MBRS-18. ALK EXPRESSION AT THE PROTEIN LEVEL IS A MARKER FOR THE DIFFERENTIATION DIAGNOSIS OF THE WNT-ACTIVATED TYPE OF PEDIATRIC MEDULLOBLASTOMA

Abstract OBJECTIVES Previous analyses indicated that expression of ALK at the RNA level is correlated with the WNT-activated type of medulloblastoma. Therefore, we investigated if ALK expression at the protein level may serve as an useful marker for identification of this type of medulloblastoma. METHODS Tumors from 134 patients were studied at diagnosis by immunohistochemistry using antibody ALK clone D5F3 #3633 (Cell Signaling). Consensus scoring was applied as the following: 0 (negative) for 0-10%, 1+ for >10-50%, 2+ for >50-100% of positive tumor cells. Transcriptional subtypes of medulloblastoma tumors were established in all cases. Thirty infratentorial ependymal tumors were also investigated to determine if ALK protein expression may serve as a marker for differential diagnosis. RESULTS All 19 WNT tumors were positive for ALK expression and in all but one positive reaction was present in >50% of tumor cells (score 2+). Only four ALK positive tumors were detected among the remaining 85 tumors: one tumor with a score 2+ in Group 4 and three tumors with a score 1+ (one in Group 3 and two in SHH group). Therefore, expression of ALK is strongly associated with the WNT type of disease (p