Posts Tagged ‘CRISPR –Cas9 system for genetic engineering’

Prime Editing as a New CRISPR Tool to Enhance Precision and Versatility


Reporter: Stephen J. Williams, PhD


CRISPR has become a powerful molecular for the editing of genomes tool in research, drug discovery, and the clinic

(see posts and ebook on this site below)


however, as discussed on this site

(see posts below)

there have been many instances of off-target effects where genes, other than the selected target, are edited out.  This ‘off-target’ issue has hampered much of the utility of CRISPR in gene-therapy and CART therapy

see posts


However, an article in Science by Jon Cohen explains a Nature paper’s finding of a new tool in the CRISPR arsenal called prime editing, meant to increase CRISPR specificity and precision editing capabilities.


By Jon Cohen | Oct 25th, 2019

Prime editing promises to be a cut above CRISPR Jon Cohen CRISPR, an extraordinarily powerful genome-editing tool invented in 2012, can still be clumsy. … Prime editing steers around shortcomings of both techniques by heavily modifying the Cas9 protein and the guide RNA. … ” Prime editing “well may become the way that disease-causing mutations are repaired,” he says.

Science Vol. 366, No. 6464; DOI: 10.1126/science.366.6464.406

The effort, led by Drs. David Liu and Andrew Anzalone at the Broad Institute (Cambridge, MA), relies on the modification of the Cas9 protein and guide RNA, so that there is only a nick in a single strand of the double helix.  The canonical Cas9 cuts both strands of DNA, and so relies on an efficient gap repair activity of the cell.  The second part, a new type of guide RNA called a pegRNA, contains an RNA template for a new DNA sequence to be added at the target location.  This pegRNA-directed synthesis of the new template requires the attachment of a reverse transcriptase enzymes to the Cas9.  So far Liu and his colleagues have tested the technology on over 175 human and rodent cell lines with great success.  In addition, they had also corrected mutations which cause Tay Sachs disease, which previous CRISPR systems could not do.  Liu claims that this technology could correct over 89% of pathogenic variants in human diseases.

A company Prime Medicine has been formed out of this effort.

Source: https://science.sciencemag.org/content/366/6464/406.abstract


Read an article on Dr. Liu, prime editing, and the companies that Dr. Liu has initiated including Editas Medicine, Beam Therapeutics, and Prime Medicine at https://www.statnews.com/2019/11/06/questions-david-liu-crispr-prime-editing-answers/

(interview by StatNews  SHARON BEGLEY @sxbegle)

As was announced, prime editing for human therapeutics will be jointly developed by both Prime Medicine and Beam Therapeutics, each focusing on different types of edits and distinct disease targets, which will help avoid redundancy and allow us to cover more disease territory overall. The companies will also share knowledge in prime editing as well as in accompanying technologies, such as delivery and manufacturing.

Reader of StatNews.: Can you please compare the pros and cons of prime editing versus base editing?

The first difference between base editing and prime editing is that base editing has been widely used for the past 3 1/2 years in organisms ranging from bacteria to plants to mice to primates. Addgene tells me that the DNA blueprints for base editors from our laboratory have been distributed more than 7,500 times to more than 1,000 researchers around the world, and more than 100 research papers from many different laboratories have been published using base editors to achieve desired gene edits for a wide variety of applications. While we are very excited about prime editing, it’s brand-new and there has only been one paper published thus far. So there’s much to do before we can know if prime editing will prove to be as general and robust as base editing has proven to be.

We directly compared prime editors and base editors in our study, and found that current base editors can offer higher editing efficiency and fewer indel byproducts than prime editors, while prime editors offer more targeting flexibility and greater editing precision. So when the desired edit is a transition point mutation (C to T, T to C, A to G, or G to A), and the target base is well-positioned for base editing (that is, a PAM sequence exists approximately 15 bases from the target site), then base editing can result in higher editing efficiencies and fewer byproducts. When the target base is not well-positioned for base editing, or when other “bystander” C or A bases are nearby that must not be edited, then prime editing offers major advantages since it does not require a precisely positioned PAM sequence and is a true “search-and-replace” editing capability, with no possibility of unwanted bystander editing at neighboring bases.

