This was a challenging but exciting side-project for Sebastian’s Masters’ degree. He wrote PnB Designer independently during the lockdown, and the rest of the lab acted as beta testers to provide suggestions. Congrats to Sebastian on your first paper, which is already being used by several people in the Zurich area. PnB Designer is completely free to use and is hosted by the Functional Genomics Center Zurich.

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When using CRISPR genome editing in stem cells, it’s far easier to break a gene with indels than to fix it with HDR. This manifests in an interesting way. If you monitor a “CD34+” population of hematopoietic stem and progenitor cells (HSPCs) from the bone marrow, indels start high and stay high but HDR alleles are lost over time. Why do these different genetic outcomes differ over time? Is HDR bad for the long-term stem cells? Or is editing in the CD34+ population actually heterogeneous, and different cells get different alleles? New work from postdoc Jenny Shin in the lab, out in Cell reports, both answers this question and finds a way to fix the problem.

Jenny and collaborators used a powerful combination of immunophenotyping, next generation sequencing, and single-cell RNA-sequencing to investigate and reprogram genome editing outcomes in subpopulations of adult human CD34+ HSPCs. These HSPCs are actually several different types of cells, including more differentiated progenitors that cycle and very “stemmy” long-term HSCs that are quiescent. The team found that there is a dramatic tension between HDR and quiescence in LT-HSCs.  Quiescent stem-enriched cells utilize NHEJ and exhibit almost no HDR. By contrast, non-quiescent cells with the same immunophenotype utilize both NHEJ and HDR. Quiescence is critical for engraftment and stem cell maintenance, so it was now clear that all cells in the CD34+ population get indels and the cycling progenitors were getting HDR alleles, but the quiescent LT-HSCs weren’t doing HDR.

Jenny then had a very creative idea. She asked if a previously reported small molecule cocktail, “XRC”, that maintains quiescence could be used after the fact to re-quiesce LT-HSCs. Using this new strategy and good timing, she found a way to get LT-HSCs with high levels of HDR by briefly allowing them to cycle during editing, and then inducing quiescence later on. This yielded a 6-fold increase in the HDR/NHEJ ratio in quiescent stem cells ex vivo and during long-term engraftment in mouse experiments. The re-quiescence strategy might in future be combined with engineered Cas9-geminin constructs that reduce NHEJ, further tipping the balance towards HDR. Jenny’s results highlight the tradeoffs between editing and fundamental cellular physiology and suggests strategies to manipulate quiescent cells for research and therapeutic genome editing. 

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Three people joined the lab, all in one day! Erman is a postdoc, interested in DNA repair and genome editing, Kinga is our new lab manager, and will be keeping us all in line. Markus is a bioinformatician, working on quantifying editing outcomes from complex datasets. Welcome to the lab!

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Are you interested in working on genome editing, DNA repair, and organelle quality control in a dynamic lab environment? We are seeking a technician/lab manager and a bioinformatics scientist. Apply at the links below.

Technician / Lab Manager

Bioinformatics Scientist

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We had an slightly belated American Thanksgiving in Zürich, complete with turkey, stuffing, mashed potatoes, and gravy. After eating more than our fill, we all walked down to a big Weinachtsmarkt for Glühwein. A great start to the holiday season!

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