Septins buffer actomyosin forces to protect the nucleus from genotoxic mechanical stress

Invasively migrating cells thread their nucleus through confined interstitial spaces. How cells protect the nucleus from intracellular forces is poorly understood. Here, we show that the septin cytoskeleton buffers the actomyosin forces that power nuclear movement. Septin filaments comprising SEPT9, a septin amplified in breast cancer, align with perinuclear actomyosin cables which exhibit higher tensile stress during 3D confined migration through narrower pores. SEPT9 depletion amplifies actin stress during confined migration and after myosin II hyper-activation in non-migrating cells causing actin and nuclear envelope ruptures. Following confined migration, DNA breaks, nuclear blebs, micronuclei and cell death increase in SEPT9-depleted cells, phenotypes rescued by the oncogenic SEPT9 isoform 1. Clinicogenomic data reveal that <i>SEPT9</i> amplification associates with lower genomic alteration in aggressive breast tumors and higher patient mortality. We propose that septins are a mechanoprotective element of the cytoskeleton, and SEPT9 amplification enhances tumor cell survival by preventing nuclear damage.