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Fibroblast growth factor 21, a biomarker for mitochondrial muscle disease

For many patients, a mitochondrial disease diagnosis is often difficult to establish with certainty, except in cases with a classical clinical presentation. Invasive and time-consuming methods, such as tissue biopsy and enzymatic studies, are frequently necessary before proceeding to molecular genetic analyses, because of the low specificity and sensitivity of existing less-invasive biomarkers, such as plasma or CSF lactate, pyruvic acid, or urinary organic acids. Moreover, histologic and biochemical investigation of muscle tissue is often nondiagnostic or may show secondary mitochondrial dysfunction due to aging or medication (e.g., HIV antiviral therapy), even in patients without mitochondrial disease. The introduction of next-generation DNA sequencing with the capability of rapid and simultaneous sequencing of mitochondrial and nuclear encoded genes further reiterates the necessity for a complementary assay with reasonable sensitivity and specificity in order to bypass current invasive testing, such as muscle biopsies.