In vivo stabilization of mutant human transthyretin in transgenic mice

Neuroprotective effect against amyloidogenic transthyretin aggregates - Induced cytotoxicity on human neuroblastoma cell by phenolic-rich Centella asiatica extract

Transthyretin (ATTR) amyloidosis is a progressive and life-threatening neurodegenerative disease caused by aggregation of the plasma transport protein, transthyretin, for which treatment is rare and cure unavailable. Centella asiatica is a small edible herb with a long history of neurological application in ethnomedicine. This work investigated whether hydrophilic extract of C. asiatica (CAB) could suppress the toxic effects of transthyretin amyloid aggregate (TTRa) in cell model derived from the same in vivo target. TTRa was prepared via thermal-induced aggregation. Chemical cross-linking and Tricine-SDS-PAGE, Thioflavin-T fluorescence, and TEM analyses confirmed that TTRa matched the profile of TTRL55P nonfibrillar amyloid aggregates. PrestoBlue cell viability assay revealed that exposure of IMR-32 human neuroblastoma cells to TTRa (2-8 μM) resulted in significant cytotoxicity. Conversely, exposure of IMR-32 cells to CAB did not adversely affect their viability. In addition, when IMR-32 cells were co-treated with TTRa and varied concentrations of CAB, the toxic effect of TTRa was significantly (p < 0.01) inhibited dose-dependently. The extract was found to possess potent radical scavenging effects, and quantitative RP-HPLC analysis showed that asiaticoside and phenolics were its main components. The cytoprotective effect against TTRa, antioxidant property, and good safety profile collectively suggest that CAB could be applied in the development of nutraceuticals or therapeutics against transthyretin amyloidosis.

Clinical and biochemical characterization of hereditary transthyretin amyloidosis caused by E61K mutation

Objects: This study was intended to find out more about the clinical characterizations of patients carrying transthyretin (TTR) E61K (p.Glu81Lys) gene mutation and the biochemical characterization of this mutant protein.

Materials and methods: Five patients who had been diagnosed with hereditary transthyretin amyloidosis and two asymptomatic carriers carrying TTR E61K gene mutation were reported. Biochemical and biophysical tests were conducted to observe the thermodynamic and kinetic stability. Fibril formation tests measured by turbidity assay were performed to explore the pathogenicity of this mutation. Kinetic stabilizer responsiveness was measured to determine the inhibitory effect on protein aggregation.

Results: The average age of onset for the five patients was 62 years, and the course of the disease ranged from 2 to 10 years. Cardiac disease was prominent in this group of patients. Nerve pathology revealed a mildly to moderately reduced myelinated fiber density and muscle pathology showed predominant neurogenic impairment accompanied by possible myogenic impairment. E61K-TTR was characterized as a kinetically destabilized protein compared to WT-TTR but its thermodynamic stability was not compromised. In addition, the subunit exchange of E61K with WT-TTR further destabilized the heterozygous tetramer. Meanwhile, the E61K:WT heterozygous tetramer exhibited a poor response to kinetic stabilizers in the fibril formation assay. Finally, the serum TTR tetramer concentration was low in E61K-TTR symptomatic patients and in one asymptomatic gene carrier. Vyndamax (Tafamidis) could increase the TTR tetramer concentration.

Conclusions: Patients with E61K mutation tended to be late-onset. The concentration of TTR tetramer in the serum might serve as a biomarker to monitor disease progress, therapeutic window time, and therapeutic response to TTR kinetic stabilizer drugs.

