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Sunday, October 25, 2020

Labroots Webinar: More comprehensive views of human genetic variation

In this BioConference Live webinar, PacBio CSO Jonas Korlach highlights how multi-kilobase reads from SMRT Sequencing can resolve many of the previously considered ‘difficult-to-sequence’ genomic regions. The long reads also allow phasing of the sequence information along the maternal and paternal alleles, demonstrated by full-length, fully phased HLA class I & II gene sequencing. In addition, characterizing the complex landscape of alternative gene products is currently very difficult with short-read sequencing technologies, and he describes how long-read, full-length mRNA sequencing can be used to describe the diversity of transcript isoforms, with no assembly required. Lastly, in the exciting area of…

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Tuesday, April 21, 2020

Large Fragment Deletions Induced by Cas9 Cleavage While Not in BEs System in Rabbit

CRISPR-Cas9 and BEs system are poised to become the gene editing tool of choice in clinical contexts, however large fragment deletion was found in Cas9-mediated mutation cells without animal level validation. By analyzing 16 gene-edited rabbit lines (including 112 rabbits) generated using SpCas9, BEs, xCas9 and xCas9-BEs with long-range PCR genotyping and long-read sequencing by PacBio platform, we show that extending thousands of bases fragment deletions in single-guide RNA/Cas9 and xCas9 system mutation rabbit, but few large deletions were found in BEs-induced mutation rabbits. We firstly validated that no large fragment deletion induced by BEs system at animal level, suggesting…

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Tuesday, April 21, 2020

Defining transgene insertion sites and off-target effects of homology-based gene silencing informs the use of functional genomics tools in Phytophthora infestans.

DNA transformation and homology-based transcriptional silencing are frequently used to assess gene function in Phytophthora. Since unplanned side-effects of these tools are not well-characterized, we used P. infestans to study plasmid integration sites and whether knockdowns caused by homology-dependent silencing spreads to other genes. Insertions occurred both in gene-dense and gene-sparse regions but disproportionately near the 5′ ends of genes, which disrupted native coding sequences. Microhomology at the recombination site between plasmid and chromosome was common. Studies of transformants silenced for twelve different gene targets indicated that neighbors within 500-nt were often co-silenced, regardless of whether hairpin or sense constructs…

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Tuesday, April 21, 2020

Optimized Cas9 expression systems for highly efficient Arabidopsis genome editing facilitate isolation of complex alleles in a single generation.

Genetic resources for the model plant Arabidopsis comprise mutant lines defective in almost any single gene in reference accession Columbia. However, gene redundancy and/or close linkage often render it extremely laborious or even impossible to isolate a desired line lacking a specific function or set of genes from segregating populations. Therefore, we here evaluated strategies and efficiencies for the inactivation of multiple genes by Cas9-based nucleases and multiplexing. In first attempts, we succeeded in isolating a mutant line carrying a 70 kb deletion, which occurred at a frequency of ~?1.6% in the T2 generation, through PCR-based screening of numerous individuals. However,…

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Tuesday, April 21, 2020

Collateral damage and CRISPR genome editing.

The simplicity and the versatility of clustered regularly interspaced short palindromic repeats/CRISPR-associated protein (CRISPR-Cas) systems have enabled the genetic modification of virtually every organism and offer immense therapeutic potential for the treatment of human disease. Although these systems may function efficiently within eukaryotic cells, there remain concerns about the accuracy of Cas endonuclease effectors and their use for precise gene editing. Recently, two independent reports investigating the editing accuracy of the CRISPR-Cas9 system were published by separate groups at the Wellcome Sanger Institute; our study-Iyer and colleagues [1]-defined the landscape of off-target mutations, whereas the other by Kosicki and colleagues…

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Tuesday, April 21, 2020

Genome-wide de novo L1 Retrotransposition Connects Endonuclease Activity with Replication.

L1 retrotransposon-derived sequences comprise approximately 17% of the human genome. Darwinian selective pressures alter L1 genomic distributions during evolution, confounding the ability to determine initial L1 integration preferences. Here, we generated high-confidence datasets of greater than 88,000 engineered L1 insertions in human cell lines that act as proxies for cells that accommodate retrotransposition in vivo. Comparing these insertions to a null model, in which L1 endonuclease activity is the sole determinant dictating L1 integration preferences, demonstrated that L1 insertions are not significantly enriched in genes, transcribed regions, or open chromatin. By comparison, we provide compelling evidence that the L1 endonuclease disproportionately cleaves…

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Tuesday, April 21, 2020

Precise therapeutic gene correction by a simple nuclease-induced double-stranded break.

Current programmable nuclease-based methods (for example, CRISPR-Cas9) for the precise correction of a disease-causing genetic mutation harness the homology-directed repair pathway. However, this repair process requires the co-delivery of an exogenous DNA donor to recode the sequence and can be inefficient in many cell types. Here we show that disease-causing frameshift mutations that result from microduplications can be efficiently reverted to the wild-type sequence simply by generating a DNA double-stranded break near the centre of the duplication. We demonstrate this in patient-derived cell lines for two diseases: limb-girdle muscular dystrophy type 2G (LGMD2G)1 and Hermansky-Pudlak syndrome type 1 (HPS1)2. Clonal…

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Tuesday, April 21, 2020

Different knockout genotypes of OsIAA23 in rice using CRISPR/Cas9 generating different phenotypes.

