Genomic and Immunologic Characterization of INI1-Deficient Pediatric Cancers.

Abstract

Several aggressive pediatric cancers harbor alterations in <i>SMARCB1</i>, including rhabdoid tumors, epithelioid sarcoma, and chordoma. As tumor profiling has become more routine in clinical care, we investigated the relationship between <i>SMARCB1</i> genetic variants identified by next-generation sequencing (NGS) and INI1 protein expression. Therapeutic approaches for INI1-deficient tumors are limited. Early reports suggest a potential role for immune checkpoint inhibition in these patients. Thus, we also investigated PD-L1 and CD8 expression in INI1-negative pediatric brain and solid tumors. We performed immunohistochemistry (IHC) for INI1 and immune markers (PD-L1, CD8, and CD163) and NGS on tumor samples from 43 pediatric patients who had tumors with INI1 loss on previous IHC or <i>SMARCB1</i> genomic alterations on prior somatic sequencing. <i>SMARCB1</i> two-copy deletions and inactivating mutations on NGS were associated with loss of INI1 protein expression. Single-copy deletion of <i>SMARCB1</i> was not predictive of INI1 loss in tumor histologies not known to be INI1-deficient. In the 27 cases with INI1 loss and successful tumor sequencing, 24 (89%) had a <i>SMARCB1</i> alteration detected. In addition, 47% (14/30) of the patients with INI1-negative tumors had a tumor specimen that was PD-L1 positive and 60% (18/30) had positive or rare CD8 staining. We report on 3 patients with INI1-negative tumors with evidence of disease control on immune checkpoint inhibitors. A significant proportion of the INI1-negative tumors express PD-L1, and PD-L1 positivity was associated with extracranial tumor site. These results suggest that clinical trials of immune checkpoint inhibitors are warranted in INI1-negative pediatric cancers.