Export or explode: Export defects cause micronucleus membrane rupture

Chromosomal instability (CIN), a hallmark of aggressive cancers, frequently stems from errors in chromosome segregation during mitosis, with anaphase lagging chromosomes being a major culprit (1). Upon mitotic exit, anaphase lagging chromosomes can be encapsulated into micronuclei (MNi), thus becoming sequestered away from the primary nucleus (1). MNi often undergo irreversible nuclear envelope (NE) rupture during interphase, exposing chromatin to the cytosol (2). This loss of compartmentalization triggers genomic abnormalities, including chromothripsis and kataegis, both frequently observed in cancer, and leads to the activation of innate immune signaling, via the cGAS–STING pathway, which can drive cancer metastasis (3, 4). Micronucleus (MN) rupture has been linked to defects in NE assembly, including reduced recruitment of lamin B1 and defective nuclear pore complex (NPC) assembly, which can generate gaps in the nuclear lamina and favor membrane rupture (2, 5). However, a recent study showed that large MNi with lamin B1 and NPC levels comparable with the main nuclei still exhibit lamina gaps and elevated rupture rates (6). Moreover, despite very low lamin B1, MNi containing gene-dense chromosomes are less likely to display lamina gaps and rupture (6). These findings suggest that additional intrinsic factors associated with high gene density impact MN stability (6). A new study by Zych and colleagues (7) now provides evidence that MN rupture is mechanistically linked to excessive membrane expansion driven by defective protein export (Fig. 1, top).

To confirm that excessive MN growth drives membrane rupture, the authors modulated the size of MNi experimentally. Inducing MN expansion via hypotonic stress triggered rupture of nearly all MNi, while inhibiting nuclear growth by expressing a GDP-locked mutant of Ran (RanT24N), which blocks protein transport, significantly reduced lamina gaps, and increased bulk MN stability. Using the optogenetic reporter NLS-mCherry-LEXY, whose light-activatable nuclear export signal enables sequential measurement of both import and export rates within individual cells, the authors also demonstrated that, although MNi showed no import defect, protein export rates were slower than in the main nuclei. This export defect was attributed to reduced levels of Ran and RCC1, a chromatin-bound enzyme that catalyzes the generation of the RanGTP gradient essential for nuclear transport. Notably, restoring RCC1 levels through overexpression increased protein export rates and protected MNi from rupture. These findings established impaired protein export, caused by RCC1 reduced levels as a driver of MN instability (Fig. 1, top).

Small MNi containing the euchromatin-rich chromosome 19 were previously found to be more stable and to display fewer nuclear lamina gaps than those containing the heterochromatin-rich chromosome 18, despite their similar sizes, NPC density, and lamin B1 recruitment (6). Zych and colleagues now show that chromosome 19 MNi exhibited lower RCC1 levels than chromosome 18 MNi, yet they were more stable. This contrasted sharply with bulk MN response, where RCC1 restoration enhanced stability, indicating that chromosome-specific effects on MN stability can be masked in mixed population analysis. Using small-molecule inhibitors to alter chromatin state, the authors proved that histone modifications modulate RCC1 recruitment to small MNi and ultimately their stability. Specifically, increasing heterochromatin-associated marks elevated RCC1 levels in chromosome 19 MNi, thereby promoting expansion and reducing stability. The authors explain this paradox by showing that severe RCC1 depletion in euchromatic MNi, linked to reduced heterochromatin histone modifications, impairs both export and import, creating a balanced transport deficit that limits cargo accumulation and delays rupture. Consistent with this model, restoring RCC1 levels in these euchromatic MNi decreased their stability, indicating that recovery of import without a corresponding rescue of export accelerates rupture. These findings by Zych and colleagues reinforce emerging evidence that chromatin landscape is a key determinant of MN stability (6, 8).

Overall, the work by Zych et al. (7) showed that, by creating structural weak points in the NE, lamina gaps are permissive of rupture. However, they are not causative, as they do not, on their own, initiate rupture. The authors also showed that the imbalance between protein import and export drives membrane expansion, thereby enlarging lamina gaps and promoting MN rupture (Fig. 1, top). However, ultimate rupture of the MN membrane likely involves additional molecular players. For instance, aberrant activation of ATR kinase, a master regulator of the DNA damage response, was shown to initiate MN membrane rupture by depleting lamin A/C from the envelope (Fig. 1, bottom right) (8). Moreover, a recent study found that export defects caused accumulation of the membrane repair protein CHMP7, which triggers MN membrane rupture upon binding to the inner nuclear membrane protein LEMD2 (Fig. 1, bottom left) (9). Together with the findings by Zych and colleagues, this suggests that nuclear export defects play a key role in MN rupture, potentially generating both the conditions that drive lamina gap expansion and the molecular cues that trigger rupture. How RCC1 intersects with ATR and CHMP7 is currently unknown (Fig. 1); dissecting the interplay between these different pathways/mechanisms of MN rupture will be an important next step in furthering our understanding of MN fate determinants.

By leveraging cutting-edge optogenetic tools and quantitative imaging, Zych and colleagues established defective protein export, driven by RCC1 depletion, as a new mechanism underlying MN rupture and provided new insights into how MN stability is shaped by chromatin composition. This is one of a growing list of mechanisms that can promote MN rupture (Fig. 1). Studies investigating MN stability have focused on MNi induced by missegregation of whole chromosomes (2, 5, 6, 7, 8, 9), which is relevant, given that anaphase lagging chromosomes are a major mechanism of CIN in cancer and that lagging chromosomes often form MNi upon mitotic exit. However, MNi can also arise from chromosome fragments or chromosome bridges (1, 10), which result from DNA damage and can increase in response to treatment with DNA damage–inducing chemotherapeutic drugs. Understanding whether the same MN rupture mechanisms are at play in MNi containing whole chromosomes vs. MNi containing chromosome arms or smaller DNA fragments will be essential to set the stage for the development of strategies that can exploit these mechanisms for therapeutic purposes. Cancer therapies that exploit the instability of MN membrane could take one of two forms: (1) preventing membrane rupture to constrain cancer progression or (2) accelerating membrane rupture to trigger extreme CIN that is incompatible with cell survival and can trigger a strong immune response. This would transform MN instability from an inevitable consequence of missegregation into a targetable process.