Rare c.302C>T TTR Variant Associated with Transthyretin Amyloidosis

Background and Objectives: Hereditary transthyretin amyloidosis (ATTRv) is a rare disease caused by pathogenic variants in the transthyretin (TTR) gene. More than 140 different disease-causing variants in TTR have been reported. Only a few individuals with a rare TTR variant, c.302C>T, p.(Ala101Val) (historically known as p.(Ala81Val)), primarily associated with cardiac ATTRv, have been described. Therefore, our aim was to analyze the clinical characteristics of individuals with the identified c.302C>T TTR variant at our center. Materials and Methods: We analyzed data from individuals with ATTRv who were diagnosed and treated at Vilnius University Hospital Santaros Klinikos. ATTRv was confirmed by negative hematological analysis for monoclonal protein, positive tissue biopsy or bone scintigraphy and a pathogenic TTR variant. Results: During 2018-2021, the TTR NM_000371.3:c.302C>T, NP_000362.1:p.(Ala101Val) variant was found in one individual in a homozygous state and in three individuals in a heterozygous state. The age of onset of symptoms ranged from 44 to 74 years. The earliest onset of symptoms was in the individual with the homozygous variant. A history of carpal tunnel syndrome was identified in two individuals. On ECG, three individuals had low QRS voltage in limb leads. All individuals had elevated NT-proBNP and hsTroponine I levels on baseline laboratory tests and concentric left ventricular hypertrophy on transthoracic echocardiography. The individual with the homozygous c.302C>T TTR variant had the most pronounced polyneuropathy with tetraparesis. Other patients with the heterozygous variant had more significant amyloid cardiomyopathy. When screening family members, the c.302C>T TTR variant was identified in two phenotypically negative relatives at the ages of 33 and 47 years. Conclusions: c.302C>T is a rare TTR variant associated with ATTRv cardiomyopathy. The homozygous state of this variant was not reported before, and is associated with earlier disease onset and neurological involvement compared to the heterozygote state.

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.

Lox'd in translation: contradictions in the nomenclature surrounding common lox-site mutants and their implications in experiments

The Cre-Lox system is a highly versatile and powerful DNA recombinase mechanism, mainly used in genetic engineering to insert or remove desired DNA sequences. It is widely utilized across multiple fields of biology, with applications ranging from plants, to mammals, to microbes. A key feature of this system is its ability to allow recombination between mutant lox sites. Two of the most commonly used mutant sites are named lox66 and lox71, which recombine to create a functionally inactive double mutant lox72 site. However, a large portion of the published literature has incorrectly annotated these mutant lox sites, which in turn can lead to difficulties in replication of methods, design of proper vectors and confusion over the proper nomenclature. Here, we demonstrate common errors in annotations, the impacts they can have on experimental viability, and a standardized naming convention. We also show an example of how this incorrect annotation can induce toxic effects in bacteria that lack optimal DNA repair systems, exemplified by Mycoplasma pneumoniae.

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.

Rescue of retinal morphology and function in a humanized mouse at the mouse retinol-binding protein locus

Retinol-binding protein RBP4 is the specific carrier for retinol in the blood. We previously produced a Rbp4-deficient (Rbp4^-/-) mouse that showed electroretinogram (ERG) abnormalities, accompanied by histological and electron-microscopic changes such as fewer synapses in the inner plexiform layer in the central retina. To address whether human RBP4 gene expression can rescue the phenotypes observed in Rbp4^-/- mice, we produced a humanized (Rbp4^{hRBP4orf/ hRBP4orf}) mouse with a human RBP4 open reading frame in the mouse Rbp4 locus using a Cre-mutant lox recombination system. In Rbp4^{hRBP4orf/hRBP4orf} mice, the tissue-specific expression pattern of hRBP4orf was roughly the same as that of mouse Rbp4. ERG and morphological abnormalities observed in Rbp4^-/- mice were rescued in Rbp4^{hRBP4orf/hRBP4orf} mice as early as 7 weeks of age. The temporal expression pattern of hRBP4orf in the liver of Rbp4^{hRBP4orf/hRBP4orf} mice was similar to that of mouse Rbp4 in Rbp4^+/+mice. In contrast, hRBP4orf expression levels in eyes were significantly lower at 6 and 12 weeks of age compared with mouse Rbp4 but were restored to the control levels at 24 weeks. The serum hRBP4 levels in Rbp4^{hRBP4orf/hRBP4orf} mice were approximately 30% of those in Rbp4^+/+ at all ages examined. In accordance with this finding, the plasma retinol levels remained low in Rbp4^{hRBP4orf/hRBP4orf} mice. Retinol accumulation in the liver occurred in control and Rbp4^{hRBP4orf/hRBP4orf} mice but was higher in Rbp4^{hRBP4orf/hRBP4orf} mice at 30 weeks of age. Mouse transthyretin expression was not altered in Rbp4^-/- or Rbp4^{hRBP4orf/hRBP4orf} mice. Taken together, 30% of the serum RBP4 level was sufficient to correct the abnormal phenotypes observed in Rbp4^-/- mice.

