Elevated Apurinic/Apyrimidinic Endonuclease Activity Significantly Contributes to DNA Instability in Multiple Myeloma.

Apurinic/apyrimidic (AP) sites are the commonest DNA damage lesions and are highly mutagenic and cytotoxic requiring immediate repair. AP endonucleases (APE) 1 and 2 are the key proteins involved in the repair of AP sites and therefore play an important role in maintaining genomic integrity. However, if APE activity is dysregulated, it may lead to increase in DNA breaks leading to genomic rearrangements. In this study we demonstrate that endonuclease activity as measured by a novel plasmid based assay is elevated over 5 fold in myeloma cell lines and primary myeloma cells relative to normal plasma cells. APE1 and 2 were identified as the major endonuclease in MM by gene expression profiling and protein studies as well as using methoxamine (MX), which specifically inhibits APE activity by reacting with the aldehyde group of an abasic site. To further investigate the role of elevated APE activity in MM, we cultured MM cells in the presence of MX and demonstrate a significant inhibition of DNA recombination activity, as well as acquisition of new mutations as assessed by change in genome-wide loss of heterozygosity (LOH) using the 100K single nucleotide polymorphism arrays. Conversely, in cell line with lower APE activity we demonstrate a significant increase in genomic rearrangement with acquisition of significant number of new LOH loci. These data suggest that dysregulated AP endonuclease activity plays an important role in ongoing accrual of mutational changes, which may be associated with progression of myeloma and/or development of drug resistance. Moreover APE1 and APE2 provide a novel target to prevent disease progression and drug resistant phenotype associated with genomic instability.