Exam 5

• Cardiomyopathies

  Diseases of the heart muscle
• Myosin binding protein-C (Maine coon vs Ragdolls)

  Understand how this impacts the structure and function of the protein

  
• ALMS-1 (Sphynx)


  Gene mutation associated with Alstrom syndrome in humans
• Hypertrophic cardiomyopathy (HCM)

  • Left ventricular concentric hypertrophy in the absence of other causes
  • Impaired diastolic function (relaxation)
  • Myocardial fibrosis
  • Myocyte disarray
  • Altered calcium kinetics
  • Coronary artery arteriosclerosis

  
• Myosin binding protein-C (MyBPC)

  • Interacts with titin, myosin, and actin
  • Regulatory protein with 11 domains
  • Helps maintain thick filament structure
  • Assists in regulating contraction - influences speed and force of contraction
  • Multiple phosphorylation sites
  • Modulated by second messengers
  • Regulates interactions between thick and thin filaments → force generation

  
• Mutations in MyBPC gene

  • Commonly cause HCM
  • In humans, ~27% of genetic cardiomyopathies are due to mutations in this gene
  • Mutations → haploinsufficiency or dysfunctional protein
• HCM in Maine Coons - MyBPC mutation

  • Single base pair substitution in codon 31 → changes alanine to proline (highly conserved)
  • Specific mutation name: A31P
  • Causes truncation, may impact binding affinity for actin or disrupt myosin's ability to generate force
  • Reduced expression of MyBPC protein (also myomesin, titin, actin)
  • Truncated MyBPC more rapidly degraded → inadequate integration into sarcomere → abnormal myosin positioning → disruption of normal sarcomere anatomy
  • Concentric hypertrophy of left ventricle

  
• HCM in Maine Coons - MyBPC mutation

  • Highly prevalent mutation – 34-41% of the population
  • Autosomal dominant with incomplete penetrance
  • Low penetrance in heterozygous cats (~6-8%)
  • Higher penetrance in homozygous positive cats (~50-60%)
  • Age of onset 5-7 years
  • May be younger if homozygous positive
• HCM in Ragdolls - MyBPC mutation

  • Single base pair substitution in codon 820 → changed arginine (R) into tryptophan (W)
  • Specific mutation name: R820W
  • Main impact = L ventricular concentric hypertrophy

  
• HCM in Ragdolls - MyBPC mutation

  • Prevalence ~20%
  • Earlier onset disease – average 15-18 months
  • Homozygous positive cats develop worse disease at a younger age compared to heterozygous positive
  • Heterozygotes unlikely to develop disease before 4-5 years of age
• HCM in the Sphynx - ALMS1 mutation

  • Mutation in exon 12
  • Glycine → arginine, changes protein structure
  • Histopathologic changes: Myofiber disarray, Interstitial fibrosis, Normal cardiomyocytes
  • Function of the protein produced by ALMS1 gene is unclear: Associated with energy metabolism and homeostasis, Intracellular trafficking, Cell signaling, Cell differentiation, Cell cycle control

  
• HCM in the Sphynx - ALMS1 mutation


  • Variable age of onset – 1-14 years
  • Highly variable disease severity
  • Incomplete penetrance
  • Not all Sphynx cats with HCM have this mutation
  • Other mutations must exist
• Known genetic mutations account for a significant proportion of HCM in breeds such as Maine Coons, Ragdolls, and Sphynx
• These breeds can develop HCM and not have the expected causative genetic mutation
• The most common breed of cat that develops HCM is the Domestic shorthair
• No known genetic mutation(s) in the Domestic shorthair breed
• Normal Cardiomyocyte Structure


  Sarcolemma, Sarcomeres, Mitochondria, Sarcoplasmic reticulum, Nucleus
• The Sarcomere

  Z disc, Actin, Troponins and tropomyosins, Myosin, Titin

  
• What Can Cause Congestive Heart Failure?

  Abnormal cardiac muscle, Abnormal valves, Abnormal blood shunting, Arrhythmias, Ischemia

  
• Types of Hypertrophy

  Dilation = eccentric (Volume overload), Thickening = concentric (Pressure overload)
• Dilated Versus Hypertrophic Cardiomyopathy

  Dilated: wall thickness down, volume up. Hypertrophic: thickness up, volume down.
• Canine Cardiomyopathies

  • Genetic mutations are the primary cause
  • Cardiomyopathies = second most common heart disease category in dogs
  • Cardiomyopathies can lead to: arrhythmias, congestive heart failure, sudden death
• Focus Of Lecture: Genetics of Canine Cardiomyopathies

  • Canine cardiomyopathies = dilated cardiomyopathy (DCM), arrhythmogenic right ventricular cardiomyopathy (ARVC)
  • Feline cardiomyopathy = hypertrophic cardiomyopathy (HCM)
  • Other cardiomyopathies: restrictive cardiomyopathy, nonspecific phenotypes
• Dilated Cardiomyopathy (DCM)

