Functional CRISPR Screening Identifies Ptprg As a Driver of Migration and Adhesion in NSD2-E1099K ALL
Authors: Kaestner Charlotte L , Sobh Amin , Li Jianping , Riva Alberto , Bennett Richard Lynn , Licht Jonathan D. 
Published: November 2021

Abstract
Background: Acute Lymphoblastic Leukemia (ALL) is the most common childhood cancer and frequently infiltrates the central nervous system (CNS). CNS-directed therapy is currently limited to intrathecal and systemic high-dose methotrexate, or less commonly craniospinal irradiation, both of which are associated with substantial neurotoxicity. A lack of mechanistic understanding of the mechanisms of CNS infiltration presents an obstacle for the development of more specific and less toxic therapeutic approaches. We previously showed that ALL cells with a specific mutation (E1099K) in the histone methyltransferase NSD2 have aggressive CNS tropism by not only infiltrating the leptomeninges but also the brain parenchyma in murine xenografts models. Analysis of cBioPortal data shows that NSD2-E1099K is associated with a higher rate of testicular involvement in ALL also suggesting more aggressive infiltration behavior of the tumor. Accordingly, using gene editing to revert mutant NSD2 back to wild-type, we also showed that NSD2-E1099K cells have an enhanced ability to migrate and adhere in vitro. RNA-seq data on four NSD2-E1099K cell lines revealed genes that may play a role in ALL brain infiltration. However, it remains unknown which of those upregulated genes could be potential therapeutic targets against CNS leukemia.
Aim: This study aims to Identify therapeutically targetable genes that are important for migration of NSD2-E1099K ALL cells
Methods: Using a focused CRISPR-gene-knockout library of 5600 sgRNAs directed against 500 genes upregulated in NSD2-E1099K cells, we ascertained the necessity of the selected genes for migration in the RCH-ACV cell line. Candidate genes were evaluated for cellular dependency using a CRISPR-loss of function screen and the cancer dependency map portal. Overexpression of the candidate genes in NSD2-E1099K cell lines was confirmed with qPCR analysis. Candidate genes were validated by individual shRNA knockdown followed by migration and adhesion assays.
Results: Our study identified genes whose knockout led to enhancement of migration and others whose knockout resulted in inhibition of migration. Protein Tyrosine Phosphatase Receptor Type G (PTPRG) was one of the top candidate genes whose knockout resulted in inhibition of migration. Dependency map analysis showed that PTPRG is not a commonly essential gene and a CRISPR-based-loss-of function screen performed in parallel to the migration screen confirmed that ALL cell survival is not dependent on PTPRG. We also found that PTPRG is overexpressed in multiple NSD2-E1099K ALL cell lines. Individual Knockdown of PTPRG in NSD2-E1099K ALL cell lines not only inhibited migration, but also led to a loss of adhesion ability to endothelial cells of the Blood Brain Barrier.
Conclusions: Our findings implicate PTPRG as an important modulator of migration and adhesion in ALL cells and a potential therapeutic target for preventing ALL brain infiltration, especially in NSD2-E1099K ALL.

Disclosures
Licht: Epizyme: Research Funding.