The mitochondrial disease biomarker GDF15 is dynamic, quantifiable in saliva, and correlates with disease severity.

Circulating growth differentiation factor 15 (GDF15) is a biomarker of mitochondrial diseases and aging, but its natural dynamics and response to acute stress in blood and other biofluids have not been well defined. Using extensive samples from MiSBIE participants with rare mitochondrial diseases (MitoD), we examined GDF15 biology in 290 plasma and 860 saliva aliquots from 40 subjects with the m.3243 A > G mutation (n = 25) or with single, large-scale mtDNA deletions (n = 15). Compared to healthy controls, both MitoD groups exhibited significantly elevated blood and saliva GDF15 (p < 0.0001). To examine the origin of GDF15 protein in saliva, we profiled GDF15 expression in 48 tissues from the GTEx dataset and identified high GDF15 expression in salivary gland secretory cells. Despite being chronically elevated in MitoD, saliva GDF15 further increased in response to experimental laboratory mental stress alone (without physical exertion), whereas the stress-induced plasma GDF15 reactivity was blunted in MitoD compared to controls. Using a home-based saliva collection protocol, we show that similar to other stress-related metabolic hormones, saliva GDF15 is highest upon awakening and declines rapidly by 61.2 % within 45 min. Elevated saliva GDF15 levels persisted throughout the day in MitoD. Clinically, saliva GDF15 correlated with neurological symptoms, fatigue, and functional capacity. Importantly, stress-evoked changes in GDF15 did not amplify noisy disease severity associations, but rather consistently increased the effects sizes for GDF15-symptoms connections, pointing to converging psychobiology underlying the responses to mitochondrial OxPhos defects and acute mental stress. These results open the door to exploring saliva GDF15 as a non-invasive monitoring approach for mitochondrial diseases and call for further studies examining the psychobiological processes linking mitochondria, mental stress, and GDF15 dynamics.