POLG and heart disorder: Non-neurological complications may also be present and prominent in some genotypes, such as severe and potentially life-threatening gastrointestinal disease (e.g. TYMP, POLG, RRM2B and MGME1 variants) [90,244,246,247,268–272], liver disease (e.g. POLG, MPV17, DGUOK and TFAM variants) [16,232,244,246–249,267,276–278] and cardiac disease (e.g. AGK, SLC25A4 and TOP3A variants) [179,287,290–292].