Despite improvements in chemotherapy that have increased the 5-year survival rates of pediatric ALL to close to 90%, 15-20% of patients may relapse with a very poor prognosis. Pediatric ALL patients, particularly those in relapse can harbor a specific point mutation (E1099K) in NSD2 (nuclear receptor binding SET domain protein 2) gene, also known as MMSET or WHSC1, which encodes a histone methyl transferase specific for H3K36me2. To understand the biology of mutant NSD2, we used CRISPR-Cas9 gene editing to disrupt the NSD2E1099K mutant allele in B-ALL cell lines (RCH-ACV and SEM) and T-ALL cell line (RPMI-8402) or insert the E1099K mutation into the NSD2WT T-ALL cell line (CEM) and B-ALL cell line (697). Cell lines in which the NSD2E1099K mutant allele is present display increased global levels of H3K36me2 and decreased H3K27me3. NSD2E1099Kcells demonstrate enhanced cell growth, colony formation and migration. NSD2E1099K mutant cell lines assayed by RNA-Seq exhibit an aberrant gene signature, mostly representing gene activation, with activation of signaling pathways, genes implicated in the epithelial mesenchymal transition and prominent expression of neural genes not generally found in hematopoietic tissues. Accordingly, NSD2E1099K cell lines showed prominent tropism to the central neural system in xenografts.
To understand why this NSD2 mutations are identified prominently in children who relapse early from therapy for ALL, we performed high-throughput screening in our isogenic cell lines with the National Center for Advancing Translation Science (NCATS) Pharmaceutical Collection and other annotated chemical libraries and found that NSD2E1099K cells are resistant to glucocorticoids (GC) but not to other chemotherapeutic agents used to treat ALL such as vincristine, doxorubicin, cyclophosphamide, methotrexate, and 6-mercaptopurine. Accordingly, patient-derived-xenograft ALL cells with NSD2E1099K mutation were resistant to GC treatment. Reversion of NSD2E1099K mutation to NSD2WT restored GC sensitivity to both B- and T-ALL cell lines, which was accompanied by cell cycle arrest in G1 and induced-apoptosis. Furthermore, knock-in of the NSD2E1099K mutation conferred GC resistance to ALL cell lines by triggering cell cycle progression, proliferation and anti-apoptotic processes. Mice with NSD2E1099K xenografts were completely resistant to GC treatment while treatment of mice injected with isogenic NSD2WT cells led to significant tumor reduction and survival benefit.
To illustrate these biological phenotypes and understand the molecular mechanism of GC resistance driven by NSD2E1099Kmutation, we investigated the GC-induced transcriptome, GC receptor (GR) binding sites and related epigenetic changes in isogenic ALL cell lines in response to GC treatment. RNA-Seq showed that GC transcriptional response was almost completely blocked in NSD2E1099K cells, especially in T-ALL cell lines, correlating with their lack of biological response. GC treatment activated apoptotic pathways and downregulated cell cycle and DNA repair pathways only in NSD2WT cells. The critical pro-apoptotic regulators BIM and BMF failed to be activated by GC in NSD2E1099K cells but were prominently activated when the NSD2 mutation was removed. Chromatin immunoprecipitation sequencing (ChIP-Seq) showed that, the NSD2E1099K mutation blocked the ability of GR and CTCF to bind most GC response elements (GREs) such as those within BIM and BMF. While GR binding in NSD2WT cells was accompanied by increased H3K27 acetylation and gene expression, this failed to occur in NSD2 mutant cells. Furthermore, we found that GR RNA and protein levels were repressed in ALL cells expressing NSD2E1099K and GC failed to induce GR expression in these cells. Paradoxically, while H3K27me3 levels were generally decreased in NSD2E1099K cells, we saw increased levels of H3K27me3 at the GRE within the GR gene body where GR itself and CTCF normally bind, suggesting a novel role for the polycomb repressive complex 2 and EZH2 inhibitors for this form of GC resistance. In conclusion, these studies demonstrate that NSD2E1099K mutation may play an important role in treatment failure of pediatric ALL relapse by interfering with the GR expression and its ability to bind and activate key target genes. Gene editing screens are being performed to understand how to overcome this resistance.
Disclosures
No relevant conflicts of interest to declare.
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