Williams syndrome: A Translational Approach to Mechanism and Treatment

National Heart, Lung, And Blood Institute

Investigators

Linked publications, trials & patents

Abstract

Using a translational approach consisting of animal models and human studies, we have sought to 1) examine the impact of elastin insufficiency on end organs, 2) investigate the cause of sudden death in this population, and 3) test potential therapies to treat elastin mediated vasculopathy. Impact of elastin insufficiency on end organs: The effect of elastin insufficiency on conducting vessels has been well described, consisting of reduced elastin content and a reciprocal increase in arterial smooth muscle cell number. This change in the cell to matrix ratio produces vessels with narrow luminal diameter and thickened, poorly compliant walls. Mice and humans with decreased elastin have increased resting blood pressure, higher pulse wave velocity (a marker for arterial stiffness), and reduced blood flow through the major conducting arteries. Subsequent investigation in Eln+/- mice using blood flow imaging has shown globally reduced brain perfusion relative to wild types. To investigate these findings in humans, we developed a human protocol to allow us to broadly evaluate the impact of elastin insufficiency on end organs. To date, 129 individuals have undergone complete evaluation at the NIH for a total of 320 separate patient visits. In the last year, data from this patient cohort yielded 4 published manuscripts with one still under review and four in preparation. Recently accepted work includes: 1) a study of lung function in this population--elastin also impacts the mechanical function of the lung and adults with Williams syndrome (WS) experience increased air trapping and obstruction (Kronquist et al, Diagnostics), 2) a study utilizing the NIH tool kit, to study executive function in people with intellectual disability (Condy et al, Amer J Intel and Dev Disabil), and 3) a descriptive study looking at ocular findings in people with WS (Huryn et al, Br J Ophthalmol). Additionally, human data from this cohort were used to investigate residual pulmonary vascular disease in people with WS with and without a history of pulmonary surgical intervention (Knutsen et al, Front Cardiovasc Med.). One study, outlining differences in heart rate variability in people with WS is still under review (see details below) while work investigating gastrointestinal outcomes, cardiac dysfunction and strain abnormalities, and retinal vascular tortuosity are in progress. Knowledge gained from these studies will impact patients with WBS/SVAS, as it will be important as we consider potential new therapies to balance a drugs impact on blood pressure and arterial stiffness to its impact on tissue perfusion and end organ impact. It will also have an impact for understanding the changes to blood flow that occur with normal aging, a process that involves gradual loss of elastin over the lifetime. Sudden death in Williams Beuren syndrome: Individuals with elastin insufficiency are reported to have a 25-100X increased risk of sudden death; this risk is enhanced during anesthesia. We questioned whether features of the heart itself may contribute to sudden death risk in this population. Evaluation of coronaries in both the WBS patients and Eln+/- mouse found increased tortuosity for the vessels and studies designed to recapitulate anesthesia hemodynamic changes in mice resulted in hemodynamic collapse and ventricular fibrillation in study animals but not controls. Further investigation of dissociated Eln+/- mouse and control cardiomyocytes showed: 1. Increased inward calcium current 2. Abnormal calcium current kinetics 3. Diminished potassium currents 4. Prolonged action potential duration (APD) and 5. Early after-depolarizations (EADs) which are a cellular correlate for increased arrhythmia in the Eln+/- mice. We conceptualize a two-hit type mechanism where coronary abnormalities underlie substrate abnormalities and cardiomyocyte abnormalities result in increased trigger formation. Together, these abnormalities give rise to increased arrhythmia, likely ventricular fibrillation (VF), or cardiac collapse, both of which we have demonstrated in animal model testing. We are currently performing in vivo cardiac electrophysiology studies to further validate sudden death phenotype and better define arrhythmia mechanism inferred by previous findings. Preliminary studies confirm increased ventricular arrhythmia in Eln+/- mice and suggest increased atrial arrhythmia as well. Following further validation of these results we will perform rescue experiments in Eln+/- mice with KATP conditionally deleted in cardiomyocytes or smooth cell cells to further test our hypotheses. We anticipate submitting the resulting manuscript at the conclusion of these experiments. Together with our heart rate variability work, we believe that our findings represent a critical sudden death biomarker and mechanistic insight that may be useful for both assessing risk as well as effectiveness of interventions designed to mitigate this risk. In particular, HRV provides a means of quantifying autonomic system abnormalities using ambulatory ECG data from human subjects and controls. Similar studies in post-myocardial infarction, stroke, and diabetes mellitus have shown that diminished HRV provides reliable means for risk-stratifying these latter patients, correlating increased morbidity and mortality with diminished HRV. Our data confirms the presence of HRV abnormalities in people with WS using time and frequency domain, as well as non-linear analyses. Each show multiple biomarkers consistent with decreased HRV and decreased parasympathetic activity. Increased sympathetic activity was inferred from increased heart rate. We are currently completing revisions and expect our manuscript will be accepted for publication shortly. Therapies for elastin mediated disease: As stated above, individuals with elastin insufficiency have stiff blood vessels with narrow caliber. Previous work by our lab showed that this combination of findings contributed to decreased cerebral perfusion in Eln+/- mice. Previous analyses showed that treatment with minoxidil, at KATP channel opener, normalized blood pressure, reduced apparent stiffness and improved brain perfusion. Once completed, the vascular remodeling remains stable for some time after the drug is removed. The lab recently published a study extending a similar line of inquiry to the pulmonary vascular tree. Pulmonary vascular abnormalities are found 40-60% of those with elastin haploinsufficiency (EH) and WS. Moreover, moderate to severe stenoses in pulmonary arterial vasculature encompasses the sub-cohort with the highest morbidity and mortality of those with EH or WS. Surgical palliation of these vessels can only be performed in a handful of centers in the US and the most severe cases have mostly been treated at one center by one surgeon, and even then, mortality is high, and results are variable. Thus, delineating mechanisms underlying this portion of disease is critical and establishing therapies tailored to these mechanisms is significant barrier to reducing morbidity and mortality in children and adults with Williams. Our recent publication (Knutsen et al, 2022, Frontiers in Cardiology) both advances our understanding of human pulmonary vascular disease phenotype and disease mechanism by forwarding quantitative human CT angiogram and echocardiographic data and demonstrating how aspects of these findings are recapitulated in the elastin haploinsufficient mouse model. Moreover, this work further demonstrates how the KATP channel opener Minoxidil (here evaluated in mice) reverses aspects of these abnormalities following chronic therapy. In addition to the above efforts, we also contributed to an editorial piece aimed at outlining optimal methods and approaches for investigating the impact of vascular disease on end organs like the brain (Muratoglu et al, ATVB).