Characterisation of a Novel Human Tyrosyl DNA phosphodiesterase

Human cells possess a class of proteins known as topoisomerases that stop DNA from becoming twisted or overwound. Occassionally, these proteins accidentally break DNA and thereby render our genetic material unstable. If not repaired properly, such DNA breaks can lead to cell death or cancer. Here, we have identified a completely novel human enzyme that can safeguard DNA against topoisomerase-induced DNA damage by repairing these breaks if they arise. We now wish to characterise the function of this novel human enzyme in detail, and investigate its importance to DNA stability and human health.

Topoisomerases regulate DNA topology and are fundamental to many aspects of chromosome metabolism. However, topoisomerase activity involves the transient cleavage of DNA, which if occurring close to sites of endogenous DNA damage or in the presence of topoisomerase poisons can result in abortive topoisomerase-induced DNA single- or double-strand breaks that threaten genome integrity. A characteristic feature of topoisomerase-induced DNA breaks is covalent linkage of the enzyme to DNA termini via a 3?- or 5?-phosphotyrosyl bond. Such breaks are implicated in hereditary human disease, chromosomal instability and cancer, and underlie the clinical efficacy of an important class of anti-tumour poisons. The importance of liberating DNA termini from trapped topoisomerase is illustrated by the progressive neurodegenerative disease observed in individuals harbouring a mutation in tyrosyl DNA phosphodiesterase-1 (TDP1), an enzyme that cleaves 3?-phosphotyrosyl bonds. Surprisingly, however, a complementary human enzyme that cleaves 5?-phosphotyrosyl bonds has not been reported, despite the impact of DNA double-strand breaks harbouring such termini on chromosome instability and cancer. Here, we identify such an enzyme and reveal that this enzyme is TTRAP, a member of the metal-dependent family of phosphodiesterases of unknown function. TTRAP is the first human 5?-tyrosyl DNA phosphodiesterase to be identified, and we propose that this enzyme facilitates the repair of DNA strand breaks arising from abortive topoisomerase activity and is thereby important for genetic stability and proper neural function, in vivo.