Abstract 705: Inhibition of H3.3 S31 phosphorylation by the pediatric glioma driver mutation, H3.3 K27M, results in chromosomal instability, loss of p53 regulation, and tumorigenesis

Abstract The heterozygous H3.3 K27M mutation is found in the majority of pediatric High-Grade Glioma’s (pHGG). H3.3 K27M is thought to promote tumor formation through altered epigenetic gene regulation. However, it is unclear if epigenetic reprogramming alone is sufficient for glioma formation. H3.3 contains a unique Serine at position 31 which is phosphorylated in early mitosis (where it functions in chromosome segregation) and again in late M/early G1 when a mitotic error has occurred (triggering p53 expression). We found that pHGG cell lines harboring H3.3 K27M have significantly reduced S31 phosphorylation as compared to both pHGG and non-transformed H3.3 WT cells. When compared to WT H3.3 in an in vitro kinase assay, H3.3 K27M exhibits an ~60% reduction in S31 phosphorylation by Chk1, the mitotic S31 kinase. In normal diploid cells, expression of K27M or non-phosphorylatable S31A mutant significantly increased chromosome missegregation. Yet expressing a phosphomimetic double mutant (K27M/S31E) did not significantly alter the rate of chromosome mis-segregation compared to WT. Furthermore, patient-derived pHGG lines harboring K27M have significantly higher rates of chromosome mis-segregation compared to an H3.3 WT pHGG line or H3.3 WT non-transformed human cells. We used CRISPR gene editing to remove the H3.3 K27M allele or replace it with WT H3.3. In both cases, loss of K27M elevated pS31 levels and reduced chromosome mis-segregations. Yet, when K27M was replaced with S31A, the mis-segregation rate remained similar to the parental K27M cells. In normal cells, chromosome missegregation stimulates p53-dependent cell cycle arrest in G1 to prevent the proliferation of aneuploid daughters. However, cells expressing H3.3 K27M or S31A failed to arrest following missegregation - despite having WT p53. To determine if the induction of chromosomal instability would alter tumor formation, we expressed H3.3 WT, K27M or S31A in combination with PDGFβ in a glioma mouse model. Expression of WT H3.3 failed to generate high-grade tumors. But 66% of mice expressing K27M and 93% of those expressing H3.3 S31A developed diffuse high-grade brain tumors by 100 days. Our results suggest that loss of phospho-S31 alone is oncogenic because H3.3 S31A-expressing cells are WT for K27me3. Our results demonstrate that H3.3 K27M inhibits Ser31 phosphorylation both in vitro and in vivo, leading to both chromosome missegregation and loss of subsequent G1 arrest - thus creating diffuse midline gliomas with both dynamic, complex karyotypes and epigenetic reprogramming. Citation Format: Charles Day, Florina Grigore, Alyssa Langfald, David J. Daniels, James Robinson, Edward Hinchcliffe. Inhibition of H3.3 S31 phosphorylation by the pediatric glioma driver mutation, H3.3 K27M, results in chromosomal instability, loss of p53 regulation, and tumorigenesis [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 705.