IMMUNOHISTOCHEMICAL EVALUATION OF CHEMICALLY-INDUCED RHABDOMYOSARCOMAS IN RATS - DIAGNOSTIC UTILITY OF MYOD1

Monoclonal antibodies (mAbs) to selected muscle proteins were assessed as potential immunohistochemical markers to assist in the definitive diagnosis of poorly differentiated soft tissue sarcomas in rats. A ser ies of 7 rat rhabdomyosarcomas (RMS) induced with nickel subsulfide we re studied by light microscopy and were evaluated for immunoreactivity to desmin, vimentin, fast (type II isoform) skeletal myosin, alpha-ac tin (smooth muscle isoform), or MyoD1 (myogenic regulatory protein) mA bs using an avidin-biotin-chromogen technique. Consecutive RMS slices were fixed in 10% neutral buffered formalin (the fixative routinely us ed in carcinogenicity bioassays) for periods of 3 days or 2 mo prior t o paraffin embedding to determine the effect ol fixation time on immun oreactivity. Desmin and vimentin mAbs bound to many cells of all tumor s, but fixation for 2 mo resulted in irretrievable loss of desmin and vimentin binding. Fast myosin and a-actin mAbs bound to many cells in 1 RMS but to <1% of the cells in the remainder. MyoD1 mAb bound to tum or cell nuclei in 5/7 RMS with no loss of staining in tissue fixed for 2 mo. Results indicate that MyoD1 immunostaining, in contrast to desm in, maintains its sensitivity following prolonged formalin fixation an d may be of value to distinguish RMS from other soft tissue sarcomas i n the rat.