Abstract BACKGROUND The contribution of rare pathogenic germline variation to central nervous system (CNS) tumor formation in pediatric patients without a family history of cancer remains unclear. METHODS We characterized the prevalence of pathogenic germline variants in 214 cancer predisposition genes (CPGs) in 837 patients profiled in the Pediatric Brain Tumor Atlas by whole genome or exome sequencing (n=820 and n=17, respectively). Rare SNVs and small INDELs were annotated as pathogenic (P) or likely pathogenic (LP) consistent with American College of Medical Genetics criteria using AutoGVP, our open-source automated pathogenicity assessment tool. We classified pathogenicity of germline structural variants (SVs) using ClassifyCNV and AnnotSV. Somatic alterations and mutational signatures from matched tumor sequencing were integrated to identify functional consequences associated with germline pathogenic variation. RESULTS We observed 206 germline P/LP SNVs/small INDELs and 18 SVs (16 deletions, 2 duplications) within 78 unique CPGs in 195 patients (23.3%). We detected syndrome-associated P/LP variants in 45/58 patients with a clinically-reported cancer predisposition syndrome and incidentally in 41 patients. Ninety-five (42%) germline P/LP variants co-occurred with at least one somatic alteration in the same gene in matched tumors: n=6 oncogenic SNVs, n=28 CNVs, n=54 loss of heterozygosity (LOH), n=26 differential gene or protein expression, and n=18 alternative splicing. NF1, TSC1, and TSC2 germline P/LP carriers exhibited significantly higher tumor LOH scores and lower gene expression in tumors relative to non-P/LP carriers. Patients with germline splice region P/LP variants exhibited significantly increased skipping of most the proximal exon in matched tumor relative to patients with non-splice region P/LP variants (W=401, p=1.1E-04). CONCLUSION We have identified rare germline P/LP variants associated with cancer predisposition syndromes and diverse functional consequences in pediatric CNS tumor patients. Efforts are underway to extend this work to include 1,801 additional probands and 1,105 parents to further characterize the prevalence and heritability of germline pathogenic variation in children diagnosed with CNS tumors.

METB-14. INTEGRATED GERMLINE AND SOMATIC PROFILING OF PEDIATRIC BRAIN TUMORS REVEALS INCIDENTAL FINDINGS AND NOVEL TUMOR BIOLOGY