Alan K. Meeker, Ph.D.
Primary Appointment in Pathology; Secondary Appointment in Urology
Telomere Shortening in Cancer
The genesis and progression of cancer is thought to depend upon genetic instability. This is clearly seen at the chromosomal level in epithelial cancers, such as prostate and breast cancers, which show chromosomal instability as reflected by aberrations in both chromosomal number and structure, yet the molecular mechanisms responsible for chromosome destabilization during carcinogenesis and progression are largely unknown. Our lab is examining defective telomeres which appear to be an important route to chromosomal instability. Telomeres are specialized chromosomal structures consisting of terminal DNA tracts and associated binding proteins. Telomeres protect chromosome ends from fusing with other chromosome ends or other chromosomes containing DNA double strand breaks. In the absence of the telomere synthetic enzyme telomerase, telomeric DNA is subject to loss during cell division. Results from telomerase knockout mouse models indicate that telomere shortening can lead to an increased incidence of cancer in these animals, thus apparently playing a role in cancer initiation.
In close collaboration with Angelo De Marzo’s lab in the John Hopkins Pathology Department, we developed a novel quantitative fluorescence microscopy technique to measure telomere lengths directly in archival tissues, in order to find out if telomere shortening is playing a role in the initiation of human cancers. Making use of this technique in collaborative efforts with other faculty members in the Johns Hopkins Department of Pathology (Drs. Pedram Argani, Theresa Chan, Christine Iacobuzio-Donahue, Elizabeth Montgomery, Bridgette Ronnett and William Westra) we found that, indeed, telomeres are abnormally short in the vast majority of microscopic cancer precursor lesions in common epithelial cancers, including those of the bladder, breast, cervix, colon, esophagus, gall bladder, oral cavity and prostate.
Telomere length abnormalities appear to be one of the earliest and most prevalent molecular genetic alterations acquired in the multi-step process of tumorigenesis. These findings support a model whereby telomere dysfunction induces a mutator phenotype that acts at the chromosomal and sub-chromosomal levels to accelerate tumor development. We hypothesize that pre-malignant lesions displaying short telomeres are poised on the edge of genetic instability, and are therefore at risk of progressing on to fully invasive carcinomas.
Practically speaking, telomere shortening may have utility in cancer diagnosis, as well as an intermediate endpoint marker in chemoprevention studies. In addition, if telomere shortening is indeed playing a causal role in cancer initiation, then it represents a valid prevention target in its own right.
Telomere Shortening in Aging
Cells normally respond to short telomeres by halting their cell division activities or by committing cellular suicide (apoptosis). It is thought that these responses evolved in long-lived multicellular organisms to prevent the outgrowth of potentially cancerous cell populations. It has been postulated that a loss of division potential due to telomere shortening in proliferating tissues might contribute to certain age-related pathologies, such as the decreased wound healing seen in the elderly. This theory has been difficult to evaluate, as previous methods of telomere length measurement only gave information on the average telomere length of a large number of cells combined.
We believe that our telomere length assay, which features single cell resolution, can be used to test the hypothesized link between telomere shortening and human aging.
Meeker, A.K., Gage, W.R., Simon, I., Coffman, J.R., Platz, E.A., March, G., and DeMarzo, A.M. Telomere Length Assessment in Human Archival Tissues: Combined Telomere Fluorescent in Situ Hybridization and Immunostaining. American Journal of Pathology. 160:1259-1268, 2002.
Meeker, A.K., Hicks, J.L., Platz, E.A., March, G.E., Bennett, C.J., and De Marzo, A.M. Telomere Shortening is an Early Somatic DNA Alteration in Human Prostate Tumorigenesis. Cancer Research. 62:6405-6409, 2002.
Montgomery, E.A., Argani, P., Hicks, J.L., DeMarzo, A.M., and Meeker, A.K. Telomere Lengths of Translocation Associated and Non-Translocation Associated Sarcomas Differ Dramatically. American Journal of Pathology. 164:1523-1529, 2004.
Meeker, A.K., Hicks, J.L., Iacobuzio-Donahue, C.A., Montgomery, E.A., Westra, W.H., Chan, T.Y., Ronnett, B.M., and DeMarzo, A.M. Telomere Length Abnormalities Occur Early in the Initiation of Epithelial Carcinogenesis. Clinical Cancer Research. 10:3317-3326, 2004.
Meeker, A.K., Hicks, J.L., Gabrielson, E., Strauss, W.M., De Marzo, A.M., and Argani, P. Telomere Shortening Occurs in Subsets of Normal Breast Epithelium as well as In Situ and Invasive Carcinoma. American Journal of Pathology. 164:925-935, 2004.