Quantitative analysis of the frequency of chromosome loss following DSB induction
Numerical abnormalities in chromosomal states, referred to as aneuploidy, is commonly observed in many cancer cells. Although numerous internal and external factors induce aneuploidy, the primary cause of aneuploidy in humans remains unclear. DNA damage is identified as a potential cause of aneuploidy by inducing chromosome segregation errors. However, a direct relationship between DNA damage and aneuploidy remains poorly understood. A major reason for this is the extremely low frequency of aneuploidy in cultured cells, making quantitative analyses challenging. In this study, we investigated the relationship between DNA damage and aneuploidy in cell lines containing minichromosomes. These chromosomes are more prone to loss than normal chromosomes, with the rate of loss substantially increased following exposure to various DNA-damaging agents. To determine whether damaged chromosomes were subjected to direct loss or whether chromosome loss occurred as an indirect consequence of a prolonged G2 phase or other factors, we used the CRISPR-Cas9 system to introduce a single DNA double-strand break (DSB) on a minichromosome. The rate of minichromosome loss increased by approximately seven-fold compared with that of the control. Furthermore, the loss rate was significantly elevated in the absence of KU70, a key factor in non-homologous end joining, and upon inhibition of ataxia telangiectasia mutated (ATM), a DNA damage checkpoint protein. Finally, two closely spaced nicks, believed to generate a 5-overhang, were also shown to induce minichromosome loss. These findings indicated that a single DSB or two closely spaced nicks can cause aneuploidy if improperly repaired in vertebrates.