Reviewer #1 (Public review):
Summary:
Perlee et al. sought to generate a zebrafish line where CRISPR-based gene editing is exclusively limited to the melanocyte lineage, allowing assessment of cell-type restricted gene knockouts. To achieve this, they knocked in Cas9 to the endogenous mitfa locus, as mitfa is a master regulator of melanocyte development. The authors use multiple candidate genes - albino, sox10, tuba1a, ptena/ptenb, tp53 - to demonstrate that their system induces lineage-restricted gene editing. This method allows researchers to bypass embryonic lethal and non-cell autonomous phenotypes emerging from whole body knockout (sox10, tuba1a), drive directed phenotypes, such as depigmentation (albino), and induce lineage-specific tumors, such as melanomas (ptena/ptenb, tp53, when accompanied with expression of BRAFV600E). The main weakness of the manuscript is that the mechanistic explanations proposed to underlie the presented phenotypes are minimally interrogated, but nonetheless interesting and motivating for future experimentation. Overall, there is a clear use for this genetic methodology, and its implementation will be of value to many in vivo researchers.
Strengths:
The strongest component of this manuscript is the genetic control offered by the mitfa:Cas9 system and the ability to make stable, lineage-specific knockouts in zebrafish. This is exemplified by the studies of tuba1a, where the authors nicely show non-cell autonomous mechanisms have obfuscated the role of this gene in melanocyte development. In addition, the mitfa:Cas9 system is elegantly straightforward and can be easily implemented in many labs. Mostly, the figures are clean, controls are appropriate, and phenotypes are reproducible. The invented method is a welcome addition to the arsenal of genetic tools used in zebrafish. The authors kindly and honestly responded to reviewer criticism, which has led to an improved manuscript and a pleasant review process.
Weaknesses:
The authors argue that the benefit of their system is the maintenance of endogenous regulatory elements. However, no direct comparison is made with other tools that offer similar genetic control, such as MAZERATI. This is a missed opportunity to provide researchers the ability to evaluate these two similar genetic approaches. There is a slight concern that tumor onset with this system is hindered by the heterozygous state it imparts to the lineage master regulator (here, mitfa). The authors do a good job at addressing these issues in the Discussion, but experimentation would have been appreciated. Additionally, the authors claim 86% of mitfa+ cells express Cas9. The image shown in Figure 1C does not do a convincing job at showing this percentage.
Another weakness of the manuscript regards minimally investigated mechanistic explanations for each biological vignette. Detailed mechanistic information is indeed out-of-scope for this manuscript, which intends to prove the efficacy of a genetic tool. Readers are cautioned to use the mechanistic insights from these vignettes as inspiration rather than bona fide truth.
The authors performed the necessary experiments to address each of the reviewers' concerns and thereby quell any substantial issues raised during the first review. They have additionally edited their language appropriately to make their claims more accurate. Their efforts during the review process are appreciated.
Conclusion:
The authors were highly receptive to reviewer comments and improved their manuscript from the first submission. The authors were successful in their goal of creating a rapid genetic approach to study cell-type specific genetic insults in vivo. They have presented multiple interesting and convincing stories to support the power of their invented methodology. The refined mechanisms underlying their observed phenotypes may be lacking but this does not take away from the methodological benefit this manuscript provides to the large field of in vivo researchers.


