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Revision as of 11:42, 30 September 2024
Staple subunit: dMbCas12a-Nucleoplasmin NLS
dMbCas12a is one crucial part of our PICasSO tool box, being able to function as a Cas staple with the fgRNA (BBa_K5237000) and the dSpCas9 (BBa_K5237002). Transactivation has been shown using this part proving the proper formation of our Cas staple. Easy employment through swift reprogramming of the fgRNA.
Contents
- 1 Sequence overview
- 2 Usage and Biology
- 3 Assembly
and part evolution
- 3.1 Qualtitative assesment of Cas12a orthologs
- 3.2 Quantitative comparison between AsCas12a and MbCas12a
- 3.3 MbCas12a tolerates co-transfection and simultaneous editing of different Cas proteins
- 3.4 MbCas12a shows editing with fgRNA
- 3.5 MbCas12a withstanding fusion to SpCas9 while staying functional
- 3.6 MbCas12a fused to SpCas9 editing utilizing a fgRNA
- 4 Results
- 5 References
The 3D organization of the genome plays a crucial role in regulating gene expression in eukaryotic cells,
impacting cellular behavior, evolution, and disease. Beyond the linear DNA sequence, the spatial arrangement of
chromatin, influenced by DNA-DNA interactions, shapes pathways of gene regulation. However, the tools to precisely
manipulate this genomic architecture remain limited, rendering it challenging to explore the full potential of the
3D genome in synthetic biology. We - iGEM Team Heidelberg 2024 - have developed PICasSO, a powerful molecular
toolbox based on various DNA-binding proteins to address this issue.
The PICasSO part collection offers a comprehensive, modular platform for precise manipulation and re-programming of DNA-DNA interactions using protein staples in living cells, enabling researchers to recreate natural 3D genomic interactions, such as enhancer hijacking, or to design entirely new spatial architectures for gene regulation. Beyond its versatility, PICasSO includes robust assay systems to support the engineering, optimization, and testing of new staples, ensuring functionality in vitro and in vivo. We took special care to include parts crucial for testing every step of the cycle (design, build, test, learn) when engineering new parts
At its heart, the PICasSO part collection consists of three categories.
(i) Our DNA-binding proteins
include our
finalized enhancer hijacking Cas staple as well as half staples that can be used by scientists to compose entirely
new Cas staples in the future. We also include our simple staples that serve as controls for successful stapling
and can be further engineered to create alternative, simpler and more compact staples.
(ii) As functional elements, we list additional parts that enhance the functionality of our Cas and Basic staples. These
consist of
protease-cleavable peptide linkers and inteins that allow condition-specific, dynamic stapling in vivo.
Besides staple functionality, we also include the parts to enable the efficient delivery of PICasSO's constructs with our
interkingdom conjugation system.
(iii) As the final component of our collection, we provide parts that support the use of our custom readout
systems. These include components of our established FRET-based proximity assay system, enabling users to
confirm
accurate stapling. Additionally, we offer a complementary, application-oriented testing system for functional
readout via a luciferase reporter, which allows for straightforward experimental simulation of enhancer hijacking.
The following table gives a complete overview of all parts in our PICasSO toolbox. The highlighted parts showed
exceptional performance as described on our iGEM wiki and can serve as a reference. The other parts in the
collection are versatile building blocks designed to provide future iGEMers with the flexibility to engineer their
own custom Cas staples, enabling further optimization and innovation.