Of course, for classes of mutations other than the four types of point mutations that base editors can make, such as insertions, deletions, and the eight other kinds of point mutations, to our knowledge prime editing is currently the only approach that can make these mutations in human cells without requiring double-stranded DNA cuts or separate DNA templates.

Nucleases (such as the zinc-finger nucleases, TALE nucleases, and the original CRISPR-Cas9), base editors, and prime editors each have complementary strengths and weaknesses, just as scissors, pencils, and word processors each have unique and useful roles. All three classes of editing agents already have or will have roles in basic research and in applications such as human therapeutics and agriculture.

Nature Paper on Prime Editing CRISPR

Search-and-replace genome editing without double-strand breaks or donor DNA (6)


Andrew V. Anzalone,  Peyton B. Randolph, Jessie R. Davis, Alexander A. Sousa,

Luke W. Koblan, Jonathan M. Levy, Peter J. Chen, Christopher Wilson,

Gregory A. Newby, Aditya Raguram & David R. Liu


Nature volume 576, pages149–157(2019)



Most genetic variants that contribute to disease1 are challenging to correct efficiently and without excess byproducts2,3,4,5. Here we describe prime editing, a versatile and precise genome editing method that directly writes new genetic information into a specified DNA site using a catalytically impaired Cas9 endonuclease fused to an engineered reverse transcriptase, programmed with a prime editing guide RNA (pegRNA) that both specifies the target site and encodes the desired edit. We performed more than 175 edits in human cells, including targeted insertions, deletions, and all 12 types of point mutation, without requiring double-strand breaks or donor DNA templates. We used prime editing in human cells to correct, efficiently and with few byproducts, the primary genetic causes of sickle cell disease (requiring a transversion in HBB) and Tay–Sachs disease (requiring a deletion in HEXA); to install a protective transversion in PRNP; and to insert various tags and epitopes precisely into target loci. Four human cell lines and primary post-mitotic mouse cortical neurons support prime editing with varying efficiencies. Prime editing shows higher or similar efficiency and fewer byproducts than homology-directed repair, has complementary strengths and weaknesses compared to base editing, and induces much lower off-target editing than Cas9 nuclease at known Cas9 off-target sites. Prime editing substantially expands the scope and capabilities of genome editing, and in principle could correct up to 89% of known genetic variants associated with human diseases.



From Anzolone et al. Nature 2019 Figure 1.

Prime editing strategy

Cas9 targets DNA using a guide RNA containing a spacer sequence that hybridizes to the target DNA site. We envisioned the generation of guide RNAs that both specify the DNA target and contain new genetic information that replaces target DNA nucleotides. To transfer information from these engineered guide RNAs to target DNA, we proposed that genomic DNA, nicked at the target site to expose a 3′-hydroxyl group, could be used to prime the reverse transcription of an edit-encoding extension on the engineered guide RNA (the pegRNA) directly into the target site (Fig. 1b, cSupplementary Discussion).

These initial steps result in a branched intermediate with two redundant single-stranded DNA flaps: a 5′ flap that contains the unedited DNA sequence and a 3′ flap that contains the edited sequence copied from the pegRNA (Fig. 1c). Although hybridization of the perfectly complementary 5′ flap to the unedited strand is likely to be thermodynamically favoured, 5′ flaps are the preferred substrate for structure-specific endonucleases such as FEN122, which excises 5′ flaps generated during lagging-strand DNA synthesis and long-patch base excision repair. The redundant unedited DNA may also be removed by 5′ exonucleases such as EXO123.