The hydrophobic residue Leu73 is crucial for the high stability and low aggregation properties of murine transthyretin

Destabilization of human transthyretin leads to its aggregation into amyloid fibrils, which causes a rare, progressive and fatal systemic disorder called ATTR amyloidosis. By contrast, murine transthyretin is known to be very stable and therefore does not aggregate into amyloid fibrils in vivo or in vitro. We examined the hydrophobic residues responsible for the high-stability and low-aggregation properties of murine transthyretin using site-directed mutagenesis. Urea-induced unfolding and thioflavin T fluorescence aggregation assay revealed that Leu73 of murine transthyretin largely contributes to its high stability and low aggregation properties: the I73L mutation stabilized human transthyretin, while the L73I mutation destabilized murine transthyretin. In addition, the I26V/I73L mutation stabilized the amyloidogenic V30M mutant of human transthyretin to the same degree as the suppressor mutation T119M, which protects transthyretin against amyloid fibril aggregation. The I73L mutation resulted in no significant differences in the overall structure of the transthyretin tetramer or the contacts of side-chains in the hydrophobic core of the monomer. We also found that Leu73 of murine transthyretin is conserved in many mammals, while Ile73 of human transthyretin is conserved in monkeys and cats. These studies will provide new insights into the stability and aggregation properties of transthyretin from various mammals.

TTR exon-humanized mouse optimal for verifying new therapies for FAP

Familial amyloidotic polyneuropathy (FAP) is caused by a mutation in the transthyretin (TTR) gene. In addition, deposition of wild-type TTR can cause senile systemic amyloidosis (SSA). To date, we have produced several transgenic mouse models for FAP and SSA by introducing TTR genes with different promoters or mutations. However, mouse TTR can associate with human TTR to produce hybrid tetramers in transgenic mice. Thus, these transgenic mice cannot be used to test the efficacy of a new therapy. In this study, we attempted to construct an optimized mouse model to verify a new therapy. The TTR gene consists of 4 exons and 3 introns. We prepared two gRNAs, one for the exon 1 and the other for exon 4, and a single donor vector carrying the whole TTR gene in which mouse exons were replaced with human exons. Using these vectors, we produced a TTR exon-humanized mouse with human exons and mouse introns using genome editing technology. These TTR exon-humanized mice showed normal TTR expression patterns in terms of serum TTR level and spatial specificity. These TTR exon-humanized mice will be useful for devising new treatment methods for FAP, including gene therapy.

Single AAV-mediated CRISPR-Nme2Cas9 efficiently reduces mutant hTTR expression in a transgenic mouse model of transthyretin amyloidosis

Transthyretin (TTR) amyloidosis is a hereditary life-threatening disease characterized by deposition of amyloid fibrils. The main causes of TTR amyloidosis are mutations in the TTR gene that lead to the production of misfolded TTR protein. Reducing the production of toxic protein in the liver is a validated strategy to treat TTR amyloidosis. In this study, we established a humanized mouse model that expresses mutant human TTR (hTTR; V30M) protein in the liver to model TTR amyloidosis. Then, we compared the efficiency of reducing the expression of mutant hTTR by dual adeno-associated virus 8 (AAV8)-mediated split SpCas9 with that by single AAV8-mediated Nme2Cas9 in this model. With two gRNAs targeting different exons, dual AAV-mediated split SpCas9 system achieved efficiencies of 37% and 34% reduction of hTTR mRNA and reporter GFP expression, respectively, in the liver. Surprisingly, single AAV-mediated Nme2Cas9 treatment resulted in 65% and 71% reduction of hTTR mRNA and reporter GFP, respectively. No significant editing was identified in predicted off-target sites in the mouse and human genomes after Nme2Cas9 targeting. Thus, we provide proof of principle for using single AAV-mediated CRISPR-Nme2Cas9 to effectively reduce mutant hTTR expression in vivo, which may translate into gene therapy for TTR amyloidosis.

Plasmin activity promotes amyloid deposition in a transgenic model of human transthyretin amyloidosis

