TI - Discussion . AB - Mouse models are valuable research tools for the study of the molecular mechanisms of oncogenesis and for the testing and screening of cancer therapeutic agents targeted to specific oncogenic lesions . In order to produce such a model for studying the effects of Pten mutations in breast cancer , we have crossed Wnt-1 TG mice to mice heterozygous for Pten . Mice transgenic for MMTV-Wnt-1 and heterozygous for Pten developed mammary adenocarcinomas much earlier than mice carrying either lesion alone . The majority of tumors in Wnt-1 TG , Pten+/- mice had lost the wild type allele of Pten , suggesting that the complete inactivation of Pten function provides an additional growth advantage to these tumors . LOH at the Pten locus has recently been reported in mammary tumors arising in Pten heterozygous mice [26] , but only 4% of the mice developed tumors before the age of 30 weeks , and , in our cross , none of the Pten+/- mice without the Wnt-1 transgene had a palpable mammary tumor before the age of 30 weeks . LOH at the Pten locus has also been reported in adrenal and lymphatic tumors , but not in uterine tumors and prostate lesions in Pten+/- mice [KP & RP , submitted ; 38] . Activated AKT was more abundant in tumors that were Wnt-1 TG , Pten +/- with LOH , as has been shown for uterine tumors in Pten heterozygous mice ( KP , RP , submitted ) . The presence of areas unstained with antibody specific for PHOSphorylated AKT suggests that additional factors other than Pten may influence the activation of AKT . Neither apoptosis nor proliferation , as determined by the TUNEL assay and KI67 staining , was significantly different between tumors heterozygous for Pten ( with or without LOH ) and those without a Pten defect . This result may seem paradoxical since increased PHOSphorylated AKT , due to inactivation of Pten , suppresses apoptosis and promotes proliferation . However , tumors in Wnt-1 TG mice in a wild type Pten background already have highly suppressed apoptosis and accelerated proliferation rates , and any incremental changes , if present , may be difficult to detect with our current assays . Nonetheless , a seemingly minor difference in either apoptosis or proliferation could have a dramatic impact on tumor growth over time . Accelerated appearance of mammary adenocarcinomas in the presence of a Pten mutation suggests a synergy between Pten inactivation and the Wnt-1 transgene . The apparent cooperation of these two genetic factors is further strengthened by our finding that the majority of tumors arising in a Pten+/- background have lost the wild type allele . To date , heterozygous inactivation of any of the other tumor suppressor genes implicated in breast cancer , including P53 , Brca1 , Brca2 and Rb , has failed to cooperate with Wnt-1 in promoting the appearance of mammary tumors [reviewed in ref 39] . Therefore , it is possible that the signal pathway controlled by Pten is one of the preferred partners of the Wnt-1 pathway in cancer progression , although it remains possible that LOH at the Pten locus is more likely to occur than for the other alleles crossed into Wnt-1 TG mice . Even though Wnt-1 has not been directly implicated in human cancers , many components of the Wnt signaling pathway ( such as beta-catenin , APC , c-myc and cyclin D1 ) are involved in human tumors including breast cancer [reviewed in ref 40] . One of the goals of mouse modeling efforts is to provide an opportunity for pre-clinical testing of generalized or mechanism-specific therapeutics in a defined genetic setting . Many human breast tumors have reduced levels of PTEN , hence inhibitors of the cell signaling pathways affected by PTEN could be tested in the model we have developed . One such candidate is the rapamycin analog CCI779 , which has been suggested to inhibit the AKT signaling pathway , probably through inhibition of mTOR ( target of rapamycin ) , a potential target of AKT [reviewed in ref 41] . CCI779 has been shown to have inhibitory effects on the growth of Pten-deficient tumors in the uterus and prostate in mice ( KP & RP , submitted ; M Neshat & C Sawyers , submitted ) . Since tumors arise rapidly after a copy of Pten is inactivated , and with predictable kinetics , this model may be useful for testing the efficacy of these therapies . Results presented here and published earlier [15,16,17,18 , 26] have clearly demonstrated the significant impact of Pten on mammary oncogenesis . Future testing will examine the tumorigenic effects caused by other kinds of genetic damage in the signaling pathways affected by Pten . The TVA -directed gene delivery system [reviewed in ref 42] can be used to introduce viral vectors carrying genes encoding an activated form of PI3K , AKT or candidate SUBstrates for AKT into the mammary tissue of Wnt-1 TG mice . This would allow the testing of therapeutic interventions directed to specific steps of the signaling pathway regulated by Pten .