p63, a p53 homologue, is a selective nuclear marker of myoepithelial cellsof the human breast

Myoepithelial cells (MCs) constitute the basal cell layer of normal mammary epithelia, and their identification is of particular diagnostic value beca use they are retained in most benign lesions while being lost in malignancy . Several MC immunocytochemical markers are currently available for diagnos tic purposes, with special reference to smooth muscle-related antigens. p63 is a member of the p53 gene family, and its germline mutations are associa ted with severe mammary developmental defects in both rodents and humans. D ifferent p63 isoforms have been identified, some of which (Delta Np63) are preferentially expressed in the epithelial basal cells of different or-ans and have been considered as possible markers of stem cells/reserve cells. W e investigated immunohistochemically 384 samples of normal and diseased hum an breast, including 300 invasive carcinomas, using four antibodies recogni zing all p63 isoforms, or the Delta Np63 isoforms. Twenty cytologic specime ns were also investigated. Furthermore, snap-frozen tissue samples from thr ee fibroadenomas and 10 invasive ductal carcinomas with their paired non-ne oplastic tissues and three corresponding lymph node metastases were evaluat ed for the expression of p63 mRNA by RT-PCR. In normal breast tissue p63 im munoreactivity was confined to the nuclei of MCs. In all benign lesions p63 -immunoreactive cells formed a continuous basal rim along the epithelial st ructures. Stromal cells, and in particular myofibroblasts, were consistentl y unreactive. Adenomyoepitheliomas showed nuclear staining in most neoplast ic cells. A peripheral rim of p63-immunoreactive cells was retained surroun ding lobular and ductal carcinoma in situ, although it was discontinuous as opposed to the normal structures. Invasive breast carcinomas were consiste ntly devoid of nuclear p63 staining, with the exception of the two adenoid- cystic carcinomas, of the two ductal carcinomas with squamous metaplasia, a nd of 11 (4.6%) ductal carcinomas not otherwise specified, showing p63 immu noreactivity in a minor fraction (5-15%) of the neoplastic cells. In compar ison with other MC markers, p63 was the most specific, being restricted exc lusively to MCs, whereas antibodies to smooth muscle actin and, to a lesser extent, calponin also decorated stromal myofibroblasts. In the cytologic p reparations p63 immunoreactivity was a consistent feature of "naked nuclei" and of a subset of cells surrounding benign epithelial clusters. RT-PCR ex periments with primers specific for different p63 isoforms documented that normal tissues and fibroadenomas preferentially expressed the Delta Np63 is oforms. Our study demonstrates that in normal and pathologic breast tissues MCs consistently express the Delta Np63 isoforms. We suggest p63 as a reli able, highly specific, and sensitive MC marker in both histologic and cytol ogic preparations. Furthermore, because p63 immunoreactivity in adult epith elia is normally restricted to progenitor cells, it can be speculated that it mi-ht be a clue for the identification of the still elusive breast proge nitor cells.