Lamins reach out to novel functions in DNA damage repair

The faithful maintenance of genomic DNA is essential for cell function and organismal life and is achieved by an intricate DNA damage recognition and repair machinery. External factors such as UV or ionizing irradiation and genotoxic chemicals as well as endogenous hazards (e.g. reactive oxygen species) and defects during DNA replication constantly damage DNA. The most severe lesions are DNA double strand breaks (DSBs), which are repaired by three pathways depending on the cell cycle phase: Homologous recombination repair (HR) is predominantly used during late S and G2 phase utilizing sister chromatids as templates for high-fidelity repair. In G1/G0 DSBs are repaired by the more error-prone but faster non homologous end joining (NHEJ) pathway, during which repair factors synapse and join the broken DNA ends. NHEJ also mediates long range end fusions of de-protected telomeres and chromosome regions¹. In addition, a less understood slow alternative NHEJ pathway (a-NHEJ) repairs DSBs via DNA end resection and microhomology-mediated ligation.

In the past years defects in lamin A – a major scaffold protein in metazoan cell nuclei – were linked to the premature ageing syndromes, Hutchinson-Gilford progeria and Werner-like syndrome^{2, 3}. Lamin-linked accelerated ageing is associated with impaired DSB repair and genomic instability^4-6. At molecular level, disease-linked lamin A mutants were found to impair recruitment of essential components of the DNA damage response pathways, including ATM (ataxia-telangiectasia mutated), ATR (ATM and Rad3 related), and p53 binding protein (53BP1) to DNA damage sites^4-6. The specific role of lamin A in DNA damage repair, however remained unclear. A recent report by Susana Gonzalo showed that lamin A also stabilizes 53BP1 protein. Lamin A loss caused 53BP1 downregulation and impaired fusion of de-protected telomeres⁷, consistent with reports on 53BP1’s role in long range NHEJ⁸. In their recent paper in Cell Cycle, the authors identify novel unexpected roles of lamin A in the repair of irradiation-induced DNA damage by short-range NHEJ and HR⁹. They found that lamin A-deficient cells have significantly reduced levels of 53BP1 and show a delayed DNA repair compared to lamin A-expressing cells. Particularly the initial, fast phase of DNA damage response, likely mediated by NHEJ was impaired in lamin-deficient cells. While 53BP1 overexpression rescued this defect, direct 53BP1 downregulation by RNA interference did not effect short range NHEJ, as seen after 53BP1 downregulation mediated by lamin A loss. These unexpected findings are consistent with a model in which 53BP1 indirectly promotes fast NHEJ by inhibiting slow repair mechanisms like HR and alternative-NHEJ. Since lower 53BP1 levels in lamin A-deficient cells did not promote HR, the authors concluded that lamin A may have additional, 53BP1-independent roles in HR. One clue on this novel role came from analyses of components of the HR pathway: Both RAD51 and BRCA1 were reduced on protein and more surprisingly also on transcript level. Based on previous reports that RAD51 and BRCA1 transcript levels are regulated by the p130/E2F4 pathway and that lamin A is involved in the regulation of the p130-related pocket proteins, pRb and p107, the authors investigated the levels of p130/E2F4 upon lamin A depletion. Loss of lamin A was found to promote formation of p130/E2F4 complexes, which in turn may repress RAD51 and BRCA1 transcription.

The molecular details how lamin A influences 53BP1 stability and inhibits formation of p130/E2F4 repressor complexes remain to be identified. Lamins may directly bind to and affect these proteins or these effects may be indirect. Altogether, the study by Redwood et al. provides a glimpse on novel functions of lamin A in both the transcription and stabilization of DNA damage repair components. Lamin A thus seems to be involved also in regulating the intricate cross talk between the different DSB repair pathways.

Figure 1.

DSBs are sensed by the MRN (MRE11-RAD50-NBS1) complex, which activates ATM. ATM phosphorylates histone H2AX that recruits mediator proteins like MDC1 and ATM itself. In HR, MRN induces strand resection together with several proteins including BRCA1 (breast cancer 1) and C-terminal binding protein (CtBP)-interacting protein (CtIP). Single strand DNA overhangs are bound by replication protein A (RPA), ATR and ATR-interacting protein (ATRIP). RAD51 then promotes homology search and strand invasion. DNA ends bound by Ku70/Ku80 are repaired by NHEJ, involving also DNA-dependent protein kinases (DNA-PK), Artemis and Ligase 4. Alternative NHEJ (A-NHEJ) can be induced by MRN-CtIP, which facilitates DNA end fusion by strand resection and usage of sequence microhomologies.

Lamin A has a dual role in these pathways: a) It stabilizes 53BP1, which inhibits HR and A-NHEJ thereby favoring short range classical NHEJ. b) Lamin A impairs formation of the p130/E2F4 complex, which represses BRCA1 and RAD51 transcription.