ProA and ProB repeat sequences shape genome organization, and enhancers open domains

There is a growing awareness that repeat sequences (RepSeq) - the main constituents of the human genome - are also prime players in its organization. Here we propose that the genome should be envisioned as a supersystem with three main subsystems, each composed of functionally redundant, cooperating elements. We define herein ProA and ProB RepSeqs as sequences that promote either the A/euchromatin or the B/heterochromatin compartment. ProA and ProB RepSeqs shape A/B partitioning, such that the relative proportions of ProA and ProB RepSeqs determine the propensity of a chromosome segment to adopt either an A or a B configuration. In human, core ProA RepSeqs are essentially made of Alu elements, whereas core ProB RepSeqs consist of young L1 and some Endogenous Retroviruses (ERVs) as well as a panel of AT-rich microsatellites and pericentromeric and telomeric satellites. Additionally, RepSeqs with more indefinite character and, importantly, their derivatives known as "transcriptional enhancers", can shift between ProA and ProB functions and thus act to open or close specific chromatin domains depending on the cellular context. In this framework, genes and their promoters appear as a special class of RepSeqs that, in their active, transcribed state, reinforce the openness of their surroundings. Molecular mechanisms involve cooperativity between ProB elements, presumably underpinned by the condensate-like properties of heterochromatin, which ProA elements oppose in several ways. We provide strong arguments that altered CpG methylation patterns in cancer including a marked loss in the B compartment, result primarily from a global imbalance in the process of CpG methylation and its erasure. Our results suggest that the resulting altered methylation and impaired function of ProB RepSeqs globally weaken the B compartment, rendering it more plastic, which in turn may confer fate plasticity to the cancer cell. ### Competing Interest Statement The authors have declared no competing interest.