Team:Penn/MethylaseCharacterization
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- | We bisulfite converted the plasmid and used our <a href="https://2013.igem.org/Team:Penn/AssayValidation">validated bisulfite sequencing primers</a> on both the target and off target site, then used the <a href="https://2013.igem.org/Team:Penn/AssayOverview">COBRA assay</a>. The controls recapitulated that our primers are biased for only bisulfite converted DNA, as desired. Unconverted DNA would have been digested by the control enzyme, otherwise. Methylation-sensitive TaqαI digested both the on and off target sites, confirming that the TALE was partially methylating both sites, as MaGellin reported (Figure 3). This validated our assay further, as MaGellin reports the same biological outcome as the published COBRA method, but at a fraction of the cost, time, and technical difficulty. | + | We bisulfite converted the plasmid and used our <a href="https://2013.igem.org/Team:Penn/AssayValidation">validated bisulfite sequencing primers</a> on both the target and off target site, then used the <a href="https://2013.igem.org/Team:Penn/AssayOverview">COBRA assay</a>. The controls recapitulated that our primers are biased for only bisulfite converted DNA, as desired. Unconverted DNA would have been digested by the control enzyme, otherwise. Methylation-sensitive TaqαI digested both the on and off target sites, confirming that the TALE was partially methylating both sites, as MaGellin reported (Figure 3). <h7>This validated our assay further, as MaGellin reports the same biological outcome as the published COBRA method, but at a fraction of the cost, time, and technical difficulty.</h7> |
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We varied induction conditions, expecting one might be more optimal than our previous inductions. As expected, the "without binding site" negative control showed significant off target effects, presumably because the TALE did not bind and the methylase is still active. However, as we induced for longer times and with more IPTG than we had originally used, MaGellin reported off target methylation increasing even for the TALE with the binding site. | We varied induction conditions, expecting one might be more optimal than our previous inductions. As expected, the "without binding site" negative control showed significant off target effects, presumably because the TALE did not bind and the methylase is still active. However, as we induced for longer times and with more IPTG than we had originally used, MaGellin reported off target methylation increasing even for the TALE with the binding site. | ||
- | </br>This makes sense, as the TALEs could saturate the binding sites on a low copy plasmid or genome. We expect this has implications for recently published TALE-effector systems, which may be noisier than expected as it is much more difficult to assay off-target effects in mammalian systems. We propose the use of split-reconstitution systems that are activated by co-localization of effector subunits at a target site. Our future direction is to design this sort of chimeric TALE, cloning and screening the construct will be quick and easy with MaGellin. Notably, we would not have noticed the off target methylation so quickly on a mammalian genome, but it was simple to de-noise this complex system with MaGellin. | + | </br>This makes sense, as the TALEs could saturate the binding sites on a low copy plasmid or genome. We expect this has implications for recently published TALE-effector systems, which may be noisier than expected as it is much more difficult to assay off-target effects in mammalian systems. We propose the use of split-reconstitution systems that are activated by co-localization of effector subunits at a target site. Our future direction is to design this sort of chimeric TALE, cloning and screening the construct will be quick and easy with MaGellin. <h7>Notably, we would not have noticed the off target methylation so quickly on a mammalian genome, but it was simple to de-noise this complex system with MaGellin.</h7> |
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MaGellin was designed to optimize the development of robust tools for site-specific methylation. To those ends, we successfully cloned and expressed three fusion methylases, two of which are novel constructs with advantages over the previously published zinc finger. Our constructs have shown methylase activity and DNA binding activity, which we could measure with our new assay. They are ready to be further optimized, using our MaGellin workflow. | MaGellin was designed to optimize the development of robust tools for site-specific methylation. To those ends, we successfully cloned and expressed three fusion methylases, two of which are novel constructs with advantages over the previously published zinc finger. Our constructs have shown methylase activity and DNA binding activity, which we could measure with our new assay. They are ready to be further optimized, using our MaGellin workflow. | ||
