The Effects Of Beta-1 And -2 Adrenergic Receptor Genotypes On Cardiopulmonary Outcomes In Duchenne Muscular Dystrophy

Abstract

Introduction: The main contributor of mortality in Duchenne muscular dystrophy (DMD) patients is cardiorespiratory failure. The beta-1 adrenergic receptor (ADRB1) has been shown to play a functional role in cardiomyocyte function with ADRB1 stimulation increasing cardiac rate, contractility, and work. Multiple polymorphisms of the ADRB1 have been identified such as the Gly49 polymorphism that includes at least one glycine (Gly) for serine (Ser) substitution at amino acid 49 resulting in either homozygous for Gly (Gly49Gly) or heterozygous (Gly49Ser) polymorphisms and a Gly for Arg substitution at amino acid 389 resulting in either homozygous for Gly (Gly389) or heterozygous (Arg389). Heart failure patients with these polymorphisms (Gly49 and Arg389) have been shown to have improved cardiac function and decreased mortality risk. Furthermore, the beta-2 adrenergic receptor (ADRB2) has been shown to influence respiratory muscle strength and function. Multiple polymorphisms of the ADRB2 have been identified as including a glycine (Gly) for arginine (Arg) substitution at amino acid 16. The Gly16 polymorphism has been shown to have higher receptor density on lymphocytes, be more resistant to receptor downregulation, and functionally demonstrate improved respiratory function when compared with the Arg16 genotype in humans. Purpose: The purpose of this dissertation was to assess the functional consequences of ADRB1 genotypes on cardiac function and ADRB2 genotypes on pulmonary function in patients with DMD. We hypothesize DMD patients with the Gly389 polymorphism would have a lower incidence of cardiac events compared with those expressing Arg389 polymorphism and that DMD patients with the Gly16 polymorphism would have a reduced risk of nocturnal ventilation (NV) use at any given age compared with those patients expressing the Arg16 polymorphism. Methods: For study 1, we performed genotyping of the ADRB1 (amino acid 49) and high-intensity, steady-state exercise on 71 healthy subjects (Ser49Ser = 52, Gly49Ser = 19). For study 2, we performed genotyping of the ADRB2 (amino acid 16) and high-intensity, steady-state exercise on 77 healthy subjects (AA = 18, AG = 25, GG = 34). Data from CINRG-DNHS including 175 DMD patients (ages 3-25 yrs) with up to 9.7 years follow-up were analyzed focusing on ADRB1 and ADRB2 functional variants for studies 3 and 4. We performed Cox proportional hazard and Kaplan-Meier time to event analyses for the age of NV use and the age of cardiac outcomes and interventions. Results: There were no differences between ADRB1 genotype groups in age, height, weight, BMI, or watts achieved in the healthy patients. Additionally, there were no differences between genotype groups for cardiac output (CO), systolic blood pressure (BPsys), or diastolic blood pressure (BPdias) at rest, maximal exercise, or in change from rest to maximal exercise. There were, however, differences between genotype groups for resting CI and SVR and for HR at peak exercise (HRmax) with the Gly49Ser genotype presenting improved CI and a lower SVR at rest, and a higher HR at peak exercise. There was a trend towards significance (p = 0.058) for the change in stroke volume from rest to peak exercise (∆SV) with the Ser49Ser genotype demonstrating a larger change in SV. There were no differences between ADRB2 genotype groups in age, height, weight, or BMI in the healthy patients. The genotype groups differed significantly in watts, and watts/VO2 with heavy exercise with the Gly16 genotype achieving higher workloads. There was a trend towards significance (p=0.058) for watts/kg. There were no differences between ADRB1 genotype groups in age, height, weight, number of ambulatory patients, or age of loss of ambulation in our DMD cohort. The Arg389 polymorphism demonstrated a higher mean corticosteroid use compared with the Gly389 polymorphism. The genotype groups differed significantly (P<0.05) in the risk of diuretics use with the Gly389 polymorphism demonstrating a 5.01-fold increased risk of diuretics use at any age compared with the Arg389 polymorphism. There were no differences between ADRB2 genotype groups in age, height, weight, corticosteroid use, number of ambulatory patients, or age of loss of ambulation in our DMD cohort. The Gly16 polymorphism demonstrated a higher probability (P<0.05) for the use of NV assistance at any given age compared with the Arg16 polymorphism. The genotype groups differed significantly in the risk of NV use with the Gly16 polymorphism demonstrating a 2.77-fold increased risk of using NV at any given age compared with the Arg16 polymorphism. Conclusion: These data suggest genetic variation in the ADRB1 gene may influence the age of diuretics use in DMD patients with DMD patients expressing the Gly389 polymorphism being more likely to use diuretics compared with patients expressing the Arg389 polymorphism. Additionally, these data suggest genetic variation in the ADRB2 gene may also influence the age of NV use in DMD patients. Specifically, DMD patients expressing the Gly16 polymorphism were more likely to use NV at any given age compared with patients expressing the Arg16 polymorphism.