A 16.7 kb deletion in Sipa1l3 is associated with juvenile cataract in mice.

Congenital or juvenile cataract is a disease condition in which opacification of the lenses is present at birth or manifests early in life. It has been attributed to different monogenic factors with a high degree of heterogeneity and is often studied using mouse models. A spontaneous mutation was identified in a mouse line selected for heat loss that influenced lens formation and resulted in juvenile cataracts in mice homozygous for the recessive allele. Genetic dissection of this selection line by combining high-density genotypes and homozygosity mapping uncovered a 906 kb fragment on MMU7 encompassing 21 SNPs split into two groups of consecutive, homozygous segments specific to the cataract phenotype. Haplotype analysis revealed a 197.5 kb segment unique to cataract-affected mice that included a single known transcript consisting of the first 14 exons of Sipa1l3. In this region, we discovered a deletion of 1114 bp at the mRNA level, spanning four coding exons, predicted to produce a truncated Sipa1l3 protein lacking a portion of a Rap-GAP domain and two other potentially vital domains. At the genome level, the deletion consisted of 16,733 bp. Genotyping across different samples confirmed that only affected mice were homozygous for the deletion and normal mice were either heterozygous or homozygous for the wild-type allele. Further studies will be required to determine the impact of the truncated Sipa1l3 domains on eye development.

Quantitative trait loci for energy balance traits in an advanced intercross line derived from mice divergently selected for heat loss.

Obesity in human populations, currently a serious health concern, is considered to be the consequence of an energy imbalance in which more energy in calories is consumed than is expended. We used interval mapping techniques to investigate the genetic basis of a number of energy balance traits in an F11 advanced intercross population of mice created from an original intercross of lines selected for increased and decreased heat loss. We uncovered a total of 137 quantitative trait loci (QTLs) for these traits at 41 unique sites on 18 of the 20 chromosomes in the mouse genome, with X-linked QTLs being most prevalent. Two QTLs were found for the selection target of heat loss, one on distal chromosome 1 and another on proximal chromosome 2. The number of QTLs affecting the various traits generally was consistent with previous estimates of heritabilities in the same population, with the most found for two bone mineral traits and the least for feed intake and several body composition traits. QTLs were generally additive in their effects, and some, especially those affecting the body weight traits, were sex-specific. Pleiotropy was extensive within trait groups (body weights, adiposity and organ weight traits, bone traits) and especially between body composition traits adjusted and not adjusted for body weight at sacrifice. Nine QTLs were found for one or more of the adiposity traits, five of which appeared to be unique. The confidence intervals among all QTLs averaged 13.3 Mb, much smaller than usually observed in an F2 cross, and in some cases this allowed us to make reasonable inferences about candidate genes underlying these QTLs. This study combined QTL mapping with genetic parameter analysis in a large segregating population, and has advanced our understanding of the genetic architecture of complex traits related to obesity.

A polymorphic glucocorticoid receptor in a mouse population may explain inherited altered stress response and increased anxiety-type behaviors.

A polymorphic glucocorticoid receptor (GR(Qn)) with an expanded CAG track and two silent mutations, when compared with the sequence of other isoform (GR(wt)), is found in two outbred mouse lines that were produced by selection for high (SH) or low (SL) stress response from high or low heat loss lines of mice, respectively. The GR(Qn) allele, which is also found in 5 of 16 commonly used inbred mouse lines, had a much higher frequency in SL mice; the GR(wt/wt) was found only in the SH line. Both GR(Qn/Qn) and GR(wt/Qn) mice had a much weaker corticosterone response to stress than the GR(wt/wt) mice. Assessment of open field activity revealed that GR(Qn/Qn) and GR(wt/Qn) mice exhibited significantly lower velocity and locomotor activity, less time in the center, and much longer duration in corner zones than the GR(wt/wt) mice. The increased anxiety-type behaviors of the GR(Qn/Qn) and GR(wt/Qn) mice were confirmed by the "elevated plus maze" test in which GR(Qn/Qn) and GR(wt/Qn) mice spent significantly less time in the "open arm" and longer duration in the "closed arm," than GR(wt/wt) mice. These results suggest this GR polymorphism plays a role in complex mechanisms leading to lower corticosterone response to stress, and may also be associated with decreased locomotive and increased anxiety-type behaviors in mice.

Genetic variance and covariance patterns for body weight and energy balance characters in an advanced intercross population of mice.

We estimated heritabilities and genetic correlations for a suite of 15 characters in five functional groups in an advanced intercross population of over 2000 mice derived from a cross of inbred lines selected for high and low heat loss. Heritabilities averaged 0.56 for three body weights, 0.23 for two energy balance characters, 0.48 for three bone characters, 0.35 for four measures of adiposity, and 0.27 for three organ weights, all of which were generally consistent in magnitude with estimates derived in previous studies. Genetic correlations varied from -0.65 to +0.98, and were higher within these functional groups than between groups. These correlations generally conformed to a priori expectations, being positive in sign for energy expenditure and consumption (+0.24) and negative in sign for energy expenditure and adiposity (-0.17). The genetic correlations of adiposity with body weight at 3, 6, and 12 weeks of age (-0.29, -0.22, -0.26) all were negative in sign but not statistically significant. The independence of body weight and adiposity suggests that this advanced intercross population is ideal for a comprehensive discovery of genes controlling regulation of mammalian adiposity that are distinct from those for body weight.

Evaluation of hypothalamic gene expression in mice divergently selected for heat loss.

Mouse lines divergently selected for heat loss were evaluated for correlated responses in the hypothalamic transcriptome. High (MH) heat loss mice have approximately 50% greater heat loss, approximately 35% less body fat, approximately 20% greater feed intake, approximately 100% greater locomotor activity levels, and higher core body temperature compared with low (ML) heat loss mice. We evaluated hypothalamic expression between inbred lines derived from MH and ML lines (IH and IL, respectively) using cDNA microarrays and selected genes previously isolated in a large differential-display PCR experiment. Northern analysis was used to confirm differences, revealing higher hypothalamic mRNA expression of oxytocin (Oxt) and tissue inhibitor of metalloproteinase 2 (Timp-2) in the IH line. Real-time PCR assays were developed for Oxt, Timp-2, and ribosomal protein L3 (Rpl3, previously found to be upregulated in IL) and confirmed differential expression of these genes with potential physiological relevance in energy balance. These results provide information on correlated responses in the transcriptome of mice selected for high and low energy expenditure and reveal new information regarding genetic regulation of energy balance.