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A‐type lamins are intermediate filament proteins that provide a scaffold for protein complexes regulating nuclear structure and function. Mutations in the LMNA gene are linked to a variety of degenerative disorders termed laminopathies, whereas changes in the expression of lamins are associated with tumourigenesis. The molecular pathways affected by alterations of A‐type lamins and how they contribute to disease are poorly understood. Here, we show that A‐type lamins have a key role in the maintenance of telomere structure, length and function, and in the stabilization of 53BP1, a component of the DNA damage response (DDR) pathway. Loss of A‐type lamins alters the nuclear distribution of telomeres and results in telomere shortening, defects in telomeric heterochromatin, and increased genomic instability. In addition, A‐type lamins are necessary for the processing of dysfunctional telomeres by non‐homologous end joining, putatively through stabilization of 53BP1. This study shows new functions for A‐type lamins in the maintenance of genomic integrity, and suggests that alterations of telomere biology and defects in DDR contribute to the pathogenesis of lamin‐related diseases.

Genomic instability due to telomere dysfunction and defective repair of DNA double‐strand breaks (DSBs) is an underlying cause of ageing‐related diseases. 53BP1 is a key factor in DNA DSBs repair and its deficiency is associated with genomic instability and cancer progression. Here, we uncover a novel pathway regulating the stability of 53BP1. We demonstrate an unprecedented role for the cysteine protease Cathepsin L (CTSL) in the degradation of 53BP1. Overexpression of CTSL in wild‐type fibroblasts leads to decreased 53BP1 protein levels and changes in its cellular distribution, resulting in defective repair of DNA DSBs. Importantly, we show that the defects in DNA repair associated with 53BP1 deficiency upon loss of A‐type lamins are due to upregulation of CTSL. Furthermore, we demonstrate that treatment with vitamin D stabilizes 53BP1 and promotes DNA DSBs repair via inhibition of CTSL, providing an as yet unsuspected link between vitamin D action and DNA repair. Given that CTSL upregulation is a hallmark of cancer and progeria, regulation of this pathway could be of great therapeutic significance for these diseases. Earlier work implicated the DNA repair factor 53BP1 in laminopathy‐linked genome stability and progeria phenotypes. Lamin A loss is now shown to cause 53BP1 proteolysis through Cathepsin L, which can be counteracted by vitamin D—thus offering therapeutic potential in these situations.

53BP1 is a tumor suppressor protein that is important in the non-homologous end joining (NHEJ) pathway for DNA double strand break repair. Mice lacking 53BP1 present with increased radiosensitivity and genomic instability and are cancer prone. Loss of 53BP1 has been implicated by our laboratory in causing genomic instability in cells that lack A-type lamins and work by others has shown that loss of 53BP1 is associated with breast cancers of the poorest prognosis—BRCA1-mutated and Triple Negative Breast Cancers (TNBC)—and contributes to their resistance to current therapies, such as PARP inhibitors (PARPi). Work in our laboratory had previously identified a novel mechanism for regulation of 53BP1 protein levels—degradation by the cysteine protease cathepsin L (CTSL). The main questions addressed in this dissertation are whether CTSL-mediated degradation of 53BP1 is a mechanism contributing to BRCA1-mutated and TNBC tumor progression and resistance to therapy and if activation of this pathway is responsible for genomic instability in progeria and other laminopathies. We found that CTSL-mediated degradation of 53BP1 is activated upon loss of BRCA1, rescuing homologous recombination (HR) and proliferation defects. Inhibiting CTSL using CTSL inhibitors or vitamin D treatment stabilizes levels of 53BP1 protein, leading to an increase in NHEJ and defects in HR. Stabilization of 53BP1 using CTSL inhibitors or vitamin D leads to increased genomic instability and compromises proliferation following ionizing radiation or treatment with PARPi, which could represent a novel therapeutic strategy for breast cancers with the poorest prognosis. Furthermore, we identified nuclear CTSL as a novel biomarker for TNBC, which correlated inversely with 53BP1. We also identified the signature of low nuclear vitamin D receptor (VDR) with low 53BP1 and high nuclear CTSL in TNBC and BRCA1-mutated tumors, revealing a novel triple biomarker for stratification of patients with these cancers. We hypothesize that these patients can be treated with vitamin D or cathepsin inhibitors to stabilize 53BP1 levels to render tumor cells susceptible to PARPi treatment or other DNA damaging strategies. We also show that the Δexon9Lmna mouse model of progeria exhibits telomere shortening and defects of heterochromatin, but not an increase in telomere deletions or an increase in genomic instability as seen upon loss of A-type lamins. Interestingly, the levels of 53BP1 are maintained in cells from these mice. These results demonstrate that different lamins mutations present with varying phenotypes and that 53BP1 status could be an important determinant for maintenance of genomic instability in some lamins-related diseases.