We examined whether excessive telomere shortening may lead to breast cancer by investigating the relationship between relative telomere length (RTL) and breast cancer risk in a large breast cancer case-control study nested within the prospective Nurses’ Health Study. We found a non-significant 25% elevation in postmenopausal breast cancer risk among women in the shortest quartile of RTL compared to women in the longest quartile. This is somewhat comparable to a case-control study of high-risk sister sets, which reported a non-significant 34% increased risk of postmenopausal breast cancer among women in the shortest versus longest quartile of relative telomere. However, the authors did not observe a consistent trend. Instead the association of telomere length with breast cancer risk, though still not statistically significant, appeared predominantly in premenopausal women (9
). All women in our study were postmenopausal at diagnosis, limiting the generalizability of our results to postmenopausal women.
Telomere length did not differ between 123 untreated newly diagnosed breast cancer cases and 108 age- and ethnicity-matched controls in a small UK study (11
), whereas a Swedish case-control study observed unexpected associations with increased breast cancer risk and decreased survival associated with longer RTL (10
). The potential for systematic measurement bias of RTL is of some concern in the Swedish study as DNA was isolated from granulocytes in 1/3 of control subjects, while buffy coat DNA was isolated from cases and the remainder of controls. Within the same individual, granulocytes may have telomeres as much as ~2–3 kb shorter than leukocyte telomeres up until age 60 (16
). As the majority of controls in the Swedish study were less than 60 years of age, the inclusion of short telomere measurements from granulocytes may have created a spurious positive association between RTL and breast cancer risk.
We also assessed the relationship between RTL and several known breast cancer risk factors. Other than the statistically significant inverse association with age at blood draw (Ptrend
<0.01), most of the established breast cancer risk factors were not significantly associated with RTL. Inverse associations between RTL and estradiol and estrone hormone levels reached statistical significance. These associations appeared stronger in cases than in controls, though no significant heterogeneity was observed by disease status (p ≥ 0.23). When cases diagnosed within the first 2 years of follow-up were excluded, we no longer observed inverse trends with RTL (data not shown) suggesting a potential effect of underlying disease. Greater local concentration of estrogens in cancerous versus normal breast tissue (17
) could have subtly increased peripheral plasma concentrations resulting in the observed associations. One study demonstrated that estradiol upregulates telomerase expression in vitro
), but how this translates to the in vivo
relationship between telomere length and plasma hormone levels among postmenopausal women has not been explored.
Besides being the largest study to date, our analyses benefit from the nested case-control design. In addition to drawing cases and controls from a well-characterized relatively homogeneous population limiting selection bias, the collection of blood specimens occurred prior to breast cancer diagnosis. This reduces the potential of generating invalid risk estimates due to the limitations of retrospective case-control studies, such as the cancer treatment and/or the disease itself influencing the phenotype of interest, i.e. telomere length. Furthermore, given that telomere length has been positively associated with survival (19
), cancer patients with shorter telomere lengths may die or be too ill to participate in a case-control study resulting in a case group with a disproportionate number of individuals with longer telomeres. The number of deaths attributed to breast cancer (n = 29) was not sufficient to investigate the relationship between RTL and breast cancer survival.
We used an economical real time PCR-based method in a high-throughput setting to measure RTL in DNA derived from peripheral blood leukocyles. While this does not provide an absolute measurement of telomere length, relative telomere lengths estimated by this method correlate well with telomere restriction fragment (TRF) lengths produced by the Southern blot assay (13
). An advantage of the PCR-based assay over the TRF assay is that it does not measure subtelomeric DNA, which can introduce up to 2 kb in variation between individuals (13
). In our study, we observed a statistically significant inverse correlation with age giving further assurance that the real-time PCR method provides a biologically meaningful measure of telomere length. We cannot be certain that RTL measured in blood reflects telomere length in breast tissue. However, statistically significant correlations found between leukocyte telomere length and other tissues from the same individual suggest blood serves as an adequate proxy for non-malignant breast tissue (20
In summary, we did not observe a significant elevation in postmenopausal breast cancer risk associated with shorter telomere lengths. Our data provide little support for an important role of telomere crisis as a crucial factor in breast carcinogenesis among postmenopausal women. Additional prospective studies are needed to confirm our finding as well as to explore the relationship between relative telomere length, premenopausal breast cancer risk, and breast cancer survival.