Smooth Muscle Myosin Heavy Chain (SMM-HC) is an antibody to the cytoplasmic structural protein, which is a component of the contractile apparatus in smooth muscle cells. SMM-HC is expressed in myoepithelial cells of the breast, which makes it a very useful stain in the differentiation of in situ and invasive lesions.
One of the big problems in the breast with smooth muscle markers is that myofibroblasts may have background expression, which can occasionally mimic myoepithelial cells. SMM-HC is one of the best markers (comparatively speaking) not to have a lot background expression in myofibroblasts. Calponin and smooth muscle actin have higher background staining of myofibroblasts.
SMM-HC is a more sensitive and specific marker in the differentiation of in situ and invasive breast carcinoma compared to CD10 [J Clin Pathol. 2007 Aug;60(8):958-9]. Another study noted that SMM-HC stained myofibroblasts in ~8% of cases compared to ~76% of cases for calponin. p63 is also a useful marker for breast myoepithelial cells, but the staining may not be as continuous as SMM-HC and occasional tumor cells may be positive for p63 (although the patter is recognizably different) [Am J Surg Pathol 2003 Jan;27(1):82-90]. SMM-HC combined with p63 as a dual stain may have synergistic and complementary effects.
Usefulness may be decreased in cases of DCIS, in which myoepithelial cells have been reported negative in up to 16% of DCIS cases (especially high grade DCIS). (Dewar, et al)
SMM-HC is also expressed in vascular smooth muscle cells.
Microscopic Images
References
Kalof AN, et. al. “Immunostaining patterns of myoepithelial cells in breast lesions: a comparison of CD10 and smooth muscle myosin heavy chain.” J Clin Pathol. 2004 vol. 57(6) pp. 625-629. doi:10.1136/jcp.2003.013227
Werling RW, et. al. “Immunohistochemical distinction of invasive from noninvasive breast lesions: a comparative study of p63 versus calponin and smooth muscle myosin heavy chain.” Am J Surg Pathol 2003 Jan;27(1):82-90.
Dewar, R., Fadare, O., Gilmore, H., & Gown, A. M. (2011). Best practices in diagnostic immunohistochemistry: myoepithelial markers in breast pathology. Archives of pathology & laboratory medicine, 135(4), 422–429. doi:10.1043/2010-0336-CP.1