Genetic Stabilization of Transthyretin, Cerebrovascular Disease, and Life Expectancy 
Louise S. Hornstrup, Ruth Frikke-Schmidt, Børge G. Nordestgaard, Anne Tybjærg-Hansen 

Objective
Transthyretin can cause amyloidosis attributable to destabilization of transthyretin tetramers in plasma. We tested the hypothesis that genetic stabilization of transthyretin associates with reduced risk of vascular disease and increased life expectancy.

Approach and Results
We included 68,602 participants from 2 prospective studies of the general population. We genotyped for 2 stabilizing genetic variants in the transthyretin gene (TTR), R104H and T119M, and determined the association of genotypes with plasma levels of transthyretin, measures of thyroid function, risk of vascular disease, and life expectancy. During a mean follow-up of 32 years, 10,636 participants developed vascular disease. We identified 321 heterozygotes for T119M (frequency, 0.47%); R104H was not detected. First, mean plasma transthyretin and thyroxine levels were increased by 17% (26 g/mL) and 20% (19 nmol/L), respectively, in heterozygotes versus noncarriers (P=0.007 and P<0.0001), demonstrating functionality of this variant in the general population. Second, corresponding hazard ratios were 0.70 (95% confidence interval, 0.51-0.97) for all vascular diseases, 0.85 (0.59-1.23) for cardiovascular disease, 0.45 (0.25-0.81) for cerebrovascular disease, 0.47 (0.25-0.88) for ischemic cerebrovascular disease, and 0.31 (0.04-2.22) for hemorrhagic stroke. The cumulative incidence of cerebrovascular disease as a function of age was decreased in heterozygotes versus noncarriers (P=0.005). Third, median age at death from all causes, from vascular and cerebrovascular diseases, and after diagnosis of vascular disease, and median age at diagnosis of vascular disease, was increased by 5 to 10 years in heterozygotes versus noncarriers (P=0.002-0.05).

Conclusions
These results are compatible with an association between genetic stabilization of transthyretin and decreased risk of cerebrovascular disease, and with increased life expectancy in the general population. (Arterioscler Thromb Vasc Biol. 2013;33:1441-1447.)

Key Words: amyloid, amyloid angiopathy, cerebrovascular disease/stroke, genetic epidemiology, vascular disease, wild-type transthyretin in many organs and in the walls of small arteries but with clinical manifestations mainly from the heart attributable to restrictive cardiomyopathy. The hereditary forms of transthyretin amyloidosis are presumably rare and caused by mutations in the transthyretin gene (TTR) that make the tetrameric protein more unstable and prone to misfold into fibrils. The hereditary forms of transthyretin amyloidosis are typically of earlier onset and more severe than in senile systemic amyloidosis. Clinically, these patients may present with polyneuropathy, cardiomyopathy, or cerebral amyloid angiopathy characterized by deposition of amyloid in cerebral blood vessels, and clinically by both hemorrhagic and ischemic lesions. Hence, both senile systemic amyloidosis and hereditary forms may have deposition of amyloid in small arteries. 

Amyloidosis is a term for a group of degenerative diseases characterized by extracellular deposition of pathological insoluble fibrillar proteins in organs and tissues. Transthyretin is primarily synthesized in the liver, but is also produced in the choroid plexus. Although the main function is to transport retinol-binding protein, transthyretin also transports 15% and 80% of thyroxine in blood and cerebrospinal fluid, respectively. In both plasma and cerebrospinal fluid, transthyretin circulates as a tetramer comprising 4 identical subunits. Destabilization of these tetramers leads to monomer misfolding and amyloidogenesis, a process that can be prevented by ligands such as thyroxine and by drugs that stabilize transthyretin tetramers. In humans, both wild-type transthyretin tetramers and tetramers comprising mutant and wild-type subunits can cause amyloidosis. Senile systemic amyloidosis is a common form characterized by wild-type transthy(Genotype CC) and 321 TTR T119M Table. Characteristics of Heterozygotes for TTR T119M (rs28933981) vs Noncarriers in the Copenhagen City Heart Study and the Copenhagen General Population Study Combined. 

TTR T119M Genotype Noncarriers (CC) Heterozygotes (CT) P Value No. of subjects (%) 68,281 (99.5) 321 (0.47) Age, y 58 (47-67) 56 (47-68) 0.71 Sex (F/M) 37,805/30,476 188/133 0.25 Total cholesterol, mmol/L 5.6 (4.9-6.4) 5.7 (5.0-6.4) 0.88 Triglycerides, mmol/L 1.4 (1.0-2.1) 1.5 (1.0-2.3) 0.41 LDL cholesterol, mmol/L 3.2 (2.6-3.9) 3.2 (2.6-4.0) 0.92 HDL cholesterol, mmol/L 1.6 (1.3-1.9) 1.5 (1.3-1.9) 0.65 Body mass index, kg/m2 26 (23-28) 26 (23-28) 0.63 Hypertension, % 53% 55% 0.81 Diabetes mellitus, % 4% 4% 0.60 Smoking, % 61% 60% 0.69 Physical activity, % 45% 45% 0.71 Lipid-lowering therapy, % 8% 7% 0.54

Values are median (interquartile range) or percentage. Diabetes mellitus was a self-reported disease, use of insulin, use of oral hypoglycemic drugs, or nonfasting plasma glucose >11 mmol/L. Smoking was active and former smokers. Hypertension was systolic blood pressure >140 mm Hg, diastolic blood pressure >90 mm Hg, or use of antihypertensive medication. Physical activity was 4 hours per week of light physical activity in leisure time. Lipid-lowering therapy was mainly statins. P values by Mann-Whitney U test or Pearson 2 test. HDL indicates high-density lipoprotein; and LDL, low-density lipoprotein.

Figure 1. Plasma levels of transthyretin in T119M heterozygotes vs noncarriers. A, All 1650 participants with transthyretin levels measured. B, Heterozygotes matched 1:7 with controls by age (in 10-year age groups) and by sex. Values are mean standard error of the mean. P values by Mann-Whitney U test. CC indicates noncarriers; and CT, T119M heterozygotes.