  • The authors reasoned that preferential 5′ flap excision and 3′ flap ligation could drive the incorporation of the edited DNA strand, creating heteroduplex DNA containing one edited strand and one unedited strand (Fig. 1c).
  • DNA repair to resolve the heteroduplex by copying the information in the edited strand to the complementary strand would permanently install the edit (Fig. 1c).
  • They had hypothesized that nicking the non-edited DNA strand might bias DNA repair to preferentially replace the non-edited strand.


  • The authors evaluated the eukaryotic cell DNA repair outcomes of 3′ flaps produced by pegRNA-programmed reverse transcription in vitro, and performed in vitro prime editing on reporter plasmids, then transformed the reaction products into yeast cells (Extended Data Fig. 2).
  • Reporter plasmids encoding EGFP and mCherry separated by a linker containing an in-frame stop codon, +1 frameshift, or −1 frameshift were constructed and when plasmids were edited in vitro with Cas9 nickase, RT, and 3′-extended pegRNAs encoding a transversion that corrects the premature stop codon, 37% of yeast transformants expressed both GFP and mCherry (Fig. 1f, Extended Data Fig. 2).
  • They fused a variant of M—MLV-RT (reverse transcriptase) to Cas9 with an extended linker and this M-MLV RT fused to the C terminus of Cas9(H840A) nickase was designated as PE1. This strategy allowed the authors to generate a cell line containing all the required components of the primer editing system. They constructed 19 variants of PE1 containing a variety of RT mutations to evaluate their editing efficiency in human cells
  • Generated a pentamutant RT incorporated into PE1 (Cas9(H840A)–M-MLV RT(D200N/L603W/T330P/T306K/W313F)) is hereafter referred to as prime editor 2 (PE2).  These were more thermostable versions of RT with higher efficiency.
  • Optimized the guide (pegRNA) using a series of permutations and  recommend starting with about 10–16 nt and testing shorter and longer RT templates during pegRNA optimization.
  • In the previous attempts (PE1 and PE2 systems), mismatch repair resolves the heteroduplex to give either edited or non-edited products. So they next developed an optimal editing system (PE3) to produce optimal nickase activity and found nicks positioned 3′ of the edit about 40–90 bp from the pegRNA-induced nick generally increased editing efficiency (averaging 41%) without excess indel formation (6.8% average indels for the sgRNA with the highest editing efficiency) (Fig. 3b).
  • The cell line used to finalize and validate the system was predominantly HEK293T immortalized cell line
  • Together, their findings establish that PE3 systems improve editing efficiencies about threefold compared with PE2, albeit with a higher range of indels than PE2. When it is possible to nick the non-edited strand with an sgRNA that requires editing before nicking, the PE3b system offers PE3-like editing levels while greatly reducing indel formation.
  • Off Target Effects: Strikingly, PE3 or PE2 with the same 16 pegRNAs containing these four target spacers resulted in detectable off-target editing at only 3 out of 16 off-target sites, with only 1 of 16 showing an off-target editing efficiency of 1% or more (Extended Data Fig. 6h). Average off-target prime editing for pegRNAs targeting HEK3HEK4EMX1, and FANCFat the top four known Cas9 off-target sites for each protospacer was <0.1%, <2.2 ± 5.2%, <0.1%, and <0.13 ± 0.11%, respectively (Extended Data Fig. 6h).
  • The PE3 system was very efficient at editing the most common mutation that causes Tay-Sachs disease, a 4-bp insertion in HEXA(HEXA1278+TATC).


  1. Landrum, M. J. et al. ClinVar: public archive of interpretations of clinically relevant variants. Nucleic Acids Res44, D862–D868 (2016).
  2. Jinek, M. et al. A programmable dual-RNA-guided DNA endonuclease in adaptive bacterial immunity. Science337, 816–821 (2012).
  3. Cong, L. et al. Multiplex genome engineering using CRISPR/Cas systems. Science339, 819–823 (2013).


