Cyanide overproduction impairs cellular bioenergetics in Down syndrome

Cyanide exerts its toxic effects primarily by inhibiting mitochondrial Complex IV (Cytochrome c oxidase, CCOx). Recent studies have shown that mammalian cells can endogenously produce cyanide from glycine via a lysosomal pathway. At low concentrations, cyanide may play regulatory roles, but at higher levels, it causes metabolic inhibition. Here we show that Down syndrome (DS) cells and tissues exhibit significant overproduction of cyanide, contributing to cellular metabolic suppression. DS rats show elevated blood cyanide levels, and their tissues generate more cyanide than wild-type controls—both under basal conditions and following glycine supplementation. Similarly, human DS fibroblasts produce higher levels of cyanide than healthy control cells. We attribute this increased cyanide production in DS to the marked downregulation of thiosulfate sulfurtransferase (TST, also known as rhodanese), the key enzyme responsible for cyanide detoxification. Importantly, suppression of lysosomal cyanide production in DS cells (through cyanide scavengers, lysosomal deacidification, or inhibition of serine/glycine conversion) improves cellular bioenergetics and/or enhances cell proliferation rates. Previous work has implicated excessive hydrogen sulfide (H 2 S) production, another endogenous gaseous signaling molecule that inhibits CCOx, in DS-associated metabolic suppression. Our current findings indicate that cyanide overproduction may also contribute to this dysfunction. Cyanide and H 2 S may act cooperatively on the same molecular target. These results open the possibility of developing therapeutic strategies that target cyanide or both cyanide and H 2 S to counteract DS-associated metabolic impairment.