Transthyretin - Role in Disease

Role in Disease

TTR misfolding and aggregation is known to be associated with the amyloid diseases senile systemic amyloidosis (SSA), familial amyloid polyneuropathy (FAP), and familial amyloid cardiomyopathy (FAC).

TTR tetramer dissociation is known to be rate-limiting for amyloid fibril formation. However, the monomer also must partially denature in order for TTR to be mis-assembly competent, leading to a variety of aggregate structures including amyloid fibrils. While wild type TTR can dissociate, misfold and aggregate leading to SSA, point mutations within TTR are known to destabilize the tetramer composed of mutant and wild type TTR subunits facilitating more facile dissociation and/or misfolding and amyloidogenesis. A replacement of valine by methionine at position 30 (TTR V30M) is the mutation most commonly associated with FAP. A position 122 replacement of valine by isoleucine (TTR V122I) is carried by 3.9% of the African-American population, and is the most common cause of FAC. SSA is estimated to affect over 25% of the population over age 80. Severity of disease varies greatly by mutation, with some mutations causing disease in the first or second decade of life, and others being more benign. Deposition of TTR amyloid is generally observed extracellularly, although TTR deposits are also clearly observed within the cardiomyocytes of the heart. Treatment of familial TTR amyloid disease has historically relied on liver transplantation as a crude form of gene therapy. Because TTR is primarily produced in the liver, replacement of a liver containing a mutant TTR gene with a normal gene is able to reduce the mutant TTR levels in the body to < 5% of pretransplant levels. Certain mutations, however, cause CNS amyloidosis, and due to the their production by the choroid plexus, the CNS TTR amyloid diseases do not respond to gene therapy mediated by liver transplantation. In 2011, the European Medicines Agency approved Tafamidis or Vyndaqel for the amelioration of FAP. Vyndaqel kinetically stabilizes the TTR tetramer preventing tetramer dissociation required for TTR amyloidogenesis and degradation of the autnomic nervous system and/or the peripheral nervous system and/or the heart.

TTR is also thought to have beneficial side, by binding to the infamous beta-amyloid protein, thereby preventing beta-amyloid's natural tendency to accumulate into the plaques associated with the early stages of Alzheimer's Disease. Preventing plaque formation is thought to enable a cell to rid itself of this otherwise toxic protein form and, thus, help prevent and maybe even treat the disease.

There is now strong genetic and pharmacologic data (see European Medicines Agency website for the Tafamidis clinical trial results) indicating that the process of amyloid fibril formation leads to the degeneration of post-mitotic tissue causing FAP and likely FAC and SSA. Evidence points to the oligomers generated in the process of amyloidogenicity leading to the observed proteotoxicity.

Transthyretin level in cerebrospinal fluid has also been found to be lower in patients with some neurobiological disorders such as schizophrenia. The reduced level of transthyretin in the CSF may indicate a lower thyroxine transport in brains of patients with schizophrenia.

Because transthyretin is made in part by the choroid plexus, it can be used as an immunohistochemical marker for choroid plexus papillomas.