title: Base excision repair of chemotherapeutically-induced alkylated DNA damage predominantly causes contractions of expanded GAA repeats associated with Friedreich's ataxia.
abstract: Expansion of GAA·TTC repeats within the first intron of the frataxin gene is the cause of Friedreich's ataxia (FRDA), an autosomal recessive neurodegenerative disorder. However, no effective treatment for the disease has been developed as yet. In this study, we explored a possibility of shortening expanded GAA repeats associated with FRDA through chemotherapeutically-induced DNA base lesions and subsequent base excision repair (BER). We provide the first evidence that alkylated DNA damage induced by temozolomide, a chemotherapeutic DNA damaging agent can induce massive GAA repeat contractions/deletions, but only limited expansions in FRDA patient lymphoblasts. We showed that temozolomide-induced GAA repeat instability was mediated by BER. Further characterization of BER of an abasic site in the context of (GAA)20 repeats indicates that the lesion mainly resulted in a large deletion of 8 repeats along with small expansions. This was because temozolomide-induced single-stranded breaks initially led to DNA slippage and the formation of a small GAA repeat loop in the upstream region of the damaged strand and a small TTC loop on the template strand. This allowed limited pol β DNA synthesis and the formation of a short 5'-GAA repeat flap that was cleaved by FEN1, thereby leading to small repeat expansions. At a later stage of BER, the small template loop expanded into a large template loop that resulted in the formation of a long 5'-GAA repeat flap. Pol β then performed limited DNA synthesis to bypass the loop, and FEN1 removed the long repeat flap ultimately causing a large repeat deletion. Our study indicates that chemotherapeutically-induced alkylated DNA damage can induce large contractions/deletions of expanded GAA repeats through BER in FRDA patient cells. This further suggests the potential of developing chemotherapeutic alkylating agents to shorten expanded GAA repeats for treatment of FRDA.
authors.affiliation: Department of Environmental Health, Sichuan University West China School of Public Health, Chengdu, Sichuan, P. R. China; Department of Chemistry and Biochemistry, Florida International University, Miami, Florida, United States of America.
authors.forename: Yanhao
authors.initials: Y
authors.lastname: Lai
authors.affiliation: Department of Chemistry and Biochemistry, Florida International University, Miami, Florida, United States of America.
authors.forename: Jill M
authors.initials: JM
authors.lastname: Beaver
authors.affiliation: Department of Chemistry and Biochemistry, Florida International University, Miami, Florida, United States of America.
authors.forename: Karla
authors.initials: K
authors.lastname: Lorente
authors.affiliation: Department of Chemistry and Biochemistry, Florida International University, Miami, Florida, United States of America.
authors.forename: Jonathan
authors.initials: J
authors.lastname: Melo
authors.affiliation: Department of Chemistry and Biochemistry, Florida International University, Miami, Florida, United States of America.
authors.forename: Shyama
authors.initials: S
authors.lastname: Ramjagsingh
authors.affiliation: Department of Cellular Biology and Pharmacology, Florida International University, Miami, Florida, United States of America.
authors.forename: Irina U
authors.initials: IU
authors.lastname: Agoulnik
authors.affiliation: Department of Environmental Health, Sichuan University West China School of Public Health, Chengdu, Sichuan, P. R. China.
authors.forename: Zunzhen
authors.initials: Z
authors.lastname: Zhang
authors.affiliation: Department of Chemistry and Biochemistry, Florida International University, Miami, Florida, United States of America.
authors.forename: Yuan
authors.initials: Y
authors.lastname: Liu
mesh_terms: D000478:Alkylation / Q000187:drug effects; D018906:Antineoplastic Agents, Alkylating / Q000494:pharmacology*; D045744:Cell Line, Tumor; D053904:DNA Breaks, Single-Stranded / Q000187:drug effects; D004249:DNA Damage / Q000187:drug effects*; D004260:DNA Repair; D004261:DNA Replication; D003606:Dacarbazine / Q000031:analogs & derivatives / Q000494:pharmacology; D005621:Friedreich Ataxia / Q000235:genetics*; D042822:Genomic Instability; D006801:Humans; D007438:Introns; D009690:Nucleic Acid Conformation; D017384:Sequence Deletion; D000077204:Temozolomide; D019680:Trinucleotide Repeat Expansion
chemical_list: D018906:Antineoplastic Agents, Alkylating; D003606:Dacarbazine; D000077204:Temozolomide