The DNA methyltransferase Dnmt3a suppresses tumorigenesis in models of leukemia and lung cancer. Conversely, deregulation of Dnmt3b is thought to generally promote tumorigenesis. However, the role of Dnmt3a and Dnmt3b in many types of cancer remains undefined. Here, we show that Dnmt3a and Dnmt3b are dispensable for homeostasis of the murine epidermis. However, loss of Dnmt3a-but not Dnmt3b-increases the number of carcinogen-induced squamous tumors, without affecting tumor progression. Only upon combined deletion of Dnmt3a and Dnmt3b, squamous carcinomas become more aggressive and metastatic. Mechanistically, Dnmt3a promotes the expression of epidermal differentiation genes by interacting with their enhancers and inhibits the expression of lipid metabolism genes, including PPAR-g, by directly methylating their promoters. Importantly, inhibition of PPAR-g partially prevents the increase in tumorigenesis upon deletion of Dnmt3a. Altogether, we demonstrate that Dnmt3a and Dnmt3b protect the epidermis from tumorigenesis and that squamous carcinomas are sensitive to inhibition of PPAR-g.

The main pathways for the repair of DNA double strand breaks (DSBs) are non-homologous end-joining (NHEJ) and homologous recombination directed repair (HDR). These operate mutually exclusive and are activated by 53BP1 and BRCA1, respectively. As HDR can only succeed in the presence of an intact copy of replicated DNA, cells employ several mechanisms to inactivate HDR in the G1 phase of cell cycle. As cells enter S-phase, these inhibitory mechanisms are released and HDR becomes active. However, during DNA replication, NHEJ and HDR pathways are both functional and non-replicated and replicated DNA regions co-exist, with the risk of aberrant HDR activity at DSBs in non-replicated DNA. It has become clear that DNA repair pathway choice depends on inhibition of DNA end-resection by 53BP1 and its downstream factors RIF1 and MAD2L2. However, it is unknown how MAD2L2 accumulates at DSBs to participate in DNA repair pathway control and how the NHEJ and HDR repair pathways are appropriately activated at DSBs with respect to the replication status of the DNA, such that NHEJ acts at DSBs in pre-replicative DNA and HDR acts on DSBs in post-replicative DNA. Here we show that MAD2L2 is recruited to DSBs in H4K20 dimethylated chromatin by forming a protein complex with 53BP1 and RIF1 and that MAD2L2, similar to 53BP1 and RIF1, suppresses DSB accumulation of BRCA1. Furthermore, we show that the replication status of the DNA locally ensures the engagement of the correct DNA repair pathway, through epigenetics. In non-replicated DNA, saturating levels of the 53BP1 binding site, di-methylated lysine 20 of histone 4 (H4K20me2), lead to robust 53BP1-RIF1-MAD2L2 recruitment at DSBs, with consequent exclusion of BRCA1. Conversely, replication-associated 2-fold dilution of H4K20me2 promotes the release of the 53BP1-RIF1-MAD2L2 complex and favours the access of BRCA1. Thus, the differential H4K20 methylation status between pre-replicative and post-replicative DNA represents an intrinsic mechanism that locally ensures appropriate recruitment of the 53BP1-RIF1-MAD2L2 complex at DNA DSBs, to engage the correct DNA repair pathway.

MAD2L2 (REV7) plays an important role in DNA double-strand break repair. As a member of the shieldin complex, consisting of MAD2L2, SHLD1, SHLD2 and SHLD3, it controls DNA repair pathway choice by counteracting DNA end-resection. Here we investigated the requirements for shieldin complex assembly and activity. Besides a dimerization-surface, HORMA-domain protein MAD2L2 has the extraordinary ability to wrap its C-terminus around SHLD3, likely creating a very stable complex. We show that appropriate function of MAD2L2 within shieldin requires its dimerization, mediated by SHLD2 and accelerating MAD2L2-SHLD3 interaction. Dimerization-defective MAD2L2 impairs shieldin assembly and fails to promote NHEJ. Moreover, MAD2L2 dimerization, along with the presence of SHLD3, allows shieldin to interact with the TRIP13 ATPase, known to drive topological switches in HORMA-domain proteins. We find that appropriate levels of TRIP13 are important for proper shieldin (dis)assembly and activity in DNA repair. Together our data provide important insights in the dependencies for shieldin activity.

Helminth parasitoses are among the most prevalent health issues worldwide. Their control depends largely on unravelling host–parasite interactions, including parasitic exploitation of the host haemostatic system. The present study undertakes a scoping review of the research carried out in this field with the aim of unifying and updating concepts. Multiple keywords combined with Boolean operators were employed to design the literature search strategy. Two online databases were used to identify original peer-reviewed articles written in English and published before 1st January 2020 describing molecular interactions between helminth parasites and the host haemostatic system. Relevant data from the selected sources of evidence were extracted and analysed. Ninety-six publications reporting 259 interactions were selected. Fifty-three proteins belonging to 32 species of helminth parasites were involved in interactions with components of the host haemostatic system. Many of these proteins from both parasite and host were conserved among the different interactions identified. Most of these interactions were related to the inhibition of the coagulation system and the activation of fibrinolysis. This was associated mainly with a potential of parasites to reduce the formation of blood clots in the host and attributed to biological processes, such as parasite nutrition, survival, invasion, evasion and migration or the appearance of pathological mechanisms in the host. A wide range of helminth parasites have developed similar strategies to exploit the haemostatic system of their hosts, which could be regarded as an evolutionary conserved mechanism that could confer benefits to parasites in terms of survival and establishment in their vertebrate hosts.

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