Chromosomal instability (CIN) and epigenetic alterations are characteristics of advanced and metastatic cancers1–4, but whether they are mechanistically linked is unknown. Here we show that missegregation of mitotic chromosomes, their sequestration in micronuclei5,6 and subsequent rupture of the micronuclear envelope7 profoundly disrupt normal histone post-translational modifications (PTMs), a phenomenon conserved across humans and mice, as well as in cancer and non-transformed cells. Some of the changes in histone PTMs occur because of the rupture of the micronuclear envelope, whereas others are inherited from mitotic abnormalities before the micronucleus is formed. Using orthogonal approaches, we demonstrate that micronuclei exhibit extensive differences in chromatin accessibility, with a strong positional bias between promoters and distal or intergenic regions, in line with observed redistributions of histone PTMs. Inducing CIN causes widespread epigenetic dysregulation, and chromosomes that transit in micronuclei experience heritable abnormalities in their accessibility long after they have been reincorporated into the primary nucleus. Thus, as well as altering genomic copy number, CIN promotes epigenetic reprogramming and heterogeneity in cancer.

Background: CRISPR-Cas systems, which provide adaptive immunity against foreign nucleic acids in prokaryotes, can serve as useful molecular tools for multiple applications in genome engineering. Diverse CRISPR-Cas systems originating from distinct prokaryotes function through a common mechanism involving the assembly of small crRNA molecules and Cas proteins into a ribonucleoprotein (RNP) effector complex, and formation of an R-loop structure upon binding to the target DNA. Extensive research on the I-E subtype established the prototypical mechanism of DNA interference in type I systems, where the coordinated action of a ribonucleoprotein Cascade complex and Cas3 protein destroys foreign DNA. However, diverse protein composition between type I subtypes suggests differences in the mechanism of DNA interference that could be exploited for novel practical applications that call for further exploration of these systems. Results: Here we examined the mechanism of DNA interference provided by the type I-F1 system from Aggregatibacter actinomycetemcomitans D7S-1 (Aa). We show that functional Aa-Cascade complexes can be assembled not only with WT spacer of 32 nt but also with shorter or longer (14-176 nt) spacers. All complexes guided by the spacer bind to the target DNA sequence (protospacer) forming an R-loop when a C or CT protospacer adjacent motif (PAM) is present immediately upstream the protospacer (at −1 or −2,−1 position, respectively). The range of spacer and protospacer complementarity predetermine the length of the R-loop; however, only R-loops of WT length or longer trigger the nuclease/helicase Cas2/ 3, which initiates ATP-dependent unidirectional degradation at the PAM-distal end of the WT R-loop. Meanwhile, truncation of the WT R-loop at the PAM-distal end abolishes Cas2/3 cleavage. Conclusions: We provide a comprehensive characterisation of the DNA interference mechanism in the type I-F1 CRISPR-Cas system, which is different from the type I-E in a few aspects. First, DNA cleavage initiation, which usually happens at the PAM-proximal end in type I-E, is shifted to the PAM-distal end of WT R-loop in the type I-F1. Second, the R-loop length controls on/off switch of DNA interference in the type I-F1, while cleavage initiation is less restricted in the type I-E. These results indicate that DNA interference in type I-F1 systems is governed through a checkpoint provided by the Cascade complex, which verifies the appropriate length for the R-loop.