α-Methylacyl-CoA racemase: a useful immunohistochemical marker of breast carcinoma with apocrine differentiation.

Carcinoma with apocrine differentiation is an androgen receptor (AR)-positive subset of triple-negative breast carcinomas. In addition to carcinoma with apocrine differentiation, other AR-positive triple-negative breast carcinomas occur, albeit less frequently. We found that α-methylacyl-CoA racemase (AMACR), also known as P504S, is overexpressed in carcinoma with apocrine differentiation and non-neoplastic apocrine metaplasia. We aimed to evaluate AMACR as a possible marker of carcinoma with apocrine differentiation. We immunohistochemically examined the expression of AMACR in carcinoma with apocrine differentiation and nonapocrine carcinomas and compared it with that of gross cystic disease fluid protein-15 (GCDFP-15). In total, 212 breast carcinomas were evaluated: 39 carcinomas with apocrine differentiation, 28 ductal carcinomas in situ with apocrine morphology (ADCIS), and 145 nonapocrine breast carcinomas. AMACR was expressed in 38 of 39 (97.4%) carcinomas with apocrine differentiation and in 27 of 28 (96.4%) ADCIS, consistent with the expression of GCDFP-15. However, in nonapocrine carcinomas, AMACR expression was observed in 32 of 145 (22.0%) lesions, whereas GCDFP-15 expression was observed in 91 of 145 (62.7%) lesions. For carcinoma with apocrine differentiation, AMACR was as sensitive as GCDFP-15 (both 97.1%) but more specific (77.9% versus 37.2%). In selected cases, AMACR messenger RNA (mRNA) levels were quantitatively determined relative to that of TATA-binding protein mRNA, and they comprised 5.23, 1.33, and 0.60 for carcinomas with apocrine differentiation, nonapocrine carcinomas, and normal breast tissue, respectively. CONCLUSION: Our findings demonstrate that AMACR expression may be used for differentiating carcinoma with apocrine differentiation from nonapocrine carcinomas and indicate that AMACR is a more sensitive carcinoma with apocrine differentiation marker than GCDFP-15.