Ricerc@Sapienza

Heterochromatin Protein 1 (HP1) and the Mre11-Rad50-Nbs1 (MRN) complex are conserved factors that play crucial role in genome stability and integrity. Despite their involvement in overlapping cellular functions, ranging from chromatin organization, telomere maintenance, to DNA replication and repair, a tight functional relationship between HP1 and the MRN complex has never been elucidated. We have recently shown that the Drosophila HP1a protein binds to the MRN complex and that loss of any of the MRN members reduces HP1a levels indicating that the MRN complex acts as regulator of HP1a stability. Moreover, overexpression of HP1a in NBS (but not in rad50 or mre11) mutant cells drastically reduces DNA damage associated with the loss of Nbs suggesting that HP1a and Nbs work in concert to maintain chromosome integrity in flies. We found that, similarly to Drosophila, human HP1alpha and NBS1 interact and that depletion of NBS1 reduces HP1alpha levels. Surprisingly, fibroblasts from Nijmegen Breakage Syndrome (NBS) patients carrying the 657del5 hypomorphic mutation in NBS1 and expressing the p26 and p70 NBS1 fragments, accumulate HP1alpha indicating that the presence of truncated NBS1 extends HP1alpha turnover and/or promotes its stability. Remarkably, a siRNA-mediated reduction of HP1alpha in NBS fibroblasts decreases the hypersensitivity to irradiation, a characteristic of the NBS syndrome. Thus, our data highlight a close interaction between HP1 and NBS1 that is essential for genome stability and identify HP1 as a potential target to counteract chromosome instability in NBS patients' cells. However, the molecular mechanisms underlying the HP1a-NBS1 evolutionarily conserved functional interaction remain still elusive. My proposal is meant to fill this gap and my results will hopefully provide new insights on the implication of HP1a in cell metabolism and in the clinical features on NBS.