2. controlled change in the DNA
DNA is deleted, modified, inserted or replaced
Randomly inserts genetic
Site specific locations
CRISPR, 2009
Easier
Simpler
Faster
Cheaper
Accurate
WHAT IS GENOME EDITING?
4. TOOLS
Science as 2015 Breakthrough of the Year
Meganucleases (MegaN)
Zinc Finger Nucleases (ZFNs)
Transcription Activator-Like Effector-based Nucleases (TALEN)
Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR/Cas9)
system
Create site-specific double-strand breaks (DSBs)
NHEJ or HR, targeted mutations (edits)
5. DOUBLE STRAND BREAK
REPAIR
Non-Homologous End Joining (NHEJ)
Homology Directed Repair (HDR)
NHEJ - directly join the DNA ends
HDR- homologous sequence as a template
Homologous to the flanking sequences
Desired change at DSB
Rate increases by at least three orders of magnitude
6. Genome editing with site-specific nucleases (SSNs). The double stranded breaks (DSBs) introduced by CRISPR/Cas9 complex can
be repaired by non-homologous end joining (NHEJ) and homologous recombination (HR).
(A)NHEJ repair can produce heterozygous mutations, biallelic mutations (two different mutations at each chromosome) and
homozygous mutations (two independent identical mutations) leading to gene insertion or gene deletion.
(B) (B) In the presence of donor DNA digested with the same endonuclease leaving behind similar overhangs, HR can be achieved
leading to gene modification and insertion.
7. 14 to 40 base pair unique or nearly-so in most genomes
Very specific (>14bp)
Best known are the LAGLIDADG family
Costly and time (major disadvantage)
Initial protein engineering stage for custom meganuclease
technically challenging and hindered by patent disputes
MEGANUCLEASES (MegaN)
8. ZINC FINGER NUCLEASE
Designed zinc finger domains
Cys2-His2 zinc finger protein
3–6 zinc finger domains (ZFPs)
3 bp long target DNA sequence
Non-specific nuclease domain FokI
ZFNs pair bind and align in reverse
fashion
Dimerization of FokI nuclease
domain
First time reported in tobacco (Wright
et al. 2005) and Arabidopsis (Lloyd et
al. 2005)
Tobacco, Maize, Arabidopsis,
Soybean, Canola and other plants
9. TAL EFFECTOR NUCLEASES
(TALENS)
Secreted proteins bacterial genus Xanthomonas
33–35 amino acid repeats
12 and 13, repeat variable di-residues (RVDs)
RVD recognizes one nucleotide
Combination of repeat number and RVDs composition in the repeats
Chimeric protein (DNA binding domain and FokI nuclease domain)
Cut in desired DNA region
Low off-target effects
Rice, wheat, maize, tomato, potato,Arabidopsis, tobacco
10. CRISPR/CAS9
Provide immunity to bacteria against bacteriophage
Adaptation
Small fragment inserted into CRISPR locus.
Cas genes (CRISP associated), helicase and nuclease activity.
Transcription
long pre-crRNA (poly-spacer precursor crRNA)
short crRNAs CRISPR RNA), 39–45 nucleotides containing one spacer sequence.
Interference
Ternary Cas9-crRNAtracrRNA complex binds to Cas proteins.
crRNA complement with protospacer sequence (PAM)
tracrRNA required for Cas-mediated DNA interference
Cas-proteins cut foreign DNA sequences, DNA degradation.
Cost-effective and easy-to-use technology.
11. Mechanism of CRISPR/Cas9 action: in the acquisition phase foreign DNA gets incorporated into the CRISPR loci of
bacterial genome. CRISPR loci is then transcribed into primary transcript and processed into crRNA with the help of
tracrRNA during crRNA biogenesis. During interference, Cas9 endonuclease complexed with a crRNA and cleaves foreign
DNA near PAM region.
12. Various genome-editing tools.
(A)Zinc-finger nucleases (ZFNs) act as dimer. Each monomer consists of a DNA binding domain and a nuclease domain. Each DNA
binding domain consists of an array of 3–6 zinc finger repeats which recognizes 9–18 nucleotides. Nuclease domain consists of type II
restriction endonuclease Fok1.
