Potential Pharmacological Chaperones for Cystathionine Beta-Synthase-Deficient Homocystinuria

Abstract

Classical homocystinuria (HCU) is the most common loss-of-function inborn error of sulfur amino acid metabolism. HCU is caused by a deficiency in enzymatic degradation of homocysteine, a toxic intermediate of methionine transformation to cysteine, chiefly due to missense mutations in the cystathionine beta-synthase (CBS) gene. As with many other inherited disorders, the pathogenic mutations do not target key catalytic residues, but rather introduce structural perturbations leading to an enhanced tendency of the mutant CBS to misfold and either to form nonfunctional aggregates or to undergo proteasome-dependent degradation. Correction of CBS misfolding would represent an alternative therapeutic approach for HCU. In this review, we summarize the complex nature of CBS, its multi-domain architecture, the interplay between the three cofactors required for CBS function [heme, pyridoxal-5'-phosphate (PLP), and S-adenosylmethionine (SAM)], as well as the intricate allosteric regulatory mechanism only recently understood, thanks to advances in CBS crystallography. While roughly half of the patients respond to treatment with a PLP precursor pyridoxine, many studies suggested usefulness of small chemicals, such as chemical and pharmacological chaperones or proteasome inhibitors, rescuing mutant CBS activity in cellular and animal models of HCU. Non-specific chemical chaperones and proteasome inhibitors assist in mutant CBS folding process and/or prevent its rapid degradation, thus resulting in increased steady-state levels of the enzyme and CBS activity. Recent interest in the field and available structural information will hopefully yield CBS-specific compounds, by using high-throughput screening and computational modeling of novel ligands, improving folding, stability, and activity of CBS mutants.

A proactive genotype-to-patient-phenotype map for cystathionine beta-synthase

“…The well-documented clinical responsiveness to vitamin B6 has not yet been fully elucidated mechanistically 13,40 . To better understand the mechanistic underpinnings of vitamin B6 remediability of human CBS variants in the yeast model, we examined six secondary structure features, finding that positions annotated with beta-strand secondary structure tended to have lower delta scores, a trend that was modest but significant (Wilcoxon test; p = 0.0036; Supplementary Fig.…”

“…This reaction eliminates the toxic metabolite homocysteine and, by using the alternative substrate cysteine, also produces hydrogen sulfide, a gaseous signaling molecule 11,12 . CBS forms tetramers and uses both heme and pyridoxal 5′-phosphate (PLP; the active form of vitamin B6) as cofactors, and S-adenosylmethionine (AdoMet) as an allosteric activator 13 .…”

The Spectrum of Mutations of Homocystinuria in the MENA Region

“…For the last three decades, CBS inactivating mutations have been extensively studied in the context of causing homocystinuria [16]. Overall, homocystinuria caused by CBS deficiency is considered a relatively rare disease with an incidence rate varying from one in every 200,000 to 335,000 live births.…”

Assessing computational predictions of the phenotypic effect of cystathionine‐beta‐synthase variants

“…More than 160 different disease‐associated variants have been identified in the CBS gene (http://cbs.lf1.cuni.cz/index.php). The majority of these are substitutions that do not involve catalytic residues, suggesting that their effect resides in structural or conformational perturbations leading to a misfolded protein (Majtan et al, ). About one‐half of homocystinuric patients respond to high doses of pyridoxine, the soluble form of PLP (Mudd et al, ) and several mutations are clearly pyridoxine remediable (B6‐responsive homocystinuria): p.A114V, p.R266K, p.R369H, p.K384E, p.L539S, and the most common substitution p.I278T, which accounts for ~20% of all homocystinuric alleles (Dimster‐Denk, Tripp, Marini, Marqusee, & Rine, ; Moat et al, ; Skovby, Gaustadnes, & Mudd, ).…”

“…The majority of these are substitutions that do not involve catalytic residues, suggesting that their effect resides in structural or conformational perturbations leading to a misfolded protein (Majtan et al, 2018).…”