  1. Mali, P. et al. RNA-guided human genome engineering via Cas9. Science339, 823–826 (2013).
  2. Kosicki, M., Tomberg, K. & Bradley, A. Repair of double-strand breaks induced by CRISPR–Cas9 leads to large deletions and complex rearrangements.  Biotechnol. 36, 765–771 (2018).
  3. Anzalone, A.V., Randolph, P.B., Davis, J.R. et al.Search-and-replace genome editing without double-strand breaks or donor DNA. Nature576, 149–157 (2019). https://doi.org/10.1038/s41586-019-1711-4

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Live Conference Coverage AACR 2020 in Real Time: Monday June 22, 2020 Mid Day Sessions

Reporter: Stephen J. Williams, PhD

This post will be UPDATED during the next two days with notes from recordings from other talks

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Register for FREE at https://www.aacr.org/




June 22-24: Free Registration for AACR Members, the Cancer Community, and the Public
This virtual meeting will feature more than 120 sessions and 4,000 e-posters, including sessions on cancer health disparities and the impact of COVID-19 on clinical trials


This Virtual Meeting is Part II of the AACR Annual Meeting.  Part I was held online in April and was centered only on clinical findings.  This Part II of the virtual meeting will contain all the Sessions and Abstracts pertaining to basic and translational cancer research as well as clinical trial findings.




Pezcoller Foundation-AACR International Award for Extraordinary Achievement in Cancer Research

The prestigious Pezcoller Foundation-AACR International Award for Extraordinary Achievement in Cancer Research was established in 1997 to annually recognize a scientist of international renown who has made a major scientific discovery in basic cancer research OR who has made significant contributions to translational cancer research; who continues to be active in cancer research and has a record of recent, noteworthy publications; and whose ongoing work holds promise for continued substantive contributions to progress in the field of cancer. For more information regarding the 2020 award recipient go to aacr.org/awards.

John E. Dick, Enzo Galligioni, David A Tuveson


Awardee: John E. Dick
Princess Anne Margaret Cancer Center, Toronto, Ontario
For determining how stem cells contribute to normal and leukemic hematopoeisis
  • not every cancer cell equal in their Cancer Hallmarks
  • how do we monitor and measure clonal dynamics
  • Barnie Clarkson did pivotal work on this
  • most cancer cells are post mitotic but minor populations of cells were dormant and survive chemotherapy
  •  only one cell is 1 in a million can regenerate and transplantable in mice and experiments with flow cytometry resolved the question of potency and repopulation of only small percentage of cells and undergo long term clonal population
  • so instead of going to cell lines and using thousands of shRNA looked at clinical data and deconvoluted the genetic information (RNASeq data) to determine progenitor and mature populations (how much is stem and how much is mature populations)
  • in leukemic patients they have seen massive expansion of a single stem cell population so only need one cell in AML if the stem cells have the mutational hits early on in their development
  • finding the “seeds of relapse”: finding the small subpopulation of stem cells that will relapse
  • they looked in BALL;;  there are cells resistant to l-aspariginase, dexamethasone, and vincristine
  • a lot of OXPHOS related genes (in DRIs) that may be the genes involved in this resistance
  • it a wonderful note of acknowledgement he dedicated this award to all of his past and present trainees who were the ones, as he said, made this field into what it is and for taking it into directions none of them could forsee

Monday, June 22

1:30 PM – 3:30 PM EDT

Virtual Educational Session

Experimental and Molecular Therapeutics, Drug Development, Cancer Chemistry

Chemistry to the Clinic: Part 1: Lead Optimization Case Studies in Cancer Drug Discovery

How can one continue to deliver innovative medicines to patients when biological targets are becoming ever scarcer and less amenable to therapeutic intervention? Are there sound strategies in place that can clear the path to targets previously considered “undruggable”? Recent advances in lead finding methods and novel technologies such as covalent screening and targeted protein degradation have enriched the toolbox at the disposal of drug discovery scientists to expand the druggable ta

