The cumulative incidence of cerebrovascular disease was reduced in T119M heterozygotes versus noncarriers (Figure 4; log rank, P=0.005). At the age of 75 years, the cumulative incidence of cerebrovascular disease was 5% in T119M heterozygotes and 12% in noncarriers (Figure 4).

TTR T119M genotype associated with a 20% (19 nmol/L) increase in mean plasma levels of total thyroxine in heterozygotes versus noncarriers. Mean total thyroxine levels were 114 nmol/L and 95 nmol/L, respectively (Figure 2; P<0.0001). In contrast, genotype did not associate with other measures of thyroid function, that is, thyroid stimulating hormone, triiodothyronine or triiodothyronine uptake test, an indirect measure of the amount of thyroxine bound to thyroid-binding globulin, (Figure 2; P=0.56-0.63), or with plasma levels of albumin (Table I in the online-only Data Supplement; P=0.89). In plasma, thyroid-binding globulin normally binds 70% to 80% of thyroxine, transthyretin binds 10% to 15%, and the rest is bound to albumin.3,24 Hence, these data suggest that T119M heterozygotes have increased total thyroxine levels attributable to an increased binding of thyroxine to transthyretin but normal thyroid function. Results were similar after adjusting for age and sex by linear regression analysis (-coefficient=18, corresponding to a mean increase in thyroxine of 18 nmol/L inThan normal. Therefore, the higher transthyretin levels indicate a protein less likely to dissociate and aggregate into fibrils.

It has been shown both by extensive physicochemical studies in vitro and by ex vivo studies of sera from T119M heterozygotes that increased binding affinity for thyroxine reflects the stabilizing effect of T119M on the transthyretin tetramer. Furthermore, adding thyroxine, a natural ligand for transthyretin, has been shown to stabilize wild-type transthyretin and inhibit fibril formation in vitro. Therefore, increased plasma levels of total thyroxine in heterozygotes is also a marker for the increased transthyretin tetramer stability attributable to T119M. We found that carriers had a 20% (19 nmol/L) increase in total thyroxine levels compared with noncarriers but normal thyroid function, indicating that the increased thyroxine level was attributable to increased binding to transthyretin. Thus, our study demonstrates that T119M is functional and stabilizes transthyretin in the general population in vivo.

Figure 4 shows the cumulative incidence of cerebrovascular disease as a function of age in T119M heterozygotes (green) vs noncarriers (red) from the general population. Cerebrovascular disease was ischemic cerebrovascular disease (transitory ischemic attacks, amaurosis fugax, or ischemic stroke) or hemorrhagic stroke. 

Postmortem studies have shown that senile systemic amyloidosis affects 25% of individuals >80 years; however, the impact of this condition in the general population is not known. Although cardiomyopathy may be the most severe manifestation of senile systemic amyloidosis, this condition has been associated with an increased risk of myocardial infarction in autopsy studies and possibly an increased risk of cardioembolic stroke. Furthermore, vascular deposits of transthyretin amyloid have been found systemically in cases with senile systemic amyloidosis, indicating that the vascular integrity may be compromised in other organs as well, including the brain. Therefore, we propose that the protective effect on vascular disease associated with the stabilizing variant T119M may be attributable to a reduced deposition of wild-type transthyretin in the wall of small arteries. In addition, in the present study, heterozygotes had increased life expectancy overall and among those who died from vascular and cerebrovascular disease, increased life expectancy after diagnosis of vascular disease and increased age at diagnosis of vascular disease compared with noncarriers. These results support that T119M may act through an effect on the vascular wall.

The sporadic form of cerebral amyloid angiopathy with deposition of A, the protein also implicated in Alzheimer disease, is a common condition affecting 40% to 50% of individuals aged 60 years and leading to an increased risk of both ischemic and hemorrhagic cerebral events. Several studies have shown that transthyretin can interact with A and possibly inhibit A amyloid deposition. Thus, another potential explanation for the apparent protective effect of T119M on cerebrovascular disease could be that T119M stabilizes transthyretin in the cerebrospinal fluid and, by binding A, protects against sporadic cerebral amyloid angiopathy.

In conclusion, these results are compatible with an association between genetic stabilization of transthyretin and decreased risk of cerebrovascular disease but need to be confirmed in other large studies.

Acknowledgments: We thank Mette Refstrup and Teresa Rozenfeld for expert technical assistance. We are indebted to the staff and participants of the Copenhagen City Heart Study and the Copenhagen General Population Study for their important contributions.

Other studies have pointed to a general role for transthyretin in neurobiology and repair. More specifically, transthyretin has been suggested to be an important factor in response to brain ischemia; however, human evidence is lacking. Although the association with cerebrovascular disease in our study seems to be attributable mainly to a reduced risk of ischemic cerebrovascular disease, firm conclusions regarding the effect on hemorrhagic stroke cannot be based on the relatively limited number of individuals with this endpoint.