Developmental regulation of telomerase activity in human fetal tissues during gestation.

Telomerase is a ribonucleoprotein that adds hexanucleotide repeats (telomeres) to the ends of linear chromosomes, compensating for the loss of telomeric DNA which occurs with DNA replication. In humans, telomerase has been previously detected in germ-line tissues, blastocysts, 16-20 week old fetal tissue, and most cancers, but not in mature sperm or ova, or in most normal somatic tissues. It has been hypothesized that telomerase is suppressed during somatic development and reactivated in malignancy. To test the hypothesis that telomerase is suppressed during somatic development, human fetal tissues of 8-21 weeks gestational age were assayed for telomerase activity. All tissues expressed telomerase at the earliest ages examined. Lung, liver, spleen, and testis maintained telomerase activity through the latest age assayed, namely 21 weeks. Brain and kidney telomerase activity was present up to the 16th week and was undetectable thereafter. Heart tissue did not display activity beyond the 12th week. Lysates of heart, brain, and kidney without telomerase activity did not inhibit the activity of known telomerase-positive cells, suggesting that suppression of telomerase activity during gestational development is due to a lack of active telomerase rather than to the presence of an inhibitor. These findings demonstrate tissue-specific and developmental regulation of telomerase in the human fetus, suggesting an important role for this ribonucleoprotein in human fetal tissue differentiation and development.