Little is known about the genes that regulate telomere length diversity between mammalian species. A candidate gene locus was previously mapped to a region on distal mouse Chr 2q. Within this region, we identified a gene similar to the dog-1 DNA helicase-like gene in C. elegans. We cloned this Regulator of telomere length (Rtel) gene and inactivated its expression in mice. Rtel(-/-) mice died between days 10 and 11.5 of gestation with defects in the nervous system, heart, vasculature, and extraembryonic tissues. Rtel(-/-) embryonic stem cells showed telomere loss and displayed many chromosome breaks and fusions upon differentiation in vitro. Crosses of Rtel(+/-) mice with Mus spretus showed that Rtel from the Mus musculus parent is required for telomere elongation of M. spretus chromosomes in F1 cells. We conclude that Rtel is an essential gene that regulates telomere length and prevents genetic instability.

Progressive telomere shortening with aging was studied in the normal liver tissue of 94 human subjects aged between 0 and 101 years old to determine the rate of telomere loss in 1 year. Telomere length demonstrated accelerated shortening with reduction of 55 base pairs (bp) per year. The mean telomere length in five neonates was 12.9 +/- 2.6 kilobase pairs (kbp), and that in one centenarian was 8.3 kbp. Mean telomere lengths by age group were 13.2 +/- 2.0 kbp (< or = 8 years; 10 subjects), 7.8 +/- 1.9 kbp (40-79 years; 29 subjects), and 7.5 +/- 2.0 kbp (> or = 80 years; 53 subjects), with reduction thus appearing to show slowing on the attainment of middle age. The difference of mean telomere lengths for two groups with or without advanced malignancies of other than liver origin was not significant in the older two groups. Despite the slow turnover of liver tissue, the overall reduction rate of telomere length decrease in 1 year was almost the same as that of digestive tract mucosa, with its very rapid renewal.

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