Integrative Principal Component–QTL Mapping Identifies Genetic Modifiers of Tumor and Metabolic Traits in Smad4-Deficient Collaborative Cross Mice

Genetic background strongly influences the penetrance and phenotypic expression of SMAD4-associated intestinal tumorigenesis, yet the underlying modifier loci remain poorly defined. To investigate the genetic architecture of tumor susceptibility and systemic physiology, we analyzed 260 Smad4+/− × Collaborative Cross (CC)-F1 mice derived from 14 CC lines using 11 quantitative traits, including longitudinal body weight, adjusted organ weights, and intestinal polyp counts across anatomical regions. Principal component analysis reduced these traits to seven components explaining more than 85% of total phenotypic variance. PC1 represented a tumor burden–metabolic axis, whereas PC2 captured systemic organ-physiology variation. Genome-wide QTL mapping of principal component scores identified significant loci for PC1 on chromosomes 1 and 4 and a female-specific locus for PC5 on chromosome 10, with additional suggestive loci supporting a polygenic architecture. Founder-effect analysis revealed strong contributions from CAST/EiJ, 129S1/SvImJ, and WSB/EiJ haplotypes. Candidate gene annotation identified biologically relevant coding and noncoding loci, including Galnt7 and Galntl6, as well as regulatory regions with potential enhancer activity. Together, these findings indicate that intestinal tumor susceptibility in Smad4+/− × CC-F1 mice is influenced by multiple coding and regulatory genetic modifiers with sex-dependent effects. This study demonstrates that integrating multivariate phenotyping with systems genetics analyses provides an effective framework for identifying the complex genetic networks underlying intestinal tumorigenesis and associated systemic physiological variation in genetically diverse mouse populations.