We have isolated several Osiaa23 rice mutants with different knockout genotypes, resulting in different phenotypes, which suggested that different genetic backgrounds or mutation types influence gene function. The Auxin/Indole-3-Acetic Acid (Aux/IAA) gene family performs critical roles in auxin signal transduction in plants. In rice, the gene OsIAA23 (Os06t0597000) is known to affect development of roots and shoots, but previous knockouts in OsIAA23 have been sterile and difficult for research continuously. Here, we isolate new Osiaa23 mutants using the CRISPR/Cas9 system in japonica (Wuyunjing24) and indica (Kasalath) rice, with extensive genome re-sequencing to confirm the absence of off-target effects. In Kasalath,…

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Tuesday, April 21, 2020

Smashing Barriers in Biolistic Plant Transformation.

A foundation of modern biotechnology is the ability to stably introduce foreign DNA into an organism. The two most widely used methods, Agrobacterium-mediated transformation and biolistics, are both steeped in a rich history of creative exploration into the molecular unknown. Agrobacterium research accelerated in the early 1970s, particularly with the discovery of the large Ti (tumor-inducing) plasmid of Agrobacterium that contained a region of transfer DNA (T-DNA). Culturing plant calli in autoclaved jelly jars, and long before the advent of PCR, Southern blots were first used to show that T-DNA fragments could stably integrate into the nuclear genome (Chilton et…

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Tuesday, April 21, 2020

Development of CRISPR-Cas systems for genome editing and beyond

The development of clustered regularly interspaced short-palindromic repeat (CRISPR)-Cas systems for genome editing has transformed the way life science research is conducted and holds enormous potential for the treatment of disease as well as for many aspects of biotech- nology. Here, I provide a personal perspective on the development of CRISPR-Cas9 for genome editing within the broader context of the field and discuss our work to discover novel Cas effectors and develop them into additional molecular tools. The initial demonstra- tion of Cas9-mediated genome editing launched the development of many other technologies, enabled new lines of biological inquiry, and motivated…

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Tuesday, April 21, 2020

Whole Genome Sequencing of the Mutamouse Model Reveals Strain- and Colony-Level Variation, and Genomic Features of the Transgene Integration Site.

The MutaMouse transgenic rodent model is widely used for assessing in vivo mutagenicity. Here, we report the characterization of MutaMouse’s whole genome sequence and its genetic variants compared to the C57BL/6 reference genome. High coverage (>50X) next-generation sequencing (NGS) of whole genomes from multiple MutaMouse animals from the Health Canada (HC) colony showed ~5 million SNVs per genome, ~20% of which are putatively novel. Sequencing of two animals from a geographically separated colony at Covance indicated that, over the course of 23 years, each colony accumulated 47,847 (HC) and 17,677 (Covance) non-parental homozygous single nucleotide variants. We found no novel…

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Tuesday, April 21, 2020

Programmable mutually exclusive alternative splicing for generating RNA and protein diversity.

Alternative splicing performs a central role in expanding genomic coding capacity and proteomic diversity. However, programming of splicing patterns in engineered biological systems remains underused. Synthetic approaches thus far have predominantly focused on controlling expression of a single protein through alternative splicing. Here, we describe a modular and extensible platform for regulating four programmable exons that undergo a mutually exclusive alternative splicing event to generate multiple functionally-distinct proteins. We present an intron framework that enforces the mutual exclusivity of two internal exons and demonstrate a graded series of consensus sequence elements of varying strengths that set the ratio of two…

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Tuesday, April 21, 2020

Linking CRISPR-Cas9 interference in cassava to the evolution of editing-resistant geminiviruses.

Geminiviruses cause damaging diseases in several important crop species. However, limited progress has been made in developing crop varieties resistant to these highly diverse DNA viruses. Recently, the bacterial CRISPR/Cas9 system has been transferred to plants to target and confer immunity to geminiviruses. In this study, we use CRISPR-Cas9 interference in the staple food crop cassava with the aim of engineering resistance to African cassava mosaic virus, a member of a widespread and important family (Geminiviridae) of plant-pathogenic DNA viruses.Our results show that the CRISPR system fails to confer effective resistance to the virus during glasshouse inoculations. Further, we find…

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Wednesday, October 23, 2019

Altering tropism of rAAV by directed evolution.

Directed evolution represents an attractive approach to derive AAV capsid variants capable of selectively infect specific tissue or cell targets. It involves the generation of an initial library of high complexity followed by cycles of selection during which the library is progressively enriched for target-specific variants. Each selection cycle consists of the following: reconstitution of complete AAV genomes within plasmid molecules; production of virions for which each particular capsid variant is matched with the particular capsid gene encoding it; recovery of capsid gene sequences from target tissue after systemic administration. Prevalent variants are then analyzed and evaluated.

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Wednesday, October 23, 2019

Optimized CRISPR-Cas9 genome editing for Leishmania and its use to target a multigene family, induce chromosomal translocation, and study DNA break repair mechanisms.

CRISPR-Cas9-mediated genome editing has recently been adapted for Leishmania spp. parasites, the causative agents of human leishmaniasis. We have optimized this genome-editing tool by selecting for cells with CRISPR-Cas9 activity through cotargeting the miltefosine transporter gene; mutation of this gene leads to miltefosine resistance. This cotargeting strategy integrated into a triple guide RNA (gRNA) expression vector was used to delete all 11 copies of the A2 multigene family; this was not previously possible with the traditional gene-targeting method. We found that the Leishmania donovani rRNA promoter is more efficient than the U6 promoter in driving gRNA expression, and sequential transfections of…

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