Length mutations in human mitochondrial DNA: direct sequencing of enzymatically amplified DNA.

A specific segment of mitochondrial DNA from 18 people was examined by two methods of direct DNA sequencing. This segment includes a small noncoding region (V) shown before by restriction analysis to exhibit length polymorphism. All 11 of the human mtDNAs previously reported to have a deletion in this region proved to lack one of the two adjacent copies of a 9-base-pair sequence normally present in human mtDNAs. Phylogenetic analysis suggests that this deletion occurred only once during the evolution of modern types of human mtDNA and that it will be a valuable anthropological marker for peoples of East Asian origin. The one human mtDNA reported to have an addition in region V differs from the wild type by two mutations in the first copy of the 9-base-pair sequence: one transition and an addition of four cytosines, thereby producing a run of 11 cytosines. One of the direct DNA sequencing methods uses a single oligonucleotide primer to facilitate dideoxy sequencing from purified mtDNA templates. The second, more successful, method first amplifies this mtDNA segment enzymatically with two flanking primers (the "polymerase chain reaction") and then uses a third primer for DNA sequencing. This latter method, which works on the DNA extracted from small amounts of blood as well as on purified mtDNA, is shown to be a rapid means of defining sequence variants without purifying and cloning the same DNA segment from many individuals.