  • Second most common heart disease in dogs
  • Affected breeds: Dobermans, boxers, great danes, newfoundlands, irish wolfhounds, english cocker spaniels
  • Myocardial disease causing systolic dysfunction → ventricular dilation, ischemia/fibrosis → arrhythmias
• Stages

  1. Stage A - at risk (normal heart, no signs of disease, +/- genetic mutation)

  2. Stage B - occult phase (disease present - structural or electrical changes, NO clinical signs)

  3. Stage C - overt phase (clinical signs present - due to congestive heart failure or arrhythmias)

  
• Causes of DCM

  • Primary: genetic, idiopathic
  • Secondary: diet-associated, toxicity (doxorubicin), tachycardia-induced, infectious disease (parvovirus, chagas disease)
• DCM in Dobermans — Genetic Basis

  • Autosomal dominant with incomplete penetrance
  • Common disease (40% prevalence in US, up to 58% in Europe)
  • Onset as young as 2-4 years; usually 5-7 years
  • Sex difference in disease manifestation (males = earlier structural changes, females = more ventricular arrhythmias)
• Quick Aside on Cardiac Metabolism

  • Heart can use both fats and carbs for energy
  • Prefers fatty acid oxidation (fats produce more ATP than glucose)
• Dobermans — PDK4 Mutation

  • Pyruvate dehydrogenase kinase 4 (PDK4) controls energy pathway
  • Mutation shifts metabolism towards glycolysis → impaired mitochondrial electron transport chain → mitochondrial damage, inadequate energy levels
  • Associated with mitochondrial abnormalities on electron microscopy
  • Genetic test available, 10x more likely to develop DCM if mutation present
• Dobermans — Titin Mutation

  • Titin = largest known protein, acts as a molecular spring
  • Genetic test available, 21x more likely to develop DCM
  • 30x more likely to develop DCM if positive for both titin and PDK4 mutations
• DCM can develop in dobermans with neither genetic mutation
• DCM in Standard Schnauzers

  • Mutation in gene for RNA-binding motif protein 20 (RMB20)
  • Autosomal recessive, highly associated with DCM in standard schnauzers
  • Heterozygous positive live more normal life, homozygous positive have significantly shortened lifespan
• Arrhythmogenic Right Ventricular Cardiomyopathy (ARVC)

  • Aka "Boxer Cardiomyopathy"
  • Fibro-fatty infiltration of myocardium (mostly right ventricle), myocyte atrophy, patchy necrosis
  • Mean age 5-7 years
  • Preclinical stage = arrhythmias, Clinical stage = syncope, Myocardial dysfunction stage = arrhythmias plus congestive heart failure - dilated and poorly contractile L ventricle
  • Many dogs are asymptomatic until developing collapse or sudden death
• ARVC

  • Autosomal dominant, incomplete penetrance
  • In humans, mutations associated with proteins in the intercalated disc (desmosome)
  • Postulated mechanisms: aberrant localization of plakoglobin, dysregulation in calcium handling by sarcoplasmic reticulum
• Genetics of ARVC

  • American boxers — strong correlation with mutation in striatin gene
  • UK boxers — mutation in striatin also present but no correlation to ARVC
  • Boxers can have ARVC without the striatin mutation
  • No apparent correlation with striatin mutation in English bulldogs
  • Sporadically present in other breeds (case reports)
• Myosin binding protein-C (Mainecoon vs. Ragdolls)

  • Understand how this impacts the structure and function of the protein
  • Understand how this impacts cardiac structure and function

  
• Hypertrophic Cardiomyopathy (HCM)

  • L ventricular concentric hypertrophy with no other causes — wall thickening, chamber lumen decreases
  • Hypertrophy may be symmetric or asymmetric
  • Hallmark findings: impaired diastolic function (relaxation), myocardial fibrosis, myocyte disarray, altered calcium kinetics, coronary artery arteriosclerosis

  
• In humans, HCM commonly due to genetic mutation in sarcomere proteins or proteins closely related to sarcomere function
• Mutations


  • Usually missense mutations — one highly conserved nucleotide is replaced by another, that replacement causes the codon to be different, leading to different AA → disruption in protein structure or misfunction
  • Poison polypeptides, haploinsufficiency
  • Individual families may have unique mutations
  • Founder mutations: same mutation found within many families
• Myosin Binding Protein-C (MyBPC)

  • Interacts with titin, myosin, and actin
  • Regulatory protein with 11 domains
  • Helps maintain thick filament structure
  • Assists in regulating contraction — influences speed and force of contraction
  • Multiple phosphorylation sites
  • Modulated by second messengers — protein kinase A or C
  • Regulates interactions between thick and thin filaments → force generation
  • Disrupted with disease