Poster Sessions

Mol-16

Gabi Gerlitz

G. Gerlitz, Y. Kim, T. Furusawa, M. Bustin

The histone code regulates the ATM intranuclear organization prior to DNA damage as well as its DNA damage induced activation

Efficient and correct response to double stranded breaks (DSBs) in chromosomal DNA is critical for maintaining genomic stability and preventing chromosomal alterations leading to cancer or cell malfunction. The generation of DSBs by ionizing irradiation (IR) or other DNA damaging agents activates the nuclear protein kinase ataxia-telangiectasia mutated (ATM), a key regulator of the cellular response to DSBs. DSBs-induced activation of ATM is associated with structural changes in chromatin and recruitment of early response proteins. However the interrelationship between DSBs-induced changes in chromatin architecture and the activation of ATM are unclear. Here we show that the nucleosome-binding protein HMGN1 modulates the interaction of ATM with chromatin both prior to and following the induction of DNA DSBs. ATM- chromatin retention assays revealed 3 fold increase in ATM retention to chromatin in HMGN1 KO cells in comparison to WT cells both prior and following DSBs formation. IR-induced ATM activation in HMGN1 KO cells is 3-4 folds lower in comparison to WT as judged by its autophosphorylation levels as well as by the phosphorylation levels of its substrates p53, Chk1, Chk2 and Smc1. Since it is the first example of opposite correlation between ATM chromatin binding and its kinase activity it seems that the increased chromatin binding of ATM in HMGN1 KO cells is in incorrect mode which does not facilitates its kinase activation. In order to understand how HMGN1 may affect the ATM chromatin binding we checked the levels of histone post-translational modifications in HMGN1 WT and KO cells. We found that IR leads to global increase in acetylation of Lys14 of histone H3 (H3K14) in an HMGN1 dependent manner. Treatment of cells with a histone deacetylase inhibitor which reduces the H3K14 acetylation level also reduces the ATM chromatin retention in HMGN1 KO cells both prior and following DSBs formation. The treatment also increases the induction of ATM kinase activity in HMGN1 KO cells as judged by the ATM autophosphorylation levels following IR. Thus suggesting that HMGN1 by regulating histone post-translational modifications mediate the efficient activation of ATM by optimizing its chromatin interactions both prior to and after DSBs formation. Our studies identify a new mediator of ATM activation and demonstrate a direct link between the steady-state intranuclear organization of ATM and the kinetics of its activation following DNA damage.