Epigenetic and genetic alterations of APC and CDH1 genes in lobular breast cancer:: Relationships with abnormal E-cadherin and catenin expression and microsatellite instability

The causes and functional consequences of E-cadherin (E-CD) loss in breast cancer are poorly understood. E-CD loss might act in concert with alterations in the APC/beta-catenin pathway to permit oncogenic beta-catenin signaling. To test this hypothesis, we have analyzed the presence of genetic and epigenetic alterations affecting E-CD (CDHI), APC and beta-catenin (CTNNBI) genes and the immunohistochemical expression of E-CD, beta- and gamma-catenin in a series of 46 infiltrating lobular breast carcinomas (ILCs). Since 80% of ILCs featured complete loss of E-CD expression, we analyzed the molecular alterations responsible for E-CD inactivation in these tumors. We found that 10 of 46 (22%) cases harbored mutations in CDHI, including 1 case with 2 different mutations (1 of which was germline). CDHI was also inactivated by loss of heterozygosity (LOH; 30/41, 73%) and promoter hypermethylation (19/46, 41%). Interestingly, LOH and mutations were also detected in the corresponding in situ lesions of the ILCs, implying that these alterations are early events in lobular cancer tumorogenesis. Additionally, the presence of a polymorphism in the CDHI promoter was found to be inversely correlated with CDHI mutations, but not with E-CD levels. We next examined whether alterations in the APC/beta-catenin pathway also occurred in the same series of ILCs. Although no CTNNBI or APC mutations were detected, promoter methylation (25/46, 52%) and LOH (7/30, 23%) of APC were found. Moreover, methylation of APC and CDHI occurred concordantly. However, beta- and gamma-catenin were severely reduced or absent in 90% of these tumors, implying that alterations in CDHI and APC genes do not promote beta-catenin accumulation in ILC. These molecular alterations were not associated with microsatellite instability. In summary, several different mechanisms (mutations, LOH, methylation) are involved in the frequent CDHI inactivation in invasive and in situ lobular breast cancer. The same tumors also show genetic and epigenetic alterations of APC gene. However, altered CDHI and APC genes do not promote beta-catenin accumulation in this tumor type. (C) 2003 Wiley-Liss, Inc.