Targeted gene knock-in reduces variation between transformants in the mushroom-forming fungus Schizophyllum commune

Gene integration in mushroom-forming fungi currently occurs by the ectopic integration of a plasmid. The locus of integration is unpredictable and, problematically, this generally results in a high variability in gene expression and phenotypes between the transformants. Here, we developed an approach for targeted gene integration (knock-in) in the basidiomycete Schizophyllum commune by replacing a 75-bp non-coding region of the genome with a selection marker and an arbitrary gene of interest using CRISPR-Cas9 ribonucleoproteins. To assess the suitability of our method, we compared targeted integration and ectopic integration of the gene encoding the red fluorescent protein dTomato. Targeted integration resulted in a higher average fluorescence intensity and less variability between the transformants. This method may be applied to any gene construct and may therefore greatly increase the efficiency of functional gene analysis in S. commune.

Drag-and-drop genome insertion without DNA cleavage with CRISPR-directed integrases

Programmable and multiplexed genome integration of large, diverse DNA cargo independent of DNA repair remains an unsolved challenge of genome editing. Current gene integration approaches require double-strand breaks that evoke DNA damage responses and rely on repair pathways that are inactive in terminally differentiated cells. Furthermore, CRISPR-based approaches that bypass double stranded breaks, such as Prime editing, are limited to modification or insertion of short sequences. We present Programmable Addition via Site-specific Targeting Elements, or PASTE, which achieves efficient and versatile gene integration at diverse loci by directing insertion with a CRISPR-Cas9 nickase fused to both a reverse transcriptase and serine integrase. Without generating double stranded breaks, we demonstrate integration of sequences as large as ~36 kb with rates between 10-50% at multiple genomic loci across three human cell lines, primary T cells, and quiescent non-dividing primary human hepatocytes. To further improve PASTE, we discover thousands of novel serine integrases and cognate attachment sites from metagenomes and engineer active orthologs for high-efficiency integration using PASTE. We apply PASTE to fluorescent tagging of proteins, integration of therapeutically relevant genes, and production and secretion of transgenes. Leveraging the orthogonality of serine integrases, we engineer PASTE for multiplexed gene integration, simultaneously integrating three different genes at three genomic loci. PASTE has editing efficiencies comparable to or better than those of homology directed repair or non-homologous end joining based integration, with activity in non-dividing cells and fewer detectable off-target events. For therapeutic applications, PASTE can be delivered as mRNA with synthetically modified guides to programmably direct insertion of DNA templates carried by AAV or adenoviral vectors. PASTE expands the capabilities of genome editing via drag-and-drop gene integration, offering a platform with wide applicability for research, cell engineering, and gene therapy.

A Novel Cre/lox-Based Genetic Tool for Repeated, Targeted and Markerless Gene Integration in Yarrowia lipolytica

The unconventional yeast Yarrowia lipolytica is extensively applied in bioproduction fields owing to its excellent metabolite and protein production ability. Nonetheless, utilization of this promising host is still restricted by the limited availability of precise and effective gene integration tools. In this study, a novel and efficient genetic tool was developed for targeted, repeated, and markerless gene integration based on Cre/lox site-specific recombination system. The developed tool required only a single selection marker and could completely excise the unnecessary sequences. A total of three plasmids were created and seven rounds of marker-free gene integration were examined in Y. lipolytica. All the integration efficiencies remained above 90%, and analysis of the protein production and growth characteristics of the engineered strains confirmed that genome modification via the novel genetic tool was feasible. Further work also confirmed that the genetic tool was effective for the integration of other genes, loci, and strains. Thus, this study significantly promotes the application of the Cre/lox system and presents a powerful tool for genome engineering in Y. lipolytica.

ASSESSMENT OF POTENTIAL SARS-CoV-2 VIRUS N GENE INTEGRATION INTO HUMAN GENOME REVEALS NO SIGNIFICANT IMPACT ON RT-qPCR COVID-19 DIAGNOSTIC TESTING

The SARS Coronavirus 2 (SARS-CoV-2) pandemic presents new scientific and scale-up challenges for diagnostic capabilities worldwide. The gold standard diagnostic for SARS-CoV-2 infection is a reverse transcription/quantitative PCR (RT-qPCR) which targets the viral genome, an assay that has now been performed on millions of patient specimens worldwide regardless of symptomatic status. Recently Zhang et al. suggested the possibility that the SARS-CoV-2 N gene could integrate into host cell DNA through the action of the LINE-1 retrotransposon, a mobile element that is potentially active in human somatic cells, thereby calling into question the veracity of N-gene based RT-qPCR for detection of SARS-CoV-2 infection. Accordingly, we assessed the potential impact of these purported integration events on nasal swab specimens tested at our clinical laboratory. Using an N-gene based RT-qPCR assay, we tested 768 arbitrarily selected specimens and identified 2 samples which resulted in a positive detection of viral sequence in the absence of reverse transcriptase, a necessary but not sufficient signal consistent with possible integration of the SARS-CoV-2 N gene into the host genome. Regardless of possible viral N gene integration into the genome, in this small subset of samples, all patients were still positive for SARS-CoV-2 infection, as indicated by a much lower Ct value for reactions performed in the presence of reverse transcriptase (RT) versus reactions performed without RT. Moreover, one of the two positives observed in the absence of RT also tested positive when using primers targeting ORF1ab, a gene closer to the 5 prime end of the genome. These data are inconsistent with the N gene integration hypothesis suggested by the studies by Zhang et al., and importantly, our results suggest little to no practical impact of possible SARS-CoV-2 genome integration events on RT-qPCR testing.

Stable Transformant of Phalaenopsis amabilis Somatic Embryo Carrying 35S::AtRKD4 Develops Into Normal Phenotype of Transgenic Plant

Phalaenopsis amabilis (L.) Blume orchid is an Indonesian national flower. The number of these orchids in their natural habitat is very limited, therefore plant propagation efforts are needed. One of the promising methods is plant propagation by inserting embryo gene AtRKD4 from a model plant Arabidopsis thaliana into the orchid genome to produce many somatic embryos. From previous research, we have obtained 28 plant P. amabilis transformants carrying the AtRKD4 gene, however, it was unknown whether these plants have normal phenotypes and growth similar to their parents. Therefore, descriptions on growth and morphology are needed. This research aimed to evaluate the phenotype of P. amabilis carrying 35S::AtRKD4 the transformants grown in greenhouse. To achieve it, AtRKD4 gene integration stability on transformants genome was analyzed. Morphology and cross-section anatomy structure on transformant and non-transformant plantswere described. The stability of AtRKD4 gene integration in the plant genome was confirmed by amplification of the AtRKD4 gene from genomic DNA with Polymerase Chain Reaction (PCR) using a specific primer for AtRKD4 and ACTIN genes as the internal control. The quantitative data from morphology and anatomy measurements were analyzed statistically using ANOVA. The results showed that AtRKD4 was stably integrated into the genome of P. amabilis transformants and all transformant plants showed similar morphology and anatomy characteristics as non-transformant plants. The AtRKD4 embryo gene was stably integrated into the orchid genome and the transformant plants grow normally without significant changes in phenotype.