Cardiac ATTR amyloidosis, a serious but much under-diagnosed form of cardiomyopathy, is caused by deposition of amyloid fibrils derived from the plasma protein transthyretin (TTR), but its pathogenesis is poorly understood and informative in vivo models have proved elusive. Here we report the generation of a mouse model of cardiac ATTR amyloidosis with transgenic expression of human TTR^S52P. The model is characterised by substantial ATTR amyloid deposits in the heart and tongue. The amyloid fibrils contain both full-length human TTR protomers and the residue 49-127 cleavage fragment which are present in ATTR amyloidosis patients. Urokinase-type plasminogen activator (uPA) and plasmin are abundant within the cardiac and lingual amyloid deposits, which contain marked serine protease activity; knockout of α₂-antiplasmin, the physiological inhibitor of plasmin, enhances amyloid formation. Together, these findings indicate that cardiac ATTR amyloid deposition involves local uPA-mediated generation of plasmin and cleavage of TTR, consistent with the previously described mechano-enzymatic hypothesis for cardiac ATTR amyloid formation. This experimental model of ATTR cardiomyopathy has potential to allow further investigations of the factors that influence human ATTR amyloid deposition and the development of new treatments.

ATTR amyloidosis causes heart failure through the accumulation of misfolded transthyretin in cardiac muscle. Here the authors report a mouse model of ATTR amyloidosis and demonstrate the involvement of protease activity in ATTR amyloid deposition.

Transthyretin Stabilization: An Emerging Strategy for the Treatment of Alzheimer's Disease?

Transthyretin (TTR), previously named prealbumin is a plasma protein secreted mainly by the liver and choroid plexus (CP) that is a carrier for thyroid hormones (THs) and retinol (vitamin A). The structure of TTR, with four monomers rich in β-chains in a globular tetrameric protein, accounts for the predisposition of the protein to aggregate in fibrils, leading to a rare and severe disease, namely transthyretin amyloidosis (ATTR). Much effort has been made and still is required to find new therapeutic compounds that can stabilize TTR ("kinetic stabilization") and prevent the amyloid genetic process. Moreover, TTR is an interesting therapeutic target for neurodegenerative diseases due to its recognized neuroprotective properties in the cognitive impairment context and interestingly in Alzheimer's disease (AD). Much evidence has been collected regarding the neuroprotective effects in AD, including through in vitro and in vivo studies as well as a wide range of clinical series. Despite this supported hypothesis of neuroprotection for TTR, the mechanisms are still not completely clear. The aim of this review is to highlight the most relevant findings on the neuroprotective role of TTR, and to summarize the recent progress on the development of TTR tetramer stabilizers.

Origins of a pervasive, erroneous idea: The "green birefringence" of Congo red-stained amyloid

Congo red was discovered to stain amyloid by accident in 1922, and Congo red-stained amyloid was shown to be birefringent on polarization microscopy in 1927. Colours, namely green and yellow, were reported under these conditions in 1945, although these are only two of various anomalous colours that may be seen, depending on the optical set-up. In 1953 there began a dogmatic insistence that in Congo red-stained amyloid between crossed polarizer and analyser green alone should be seen, and the finding of any other colour was a mistake. The idea that green, and only green, is essential for the diagnosis of amyloid has persisted almost universally, and virtually all mentions of Congo red-stained amyloid say that it just shows "green birefringence" or "apple-green birefringence." This idea is wrong and is contrary to everyday experience, because green is seldom seen on its own under these conditions of microscopy, and often, there is no green at all. How observers maintain this unscientific position is explained by a study of its historical origins. Most of the early literature was in German or French and was usually quoted in English at second hand, which meant that misquotations, misattributions and misunderstandings were common. Few workers reported their findings accurately, hardly any attempted to explain them, and until 2008, none gave a completely satisfactory account of the physical optics. The history of Congo red-stained amyloid is an instructive example of how an erroneous belief can become widely established even when it is contradicted by simple experience.

Treatment of hereditary and acquired forms of transthyretin amyloidosis in the era of personalized medicine: the role of randomized controlled trials

