Genomes contain many sequences that are intrinsically difficult to replicate. Tracts of tandem guanines, for instance, have the potential to adopt stable G-quadruplex structures, which are prone to cause genome alterations. Here we describe G4 DNA-induced mutagenesis in Caenorhabditis elegans and identify a non-canonical DNA break repair mechanism that generates deletions characterized by an extremely narrow size distribution, minimal homology of exactly one nucleotide at the junctions, and by the occasional presence of templated insertions. This typical mutation profile is fully dependent on the A-family polymerase Theta, the absence of which leads to profound loss of sequences surrounding G4 motifs. Theta-mediated end-joining prevails over non-homologous end joining and homologous recombination and prevents genomic havoc at replication fork barriers at the expense of small deletions. G4 DNA-induced deletions also manifest in the genomes of wild isolates of C. elegans, indicating a protective role for this pathway during evolution.

Aneuploidy is found in most solid tumours, but it remains unclear whether it is the cause or the consequence of tumorigenesis. Using Plk4 overexpression (PLK4OE) during epidermal development, we assess the impact of centrosome amplification and aneuploidy on skin development and tumorigenesis. PLK4OE in the developing epidermis induced centrosome amplification and multipolar divisions, leading to p53 stabilization and apoptosis of epidermal progenitors. The resulting delayed epidermal stratification led to skin barrier defects. Plk4 transgene expression was shut down postnatally in the surviving mice and PLK4OE mice never developed skin tumours. Concomitant PLK4OE and p53 deletion (PLK4OE/p53cKO) rescued the differentiation defects, but did not prevent the apoptosis of PLK4OE cells. Remarkably, the short-term presence of cells with supernumerary centrosomes in PLK4OE/p53cKO mice was sufficient to generate aneuploidy in the adult epidermis and triggered spontaneous skin cancers with complete penetrance. These results reveal that aneuploidy induced by transient centrosome amplification can accelerate tumorigenesis in p53-deficient cells.

Human-nucleotide-excision repair (NER) deficiency leads to different developmental and segmental progeroid symptoms of which the pathogenesis is only partially understood. To understand the biological impact of accumulating spontaneous DNA damage, we studied the phenotypic consequences of DNA-repair deficiency in Caenorhabditis elegans. We find that DNA damage accumulation does not decrease the adult life span of post-mitotic tissue. Surprisingly, loss of functional ERCC-1/XPF even further extends the life span of long-lived daf-2 mutants, likely through an adaptive activation of stress signaling. Contrariwise, NER deficiency leads to a striking transgenerational decline in replicative capacity and viability of proliferating cells. DNA damage accumulation induces severe, stochastic impairment of development and growth, which is most pronounced in NER mutants that are also impaired in their response to ionizing radiation and inter-strand crosslinks. These results suggest that multiple DNA-repair pathways can protect against replicative decline and indicate that there might be a direct link between the severity of symptoms and the level of DNA-repair deficiency in patients.