DNA QUANTIFICATION TECHNIQUE AS A INTERPRETATION TOOL IN MITOCHONDRIAL DISEASES

2622/5000 Mitochondrial diseases (MM) are the most common metabolic diseases. Since these pathologies are very heterogeneous in clinical terms, only the identification of mutations in nuclear genes or mitochondrial DNA confirms the diagnosis. The full-scale study of mtDNA by high-throughput sequencing (NGS) is a first step in the diagnostic approach. The recent introduction of this revolutionary new technology has greatly increased the efficiency of mutation identification. However, in addition to known pathogenic mutations, NGS reveals numerous variants whose significance is currently unknown. A major challenge to obtain a reliable diagnosis is therefore the interpretation of the clinical impact of these new rare variants which proves to be very difficult. Pathogenicity criteria allow the classification of variants from benign to pathogenic. One of the major pathogenicity criteria is a good correlation of heteroplasmic level with tissue or cellular involvement. Indeed, mtDNA mutations are generally heteroplasmic, which corresponds to the coexistence of normal and mutated molecules in the same cell or tissue, the most affected tissues having a high rate of mutation. On a muscle biopsy of an affected patient, the fibers often present an enzyme deficiency in cytochrome c oxidase (COX-negative) which can be demonstrated in immunohistochemistry. The single fiber study allows to isolate the deficient fibers and to quantify the heteroplasmic rate of a variant. The presence of a high level of heteroplasm in the COX-negative fibers, unlike fibers without deficit, is a strong argument in favor of the pathogenicity of this variant. Currently, this technique is not used routinely in diagnostic laboratories but only occasionally in a research framework in some laboratories. It is a heavy technique that consists of a first stage of laser microdissection of the various muscle fibers followed by a second step of quantification of the variant from each fiber. This second step req