High-Throughput Screening of Mouse Gene Knockouts Identifies Established and Novel High Body Fat Phenotypes

Abstract

Purpose Obesity is a major public health problem. Understanding which genes contribute to obesity may better predict individual risk and allow development of new therapies. Because obesity of a mouse gene knockout (KO) line predicts an association of the orthologous human gene with obesity, we reviewed data from the Lexicon Genome5000 TM high throughput phenotypic screen (HTS) of mouse gene KOs to identify KO lines with high body fat. Materials and Methods KO lines were generated using homologous recombination or gene trapping technologies. HTS body composition analyses were performed on adult wild-type and homozygous KO littermate mice from 3758 druggable mouse genes having a human ortholog. Body composition was measured by either DXA or QMR on chow-fed cohorts from all 3758 KO lines and was measured by QMR on independent high fat diet-fed cohorts from 2488 of these KO lines. Where possible, comparisons were made to HTS data from the International Mouse Phenotyping Consortium (IMPC). Results Body fat data are presented for 75 KO lines. Of 46 KO lines where independent external published and/or IMPC KO lines are reported as obese, 43 had increased body fat. For the remaining 29 novel high body fat KO lines, Ksr2 and G2e3 are supported by data from additional independent KO cohorts, 6 ( Asnsd1, Srpk2, Dpp8, Cxxc4, Tenm3 and Kiss1 ) are supported by data from additional internal cohorts, and the remaining 21 including Tle4, Ak5, Ntm, Tusc3, Ankk1, Mfap3l, Prok2 and Prokr2 were studied with HTS cohorts only. Conclusion These data support the finding of high body fat in 43 independent external published and/or IMPC KO lines. A novel obese phenotype was identified in 29 additional KO lines, with 27 still lacking the external confirmation now provided for Ksr2 and G2e3 KO mice. Undoubtedly, many mammalian obesity genes remain to be identified and characterized.

Mice Lacking Gpr75 are Hypophagic and Thin

“…Early in life as their thin phenotype developed, Gpr75 KO mice ate less than their WT littermates but had comparable activity levels, oxygen consumption, and energy expenditure, suggesting that Gpr75 KO mice were thin due to differences in energy intake rather than energy expenditure. These data, along with the association of increased energy intake and obesity in mice and humans with inactivating mutations in 18 monogenic obesity genes 8 and with the likelihood that the obesity pandemic is driven primarily by increased energy intake, 5,39 support the idea that mammalian body fat stores are regulated mainly by energy intake and suggest that GPR75 may play a role in this process. GPR75 is expressed primarily DAGLA and CNR1 are protein members of the endocannabinoid pathway that regulates food intake, other metabolic parameters and some emotional behaviors.…”

“…Hypophagia and low body fat are common findings in many disease states, often associated with decreased body length and LBM. 8,45 Our Gpr75 KO mice appeared healthy, and our HTS did not identify any abnormalities that suggest the hypophagia was caused by an underlying chronic disease state. The lower body weight of Gpr75 KO mice was due primarily to lower body fat rather than lower LBM, and their body length appeared normal, both consistent with healthy mice.…”

“…5,6 Most prior studies focused on identifying genetic variants that increase body fat. 7,8 The value in identifying these genetic variants is the potential to predict individual risk for future obesity and the potential to identify targets for therapeutic intervention. Unfortunately, the genetic component of obesity is polygenic with >250 GWAS loci providing minor contributions, 5,[9][10][11][12] and identifying these loci has not resulted in models with acceptable ability to predict obesity risk.…”

“…Unfortunately, the genetic component of obesity is polygenic with >250 GWAS loci providing minor contributions, 5,[9][10][11][12] and identifying these loci has not resulted in models with acceptable ability to predict obesity risk. 13 Also, a few genes with variants that result in severe obesity have been identified, 7,8 but to date studying these genes has not led to effective therapeutics for polygenic obesity.…”

“…[17][18][19][20][21] In humans and mice, inactivating mutations in at least 18 genes result in a shared obese phenotype. 8 These mouse models provide insight into mechanisms behind development of obesity in affected humans. The low body fat reported in Gpr75 knockout (KO) mice 16 suggests that this conservation between humans and mice extends to thin phenotypes.…”