On 2016-04-28 14:19:27, user Lionel Christiaen wrote:
Reviewed by third journal submitted to.<br />
Journal decision, after reviews: REJECT, <br />
reviews below (simple copy-paste; we'll respond and comment on modifications with next version).
Reviewer 1 Advance Summary and Potential Significance to Field:
Reviewer 1 Comments for the Author:
This study is well done and very convincing, and that revisions are not
necessary for resubmission to a more appropriate journal.
*****
Reviewer 2 Advance Summary and Potential Significance to Field:
Reviewer 2 Comments for the Author:
I have a number of comments/questions/concerns about this manuscript.
Electroporation of sgRNA drivers. The authors report that pools of
electroporated embryos were used for further analysis. Does electroporation
efficiency vary significantly? How many replicates per sgRNA were performed – it
seems like just a single electroporation was reported for each sgRNA. What
proportion of the resulting embryos were transgenic (aren’t DNA constructs
mosaic ally expressed in Ciona)? Were all of these electroporations from a
single batch of embryos? If not, is there variation from batch to batch? If
there is variation, how does this effect your downstream analysis?
Mutagenesis frequency (pg. 6). This is really the mutagenesis frequency per
haploid allele. The authors should describe how this correlates to a mutagenesis
frequency per embryo, given that not all embryos express the constructs
(electroporation efficiency) and not all cells within a given transgenic embryo
express the transgenes (mosaic transgene expression). Presumably, there is a
distribution of mutations in either allele as well as mutations in both copies
of an allele within any given cell. Is there any information on what this
distribution looks like?
Off-target effects (pg 7). This seems more like a specificity assay rather than
an off-target assay. Mismatches outside of the PAM-proximal sequence can be
tolerated and have been shown to produce DSBs – do any of your predicted sgRNAs
share similar or identical PAM-proximal sequences? Have you tested DSBs on any
genomic regions that have well-conserved PAM-proximal sequences, but have
mismatches outside of this region? Did you consider single nt mismatches to the
PAM proximal sequence, rather than just 2 or 4 nt differences? (2 or 4 nt
mismatches are not likely to be effective in any case). This would be a more
accurate assessment of off target cleavage. One of the papers you reference, Hsu
et al., assay substantially more potential off-target genomic locations; it
would be prudent to include a much larger number of genomic sites to assay
off-target effects. It would be useful to include a supplementary table that
describes these potential off-target sites and the resulting analysis of such sites.
On page 9 you mention that CRISPRScan is a tool for rational sgRNA based on
zebrafish data. You developed your own algorithm (TuniCUT) because you
hypothesized that there would be differences between Ciona and zebrafish.
However, you never compare Ciona sgRNAs designed with CRISPRScan to those
designed with TuniCUT. Do they differ substantially? If not, then what is the
significance of your algorithm? If they are similar, what does that say about
the mechanism of Cas9 activity in Ciona vs. other species? If there is not a
significant difference, then the AT-rich nature of the Ciona genome has little
to no effect on the CRISPR/Cas9 process, only on the ability to locate a
suitable CRISPR target sequence.
Is the training set large enough to provide sufficient discrimination of “good”
and “bad” sites for your algorithm? CRISPRScan used >1200 sgRNAs; you
essentially analyzed < 50 sgRNAs (~20 good and ~20 bad – 25% of the 83 total
sgRNAs). It would seem that this small number of analyzed sites could
significantly skew your results. Again, with no comparisons to CRISPRScan, it is
unclear if your algorithm provides any advantages over existing sgRNA design
algorithms.
On page 9 you mention that you added an arbitrary error of 10% based on plasmid
DNA uptake. What exactly does this mean, and how did you settle on a value of 10%?
It is not clear that you have experimentally demonstrated that your algorithm
can accurately design functional sgRNAs as it seems most of the sgRNAs you
report in this study were the same ones used as inputs to your algorithm. If
this is not the case, then this was not clear from the text. I would have
expected that you would test your algorithm by comparing say one or two dozen
novel sgRNAs predicted to work well vs. one or two dozen sgRNAs that have much
lower expected function. This comparison would at least provide experimental
evidence to support your scoring scheme. Ideally, this should also be compared
with sgRNAs identified by CRISPRScan to assess whether your algorithm is a
better predictor of functional sgRNAs in Ciona.
On pages 10-11, you report on producing large deletions by using multiplexed
sgRNAs. However, your assay only detects the presence of a deletion product. Do
you know what percentage of alleles are deleted within an embryo? Is this a rare
event? Have you analyzed/quantitated the spatial distribution of these events?
Can you compare the relative amounts of “wild-type” to deleted regions as a more
accurate measure of efficiency? In other words, compare short PCR products from
your specific deleted region to similarly-sized PCR products produced from
non-deleted regions. This should provide a more accurate, quantitative
description of your mutation efficiencies.
On page 12 you describe the use of linear PCR products to express sgRNAs. It is
a common practice when generating stable cell lines to linearize a plasmid
before transfection to increase the probability of genomic integration.
