Failed Metabolic Adaptation to Stress Underlies Pathogenesis in a Heterozygous Genetic Disorder
Disorders linked to heterozygous variants occupy a continuum in terms of the timing and severity of phenotypic emergence. An important question regarding this variability entails the effect stress has on the residual protein function. Using Darier disease (DD), caused by heterozygous variants of the SERCA2 calcium pump, as a model, we uncovered a potential connection between extrinsic stress and pathogenesis. The skin lesions characteristic of DD entail loss of intercellular adhesion and rarely appear pre-adolescence, suggesting that factors beyond heterozygosity contribute to disease pathogenesis. Testing whether age-related stressors contribute to DD, we show that DD patient-derived keratinocytes subjected to stress yield twice the reactive oxygen species of controls, accompanied by greater disruption of intercellular adhesion. Metabolic analysis of DD cells revealed perturbation of the pentose phosphate pathway (PPP), a stress response system responsible for regenerating antioxidants like glutathione. At baseline, DD cells had less free glutathione but an increase in protective glutathione-based modifications of SERCA2, a reversible form of protein oxidation. With stress, DD cells form an aberrant, heavily glutathionylated perinuclear halo consisting of keratin and the intercellular adhesion component, desmoplakin. We propose a model whereby SERCA2 heterozygosity causes mild oxidative stress that under homeostatic conditions can be buffered by glutathionylation. When stressed, the depleted glutathione store is shunted towards the desmoplakin-intermediate filament system at the expense of SERCA2, rendering it vulnerable to damage. A lesional flare, then, would represent a case of more complete SERCA2 inhibition and a novel example of how heterozygous disorders interact with stress to disrupt intercellular adhesion.