Stefan N Gradl, Elena S Koltun, Scott D Edmondson, Matthew A. Marx, Joachim Rudolph


Monday, June 22

1:30 PM – 3:30 PM EDT

Virtual Educational Session

Bioinformatics and Systems Biology, Molecular and Cellular Biology/Genetics

Informatics Technologies for Cancer Research

Cancer researchers are faced with a deluge of high-throughput data. Using these data to advance understanding of cancer biology and improve clinical outcomes increasingly requires effective use of computational and informatics tools. This session will introduce informatics resources that support the data management, analysis, visualization, and interpretation. The primary focus will be on high-throughput genomic data and imaging data. Participants will be introduced to fundamental concepts

Rachel Karchin, Daniel Marcus, Andriy Fedorov, Obi Lee Griffith


  • Variant analysis is the big bottleneck, especially interpretation of variants
  • CIVIC resource is a network for curation, interpretation of genetic variants
  • CIVIC curators go through multiple rounds of editors review
  • gene summaries, variant summaries
  • curation follows ACSME guidelines
  • evidences are accumulated, categories by various ontologies and is the heart of the reports
  • as this is a network of curators the knowledgebase expands
  • CIVIC is linked to multiple external informatic, clinical, and genetic databases
  • they have curated 7017 clinical interpretations, 2527 variants, using 2578 papers, and over 1000 curators
  • they are currently integrating with COSMIC ClinVar, and UniProt
  • they are partnering with ClinGen to expand network of curators and their curation effort
  • CIVIC uses a Python interface; available on website


The Precision Medicine Revolution

Precision medicine refers to the use of prevention and treatment strategies that are tailored to the unique features of each individual and their disease. In the context of cancer this might involve the identification of specific mutations shown to predict response to a targeted therapy. The biomedical literature describing these associations is large and growing rapidly. Currently these interpretations exist largely in private or encumbered databases resulting in extensive repetition of effort.

CIViC’s Role in Precision Medicine

Realizing precision medicine will require this information to be centralized, debated and interpreted for application in the clinic. CIViC is an open access, open source, community-driven web resource for Clinical Interpretation of Variants in Cancer. Our goal is to enable precision medicine by providing an educational forum for dissemination of knowledge and active discussion of the clinical significance of cancer genome alterations. For more details refer to the 2017 CIViC publication in Nature Genetics.

U24 funding announced: We are excited to announce that the Informatics Technology for Cancer Research (ICTR) program of the National Cancer Institute (NCI) has awarded funding to the CIViC team! Starting this year, a five-year, $3.7 million U24 award (CA237719), will support CIViC to develop Standardized and Genome-Wide Clinical Interpretation of Complex Genotypes for Cancer Precision Medicine.

Informatics tools for high-throughput analysis of cancer mutations

Rachel Karchin
  • CRAVAT is a platform to determine, categorize, and curate cancer mutations and cancer related variants
  • adding new tools used to be hard but having an open architecture allows for modular growth and easy integration of other tools
  • so they are actively making an open network using social media

Towards FAIR data in cancer imaging research

Andriy Fedorov, PhD

Towards the FAIR principles

While LOD has had some uptake across the web, the number of databases using this protocol compared to the other technologies is still modest. But whether or not we use LOD, we do need to ensure that databases are designed specifically for the web and for reuse by humans and machines. To provide guidance for creating such databases independent of the technology used, the FAIR principles were issued through FORCE11: the Future of Research Communications and e-Scholarship. The FAIR principles put forth characteristics that contemporary data resources, tools, vocabularies and infrastructures should exhibit to assist discovery and reuse by third-parties through the web. Wilkinson et al.,2016. FAIR stands for: Findable, Accessible, Interoperable and Re-usable. The definition of FAIR is provided in Table 1:

