A P2X7R Single Nucleotide Mutation Promotes Chronic Allograft Vasculopathy
Condition: Cardiac Allograft Vasculopathy
MeSH terms: Vascular Diseases
Study type: Observational

Summary: 
      Heart transplantation (HT) is a lifesaving procedure for patients with end-stage heart
      failure and provides a better survival and quality of life if compared to medical treatment.
      HT is subject to alloimmune response, which, if left uncontrolled, is capable of jeopardizing
      long-term cardiac function. Advances in immunosuppression have enhanced the survival of HT
      patients. Nearly 2500 HT per year have been performed in the US during the last 10 years and
      despite significant improvements, long-term survival rates remain poor. More than 20% of
      patients do not survive more than 3 years, and those who survive are afflicted by long-term
      complications of alloimmunity and chronic immunosuppression. Life expectancy of patients who
      lose cardiac allografts is dramatically poor due to the absence of any therapeutic tool apart
      from re-transplantation, which is plagued by poor outcomes. The identification of novel
      therapeutic targets is thus mandatory. ATP/P2X7R signaling in T cells is highly relevant for
      cardiac allograft survival. ATP is a small molecule present at high concentrations inside
      cells; it is released as extracellular ATP (eATP) following cell damage or death where it
      acts as a danger signal. ATP is sensed by the P2X receptors (seven receptors named P2X1-7),
      mainly expressed by T lymphocytes. We have recently demonstrated that the ATP/P2X7R axis has
      a key role in cardiac allograft survival in humans and mice. Cardiac allograft vasculopathy
      (CAV) is a major limiting factor for HT survival; indeed CAV occurs in 50% of HT recipients
      by 5 years after transplantation and invariably results in allograft failure. CAV is clearly
      of immunological origin, as syngeneic murine grafts do not develop it. Once CAV occurs, the
      most definitive treatment is re-transplantation, but survival remains poor. We hypothesize
      that a single nucleotide polymorphysm (SNP) loss-of-function P2X7R mutation (p.Glu496Ala /
      c.1513A>C, rs3751143) generates a compensatory upregulation of the other purinergic receptors
      (P2XsR), thus creating a state of hypersensitivity to eATP. This eATP hypersensitivity
      results in an abnormal generation of Th1/Th17 cells, that leads to CAV and early cardiac
      allograft loss. Our study will answer a fundamental question: What is the effect of the P2X7R
      loss-of-function mutation on the immune system? Our goal is to generate the first
      targeted-therapy for a selected group of cardiac transplant recipients.
    

Detailed description: 
      Heart transplantation is a lifesaving procedure however, more then 20% of patients do not
      survive beyond 3 years, being the cardiac allograft afflicted by cardiac allograft
      vasculopathy (CAV), which results in allograft loss. The purine adenosine 5'-triphosphate
      (ATP), released during cell damage/inflammation, is sensed by the ionotropic purinergic P2X7
      receptor (P2X7R), which is expressed primarily, though not exclusively, on lymphocytes, thus
      regulating T cell activation. Loss-of-function single nucleotide mutations (SNPs) have been
      detected for P2X7R gene; particularly the Glu496 to Ala 1513A>C (rs3751143) P2X7R
      loss-of-function mutation is relatively common (1-3% of individuals are mutated omozygous and
      25% are heterozygous). Our central hypothesis is that a loss-of-function P2X7R mutation
      identifies a group of cardiac transplanted patients at high risk for CAV and cardiac
      allograft loss because of a compensatory overexpression of P2X1R/P2X4R, which induces a
      disregulation of T-bet/ROR-g, ultimately leading to the abnormal generation of Th1/Th17
      cells. Our primary goal is to define the effect of the P2X7R loss-of-function mutation on
      clinical end points in the CTOT-05 cohort of cardiac transplant recipients (200 patients) and
      to explore the effects of the mutation on the immune system. Our preliminary data
      demonstrated that P2X7R increases during cardiac transplant rejection in vivo in mice and in
      humans and it is activated by ATP released by cardiac cells, thus triggering activation of
      Th1/Th17 cells. However, while short-term disruption of the P2X7R pathway prolongs cardiac
      allograft survival, the genetic deletion of P2X7R accelerates CAV and shortens cardiac
      allograft survival. This was evident in P2X7R KO-B6 mice and in a group of cardiac transplant
      recipients bearing the P2X7R loss-of-function mutation. Based on our published results and
      our novel observations, we have developed the following working hypothesis: P2X7R
      loss-of-function mutation generates a compensatory overexpression of the other ionotropic
      purinergic receptors (P2X1/P2X4) with chronic delivery of ATP immunity, hyperactivation
      T-bet/ROR-g, abnormal generation of Th1/Th17 cells and ultimately leading to accelerated CAV
      and to cardiac allograft loss. To test this hypothesis, we will follow two main paths: i) we
      will evaluate in the CTOT-05 cohort of cardiac transplant recipients the effect of the Glu496
      to Ala 1513A>C (rs3751143) P2X7R loss-of-function mutation on clinical end points
      (development of coronary artery vasculopathy, death, re-transplantation or re-listed for
      transplantation, any rejection) in the first year post transplant; ii) we will explore in
      vivo and ex vivo in the CTOT-05 cohort of cardiac transplant recipients the effects of P2X7R
      loss-of-function mutation on the immune system.
    

Observational model: Case-Control
Time perspective: Cross-Sectional
Sampling method: Non-Probability Sample
Study population: 
        Patients from the CTOT-05 cohort of cardiac transplant recipients will be stratified
        according to the genotype (P2X7R+/+, P2X7R+/- and P2X7R-/-) and compared for the
        development of CAV
      
Biospecimen description: 
      Peripheral blood mononulcear cells