Candidate Genes
The Berlin Fat Mouse Inbred line 860 (BFMI860) is a model for juvenile obesity. Previously, a recessive major effect locus (jObes1) on chromosome 3 between 34 and 44 Mb had been found to be responsible for 39% of the variance of total fat mass at 10 weeks in a (BFMI860/Hber x C57BL/6NCrl) F2 population. The aim of the current study was to fine map the jObes1 locus. Curator Note: The previous study mapping jObes1 was J:243037; the jObes1 QTL identified as mapping the total fat mass locus was assigned official nomenclature and is now Jobes1b.An advanced intercross line (AIL) was generated from the initial F2 mapping population. Three hundred and forty-four male mice of generation 28, Hber:B6N,BFMI860-G28, were excessively phenotyped and genotyped using the MegaMuga mouse chip containing 22,164 informative single-nucleotide polymorphisms. The high mapping resolution of the AIL reduced the confidence interval for Jobes1b from 10 to 0.37 Mb between 36.48 and 36.85 Mb (GRCm38). The region was highly significantly (LOD score >50) associated with total fat mass starting at puberty (6 weeks). Male homozygous carriers of the Jobes1b BFMI allele had 3 g more fat than the other genotypes.Surprisingly, the genotype class showed lower body mass until weaning at 3 weeks (LOD = 3.2). The mapped interval contained four genes [Table 2]. Bbs7, the most likely candidate gene that also caused obesity in complementation tests was differentially expressed in all tissues examined, whereas the neighboring cyclin A2 (Ccna2) gene showed differential expression in gonadal adipose tissue.Many of the small effect QTLs for juvenile obesity that were found at the genome-wide 5% significance level in the initial F2 mapping population as reported by Neuschl et al. [J:243037] could not be confirmed in this study. Possibly the QTLs reported for the F2 population were either spurious or their effects got lost because epistatic interaction or joined effects of genes in close proximity were interrupted through the many recombination events that occurred during the pedigree history of the advanced intercross population until generation 28.
Mapping and Phenotype information for this QTL,
its variants and associated markers
The aim of the current study was the mapping and estimation of genetic and sex effects contributing to the obese phenotype of the Berlin Fat Mouse Inbred line 860 (BFMI860). The mouse line is predisposed for juvenile obesity. BFMI860 mice accumulate 24% total fat mass at 10 weeks of age under a standard maintenance diet, where as C57BL/6NCrl (B6) mice disposed of only 3% fat at ten weeks under the same diet. A total of 471 mice (222 males and 249 females) from a (BFMI860 x C57BL/6NCrl) F2 intercross population were fed a standard maintenance diet and were analyzed for body composition at 10 weeks when they had finished their rapid growth phase. In all, 5 males and 5 females of each of the two parental lines BFMI860 and B6, 11 male and 13 female F1 and all F2 animals were phenotyped at 10 weeks of age. In addition, body weight, total fat mass and total lean mass were measured weekly on the basis of the birthdays of animals from weeks 4 to 9. Total fat mass and total lean mass were determined in non-anaesthetized mice by quantitative magnetic resonance interference analysis. Total fat mass represented the sum of all fat in the body. Total lean mass included mainly muscle and inner organs. Skeletal muscle mass accounted for the largest portion of total lean mass. At 71 days and after a fasting period of 2h, mice were anaesthetized and white adipose tissues were dissected and weighed along with brown adipose tissue and the liver. DNA was extracted from tail tips, out of 471 phenotyped F2 animals, 365 were genotyped at 132 informative markers covering all chromosomes except Y with an average distance of 21.08 Mb. QTLs were mapped with Grid QTL (http://www.gridqtl.org.uk), in which the multiple regression least squares approach is implemented. The program enables QTL mapping in outbred populations and thus the inclusion of heterozygous parental marker genotypes into linkage analyses. For each significant QTL identified with the standard model, sex was fitted as additional interaction term into the genetic model. The QTL was assumed to be affected by sex if the difference between the F-value of the standard and the interaction model was > 4.6. An F-value of approximately 4.6 corresponded to a LOD score of 2.0 in the F2 population.The following thresholds were taken from the resulting test statistic distribution: genome-wide highly significant (P=0.01) corresponded to an F-value of 10.2; genome-wide significant (P=0.05) corresponded to F=8.2; and genome-wide suggestive (P=0.63) corresponded to chromosome-wide significant (P=0.05) and ranged between 4.2Curator Note: Each significant trait within each of the larger overlapping QTL regions was given a unique identifier. Table 2:QTL Jobes1 (juvenile obesity 1) mapped to Chromosome 3, with a peak nearest marker D3Mit21 and a 95% confidence interval spanning from 30-100 Mb. Jobes1 was comprised of the following individual traits, each assigned a unique identifier within the larger QTL:QTL Jobes1a (juvenile obesity 1a) was a measure of body weight, mapping to 40.0 Mb nearest marker D3Mit21, F=34.4, with a CI spanning from 36-54 Mb and accounted for 19.90 % of trait