Our part collection includes:
DNA-binding proteins: The building blocks for engineering of custom staples for DNA-DNA interactions with a modular system ensuring easy assembly. | ||
BBa_K5237000 | fgRNA Entryvector MbCas12a-SpCas9 | Entryvector for simple fgRNA cloning via SapI |
BBa_K5237001 | Staple subunit: dMbCas12a-Nucleoplasmin NLS | Staple subunit that can be combined to form a functional staple, for example with fgRNA and dCas9 |
BBa_K5237002 | Staple subunit: SV40 NLS-dSpCas9-SV40 NLS | Staple subunit that can be combined to form a functional staple, for example with our fgRNA or dCas12a |
BBa_K5237003 | Cas-Staple: SV40 NLS-dMbCas12a-dSpCas9-Nucleoplasmin NLS | Functional Cas staple that can be combined with sgRNA or fgRNA to bring two DNA strands in close proximity |
BBa_K5237004 | Staple subunit: Oct1-DBD | Staple subunit that can be combined to form a functional staple, for example with TetR. Can also be combined with a fluorescent protein as part of the FRET proximity assay |
BBa_K5237005 | Staple subunit: TetR | Staple subunit that can be combined to form a functional staple, for example with Oct1. Can also be combined with a fluorescent protein as part of the FRET proximity assay |
BBa_K5237006 | Simple taple: TetR-Oct1 | Functional staple that can be used to bring two DNA strands in close proximity |
BBa_K5237007 | Staple subunit: GCN4 | Staple subunit that can be combined to form a functional staple, for example with rGCN4 |
BBa_K5237008 | Staple subunit: rGCN4 | Staple subunit that can be combined to form a functional staple, for example with rGCN4 |
BBa_K5237009 | Mini staple: bGCN4 | Assembled staple with minimal size that can be further engineered | Functional elements: Protease cleavable peptide linkers and inteins are used to control and modify staples for further optimization for custom applications. |
BBa_K5237010 | Cathepsin B-Cleavable Linker (GFLG) | Cathepsin B cleavable peptide linker, that can be used to combine two staple subunits ,to make responsive staples |
BBa_K5237011 | Cathepsin B Expression Cassette | Cathepsin B which can be selectively express to cut the cleavable linker |
BBa_K5237012 | Caged NpuN Intein | Undergoes protein transsplicing after protease activation, can be used to create functionalized staple units |
BBa_K5237013 | Caged NpuC Intein | Undergoes protein transsplicing after protease activation, can be used to create functionalized staple units |
BBa_K5237014 | fgRNA processing casette | Processing casette to produce multiple fgRNAs from one transcript, can be used for multiplexing |
BBa_K5237015 | Intimin anti-EGFR Nanobody | Interkindom conjugation between bacteria and mammalian cells, as alternative delivery tool for large constructs | Readout Systems: FRET and enhancer recruitment to measure proximity of stapled DNA in bacterial and mammalian living cells enabling swift testing and easy development for new systems. |
BBa_K5237016 | FRET-Donor: mNeonGreen-Oct1 | Donor part for the FRET assay binding the Oct1 binding cassette. Can be used to visualize DNA-DNA proximity |
BBa_K5237017 | FRET-Acceptor: TetR-mScarlet-I | Acceptor part for the FRET assay binding the TetR binding cassette. Can be used to visualize DNA-DNA proximity |
BBa_K5237018 | Oct1 Binding Casette | DNA sequence containing 12 Oct1 binding motifs, can be used for different assays such as the FRET proximity assay |
BBa_K5237019 | TetR Binding Cassette | DNA sequence containing 12 Oct1 binding motifs, can be used for different assays such as the FRET proximity assay | BBa_K5237020 | Cathepsin B-Cleavable Trans-Activator: NLS-Gal4-GFLG-VP64 | Readout system that responds to protease activity. It was used to test Cathepsin-B cleavable linker. |
BBa_K5237021 | NLS-Gal4-VP64 | Trans-activating enhancer, that can be used to simulate enhancer hijacking. | BBa_K5237022 | mCherry Expression Cassette: UAS, minimal Promotor, mCherry | Readout system for enhancer binding. It was used to test Cathepsin-B cleavable linker. |
BBa_K5237023 | Oct1 - 5x UAS binding casette | Oct1 and UAS binding cassette, that was used for the simulated enhancer hijacking assay. |
BBa_K5237024 | TRE-minimal promoter- firefly luciferase | Contains Firefly luciferase controlled by a minimal promoter. It was used as a luminescence readout for simulated enhancer hijacking. |
1. Sequence overview
Sequence and Features
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In 2012, Jinek et al. discovered the use of the Clustered Regularly Interspaced Short Palindromic Repeats
(CRISPR)/Cas system to induce double-strand breaks in DNA. Since then, the system has been well established as a
tool for genome editing. The CRISPR/Cas system, which originates from the bacterial immune system, is constituted
by a ribonucleoprotein complex. For class 1 CRISPR systems, the RNA is complexed by multiple Cas proteins, whereas
class 2 systems consist of a singular protein and RNA. The class 2 type II system describes all ribonucleoprotein
complexes with Cas9 (Pacesa et al., 2024). They include a CRISPR RNA (crRNA), which specifies the target with a 20
nucleotide (nt) spacer sequence, and a transactivating CRISPR RNA (tracrRNA), which induces the processing by the
Cas protein (Jinek et al., 2012) (Figure 2 A). Furthermore, a specific three
nucleotide sequence (NGG) on the 3' end in the targeted DNA is needed for binding and cleavage. This is referred
to as the protospacer adjacent motif (PAM) (Sternberg et al., 2014). The most commonly used Cas9 protein is SpCas9
or SpyCas9, which originates from Streptococcus pyogenes (Pacesa et al., 2024).