- | <p>We picked up on the noisiness of our TALE-M.SssI using MaGellin, which could have implications for the noisiness of other TAL-Effector systems being used in mammalian systems. To do so, we used MaGellin to its full extent: swapping out DNA binding domains and binding sites, varying induction conditions, applying COBRA, and depending on our original algorithm to properly predict methylation-sensitive digestion patterns. Importantly, we could not have reached this result without MaGellin, because the one-plasmid system in a noiseless chassis makes it simple, even unavoidable, to detect off target methylation. Conversely, for the previously published work in mammalian systems, it was not as feasible to detect off target effects across a long genome with background signal. Based on our data, future improvements on genome engineering tools should include the construction of systems including two targeted fusions with subunits of effectors that only dimerize and show activity at the binding sites, along the lines of how TALE-Nucleases cleave DNA. That could be the best way to construct epigenetic engineering tools with the specificity necessary for clinical applications. | + | <p>We picked up on the noisiness of our TALE-M.SssI using MaGellin, which could have implications for the noisiness of other TAL-Effector systems being used in mammalian systems. To do so, we used MaGellin to its full extent: swapping out DNA binding domains and binding sites, varying induction conditions, applying COBRA, and depending on our original algorithm to properly predict methylation-sensitive digestion patterns. <h7>Importantly, we could not have reached this result without MaGellin,</h7> because the one-plasmid system in a noiseless chassis makes it simple, even unavoidable, to detect off target methylation. Conversely, for the previously published work in mammalian systems, it was not as feasible to detect off target effects across a long genome with background signal. Based on our data, future improvements on genome engineering tools should include the construction of systems including two targeted fusions with subunits of effectors that only dimerize and show activity at the binding sites, along the lines of how TALE-Nucleases cleave DNA. That could be the best way to construct epigenetic engineering tools with the specificity necessary for clinical applications. |
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Revision as of 01:33, 29 October 2013
Methylase Characterization
Zinc Finger-M.SssI Fusion
The zinc finger is a small DNA binding domain, with limited sequence specificity. Previous studies showed it was prone to off-target methylation, which we verified. This was also validation that the MaGellin assay accurately reports the site-specificity of methylation, effectively demonstrating our assay does everything we need it to do. SHOW ZINC FINGER DATA Figure 1: The ZF-M.SssI was cloned into MaGellin with and without its binding site present. We ran the standard MaGellin assay on both plasmids, using methylation sensitive restriction enzymes to report the methylase activity. To be sure of the targeting specificity, we cloned the MaGellin plasmid with and without the zinc finger’s binding site present at the target cut site. This demonstrated how the presence of a zinc finger binding site shifts the methylation pattern (Figure 1).TALE-M.SssI Fusion
TALEs have a greater sequence specificity than zinc fingers, and are easier to customize and less expensive to construct. They have already been validated for use in genome engineering and are replacing zinc fingers for some applications. We followed the MaGellin protocol to clone a TALE-M.SssI fusion and induced its expression. We repeated this experiment numerous times and found the TALE-M.SssI was methylating at both sites, as reported by the MaGellin software.TALE-M.SssI actively methylates DNA as reported by our MaGellin Assay
MaGellin reported our TALE-M.SssI was detectably methylating DNA at both the target site and off-target site. We expected a certain degree of off target methylation simply because the TALEs could occupy all the binding sites on our low copy plasmid; the molar ratio is one of the problems in developing site-specific methylases that the inducible MaGellin system is designed to address. MaGellin is designed to screen multiple fusion protein constructs in a high-throughput manner, and a user would normally select only constructs that methylate in a highly site-specific manner. However, we were interested in using MaGellin to study the TALE-M.SssI further before going back to the drawing board to redesign the linker length, binding site, and other variables.
Validated COBRA is in agreement with our new MaGellin Assay
Varied Induction Conditions Suggests TALE-M.SssI is Prone to Off Target Activity
Summary
MaGellin was designed to optimize the development of robust tools for site-specific methylation. To those ends, we successfully cloned and expressed three fusion methylases, two of which are novel constructs with advantages over the previously published zinc finger. Our constructs have shown methylase activity and DNA binding activity, which we could measure with our new assay. They are ready to be further optimized, using our MaGellin workflow.We picked up on the noisiness of our TALE-M.SssI using MaGellin, which could have implications for the noisiness of other TAL-Effector systems being used in mammalian systems. To do so, we used MaGellin to its full extent: swapping out DNA binding domains and binding sites, varying induction conditions, applying COBRA, and depending on our original algorithm to properly predict methylation-sensitive digestion patterns.