Researchers have discovered what could be a new treatment option for high-risk myelodysplastic neoplasms (MDS) and acute myeloid leukemia (AML), based on their investigations of TOPORS, a dual ubiquitin E3 and SUMO ligase. The therapy would be a combination of hypomethylating agents (HMA) as well as inhibition of SUMOylation or TOPORS, they describe in Natural communications.¹

The authors found that TOPORS depletion mediates sensitivity to hypomethylating agents. | Image credit: © Andrey – stock.adobe.com

Because TOPORS is not directly druggable at this time, the team proposes low-dose HMA in combination with TAK-981 as a viable therapeutic strategy to consider for these diseases.² This workaround inhibits SUMOylation of the protein with TAK-981, which they suggest partially phenocopies HMA sensitivity and DNA damage response (DDR). Another advantage could be that in immunosufficient AML patients, TAK-981 combination therapy could potentially activate anti-tumor T cells and natural killer cells.

Although there are of course first-line therapies for MDS and AML, particularly HMA, acquired resistance and treatment failure are common. The investigators sought to address this problem by performing a genome-wide CRISPR-Cas9 screen in a human MDS-derived cell line called MDS-L, they explained. Therefore, they identified TOPORS as a loss-of-function target that synergizes with HMAs, thereby reducing leukemic burden and improving survival in xenograft models.

They found that depletion of TOPORS attenuates sensitivity to HMA. It does so, they described, “by predisposing leukemic blasts to impaired DDR accompanied by accumulation of SUMOylated DNMT1 in HMA-treated TOPORS-depleted cells.” This hypersensitive phenotype does not depend on del5q or TP53 mutational status, they said. Furthermore, the combination of HMAs with TOPORS targeting does not harm healthy hematopoiesis.

TAK-981 is an effective substitute for TOPORS editing, the investigators said, and it demonstrates cytotoxic synergy in combination with low-dose HMAs, but the reduction in TOPORS activity synergizes with HMAs much more specifically. as the reduction of upstream SUMOylation activity mediated by TAK-981. “Complete inactivation of SUMOylation is lethal, whereas inactivation of TOPORS E3 ligase alone is not,” they pointed out.

This is one of the reasons why TOPORS constitutes an attractive target for the development of specific inhibitors with a potentially improved therapeutic index compared to TAK-981. SUMOylation plays a critical role in the transcriptional repression of inflammatory cytokines through its influence on chromatin architecture, beyond its expanded role in modulating the DDR.

The team’s CRISPR work confirmed that DNMT1 was the dominant mediator of HMA cytotoxicity in AML cells. “Against our initial expectations, we found that TOPORS was not required for SUMOylation of added DNMT1,” they said. Nevertheless, HMA-induced DNMT1 degradation was effectively reduced in TOPORS-edited cells, and other investigators found that TOPORS “acts in semi-redundant concert with RNF4 to mediate efficient proteasomal degradation of DNMT1 added to l ‘DNA’.

Ubiquitylation of SUMOylated DNMT1 adducts may not be the only way TOPORS protects HMA-exposed cells from DDR-induced apoptosis, the researchers explained. Their proteomic investigation identified RNA splicing factors as candidates for SUMOylation or TOPORS-mediated ubiquitylation in AML cells, even in the absence of HMA. They also identified mis-splicing of DNA repair genes in TOPORS-edited cells. This may be due, in part, to aberrant SUMO or ubiquitin modulation of interacting splicing factors.

The researchers noted that their data contrasted with previous clinical studies, in which other inhibitors of post-translational mechanisms, including the proteasome inhibitor bortezomib and pevonedistat (which inhibits the activating enzyme NEDD8), in combination with azacitidine, did not allow survival. advantage over azacitidine monotherapy. “A possible explanation for these results could be that these agents exhibit strong antimitotic properties, which could oppose the incorporation of HMA into tumor DNA,” they argued.

References

1. Truong P, Shen S, Joshi S et al. TOPORS E3 ligase mediates resistance to cytotoxicity of hypomethylating agents in acute myeloid leukemia cells. Nat Common. 2024;15(1):7360. doi:10.1038/s41467-024-51646-6

2. Gabellier L, De Toledo M, Chakraborty M et al. The SUMOylation inhibitor TAK-981 (subasumstat) synergizes with 5-azacytidine in preclinical models of acute myeloid leukemia. Haematological. 2024;109(1):98-114. doi:10.3324/hematol.2023.282704