Targeting Free Light Chain Secretion Via Botulinum Neurotoxin Is a Novel Therapeutic Strategy in AL Amyloidosis By Inducing a Terminal Unfolded Protein Response

Background

AL amyloidosis (AL) is an incurable plasma cell (PC) disorder. The solely pathogenic mechanism in AL is deposition of immunoglobulin free light chains (FLC) organized in fibrils in target organs. Surprisingly, therapeutic strategies directly targeting FLC secretion are not available. SNARE proteins, which are the specific target of botulinum neurotoxin (BoNT), are involved in the docking and fusion of secretory vesicles. We hypothesized that certain BoNT serotypes may block FLC exocytosis, causing retention of FLC-loaded vesicles and triggering a terminal unfolded protein response (UPR).

Materials and Methods

Gene expression profiling in a cohort of 170 newly diagnosed multiple myeloma (MM) patients (IFM170) was used to interrogate SNAREs expression in malignant plasma cells. Western blotting (WB) was used to assess SNARE expression across MM and AL cell lines. We developed tetracycline inducible, lentiviral vectors expressing distinct BoNT serotypes (BoNT/A-F), T2A and GFP. Lentivirally transduced cells would express BoNT in a 1:1 stochiometric ratio with GFP, upon doxycycline (dox) administration, allowing for flow cytometry-based analysis. A vector comprising solely T2A and GFP was used as negative control. We transduced AL cell lines with Tet-On lentivirus expressing 7 distinct BoNTs and performed two sets of experiments. First, we performed time-course viability assays on polyclonally transduced cells and compared relative proportion of GFP+ cells over time. Then, we single-cell sorted transduced cells, triggered BoNT expression and assessed GFP kinetic and apoptosis via AnnexinV/DAPI flow cytometry at 24, 48 and 72 hours post dox. SNAREs cleavage following induction of BoNT expression was evaluated via WB in GFP+ clones. To assess if BoNT cytotoxicity correlated with cessation of FLC secretion, we performed a secretion assay in monoclones expressing distinct BoNTs.

Results

IFM170 GEP analysis showed VAMP2, VAMP3 and SNAP23 as the top expressed SNAREs. This was further confirmed in AL/MM cell lines. By using polyclonally transduced cells, we show that GFP+ cells are rapidly depleted over time after dox, across all serotypes, except BoNT/B, consistent with cytotoxic effect. Similarly, we observed rapid apoptosis in monoclones expressing any BoNT serotypes, except BoNT/B. We noted an association between SNAP23 and VAMP3 cleavage and BoNT toxicity, suggesting that dual targeting of SNAP23/VAMP3 may be necessary to mediate BoNT cytotoxicity. We next show that only BoNTs causing early cytotoxicity significantly inhibited FLC secretion. Cytotoxic BoNTs, activated PERK pathway with eIF2a phosphorylation (p-eIF2a); CHOP and GADD34 upregulation, presumably through FLC retention.

Conclusions

We show that cytotoxic BoNTs block FLC secretion, trigger a terminal UPR and induce apoptosis in AL and MM models. We provide proof of concept that targeting FLC secretion has a potential clinical translatability.