A new marker for rhabdomyosarcoma. Insulin-like growth factor II

Accurate diagnosis of soft tissue tumors is often difficult and considered important because the clinical outcome and the modality of therapy vary according to tumor type. We have realized that rhabdomyoblasts seen in occasional Wilms' tumors express high levels of insulin-like growth factor II (IGF-II) transcripts. This observation prompted us to see if IGF-II could be a useful marker for differential diagnosis of rhabdomyosarcoma (RMS) from other soft tissue sarcomas.

Five cases of RMSs (including embryonal, botryoid, and alveolar types), various other soft tissue tumors, and 10-week gestation fetal muscle tissue were studied to demonstrate IGF-II transcripts by means of Northern blot hybridization and in situ mRNA hybridization using radio-labeled probes. Hybridization results were then compared with data obtained from immunohistochemistry using antibodies against desmin and vimentin.

Northern blot hybridization demonstrated that all five RMSs contained three major sizes of IGF-II transcripts. In situ hybridization indicated that all RMSs showed highly strong and specific IGF-II mRNA expression, whereas other soft tissue tumors showed very low or no signal. A characteristic finding was that the expression of IGF-II mRNA was inversely correlated with the degree of tumor cell differentiation. Thus poorly differentiated RMSs showed the highest signal, whereas well-differentiated RMSs showed low expression, albeit, still significantly higher than normal skeletal muscle fibers. Among five RMSs, desmin was seen in three tumors and vimentin in all tumors. Desmin was negative in other nonskeletal muscle origin tumors while vimentin was present in all tumors examined.

IGF-II has potential as a new marker for differential diagnosis of RMS from other soft tissue tumors and could be especially useful in differential diagnosis of small blue cell tumors.