MBRS-55. MOLECULAR CLASSIFICATION OF MEDULLOBLASTOMAS: NANOSTRING nCOUNTER ASSAY VS A COMBINATION OF IMMUNOHISTOCHEMISTRY AND FLUORESCENCE IN-SITU HYBRIDISATION

INTRODUCTION: WHO 2016 classification recognizes four molecular-subgroups of medulloblastoma(MB) viz. WNT,SHH,Group3 and 4. This molecular classification is prognostically and therapeutically important and helps in better risk-stratification of patients. Translation of this molecular data to routine practice is a challenge. Immunohistochemistry(IHC), FISH and Nanostring are methods which can be done using FFPE tumor-tissue. OBJECTIVES: (a)To compare MB subgrouping by three-panel IHC and FISH vs Nanostring (b)To categorise non-WNT/non-SHH into Group3 and 4. METHODS: Forty-seven cases of MB with adequate tissue were retrieved. IHC was performed for beta-catenin,GAB1 and YAP1. MYC amplification was done using FISH. RNA was extracted from FFPE using Invitrogen-RecoverAll(TM) total nucleic-acid isolation, and then subjected to Nanostring-Assay. RESULTS: Among 47 cases, 39 were pediatric and 8 adult. IHC results revealed 5WNT, 8SHH, and 32non-WNT/non-SHH. MYC amplification was identified in 8 cases. Nanostring classified MBs into 6 WNT, 6 SHH, 7 Group3 and 28 Group4. Seven cases were unclassifiable; all of which had >8 year old blocks. Hence, successful subgroup assignment was seen in 40/47(85.1%)cases. Concordant subgroup assignment was noted in 35/40 cases(87.5%). Subgroup switching was identified in 5 cases. However, on correlation with follow-up, there was significant difference between MYC-amplified(worse) and non-amplified Non-WNT/SHH, and also between Group3(worse) and 4. CONCLUSIONS: Both IHC supplemented by FISH and Nanostring are robust methods for molecular subgrouping, albeit with few disadvantages. IHC cannot differentiate between group3 and 4, while Nanostring cannot classify older-archived tumors,and is not available at most centres. Thus, a neuropathologist can perform molecular subgrouping using IHC alone, IHC supplemented by FISH, or Nanostring, depending on availability of resources.