p73 in colorectal cancer

Colorectal cancer (CRC) is the third most common cancer in the world, with about 5000 new cases in Sweden every year. CRC is caused by mutation (inherited or acquired) in genes, by gene variants and changed expression of proteins. The primary way to achieve a curative result for CRC is to remove the tumor by surgery. To reduce risk of recurrence chemoor radiotherapy are given as a complement to surgery. p73 is a structural and functional homologue of tumor suppressor p53. However, p73 is rarely mutated in tumors, but rather overexpressed as compared to normal tissue. There are two main isoforms of p73, the transactivation capable TAp73 and the truncated ΔNp73, which are involved in an autoregulatory loop with TAp73 and p53. The aim of this study was to investigate the role of p73 and related proteins in the development and treatment of CRC. A G4C14-to-A4T14 polymorphism of p73 was studied in CRC patients and healthy controls (Paper I), and rectal cancer patients who were randomized to treatment with either surgery alone or preoperative radiotherapy and surgery (Paper II). The AT/AT genotype of the p73 polymorphism may increase risk of CRC development and CRC patients with the AT allele had a better prognosis. When dividing the cases into colon and rectal cancer it was seen that in colon cancer the AT allele tended to be more favorable for overall survival, while in rectal cancer the GC allele seemed to be more favorable. Rectal cancer patients, with a combination of GC/GC genotype, wild type p53 and weak survivin expression survived longer after preoperative radiotherapy. This was not observed in the patients only receiving surgery. The protein expression of p73 was further studied in the rectal cancer patients randomized to treatment with either surgery alone or preoperative radiotherapy and surgery (Paper III). p73 was expressed higher in tumor tissue than in normal mucosa. Patients with p73 negative tumors had a lower risk of local recurrence after radiotherapy, as opposed to patients that had p73 positive tumors or patients with p73 negative tumors that did not receive radiotherapy. Effects of γ-radiation was further studied in colon cancer cell lines KM12C, KM12SM and KM12L4a regarding cell cycle, survival fraction (clonogenicity), apoptosis and protein expression patterns of mutated p53, TAp73, ΔNp73, survivin and PRL-3 (Paper IV). KM12C displayed low survival fraction, low apoptosis, no cell cycle arrest and an upregulation of the anti-apoptotic ΔNp73 after irradiation. KM12L4a showed a high survival fraction, but high apoptosis, arresting of the cell cycle and upregulation of the radioresistance factor survivin. The effects of overexpression and knockdown of survivin on TAp73, ΔNp73 and p53 expression in colon cancer cell lines HCT-116 and HCT-116 with and without γ-radiation were studied (Paper V). Overexpression of survivin decreased wild type p53, whilst downregulation of survivin lead to a simultaneous downregulation of TAp73 and ΔNp73, mRNA and protein, both with and without γ-radiation. Knockdown of survivin also demonstrated an increase in apoptosis. In conclusion, we showed that the G4C14-to-A4T14 polymorphism of p73 and p73 protein expression may be involved in CRC development, radiotherapy response and survival. We further showed that TAp73, ΔNp73 and p53 were regulated by survivin in colon cancer cells.