The Fanconi anemia pathway and HELQ work alongside dormant replication origins to suppress replication-associated genome instability

Abstract

DNA replication is continually impeded by endogenous lesions that cause the stalling of replication forks. If left unchecked, this threatens the integrity of the genome and may be a driver of cancer development. Utilizing the <italic>Mcm4<super>chaos3</super></italic> mouse model, we found that dormant replication origins, which act as backup initiation sites, play a critical role in the recovery of stalled replication forks. A reduced number of dormant origins in these mice led to persistently stalled forks, incomplete replication and the mis-segregation of sister chromatids in mitosis, causing elevated genome instability. <italic>Mcm4<super>chaos3/chaos3</super></italic> cells also displayed intrinsic activation of the Fanconi anemia (FA) pathway, suggesting that it too plays a functional role in fork progression. Indeed, disruption of FA pathway activation in the <italic>Mcm4<super>chaos3/chaos3</super></italic> background led to an even higher number of persistently stalled forks. Furthermore, we discovered that a lack of dormant origins also leads to delayed replication, as seen by extremely late DNA synthesis. Accordingly, concomitant loss of both mechanisms led to heightened genomic instability, causing mice to either die shortly after birth or exhibit accelerated tumorigenesis. Finally, we investigated if <italic>HELQ</italic> is perhaps another FA gene by characterizing the first <italic>Helq</italic> mutant mouse model (<italic>Helq<super>gt</super></italic>). <italic>Helq<super>gt/gt</super></italic> cells/mice displayed modest FA-like phenotypes such as interstrand crosslink hypersensitivity and hypogonadism, but not defects in homologous recombination repair. Rather, HELQ was found to work in parallel to the FA protein FANCC to suppress replication-associated genome instability.