Le gène BAG3 a été identifié comme étant un nouveau gène responsable de cardiomyopathie dilatée (CMD), première cause d'insuffisance cardiaque (IC). La protéine BAG3 est une co-chaperonne qui participe au contrôle de l'homéostasie protéique via son rôle dans l'autophagie, protégeant ainsi les cellules contre la protéotoxicité induite par les protéines dégradées ou mal repliées. L'hypothèse qu'une inactivation de la voie autophagique contrôlée par BAG3 induirait une protéotoxicité cardiomyocytaire à l'origine de la CMD apparait particulièrement attractive et constitue l'hypothèse centrale de ce travail. Nos résultats indiquent que les mutations de BAG3 abolissent l'interaction avec la chaperonne HSP70, une protéine centrale du contrôle qualité des protéines. Nous avons observé une cytotoxicité des mutants BAG3, une altération de la fonction chaperonne HSP70-dépendante et une absence de réponse autophagique en condition de stress (jeun, choc thermique, expression d'une protéine pro-agrégante). In vivo (modèle poisson-zèbre) l'extinction de l'expression de BAG3 ou la surexpression des mutants conduisent à l'apparition d'un phénotype d'insuffisance cardiaque (¿dème péricardique) chez les embryons injectés. Par édition génomique, nous développons également un modèle de cardiomyocytes dérivés de cellules iPS porteurs ou non de la mutation afin d'explorer plus en avant la fonction contractile de ces cellules. Nos résultats confirment donc le rôle de BAG3 dans la CMD et indiquent que l'altération de la fonction protéostasique serait à l'origine de la maladie. Cette nouvelle voie physiopathologique dans la CMD pourrait s'avérer être, plus généralement, une voie centrale dans l'IC. / The BAG3 gene was identified as a novel gene responsible for dilated cardiomyopathy (DCM), a major cause of heart failure (HF). The BAG3 protein is a co-chaperone that participates in the control of protein homeostasis via its role in autophagy, protecting cells against the proteotoxicity induced by degraded or misfolded proteins. The hypothesis that inactivation of the autophagic pathway controlled by BAG3 would induce cardiomyocyte proteotoxicity behind the CMD appears particularly attractive and is the central hypothesis of this work. Our results indicate that BAG3 mutations abolish the interaction with the chaperone HSP70, a central actor of the protein quality control. We observed cytotoxicity of BAG3 mutants, an impaired HSP70-dependent chaperone function and absence of autophagic response under stress conditions (starvation, heat shock, expression of a pro-aggregating protein). In vivo (zebrafish model) the extinction of BAG3 expression or mutants overexpression lead to the occurrence of a heart failure phenotype (pericardial edema) in injected embryos. Through genomic edition, we also develop a model of iPS-derived cardiomyocytes carrying or not the mutation in order to further explore the contractile function of these cells. Our results confirm the role of BAG3 in DCM and indicate that the alteration of the proteostasis function is the cause of the disease. This new pathophysiological pathway in DCM may prove to be more generally, a central line in the IC.

HSP70 plays an important role in cancer development. However, the molecular role of HSP70 in cancer is poorly understood. Previous work from our laboratory demonstrated that HSP70 is essential for initiation of Her2-positive breast cancer by controlling oncogene-induced senescence. Here we demonstrate that HSP70 is critical for both initiation and progression of mammary cancer. Interestingly, the role of HSP70 in cancer development did not involve its canonical function as a molecular chaperone. Instead, HSP70 had multiple effects on signaling pathway components related to tumor initiation, growth, and metastasis, such as FOXM1, HIF1, NF-𝜅B, and SRC. HSP70 regulated signaling networks via association with the co-chaperone, BAG3, a scaffold protein with capacity to interact with multiple key regulators of cell signaling. Using SRC as a model, we demonstrated that association with HSP70 attenuates BAG3's interaction with the SH3 domain of SRC. We also show that an HSP70-interacting small molecule, YM-1, can specifically inhibit the HSP70-BAG3 signaling axis, leading to selective inhibition of tumor growth in vivo and in vitro. This compound mimicked the effects seen with depletion of HSP70 in a dose dependent manner, providing a proof of principle that the association of HSP70 and BAG3 is needed for regulation of these pathways. Additionally, a second generation of YM-1 analogs, JG-98 and JG-84, were shown to be more potent than YM-1 while acting in a similar fashion on signaling pathways. A less potent analog, JG-36, was not able to modulate these pathways as effectively. These studies demonstrate that the HSP70-BAG3 axis is a major regulator of cancer signaling and suggest that targeting the interface between HSP70 and BAG3 is a novel therapeutic approach.

Heart disease has been the leading cause of death in the United States for more than 90 years. The leading cause of death in individuals aged 65 and older has remained diseases of the heart from 1950 to the current time. According to the CDC, once diagnosed with heart disease, individuals have an approximately 50% chance of dying within 5 years, regardless of race. Mortality related to heart disease increased dramatically from the start of the 1900s to 1921, but subsequently experienced a steady decline from the mid-1960’s to 2000. However, when the decrease in heart disease is examined at the level of race it is clear that the decrease is not equally shared. While the leading cause of death among both Caucasian American men and women and African American men and women remains heart disease, the decrease in incidence of coronary heart disease among African American men was only half of the decrease in incidence among Caucasian American men. Genetic variants in BAG3 (Bcl-2 associated athanogene 3), a highly evolutionarily conserved gene that has recently emerged as a major dilated cardiomyopathy locus, are prevalent in isolated populations. This led us to hypothesize that variants in BAG3 might contribute to the increased prevalence of IDC in individuals of African ancestry. Expressed predominantly in the heart, the skeletal muscle and in many cancers, BAG3 has pleotropic effects in the heart. It inhibits apoptosis by binding to Bcl-2, facilitates protein quality control by binding to both large and small heat shock proteins, mediates adrenergic responsiveness by coupling the β-adrenergic receptor and the L-type Ca2+ channel, and maintains the integrity of the sarcomere by anchoring actin filaments to the Z disc. However, a paucity of subjects of African ancestry have been included in cohorts of probands with familial dilated cardiomyopathy whose exomes or genomes have been sequenced. Based on our previous observations and reports from other groups we postulated: 1) that mice with haplo-insufficiency of BAG3 will re-capitulate disease seen in humans and serve as a model for studying the pathogenesis of BAG3. 2) The prevalence or identification of specific BAG3 variants will differ by race and/or ethnicity. 3) SNVs of BAG3 may contribute to disease progression and thereby be pathogenic. Our study points out that we cannot understand population-based differences without enhancing the diversity of populations included in genomic studies. Similarly, in the era of big data, efforts must be undertaken to assess the genetic profile of both probands and their family members as without the ability to measure segregation, penetrance and plasticity we can only ascribe associations to functional genetic variants. / Biomedical Sciences