Mice that are homozygous for the targeted mutation are viable and fertile. During the generation of this allele, a 3' homologous recombination and insertion occurred with a duplication of exon 3 between exons 5 and 6. The exon 3 insertion, which was confirmed by RT-PCR, results in a mutant transcript that encodes a truncated protein due to a stop codon at amino acid 255. Normal gene product (mRNA) containing an exon 5?6 border is not detected. If the truncated protein gene product is stable, it could theoretically have some activity, given that it would contain nuclear receptor-interacting domains and the amino-terminal activation domain. Total body weights for homozygotes are reduced 15-20% for the first week after birth, but become normal by 3 weeks of age. By 18 weeks of age, homozygotes have heavier body weights and increased percent body fat than wildtype controls. Heart and slow twitch skeletal muscle (gastrocnemius, soleus) weights are lower in homozygotes than controls. Electron microscopy analysis reveals lower density and smaller mitochondria in soleus muscle. The state 3 (ADP-stimulated) mitochondrial respiration rate is diminished. Overall activity is decreased and thigmotaxis behavior is increased. Mutant mice exhibit reduced strength, activity endurance capacity, muscle fatigue and reduced cardiac output. Homozygotes are more sensitive to cold temperatures. 24 hour fasting results in hepatic steatosis. 4.5 month old homozygous females exhibit increased body weight, but normal glucose tolerance and insulin sensitivity on standard chow diet. Female homozygotes on high fat chow exhibit mildly increased glucose tolerance and insulin sensitivity. Histological examination of the parietal cerebral cortex, hippocampus, brainstem and basal ganglia reveals microvacuolation. This mutant mouse strain may be useful in studies of mitochondrial respiration and physiology, muscle physiology, and energy metabolism.
