Nailing down the prevalence of hereditary TAC: the DISCOVERY study
TAC occurs when transthyretin, a transport protein, becomes destabilized and misfolds, promoting deposition of amyloid fibrils in the myocardium and elsewhere. In the heart, the result is progressive ventricular wall thickening and stiffness, manifest as restrictive cardiomyopathy and progressive nonischemic heart failure. The cause of transthyretin destabilization can be either autosomal dominant inheritance of any of more than 100 pathogenic mutations in the transthyretin gene identified to date or a spontaneous wild-type protein.
Dr. Damy was a coinvestigator in the recently published multicenter DISCOVERY study, in which 1,001 patients with clinically suspected cardiac amyloidosis, the great majority of them from the United States, were screened for pathogenic transthyretin genetic mutations. The overall prevalence of such mutations was 8% in the American patients, with the Val122Ile mutation being identified in 11% of African Americans (Amyloid. 2020 May 26;1-8).
The prevalence of wild-type amyloidosis causing TAC hasn’t yet been studied with anything approaching the rigor of DISCOVERY, but the available evidence suggests the wild-type version is roughly as common as the hereditary forms.
Although DISCOVERY and other studies indicate that TAC is far more common than generally realized, Pfizer has priced Vyndaqel and Vyndamax as though TAC is a rare disease, with a U.S. list price of around $225,000 per year.
“Obviously, the cost will go down over time,” Dr. Seferovic predicted.
Diagnosing TAC
Audience members mostly wanted to know how to identify individuals with TAC who are buried within the huge population of patients with heart failure with preserved ejection fraction. Dr. Damy said it’s actually a simple matter using a screening framework developed by an 11-member TAC expert panel on which he served. A definitive diagnosis can usually be achieved noninvasively at a low cost using bone scintigraphy, he added.
The panel recommended screening via bone scintigraphy in patients with an increased left ventricular wall thickness of 14 mm or more in men over age 65 and women older than 70 who either have heart failure or red flag symptoms.
These red flags for TAC include an echocardiographic finding of reduced longitudinal strain with relative apical sparing, a discrepancy between left ventricular wall thickness on imaging and normal or low-normal voltages on a standard 12-lead ECG, diffuse gadolinium enhancement or marked extracellular volume expansion on cardiac magnetic resonance imaging, a history of bilateral carpal tunnel syndrome, symptoms of polyneuropathy, and mildly increased serum troponin levels on multiple occasions (JACC Heart Fail. 2019 Aug;7[8]:709-16).
Dr. Damy reported receiving institutional research grant support from Pfizer, the study sponsor, and serving on a scientific advisory board for the company.