A significant enhancement of this system was the introduction of single guide RNA (sgRNA)s, which combine the
functions of a tracrRNA and crRNA (Mali et al., 2013).
Moreover, Cong (2013) established precise targeting of human endogenous loci by designing the 20 nt spacer sequence accordingly.
To select a suitable Cas12a ortholog for cronstructing the Cas sstaple, three different orhtologs were ordered from Addgene:
AsCas12a (#69982), LbCas12a (#69988),
and MbCas12a (#115142).
Since the T7EI assay can only be used to qualitatively compare editing efficiencies, it was hard to distinguish
between the better editing ortholog.
Wanting to employ the MbCas12a as part of a Cas staple, our goal was to find out how well MbCas12a can stay active while
being transfected together with SpCas9. Therefore we engineered the dual luciferase assay to allow us for testing two
catalytically active Cas proteins at once. Including a well established protospacer, CCR5, into the renilla luciferase
gene enables us to test the editing rates of two Cas proteins simultaneously (Fig.5 ).
For the MbCas12a we have a highly significant editing rate in the single cut, meaning both Cas proteins are transfected,
but only MbCas12a has a targeting gRNA. When introducing a targeting gRNA for SpCas9 we see no reduction in the highly
significant editing of MbCas12a, strongly suggesting both Cas proteins to be able to edit simultaneously.
To further confirm if MbCas12a is compatible with our Cas staples, editing rates were tested using a fusion guide RNA
(fgRNA, BBa_K5237000)
targeting two different loci: FANCF and VEGFA. To better assess the impact that the utilization of a
fgRNA has on the editing rates, the sgRNAs were tested separately and in one sample.
To further assess the effect of the genomic locus on the editing rate, we included CCR5 as an additional gene target.
For this assay, a fgRNA with a 20 nt long linker was included between the two spacers.
The editing rate for VEGFA was again relatively consistent throughout the samples (Fig. 7).
Testing showed simultaneous editing of MbCas12a and SpCas9. Therefore we wanted to test protein-protein fusion,
potentially giving us another way of Cas stapling. Our goal was to find out how well MbCas12a can stay active while
being fused to SpCas9. Therefore we employed the previously engineered dual luciferase assay to allow us for testing two
catalytically active Cas proteins at once, this time being fused to each other (Fig. 8).
For the MbCas12a we have a highly significant editing rate in the single cut, meaning only MbCas12a has a targeting
gRNA. When introducing a targeting gRNA for SpCas9 we see no reduction in the highly significant editing of MbCas12a,
strongly suggesting both Cas proteins in the fusion Cas to be able to edit simultaneously.
The capability of MbCas12 in the fusion Cas was tested by assessing the editing rates via a T7EI assay. For this, the
same target sequences as before were used, namely FANCF and VEGFA in both configurations. We included biological
duplicates in this assay.
We extensively characterized the catalytically active MbCas12a in several assays determining the best ortholog out of
three for our Cas staples, assert functional editing with a fusion guide RNA (BBa_K5237000), showing high activity
while being co-transfected with SpCas9, our second staple protein's active version, and lastly a functioning fusion to
SpCas9.
The next step was to use the MbCas12a as part of a Cas staple to staple two DNA loci together, and thereby induce
proximity between two separate functional elements. For this, an enhancer plasmid and a reporter plasmid was used. The
reporter plasmid has firefly luciferase behind several repeats of a Cas9 targeted sequence. The enhancer plasmid has a
Gal4 binding site behind several repeats of a Cas12a targeted sequence. By introducing a fgRNA staple and a Gal4-VP64,
expression of the luciferase is induced (Fig. 10 A).
