Cancer arises from an accumulation of mutations that promote the selection of cells with progressively malignant phenotypes. Previous studies have shown that genomic instability, a hallmark of cancer cells, is a driving force in this process. In the present study, two markers of genomic instability, telomere DNA content and allelic imbalance, were examined in two independent cohorts of mammary carcinomas. Altered telomeres and unbalanced allelic loci were present in both tumors and surrounding histologically normal tissues at distances at least 1 cm from the visible tumor margins. Although the extent of these genetic changes decreases as a function of the distance from the visible tumor margin, unbalanced loci are conserved between the surrounding tissues and the tumors, implying cellular clonal evolution. Our results are in agreement with the concepts of ''field cancerization'' and ''cancer field effect,'' concepts that were previously introduced to describe areas within tissues consisting of histologically normal, yet genetically aberrant, cells that represent fertile grounds for tumorigenesis. The finding that genomic instability occurs in fields of histologically normal tissues surrounding the tumor is of clinical importance, as it has implications for the definition of appropriate tumor margins and the assessment of recurrence risk factors in the context of breast-sparing surgery. ' 2006 Wiley-Liss, Inc.Key words: telomere loss; allelic imbalance; genomic instability; cancer field effect; breast cancer Genomic instability is an important factor in the progression of human cancers. [1][2][3][4] One mechanism that underlies genomic instability is loss of telomere function. [5][6][7] Telomeres are nucleoprotein complexes located at the ends of eukaryotic chromosomes. Telomeres in human somatic cells are composed of 1,000 to 2,000 tandemly repeated copies of the hexanucleotide DNA sequence, TTAGGG. 8 Numerous telomere binding proteins are associated with these repeat regions and are important for telomere maintenance. 9,10 Telomeres stabilize chromosome ends and prevent them from being recognized by the cell as DNA double-strand breaks, thereby preventing degradation and recombination. 11 However, telomeres can be critically shortened, and thereby become dysfunctional, by several mechanisms, including incomplete replication of the lagging strand during DNA synthesis, 12 loss or alterations of telomere-binding proteins involved in telomere maintenance, 13 and oxidative stress leading to DNA damage. 14 Alternatively, telomere loss may be compensated for by recombination 15,16 or, as seen in the majority of human cancers, by the enzyme telomerase. 17,18 Telomeres in tumors are frequently shorter than in the matched adjacent normal tissues, presumably reflecting their extensive replicative histories. [19][20][21] The cause-and-effect relation between dysfunctional telomeres and genomic instability implies that shortened telomeres are also associated with altered gene expression. The latter is a primary source of p...

TC in breast cancer tissue is an independent predictor of clinical outcome and survival interval, and may discriminate by stage.

Decreased TC in prostate tissues obtained by radical prostatectomy predicts prostate cancer recurrence independent of age at diagnosis, Gleason sum and pelvic node involvement. TC in tumor tissue is also associated with TC in NHN prostate tissue. Thus, mechanisms known to generate genomic instability are operative in fields of cells beyond the tumor margins prior to histological changes.

Studies of human mammary epithelial cells from healthy individuals are providing novel insights into how early epigenetic and genetic events affect genomic integrity and fuel carcinogenesis. Key epigenetic changes, such as the hypermethylation of the p16 (INK4a) promoter sequences, create a previously unappreciated preclonal phase of tumorigenesis in which a subpopulation of mammary epithelial cells are positioned for progression to malignancy (Romanov et al. , 2001, Nature , 409:633-637; Tlsty et al. , 2001, J. Mammary Gland Biol. Neoplasia , 6:235-243). These key changes precede the clonal outgrowth of premalignant lesions and occur frequently in healthy, disease-free women. Understanding more about these early events should provide novel molecular candidates for prevention and therapy of breast cancer that target the process instead of the consequences of genomic instability. This review will highlight some of the key alterations that have been studied in human mammary epithelial cells in culture and relate them to events observed in vivo and discussed in accompanying reviews in this volume.