Clinical utility of the ATP hydrolysis assay for the diagnosis of complex V deficiency in cultured skin fibroblasts.

Complex V catalyzes the formation of ATP from ADP and Pi through the dissipation of the proton gradient generated during the process of oxidative phosphorylation. Most complex V genetic disorders are caused by missense mutations in the mtDNA-encoded subunits, a (MT-ATP6) and A6L (MT-ATP8). Nuclear DNA-encoded gene mutations are increasingly recognized as causes of complex V defects and exhibited both autosomal recessive and autosomal dominant inheritance. Most identified variants are novel and confirmation by functional assays is important especially for variants demonstrating autosomal dominant inheritance. A kinetic spectrophotometric assay of the Complex V enzymatic hydrolysis activity has been reported. Here we report the clinical utility of this assay for the diagnosis of complex V deficiency after optimization and validation for the diagnosis of isolated complex V disorders due to both nuclear and mitochondrial DNA encoded variants and also for use in combined respiratory chain deficiencies. This assay was able to identify all nuclear DNA-encoded complex V deficiencies, whereas a decrease in complex V activity was observed in some patient cell lines with combined deficiencies. However, this assay only identified 50% of the mitochondrial DNA-encoded complex V deficiencies due to pathogenic variants in MT-ATP6/8. In conclusion, the enzymatic assay of complex V has best clinical utility for nuclear DNA-encoded complex V defects.