Difference between C57BL/6J and C57BL/6N

提到C57BL/6小鼠，大家都不会陌生，它的历史可以追溯到1921年C.C. Little对Miss Abby Lathrop品系（C57BR和C57L也源于该品系）的交配繁育；在1937年又分离出了6号系和10号系；而在1948年, Jackson实验室从Hall引入6号系，培育出C57BL/6小鼠。1951年，引入NIH。
经过多年繁育，C57BL/6形成了两个主要的亚系：C57BL/6J与C57BL/6N。目前，这两个亚系在全球多个生产单位保种，而且都已繁育220代左右，广泛应用于基因工程鼠的制作、免疫学、行为学、代谢机制、老龄鼠等方面的研究。
实际上，从基因型到表型，不同亚系间是存在一定程度差异的。我们在选择实验动物的时候，也需要重视不同亚系之间的差异，那么6N和6J这两个常见亚系之间究竟有哪些不同呢？
1、基因序列差异：编码区内有34个SNPs差异，2个核苷酸碱基的缺失，15个与基因相关的结构变异体(Michelle MS. et al. 2013)。
2、Crb1rd8突变：C57BL/6N是Crb1rd8突变(视网膜变性8突变)的纯合子，可在眼底观测到由视网膜折叠和假菊形团形成的感光器退化的斑点；C57BL/6J无该突变。
3. Nnt基因：C57BL/6J由于碱基缺失突变导致无Nnt基因（烟酰胺核苷酸转氢酶），而C57BL/6N不存在Nnt基因缺失。Nnt基因与葡萄糖代谢相关。若进行代谢机制方面的研究，需明确这一点(Freeman,H.C. et al. 2006)。
4. Cyfip2M1N突变：C57BL/6N是Cyfip2M1N突变的纯合子（此突变导致细胞质FMR1相互作用蛋白2在968位点由苯丙谷氨酸代替丝氨酸），突变降低可卡因对动物的应激反应（以运动活跃度测量）约45%左右；C57BL/6J无该突变。
5. Nlrp12突变：C57BL/6N中Nlrp12的突变导致富亮氨酸重复域C末端1034位氨基酸由亮氨酸变为精氨酸，影响未知。
6. 免疫学方面：Envigo（原Harlan Sprague Laboratories）C57BL/6NHsd亚系含有自发性Dock2 突变，导致其免疫表型发生变化：外周B细胞缺失、记忆性CD8 T细胞增多，从而影响免疫学研究；但其它C57BL/6亚系（包括C57BL/6J、C57BL/6NJ、C57BL/6Ncrl、C57BL/6NTac）没有发生该突变(Vinay S. Mahajan. et al.2016)。
7. 领地意识：C57BL/6N雄性小鼠领地意识要强于C57BL/6J小鼠，性成熟后更易出现因为争抢领地而导致的打斗现象。
8. 生理异常：C57BL/6J雌性小鼠的剃毛、掉毛现象要高于C57BL/6N小鼠。C57BL/6J雌性小鼠出现阴道纵膈的比例为4%-11.3%，而C57BL/6N雌性小鼠阴道隔比例很低，小于1% (Gearhart, S. et al. 2004)。

总结如下表：

项目（基因序列差异）	C57BL/6N	C57BL/6J	备注
Crb1^rd8	突变，纯合子	无突变
Nnt基因	有此基因	碱基缺失造成无此基因	Nnt为烟酰胺核苷酸转氢酶基因与葡萄糖代谢相关
Cyfip2^M1N突变	突变，纯合子	无突变	突变降低可卡因对动物的应激反应约45%
NIrp12突变	突变，纯合子	无突变	突变导致亮氨酸变为精氨酸
免疫性能	6NHsd亚系自发性Dock2突变，造成外周B细胞缺失记忆性CD8T增多	无突变	其他亚系也无突变
领地意识	6N雄性强	6J雄性弱	N雄性领地意识强于J,性成熟后更易打斗
生理异常	6N雌性低	6J雌性高	6J雌雄小鼠剃毛、掉毛现象高于6N
提示： 1.N即NIH，J即Jackson； 2.维通利华供应两个亚系：C57BL/6NCrl（Charles River ）和C57BL/6JCnc（中国国家啮齿类实验动物种子中心）

以下摘录只JAX网站，修改如下：

Not all C57BL/6 mice are the same: It's what's on the inside that counts…

C57BL/6 or B6 mice are the most commonly used inbred mice in biomedical research. Two of the most common substrains used to make gene targeted mouse models are C57BL/6J (J= Jackson) and C57BL/6N (N= NIH). Although mice from both substrains appear black, they differ genetically by single nucleotide polymorphisms (SNPs), insertions and deletions (Indels), copy number variations (CNVs) and/or spontaneous mutations. These genetic differences can translate into phenotypic differences that can affect properly interpreting and reproducing your research results. Phenotypic differences have specifically been noted in the fields of neurobiology (behavior, vision, hearing), metabolism, and immunology, to name a few.

For Example:

Neurological Differences：

Mice on the C57BL/6N genetic background carry the Crb1rd8 mutation, causing progressive, spotty retinal degeneration in mice. The C57BL/6J strain does not carry the rd8 mutation. Using mice with the B6N background can interfere with your experiments if your mice are required to interpret visual cues or if you are trying to characterize a new mutation that is expected to affect vision.

Metabolic Differences：

C57BL/6J mice have a mutation in the Nnt gene which may affect metabolism due to inappropriate glucose homeostasis in B6J mice. C57BL/6N lines do not have this mutation. In looking into the effect of the Nnt allele on diet induced obesity (DIO) responsiveness, C57BL/6J males had a significantly more robust DIO response (higher weight gain, more adiposity, and more severe impaired glucose tolerance) than C57BL/6N males.

Immunological Differences：

In response to the bacterial infection Listeria monocytogenes, C57BL/6J shows greater susceptibility to Listeria spp while C57BL/6N shows a significant pro-inflammatory response. Also, when both substrains were challenged with dinitrofluorobenzene, C57BL/6J showed a greater inflammatory response compared to C57BL/6N.

So which B6 substrain is right for your research? ingenious has the solution!
As the difference between the two main substrains became evident, ingenious targeting laboratory adapted to these findings by developing a 50:50 mix of N and J where one allele comes from the N substrain, and the other comes from the J substrain. From this, we can mate the resulting chimeras to the desired pure substrain for your research and subsequently genotype for the loss of the mutation. We are thus able to deliver mice that are not only germline confirmed for the desired knockout or knockin, but also negative for the unwanted mutation.

Ingenious C57BL/6 cell line success story
Ingenious was the pioneering company to offer gene targeting in the C57BL/6 strain in 2004 and we have generated hundreds of models in the pure B6 strain. All our embryonic stem cell strains, including the C57BL/6 lines are highly robust, consistently showing a >90% euploidy rate compared to the 70% industry standard. Over 1,400 mouse models and targeted mouse embryonic stem cell lines have been delivered from ingenious ES cell lines.

References:
1. Jackson Laboratory webinar- Protect your research: Know your B6 mouse.
2. Mehalow, AK et al. 2003. Hum. Mol. Genet. 12 (17): 2179-2189.
3. Leiter, EH. 2008. Does a mutant NAD nucleotide transhydrogenase (Nnt) gene in C57BL/6J impair responsiveness to diet-induced obesity? The Jackson Laboratory.
4. Simon, MM et al. 2013. Genome Biology. 14:R82