CSP-1103 (CHF5074) stabilizes human transthyretin in healthy human subjects

Hereditary amyloid polyneuropathy is a type of protein misfolding disease. Transthyretin (TTR) is a homotetrameric serum protein and TTR tetramer dissociation is the limiting step in amyloid fibril formation. Thus, prevention of TTR dissociation is a promising therapeutic approach and some TTR stabilizers have been approved for the treatment of TTR amyloidosis. CSP-1103 (CHF5074) is a non-steroidal anti-inflammatory derivative that lacks cyclooxygenase inhibitory activity. In vitro, CSP-1103 stabilizes the TTR tetramer by binding to the thyroxine (T4) binding site. We have previously shown that serum TTR levels were increased by oral CSP-1103 administration through stabilization of TTR tetramers in humanized mice at both the Ttr locus and the Rbp4 locus. To determine whether CSP-1103 stabilizes TTR tetramers in humans, multiple CSP-1103 oral doses were administered for two weeks to 48 healthy human volunteers in a double-blind, placebo-controlled, parallel-group study. CSP-1103 treatment stabilized TTR tetramers in a dose-dependent manner under normal or denaturing stress conditions, thereby increasing serum TTR levels. Preincubation of serum with CSP-1103 or diflunisal in vitro increased the TTR tetramer stability. Computer simulation analysis revealed that the binding affinities of CSP-1103 with TTR at pH 7.0 were similar to those of tafamidis, thus confirming that CSP-1103 has potent TTR-stabilizing activity.

Severe ocular phenotypes in Rbp4-deficient mice in the C57BL/6 genetic background

Retinol-binding protein 4 (RBP4) is a specific carrier for retinol in the blood. In hepatocytes, newly synthesized RBP4 associates with retinol and transthyretin and is secreted into the blood. The ternary transthyretin-RBP4-retinol complex transports retinol in the circulation and delivers it to target tissues. Rbp4-deficient mice in a mixed genetic background (129xC57BL/6J) have decreased sensitivity to light in the b-wave amplitude on electroretinogram. Sensitivity progressively improves and approaches that of wild-type mice at 24 weeks of age. In the present study, we produced Rbp4-deficient mice in the C57BL/6 genetic background. These mice displayed more severe phenotypes. They had decreased a- and b-wave amplitudes on electroretinograms. In accordance with these abnormalities, we found structural changes in these mice, such as loss of the peripheral choroid and photoreceptor layer in the peripheral retinas. In the central retinas, the distance between the inner limiting membrane and the outer plexiform layer was much shorter with fewer ganglion cells and fewer synapses in the inner plexiform layer. Furthermore, ocular developmental defects of retinal depigmentation, optic disc abnormality, and persistent hyaloid artery were also observed. All these abnormalities had not recovered even at 40 weeks of age. Our Rbp4-deficient mice accumulated retinol in the liver but it was undetectable in the serum, indicating an inverse relation between serum and liver retinol levels. Our results suggest that RBP4 is critical for the mobilization of retinol from hepatic storage pools, and that such mobilization is necessary for ocular development and visual function.

