Isogenic Loss of TNFAIP3 in Waldenstrom Macroglobulinemia Enhances MYD88_L265P-Driven Signaling

Recent next generation sequencing of paired normal and tumor lymphoma tissue highlights a significant role for the MYD88_L265P mutations in Waldenstrom Macroglobulinemia (WM) and diffuse large B-cell lymphoma (DLBCL). In recent work we have found that MYD88_L265P signaling is constitutively active in both WM and DLBCL cells leading to heightened MYD88_L265P, IRAK and TRAF6 oligomerization and TAK1 and NF-kB activation. However, little else is know about the full impact of the mutation, especially in combination with other genetic events. Recent mouse models suggest that MYD88_L265P alone may not be sufficient to induce tumor formation and that the pathway requires additional genetic hits. Interplay between MYD88_L265P and other oncogenic events are further supported by the fact that TNFAIP3 (A20) inactivation often accompanies MYD88_L265P. These data, combined with the finding that loss of TNFAIP3 promotes accumulation of MYD88_L265PB-cells in a mouse model, suggest a potential relationship between these genetic events, and more importantly, a possible clinical impact. The aim of this study was to generate a human cell line model which mimics the effect of MYD88_L265P andTNFAIP3 loss to further understand the cellular consequence of these genomic alterations in lymphoma.

To mimic the TNFAIP3 loss in combination with the MYD88_L265P mutation,we used the WM cell line MWCL1, which harbors a heterozygous mutation in MYD88. For the TNFAIP3 specific knock out, we used the transcription activator-like effector nucleases (TALENs) genome editing approach to induce a double stranded break in exon 5 of the TNFAIP3 gene. TALENs were expressed in MWCL1 cells by transient transfection, and single-cell clones were isolated. Sanger sequencing of the TNFAIP3 gene was performed to identify clones that carried the deletion and all clones were screened by Western blot to confirm knock down of A20.

We successfully generated a panel of MWCL1 cell lines that harbor a heterozygous deletion in the TNFAIP3 gene. Western blot analysis confirm a 2.4 fold reduction in A20 expression in the heterozygous cell line (MWCL1_A20-/A20+), Because A20 has been shown the be a central negative regulator of NF-κB activation, we first measured the impact of A20 loss on phosphorylation of NF-κB p65 and found that MWCL1_A20-/A20+ cells had higher base line levels of pNF-κB compared to the wild type (WT) cell line (1.77 fold). Furthermore, time course stimulation experiments with the TLR ligand lipopolysaccharide (LPS) showed that MWCL1_A20-/A20+ cells had a higher amount of pNF-κB after 30 (3 fold) and 60 (1.5 fold) minutes of stimulation. We next looked at expression of NF-κB target genes by quantitative real time PCR and found that IL-6 (2.5 fold; p≤ 0.05) and CXCL10 (IP10) (4 fold; p≤ 0.05) were upregulated in MWCL1_A20-/A20+cells compared to WT.

In summary, we have established a new WM cell line model which mimics the effect of the MYD88_L265P mutation in combination with a heterozygous loss of the TNFAIP3 gene. We show that loss of TNFAIP3 results in a higher baseline and TLR-stimulated phosphorylation of NF-κB as well as in higher expression IL-6 and IP10. Overall, this cell line will be a useful tool to study biological consequences of TNFAIP3loss in WM patients as well as novel therapeutic strategies.