The role of survivin for radiation oncology: moving beyond apoptosis inhibition.

Alterations in the expression of apoptosis-related proteins, like the inhibitor of apoptosis (IAP) protein family, display a pivotal pathway by which cancer cells acquire resistance to therapeutic treatment. Among this family, survivin, the smallest and structural unique member, deserves growing attention due to its universal over-expression in human tumors, and its prominent role in disparate networks of cellular division, intracellular signaling and apoptosis. Several preclinical studies have demonstrated that targeting survivin expression by the use of small interfering RNAs, dominant negative mutants, antisense-oligonucleotides and small molecule repressors sensitized tumor cells towards chemotherapy and irradiation and reduced tumor growth potential. Due to these properties, survivin has been proposed as a molecular target for anticancer therapies. Recent studies further revealed that radio-sensitization achieved by survivin inhibition seems to be multifaceted and involves caspase-dependent and caspase-independent mechanisms. In general, an enhanced rate of apoptosis, and pronounced cell cycle arrest have been observed. More recently, a hampered DNA-damage response has been noted, indicating a distinct role of the protein in radiation-induced double strand break repair. These properties were linked to a nuclear import and physical interrelationship with members of the DNA-DSB repair machinery such as phospho-histone H2AX and DNA dependent Protein Kinase (DNA-PKcs). The applicability of survivin-driven strategies in clinical practice is currently under investigation as the first survivin inhibitors successfully entered phase I/II trials. Although these trials do not include radiation therapy at present, survivin inhibitors may represent a novel type of molecular antagonists to improve the effectiveness of radiation therapy or chemoradio-therapy.