Clustering of mutant mitochondrial DNA copies suggests stem cells are common in human bronchial epithelium

Tissue maintenance stem cells, as opposed to transition and/or terminal cells in the epithelium, are possible progenitor cells for human tumors, but little is known about their frequency in human tissues. It occurred to us that the colonies of mutants that should be created when a stem cell mutates and transmits the rare mutation to its descendent transition and terminal cells should, given a quantitative mutation assay, define the average number of cells in a maintenance turnover unit and permit calculation of stem cell number. To test this concept we used a combination of high fidelity PCR and constant denaturant capillary electrophoresis to enumerate mitochondrial point mutations and define their number and distribution among multiple small samples of approximately one million cells containing about 400 million copies of mitochondrial DNA. The bulk of the data were best explained by a model in which most stem cells, defined here as long-lived cells, give rise to colonies of approximately 8-128 cells. In addition, we found that about 1.5% of colonies contained hundreds or even thousands of homoplasmic mutant cells. These expanded turnover units suggest the bronchial epithelium may contain large clusters of cells with mutations, and possibly phenotypic alterations as well.