PCNA Ubiquitination Is Important, But Not Essential for Translesion DNA Synthesis in Mammalian Cells

Translesion DNA synthesis (TLS) is a DNA damage tolerance mechanism in which specialized low-fidelity DNA polymerases bypass replication-blocking lesions, and it is usually associated with mutagenesis. In Saccharomyces cerevisiae a key event in TLS is the monoubiquitination of PCNA, which enables recruitment of the specialized polymerases to the damaged site through their ubiquitin-binding domain. In mammals, however, there is a debate on the requirement for ubiquitinated PCNA (PCNA-Ub) in TLS. We show that UV-induced Rpa foci, indicative of single-stranded DNA (ssDNA) regions caused by UV, accumulate faster and disappear more slowly in Pcna(K164R/K164R) cells, which are resistant to PCNA ubiquitination, compared to Pcna(+/+) cells, consistent with a TLS defect. Direct analysis of TLS in these cells, using gapped plasmids with site-specific lesions, showed that TLS is strongly reduced across UV lesions and the cisplatin-induced intrastrand GG crosslink. A similar effect was obtained in cells lacking Rad18, the E3 ubiquitin ligase which monoubiquitinates PCNA. Consistently, cells lacking Usp1, the enzyme that de-ubiquitinates PCNA exhibited increased TLS across a UV lesion and the cisplatin adduct. In contrast, cells lacking the Rad5-homologs Shprh and Hltf, which polyubiquitinate PCNA, exhibited normal TLS. Knocking down the expression of the TLS genes Rev3L, PolH, or Rev1 in Pcna(K164R/K164R) mouse embryo fibroblasts caused each an increased sensitivity to UV radiation, indicating the existence of TLS pathways that are independent of PCNA-Ub. Taken together these results indicate that PCNA-Ub is required for maximal TLS. However, TLS polymerases can be recruited to damaged DNA also in the absence of PCNA-Ub, and perform TLS, albeit at a significantly lower efficiency and altered mutagenic specificity

Quantifying Genome-Editing Outcomes at Endogenous Loci with SMRT Sequencing

Targeted genome editing with engineered nucleases has transformed the ability to introduce precise sequence modifications at almost any site within the genome. A major obstacle to probing the efficiency and consequences of genome editing is that no existing method enables the frequency of different editing events to be simultaneously measured across a cell population at any endogenous genomic locus. We have developed a method for quantifying individual genome-editing outcomes at any site of interest with single-molecule real-time (SMRT) DNA sequencing. We show that this approach can be applied at various loci using multiple engineered nuclease platforms, including transcription-activator-like effector nucleases (TALENs), RNA-guided endonucleases (CRISPR/Cas9), and zinc finger nucleases (ZFNs), and in different cell lines to identify conditions and strategies in which the desired engineering outcome has occurred. This approach offers a technique for studying double-strand break repair, facilitates the evaluation of gene-editing technologies, and permits sensitive quantification of editing outcomes in almost every experimental system used

High-throughput Imaging of CRISPR- and Recombinant Adeno-associated Virus-induced DNA Damage Response in Human Hematopoietic Stem and Progenitor Cells

CRISPR-Cas technology has revolutionized gene editing, but concerns remain due to its propensity for off-target interactions. This, combined with genotoxicity related to both CRISPR-Cas9-induced double-strand breaks and transgene delivery, poses a significant liability for clinical genome-editing applications. Current best practice is to optimize genome-editing parameters in preclinical studies. However, quantitative tools that measure off-target interactions and genotoxicity are costly and time-consuming, limiting the practicality of screening large numbers of potential genome-editing reagents and conditions. Here, we show that flow-based imaging facilitates DNA damage characterization of hundreds of human hematopoietic stem and progenitor cells per minute after treatment with CRISPR-Cas9 and recombinant adeno-associated virus serotype 6. With our web-based platform that leverages deep learning for image analysis, we find that greater DNA damage response is observed for guide RNAs with higher genome-editing activity, differentiating even single on-target guide RNAs with different levels of off-target interactions. This work simplifies the characterization and screening process of genome-editing parameters toward enabling safer and more effective gene-therapy applications

Uridine Depletion and Chemical Modification Increase Cas9 mRNA Activity and Reduce Immunogenicity without HPLC Purification

The Cas9/guide RNA (Cas9/gRNA) system is commonly used for genome editing. mRNA expressing Cas9 can induce innate immune responses, reducing Cas9 expression. First-generation Cas9 mRNAs were modified with pseudouridine and 5-methylcytosine to reduce innate immune responses. We combined four approaches to produce more active, less immunogenic second-generation Cas9 mRNAs. First, we developed a novel co-transcriptional capping method yielding natural Cap 1. Second, we screened modified nucleotides in Cas9 mRNA to identify novel modifications that increase Cas9 activity. Third, we depleted the mRNA of uridines to improve mRNA activity. Lastly, we tested high-performance liquid chromatography (HPLC) purification to remove double-stranded RNAs. The activity of these mRNAs was tested in cell lines and primary human CD34+ cells. Cytokines were measured in whole blood and mice. These approaches yielded more active and less immunogenic mRNA. Uridine depletion (UD) most impacted insertion or deletion (indel) activity. Specifically, 5-methoxyuridine UD induced indel frequencies as high as 88% (average ± SD = 79% ± 11%) and elicited minimal immune responses without needing HPLC purification. Our work suggests that uridine-depleted Cas9 mRNA modified with 5-methoxyuridine (without HPLC purification) or pseudouridine may be optimal for the broad use of Cas9 both in vitro and in vivo. Keywords: mRNA, capping, Cas9, innate immunity, CRISPR, CleanCap, mRNA, uridine depletion, ARCA, Cap

Stepwise-edited, human melanoma models reveal mutations’ effect on tumor and microenvironment

Establishing causal relationships between genetic alterations of human cancers and specific phenotypes of malignancy remains a challenge. We sequentially introduced mutations into healthy human melanocytes in up to five genes spanning six commonly disrupted melanoma pathways, forming nine genetically distinct cellular models of melanoma. We connected mutant melanocyte genotypes to malignant cell expression programs in vitro and in vivo, replicative immortality, malignancy, rapid tumor growth, pigmentation, metastasis, and histopathology. Mutations in malignant cells also affected tumor microenvironment composition and cell states. Our melanoma models shared genotype-associated expression programs with patient melanomas, and a deep learning model showed that these models partially recapitulated genotype-associated histopathological features as well. Thus, a progressive series of genome-edited human cancer models can causally connect genotypes carrying multiple mutations to phenotype.</jats:p