E-cadherin mutational landscape and outcomes in breast invasive lobular carcinoma.

Invasive lobular carcinomas (ILC) are characterized by a loss of E-cadherin expression and CDH1 gene inactivation. Diagnostic reproducibility for this tumor type is currently suboptimal and could be improved by a better understanding of its histomolecular and clinical heterogeneity. We have analyzed the relationship between presence, type or position of CDH1 mutations, E-cadherin expression and clinicopathological features (including outcome) in a retrospective series of 251 primary ILC with long follow-up (median: 9.5 years). The mutational status of E-cadherin gene (CDH1) was determined by RNA sequencing from frozen tumor samples. E-cadherin immunohistochemistry (IHC) was performed with antibodies directed against the intracellular domain (clone 4A2C7) and the extracellular domain (clone NCH38). IHC expression of p120 and β-catenin was also assessed in E-cadherin diffusely positive cases. Three major patterns of E-cadherin membrane expression were identified by IHC, with good agreement between the two clones (overall concordance: 83.8%, Kappa 0.67): null/focal expression (≤10%) (72.8% of cases for 4A2C7, 83.8% for NCH38), heterogeneous expression (11-89%) (19.2% of cases for 4A2C7, 6.9% for NCH38) and diffuse expression (≥90%) (8% of cases for 4A2C7, 9.3% for NCH38). E-cadherin membranous expression, when present, was abnormal (incomplete labeling and/or reduced intensity). ILC with diffuse E-cadherin expression showed abnormal β-catenin or p120-catenin staining in 21% of cases. Interestingly, these cases with diffusely expressed E-cadherin had a CDH1 mutation rate as high as the E-cadherin null/focal cases (∼70%), but were enriched in non-truncating mutations. Regarding CDH1 mutation location, intracytoplasmic domain mutations correlated with a divergent E-cadherin IHC phenotype between the two antibodies (4A2C7 ≤10% / NCH38 ≥10%). Clinico-pathological correlation analyses found that stromal amount (inversely correlated with tumor cellularity) and TILs were less abundant in ILC with E-cadherin null/focal cases. In addition, CDH1 truncating mutations were associated with radio-histological size discordance, and were identified in multivariate survival analysis as an independent poor prognosis factor in terms of metastasis risk and breast cancer related mortality. Overall, our study highlights the importance of the precise mutational status of CDH1 in the clinical, radiological, histological and phenotypic expression of lobular carcinomas. These findings should be taken into account in future attempts to improve diagnostic criteria or methods for ILC, as well as for clinico-biological studies dedicated to this tumor type.