Targeted cancer therapies represent a milestone towards personalized treatment as they function via inhibition of cancer-specific alterations. Polymerase θ (POLQ), an error-prone translesion polymerase, also involved in DNA doublestrand break (DSB) repair, is often upregulated in cancer. POLQ is synthetic lethal with various DNA repair genes, including known cancer drivers such as BRCA1/2, making it essential in homologous recombination-deficient cancers. Thus, POLQ represents a promising target in cancer therapy and efforts for the development of POLQ inhibitors are actively underway with first clinical trials due to start in 2021. This review summarizes the journey of POLQ from a backup DNA repair enzyme to a promising therapeutic target for cancer treatment. POLQ: Exploiting a Cancer Vulnerability for TherapyTo increase efficiency and lower the burden of toxic side effects, a major goal of cancer therapy is to progress from a 'one-drug-fits-all' to an individualized treatment approach tailored to the tumor-specific molecular features. Two main targeted therapeutic strategies are currently utilized in cancer treatment, both exploiting cancer-specific vulnerabilities. In the first approach, therapeutic suppression of aberrantly upregulated oncogenes alleviates the growth advantage of cancer cells. The second approach is based on the phenomenon that genetic alterations acquired by tumor cells cause their dependency on other compensatory pathways, loss of which leads to synthetic lethality (see Glossary). Therefore, therapeutic inhibition of pathways that are synthetic lethal with a cancer-specific alteration evokes cellular death in tumor cells while leaving normal cells unharmed [1]. The recent advent of genome-wide genetic interaction studies has demonstrated the extensive number of synthetic lethal interactions in cancer, many of which can potentially be translated to targeted cancer therapies [2].Cancer cells frequently acquire mutations in DNA repair genes and respond by rewiring their DNA repair network to utilize compensatory pathways for survival. Dependency on compensatory DNA repair pathways opens room for the development of cancer-specific small molecule inhibitors. A group of successful drugs that use this mode of action are poly(ADP-ribose) polymerase (PARP) inhibitors, approved for the treatment of BRCA-deficient cancers. The essentiality of PARP for cancer cells with loss-of-function mutations in BRCA1/2 is remarkable as such cancer cells are up to 1000 times more sensitive to PARP inhibitors than healthy cells [3,4]. Although challenges such as the acquisition of drug resistance need to be faced, the clinical success of inhibitory drugs targeting DNA repair enzymes is highly encouraging. In this context, the DNArepair enzyme polymerase θ (POLQ) has received increasing attention. POLQ is upregulated in numerous cancers and its overexpression is associated with poor prognosis [5][6][7][8][9]. Moreover, synthetic lethal interactions between POLQ and multiple DNA repair genes, including factors involv...

Immune responses need to be controlled tightly to prevent autoimmune diseases, yet underlying molecular mechanisms remain partially understood. Here, we identify biallelic mutations in three patients from two unrelated families in differentially expressed in FDCP6 homolog (DEF6) as the molecular cause of an inborn error of immunity with systemic autoimmunity. Patient T cells exhibit impaired regulation of CTLA-4 surface trafficking associated with reduced functional CTLA-4 availability, which is replicated in DEF6 -knockout Jurkat cells. Mechanistically, we identify the small GTPase RAB11 as an interactor of the guanine nucleotide exchange factor DEF6, and find disrupted binding of mutant DEF6 to RAB11 as well as reduced RAB11 + CTLA-4 + vesicles in DEF6 -mutated cells. One of the patients has been treated with CTLA-4-Ig and achieved sustained remission. Collectively, we uncover DEF6 as player in immune homeostasis ensuring availability of the checkpoint protein CTLA-4 at T-cell surface, identifying a potential target for autoimmune and/or cancer therapy.

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In enteroviruses, the inhibition of protein synthesis from capped host cell mRNA is catalyzed by the virally encoded 2A proteinase (2A), which cleaves eukaryotic initiation factors (eIF) 4GI and 4GII. Despite much investigation, the exact mechanism of 2A cleavage remains however unclear. Here, we identify the domains responsible for the eIF4E/HRV2 2A interaction using molecular modelling and describe mutations that impair this interaction and delay in vitro cleavage of eIF4G isoforms. Furthermore, we produced HRV1A viruses bearing the mutation L17R, Y32A or Y86A in the 2A sequence. All three viruses showed reduced yield and were appreciably delayed during infection in eIF4GI cleavage. Thus, we propose for genetic group A HRVs that the eIF4E/2A interaction is essential for successful viral replication. In contrast, HRV4 2A and coxsackievirus B4 2A failed to form complexes with eIF4E, suggesting that the mechanism of eIF4G isoform cleavage in these and related viruses is different.

Muscle function requires unique structural and metabolic adaptations that can render muscle cells selectively vulnerable, with mutations in some ubiquitously expressed genes causing myopathies but sparing other tissues. We uncovered a muscle cell vulnerability by studying miR-1, a deeply conserved, muscle-specific microRNA whose ablation causes various muscle defects. Using Caenorhabditis elegans, we found that miR-1 represses multiple subunits of the ubiquitous vacuolar adenosine triphosphatase (V-ATPase) complex, which is essential for internal compartment acidification and metabolic signaling. V-ATPase subunits are predicted miR-1 targets in animals ranging from C. elegans to humans, and we experimentally validated this in Drosophila. Unexpectedly, up-regulation of V-ATPase subunits upon miR-1 deletion causes reduced V-ATPase function due to defects in complex assembly. These results reveal V-ATPase assembly as a conserved muscle cell vulnerability and support a previously unknown role for microRNAs in the regulation of protein complexes.

Muscles are not only essential for force generation but are also key regulators of systemic energy homeostasis1. Both these roles rely heavily on mitochondria and lysosome function as providers of energy and building blocks, but also as metabolic sensors2-4. Perturbations in these organelles or their crosstalk lead to a wide range of pathologies5. Here, we uncover a deeply conserved regulon of mitochondria and lysosome homeostasis under control of the muscle-specific microRNA miR-1. Animals lacking miR-1 display a diverse range of muscle cell defects that have been attributed to numerous different targets6. Guided by the striking conservation of miR-1 and some of its predicted targets, we identified a set of direct targets that can explain the pleiotropic function of miR-1. miR-1-mediated repression of multiple subunits of the vacuolar ATPase (V-ATPase) complex, a key player in the acidification of internal compartments and a hub for metabolic signaling7,8, and of DCT-1/BNIP3, a mitochondrial protein involved in mitophagy and apoptosis9,10, accounts for the function of this miRNA in C. elegans. Surprisingly, although multiple V-ATPase subunits are upregulated in the absence of miR-1, this causes a loss-of-function of V-ATPase due to altered levels or stoichiometry, which negatively impact complex assembly. Finally, we demonstrate the conservation of the functional relationship between miR-1 and the V-ATPase complex in Drosophila.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.