Detection of FOXO1 break-apart status by fluorescence in situ hybridization in atypical alveolar rhabdomyosarcoma.

The morphologies of alveolar rhabdomyosarcoma (ARMS) are various. Some cases entirely lack an alveolar pattern and instead display a histological pattern that overlaps with embryonal rhabdomyosarcoma (ERMS). The method of pathological diagnosis of ARMS and ERMS has been updated in the 4th edition of the World Health Organization's guidelines for classification of skeletal muscle tumors. Under the new guidelines, there is still no molecular test to distinguish between these two subtypes of rhabdomyosarcoma (RMS). In the present study, we applied fluorescent in situ hybridization (FISH) and found that the Forkhead box O1 (FOXO1) gene broke apart, amplified, and displayed an aneuploid signal that was related to the RMS pathological subtype. Aside from the fact that FOXO1 break-apart and its amplification were correlated with atypical ARMS, aneuploidies were usually found in atypical ERMS. In conclusion, our results detail a potential biomarker to improve the accuracy of pathological diagnosis by discriminating between atypical ARMS and atypical ERMS.