40% of colorectal cancer (CRC) patients undergoing curative resection of the primary tumor will develop metastases in the following years 1 . Therapies to prevent disease relapse remain an unmet medical need. Here we uncover the identity and features of the residual tumor cells responsible for CRC relapse. Analysis of single-cell transcriptomes of CRC patient samples revealed that the majority of poor prognosis genes are expressed by a unique tumor cell population that we named High Relapse Cells (HRCs). We established a human-like mouse model of microsatellite stable CRC that undergoes metastatic relapse following surgical resection of the primary tumor. Residual HRCs occult in mouse livers after primary CRC surgery gave rise to multiple cell types over time, including Lgr5+ stemlike tumor cells 2-4 , and caused overt metastatic disease. Using Emp1 (epithelial membrane Competitiveness (MINECO). HH is a Miguel Servet (CP14/00229) researcher funded by the

The standard of care for advanced colorectal cancer (CRC) includes treatment with chemotherapeutic drugs that target the cell proliferation machinery 1 . In CRC patients with overt metastases, chemotherapy initially halts tumor growth but, almost inevitably, disease progresses after some cycles of treatment. Adjuvant chemotherapy is also administered to eliminate minimal residual disease, yet it only diminishes the risk of relapse by 10-25% 2 . Previous studies have shown that patient-derived organoids predict responses to chemotherapy 3-6 . Therefore, we used them as models to investigate the mechanisms behind the limited benefit of these treatments. Whereas CRC organoids expand from highly proliferative Lgr5+ tumor cells, we discovered that lack of optimal stem cell growth conditions specifies a latent Lgr5+ cell population. These cells expressed the gene Mex3a, were largely insensitive to chemotherapy and regenerated the organoid culture after treatment. In mouse models of metastatic latency, Mex3a+ cells contributed marginally to metastatic outgrowth. However, after chemotherapy treatment, Mex3a+ cells produced large cell clones that regenerated metastatic disease. Using lineage-tracing analysis combined with single cell profiling, we showed that drug-tolerant persister Mex3a+ cells downregulate the WNT/Lgr5+ stem cell program immediately after chemotherapy and adopt a transient regenerative state

Duchenne muscular dystrophy (DMD) is caused by mutations in the DMD gene leading to the presence of premature termination codons (PTC). Previous transcriptional studies have shown reduced DMD transcript levels in DMD patient and animal model muscles when PTC are present. Nonsense-mediated decay (NMD) has been suggested to be responsible for the observed reduction, but there is no experimental evidence supporting this claim. In this study, we aimed to investigate the mechanism responsible for the drop in DMD expression levels in the presence of PTC. We observed that the inhibition of NMD does not normalize DMD gene expression in DMD. Additionally, in situ hybridization showed that DMD messenger RNA primarily localizes in the nuclear compartment, confirming that a cytoplasmic mechanism like NMD indeed cannot be responsible for the observed reduction. Sequencing of nascent RNA to explore DMD transcription dynamics revealed a lower rate of DMD transcription in patient-derived myotubes compared to healthy controls, suggesting a transcriptional mechanism involved in reduced DMD transcript levels. Chromatin immunoprecipitation in muscle showed increased levels of the repressive histone mark H3K9me3 in mdx mice compared to wild-type mice, indicating a chromatin conformation less prone to transcription in mdx mice. In line with this finding, treatment with the histone deacetylase inhibitor givinostat caused a significant increase in DMD transcript expression in mdx mice. Overall, our findings show that transcription dynamics across the DMD locus are affected by the presence of PTC, hinting at a possible epigenetic mechanism responsible for this process.