(B)Transcription activator-like nucleases (TALENs): these are dimeric enzymes similar to ZFNs. Each subunit consists of DNA
binding domain (highly conserved 33–34 amino acid sequence specific for each nucleotide) and Fok1 nuclease domain.
(C)CRISPR/Cas9: Cas9 endonuclease is guided by sgRNA (single guide RNA: crRNA and tracrRNA) for target specific cleavage. 20
nucleotide recognition site is present upstream of protospacer adjacent motif (PAM).
13. CRISPR/CAS SYSTEM
Cas protein in the ribonucleoprotein system can vary.
Makarova et al. (2011) classified CRISPR/Cas systems into three types:
type I, type II, and type III
presence of signature Cas3, Cas9 and Cas10 proteins respectively.
Further modified into two class-five type classification systems
Class 1 CRISPRs have multiple subunit effector complexes
Class 2 CRISPRs concentrates most of their functions with single protein
effectors
17. CRISPR-CPF1
CRISPR-Cpf1, (Prevoltella and Francisella), advanced tool
Uses single Cpf1 protein for crRNA processing, target site recognition, and DNA
cleavage.
Differs substantially in many aspects.
Ribonuclease that processes precursor crRNA
Recognizes thymine rich (like 5’-TTTN-3’) PAM sites.
Cpf1 generates 4 bp overhangs.
Sticky ends provide more efficient genomic insertions.
21. CRISPR SPECIFICATIONS IN PLANTS
Optimal promoters for expression.
Suitable vector system
Efficient target sites and transformation method.
Specific expression vectors
sgRNA regulated promoters AtU6, TaU6 etc
Cas9 promoters like ubiquitin promoters.
Choice varies
sgRNA and Cas9 can be co-expressed in a single plasmid ex. pFGC-pcoCas9,
pRGEB32, pHSE401.
https://www.addgene.org/ crispr/plant/
22. WEB BASED TOOLS FOR GUIDE
RNA SYNTHESIS ALONG WITH OFF
TARGET PREDICTION
24. Minimum or No off-target effects.
COSMID (CRISPR Off-target Sites with Mismatches, Insertions, and
Deletions).
potential off-targets 3% (soyabean).
No detectable off-targets found in A. thaliana, wheat, rice and sweet orange.
Target specific oligonucleotides (20 nt)
sgRNA + Cas9 sequence (a binary vector)
Individually
Transformed using a suitable method.
Delivery systems vary based on plant species, research purpose, and
requirements.
Restriction enzyme digestion suppressed PCR (RE-PCR) method.
Whole genome sequencing.
SELECTION OF TARGET SITE
25. Simplified flow chart representing CRISPR/Cas9 mediated plant genome editing. After the selection of the target site, sgRNAs are
designed using various bioinformatic softwares and packed into specific vectors along with codon optimized Cas9. After delivery
into plant cells, putative transformants can be screened by multiple assays and used for further analysis.
31. DNA FREE MODIFICATIONS OF
PLANT GENOME
Extra DNA frequently integrate into the plant genome.
Disruption, plant mosaicism and off target disruptions.
Lessens the efficiency of gene editing and gene insertion.
Uses Cas9 ribonucleoproteins (RNPs).
Cas9 RNPs are in vitro pre-integrated Cas9 nucleases and gRNA, delivered
into plant
Equally efficient to plasmid based expression.
RNPs are delivered in isolated plant protoplasts.
Tobacco, Arabidopsis, lettuce, rice, Petunia, and wheat.
32. Proposed workflow for DNA free genome editing. Cas9 is expressed purified from E. coli. In vitro transcription of single guide RNA
(sgRNA) and transcribed in vitro and RNP complex formation. RNPs and DNA precipitation onto 0.6 µm gold particles followed by
Particle bombardment in targeted cells. Plants regeneration without any selective agent from bombarded cells and screened for mutations
via PCR/restriction enzyme assay and deep sequencing.
34. POWDERY MILDEW
Fungal disease.
Erysiphales, with Podosphaera xanthii (a.k.a. Sphaerotheca fuliginea), most
commonly reported cause.
Easy to identify.
Display white powdery spots on the leaves and stems.
