The common forms of human type 2 diabetes are caused by the interaction of multiple genes and environmental factors. In the present study we attempted the genetic dissection of loci contributing to impaired glucose tolerance, hyperglycaemia, hyperinsulinaemia and obesity by using the SMXA-5 mouse, which develops diabetes by the coexistence of diabetogenic alleles originating in the SM/J and A/J strains, under feeding on a high-fat diet. Although about half of the SMXA-5 genome is composed of the SM/J genome, the SMXA-5 mouse is susceptible to diabetes induced by a high-fat diet, whereas the SM/J mouse is resistant. Therefore, we decided to use the intercross between SMXA-5 and SM/J mice for QTL analysis. Using this intercross, the diabetogenic loci existing in the A/J regions of the SMXA-5 genome can be analysed, because the SMXA-5 has a mosaic genome derived from the non-diabetic SM/J and A/J strains.
QTL analysis for free-fed blood glucose concentration and glucose tolerance revealed the existence of a highly significant QTL on chromosome 2: T2dm2sa. We previously reported that QTL analysis of 19 SMXA RI strains fed a high-carbohydrate diet revealed several diabetogenic loci on four chromosomes (chromosomes 2, 6, 10 and 18), and the SMXA-5 mouse seems to possess these loci . Except for the QTL on chromosome 2, the diabetogenic QTLs identified by using 19 SMXA RI strains were different from those identified in this study using (SM×SMXA-5)F2 mice. With regard to glucose tolerance, free-fed blood glucose concentration and serum insulin concentration, this study identified significant and suggestive QTLs on nine chromosomes. Moreover, the present results indicate that the QTL on chromosome 2, T2dm2sa, contributes the most strongly among these QTLs to the development of diabetes-related traits, but the effect of the QTL on chromosome 2 on these traits was weak among 19 SMXA RI strains. A high-fat diet might strengthen the effect of T2dm2sa on the development of diabetes-related traits in (SM×SMXA-5)F2 mice.
Using different polygenic type 2 diabetes mouse models, three groups have reported diabetes-related QTLs on chromosome 2 near T2dm2sa, which we mapped in this study (D2Mit15; 50 cM, 92.7 Mb). Firstly, Hirayama et al. reported Nidd5, a QTL that affects body weight and plasma insulin concentration, by using (TSOD×BALB/c)F2 intercross mice . Secondly, Mehrabian et al. reported Mob (multigenic obesity) loci, a QTL that affects body weight, subcutaneous fat mass and plasma insulin, by using (C57BL/6×CAST/Ei)F2 intercross mice . In addition, three congenic mouse strains with Mob loci were created, and differences in obesity and insulin traits consistent with the original QTL analysis were shown . Thirdly, Stoehr et al. identified T2dm3, a QTL that affects the fasting plasma insulin concentration, by using a (BTBR-+/+×C57BL/6J-ob/+)F2 intercross , and mapped Moo1 (modifier of obese) in the centromeric region of T2dm3, a QTL that controls body mass . T2dm2sa also has suggestive or significant linkage to free-fed insulin concentration or fasting insulin concentration, respectively, as did Nidd5, Mob and T2dm3. Although these three QTLs and T2dm2sa were mapped on the middle of mouse chromosome 2 by using different diabetic mouse strains, it is unclear at present whether T2dm2sa possesses the same diabetogenic gene that these QTLs do.
The chromosomal region with significant linkage of blood glucose concentration at 120 min during IPGTT is homologous to human chromosomes 2q11–q32, 9q33–34, 11p14–q12, 15q13–q21 and 20p13–11, and to rat chromosome 3. Human chromosomes 2  and 20 [36–39] encompass the region associated with insulin resistance. In the WOKW rat, the homologous region on rat chromosome 3 contains a QTL that affects AUC during IPGTT . These mapping data in the human, rat, and mouse indicate that this homologous region may possess orthologous diabetogenic genes.
