Age of diabetes onset in the mutant proinsulin syndrome correlates with mutational impairment of protein foldability and stability

Abstract

Diverse heterozygous mutations in the human insulin gene cause a monogenic diabetes mellitus (DM) syndrome due to toxic misfolding of the variant proinsulin. Whereas mutations that add or remove cysteines (thereby leading to an odd number of thiol groups) generally lead to neonatal-onset DM, non-Cys-related mutations can be associated with a broad range of ages of onset. Here, we compare two mutations at a conserved position in the central B-chain α-helix: one neonatal in DM onset (ValB18→Gly) and the other with onset delayed until adolescence (AlaB18). The substitutions were introduced within a 49-residue single-chain insulin precursor optimized for folding efficiency (Zaykov, A., et al. ACS Chem. Biol. 9, 683-91 (2014)). Although mutations are each unfavorable, GlyB18 (a) more markedly perturbs DesDi folding efficiency in vitro than does AlaB18 and (b) more severely induces endoplasmic reticulum (ER) stress in cell-based studies of the respective proinsulin variants. In corresponding two-chain hormone analogs, GlyB18 more markedly perturbs structure, function and thermodynamic stability than does AlaB18. Indeed, the GlyB18-insulin analog forms a molten globule with attenuated α-helix content whereas the AlaA18 analog retains a nativelike cooperative structure with reduced free energy of unfolding (ΔΔGu 1.2(±0.2) kcal/mole relative to ValB18 parent). We propose that mutations at B18 variably impede nascent pairing of CysB19 and CysA20 to an extent correlated with perturbed core packing once native disulfide pairing is achieved. Differences in age of disease onset (neonatal or adolescent) reflect relative biophysical perturbations (severe or mild) of an obligatory on-pathway protein folding intermediate.