Layer of mildew made up of many spores forms across
the top of the leaves.
Chemical methods, bio organic methods,
genetic resistance.
Slow down the growth of plant, in severe cases reduces
fruit yield and quality.
35. BACKGROUND
MILDEW RESISTANT LOCUS O (Mlo), encodes a membrane-associated protein
with seven transmembrane domains.
Confer susceptibility to Oidium neolycopersici.
Homozygous loss-of-function mutations (mlo) result powdery mildew resistance.
16 Mlo genes, SlMlo1 to SlMlo16, SlMlo1 major contributor to powdery mildew
susceptibility.
Natural non-transgenic loss-of-function slmlo1 mutants, but introgression mutations
is lengthy and laborious process.
Generated transgene-free genetically edited slmlo1 tomato (Tomelo) variety using
the CRISPR/Cas9 system.
Deleted SlMlo1 locus using the double sgRNA strategy.
36. METHODS
Plasmid pAGM4723::Cas9_sgRNA1_sgRNA2
Plant transformation
GCR7589, a derivative of tomato cultivar Moneymaker, was
transformed
T0 transgenic plants were selected with kanamycin medium medium
and then transferred
into soil.
Plant DNA extraction.
Detection of Cas9-induced deletions and T-DNA in plant genomic DNA using
PCR
Whole genome Illumina sequencing.
37. 10 primary transformants (T0) carrying T-DNA expressing Cas9 and sgRNAs,
analysed for altered electrophoretic mobility, indication SlMlo1 modifications.
8 out of 10 tested T0 transformants showed a mobility shift indicating the
presence of mutations.
3 transformants (2, 8 and 10), showed band shift consistent with the expected
deletion of 48 bp.
All three transformants that showed the expected band shift proved to carry
homozygous (transformants 2 and 8) or biallelic (transformant 10) mutations
at the SlMlo1 locus.
To generate non-transgenic slmlo1 tomato lines, T-DNA segregated by selfing T0
transformant and cultivating next generation (T1) plants.
5 slmlo1 T-DNA-free individuals identified.
RESULTS
38. Disease resistance assays revealed all the slmlo1 mutant plants fully resistant.
All tested wild-type plants were susceptible.
slmlo1 mutant plants morphologically similar and produced harvested fruit weight
similar to the wild type.
39. Generating non-transgenic slmlo1
tomato lines resistant to powdery
mildew.
(a) The SlMlo1 locus
was targeted by two sgRNAs
(b) T0 tomato transformants were
tested for the presence of deletions
using PCR
(c) Selected T0 transformants
genotyped using the PCR band
shift assay alongside wild
type (WT)
(d) SlMlo1 sequencing reads from
selected T0 transformants
(e) Leaves of tomato plants
inoculated with Oidium
neolycopersici (5 weeks post
inoculation)
(f) PCR genotyping of the T1
generation for the presence
T-DNA and the slmlo1 mutation.
The agarose gels presented in panels
(b and c) were cropped.
40. Illumina sequencing data
(a) Quantification of Illumina
sequencing reads matching the
T-DNA or vector backbone in
wild type and slmlo1 T1
progeny lines;
(b) Coverage of the T-DNA by
Illumina reads
(c) Coverage of the SlMlo1 locus
by Illumina reads.
41. CONCLUSION
Named the powdery mildew resistant slmlo1 tomato variety Tomelo.
9.5 months from the DNA transformation step.
slmlo1 mutation could be readily introduced into elite or locally adapted varieties in
less than a year with relatively little effort or investment.
42. FUTURE PROSPECTS
Recent advances have revolutionized genome editing but certain issues and challenges
like the SSN/DNA delivery methods, off-target effects needs to be addressed for better
efficiency and output.
Some aspects of the editing system are still unclear, including the catalytic activity of
Cas9, target sites identification and the importance of PAM sites.
Approaches like Digenome-seq and GUIDE-seq have been developed to detect off
target activities in human cells and need to be adapted to plants too for better efficiency
and specificity of Cas9.
Till date, the genome editing systems are mainly used to destroy genes in plants by
inducing DSBs following NHEJ repair that introduces Indels at the target site.