Lamins as caretakers of the genome
Nuclear lamins orchestrate genome organization, forming a scaffold for tethering chromatin and protein complexes regulating many nuclear functions. Lamins dysfunction impacts nuclear architecture, chromatin organization, as well as DNA transactions such as transcription, replication and repair. These data, and the association of lamins dysfunction with degenerative disorders, premature aging and cancer, provide evidence for these proteins operating as “caretakers of the genome”. Our research focuses on understanding the molecular mechanisms whereby lamins regulate genome stability and function, as these mechanisms are key to identify therapies that restore genome function and ameliorate disease progression in patients with laminopathies. Specifically, we focus on the role of lamins in the maintenance of telomere homeostasis, chromatin compartmentalization and dynamics, and efficiency of DNA replication and repair of DNA damage.
(A) Hallmarks of genomic instability identified in A-type lamins-deficient cells include: (a) Aneuploidy and increased chromosomal aberrations and DNA damage; (b) Defects in short-range and long-range NHEJ mechanism of DNA DSB repair; (c) Telomere shortening and decompartmentalization of telomeres in the 3D nuclear space.
(B) At a molecular level, reduced expression of A-type lamins causes transcriptional upregulation of CTSL (cathepsin L) gene and increased global CTSL activity, which is responsible for the degradation of 53BP1 and Rb family members pRb and p107, key factors in DNA repair and cell cycle regulation, respectively. In addition, lamins deficiency elicits downregulation of VDR (vitamin D receptor) and BRCA1 gene expression, events that impair HDR repair.
Pathophysiology of Hutchinson Gilford Progeria Syndrome
We are using HGPS patients-derived cells and mouse models of the disease to define mechanisms contributing to the pathophysiology of this devastating accelerated aging disease. In addition, we are searching for strategies that ameliorate disease severity.
HGPS is caused by mutation in the LMNA gene and expression of a mutant lamin A protein “progerin” that elicits cellular and organismal decline. HGPS patients usually die in their teens from cardiovascular complications. Our studies are revealing new pathways contributing to progerin toxicity. In particular, we find that replication stress is a major cause of genomic instability in progeria, that also triggers innate immune responses to self-DNA, namely the cGAS/STING pathway and an interferon (IFN) response. How this cascade contributes to the pathophysiology of HGPS and other laminopathies is an area of great of interest for our team. Moreover, we find that the hormonal form of vitamin D (calcitriol) has a robust beneficial effect in HGPS cells, reducing progerin toxicity. We are translating these studies in vivo by testing the effect of calcitriol treatment in disease progression in progeria mice.
Expression of progerin alters nuclear organization and genome stability.
Cells from HGPS patients are characterized by reduced expression of ECM components, nuclear envelope defects, clustering of nuclear pore complexes, loss of peripheral heterochromatin, and higher levels of ROS and DNA damage. Progerin expression also affects dynamics of nuclear envelope transmembrane proteins (NETs) and their interactions with chromatin-associated proteins such as BAF, transcription factors, and chromatin modifiers.
Nuclear defects in HGPS cells.
Immunofluorescence performed in primary normal human fibroblasts (NF) and HGPS patient-derived fibroblasts shows how HGPS cells exhibit nuclear morphological abnormalities, decreased levels of the histone modification H3K9me3, and accumulation of basal levels of unrepaired DNA damage, when compared to NF.
Lastly, we are testing how diets of different caloric and fat content impact healthspan and lifespan in progeria mice. Excitingly, we found that only by feeding progeria mice a high-caloric/high-fat diet were we able to improve their health and double their lifespan. During their extended lifespan, progeria mice develop aging phenotypes with a severity that mirrors for the first time the human syndrome. This model (progeria mice on high-fat diet) gives us a unique opportunity to identify molecular mechanisms of disease and to test therapies.
Pictures of progeria mice, which carry the human HGPS mutation in homozygosis (LmnaG609G/G609G mice or G609G).
Mice that are fed a regular chow diet (RC) have an average lifespan of ~100 days. However, mice fed a high-fat diet (HFD) nearly double their lifespan and mirror the human syndrome.
Nuclear lamins in cancer and immunotherapy
Alterations in nuclear architecture are highly prevalent in cancer and are one of the features for diagnosis. Lamin A/C has clear roles in preserving nuclear architecture and genome stability, in addition to regulating a variety of signaling pathways implicated in tumorigenesis and cell migration/metastasis. These roles, and the fact that reduced expression of lamin A/C is often associated with poor prognosis in cancers, suggest that lamin A/C loss promotes malignancy.
Our studies aim to improve our understanding of cause-and-effect relationships between lamins loss and oncogenic mechanisms, which could reveal vulnerabilities to be exploited therapeutically. In recent years, checkpoint blockade immunotherapy (CBI) has revolutionized cancer treatment. Through inhibition of negative regulators of T cells, CBI harnesses the adaptive immune response to fight the tumor. However, many cancer patients do not generate adequate and durable antitumor immunity, resulting in patients not responding to CBI, or the response not deriving in long-term survival benefit.
Our team is testing a model whereby alterations in nuclear architecture and genomic instability, accumulating tumor cells during replication and in stromal cells during aging, cooperate on triggering key mechanisms in antitumor immunity and response to CBI. We are using immunocompetent mouse models of laminopathies to monitor the growth of syngeneic orthotopic tumors and their response to CBI.
