Hypothesis

Core idea: Lamin A/C functions as a phosphorylation‑regulated switch that governs the detachment of lamina‑associated domains (LADs) from the nuclear envelope during DNA damage response and replication stress. In physiological aging, declining CDK1/cyclin B activity leads to hypophosphorylated lamin A/C that remains tightly bound to LADs, impairing the ATM‑LINC‑mediated detachment pathway and causing persistent fork stalling and genomic instability.

Mechanistic rationale

• During mitosis, CDK1/cyclin B phosphorylates lamin A/C on serine 22 and serine 392 (sites previously shown to reduce lamin‑chromatin affinity) [1]. This phosphorylation weakens lamin A/C interactions with LAD‑binding proteins such as LBR and emerin, facilitating the rapid LINC‑driven mobilization of damaged LADs observed after ATM activation [3].
• In aged cells, oxidative stress and reduced cyclin B expression lower CDK1 activity, resulting in hypophosphorylated lamin A/C that exhibits increased residence time at LADs. Consequently, ATM signaling still occurs, but the downstream LINC complex fails to generate sufficient force to detach LADs, prolonging tethering and exacerbating replication stress via PCNA sequestration [2].
• Persistent LAD tethering prevents efficient homologous recombination repair, leading to increased micronuclei formation and cytokinesis failure, as seen when lamin A/C is depleted [4]. Moreover, the stalled forks trigger chronic activation of the ATR‑CHK1 axis, which further suppresses cyclin B synthesis, creating a vicious feedback loop.

Testable predictions

1. Phosphomimetic rescue: Expressing a lamin A/C mutant with aspartic acid substitutions at S22/S392 (lamin A/C-S22D/S392D) in senescent human fibroblasts will:
   • Increase the rate of LAD detachment after ionizing radiation (measured by live‑cell LINC‑LAD imaging) [3].
   • Reduce PCNA foci accumulation and restore replication fork speed (DNA fiber assay) [2].
   • Lower micronuclei and aneuploidy rates compared with wild‑type lamin A/C expression [4].
2. Kinase inhibition: Treating young proliferating cells with a CDK1 inhibitor (RO‑3306) will phenocopy aging‑associated LAD tethering and replication stress, an effect reversible by expressing phosphomimetic lamin A/C.
3. In vivo validation: Lmna^L648R^ mice crossed with a knock‑in of the phosphomimetic lamin A/C allele will show delayed onset of age‑related pathology (e.g., reduced hematopoietic stem cell decline and improved tissue homeostasis) relative to Lmna^L648R^ controls [1].

Falsifiable outcomes

• If phosphomimetic lamin A/C fails to improve LAD detachment kinetics or replication metrics, the hypothesis that CDK1‑dependent phosphorylation directly regulates LAD dynamics is unsupported.
• If CDK1 inhibition does not increase LAD tethering in young cells, alternative kinases (e.g., PKC, MAPK) may be the primary regulators, redirecting mechanistic focus.

Therapeutic implication

Targeting the lamin A/C phosphorylation state—either via CDK1 activators, phosphatase inhibitors, or direct delivery of phosphomimetic lamin A/C mRNA—could restore the lamina’s signaling function without globally altering farnesylation, offering a complementary strategy to FTIs or ZMPSTE24 enhancers [6].