G1 checkpoint establishment in vivo during embryonic liver development

Sexually dimorphic DNA damage responses and mutation avoidance in the mouse germline

In this study, Bloom and Schimenti examine the response of primordial germ cells to DNA damage. Using both environmental and genetic stresses, the authors reveal the importance of the G1 checkpoint in preventing accumulation of complex mutations in the germline, and the differentiation of the DNA damage response during germ cell development.

Germ cells specified during fetal development form the foundation of the mammalian germline. These primordial germ cells (PGCs) undergo rapid proliferation, yet the germline is highly refractory to mutation accumulation compared with somatic cells. Importantly, while the presence of endogenous or exogenous DNA damage has the potential to impact PGCs, there is little known about how these cells respond to stressors. To better understand the DNA damage response (DDR) in these cells, we exposed pregnant mice to ionizing radiation (IR) at specific gestational time points and assessed the DDR in PGCs. Our results show that PGCs prior to sex determination lack a G1 cell cycle checkpoint. Additionally, the response to IR-induced DNA damage differs between female and male PGCs post-sex determination. IR of female PGCs caused uncoupling of germ cell differentiation and meiotic initiation, while male PGCs exhibited repression of piRNA metabolism and transposon derepression. We also used whole-genome single-cell DNA sequencing to reveal that genetic rescue of DNA repair-deficient germ cells (Fancm^−/−) leads to increased mutation incidence and biases. Importantly, our work uncovers novel insights into how PGCs exposed to DNA damage can become developmentally defective, leaving only those genetically fit cells to establish the adult germline.

XLF-mediated NHEJ activity in hepatocellular carcinoma therapy resistance

Background

DNA repair pathways are used by cancer cells to overcome many standard anticancer treatments, causing therapy resistance. Here, we investigated the role of XRCC4-like factor (XLF), a core member of the non-homologous end joining (NHEJ) repair pathway, in chemoresistance in hepatocellular carcinoma (HCC).

Methods

qRT-PCR analysis and western blotting were performed to detect expression levels of genes and proteins related to NHEJ. NHEJ repair capacity was assessed in vitro (cell-free) and in vivo by monitoring the activity of the NHEJ pathway. Cell viability and IC50 assays were used to measure sensitivity to drug therapy. A xenograft HCC model was used to develop methods of targeting XLF-induced chemosensitization. Clinicopathological analysis was conducted on patients with HCC treated with transarterial chemoembolization (TACE).

Results

Many conventional cancer chemotherapeutics induce DNA double-strand breaks (DSBs). HCC cells respond to these breaks by increasing their NHEJ activity, resulting in resistance. XLF-knockdown cells show an inhibition of NHEJ activity in both cell-free and live-cell assays as well as a high level of unrepaired cellular DSBs. These results indicate that XLF facilitates DNA end-joining and therefore promotes NHEJ activity in cancer cells. Consequently, knockdown of XLF significantly chemosensitized resistant cells both in vitro and in xenograft tumors. A low rate of XLF genomic alteration was found in patients with primary HCC, but XLF expression was induced after drug treatment. Clinically, a high level of XLF expression is significantly associated with advanced HCC and shorter overall survival.

Conclusion

Chemotherapy-induced overexpression of XLF and XLF-mediated enhancements in NHEJ activity contribute to chemoresistance in HCC cells and patients with HCC. Targeting XLF to modulate DSB repair could enhance drug sensitivity and may be a therapeutically useful addition to conventional therapy.

Electronic supplementary material

The online version of this article (doi:10.1186/s12885-017-3345-y) contains supplementary material, which is available to authorized users.