Succinate dehydrogenase (SDH) inhibition leads to cancer through oxidative stress

Xavier Coumoul

Sylvie Bortoli

Arnaud Tête

Judith Favier

Karine Audouze

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. A complete inactivation of SDH leads to cancerous pathologies in young adults (paragangliomas, pheochromocytomas, renal cancers and gastrointestinal stromal tumors). In neuroendocrine tumors, SDH genetic inactivation induces an oxidative stress. Oxidative stress has been linked to genetic mutations and therefore the risk of cancer. 

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 genetic 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 multitude of mechanisms of action that can be brought into play by the inhibition of SDH, a key enzyme in mitochondrial metabolism (Krebs cycle and respiratory chain); this inhibition may be genetic in origin, but it may also be chemical, due to the existence of man-made molecules that inhibit its catalytic cycle (in particular those targeting moulds); the initial consequences of inhibition include oxidative stress or an accumulation of succinate; some cancers are known to be associated with genetic invalidation. The development strategy for this AOP is part of the development of an AON with three independent AOPs.

The biological plausibility of KERs is defined by the OECD as the « understanding of the fundamental biological processes involved and whether they are consistent with the causal relationship being proposed in the AOP ». The biological plausibility is strong due to the presence of overwhelming evidence present in different studies. SDH genetic inhibition leads for example to paragangliomas.

The essentiality of KEs refers to « experimental data for whether or not downstream KEs or the AO are prevented or modified if an upstream event is blocked ». The essentiality of KEs is moderate: most works converge to imply SDH inactivation with tumorigenic outcomes. One setback would be that SDH inhibition leads to several molecular outcomes including succinate accumulation which is developed in another AOP. The essentiality would be strong considering all AOP starting from SDH inhibition and converging towards cancer through an AON (AOP network).

Finally, the empirical support of KERs, is often « based on toxicological data derived by one or more reference chemicals where dose–response and temporal concordance for the KE pair can be assessed ». The overall assessment of the empirical support of our KERs is moderate. 

The AOP begins with an 'SDH inhibition' MIE and could therefore be used to test any molecule likely to directly inhibit succinate dehydrogenase. 

Its simple structure could also be used for any molecule likely to cause oxidative stress (i.e. at the level of KEs).

To be included