Death becomes her: FBH1, DNA damage and apoptosis.

The process of DNA replication is fraught with danger: there are nicks and harmful modifications to the DNA template strand that can impede fork progression, alterations and imbalances in the dNTP pool can slow down fork progression and a lack of communication between leading and lagging strand replication can thwart the entire process. When replication forks stall, the first step involves a restart process, which tries, in the most effective but least drastic way, to get replication back on track. If the blocks to replication persist, and homologous recombination does not reassemble the fork, then replication can restart by repairing DNA breaks through homologous recombination and using new origins to continue replication. However, when the damage to the template strand is particularly severe, the cell instead makes DNA breaks through an active process that enhances the DNA damage response and targets the cell for apoptosis. The key players in DNA break formation are the 3′–5′ helicase FBH1- and the nuclease MUS81.

As expected from such a scenario, disruption of FBH1 or MUS81 in cells leads to resistance to replication stress and reduced formation of DNA breaks. Why would cells have pathways that willfully damage DNA and then target the cell for apoptosis? An answer to this comes from a recent study that showed that oncogenic stress, resulting in continued replication stress, is counteracted by FBH1 action. In this most altruistic behavior, apoptosis destroys the individual cell but preserves the tissue or organism. Thus it would be expected that mutations in FBH1, while providing resistance to DNA damage, would ultimately be harmful, as they would prevent very damaged cells from undergoing apoptosis and, instead, would promote propagation of cells with grossly damaged and rearranged genomes (Fig. 1). In a survey of 19 melanoma cells lines, Jeong et al. found that 55% had deletions of FBH1. Moreover, the FBH1 locus was deleted in a large fraction of melanomas in the NCBI-GEO database. Melanocytes are under constant exposure to UV damage. Hence FBH1 would be needed to destroy highly damaged cells, and loss of FBH1 seems to promote transformation to melanomas. Other tissues highly susceptible to UV damage, including lung and lung cancer cell lines, are also more often deleted for FBH1 than other tissue cancer cell lines. In Figure 1, we suggest that both translesion polymerase Polη and FBH1 allow either error-free bypass of UV damage or destruction of DNA with multiple damage sites. In the absence of FBH1, we suggest that translesion polymerase Polη is not sufficient for replication bypass of all the UV damage sites, and hence error-prone translesion polymerases are used, resulting in increased mutagenesis.

Figure 1. UV radiation causes lesions in DNA that block replication machinery, resulting in fork stalling. The cell employs a mechanism whereby 3′–5′ helicase FBH1 (green) unwinds lagging strand DNA, creating a substrate for a nuclease, such as MUS81, that cleaves (red circles) and thus promotes the formation of double-stranded breaks. These double-stranded breaks activate the DNA damage response, and if the DNA damage is extensive enough, this will eventually result in apoptosis of the cell. When FBH1 is absent or depleted, cells continue to replicate damaged DNA, possibly through mutagenic pathways, leading to increased genome instability that can result in cellular transformation and cancer.

The screening results of the melanoma cells lines are strongly suggestive, but only correlative, of the role that FBH1 helicase plays in preventing the growth of cells with highly damaged genomes and cell transformation. To further establish the connection between FBH1 and prevention of transformation, the authors induced transformation by UV irradiation in FBH1-depleted primary human melanocytes. Without UV exposure, the cells did not show any phenotypes related to transformation, but the FBH1-depleted cells showed a rapid increase in transformation promoted by UV irradiation, demonstrating the protective action of FBH1. Thus, treating melanomas with agents that cause replication stress may be counterproductive and potentially harmful if those tumors have reduced FBH1 levels or mutations that inactivate FBH1. It is possible that replication in the presence of UV damage and in the absence of FBH1 leads to mutagenic bypass or processing, leading to mutations and genome rearrangements. These studies also point out one marker that could be easily assessed prior to treatment. As the goal of chemotherapy is to induce apoptosis to kill tumor cells, the genetic makeup, including the FBH1 status of these cells must be taken into consideration to insure that appropriate treatment options are chosen.