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

Abstract

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-RecoverAllTM 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.