Telomere and Telomerase for the Regulation of Stem Cells

Telomeres, guanine-rich tandem DNA repeats of chromosomal ends, provide chromosomal stability, and cellular replication causes their loss. In somatic cells, the activity of telomerase, a reverse transcriptase that can elongate telomeric repeats, is usually diminished after birth so that the telomere(definition) length is gradually shortened with cell divisions and triggers cellular senescence(definition). In embryonic stem cells, telomerase is activated and maintains telomere length and cellular immortality. On the other hand, in adult stem cells, the level of telomerase activity is low and insufficient to maintain telomere length. Thus, even in stem cells, except for embryonic stem cells and cancer stem cells, telomere shortening occurs during replicative aging, possibly at a slower rate than that in normal somatic cells. In the past few years, the importance of telomere maintenance in human stem cells has been highlighted by the studies on dyskeratosis congenita, aplastic anemia, and idiopathic pulmonary fibrosis, a part of which are genetic disorders in the human telomerase component and are characterized by premature loss of tissue regeneration with stem cell dysfunction. The regulation of telomere length and telomerase activity is a complex and dynamic process that is tightly linked to cell cycle regulation in human stem cells. Here we review the role of telomeres and telomerase in human stem cells.