Different linker lengths were tested. Cells were again normalized against ubiquitous renilla expression. Further
information on our learnings from this assay can be found in the Cas staple engineering cycle iteration 2.
Using no linker between the two spacers showed similar relative luciferase activity to the baseline control (Fig. 10 B).
An extension of the linker from 20 nt up to 40 nt resulted in an increasingly higher expression of the reporter gene.
Kampmann, M. (2017). CRISPRi and CRISPRa Screens in Mammalian Cells for Precision Biology and Medicine. ACS Chemical Biology, 13, 406–416. https://doi.org/10.1021/acschembio.7b00657. Kleinstiver, B. P., Sousa, A. A., Walton, R. T., Tak, Y. E., Hsu, J. Y., Clement, K., Welch, M. M., Horng, J. E., Malagon-Lopez, J., Scarfò, I., Maus, M. V., Pinello, L., Aryee, M. J., and Joung, J. K. (2019). Engineered CRISPR–Cas12a variants with increased activities and improved targeting ranges for gene, epigenetic and base editing. Nature Biotechnology, 37, 276–282. https://doi.org/10.1038/s41587-018-0011-0. Koonin, E. V., Gootenberg, J. S., and Abudayyeh, O. O. (2023). Discovery of Diverse CRISPR-Cas Systems and Expansion of the Genome Engineering Toolbox. Biochemistry, 62, 3465–3487. https://doi.org/10.1021/acs.biochem.3c00159. Pacesa, M., Pelea, O., and Jinek, M. (2024). Past, present, and future of CRISPR genome editing technologies. Cell, 187, 1076–1100. https://doi.org/10.1016/j.cell.2024.01.042. Paul, B., and Montoya, G. (2020). CRISPR-Cas12a: Functional overview and applications. Biomedical Journal, 43, 8–17. https://doi.org/10.1016/j.bj.2019.10.005. Zetsche, B., Gootenberg, J. S., Abudayyeh, O. O., Slaymaker, I. M., Makarova, K. S., Essletzbichler, P., Volz, S. E., Joung, J., van der Oost, J., Regev, A., Koonin, E. V., and Zhang, F. (2015). Cpf1 Is a Single RNA-Guided Endonuclease of a Class 2 CRISPR-Cas System. Cell, 163, 759–771. https://doi.org/10.1016/j.cell.2015.09.038.2. Usage and Biology
3. Assembly and part evolution
3.1 Qualtitative assesment of Cas12a orthologs
We assessed their performance using a T7 Endonuclease I (T7EI) assay on three genomic loci: RUNX1, DNMT1, and FANCF. For
comparison, we also included SpCas9 editing the CCR5 locus (Fig. 3). Editing was only observed at the
RUNX1 locus, where we used the qualitative assay to compare the editing efficiencies of the Cas12a orthologs.
3.2 Quantitative comparison between AsCas12a and MbCas12a
To accurately quantify the editing efficiency, we concted a dual luciferase assay. This assay measures the luminescence
of Firefly luciferase, which decreases proportionally to the editing efficiency at the target site.
To account for variations in cell count and transfection efficiency, the luminescence is normalized
to Renilla luciferase, which acts as an internal control (Fig. 4).
The results revealed a clear difference, with MbCas12a demonstrating significantly higher editing efficiency compared to
AsCas12a (p=0.005). Based on these findings, we decided to proceed with MbCas12a.
3.3 MbCas12a tolerates co-transfection and simultaneous editing of different Cas proteins
3.4 MbCas12a shows editing with fgRNA
Having the sgRNA with single Cas proteins in the same sample resulted in no clear
difference in the editing rates (Fig. 6). The fusion of the gRNAs resulted in a lower editing rate
overall. While the editing for VEGFA stayed at about 20% in all
cases, the editing for FANCF dropped significantly. Nonetheless we were able to show MbCas12a editing utilizing a fgRNA.
3.5 MbCas12a tolerates fusion to SpCas9
3.6 MbCas12a fused to SpCas9 editing utilizing a fgRNA
MbCas12a editing rates were significantly lower, dropping to about 1%. At the same time, when targeting VEGFA they
resulted in a higher editing efficiency than FANCF.
4. Results
After all these successful test we were confident to test the Cas staples in action.
4.1 dMbCas12a Transactivation as Part of Cas Staple
5. References