Number Principle
F Findable
F1 (meta)data are assigned a globally unique and persistent identifier
F2 data are described with rich metadata
F3 metadata clearly and explicitly include the identifier of the data it describes
F4 (meta)data are registered or indexed in a searchable resource
A Accessible
A1 (meta)data are retrievable by their identifier using a standardized communications protocol
A1.1 the protocol is open, free, and universally implementable
A1.2 the protocol allows for an authentication and authorization procedure, where necessary
A2 metadata are accessible, even when the data are no longer available
I Interoperable
I1 (meta)data use a formal, accessible, shared, and broadly applicable language for knowledge representation.
I2 (meta)data use vocabularies that follow FAIR principles
I3 (meta)data include qualified references to other (meta)data
R Reusable
R1 meta(data) are richly described with a plurality of accurate and relevant attributes
R1.1 (meta)data are released with a clear and accessible data usage license
R1.2 (meta)data are associated with detailed provenance
R1.3 (meta)data meet domain-relevant community standards

A detailed explanation of each of these is included in the Wilkinson et al., 2016 article, and the Dutch Techcenter for Life Sciences has a set of excellent tutorials, so we won’t go into too much detail here.

  • for outside vendors to access their data, vendors would need a signed Material Transfer Agreement but NCI had formulated a framework to facilitate sharing of data using a DIACOM standard for imaging data

Monday, June 22

1:30 PM – 3:01 PM EDT

Virtual Educational Session

Experimental and Molecular Therapeutics, Cancer Chemistry, Drug Development, Immunology

Engineering and Physical Sciences Approaches in Cancer Research, Diagnosis, and Therapy

The engineering and physical science disciplines have been increasingly involved in the development of new approaches to investigate, diagnose, and treat cancer. This session will address many of these efforts, including therapeutic methods such as improvements in drug delivery/targeting, new drugs and devices to effect immunomodulation and to synergize with immunotherapies, and intraoperative probes to improve surgical interventions. Imaging technologies and probes, sensors, and bioma

Claudia Fischbach, Ronit Satchi-Fainaro, Daniel A Heller


Monday, June 22

1:30 PM – 3:30 PM EDT

Virtual Educational Session


Exceptional Responders and Long-Term Survivors

How should we think about exceptional and super responders to cancer therapy? What biologic insights might ensue from considering these cases? What are ways in which considering super responders may lead to misleading conclusions? What are the pros and cons of the quest to locate exceptional and super responders?

Alice P Chen, Vinay K Prasad, Celeste Leigh Pearce


Monday, June 22

1:30 PM – 3:30 PM EDT

Virtual Educational Session

Tumor Biology, Immunology

Exploiting Metabolic Vulnerabilities in Cancer

The reprogramming of cellular metabolism is a hallmark feature observed across cancers. Contemporary research in this area has led to the discovery of tumor-specific metabolic mechanisms and illustrated ways that these can serve as selective, exploitable vulnerabilities. In this session, four international experts in tumor metabolism will discuss new findings concerning the rewiring of metabolic programs in cancer that support metabolic fitness, biosynthesis, redox balance, and the reg

Costas Andreas Lyssiotis, Gina M DeNicola, Ayelet Erez, Oliver Maddocks


Monday, June 22

1:30 PM – 3:30 PM EDT

Virtual Educational Session

Other Articles on this Open Access  Online Journal on Cancer Conferences and Conference Coverage in Real Time Include

Press Coverage

Live Notes, Real Time Conference Coverage 2020 AACR Virtual Meeting April 28, 2020 Symposium: New Drugs on the Horizon Part 3 12:30-1:25 PM

Live Notes, Real Time Conference Coverage 2020 AACR Virtual Meeting April 28, 2020 Session on NCI Activities: COVID-19 and Cancer Research 5:20 PM

Live Notes, Real Time Conference Coverage 2020 AACR Virtual Meeting April 28, 2020 Session on Evaluating Cancer Genomics from Normal Tissues Through Metastatic Disease 3:50 PM

