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

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 µg/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,6–8 but with clinical manifestations mainly from the heart attributable to restrictive cardiomyopathy.6,9–11 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.12 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.12,13 Hence, both senile systemic amyloidosis and hereditary forms may have deposition of amyloid in small arteries.

degenerative disease amyloidosis, a term for a group of dis- eases characterized by extracellular deposition of pathological insoluble fibrillar proteins in organs and tissues.1 Transthyretin is primarily synthesized in the liver, but is also produced in the choroid plexus.1–3 Although the main function is to transport retinol-binding protein, transthyretin also transports ≈15% and 80% of thyroxine in blood and cerebrospinal fluid, respectively.3 In both plasma and cerebrospinal fluid, transthyretin circulates as a tetramer comprising 4 identical subunits. Destabilization of these tetramers leads to monomer misfolding and amyloido- genesis, a process that can be prevented by ligands, such as thyroxine, and by drugs that stabilize transthyretin tetramers.4,5 In humans, both wild-type transthyretin tetramers and

tetramers comprising mutant and wild-type subunits can cause amyloidosis.4 Senile systemic amyloidosis is a common

From the Department of Clinical Biochemistry, Rigshospitalet, Copenhagen, Denmark (L.S.H., R.F.-S., A.T.-H.); The Copenhagen City Heart Study, Frederiksberg Hospital, Frederiksberg, Denmark (B.G.N., A.T.-H.); Department of Clinical Biochemistry (B.G.N.), and The Copenhagen General Population Study (R.F.-S., B.G.N., A.T.H.), Herlev Hospital, Herlev, Denmark; and all Copenhagen University Hospitals and Faculty of Health and Medical Sciences, University of Copenhagen, Copenhagen, Denmark (L.S.H., R.F.-S., B.G.N., A.T.H.).

Correspondence to Anne Tybjærg-Hansen, MD, DMSc, Professor, Chief Physician, Department of Clinical Biochemistry KB 3.01.1, Section for Molecular Genetics, Rigshospitalet, Copenhagen University Hospital, Blegdamsvej 9, DK-2100 Copenhagen, Denmark. E-mail anne.tybjaerg.hansen@regionh.dk © 2013 American Heart Association, Inc.

Arterioscler Thromb Vasc Biol is available at http://atvb.ahajournals.org

Although most mutations in TTR increase the amyloidogenic potential of the protein by destabilizing the tetramers, 2 vari- ants, R104H (rs121918095) and T119M (rs28933981), have been shown to stabilize both wild-type transthyretin tetramers and tetramers comprising mutant and wild-type subunits, and thereby to prevent amyloidogenesis in vitro.14–17 These 2 muta- tions were first identified in compound heterozygotes, carrying one of these variants together with a known disease-causing mutation in Portuguese (T119M) and Japanese (R104H) fami- lies, respectively, and were reported to have milder disease than heterozygotes for the disease-causing mutation alone.14,18 Finally, some family studies have suggested increased thyrox- ine binding or increased levels of thyroxine or transthyretin in carriers of these 2 stabilizing variants.14,18–21