Genetic analysis of intestinal polyp development in Collaborative Cross mice carrying the Apc (Min/+) mutation

Abstract

Background: Colorectal cancer is an abnormal tissue development in the colon or rectum. Most of CRCs develop due to somatic mutations, while only a small proportion is caused by inherited mutations. Familial adenomatous polyposis is an inherited genetic disease, which is characterized by colorectal polyps. It is caused by inactivating mutations in the Adenomatous polyposis coli gene. Mice carrying and non-sense mutation in Adenomatous polyposis coli gene at site R850, which designated Apc (R850X/+) (Min), develop intestinal adenomas, while the bulk of the disease is in the small intestine. A number of genetic modifier loci of Min have been mapped, but so far most of the underlying genes have not been identified. In our previous studies, we have shown that Collaborative Cross mice are a powerful tool for mapping loci responsible for phenotypic variation. As a first step towards identification of novel modifiers of Min, we assessed the phenotypic variation between 27 F1 crosses between different Collaborative cross mice and C57BL/6-Min lines.

Results: Here, C57BL/6-Min male mice were mated with females from 27 Collaborative cross lines. F1 offspring were terminated at 23 weeks old and multiple phenotypes were collected: polyp counts, intestine length, intestine weight, packed cell volume and spleen weight. Additionally, in eight selected F1 Collaborative cross-C57BL/6-Min lines, body weight was monitored and compared to control mice carry wildtype Adenomatous polyposis coli gene. We found significant (p < 0.05) phenotypic variation between the 27 F1 Collaborative cross-C57BL/6-Min lines for all the tested phenotypes, and sex differences with traits; Colon, body weight and intestine length phenotypes, only. Heritability calculation showed that these phenotypes are mainly controlled by genetic factors.

Conclusions: Variation in polyp development is controlled, an appreciable extent, by genetic factors segregating in the Collaborative cross population and suggests that it is suited for identifying modifier genes associated with Apc (Min/+) mutation, after assessing sufficient number of lines for quantitative trait loci analysis.

Fig. 1

Body weight (±SE) of the a control and b Apc ^Min/+ groups at 5 time points (8, 12, 16, 19 and 23 weeks old) of 8 different set of CC lines. The X-axis represents the time points (in weeks) while the Y-axis represents values of body mass (g). One-way ANOVA performed for statistical analysis, p < 0.05

Fig. 2

Average number of polyps (±SE) in 27 F1 CC-C57BL/6-Min lines (n = 1-16 mice/line). The X-axis represents CC lines, C57BL/6 strain carrying the Apc ^Min/+ mutation (first black column) and CC-Mean (black column ~ in the middle), while the Y-axis number of polyps. Continuous lines under the graph represent 9 clusters based on Duncan’s Least Significant Range (LSR) test. One-way ANOVA performed for statistical analysis, p < 0.05

Fig. 3

Distribution of all counted polyp in three parts of small intestine: SB1-proximal, SB2-middle, SB3-distal and colon. The X-axis represents CC lines, C57BL/6 strain carrying the Apc ^Min/+ mutation (first black column) and CC-Mean (black column ~ in the middle), while the Y-axis number of polyps. One-way ANOVA performed for statistical analysis, p < 0.05

Fig. 4

Scatter plot with tread lines of total polyps vs. a. Intestine length (R² = 0.346), b. Intestine %Weight (R² = 0.484), c. % PCV (R² = 0.707), d. Spleen %Weight (R² = 0.553) and e. Polyps per cm (R² = 0.995)