Understanding COMMD1 function: Novel perspectives from in vitro and in vivo studies

Abstract

The transition metal copper is essential for all aerobic organisms as it is required as cofactor in a diversity of cellular processes, including mitochondrial respiration, pigmentation, and iron homeostasis. Controversially, copper can be extremely toxic due to the formation of reactive oxygen species. Therefore, a homeostatic control of intracellular copper concentrations is needed, regulated by processes mediating copper import, distribution, storage and excretion. When one of these processes is distorted, copper-related diseases, characterized by either copper deficiency or accumulation, can develop. However, the exact mechanism of cellular copper excretion is at present incompletely understood. This is illustrated by a range of copper storage disorders with unknown etiology, in which patients are highly sensitive to hepatic copper accumulation when the dietary copper intake is enhanced. We focused primarily on the copper overload disorder copper toxicosis. In dogs, an exon 2 deletion of the COMMD1 gene is associated with this disease. In this thesis, we aimed to provide more insight into the role of COMMD1 in copper homeostasis. Since Commd1 knockout mice are embryonic lethal, and thus an appropriate genetic mouse model to study the role of Commd1 in copper toxicosis was lacking, we generated a hepatocyte-specific Commd1-deficient mouse model. Similar to a diversity of human copper overload disorders, Commd1 deficiency only resulted in a progressive copper accumulation (10- to 20-fold relative to controls) when the mice were fed a copper-rich diet. This suggests that Commd1 is a rate-limiting factor in hepatic copper excretion and genetically underlines the importance of Commd1 in normal hepatic copper excretion. Using a biochemical screen to identify novel protein interactions of COMMD1, we established an interaction between COMMD1 and ATP7A. ATP7A is a copper transporting P-type ATPase involved in copper transport along the gastro-intestinal tract and across the blood-brain barrier. Mutations in ATP7A are associated with the copper deficiency disorder of Menkes disease. COMMD1 was demonstrated to partially restore the impaired copper transporting function of ATP7A mutant proteins, and thus might be considered as a potential modifier of Menkes disease pathogenesis. In this same biochemical screen, we also identified an interaction between COMMD1 and the antioxidant SOD1. Mutations in SOD1 are associated with the neurodegenerative disorder Amyotrophic Lateral Sclerosis (ALS). COMMD1 was shown to impair the SOD1 scavenging function as result of abrogated SOD1 maturation. COMMD1 interfered with the process of SOD1 homodimerization resulting in enhanced cytotoxicity. Subsequently, we examined the effect of COMMD1 on ALS pathogenesis. A pathophysiological hallmark of ALS is the formation of large protein aggregates in the spinal cords of patients, leading to motoneuron damage. Strikingly, binding of COMMD1 to SOD1 mutants resulted in induced aggregate formation of mutant SOD1 proteins. This was accompanied by a marked increase in cellular damage. These findings therefore might suggest an important function for COMMD1 in ALS disease progression. In conclusion, the findings presented in this thesis contribute significantly to our understanding of COMMD1 function. Further, the Commd1-deficient mouse model will be of high value for future studies addressing the role of COMMD1 in copper homeostasis and other cellular processes.