By Charles Parker, MD

2008-11-01

Dr. Parker indicated no relevant conflicts of interest.

Gu B-W, Bessler M, Mason PJ. A pathogenic dyskerin mutation impairs proliferation and activates DNA damage response independent of telomere length in mice. Proc Natl Acad Sci USA. 2008;105:10173-8.

Although the importance of the terminal region of the chromosome in maintaining genome integrity had been recognized in the 1930s and 1940s by Hermann Muller (who coined the term telomere) and Barbara McClintock, it was the pioneering work of Elizabeth Blackwell, Jack Szostak, and Carol Greider in the 1970s and 1980s that led both to detailed characterization of the structure of telomeres and to discovery of the enzyme complex (telomerase) that is responsible for maintaining telomeric structure. Basic and medical research focused on telomerase and on the properties of telomeres has remained vibrant, and the achievements of Drs. Blackwell, Szostak, and Greider were recognized with the 2006 Albert Lasker Award for Basic Medical Research.

Because DNA polymerase can function only in the 5'-to-3' direction, the antiparallel structure of the two strands of duplex DNA (one strand being oriented 5'-to-3' and the other 3'-to-5') poses a problem for replication. Nature has largely solved this problem by using a series of RNA templates that anneal to the parental DNA (called the lagging strand) that is oriented 5'-to-3' with respect to the direction of the replication fork. These RNA templates serve as primers for DNA polymerase, and, consequently, the lagging strand is replicated discontinuously and backward with respect to the replication fork. After the RNA primers are removed, DNA polymerase fills in the resulting gaps, and replication is complete except for the sequence to which the most 3' RNA primer was annealed. Left unresolved, this end-replication problem would result in continuous shortening of the 3'-end of the chromosome with each cell division. Chromosomes with truncated ends are unstable and are subject to recombination, end-to-end fusion, and recognition as damaged DNA. This latter process activates the ATM-p53 DNA damage pathway, leading to cell cycle arrest or cell death. The end-replication problem is solved by telomerase, an enzyme complex that uses an RNA template (TERC) and reverse transcriptase (TERT) to elongate the 3'-end of telomeric DNA and thereby seal the ends of chromosome so as to maintain structural integrity. The telomerase complex is made up of components in addition to TERC and TERT including dyskerin, a pseudouridine synthase that is a component of a box H/ACA ribonuclearprotein particle required for stability of the telomerase complex.

Bone marrow failure is a common clinical feature of patients with mutations in telomerase components and mutations in dyskerin, and in some domains of TERC cause the syndrome of dyskeratosis congenita (see Table).¹ As anticipated, short telomeres are characteristic of all patients with mutations in dyskerin, TERC, and TERT, but whether mutations in telomerase components affect cell viability independent of telomere shortening has been speculative. Now, through a series of rigorous genetic experiments in mice, Gu and colleagues have shown that mutant dyskerin induces the ATM-p53 pathway of DNA damage recognition in a telomerase-dependent process that is independent of telomere length. Laboratory mice are particularly good models for studying the effects of mutations in telomerase components that occur independent of telomere length because they have long telomeres that, even in the absence of telomerase activity, do not become critically short for several generations. The mouse model developed by Gu, et al. carried a dyskerin (Dkc) deletion mutation similar to one identified in a family with X-linked dyskeratosis congenita. Male mice hemizygous for mutant Dkc showed no signs of bone marrow failure, nail dystrophy, skin pigmentation problems, or other stigmata of human dyskeratosis congenita. Due to X-chromosome inactivation, females have only one functional X-chromosome in somatic tissues, and, therefore, females that are heterozygous for the mutant Dkc gene are mosaics with cells expressing either wild-type Dkc or mutant Dkc. Gu and colleagues took advantage of this mosaicism to demonstrate that cells with mutant Dkc have a proliferative disadvantage that was more apparent in cells from organs with a higher growth rate (spleen, thymus, and bone marrow) compared to those with lower cell turnover (brain and liver) and was dependent on telomerase integrity but not on telomere length.

Further studies showed that Dkc mutant cells have enhanced DNA damage response mediated by the ATM-p53 pathway with damage foci localized to telomeric ends. Together, these experiments demonstrate that mutations affecting the telomerase complex can induce DNA damage independent of telomere length, although telomere shortening appears to be required for full manifestation of the clinical phenotype (see Table).

OK, so no more jokes about guys with short telomeres.

References

1. Yamaguchi H, Calado RT, Ly H, et al. Mutations in TERT, the gene for telomerase reverse transcriptase, in aplastic anemia. N Engl J Med. 2005;352:1413-24.

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