Succinate dehydrogenase inactivation leads to cancer by promoting EMT

Arnaud Tête

Judith Favier

Xavier Coumoul

Karine Audouze

Sylvie Bortoli

Succinate dehydrogenase (SDH) is a key enzymatic complex involved in two interconnected metabolic processes for energy production: the transfer of electrons in the mitochondrial respiratory chain and the oxidation of succinate to fumarate in the Krebs cycle. In humans, inherited SDH deficiencies may cause major pathologies including cancers. The cellular and molecular mechanisms related to genetic SDH inactivation have been well described in neuroendocrine tumors, in which it induces an oxidative stress, a pseudohypoxic phenotype, a metabolic, epigenetic and transcriptomic remodeling, and alterations in the migration and invasion capacities of cancer cells, in connection with the accumulation of succinate, an oncometabolite, substrate of the SDH. SDH complex is the molecular target of Succinate Dehydrogenase Inhibitors (SDHi), a family of pesticides widely used to limit the proliferation of pathogenic fungi. This AOP aims to describe the relationship between SDH inactivation and cancer development.

Succinate dehydrogenase (SDH) is a key enzyme of mitochondria, organelles that play a crucial role in the production of energy, the metabolic and calcium homeostasis, the control of apoptosis, and the production of reactive oxygen species. SDH is involved in two interconnected metabolic processes for energy production: 1) cellular respiration, where it allows the transfer of electrons to ubiquinone as complex II of the mitochondrial respiratory chain, and 2) the Krebs cycle, where it catalyzes the oxidation of succinate to fumarate.

Numerous studies show that a complete inactivation of SDH caused by a first constitutional mutation associated with a second somatic mutation, leads to cancerous pathologies in young adults, including particularly aggressive forms of cancer such as paragangliomas (neuroendocrine tumors of the head and neck, thorax, abdomen and pelvis), pheochromocytomas (tumors of the adrenal medulla), renal cancers and gastrointestinal stromal tumors. The cellular and molecular mechanisms related to the genetic inactivation of SDH have been well described in neuroendocrine tumors, where it induces an oxidative stress, a pseudohypoxia phenotype, a metabolic, epigenetic and transcriptional remodeling, and alterations in tumor cell migration and invasion capacities, in connection with the accumulation of succinate, the substrate of SDH.

The succinate dehydrogenase inhibitors (SDHi) are fungicides used to control the proliferation of pathogenic fungi in cereal, fruit and vegetable crops, with a mode of action based on blocking the activity of SDH. The analysis of literature data shows that the impact of SDHi on health remains largely unexplored to date, despite a growing number of studies reporting toxic effects in non-target organisms. This is supported by our recent work highlighting 1) the high degree of conservation of the SDH catalytic site (i.e. the SDHi binding site) during the evolution and 2) the ability of SDHi to inhibit SDH in the mitochondria of non-target species, including humans (PMID: 31697708). These observations show that SDHi are not specific to fungal SDH and that their use may present a risk to human health, particularly in the context of chronic exposure through the diet. Moreover, the analysis of regulatory assessment reports shows that most SDHi induce tumors in animals without evidence of genotoxicity. Thus, for these substances, the mechanisms of carcinogenicity are, to date, not clearly established. 

Our hypothesis is that, if SDHi fungicides are able to alter SDH activity in humans, the consequences of SDHi exposure on cellular and mitochondrial functions may resemble those observed in SDH-mutated tumors and SDH-deficient cells. We assume that the development of an AOP deciphering the different steps leading to cancer following a genetically-SDH inactivation could help to propose the exploration of relevant key events and adverse effects upon chronic exposure to SDHi fungicides.

The development strategy for this AOP is based on the current knowledge on molecular and cellular events triggered by a genetic inactivation of SDH, and on the hypothesis that a chemical SDH inactivation may lead to similar events.

This AOP will be part of the development of an AON with AOP 534.

This AOP describes the molecular and cellular consequences of succinate dehydrogenase (SDH) inactivation, leading to adverse biological outcomes. It integrates evidence from genetic and chemical inhibition models, supporting the hypothesis that chemical inactivation of SDH can mimic the effects observed in genetic SDH deficiencies.

This AOP is broadly applicable across multiple taxa, as SDH is a highly conserved enzyme among eukaryotes. The key events have been observed in mammalian models (rodents, humans) as well as in some invertebrate systems, though most empirical evidence comes from vertebrate studies. The AOP is relevant to both sexes, as SDH-related dysfunctions have not shown sex-specific susceptibility.

Regarding life stage, available data suggest that embryonic and juvenile stages may be more vulnerable to SDH inactivation due to their higher metabolic demands and reliance on mitochondrial function. However, further studies are needed to delineate life-stage-dependent sensitivities. 

The overall weight of evidence for this AOP is moderate to strong, with well-established mechanistic understanding of early molecular events but some uncertainties in later-stage key event relationships.   - Biological plausibility: Strong, as SDH inactivation is well-documented to disrupt mitochondrial metabolism, leading to accumulation of oncometabolites and altered cellular signaling.   - Empirical support: Moderate to strong, particularly for the initial key events. In vitro and in vivo studies demonstrate consistency in key event progression, but quantitative understanding of dose-response relationships remains limited.   - Concordance and consistency: Moderate to strong, with observed reproducibility across multiple experimental models, though species-specific differences in metabolic compensation require further investigation.  

Regulatory Considerations This AOP contributes to a broader Adverse Outcome Network (AON) alongside AOP 534, providing a mechanistic basis for regulatory applications such as:   - Priority setting: identifying chemicals that may interfere with SDH function as potential environmental and occupational hazards. This is the case for SDHi fungicides. - Testing strategies: informing the development of in vitro assays targeting early key events to reduce reliance on animal models (SDH activity) - Risk assessment: Supporting the integration of mitochondrial toxicity (mitotoxicity) into chemical risk assessment frameworks, particularly for compounds suspected of disrupting cellular metabolism.