β2-spectrin depletion impairs DNA damage repair

beta 2-Spectrin (beta 2SP/SPTBN1, gene SPTBN1) is a key TGF-beta/SMAD3/4 adaptor and transcriptional cofactor that regulates TGF-beta signaling and can contribute to liver cancer development. Here we report that cells deficient in beta 2-Spectrin (beta 2SP) are moderately sensitive to ionizing radiation (IR) and extremely sensitive to agents that cause interstrand cross-links (ICLs) or replication stress. In response to treatment with IR or ICL agents (formaldehyde, cisplatin, camptothecin, mitomycin), beta 2SP deficient cells displayed a higher frequency of cells with delayed. gamma-H2AX removal and a higher frequency of residual chromosome aberrations. Following hydroxyurea (HU)-induced replication stress, beta 2SP-deficient cells displayed delayed disappearance of gamma-H2AX foci along with defective repair factor recruitment (MRE11, CtIP, RAD51, RPA, and FANCD2) as well as defective restart of stalled replication forks. Repair factor recruitment is a prerequisite for initiation of DNA damage repair by the homologous recombination (HR) pathway, which was also defective in beta 2SP deficient cells. We propose that beta 2SP is required for maintaining genomic stability following replication fork stalling, whether induced by either ICL damage or replicative stress, by facilitating fork regression as well as DNA damage repair by homologous recombination.