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Researchers investigated the role of cohesin complex proteins SA1 and SA2 in organizing chromatin loops during V(D)J recombination, the process that generates antibody diversity in immune cells. Using arrested pro-B cells, they found that eliminating both SA1 and SA2 proteins abolished most positioned chromatin loops genome-wide and blocked long-range recombination at the Igh antibody locus, but surprisingly, robust recombination continued at the Igκ locus. This demonstrates that loop extrusion can proceed without SA1/2 proteins, and that strong recombination signal sequences can compensate for loss of precise chromatin loop architecture in large genomic regions.
Why it matters
This research challenges current understanding of how chromatin organization controls gene regulation, suggesting that precise 3D genome structure may be less critical than previously thought when strong regulatory signals are present. The findings could inform therapeutic strategies targeting cohesin complexes in cancer and developmental disorders, and broaden understanding of how genes are regulated in non-dividing cells.
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⚠️ Preprint – Noch nicht peer-reviewed
Dieser Artikel wurde noch nicht von unabhängigen Experten begutachtet. Die Ergebnisse sind vorläufig und sollten mit Vorsicht interpretiert werden.
The cohesin ring complex organizes and mediates functional activity of the genome through chromatin loop extrusion1,2. Cohesin comprises a core SMC1-SMC3-RAD21 ring associated with multiple regulatory proteins, including SA1 and SA2 paralogs that bind mutually-exclusively to a RAD21 site3,4. Functional studies of cohesin complexes in vivo often employ inactivation of particular components2,3. However, inactivation of SA1 or SA2 in mice is limited by mouse embryonic lethality5-7 and, for dual SA1/2-inactivation by cycling cell lethality8-10. For these reasons, it remains unknown whether the requirement for SA1/2 in loop extrusion in vitro11,12 applies in vivo1,2, and whether these factors have overlapping or distinct functions in regulating this process3. Here, we employ G0/G1-arrested v-Abl pro-B lines, which are viable in the absence of cohesin-complex factors13,14, to dissect SA1 and SA2 functions in V(D)J recombination, a strictly G0/G1-specific process that harnesses loop extrusion to effect distinct Igh and Ig{kappa} locus recombination mechanisms15,16. Robust long-range Igh or Ig{kappa} V(D)J recombination continued unabated in the absence of SA1 or SA2. However, inactivation of both abrogated most positioned chromatin loops genome-wide and also abolished long-range Igh V(D)J recombination with a phenotype similar to RAD21 (cohesin) inactivation13. Yet, robust Ig{kappa} V(D)J recombination continued upon dual SA1/2-inactivation, which along with Hi-C findings, demonstrates ongoing loop-extrusion in the absence of SA1/2. Correspondingly, replacement of weak Igh V(D)J recombination signal sequences (RSSs) that target V(D)J recombination activity, with strong Ig{kappa} counterparts15 restored long-range Igh V(D)J recombination in SA1/2 inactivated cells. We conclude that SA1/2 together maintain loop extrusion impediments and genome-wide architecture in G0/G1-arrested cells; and, in their absence, loop extrusion proceeds unimpeded. Remarkably, strong RSSs allow Ig{kappa} to function normally in the absence of internal positioned loop structure across the 3.1-mega-base Ig{kappa} locus. We discuss implications of our findings for long-range gene regulation in G0/G1-arrested cells more broadly.
Source: Robust V(D)J Recombination in the Absence of SA1/2-mediated Chromatin Loops