There have now been randomized controlled trials of four different therapeutics for hereditary amyloid polyneuropathy related to transthyretin (TTR) deposition and one for amyloidotic cardiomyopathy of both genetic and sporadic origin. It is likely that in the next few months those not already approved by either the US Food and Drug Administration (FDA) and/or the European Medicines Authority (EMA) will receive similar approvals for treatment for all or particular groups of patients. This is a far cry from circumstances less than 10 years ago when the only available therapy was gene replacement by liver transplant. The randomized controlled trials have shown that all the treatments (tafamidis, diflunisal, patisiran, and inotersen) are effective in the context of a clinical trial. However, we have very little idea of whether individual patients will respond in an equally positive way to all the drugs or whether there will be some who respond better to one or another or not respond at all, nor do we know whether combinations will be additive or synergistic. We lack validated markers of clinical response. While the small molecule TTR stabilizers increase serum TTR levels, the RNA-based drugs lower serum TTR. In the latter case, it is not clear that the reduction in serum TTR is related to the clinical response in a 1:1 fashion. Pharmaceutical companies have made substantial investments in the development of these agents and will clearly attempt to recoup those investments quickly. It is incumbent upon those of us who care for these patients to develop ways to assess the effects of therapy in the shortest possible time at the lowest possible cost. The better we are able to accomplish this the more likely it is that we will be able to treat the most patients in the most clinically efficient fashion regardless of their economic status. We now have the drugs we just have to figure out who should get them and when.

Contributions of Animal Models to the Mechanisms and Therapies of Transthyretin Amyloidosis

Transthyretin amyloidosis (ATTR amyloidosis) is a fatal systemic disease caused by amyloid deposits of misfolded transthyretin, leading to familial amyloid polyneuropathy and/or cardiomyopathy, or a rare oculoleptomeningeal amyloidosis. A good model system that mimic the disease phenotype is crucial for the development of drugs and treatments for this devastating degenerative disorder. The present models using fruit flies, worms, rodents, non-human primates and induced pluripotent stem cells have helped researchers understand important disease-related mechanisms and test potential therapeutic options. However, the challenge of creating an ideal model still looms, for these models did not recapitulates all symptoms, particularly neurological presentation, of ATTR amyloidosis. Recently, knock-in techniques was used to generate two humanized ATTR mouse models, leading to amyloid deposition in the nerves and neuropathic manifestation in these models. This review gives a recent update on the milestone, progress, and challenges in developing different models for ATTR amyloidosis research.

Amyloid deposition in a mouse model humanized at the transthyretin and retinol-binding protein 4 loci

Familial amyloidotic polyneuropathy is an autosomal dominant disorder caused by a point mutation in the transthyretin (TTR) gene. The process of TTR amyloidogenesis begins with rate-limiting dissociation of the TTR tetramer. Thus, the TTR stabilizers, such as Tafamidis and Diflunisal, are now in clinical trials. Mouse models will be useful to testing the efficacy of these drugs. Although several mouse models have been generated, they all express mouse Rbp4. Thus, human TTR associates with mouse RBP4, resulting in different kinetic and thermodynamic stability profiles of TTR tetramers. To overcome this problem, we previously produced humanized mouse strains at both the TTR and Rbp4 loci (Ttr ^hTTRVal30 , Ttr ^hTTRMet30 , and Rbp4 ^hRBP4 ). By mating these mice, we produced double-humanized mouse strains, Ttr ^{hTTRVal30/hTTRVal30} :Rbp4 ^hRBP4/hRBP4 and Ttr ^{hTTRVal30/Met30} :Rbp4 ^hRBP4/hRBP4 . We used conventional transgenic mouse strains on a wild-type (Ttr ^+/+ :Tg[6.0hTTRMet30]) or knockout Ttr background (Ttr^-/-:Tg[6.0hTTRMet30]) as reference strains. The double-humanized mouse showed 1/25 of serum hTTR and 1/40 of serum hRBP4 levels. However, amyloid deposition was more pronounced in Ttr ^{hTTRVal30/Met30} :Rbp4 ^hRBP4/hRBP4 than in conventional transgenic mouse strains. In addition, a similar amount of amyloid deposition was also observed in Ttr ^{hTTRVal30/ hTTRVal30} :Rbp4 ^{hRBP4/ hRBP4} mice that carried the wild-type human TTR gene. Furthermore, amyloid deposition was first observed in the sciatic nerve without any additional genetic change. In all strains, anti-TTR antibody-positive deposits were found in earlier age and at higher percentage than amyloid fibril deposition. In double-humanized mice, gel filtration analysis of serum revealed that most hTTR was free of hRBP4, suggesting importance of free TTR for amyloid deposition.

