2/ For the uninitiated, the 37 genes in mitochondrial genome encode proteins and RNAs necessary for the electron transport chain to generate cells' ATP. mtDNA mutations are well-known to cause potent disease (e.g. MELAS), but poorly understood in cancer.
3/ Why? chrM usually isn't targeted when sequencing tumors, so progress was slow until large cohorts of whole-genome sequenced tumors became possible, and even with PCAWG, sample size is still limited for finding recurrent mutations and phenotypic associations.