Navigating ATTR-CM Diagnosis and Biology

CardioNerds Cardiac Amyloidosis Series Chair Dr. Rick Ferraro and Episode Lead Dr. Anna Radakrishnan discuss the biology of transthyretin amyloid cardiomyopathy (ATTR-CM ) with Dr. Daniel Judge.  Notes were drafted by Dr. Anna Radakrishnan. The audio was engineered by student Dr. Julia Marques. 

This episode provides a comprehensive overview of transthyretin (ATTR) cardiac amyloidosis, a complex and rapidly evolving disease process. The discussion covers the key red flags for cardiac amyloidosis, the diagnostic pathway, and the implications of hereditary versus wild-type ATTR. Importantly, the episode delves into the current and emerging therapies for ATTR, including stabilizers, gene silencers, and promising treatments like CRISPR-Cas9 and antibody-based approaches. Dr. Judge shares his insights and excitement about the rapidly advancing field, highlighting the need for early diagnosis and the potential to improve long-term outcomes for patients with this condition. 

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Pearls: – Biology of Transthyretin amyloid cardiomyopathy

1. Maintain a high index of suspicion! Look for subtle (yet telling) signs like ventricular hypertrophy, discordant EKG findings, bilateral carpal tunnel syndrome, and spontaneous biceps tendon rupture. 

2. Utilize the right diagnostic tests. Endomyocardial biopsy remains the gold standard, but non-invasive tools like PYP scan with SPECT imaging and genetic testing are essential for accurate diagnosis. 

3. Differentiating hereditary from wild-type ATTR is critical, as genetic forms may have a more aggressive course and familial implications. 

4. Early diagnosis and intervention significantly improve prognosis, making vigilance in screening and prompt treatment initiation essential. 

5. The future is now! Cutting-edge therapies are transforming the treatment landscape, including TTR stabilizers, gene silencers, and emerging technologies like CRISPR-Cas9 and antibody-based treatments. 

Notes – Biology of Transthyretin amyloid cardiomyopathy

1. What is transthyretin amyloid (aTTR) and how is it derived? 

• Transthyretin (TTR) is a transport protein primarily synthesized by the liver, responsible for carrying thyroid hormones (thyroxine) and retinol (vitamin A) in the blood. It circulates as a tetramer, composed of four identical monomers, which is essential for its stability and function. 

• In transthyretin amyloid (ATTR) amyloidosis, the TTR protein becomes unstable, leading to its dissociation into monomers. These monomers misfold and aggregate into insoluble amyloid fibrils, which deposit extracellularly in tissues such as the heart, nerves, and gastrointestinal tract. This progressive amyloid deposition leads to organ dysfunction, including restrictive cardiomyopathy and neuropathy. 

• There are two main forms of ATTR amyloidosis: hereditary (variant) and wild-type (senile) ATTR. 

• Hereditary ATTR (ATTRv) is caused by mutations in the TTR gene. These mutations destabilize the TTR tetramer, making it more prone to dissociation. This increases misfolding and amyloid fibril formation, resulting in systemic amyloid deposition.  

• Wild-type ATTR (ATTRwt) occurs without genetic mutations and is primarily age-related. Over time, even normal TTR tetramers can become unstable, leading to gradual misfolding and amyloid deposition, particularly in the heart. ATTRwt is a common but often underdiagnosed cause of heart failure with preserved ejection fraction (HFpEF) in elderly individuals. 

2. How does aTTR lead to deleterious effects in the heart and other organ systems?   

• Transthyretin amyloidosis leads to organ dysfunction through the deposition of misfolded TTR protein as amyloid fibrils, which accumulate extracellularly and disrupt normal tissue architecture and function. These deposits cause progressive damage by increasing stiffness, inducing oxidative stress, and impairing normal cellular function. 

• Cardiac manifestations include amyloid deposition in the myocardial interstitium, leading to increased stiffness, diastolic dysfunction, and restrictive cardiomyopathy. As the disease progresses, systolic dysfunction may develop. Amyloid infiltration can also cause arrhythmia, conduction abnormalities such as atrioventricular block and atrial fibrillation, valvular thickening, coronary ischemia, and pericardial effusion. Disruption of transverse tubules in cardiomyocytes contributes to heart failure and arrhythmia. 

• Systemic involvement depends on the culprit amylodogenic protein. AL amyloidosis caused by deposition of immunoglobulin light chains may deposit in and disrupt the function of any tissue/organ except for the central nevous system. ATTR amyloidosis primarily affects the heart, peripheral nerves, and the musculoskeletal system.  

• Peripheral neuropathy can cause sensory loss, pain, and motor weakness, while autonomic dysfunction may lead to orthostatic hypotension, gastroparesis, and urinary retention. Carpal tunnel syndrome is a common early sign. Gastrointestinal amyloid deposits (specifically for AL but not ATTR) can cause gastroparesis, diarrhea, constipation, and malabsorption, leading to weight loss and malnutrition. Renal involvement (specifically for AL but not ATTR), though less common, can present as proteinuria and renal dysfunction. Amyloid deposition in soft tissues and the lungs may lead to hoarseness and musculoskeletal stiffness. 

• As the disease progresses, continued amyloid accumulation leads to worsening organ dysfunction and failure. Early diagnosis and intervention are essential to slowing disease progression and managing symptoms effectively. 

3. When and why is aTTR cardiac amyloidosis hereditary versus obtained sporadically? 

• Hereditary aTTR is caused by genetic mutations in the TTR gene, which are often autosomal dominant.  

• Common mutations include V122I (more common in African Americans) and V30M (more common in certain regions like Portugal).  

• Hereditary aTTR typically presents at an earlier age and may have a more aggressive course, with a higher likelihood of neuropathic involvement.  

• Wild-type aTTR, or senile systemic amyloidosis, occurs sporadically and is more common in older individuals, typically without a family history.  

• The exact reasons for the development of wild-type aTTR are not fully understood, but factors like chronic inflammation and exercise may play a role in the misfolding and aggregation of the normal TTR protein. 

References: Biology of Transthyretin amyloid cardiomyopathy

1. Ruberg FL, Maurer MS. Cardiac Amyloidosis Due to Transthyretin Protein. JAMA. 2024;331(9):778-778. https://doi.org/10.1001/jama.2024.0442  

2. Ruberg FL, Grogan M, Hanna M, Kelly JW, Maurer MS. Transthyretin Amyloid Cardiomyopathy. Journal of the American College of Cardiology. 2019;73(22):2872-2891. https://doi.org/10.1016/j.jacc.2019.04.003  

3. ‌Maurer MS, Bokhari S, Damy T, et al. Expert Consensus Recommendations for the Suspicion and Diagnosis of Transthyretin Cardiac Amyloidosis. Circulation: Heart Failure. 2019;12(9). https://doi.org/10.1161/circheartfailure.119.006075  

4. ‌Griffin JM, Rosenthal JL, Grodin JL, Maurer MS, Grogan M, Cheng RK. ATTR Amyloidosis: Current and Emerging Management Strategies. JACC: CardioOncology. 2021;3(4):488-505. https://doi.org/10.1016/j.jaccao.2021.06.006