a The PUFA biosynthesis pathways. The desaturation reactions of ω3 and ω6 PUFAs are catalyzed by FADS1 (cyan arrows) or FADS2 (yellow arrows). b A strategy for simultaneous generation of alleles using the CRISPR/Cas9 system. A CRISPR/Cas9 cocktail containing two gRNAs that targeted the Fads1/2 genes and two single-stranded DNA donor templates with the loxP sequence (Supplementary Table 3) was microinjected into mouse fertilized eggs (C57BL/6JJcl strain). Nonhomologous end joining (NHEJ) resulted in the deletion of ~124 kb (Fads(Δ)) and homology-directed repair (HDR) generated the floxed allele. c Body weight of heterozygous and homozygous Fads1/2 deficient mice. Fads(Δ/Δ) mice were significantly leaner than the Fads(Δ/+) and WT (Fads(+/+)) mice (*P < 0.05, d = 0.80 and 1.24 [large effect size (ES)], respectively, t-test with Bonferroni correction). Males, 18–33 weeks old. The box length and a horizontal bar show the interquartile range (IQR) and median, respectively. The length of the whiskers is defined as 1.5 times the upper and lower limits of the IQR. d Food intake. Daily food intake did not differ by genotype.

Even before this FADS1/2 genomic region attracted attention as a locus of BD susceptibility shared across populations, it had seized the spotlight due to the significant changes in haplotype diversity (Supplementary Fig. 1b) since humans commenced crop agriculture [4,5,6]. The increased intake of grain oils has resulted in an increase in the proportion of people carrying a haplotype associated with higher FADS1/2 activity (Haplotype D) [4]. This haplotype has a protective effect against BD [1,2,3]. GWASs of blood lipid composition have also demonstrated a pivotal role of the FADS1/2locus in the plasma levels of ω3 and ω6 PUFAs [7]. Consistent with these results, expression quantitative trait loci (eQTL) analysis suggested that the other major haplotype (Haplotype A) conferring susceptibility to BD was associated with decreased expression of FADS1/2 and likely with lower enzyme activity [8]. Although the odds ratio for the susceptible alleles is not high (at most 1.18) [1,2,3], these evolutional and functional underpinnings of the locus collectively suggest that an animal mimicking a decreased, but not completely nullified, activity of both FADS1/2 can be a valid model and contribute to a better understanding of the pathogenesis of BD. In the present study, to generate such a model, we deleted the region containing the mouse Fads1 and Fads2 genes in a heterozygous manner (referred to as Fads(Δ/+) mice).

Single knockout (KO) mice deficient in either Fads1 or Fads2 have been investigated prior to our study; [9,10,11,12,13,14,15] however, if these mice are crossed, double KO mice cannot be generated because these genes are located only approximately 100 kb apart. Additionally, since studies using single KO mice were conducted from the perspective of nutrition, they mainly analyzed homozygous KO mice fed semipurified diets [9,10,11,12,13,14,15]. Data on behavioral phenotypes were not reported so far.

The inverse relationship between seafood consumption and the prevalence of depression and BD highlights a possible nutritional or pharmacological effect of ω3 PUFAs, such as DHA and EPA [16]. Several previous studies reported the therapeutic efficacy of ω3 PUFAs for depressive episodes in BD [17, 18], but the results from randomized clinical trials were debatable [19]. Data on plasma PUFA levels in patients are also inconsistent; however, recent studies with the largest sample size to date have reported low levels of EPA and DHA and high levels of AA in patients with BD [20]. In addition, studies of lipids in postmortem brains are limited, with small sample sizes and inconsistent results [21,22,23]. Moreover, food and medication can be major confounding factors in human studies, requiring analysis using animal models that provide greater experimental control.

In this study, we established Fads(Δ/+) mice as a preclinical model of the GWAS-identified risk factor in BD; these mice exhibited both mania- and depression-like episodic behavioral changes. To detect these infrequent episodic behavioral changes, we monitored the wheel-running activity of Fads(Δ/+) mice continuously for six months. Unlike other locomotor activities, wheel running in mice is a strongly goal-directed behavior having a significant reward value [24, 25]; thus, a reduction in wheel running is associated with “markedly diminished pleasure (anhedonia)”, a core symptom of a depressive episode [26]. In addition, we provided proof of concept that long-term supplementation with DHA improved the behavioral abnormalities in the model mice.