D18G transthyretin is monomeric, aggregation prone, and not detectable in plasma and cerebrospinal fluid: a prescription for central nervous system amyloidosis?
Over 70 transthyretin (TTR) mutations facilitate amyloidosis in tissues other than the central nervous system (CNS). In contrast, the D18G TTR mutation in individuals of Hungarian descent leads to CNS amyloidosis. D18G forms inclusion bodies in Escherichia coli, unlike the other disease-associated TTR variants overexpressed to date. Denaturation and reconstitution of D18G from inclusion bodies afford a folded monomer that is destabilized by 3.1 kcal/mol relative to an engineered monomeric version of WT TTR. Since TTR tetramer dissociation is typically rate limiting for amyloid formation, the monomeric nature of D18G renders its amyloid formation rate 1000-fold faster than WT. It is perplexing that D18G does not lead to severe early onset systemic amyloidosis, given that it is the most destabilized TTR variant characterized to date, more so than variants exhibiting onset in the second decade. Instead, CNS impairment is observed in the fifth decade as the sole pathological manifestation; however, benign systemic deposition is also observed. Analysis of heterozygote D18G patient's serum and cerebrospinal fluid (CSF) detects only WT TTR, indicating that D18G is either rapidly degraded postsecretion or degraded within the cell prior to secretion, consistent with its inability to form hybrid tetramers with WT TTR. The nondetectable levels of D18G TTR in human plasma explain the absence of an early onset systemic disease. CNS disease may result owing to the sensitivity of the CNS to lower levels of D18G aggregate. Alternatively, or in addition, we speculate that a fraction of D18G made by the choroid plexus can be transiently tetramerized by the locally high thyroxine (T(4)) concentration, chaperoning it out into the CSF where it undergoes dissociation and amyloidogenesis due to the low T(4) CSF concentration. Selected small molecule tetramer stabilizers can transform D18G from a monomeric aggregation-prone state to a nonamyloidogenic tetramer, which may prove to be a useful therapeutic strategy against TTR-associated CNS amyloidosis.
Hammarström
Per
P
The Skaggs Institute of Chemical Biology and Department of Chemistry, The Scripps Research Institute, BCC265, 10550 North Torrey Pines Road, La Jolla, California 92037, USA.
Sekijima
Yoshiki
Y
White
Joleen T
JT
Wiseman
R Luke
RL
Lim
Amareth
A
Costello
Catherine E
CE
Altland
Klaus
K
Garzuly
Ferenc
F
Budka
Herbert
H
Kelly
Jeffery W
JW
D000595:Amino Acid Sequence; D028227:Amyloid Neuropathies, Familial / Q000097:blood* / Q000134:cerebrospinal fluid* / Q000235:genetics; D001224:Aspartic Acid / Q000235:genetics; D001812:Blood-Brain Barrier / Q000502:physiology; D002493:Central Nervous System Diseases / Q000097:blood* / Q000134:cerebrospinal fluid* / Q000235:genetics; D002498:Centrifugation / Q000379:methods; D002942:Circular Dichroism; D029968:Escherichia coli Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D014644:Genetic Variation; D005998:Glycine / Q000235:genetics; D006801:Humans; D006863:Hydrogen-Ion Concentration; D002479:Inclusion Bodies / Q000378:metabolism; D008958:Models, Molecular; D011228:Prealbumin / Q000737:chemistry* / Q000235:genetics* / Q000378:metabolism; D011487:Protein Conformation; D021122:Protein Subunits / Q000737:chemistry / Q000235:genetics; D011994:Recombinant Proteins / Q000097:blood / Q000134:cerebrospinal fluid / Q000737:chemistry / Q000235:genetics
D029968:Escherichia coli Proteins; D011228:Prealbumin; D021122:Protein Subunits; D011994:Recombinant Proteins; D001224:Aspartic Acid; D005998:Glycine