Live Notes, Real Time Conference Coverage 2020 AACR Virtual Meeting April 28, 2020 Session on Novel Targets and Therapies 2:35 PM

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Human gene editing continues to hold a major fascination within a biomedical and biopharmaceutical industries. It’s extraordinary potential is now being realized but important questions as to who will be the beneficiaries of such breakthrough technologies remained to be answered. The session will discuss whether gene editing technologies can alleviate some of the most challenging unmet medical needs. We will discuss how research advances often never reach minority communities and how diverse patient populations will gain access to such breakthrough technologies. It is widely recognize that there are patient voids in the population and we will explore how community health centers might fill this void to ensure that state-of-the-art technologies can reach the forgotten patient groups . We also will touch ethical questions surrounding germline editing and how such research and development could impact the community at large.

Please follow LIVE on TWITTER using the following @ handles and # hashtags:





# Hashtags

#BIO2019 (official meeting hashtag)

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CRISPR’s Unwanted off-target effects: Need for safety study designs with Gene-Editing

Reporter: Stephen J. Williams, Ph.D.

From CafePharma at https://www.statnews.com/2016/07/18/crispr-off-target-effects/

Do CRISPR enthusiasts have their head in the sand about the safety of gene editing?

WASHINGTON — At scientific meetings on genome-editing, you’d expect researchers to show pretty slides of the ribbony 3-D structure of the CRISPR-Cas9 molecules neatly snipping out disease-causing genes in order to, everyone hopes, cure illnesses from cancer to muscular dystrophy. Less expected: slides of someone kneeling on a beach with his head in the sand.

Yet that is what Dr. J. Keith Joung of Massachusetts General Hospital showed at the American Society of Hematology’s workshop on genome-editing last week in Washington. While the 150 experts from industry, academia, the National Institutes of Health, and the Food and Drug Administration were upbeat about the possibility of using genome-editing to treat and even cure sickle cell disease, leukemia, HIV/AIDS, and other blood disorders, there was a skunk at the picnic: an emerging concern that some enthusiastic CRISPR-ers are ignoring growing evidence that CRISPR might inadvertently alter regions of the genome other than the intended ones.

“In the early days of this field, algorithms were generated to predict off-target effects and [made] available on the web,” Joung said. Further research has shown, however, that such algorithms, including one from MIT and one calledE-CRISP, “miss a fair number” of off-target effects. “These tools are used in a lot of papers, but they really aren’t very good at predicting where there will be off-target effects,” he said. “We think we can get off-target effects to less than 1 percent, but we need to do better,” especially if genome-editing is to be safely used to treat patients.

Off-target effects occur because of how CRISPR works. It has two parts. RNA makes a beeline for the site in a genome specified by the RNA’s string of nucleotides, and an enzyme cuts the genome there. Trouble is, more than one site in a genome can have the same string of nucleotides. Scientists might address CRISPR to the genome version of 123 Main Street, aiming for 123 Main on chromosome 9, only to find CRISPR has instead gone to 123 Main on chromosome 14.

In one example Joung showed, CRISPR is supposed to edit a gene called VEGFA (which stimulates production of blood vessels, including those used by cancerous tumors) on chromosome 6. But, studies show, this CRISPR can also hit genes on virtually every one of the other 22 human chromosomes. The same is true for CRISPRs aimed at other genes. Although each CRISPR has zero to a dozen or so “known” off-target sites (where “known” means predicted by those web-based algorithms), Joung said, there can be as many as 150 “novel” off-target sites, meaning scientists had no idea those errors were possible.

One reason for concern about off-target effects is that genome-editing might disable a tumor-suppressor gene or activate a cancer-causing one. It might also allow pieces of two different chromosomes to get together, a phenomenon called translocation, which is the cause of chronic myeloid leukemia, among other problems.

