After HMA Resistance in AML, a New Possible Therapy Option

Researchers have found what may be a novel treatment option for high-risk myelodysplastic neoplasms (MDS) and acute myeloid leukemia (AML), based on their investigations into TOPORS, a dual E3 ubiquitin and SUMO ligase. The therapy would be a combination of hypomethylating agents (HMAs) along with the inhibition of SUMOylation or TOPORS, they describe in Nature Communications.¹

Depleting TOPORS, the authors found, mediates sensitivity to hypomethylating agents. | Image Credit: © Andrey - stock.adobe.com

Because TOPORS is not directly druggable at present, the team proposes low-dose HMA in combination with TAK-981 as a viable therapeutic strategy to be considered for these diseases.² This “workaround” solution inhibits protein SUMOylation with TAK-981, which, they said, partially phenocopies HMA-sensitivity and DNA damage response (DDR) impairment. A further benefit might be that in immune-sufficient patients with AML, TAK-981 combination therapy could potentially activate anti-tumor T and natural killer cells.

While frontline therapies for MDS and AML—specifically, HMAs—do of course exist, acquired resistance and treatment failure are common. The investigators sought to address this issue by performing a genome-wide CRISPR-Cas9 screen in a human MDS-derived cell line called MDS-L, they explained. Consequently, they pinpointed TOPORS as a loss-of-function target that synergizes with HMAs, in turn reducing leukemic burden and improving survival in xenograft models.

Depleting TOPORS, they found, mediates sensitivity to HMAs. It does so, they described, “by predisposing leukemic blasts to an impaired DDR accompanied by an accumulation of SUMOylated DNMT1 in HMA-treated TOPORS-depleted cells.” This hypersensitive phenotype is not dependent on del5q or TP53 mutational status, they said. Further, combining HMAs with targeting of TOPORS does not impair healthy hematopoiesis.

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

That is among the reasons TOPORS is an attractive target for development of specific inhibitors with a potentially improved therapeutic index, compared with TAK-981. SUMOylation plays a critical role in transcriptionally repressing inflammatory cytokines through its influence on chromatin architecture—beyond its broad 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 adducted DNMT1,” they said. Nonetheless, HMA-induced DNMT1 degradation was indeed reduced in TOPORS-edited cells, and other investigators have discovered that TOPORS “acts in semi-redundant concert with RNF4 to mediate efficient proteasomal degradation of DNA-adducted DNMT1.”

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

The researchers noted that their data were in contrast to those of previous clinical studies, in which other inhibitors of posttranslational mechanisms, including the proteasomal inhibitor bortezomib and pevonedistat (which inhibits the NEDD8 activating enzyme), in combination with azacitidine, did not provide a survival advantage over azacitidine monotherapy. “A possible explanation for these results could be that these agents exhibit strong antimitotic properties, which might antagonize incorporation of HMAs into tumor DNA,” they posited.

References

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

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