Are other B6 strains at Taconic affected?
No. The C57BL/6NTac strain does not carry this mutation. Taconic has many GEM lines on B6 backgrounds, but none on the C57BL/6JBomTac substrain. Thus no commercial GEM lines are affected.
Are other strains or stocks at Taconic affected?
We have no evidence that other stocks or strains at Taconic have mutations in the Y chromosome. Taconic has tested C57BL/6NTac, FVB/NTac, NOD/MrkTac, 129S6/SvEvTac and C3H/HeNTac with a 143,000 SNP panel and no abnormalities were detected. Taconic is pursuing whole genome sequencing of its inbred strains. Data for some strains is now being analyzed and will be available in the future. Due to the nature of biological processes, it is possible that during DNA replication, spontaneous mutations arise. To decrease the potential for spontaneous mutations becoming fixed in a population, Taconic has cryopreserved stock for all its inbred strains. Taconic refreshes all inbred strains from cryopreserved stock every 5 years or 10 generations, whichever comes first. The initial cryopreservation for each strain took place over a number of years. The C57BL/6JBomTac strain was first cryopreserved in 2010.
Why did Taconic not detect the mutation sooner?
The Taconic genetic quality program has several components, including major quality control programs in genetic integrity and strain harmonization. These two programs are designed to maintain genetic integrity of each strain and ensure that animals obtained from one Taconic site are as genetically similar as possible to those obtained from other Taconic sites.
Additionally, Taconic maintains cryopreserved stocks of its main inbred strains in order to avoid genetic drift. As spontaneous mutations are not preventable, best practices in strain management dictate that a strain be genetically refreshed through recovery of embryos from a set stock with some frequency. Taconic follows this best practice, and maintains cryopreserved stocks of its main inbred strains in order to avoid genetic drift. The C57BL/6JBomTac strain was refreshed in 2015 using embryos from a 2010 cryopreservation. Unfortunately, the refreshed C57BL/6JBomTac stock also contained the deletion, indicating that the mutation occurred prior to 2010.
Spontaneous mutations occur in inbred mouse strains on an ongoing basis. This is a natural process that cannot be prevented and can be difficult or impossible to detect. Taconic maintains an extensive genetic monitoring program for all commercial lines using single nucleotide polymorphism (SNP) panels at varying densities during the maintenance of the breeding colonies (96 GenMon SNP Panel) and including genetic characterization profile of embryo derived refreshed colonies (1,449 SNP Panel). The majority of SNP panels available have poor or no coverage for the Y chromosome due to its highly repetitive nature and the subsequent challenge in testing of this region. The standard panel of 1,449 SNPs used by Taconic for genetic monitoring does not include any SNPs on the Y chromosome. The 96 GenMon SNP Panel includes one Y chromosome SNP marker, which is present in the C57BL/6JBomTac. However, the mutation is outside of this marker. Thus our routine genetic monitoring did not detect this deletion. Taconic is in the process of developing additional higher density SNP panels to enhance its genetic monitoring program.
Detection of spontaneous mutations in inbred strains is challenging. Unless there is an observable phenotype, nothing less than genome sequencing would detect most spontaneous mutation. Taconic has already begun a whole genome sequence program of its most popular inbred strains to add to the available genetic data on these strains. For more information on the Taconic genetic monitoring program see https://www.taconic.com/quality/genetic-integrity
Why did Taconic not detect a sex ratio discrepancy in barrier production colonies?
Taconic does monitor sex ratios of all breeding colonies. A 55% female to 45% male offspring ratio was observed in the C57BL/6JBomTac breeding colonies. Our genetic monitoring program did not detect any abnormalities with the C57BL/6JBomTac strain. Additionally, there were no phenotypic anomalies reported for this line when used in studies and when used as the background strain for various GEM lines. Given the observations available to us at the time, the shift in sex ratio was not considered substantial and was considered to be a normal trait of the C57BL/6JBomTac strain.
How do I know if my line carries the mutation?
If you purchased C57BL/6JBomTac mice from Taconic from 2009 until now, we expect that the mice you received have the mutation. If you used C57BL/6JBomTac to maintain a GEM line, the mutation may or may not be present depending on the details of your mating scheme. If you wish to analyze whether the mutation is present in your GEM line, we recommend testing your line by one of the two following methods:
- Detection of Rbm31y gene (please complete the short form below to access the document):
- Testing via the GVG STR Panel. Please note Taconic cannot independently verify the STR results reported by GVG Genetic Monitoring, but this panel is available for use by the public.
My experiments are working fine. Do I need to do anything?
It is likely that the mutation does not affect your organ system of interest unless you are studying reproduction. If you are in doubt, we suggest you use another B6 substrain available for comparison to confirm that there are no differences.
I have a knockout on the C57BL/6BomTac background. Can I just start mating to C57BL6/NTac or C57BL/6J?
You can transfer your mutation of interest to another B6 substrain background, but we would advise performing multigenerational backcrosses to assure you have a consistent, congenic background. Mixed substrain backgrounds can confound experiments.
Are there any other chromosomal deletions or mutations that I should be aware of?
Spontaneous mutations are a natural process that cannot be prevented. Various mutations have become fixed in commonly used inbred strains, and as whole genome sequencing becomes more common, additional mutations are likely to be discovered. For example, in B6 substrains, the following mutations are known:
| || ||Mutation || || || ||
|Substrain ||Vendor ||NntC57BL/6 ||Snca deletion ||Crb1rd81> ||Mmrn1 deletion ||Cyfip2M1N ||Dock2Hsd ||Nlrp12 mutation
|C57BL/6NTac ||Taconic || || ||Yes || ||Yes || ||
|C57BL/6JBomTac ||Taconic || || || || || || ||
|C57BL/6NHsd ||Envigo || || ||Yes || ||Yes ||Yes ||
|C57BL/6JOlaHsd ||Envigo || ||Yes || ||Yes || || ||
|C57BL/6RccHsd ||Envigo || || || || || || ||
|C57BL/6J ||Jax ||Yes || || || || || ||Yes
|C57BL/6NJ ||Jax || || ||Yes || ||Yes || ||
|C57BL/6NCrl ||CRL || || ||Yes || ||Yes || ||
|Reference ||- ||6 ||7 ||8 ||9 ||10 ||11 ||12
I made a KO mouse on a C57BL6/NTac strain and then backcrossed it to a C57BL/6JBomTac strain and lost my phenotype. Is that due to the deletion?
This is unlikely, as no problems with organ systems have been reported with C57BL/6JBomTac. This could be due to many other reasons.
I made a KO mouse on a BALB/c strain and then backcrossed it to a C57BL/6JBomTac strain and lost my phenotype. Is that due to the deletion?
No, this is likely due to different genetic modifiers of each strain. There are many reports in the scientific literature that changing the genetic background alters the phenotype of a mutation.
Given this discovery, will you now monitor for Y chromosome deletions in all your other strains of mice?
Yes, as technology becomes available and new markers are developed, Taconic is constantly assessing new platforms/assays to improve strain monitoring.
Are other chromosomes subject to the same kind of mutation?
Spontaneous mutations of various types, including deletions, have been reported in various laboratory mouse strains on other chromosomes.
Is it possible that only the individual mice that you have tested are subject to the deletion? Is it possible that some mice don't have the deletion?
This would have been possible for animals that were sold between 2008--2009 when the mutation arose in the C57BL/6JBomTac Foundation Colony (the source colony for all C57BL/6JBomTac stock). However, once the mutation was fixed in the Foundation Colony, all C57BL/6JBomTac production sites would have received breeders from the Foundation Colony and would therefore carry the mutation.