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Telomeres are protective structures at the end of our chromosomes, composed of multiple repeats of the DNA sequence TTAGGG. They are essential for maintaining chromosomal stability by preventing damage and degradation of the chromosome ends. Telomeres are normally shortened with each cell division until a critical length is reached, at which stage cell cycle arrest is induced. Telomere shortening can be prevented in the presence of the telomere-­‐elongating enzyme telomerase. Telomerase is expressed during embryogenesis and in certain normal cell types, but most somatic cells exhibit undetectable levels of telomerase activity. In contrast, most cancer cells express telomerase enabling them to proliferate indefinitely.

There is a search for reliable molecular markers that can be used to help predict cancer risk and outcome. The interest of investigating telomere length as a potential biomarker in malignancy has grown rapidly, and both tumors and normal tissues have been in focus for telomere length measurements. In this thesis, telomere length was investigated in breast cancer patients and in patients with renal cell carcinoma (RCC). The breast cancer patients were found to have significantly longer mean telomere length in peripheral blood cells (i.e. immune cells) compared to a tumor-­‐free control group. Moreover, patients with the longest blood telomere length had a significantly worse outcome compared to patients with shorter blood telomeres. In a patient group with clear cell RCC, telomere length was investigated in peripheral blood cells, in tumors and in corresponding kidney cortex. Again, patients with the longest blood telomere length had a significantly worse prognosis compared to those with shorter blood telomeres. In contrast, telomere length in tumor and kidney cortex tissues did not predict outcome per se.

Immunological components may play a role in telomere length dynamics as well as in cancer development. We aimed to investigate a possible association between telomere length and certain immunological parameters, including various cytokines and peripheral levels of a blood cell type with suppressor function [regulatory T cells (Tregs)]. In our patients with clear cell RCC, three cytokines correlated significantly with tumor telomere length, but not with telomere length in peripheral blood cells. In a separate patient group with various RCC tumors, blood telomere length correlated positively with the amount of Tregs. It might be speculated that a subset of patients with long blood telomeres has a less efficient immune response due to high Treg levels, contributing to a worse prognosis.

Another aim of this thesis was to explore telomere length changes over time. Evaluation of blood samples collected at a 6-­‐month interval from 50 individuals, showed that half of the participants experienced a decline in mean telomere length during the time period. This group had longer telomere length at baseline compared to those who demonstrated increased/stable telomere length. In a separate group of five blood donors, a remarkable drop in telomere length was detected in one donor over a 6-­‐month period, whereas the other donors exhibited only small fluctuations in telomere length.

In conclusion, the results of this thesis indicate that blood telomere length has potential to act as an independent prognostic marker in malignancy. Adding to the complexity is the fact that changes in blood telomere length might occur within relatively short time spans, indicating that telomere length is a dynamic character.