The Role of LAMP5 in Innate Immune Signaling Is Critical for the Survival of MLL Leukemias

Introduction

Chromosomal translocations involving the Mixed Lineage Leukemia (MLL, also called KMT2A) gene account for around 80% of the acute lymphoid leukemia (ALL) and 35%-50% of the acute myeloid leukemia (AML) diagnosed in infants, and are associated with poor outcomes. Despite improvements of conventional chemotherapies and treatments, patients with MLL-rearranged leukemia have weak response to treatment and poor prognosis. There is thus an urgent need to understand the molecular pathogenesis of these leukemias to develop more effective therapies. Gene expression studies show that MLL-leukemias uniquely display over-expression of Lysosome-associated Membrane Protein 5 (LAMP5) in both ALL and AML In normal hematopoiesis, LAMP5 expression is restricted to non-activated plasmacytoid dendritic cells (pDC), where it has been shown to be required for transport of TLR9 from VAMP3+/LAMP2+/LAMP1- endolysosomal vesicles to the LAMP1+ late lysosomal compartments, modulating the signaling and production of Type I interferon to TNF production. Here, we tested the hypothesis that LAMP5 is required for MLL leukemia wherein it modulates innate immune signaling.

Methods and Results

In previous studies, we found LAMP5 expression being restricted to MLL-fusion leukemia cell lines. Additionally, in an inducible model, LAMP5 expression directly correlated with that of the MLL-fusion protein MLL-AF9. . Knockdown on LAMP5 in MLL-fusion leukemia cells inhibited their propagation both in vitro and in vivo whereas the non-MLL leukemias were unaffected. To determine the mechanisms by which LAMP5 promotes growth in MLL leukemias, first we studied the localization of LAMP5 in these cells. Using immunofluorescent confocal microscopy we detected LAMP5 mainly in LAMP2+/LAMP1+ compartments. Additionally, LAMP5 co-localized with MYD88, a known scaffold protein required for IL1R/TLR signaling. Accordingly, LAMP5-knockdown abrogated TLR signaling as evidenced by reduced activation of JNK, MAPK p38, IRF7 and NFKB. Conversely, over-expression of LAMP5 in the non-MLL leukemia cell line Kasumi-1 led to increased activation of JNK, p38, IRF7 and NFKB, indicating that the innate-immune signaling pathway is regulated by LAMP5 expression in leukemias.r

Conclusions

Collectively, these results demonstrate that LAMP5 is required for the survival of MLL leukemia and that it plays an important role in the activation of the IL1/TLR signaling pathway. Overall, based on our results and the limited expression in normal hematopoiesis, we propose that LAMP5 could potentially serve as a therapeutic target with a wide therapeutic-window to treat MLL-leukemias.