Transthyretin (TTR) is a protein made in the liver and the choroid plexus of the brain, resulting in its presence in the blood plasma and cerebrospinal blood respectively. It the principal thyroid hormone transport protein in cerebrospinal fluid and is responsible for delivering the hormone to the brain. Expression levels in humans compared with "great apes" appear to be increased both in circulating blood and in cerebrospinal blood, and this is presumed to be due to increased gene expression at the sites of synthesis mentioned. There may be some connection to apparent differences in thyroid hormone metabolism between humans and chimpanzees. The biological significance of this event is unknown. One comparative study between humans and chimpanzees suggested that TTR has has undergone recent positive selection in humans. Notably transthyretin is also the carrier for retinol binding protein, which keeps retinol in the circulation.
Type of Human-Specific Changes in TTR
Tissue Expression Pattern Change
Interspecies subtractive hybridization of cDNA from human and chimpanzee brains., , Dokl Biochem Biophys, 2001 Nov-Dec, Volume 381, p.415-8, (2001)
Proteomic comparison of human and great ape blood plasma reveals conserved glycosylation and differences in thyroid hormone metabolism., , Am J Phys Anthropol, 2001 Jun, Volume 115, Issue 2, p.99-109, (2001)