Sensory nerve degeneration in a mouse model mimicking early manifestations of familial amyloid polyneuropathy due to transthyretin Ala97Ser

AIMS

Sensory nerve degeneration and consequent abnormal sensations are the earliest and most prevalent manifestations of familial amyloid polyneuropathy (FAP) due to amyloidogenic transthyretin (TTR). FAP is a relentlessly progressive degenerative disease of the peripheral nervous system. However, there is a lack of mouse models to replicate the early neuropathic manifestations of FAP.

METHODS

We established human TTR knock-in mice by replacing one allele of the mouse Ttr locus with human wild-type TTR (hTTR^wt ) or human TTR with the A97S mutation (hTTR^A97S ). Given the late onset of neuropathic manifestations in A97S-FAP, we investigated nerve pathology, physiology, and behavioural tests in these mice at two age points: the adult group (8 - 56 weeks) and the ageing group (> 104 weeks).

RESULTS

In the adult group, nerve profiles, neurophysiology and behaviour were similar between hTTR^wt and hTTR^A97S mice. By contrast, ageing hTTR^A97S mice showed small fibre neuropathy with decreased intraepidermal nerve fibre density and behavioural signs of mechanical allodynia. Furthermore, significant reductions in sural nerve myelinated nerve fibre density and sensory nerve action potential amplitudes in these mice indicated degeneration of large sensory fibres. The unaffected motor nerve physiology replicated the early symptoms of FAP patients, that is, sensory nerves were more vulnerable to mutant TTR than motor nerves.

CONCLUSIONS

These results demonstrate that the hTTR^A97S mouse model develops sensory nerve pathology and corresponding physiology mimicking A97S-FAP and provides a platform to develop new therapies for the early stage of A97S-FAP.

Preclinical evaluation of RNAi as a treatment for transthyretin-mediated amyloidosis

ATTR amyloidosis is a systemic, debilitating and fatal disease caused by transthyretin (TTR) amyloid accumulation. RNA interference (RNAi) is a clinically validated technology that may be a promising approach to the treatment of ATTR amyloidosis. The vast majority of TTR, the soluble precursor of TTR amyloid, is expressed and synthesized in the liver. RNAi technology enables robust hepatic gene silencing, the goal of which would be to reduce systemic levels of TTR and mitigate many of the clinical manifestations of ATTR that arise from hepatic TTR expression. To test this hypothesis, TTR-targeting siRNAs were evaluated in a murine model of hereditary ATTR amyloidosis. RNAi-mediated silencing of hepatic TTR expression inhibited TTR deposition and facilitated regression of existing TTR deposits in pathologically relevant tissues. Further, the extent of deposit regression correlated with the level of RNAi-mediated knockdown. In comparison to the TTR stabilizer, tafamidis, RNAi-mediated TTR knockdown led to greater regression of TTR deposits across a broader range of affected tissues. Together, the data presented herein support the therapeutic hypothesis behind TTR lowering and highlight the potential of RNAi in the treatment of patients afflicted with ATTR amyloidosis.

Quantification of quaternary structure stability in aggregation-prone proteins under physiological conditions: the transthyretin case

The quaternary structure stability of proteins is typically studied under conditions that accelerate their aggregation/unfolding processes on convenient laboratory time scales. Such conditions include high temperature or pressure, chaotrope-mediated unfolding, or low or high pH. These approaches have the limitation of being nonphysiological and that the concentration of the protein in solution is changing as the reactions proceed. We describe a methodology to define the quaternary structure stability of the amyloidogenic homotetrameric protein transthyretin (TTR) under physiological conditions. This methodology expands from a described approach based on the measurement of the rate of subunit exchange of TTR with a tandem flag-tagged (FT₂) TTR counterpart. We demonstrate that subunit exchange of TTR with FT₂·TTR can be analyzed and quantified using a semi-native polyacrylamide gel electrophoresis technique. In addition, we biophysically characterized two FT₂·TTR variants derived from wild-type and the amyloidogenic variant Val122Ile TTR, both of which are associated with cardiac amyloid deposition late in life. The FT₂·TTR variants have similar amyloidogenic potential and similar thermodynamic and kinetic stabilities compared to those of their nontagged counterparts. We utilized the methodology to study the potential of the small molecule SOM0226, a repurposed drug under clinical development for the prevention and treatment of the TTR amyloidoses, to stabilize TTR. The results enabled us to characterize the binding energetics of SOM0226 to TTR. The described technique is well-suited to study the quaternary structure of other human aggregation-prone proteins under physiological conditions.

