Biphasic papillary and lobular breast carcinoma with PIK3CA and IDH1 mutations.

Morphologic "special types" of breast carcinomas have been recognized for many years, and their molecular and genetic properties have not been specifically studied until recently. Lobular carcinoma lacks functional E-cadherin expression but shares molecular similarities with low-grade invasive ductal carcinomas. Papillary carcinoma is relatively rare, and molecular features are just being elucidated. We report a case of concurrent invasive lobular and papillary carcinoma, the latter with extensive nodal involvement. Multiplex screening for activating point mutations identified different point mutations in the distinct morphologic components: lobular PIK3CA H1047R, papillary; PIK3CA Q546P, and IDH1 R132H. These molecular data favor coincidental "collision tumors" over clonal evolution. The IDH1 R132H point mutation is common in gliomas and acute myelogenous leukemia, but this has not been previously reported in breast carcinoma. The characterization of activating point mutations in morphologic special types of breast carcinoma may suggest avenues amenable to targeted therapy.