Dear Ifor,

I just ran into the same questions. Did you get any informations about the integration site?

kind regards

Tilman

No, I have not gotten an answer to this question. The reason I posted the question is that when the villin-cre transgene integration site is near the site of a floxed target gene to be conditionally deleted, getting a successful recombination event between the transgene and the floxed gene (a requirement to make the conditional KO) may turn out to be a very low frequency event. Jax now offers a second villin-cre transgenic line from Dr. Gumucio's lab: B6.Cg-Tg(Vil1-cre)1000Gum/J. This transgenic line has an independent integration site; this line also has not so far been associated with any examples of germ-line deletion of a floxed gene due to low expression of the cre transgene in the testis (this is a documented, although fortunately rare, problem with the 997Gum/J mice).

Dear Ifor,

if I got your post right, a close proximity of the Cre integration site and the floxed gene may decrease recombination efficiency? This would be the first reasonable explanation for the phenomenon that, in our lab, crossbreeding of Villin-Cre mice with flox strain A leads to a perfect deletion of the target gene and crossbreeding with strain B leads to a pronounced mosaicism. If crossbred with another Cre-line, strain B shows also a complete deletion of the floxed gene. It would be great if you could maybe help me with a literature source regarding this topic.

Best wishes

Anna-Lena

Hi Anna-Lena, I did not intend to suggest that close proximity of a Cre transgene integration site and a floxed gene would decrease efficiency of the floxed gene being deleted. I can't think of a mechanism by which that would happen. My point was that proximity of a Cre transgene integration site and a floxed gene would result in a low rate of recombination during generation of gametes that might prevent the possibility of getting a successful recombinant that had the cre transgene and the floxed gene on the same chromosome. The strain A and B phenomenon you describe in your comment is interesting. There is known to be a small amount of mosaicism with the villin-cre transgene (B6.SJL-Tg(Vil-cre)997Gum/J). This is much more apparent in the cecum and colon than in the small intestine. Are the loxP sites further apart from each other in floxed strain B than floxed strain A? I wonder if there is something about the location of the loxP sites in the strain B situation that causes a substantial decrease in the efficiency with which the floxed allele is deleted when the amount of cre recombinase is limiting.

In my case , my main interest in knowing location is to be able to know zygosity . It would be a great advantage when it comes to set up breeding pairs. I found this paper, 

http://onlinelibrary.wiley.com/doi/10.1002/gene.20042/epdf

Hi Irina, knowing whether villin-cre mice are homozygous or heterozygous for the transgene would certainly help with breeding. In some cases real time PCR assays for the transgene can make this distinction if you have good controls. My personal experience is that the B6.SJL-Tg(Vil-cre)997Gum/J cannot be bred to homozygosity. This may be due to effects of homozygous interruption of the gene at the transgene integration site. By the way, the paper you posted describes a villin-cre transgenic line produced by Sylvie Robine's lab which is distinct from the one I originally posted about. The B6.SJL-Tg(Vil-cre)997Gum/J strain developed by Deb Gumucio is the most widely used villin-cre transgenic line because it is the main one that The Jackson Lab distributes. Good luck with your mouse breeding and research project.

I was recently informed that a paper published in early 2017 in Nucleic Acids Research precisely mapped the integration site of the same villin-cre transgene I was asking about (B6.SJL-Tg(Vil-cre)997Gum/J) to a specific portion of mouse chromosome 17 (chr17:55,466,255 to be specific). This paper by Cain-Hom et al. also showed that the copy number of this transgene is around 16, that there is a small DNA deletion around the site of transgene integration, and that mice homozygous for this transgene are viable. The authors of this paper also mapped the integration sites of 6 other widely used cre transgenic lines using the same technique. A link to the paper can be found below.

https://academic.oup.com/nar/article/45/8/e62/2798189/Efficient-mapping-of-transgene-integration-sites