In vivo analysis of telomerase in mammary stem cell self-renewal and maintenance.

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Telomeres are nucleoprotein structures comprised of simple repeat sequences located at the ends of chromosomes. They shorten upon cell division and are restored by a specialized ribonucleoprotein complex called telomerase, which is made up of essential protein and RNA components. It has been proposed that the erosion of telomere length is a limiting factor in replicative capacity and an important player in cell senescence. In human mammary epithelial cell cultures that have achieved replicative immortality, telomerase activity is essential. To determine if this was essential in the mouse mammary gland in vivo, we serially transplanted mammary fragments from TER +/+, TER +/−, and TER −/− mammary tissues. In earlier work, we demonstrated that mammary transplants are clonal dominant populations, supported by stem cells that divide and self renew up to 44 times before reaching senescence. We set up a series of transplants using homozygous, heterozygous, and wild type telomerase fragments and examined them over 6 transplant generations to determine if the rate of reaching growth senescence was adversely affected by the absence of telomerase activity. We observed that individual implants from both homozygous and heterozygous TER null outgrowth showed growth senescence beginning two transplant generations earlier than implants from TER +/+ mammary glands. This result suggests that either mammary epithelial stem cells must maintain their telomere length in order to self renew, or that the absence or reduction of telomerase activity results in more frequent death/extinction of stem cells during asymmetric divisions.