Understanding the Impact of Mutations in the Cystathionine Beta-Synthase Gene: Towards Novel Therapeutics for Homocystinuria.

Protein misfolding and conformational instability drive protein conformational disorders, causing either accelerated degradation and loss-of-function, as in inherited metabolic disorders like lysosomal storage disorders, or toxic aggregation and gain-of-function, as in neurodegenerative diseases like Alzheimer's disease or amyotrophic lateral sclerosis. Classical homocystinuria (HCU), an inborn error of sulfur amino acid metabolism, results from cystathionine beta-synthase (CBS) deficiency. CBS regulates methionine conversion into metabolites critical for redox balance (cysteine, glutathione) and signaling (H₂S). Pathogenic missense mutations in the CBS gene often impair folding, cofactor binding, stability or oligomerization rather than targeting the key catalytic residues of the CBS enzyme. Advances in understanding of CBS folding and assembly as well as CBS interactions with cellular proteostasis network offer potential for therapies using pharmacological chaperones (PCs), i.e., compounds facilitating proper folding, assembly or cellular trafficking. This review discusses progress in identifying PCs for HCU, including chemical chaperones, cofactors, and proteasome inhibitors. We outline future directions, focusing on high-throughput screening and structure-based drug design to develop CBS-specific PCs. These could stabilize mutant CBS, enhance its stability and restore activity, providing new treatments for HCU and possibly other conditions related to dysregulated CBS, such as cancer or Down's syndrome.