5',8-cyclo-2'-deoxynucleosides such as a 5',8-cyclo-dA or 5',8-cyclo-dG are common forms of oxidized DNA base lesions that are usually induced by hydroxyl radicals produced via endogenous and environmental oxidative stress as well as ionizing radiation. 5', 8-cyclo-deoxynucleosides can only be repaired by nucleic excision repair, but with a low efficiency. In addition, the base lesions are poorly bypassed by translesion DNA polymerases. This subsequently results in the accumulation of the damage in the genomic DNA that can severely inhibit DNA synthesis by DNA polymerases during DNA replication and repair, thereby causing mutations and genome instability. Trinucleotide repeats (TNR) are tandem repeats of Gs, As and Cs, thus they are susceptible to oxidative DNA damage that include 5',8-cyclo-deoxynucleosides. We previously found that an oxidized DNA base lesion can induce TNR deletion and expansion that are associated with prostate cancer and neurodegeneration. These are mediated by bypass of a TNR hairpin by DNA polymerase ? (pol ?) during DNA base excision repair (BER) in a damage position-dependent manner. In this study, we asked if pol ? can mediate TNR instability via its bypassing of an oxidized 5',8-cyclonucleoside, 5',8-cyclo-dA in a CAG repeat tract. We made the first discovery that pol ? efficiently bypassed both 5'R,8-cyclo-dA and 5'S,8-cyclo-dA in a (CAG)10 tract. This exclusively resulted in a CTG repeat deletion. We further demonstrated that the repeat deletion was caused by the looping out of the 5',8-cyclo-dA lesion on the template strand during pol ? bypass synthesis. Moreover, we found that pol ? bypass of the template 5',8-cyclo-dA resulted in repair deletion products with different sizes, suggesting the formation of damage-containing template loops with different sizes that subsequently facilitates the polymerase to skip the damage. Our results indicate that pol ? bypass of a 5',8-cyclo-deoxynucleoside lesion can induce TNR deletion. Because of their resistance to DNA repair, we suggest that a long-term accumulation of 5',8-cyclo-deoxynucleosides in the human genome can lead to TNR instability that can ultimately cause human cancer and neurodegeneration. Our study provides a new insight to the mechanisms of TNR instability induced by oxidative DNA damage via BER.
Bypass of a 5',8-cyclo-2'-deoxynucleoside by DNA polymerase beta leads to trinucleotide repeat deletion.
M Terzidis;A Masi;C Chatgilialoglu;
2014
Abstract
5',8-cyclo-2'-deoxynucleosides such as a 5',8-cyclo-dA or 5',8-cyclo-dG are common forms of oxidized DNA base lesions that are usually induced by hydroxyl radicals produced via endogenous and environmental oxidative stress as well as ionizing radiation. 5', 8-cyclo-deoxynucleosides can only be repaired by nucleic excision repair, but with a low efficiency. In addition, the base lesions are poorly bypassed by translesion DNA polymerases. This subsequently results in the accumulation of the damage in the genomic DNA that can severely inhibit DNA synthesis by DNA polymerases during DNA replication and repair, thereby causing mutations and genome instability. Trinucleotide repeats (TNR) are tandem repeats of Gs, As and Cs, thus they are susceptible to oxidative DNA damage that include 5',8-cyclo-deoxynucleosides. We previously found that an oxidized DNA base lesion can induce TNR deletion and expansion that are associated with prostate cancer and neurodegeneration. These are mediated by bypass of a TNR hairpin by DNA polymerase ? (pol ?) during DNA base excision repair (BER) in a damage position-dependent manner. In this study, we asked if pol ? can mediate TNR instability via its bypassing of an oxidized 5',8-cyclonucleoside, 5',8-cyclo-dA in a CAG repeat tract. We made the first discovery that pol ? efficiently bypassed both 5'R,8-cyclo-dA and 5'S,8-cyclo-dA in a (CAG)10 tract. This exclusively resulted in a CTG repeat deletion. We further demonstrated that the repeat deletion was caused by the looping out of the 5',8-cyclo-dA lesion on the template strand during pol ? bypass synthesis. Moreover, we found that pol ? bypass of the template 5',8-cyclo-dA resulted in repair deletion products with different sizes, suggesting the formation of damage-containing template loops with different sizes that subsequently facilitates the polymerase to skip the damage. Our results indicate that pol ? bypass of a 5',8-cyclo-deoxynucleoside lesion can induce TNR deletion. Because of their resistance to DNA repair, we suggest that a long-term accumulation of 5',8-cyclo-deoxynucleosides in the human genome can lead to TNR instability that can ultimately cause human cancer and neurodegeneration. Our study provides a new insight to the mechanisms of TNR instability induced by oxidative DNA damage via BER.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
