Immunohistochemical Screening for Mismatch Repair Protein Deficiency in Paediatric High-grade Gliomas - Institutional Experience and Review of Literature

Purpose: Immunohistochemical (IHC) testing for mismatch repair (MMR) deficiency (MMRD) is used as a screening tool to identify microsatellite instability in various cancers (especially colon). This not only identifies hereditary cancer syndromes like Lynch and constitutional mismatch repair deficiency (CMMRD) but also aids in prognostication and prediction of sensitivity to checkpoint inhibitor drugs. There are very few reported studies on MMRD status of pediatric high-grade gliomas (pHGG) and none from the Indian subcontinent. The aim of this study is to evaluate the frequency of MMRD in pHGG and to assess if there is a need for universal screening with immunohistochemistry.

Methods: Paraffin blocks of consecutive cases of pHGG (< 18 years) were retrieved from 2 centres, and IHC with four MMR antibodies - MLH1, PMS2, MSH2 and MSH6 - was performed using tissue microarray-based technique.

Results: Three out of nine cases (33%) studied showed loss of staining. One case had loss of MSH2 and MSH6 confirmed by gene sequencing. Eight of the cases were glioblastoma. One case of IDH1-mutated anaplastic astrocytoma showed loss of MLH1 and PMS2 staining. Isolated PMS2 loss was noted in 1 case, where the non-tumour cells also showed loss of staining, indicative CMMRD syndrome. This patient had prior colon cancer with isolated PMS2 loss and responded to check-point inhibitor therapy with nivolumab.

Conclusion: Our study shows that the frequency of MMRD to be about one-third of pHGG. Universal IHC screening for MMRD in all pHGGs may benefit early diagnosis and play a role in therapeutic decisions. A larger multi-institutional study will help better assess the prevalence and treatment implications in MMRD tumours.