Li-Fraumeni syndrome is a rare inherited disorder that greatly increases the risk of developing several types of cancer, particularly in children and young adults. The cancers associated with Li-Fraumeni syndrome include osteosarcoma, soft tissue sarcoma, breast cancer, brain tumors, adrenocortical carcinoma, and leukemia. The tumor suppressor p53 is mutated in approximately 70% of Li-Fraumeni syndrome (LFS) families; thus mutations in other genes may lead to the predisposition to tumors in other families.
We developed a mouse model to search for other tumor suppressors that may be involved in the syndrome. Inbred CE/J mice, which succumb to multiple types of tumors similar to those found in LFS, were crossed with the p53-null 129-P53tm1Tyj mouse. We monitored the tumor onset and type and found a significantly earlier tumor onset in CE/J:129-P53tm1Tyj mice compared to 129-P53tm1Tyj mice with a p53-null allele. Additionally, in CE/J:129-P53tm1Tyj-p53+/- mice, the tumors metastasize which does not occur in other strains of mice. Using simple-sequence length polymorphism analysis for loss of heterozygosity in tumors, we identified a putative tumor suppressor locus within a 1 cM region on mouse chromosome 11.
We did real time PCR experiments to analyze expression profiles of genes within the identified locus and interestingly found that the signal transducer and activator of transcription 5a (Stat5a) is expressed two fold less in the tumors that arose in the FI CE/129 mice compared to normal tissues. Sequencing of Stat5a from CE, 129 and BALB/C mice identified a point mutation in the C terminus of the CE Stat5a gene. Follow up experiments on activation and cellular localization of Stat5a in CE and 129 mice showed that the mutant Stat5a in the CE mice after activation with prolactin (PRL) does not translocate to the nucleus but remains diffuse throughout the cell. Stat5a activation was lost during metastatic progression in mammary tumors. Based on this, we assayed for E-cadherin activation in CE and 129 cells. Interestingly, we found that E-cadherin translocates to the cell membrane after Stat5a activation in 129 cells while it remains diffused in CE cells. We also knocked down the expression of Stat5a in 129 cells and found that E-cadherin remained diffused throughout the cell even after Stat5a activation.