Strong immunostaining for myogenin in rhabdomyosarcoma is significantly associated with tumors of the alveolar subclass

Rhabdomyosarcomas are a heterogeneous group of tumors with respect to their molecular basis, degree of differentiation, histology, and clinical behavi or. Because of the wide variation of tumor morphology, it is often difficul t to distinguish between the distinct subtypes of rhabdomyosarcomas, By usi ng cryosections of tumor specimens and immunohistochemistry, in the present study we show that strong expression of myogenin in rhabdomyosarcoma is as sociated with alveolar histology (P = <0.0001, Fisher's exact test). Althou gh staining for myogenin. was observed in 22 of 26 rhabdomyosarcomas, all a lveolar rhabdomyosarcomas (nine of nine) showed high levels of staining for myogenin, as defined by the frequency and intensity of staining of the tum or cells. The staining pattern suggests that the tumor cells are clonally d erived from myogenin-positive progenitor cells. In contrast, most embryonal rhabdomyosarcomas (13 of 15) were either negative or showed a low level of staining for myogenin, In these tumors a larger proportion of tumor cells were distinctly negative for myogenin. Six of seven alveolar rhabdomyosarco mas that strongly stained for myogenin were also positive for Pax3-7/Forkhe ad (FKHR) by polymerase chain reaction/reverse transcriptase-polymerase cha in reaction. One of two embryonal rhabdomyosarcomas that strongly stained f or myogenin was retrospectively found to be positive for Pax3/FKHR transcri pts. Quantitative analysis for myogenin by Western blotting using a smaller subset of rhabdomyosarcomas revealed that in general there was a good corr elation between immunohistochemical staining and Western blotting (P = 0.01 , Pearson Correlation), although the former technique was more sensitive fo r detecting tumors with low levels of the protein. On average, alveolar rha bdomyosarcomas expressed at least threefold more myogenin than embryonal rh abdomyosarcomas. Our data show that staining for myogenin will be a simple, rapid, and accurate adjunct for distinguishing between alveolar and embryo nal rhabdomyosarcomas, We propose that embryonal rhabdomyosarcomas result f rom an early block in myogenesis, before the expression of myogenin. In con trast, we propose that alveolar rhabdomyosarcomas either originate from a l ate block in myogenesis (after expression of myogenin) or that the patholog ical mechanisms involved in these neoplasms also induce strong expression o f this protein.