PIM1 Point Mutations Increase Migration of Diffuse Large B-Cell Lymphoma (DLBCL) Cells

PIM1 serine-threonine kinase is known proto-oncogene, highly expressed in lymphoid malignancies. It regulates crucial processes for the cancer cell, like proliferation, apoptosis, metabolism or migration. In DLBCLs, PIM1 gene is a target of somatic hypermutation and one of the most frequently mutated genes in DLBCL patients. PIM1 mutations are more frequent in ABC-type tumors, which have worse survival prognosis than GCB-type, and often co-occur with mutations in MYD88 and CD79B. Analyses of the available clinical data show that the most significant differences between the groups of DLBCL patients with or without PIM1 mutations are tumor mutational burden, age at diagnosis and extranodal (testicular) involvement. In line with this finding, PIM1, MYD88 and CD79B mutations have also been reported to be highly prevalent in extranodal B cell lymphomas, localized in central nervous system (PCNSL), testicles (PTL) or intravascular space (IVLBCL). Our goal was to assess the role of the most common PIM1 point mutations in the biology and pathogenesis of DLBCL cells.

The HBL-1 cell line was chosen as a model of MYD88^L265P-positive ABC-DLBCL subtype. PIM1 knock-out was performed using CRISPR/Cas9 method. Sequences of wild type PIM1, kinase-dead PIM1K67M, serving as a an inactive kinase control, and 7 most frequent mutants (PIM1G28D, PIM1E70K, PIM1P81L, PIM1S97N, PIM1P125A, PIM1L164F, PIM1L184F) were reintroduced to the HBL-1 PIM1 KO cells using lentiviral vectors. PIM1 mutations did not affect the proliferation of DLBCL cells and did not increase resistance to doxorubicin, excluding the proliferation rate or treatment resistance as a cause of the poor prognosis associated with PIM1 mutations. We performed RNA-seq analysis of the HBL-1-PIM1-WT and HBL-1-PIM1-mutants. The most upregulated pathways in PIM1 mutants (including PIM1K67M) were those related to Interferon gamma, IL-15, STAT3, cMET and semaphorin-plexin signaling (involved in proliferation, actin dynamics and migration of cancer cells). We also found that PIM1 mutants universally overexpress cMET oncogene. Moreover, gene set and pathway enrichment analyses indicated that the PIM1-mutant cells exhibit features associated with altered actin polymerization, RHO GTPases and cell membrane dynamics, suggesting the increased migratory potential of the cells and their propensity to spread to extranodal sites. Higher surface level of cMET in PIM1 mutant cells was confirmed using FACS analysis. Since cMET and CXCR4 cooperate in signaling and trigger AKT/mTOR, which is required for CXCR4-mediated cell motility, we evaluated whether PIM1 mutant cells would exhibit increased phosphorylation of AKT. In line with this hypothesis, we noted higher pAKT and pERK1/2 in PIM1 mutant cells than in PIM1 WT cells. Finally, in transwell assays, PIM1 mutant cells exhibited markedly increased migratory potential.

Taken together, these studies indicate that PIM1 mutants are not likely associated with differential drug responses, but rather facilitate tumor dissemination and are thus associated with inferior prognosis in DLBCL patients harboring PIM1 mutations.