Nr1h3, a possible candidate gene in T2dm2sa, encodes liver X receptor-α (LXR-α). This transcription factor is known to play an important role in the regulation of cholesterol metabolism [41, 42]. Nr1h3 is expressed in liver, kidney, intestine, adipose tissue and adrenals. Recently, LXR-α was also reported to play an important role in the regulation of gluconeogenesis. In the db/db mouse and the insulin-resistant Zucker fa/fa rat, the activation of LXR by a specific agonist lowered the plasma glucose level  by suppressing the expression of genes involved in hepatic gluconeogenesis, including phosphoenolpyruvate carboxykinase (PEPCK). The gene expression profile in the liver of LXR-α and -β knockout mice showed that LXRs suppressed the expression of gluconeogenic genes encoding such substances as PEPCK, fructose-1,6-bisphosphatase, and glucose-6-phosphatase [43, 44]. In addition, LXR-α also regulates glucose uptake in peripheral tissue (muscle and adipose tissue) by upregulating the expression of the gene encoding GLUT4 [45, 46]. Thus, at present, we consider Nr1h3 the most plausible candidate gene for T2dm2sa in SMXA-5 mice.
On chromosome 12, we mapped a significant QTL that affects BMI and free-fed serum insulin but not glucose tolerance or blood glucose concentration. Therefore, this locus might be linked to obesity and insulin resistance. Two QTLs, Afpq10 (abdominal fat percent QTL 10; 18.0 cM) and Afw1 (abdominal fat weight QTL 10; 21.0 cM) were mapped around our QTL region by using crosses between DU6i mice and DBA/2 mice . It is suggested that the gene commonly contributing to obesity may exist in the centromeric region of chromosome 12. However, there is no QTL for obesity on chromosome 12 in the studies using SMXA RI substrains or (SM×A)F2 mice [19, 48, 49]. Therefore, this QTL may be a locus specifically activated under a high-fat diet.
For our next step in QTL analysis, we chose congenic mapping as a strategy to dissect T2dm2sa. The glucose tolerance, free-fed blood glucose concentration, BMI and accumulation of mesenteric fat of SM.A-T2dm2sa mice differed significantly from those of mice with the SM/J background. This result clearly verified that T2dm2sa exerts definitive effects on the development of diabetes-related phenotypes and obesity. Although A/J mice possess a diabetogenic allele at T2dm2sa, they show no obviously diabetic phenotypes, unlike SM.A-T2dm2sa mice. These findings suggest that the coexistence of the T2dm2sa and other diabetogenic loci, which exist outside T2dm2sa, leads to diabetes in SM.A-T2dm2sa mice. In the present QTL analysis in (SM×SMXA-5)F2 mice, diabetogenic loci within the SM/J regions of the SMXA-5 genome cannot be detected. Therefore, unknown diabetogenic loci in SMXA-5 mice may exist within the SM/J regions of the SMXA-5 genome. At present, we are trying to dissect such unknown diabetogenic loci involved in the development of diabetes in both SMXA-5 and SM.A-T2dm2sa mice.
In conclusion, we detected nine loci associated with several diabetes-related traits and/or obesity on chromosomes 2, 5, 8, 11, 12, 14, 15, 17 and 19 by QTL analysis in F2 intercross mice derived from SM/J and diabetic SMXA-5 mice. Locus T2dm2sa, which strongly affected glucose tolerance, blood glucose concentration and obesity, was mapped on chromosome 2. Moreover, although A/J (donor) and SM/J (recipient) are non-diabetic strains, the congenic strain, SM.A-T2dm2sa, developed overt impaired glucose tolerance. The results of the present study suggest that both SM/J and A/J mouse strains possess latent diabetic genes and that the coexistence of these alleles is necessary for eliciting diabetes in SMXA-5 mice. The dissection of diabetogenic genes of this kind, which have epistatic effects, will contribute to the elucidation of the complex mechanisms underlying human diabetes.