Cell-in-cell (CIC) structures are commonly seen in tumours. Their biological significance remains unclear, although they have been associated with more aggressive tumours. Here we report that mutant p53 promotes CIC via live cell engulfment. Engulfed cells physically interfere in cell divisions of host cells and for cells without p53 this leads to host cell death. In contrast, mutant p53 host cells survive, display aberrant divisions, multinucleation and tripolar mitoses. In xenograft studies, CIC-rich p53 mutant/null co-cultures show enhanced tumour growth. Furthermore, our results show that CIC is common within lung adenocarcinomas, is an independent predictor of poor outcome and disease recurrence, is associated with mutant p53 expression and correlated to measures of heterogeneity and genomic instability. These findings suggest that pro-tumorigenic entotic engulfment activity is associated with mutant p53 expression, and the two combined are a key factor in genomic instability.

Genomic integrity is constantly threatened by problems encountered by the replication fork. BRCA1, BRCA2 and a subset of Fanconi Anaemia proteins protect stalled replication forks from nuclease degradation through pathways involving RAD51. The contribution and regulation of BRCA1 in replication fork protection, and whether this relates to, or differs from, BRCA1's role in homologous recombination (HR) is not clear. Here we show that the canonical BRCA1-PALB2 interaction is not required for fork protection but instead BRCA1-BARD1 is regulated through a conformational change mediated by the phosphorylation-directed prolyl isomerase, PIN1. PIN1 activity enhances BRCA1-BARD1 interaction with RAD51 and consequently RAD51's presence at stalled replication structures. We identify patient missense variants in the regulated BRCA1-BARD1 regions which show poor nascent strand protection but remain proficient for HR, defining novel domains required for fork protection associated with cancer development. Together these findings reveal a previously unrecognised pathway that governs BRCA1-mediated replication fork protection. Main Text Fork progression can be slowed by conflicts with transcription, deoxyribonucleotide (dNTP) shortage or by difficult to replicate sequences, frequently causing fork stalling 1. In order to prevent stalled forks collapsing into DNA double strand breaks (DSBs), a number of responses are elicited including fork remodelling and subsequent nascent strand protection. Agents that cause replicative stress or compromise DNA Polymerase-α function result in a proportion of forks reversing (reviewed in 2,3). The regressed arm of nascent DNA in reversed forks resembles a single-ended DNA DSB which is protected from excessive resection by RAD51. Several factors contribute to RAD51-mediated fork protection including BRCA1/2, FANCA/D2, RAD51 paralogs, BOD1L, SETD1A, WRNIP and Abro1 2 .

Cell-in-cell (CIC) is a term used to describe the presence of one, usually living, cell inside another cell that is typically considered non-phagocytic. Examples of this include tumour cells inside tumour cells (homotypic), mesenchymal stem cells inside tumour cells (heterotypic) or immune cells inside tumour cells (heterotypic). CIC formation can occur in cell lines and in tissues and it has been most frequently observed during inflammation and in cancers. Over the past 10 years, many researchers have studied CIC structures and a few different models have been proposed through which they can be formed, including entosis, cannibalism and emperipolesis among others. Recently, our laboratory discovered a role for mutant p53 in facilitating the formation of CIC and promoting genomic instability. These data and research by many others have uncovered a variety of molecules involved in CIC formation and have started to give us an idea of why they are formed and how they could contribute to oncogenic processes. In this perspective, we summarise current literature and speculate on the role of CIC in cancer biology.

SUMOylation (small ubiquitin-like modifier) in the DNA double-strand break (DSB) response regulates recruitment, activity, and clearance of repair factors. However, our understanding of a role for deSUMOylation in this process is limited. Here we identify different mechanistic roles for deSUMOylation in homologous recombination (HR) and nonhomologous end joining (NHEJ) through the investigation of the deSUMOylase SENP2. We found that regulated deSUMOylation of MDC1 prevents excessive SUMOylation and its RNF4-VCP mediated clearance from DSBs, thereby promoting NHEJ. In contrast, we show that HR is differentially sensitive to SUMO availability and SENP2 activity is needed to provide SUMO. SENP2 is amplified as part of the chromosome 3q amplification in many cancers. Increased SENP2 expression prolongs MDC1 focus retention and increases NHEJ and radioresistance. Collectively, our data reveal that deSUMOylation differentially primes cells for responding to DSBs and demonstrates the ability of SENP2 to tune DSB repair responses.