The genetic background modifies the spontaneous and X-ray-induced tumor spectrum in the Apc1638N mouse model

The effect of the genetic background on the tumor spectrum of Apc1638N, a mouse model for attenuated familial adenomatous polyposis (FAP), has been investigated in X‐irradiated and untreated F1 hybrids between C57BL/6JIco‐Apc1638N (B6) and A/JCrlBR (A/J), BALB/cByJIco (C) or C3H/HeOuJIco (C3). Similar to the ApcMin model, the Apc1638N intestinal tumor multiplicity seems to be modulated by Mom1. Moreover, several additional (X‐ray–responsive) modifier loci appear also to affect the Apc1638N intestinal tumor number. The genetic background did not significantly influence the number of spontaneous desmoids and cutaneous cysts in Apc1638N. In general, X‐irradiation increased the desmoid multiplicity in Apc1638N females but had no effect in males. The opposite was noted for the cyst multiplicity after X‐rays. Surprisingly, X‐irradiated CB6F1‐Apc1638N females were highly susceptible to the development of ovarian tumors, which displayed clear loss of the wild‐type Apc allele. Genes Chromosomes Cancer 24:191–198, 1999. © 1999 Wiley‐Liss, Inc.