Induced pluripotent stem cell modeling of multisystemic, hereditary transthyretin amyloidosis

Familial transthyretin amyloidosis (ATTR) is an autosomal-dominant protein-folding disorder caused by over 100 distinct mutations in the transthyretin (TTR) gene. In ATTR, protein secreted from the liver aggregates and forms fibrils in target organs, chiefly the heart and peripheral nervous system, highlighting the need for a model capable of recapitulating the multisystem complexity of this clinically variable disease. Here, we describe the directed differentiation of ATTR patient-specific iPSCs into hepatocytes that produce mutant TTR, and the cardiomyocytes and neurons normally targeted in the disease. We demonstrate that iPSC-derived neuronal and cardiac cells display oxidative stress and an increased level of cell death when exposed to mutant TTR produced by the patient-matched iPSC-derived hepatocytes, recapitulating essential aspects of the disease in vitro. Furthermore, small molecule stabilizers of TTR show efficacy in this model, validating this iPSC-based, patient-specific in vitro system as a platform for testing therapeutic strategies.

Systemic amyloidoses

The amyloidoses are a group of protein misfolding diseases in which the precursor protein undergoes a conformational change that triggers the formation of amyloid fibrils in different tissues and organs, causing cell death and organ failure. Amyloidoses can be either localized or systemic. In localized amyloidosis, amyloid deposits form at the site of precursor protein synthesis, whereas in systemic amyloidosis, amyloid deposition occurs distant from the site of precursor protein secretion. We review the type of proteins and cells involved and what is known about the complex pathophysiology of these diseases. We focus on light chain amyloidosis to illustrate how biochemical and biophysical studies have led to a deeper understanding of the pathogenesis of this devastating disease. We also review current cellular, tissue, and animal models and discuss the challenges and opportunities for future studies of the systemic amyloidoses.

The heat shock response modulates transthyretin deposition in the peripheral and autonomic nervous systems

Familial amyloidotic polyneuropathy (FAP) is a neurodegenerative disease that selectively affects the peripheral nervous system. The putative cause of this life threatening pathology is tissue deposition of mutant transthyretin (TTR), initially as non-fibrillar deposits and later as fibrillar material. The mouse models currently available do not recapitulate the human whole features, since the peripheral nervous tissue is spared. We have characterized a new mouse model expressing the human transthyretin V30M in a heat shock transcription factor 1 (Hsf1) null background. The lack of HSF1 expression leads to an extensive and earlier non-fibrillar TTR, evolving into fibrillar material in distinct organs including the peripheral nervous system. Furthermore, inflammatory stress and a reduction in unmyelinated nerve fibers were observed, as in human patients. These results indicate that HSF1 regulated genes are involved in FAP, modulating TTR tissue deposition. The novel mouse model is of the utmost importance in testing new therapeutic strategies and in addressing the influence of the stress response in misfolding diseases.

Animal models of human amyloidoses: are transgenic mice worth the time and trouble?

The amyloidoses are the prototype gain of toxic function protein misfolding diseases. As such, several naturally occurring animal models and their inducible variants provided some of the first insights into these disorders of protein aggregation. With greater analytic knowledge and the increasing flexibility of transgenic and gene knockout technology, new models have been generated allowing the interrogation of phenomena that have not been approachable in more reductionist systems, i.e. behavioral readouts in the neurodegenerative diseases, interactions among organ systems in the transthyretin amyloidoses and taking pre-clinical therapeutic trials beyond cell culture. The current review describes the features of both transgenic and non-transgenic models and discusses issues that appear to be unresolved even when viewed in their organismal context.

