1. ¹Department of Pathology, Josephine Nefkens Institute, ErasmusMC, Rotterdam, The Netherlands
2. ²Center for Human and Clinical Genetics, Leiden University Medical Center, Leiden, The Netherlands
3. ³Department of Experimental Animal Pathology, Netherlands Cancer Institute, Amsterdam, The Netherlands
4. ⁴MRC Human Genetics Unit, Edinburgh, UK

Correspondence: Professor R Fodde, Department of Pathology, ErasmusMC, PO Box 1738, 3000 DR Rotterdam, The Netherlands. E-mail: r.fodde@erasmusmc.nl

Received 6 June 2005; Revised 11 August 2005; Accepted 29 September 2005; Published online 14 November 2005.

Abstract

The Smad4^+/E6sad mouse carries a null mutation in the endogenous Smad4 gene resulting in serrated adenomas and mixed polyposis of the upper gastrointestinal (GI) tract with 100% penetrance. Here, we show by loss of heterozygosity (LOH) analysis and immunohistochemistry (IHC) that, although the majority of the tumors appear at 9 months of age, somatic loss of the wild-type Smad4 allele occurs only at later stages of tumor progression. Hence, haploinsufficiency underlies Smad4-driven tumor initiation in the GI tract. As both the Apc and Smad4 tumor suppressor genes map to mouse chromosome 18, we have bred Smad4^+/E6sad with the Apc^+/1638N model to generate two distinct compound heterozygous lines carrying both mutations either in cis (CAS) or in trans (TAS). Strikingly, both models show increased tumor multiplicities when compared with the single mutant littermates, although CAS mice are more severely affected and became moribund at only 5–6 weeks of age. Phenotypic and molecular analyses indicate that Smad4 haploinsufficiency is sufficient to significantly affect tumor initiation and progression both prior to and upon loss of Apc function. Moreover, complete loss of Smad4 strongly enhances Apc-driven tumor formation.