|Name||Mr. Eduardo Laverde|
|Organization or Institution||Florida International University|
|Topic||Biochemistry / Chem Bio.|
GAA repeat R-loops mediates GAA repeat instability through the modulation of BER enzymes activity
E Laverde1,2, Y Lai2 and Y Liu 1,2,3.
1Biochemistry Ph.D. Program, Florida International University, Miami, FL, 33199, USA
GAA repeat expansion is the cause of Friedreich’s ataxia, an autosomal recessive neurodegenerative disorder. The expanded GAA repeat tract interferes with the expression of frataxin, a mitochondrial protein involved in the assembly of iron-sulfur clusters and energy metabolism. Expanded GAA repeats form stable RNA/DNA hybrids during transcription, resulting in the non-template single-stranded DNA, i.e., R-loops susceptible to DNA base damage. Our previous studies have shown that temozolomide, an alkylating DNA base damaging agent can promote large deletions of expanded GAA repeats in Friedreich’s ataxia patient cells through base excision repair (BER). However, it is unknown if BER within GAA repeat R-loops can contribute to GAA repeat instability. In this study, we initially examined the activities of BER core enzymes in a GAA repeat R-loop during BER of a base lesion located at different sites of a (GAA)20 repeat tract. We found that the activities of APE1 cleavage and pol b DNA synthesis in the R-loop with a base lesion in the middle was significantly reduced while FEN1 DNA cleavage was significantly increased compared with those in duplex DNA. However, APE1 and Pol b activities were not significantly affected during BER of a lesion at the 5’-side of the R-loop, while FEN1 cleavage was stimulated. The results further demonstrated that imbalanced BER enzymatic activities in a GAA repeat R-loop facilitated GAA repeat instability. We further suggest that R-loops can be developed as a new target for treatment of Friedreich’s ataxia through contractions of expanded GAA repeats at the frataxin gene.