Iodine atoms: a new molecular feature for the design of potent transthyretin fibrillogenesis inhibitors

The thyroid hormone and retinol transporter protein known as transthyretin (TTR) is in the origin of one of the 20 or so known amyloid diseases. TTR self assembles as a homotetramer leaving a central hydrophobic channel with two symmetrical binding sites. The aggregation pathway of TTR into amiloid fibrils is not yet well characterized but in vitro binding of thyroid hormones and other small organic molecules to TTR binding channel results in tetramer stabilization which prevents amyloid formation in an extent which is proportional to the binding constant. Up to now, TTR aggregation inhibitors have been designed looking at various structural features of this binding channel others than its ability to host iodine atoms. In the present work, greatly improved inhibitors have been designed and tested by taking into account that thyroid hormones are unique in human biochemistry owing to the presence of multiple iodine atoms in their molecules which are probed to interact with specific halogen binding domains sitting at the TTR binding channel. The new TTR fibrillogenesis inhibitors are based on the diflunisal core structure because diflunisal is a registered salicylate drug with NSAID activity now undergoing clinical trials for TTR amyloid diseases. Biochemical and biophysical evidence confirms that iodine atoms can be an important design feature in the search for candidate drugs for TTR related amyloidosis.

Transthyretin amyloid goiter in a renal allograft recipient

Amyloid deposition in the follicular, perifollicular blood vessels, and thyroid stroma can occur in systemic forms of amyloidosis, although diffuse enlargement of the thyroid is generally not present. Marked, widespread enlargement of the thyroid gland with amyloid deposits or amyloid goiter is a rare condition reported in association with primary and secondary amyloidosis but has not been described in association with transthyretin amyloid deposition. Senile transthyretin amyloidosis is primarily associated with amyloid deposits in the heart, while the familial forms of amyloidosis due to transthyretin gene mutations are associated with deposits of amyloid in multiple tissues, classically giving rise to polyneuropathy. In this report, we describe the findings of parathyroid and lymph node amyloid deposits and amyloid goiter with transthyretin reactivity in a recipient of a kidney allograft, reportedly for renal amyloidosis, initially assumed clinically to be due to inflammatory bowel disease-related secondary amyloid deposition. This case underscores the importance of routine immunohistochemical classification of amyloid deposits for accurate diagnosis and to guide clinical management decisions.

Human-murine transthyretin heterotetramers are kinetically stable and non-amyloidogenic. A lesson in the generation of transgenic models of diseases involving oligomeric proteins

The transthyretin amyloidoses appear to be caused by rate-limiting tetramer dissociation and partial monomer unfolding of the human serum protein transthyretin, resulting in aggregation and extracellular deposition of amorphous aggregates and amyloid fibrils. Mice transgenic for few copies of amyloid-prone human transthyretin variants, including the aggressive L55P mutant, failed to develop deposits. Silencing the murine transthyretin gene in the presence of the L55P human gene resulted in enhanced tissue deposition. To test the hypothesis that the murine protein interacted with human transthyretin, preventing the dissociation and partial unfolding required for amyloidogenesis, we produced recombinant murine transthyretin and human/murine transthyretin heterotetramers and compared their structures and biophysical properties to recombinant human transthyretin. We found no significant differences between the crystal structures of murine and human homotetramers. Murine transthyretin is not amyloidogenic because the native homotetramer is kinetically stable under physiologic conditions and cannot dissociate into partially unfolded monomers, the misfolding and aggregation precursor. Heterotetramers composed of murine and human subunits are also kinetically stable. These observations explain the lack of transthyretin deposition in transgenics carrying a low copy number of human transthyretin genes. The incorporation of mouse subunits into tetramers otherwise composed of human amyloid-prone transthyretin subunits imposes kinetic stability, preventing dissociation and subsequent amyloidogenesis.