Supercoiled plasmids are much less efficiently integrated. Do you know if linear
DNA/PCR products integrate into the Ciona genome? What about supercoiled
plasmids? Is this a cause of concern – the potential introduction of additional
mutations due to the random integration of linear DNAs?
Secondly, you only report a single sgRNA introduced as a PCR product (Ebf.3). Do
you know if this works for a wide variety of sgRNAs? Do the linear products work
better than the corresponding plasmid product? Do you know how much sgRNA
product is produced from the PCR product vs. the plasmid form? Do pools of PCR
products work? It seems that there is far less certainty about the usefulness
of PCR products than plasmid products.
Supplemental protocol (page 50). In your supplementary protocol, you explain
that your pre-designed sgRNAs must first be checked with CRISPRdirect to
identify off-targets and then you must check for polymorphisms with the Ciona
genome browser. Shouldn’t your design algorithm already include this information
for the end user?
*****
Reviewer 3 Advance Summary and Potential Significance to Field:
This manuscript describes an attempt to design high efficient guide RNAs for
Crispr/Cas9 based mutagenesis in the ascidian Ciona. First, authors designed 83
guide RNAs that target genes expressed in the heart precursor cells in order to
measure their mutation efficiencies. Based on this dataset, sequence
preferences of good and not good guide RNAs were extracted. The information
were then used to establish a program designing guide RNAs that are potent to
have good mutation efficiencies in Ciona. Using the program authors made a list
of recommended guide RNAs that covers almost entire region of Ciona genome.
Crispr/Cas9 system provides us an easy method to knockout genes, and the
simplicity of the method is good at genome wide analyses. Ciona is an excellent
model for the genome wide analyses due to its small number of genes encoded in
the genome. The program and the guide RNA sequences described here will be good
resources to support future knockout analyses of this organism.
The experiments were done relatively thoroughly and extensively. Honestly
speaking, I felt that the results are not so much novel, because the sequence
preference of good sgRNA in Ciona generally confirms the previous data in other
metazoans, and the gene functions described here are already known ones.
However, the presented algorism and Ci2KO dataset will greatly facilitate gene
knockouts in Ciona and therefore this manuscript will be quite valuable for
Ciona community. For non-Ciona researchers the methods presented here will be
very helpful for constructing similar dataset and extracting the tendencies of
good guide RNAs for the organisms. For these reasons I favor this manuscript
for the techniques and resource section of Development. The text was written
quite plainly enough for easy understanding.
Reviewer 3 Comments for the Author:
I found several relatively minor but essential flaws that need to be corrected
or addressed before considering acceptance. I hope authors find my comments
useful for improving the manuscript.
- The authors initially evaluated the mutation efficiency of 83 guide RNAs.
How were these 83 sites chosen? I understand that most of the targeted genes
are related to cardiovascular system. I would like to know whether there was
any bias, or they were randomly selected, otherwise all were experimented?,
when picking up some from many potential target sites of a gene. If there is a
bias, I would worry about the possibility that the bias might have influenced
the following experiments such as extracting sequence preferences of guide RNAs.
- Controls of FoxF expression. Authors showed the occurrence of large
deletions of FoxF gene by simultaneously introducing two sgRNAs targeting FoxF.
To show the bi-allelic deletions of the gene, in situ hybridization was carried
out. As the control of the experiment, a sgRNA that is not related to FoxF was
used. This is not a good control. Controls should be single FoxF sgRNAs-
introduced embryos. The numbers of examined animals and FoxF-reduced animals
were not shown (or I failed to find them). Adding them is necessary. In Figure
3, colonies 03, 04, 06 are indicated but I could not fully understand what does
the information mean. If authors attempted to show the frequencies of large
deletions, it is an important data and please explain more clearly.
- I am interested in whether the mutation rates of many sgRNAs (the threshold
of good one is 25%, or 37% if the underestimation is considered) can or cannot
satisfy researchers, because I felt that mutation rates (or rates of phenotype
appearance) in this study and related manuscripts would not always be so high
enough to find novel phenotypes. Focusing on a specific phenomenon (like
cardiovascular system in this manuscript), meaning that analyzing with expected
phenotyeps, and using more efficient knockout/knockdown methods in parallel
with Crispr/Cas9 would be a practical use of Ci2KO sgRNAs, and discussion like
this kind may help readers' understanding. This comment was done because the
manuscript focuses on the genome wide analyses: an important focus of this kind
of studies is finding unexpected functions of genes. How about the mutation
rate in other organisms in which F0-based analyses have been adopted? Some
comparisons with the data of other animals would be useful too.
- The statement of GC contents of human genome is wrong. Human genome is AT-
rich like that of Ciona. References are necessary for the GC contents of Ciona
and human.
- I was surprised at the efficient electroporation of PCR product even though
the amount of DNA is quite low. Is the linearization the cause? What does
happen if plasmid DNAs are linearized before electroporation? The comparison
may reveal why low amount PCRed DNA can express genes in high efficiency, and
will be useful to improve electroporation, one of the greatest techniques in
Ciona.