Recent studies have identified interstitial deletions in the
cancer genome as a radiation-related mutational signature, although most of them do not fall on
cancer driver genes. Pioneering studies in the field have indicated the presence of loss of heterozygosity (LOH) spanning Apc in a subset of sporadic and radiation-induced intestinal
tumors of ApcMin/+ mice, albeit with a substantial subset in which LOH was not detected; whether copy number losses accompany such LOH has also been unclear. Herein, we analyzed intestinal
tumors of C3B6F1 ApcMin/+ mice that were either left untreated or irradiated with 2 Gy of γ-rays. We observed intratumor mosaicism with respect to the nuclear/cytoplasmic accumulation of immunohistochemically detectable β-
catenin, which is a hallmark of Apc+ allele loss. An immunoguided
laser microdissection approach enabled the detection of LOH involving the Apc+ allele in β-
catenin-overexpressing cells; in contrast, the LOH was not observed in the non-overexpressing cells. With this improvement, LOH involving Apc+ was detected in all 22
tumors analyzed, in contrast to what has been reported previously. The use of a
formalin-free
fixative facilitated the LOH and microarray-based
DNA copy number analyses, enabling the classification of the aberrations as nondisjunction/mitotic recombination type or interstitial deletion type. Of note, the latter was observed only in radiation-induced
tumors (nonirradiated, 0 of 8; irradiated, 11 of 14). Thus, an analysis considering intratumor heterogeneity identifies interstitial deletion involving the Apc+ allele as a causative radiation-related event in intestinal
tumors of ApcMin/+ mice, providing an accurate approach for attributing individual
tumors to radiation exposure.