This is a paper on the disorder Charlie Gard has.

ncbi.nlm.nih.gov

As I understand it (I only have a high school education), mitochondrial DNA is created by salvaging what the mitochondria have available rather than the normal replication of DNA in the cell nucleus. The genetic disorder prevents one or more enzymes required for salvaging the materials in mitochondria necessary to create mitochondrial DNA, resulting in gradual failure of cells.

There are several different versions of the disorder, and some people can live well past twenty years old. The version the infant in the UK has will rapidly kill a child. If I read the report from NIH correctly, fewer than two hundred people have ever been diagnosed with any form of the disorder.