Many researchers, including those planning clinical trials, are using web-based algorithms to predict which regions of the genome might get accidentally CRISPR’d. They include the scientists whose proposal to use CRISPR in patients was the first to be approved by an NIH committee. When scientists assure regulators that they looked for off-target effects in CRISPR’d cells growing in lab dishes, what they usually mean is that they looked for CRISPR’ing of genes that the algorithms flagged.

As a result, off-target effects might be occurring but, because scientists are doing the equivalent of the drunk searching for their lost keys only under the lamppost, they’re not being found.

Other articles on CRISPR and Gene Editing on this Open Access Journal Include:

FDA Cellular & Gene Therapy Guidances: Implications for CRSPR/Cas9 Trials

CRISPR/Cas9 Finds Its Way As an Important Tool For Drug Discovery & Development

CRISPR, the Genome Editing Technology is Nearing Human Trials: Human T cells will soon be modified using the CRISPR technique in a clinical trial to attack cancer cells

Use of CRISPR & RNAi for Drug Discovery, CHI’s World PreClinical Congress – Europe, November 14-15, 2016, Lisbon, Portugal

CRISPR: A Podcast from Nature.com on Gene Editing

AND Please See Our Following ebooks available on Amazon containing interviews with Dr. Jennifer Duodna

Volume One: Genomics Orientations for Personalized Medicine

Volume Two: Latest in Genomics Methodologies for Therapeutics: Gene Editing, NGS & BioInformatics, Simulations and the Genome Ontology

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A Conversation with Jennifer Doudna, Interviewer: Jan Witkowski, Executive Director, Banbury Center at Cold Spring Harbor Laboratory

Reporter: Aviva Lev-Ari, PhD, RN

A Conversation with Jennifer Doudna

INTERVIEWER: JAN WITKOWSKI Executive Director, Banbury Center at Cold Spring Harbor Laboratory

Jennifer Doudna is a Professor in the Department of Chemistry and the Department of Molecular and Cell Biology at the University of California –Berkeley.

Jan Witkowski: People know of you primarily through your work on the CRISPR –Cas9 system for genetic engineering. Can you go over the biology of the system and how you got involved in working on it?

Dr. Doudna: We started working on CRISPR (clustered regularly interspaced short palindromic repeats) biology about 10 years ago. A colleague of mine at Berkeley, Jillian Banfield, was doing research on bacterial comJmunities and the viruses that infect them. She had noticed a lot of repetitive sequences in their genomic data and wondered if these were being used in the form of RNA molecules to protect the bacteria from viral infection. This led to our work with Emmanuelle Charpentier to understand the function of a particular protein called Cas9 (CRISPRassociated protein 9), which turns out to be an RNAguided DNA cutting enzyme. This is a great way for bacteria to fight viruses. It’s an adaptive immune system. The bacteria acquire genetic material from viruses and insert them into these CRISPR sequences. They can transcribe the stored sequence into RNA, and then those RNA molecules can base-pair the matching viral DNAs sequences. They use RNA molecules to target the viral sequences and Cas9 cuts the viral DNA. About half of the sequenced bacterial genomes have one or more CRISPR loci in the genome. Jan

Witkowski: Why is it not more widespread?

Dr. Doudna: Bacteria have a lot of ways to avoid viruses. CRISPR systems operate in certain kinds of bacteria, perhaps in certain environments where they’re particularly advantageous. Other bacteria simply might not need them because they have other ways of fighting the viruses they encounter.

Jan Witkowski: Bacteria have different enzymes depending on the type of CRISPR system, but Cas9 is the one that caught people’s attention for genome engineering. Why is that particularly useful?

Dr. Doudna: It’s programmable. It can be targeting using a short sequence of RNA that provides the base-pairing information to recognize DNA molecules with a matching or complementary sequence. Cas9 is also useful because the enzyme cuts both strands of double-stranded DNA. 





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