Abstract
Micronuclei represent the cellular attempt to compartmentalize DNA to maintain genomic integrity threatened by mitotic errors and genotoxic events. Some micronuclei show aberrant nuclear envelopes (NEs) that collapse, generating damaged DNA that can promote complex genome alterations. However, ruptured micronuclei also provide a pool of cytosolic DNA that can stimulate antitumor immunity, revealing the complexity of micronuclear impact on tumor progression. The ESCRT-III (Endosomal Sorting Complex Required for Transport-III) complex ensures NE reseals during late mitosis and is repaired in interphase. Therefore, ESCRT-III activity maybe crucial for maintaining the integrity of other genomic structures enclosed by a NE. ESCRT-III activity at the NE is coordinated by the subunit CHMP7. We show that CHMP7 and ESCRT-III protect against the genomic instability associated with micronuclei formation. Loss of ESCRT-III activity increases the population of micronuclei with ruptured NEs, revealing that its NE repair activity is also necessary to maintain micronuclei integrity. Surprisingly, aberrant accumulation of ESCRT-III are found at the envelope of most acentric collapsed micronuclei, suggesting that ESCRT-III is not recycled efficiently from these structures. Moreover, CHMP7 depletion relieves micronuclei from the aberrant accumulations of ESCRT-III. CHMP7-depleted cells display a reduction in micronuclei containing the DNA damage marker RPA and a sensor of cytosolic DNA. Thus, ESCRT-III activity appears to protect from the consequence of genomic instability in a dichotomous fashion: ESCRT-III membrane repair activity prevents the occurrence of micronuclei with weak envelopes, but the aberrant accumulation of ESCRT-III on a subset of micronuclei appears to exacerbate DNA damage and sustain proinflammatory pathways.
Highlights
Micronuclei are cytosolic chromatin structures that are compartmentalized by a nuclear envelope (NE)
CHMP7 accumulations in VPS4-depleted cells contained CHMP4B (Fig. 1f, h), whereas CHMP4B was absent when cells were co-depleted for VPS4 and CHMP7 (Fig. 1h), in agreement with CHMP7 coordinating the recruitment of CHMP4B at the NE
(see figure on previous page) Fig. 7 CHMP7 is important for generating damaged DNA at micronuclei. a Control HeLa cells containing micronuclei positive or negative for RPA70 and CHMP4B
Summary
Micronuclei are cytosolic chromatin structures that are compartmentalized by a nuclear envelope (NE). Micronuclei persist through multiple cell divisions, Willan et al Oncogenesis (2019)8:29 but their NE can collapse within the G2 phase of the cell cycle[2]. The cause of such collapse remains unclear, but correlates with a lack of nuclear lamina integrity[2]. Such loss of micronuclear compartmentalization causes cytosolic enzymes to enter the micronucleus, generating further DNA damage and chromosome pulverization[3]. An intact NE around a micronucleus maintains the integrity of its genetic material[4,5,6] and thereby protects against chromothripsis
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