Mitochondrial DNA-related disorders
Mitochondrial disorders were first genetically characterized by identifying deletions and point mutations within mithochondrial DNA (mtDNA).
Before a cell expresses an abnormal phenotype, the proportion of mtDNA pathogenic variants must exceed a critical threshold level. Additionally, there might also be different tissue segregation patterns of pathogenic mtDNA variants and mtDNA exists in multiple copies per cell. For these reasons, clinical phenotypes of mtDNA disorders are quite different from one patient to another.
Mitochondrial DNA disorders have been classificed in three subgroups, encompassing disorders associated with protein-coding genes, genes encoding mitochondrial tRNAs and rRNAs, and disorders associated with single large-scale mtDNA deletions.