This discrepancy is what makes the findings of Trapp et al. in this issue (p. 1307) so important to the study of telomerase and cancer (12
). The authors set out to address whether the vTR genes in MDV are important in malignant transformation by the virus. The two vTR genes are 88% identical to the chicken TR gene (cTR) (4
). The vTR gene, when expressed in mouse cells deficient in mouse TR, reconstituted telomerase activity, indicating that vTR is capable of supporting telomerase enzymatic activity. Trapp et al. deleted both copies of vTR in an oncogenic strain of MDV and found that lymphomagenesis in infected chickens was reduced by 60% in the vTR deletion strains compared with wild-type virus. Furthermore, when animals infected with the vTR deletion strains did develop lymphoma, these cancers were smaller in size and less widely disseminated. Importantly, deletion of vTR did not impair lytic replication of the virus, indicating that vTR serves a role supporting lymphomagenesis, rather than viral replication (12
). Thus, the vTR gene exhibits attributes of an oncogene, enhancing the incidence and severity of lymphoma caused by MDV.
These important findings raise a series of new questions centered on the problem of how and why expression of vTR in MDV is oncogenic. The most straightforward interpretation is that vTR enhances telomerase activity leading to stabilization of telomeres. According to this model, a subset of telomeres in lymphoid cells infected with MDV must become critically short. In the absence of vTR, telomere uncapping in these lymphocytes would reduce tumor formation, whereas in wild-type MDV strains that express vTR, lengthening of these short telomeres would allow full malignant potential. Consistent with this idea, vTR was shown to yield increased telomerase activity compared with cTR, when combined in vitro with recombinant chicken TERT protein (13
). This model implies that in the target lymphocyte population cTR levels are limiting and TERT protein is in excess, such that telomerase activity rises when vTR is expressed from the virus. Alternatively, another viral gene may lead to stimulation of endogenous TERT levels enabling increased telomerase when coupled with expression of vTR.
Another distinct possibility is that vTR acts through a mechanism that is independent of telomere synthesis. Emerging evidence from several laboratories has indicated that telomerase has additional roles independent of telomere length. Transgenic expression of TERT in mice led to an increased number of carcinogen-induced skin papillomas (14
) and to an elevated incidence of spontaneous breast cancers (15
). Since mouse telomeres are sufficiently long that telomere uncapping does not occur in mouse tissues, these prooncogenic effects of TERT are not thought to require telomere elongation. In human cells that maintain their telomeres through a telomerase-independent mechanism (known as alternative lengthening of telomeres [ALT]), expression of TERT was necessary for malignant transformation and growth of tumors in immunocompromised mice (16
). This effect of TERT in ALT cells appears to extend to TR as well, as overexpression of mouse TR was necessary to enable mouse ALT cells from TR−/−
mice to grow efficiently as metastatic nodules in lung (17
). These results are reminiscent of the current findings in MDV.
In addition to these cancer-causing activities, telomerase has recently been found to exhibit profound effects on stem cells in mouse skin (18
). Conditional expression of TERT in mouse skin led to activation of quiescent stem cells in the hair follicle and a rapid developmental change in the follicle from the resting phase (telogen) to the active phase of the hair follicle cycle (anagen) (18
). Induction of anagen by TERT facilitated robust hair growth. These effects of TERT did not require TR and were therefore genetically separable from TERT's well-understood role in elongating telomeres. Could TR also exhibit activities independent of its role in serving as a template for telomere addition that might explain its transforming activity in MDV? Recent loss-of-function data in human cancer cells lines support this idea. Depletion of human TR (hTR) through RNA interference in human cancer cell lines reduced the rate of cell proliferation. This effect of hTR depletion was not caused by telomere uncapping, as there was no evidence of telomere dysfunction, such as an increase in DNA damage foci (20
). Instead, cells treated with hTR siRNA showed a marked change in gene expression profiles that may explain the effect of hTR depletion. Thus, the deleterious, telomere length-independent effect of hTR loss on cancer cell proliferation could represent the flip side of the prooncogenic effects of vTR overexpression in MDV.