Myogenin belongs to a group of myogenic regulatory
proteins whose expression determines commitment and differentiation of primitive mesenchymal cells into skeletal muscle. The expression of
myogenin has been demonstrated to be extremely specific for rhabdomyoblastic differentiation, which makes it a useful marker in the differential diagnosis of
rhabdomyosarcomas (RMS) from other malignant small round cell
tumors of childhood. Commercially available
antibodies capable of detecting
myogenin in routinely processed
formalin-fixed
paraffin-embedded (FFPE) tissue are now available. In this study, we evaluated
myogenin expression using the monoclonal myf-4 antibody (Novocastra Labs) on FFPE in a large number of pediatric
tumors in order to define the clinical utility of this marker. A total of 119
tumors were studied. These included 48 alveolar RMS (ARMS), 20 embryonal RMS (ERMS), one spindle cell RMS, 16 Ewing's
sarcomas (ES), six
nephroblastomas, two ectomesenchymomas, seven precursor
hematopoietic neoplasms, five
olfactory neuroblastomas, three
neuroblastomas, six desmoplastic small round cell
tumors, and five
rhabdoid tumors. Distinct nuclear staining for
myogenin was noted in all 69 RMS. Notably, the number of positive
tumor cells differed between the ARMS and ERMS. In ARMS, the majority of
tumor cells (75 to 100%) were positive, in contrast to ERMS, in which the positivity ranged from rare + to 25% in all but three
tumors. Additionally,
myogenin positivity was seen in two of two ectomesenchymomas and in two
nephroblastomas with myogenous differentiation. All other
tumors were clearly negative. Our results indicate that staining for
myogenin is an extremely reliable and specific marker for rhabdomyoblastic differentiation. It gives consistent and easily interpretable results in routinely fixed tissues.