<p>LRRK2-G2019S is the most common pathogenic LRRK2 mutation which accounts for up to 13% of cases of familial Parkinson’s disease. The LRRK2-G2019S mutation has incomplete penetrance which increases with age. Molecular mechanisms which contribute to the disease status in LRRK2-G2019S mutation carriers are yet to be fully defined. Here, we aimed to further investigate the specific mitochondrial effects of LRRK2-G2019S penetrance in a cohort of patient-derived fibroblasts from manifesting and non-manifesting LRRK2-G2019S carriers compared to controls to further elucidate the pathogenic mechanism of the mutation. We find a significant reduction of 50% in the expression of the complex IV assembly factor SCO2 in LRRK2-G2019S manifesting fibroblasts. In contrast, SCO2 levels remained similar to controls in non-manifesting LRRK2-G2019S carriers. A small reduction in complex IV subunit expression accompanied this reduction in SCO2 in manifesting LRRK2-G2019S carriers. Despite the role of SCO2 in copper incorporation into complex IV, we identified no differences in the unbound mitochondrial copper content in a limited number of manifesting or non-manifesting LRRK2-G2019S carriers compared to controls. However, LRRK2-G2019S carriers exhibit variable cellular phenotypes in mitochondrial morphology, mitochondrial membrane potential and cellular ATP or ROS production which does not differ significantly between manifesting and non-manifesting carriers. We conclude that mitochondrial complex IV deficiency could be a pathogenic mechanism of the LRRK2-G2019S mutation which may be attributed to a reduction in SCO2, however there is evident heterogeneity in the cellular phenotype of LRRK2-G2019S carriers which may suggest underlying compensatory mechanisms. </p>