variance. A significant interaction with sex was also detected at this locus, F=16.6.QTL Jobes1b (juvenile obesity 1b) was a measure of total fat mass, mapping to 40.0 Mb nearest marker D3Mit21, F=87.7, with a CI spanning from 34-44 Mb and accounted for 38.70 % of trait variance. A significant interaction with sex was also detected at this locus, F=43.3.QTL Jobes1c (juvenile obesity 1c) was a measure of reproductive adipose tissue, mapping to 36.0 Mb nearest marker D3Mit21, F=58.6, with a CI spanning from 34-42 Mb and accounted for 29.49 % of trait variance. A significant interaction with sex was also detected at this locus, F=27.3.QTL Jobes1d (juvenile obesity 1d) was a measure of renal adipose tissue, mapping to 40.0 Mb nearest marker D3Mit21, F=72.5, with a CI spanning from 34-44 Mb and accounted for 33.96 % of trait variance. A significant interaction with sex was also detected at this locus, F=35.7.QTL Jobes1e (juvenile obesity 1e) was a measure of subcutaneous adipose tissue, mapping to 36.0 Mb nearest marker D3Mit21, F=82.2, with a CI spanning from 34-42 Mb and accounted for 37.15 % of trait variance. A significant interaction with sex was also detected at this locus, F=40.6.QTL Jobes1f (juvenile obesity 1f) was a measure of brown adipose tissue, mapping to 38.0 Mb nearest marker D3Mit21, F=64.8, with a CI spanning from 34-42 Mb and accounted for 31.57 % of trait variance. A significant interaction with sex was also detected at this locus, F=30.4.QTL Jobes1g (juvenile obesity 1g) was a measure of liver weight, mapping to 40.0 Mb nearest marker D3Mit21, F=15.4, with a CI spanning from 30-100 Mb and accounted for 9.82 % of trait variance. A significant interaction with sex was also detected at this locus, F=7.6.The genetic effects of the obesity-inducing allele BFMI860 at Jobes1 and at each of its sub-loci was recessive.QTL Jbw1 (juvenile body weight 1) mapped to Chromosome 4 at 40.0 Mb nearest SNP rs27781503, F=10.3 and accounted for 6.95 % of trait variance. The effect of the obesity-inducing BFMI860 allele was recessive. QTL Jlivw1 ( juvenile liver weight 1) mapped to Chromosome 4 at 72.0 Mb nearest SNP rs28056583, F=14.0 and accounted for 9.01 % of trait variance. A significant interaction with sex was also detected at this locus, F=6.5. The effect of the obesity-inducing BFMI860 allele was recessive.QTL Jobes2 ( juvenile obesity 2) mapped to Chromosome 5, with a 95% confidence interval spanning from 46-140 Mb. Jobes2 was comprised of the following individual traits, each assigned a unique identifier within the larger QTL:QTL Jobes2a ( juvenile obesity 2a) was a measure of renal adipose tissue, mapping to 98 Mb nearest SNP rs31585424, F=10.2, with a CI spanning from 46-140 Mb and accounted for 6.76 % of trait variance. The effect of the obesity-inducing BFMI860 allele was recessive.QTL Jobes2b ( juvenile obesity 2b) was a measure of renal adipose tissue, mapping to 106 Mb nearest peak marker rs13478514, F=10.9, with a CI spanning from 68-140 Mb and accounted for 7.18% of trait variance. The effect of the obesity-inducing BFMI860 allele was dominant.QTL Jobes3 ( juvenile obesity 3) was a measure of brown adipose tissue mapping to Chromosome 5 at 70 Mb nearest marker SNP rs29635956, F=12.4 Mb, with a CI spanning from 36-88 Mb and accounted for8.08 % of trait variance. The effect of the obesity-inducing BFMI860 allele was dominant.QTL Jobes4 (juvenile obesity 4) mapped to Chromosome 6, with peak nearest marker rs13478670 and a 95% confidence interval spanning 12 136 Mb. Jobes4 was comprised of the following individual traits, each assigned a unique identifier within the larger QTL:QTL Jobes4a (juvenile obesity 4a) was a measure of body weight, mapping to 28 cM nearest SNP rs13478670, F=16.0, with a CI spanning from 20-70 Mb and accounted for 10.37 % of trait variance. A significant interaction with sex was also detected at this locus, F=7.7. The effect of the obesity-inducing BFMI860 allele was recessive.QTL Jobes4b (juvenile obesity 4b) was a measure of total fat mass, mapping to 26 cM nearest SNP rs13478670, F=10.8, with a CI spanning from 14-134 Mb and accounted for 7.24 % of trait variance. A significant interaction with sex was also detected at this locus, F=5.0. The effect of the obesity-inducing BFMI860 allele was recessive.QTL Jobes4c (juvenile obesity 4c) was a measure of total lean mass, mapping to 26 cM nearest SNP rs13478670, F=13.3, with a CI spanning from 20-116 Mb and accounted for 8.65 % of trait variance. A significant interaction with sex was also detected at this locus, F=6.3. The effect of the obesity-inducing BFMI860 allele was recessive.QTL Jobes4d (juvenile obesity 4d) was a measure of reproductive adipose tissue, mapping to 28 cM nearest SNP rs13478670, F=12.2, with a CI spanning from 14-110 Mb and accounted for 8.03 % of trait variance. A significant interaction with sex was also detected at this locus, F=5.3. The effect of the obesity-inducing BFMI860 allele was recessive.QTL Jobes4e (juvenile obesity 4e) was a measure of subcutaneous adipose tissue, mapping to 26 cM nearest SNP rs13478670, F=10.5, with a CI spanning from 12-136 Mb and accounted for 6.99 % of trait variance. A significant interaction with sex was also detected at this locus, F=5.1. The effect of the obesity-inducing BFMI860 allele was recessive.
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