Roles of hepatic glucokinase in intertissue metabolic communication: Examination of novel liver-specific glucokinase knockout mice

Glucokinase is expressed principally in pancreatic β-cells and hepatocytes, and catalyzes the phosphorylation of glucose to glucose-6-phosphate, a rate-limiting step of glycolysis. To better understand the roles of hepatic glucokinase, we generated Gck knockout mice by ablating liver-specific exon 1b. The knockout mice exhibited impaired glucose tolerance, decreased hepatic glycogen content, and reduced Pklr and Fas gene expression in the liver, indicating that hepatic glucokinase plays important roles in glucose metabolism. It has also been reported that hepatic glucokinase regulates the expression of thermogenesis-related genes in brown adipose tissue (BAT) and insulin secretion in response to glucose. However, the liver-specific Gck knockout mice displayed neither altered expression of thermogenesis-related genes in BAT nor impaired insulin secretion by β-cells under a normal chow diet. These results suggest that chronic suppression of hepatic glucokinase has a small influence on intertissue (liver-to-BAT as well as liver-to-β-cell) metabolic communication.

CHF5074 (CSP-1103) stabilizes human transthyretin in mice humanized at the transthyretin and retinol-binding protein loci

Familial amyloidotic polyneuropathy is one type of protein misfolding disease. Transthyretin (TTR) tetramer dissociation is the limiting step for amyloid fibril formation. CHF5074 (CSP-1103) stabilizes TTR tetramer in vitro by binding to the T4 binding site. Here, we used three strains of double humanized mice (mTtr(hTTRVal30/hTTRVal30), mTtr(hTTRVal30/hTTRMet30), and mTtr(hTTRMet30/hTTRMet30)) to assess whether CHF5074 stabilizes TTR tetramers in vivo. Treatment of mice with CHF5074 increased serum TTR levels by stabilizing TTR tetramers. Although the binding affinities of CHF5074 and diflunisal with TTRMet30 were similar, CHF5074 bound TTRVal30 more strongly than did diflunisal, suggesting the potent TTR-stabilizing activity of CHF5074.

Ectopic expression of Ptf1a induces spinal defects, urogenital defects, and anorectal malformations in Danforth's short tail mice

Danforth's short tail (Sd) is a semidominant mutation on mouse chromosome 2, characterized by spinal defects, urogenital defects, and anorectal malformations. However, the gene responsible for the Sd phenotype was unknown. In this study, we identified the molecular basis of the Sd mutation. By positional cloning, we identified the insertion of an early transposon in the Sd candidate locus approximately 12-kb upstream of Ptf1a. We found that insertion of the transposon caused overexpression of three neighboring genes, Gm13344, Gm13336, and Ptf1a, in Sd mutant embryos and that the Sd phenotype was not caused by disruption of an as-yet-unknown gene in the candidate locus. Using multiple knockout and knock-in mouse models, we demonstrated that misexpression of Ptf1a, but not of Gm13344 or Gm13336, in the notochord, hindgut, cloaca, and mesonephros was sufficient to replicate the Sd phenotype. The ectopic expression of Ptf1a in the caudal embryo resulted in attenuated expression of Cdx2 and its downstream target genes T, Wnt3a, and Cyp26a1; we conclude that this is the molecular basis of the Sd phenotype. Analysis of Sd mutant mice will provide insight into the development of the spinal column, anus, and kidney.

Caudal regression syndrome (CRS) is a congenital heterogeneous constellation of caudal anomalies that includes varying degrees of agenesis of the spinal column, anorectal malformations, and genitourinary anomalies. Its pathogenesis is unclear. However, it could be the result of excessive physiologic regression of the embryonic caudal region based on analyses of the various mouse mutants carrying caudal agenesis. Among the mouse mutants, the Danforth's short tail (Sd) mouse is considered a best model for human CRS. Sd is a semidominant mutation, characterized by spinal defects, urogenital defects, and anorectal malformations, thus showing phenotypic similarity to human CRS. Although Sd is known to map to mouse chromosome 2, little is known about the molecular nature of the mutation. Here, we demonstrate an insertion of one type of retrotransposon near the Ptf1a gene. This resulted in ectopic expression of Ptf1a gene in the caudal region of the embryo and downregulation of Cdx2 and its downstream targets, leading to characteristic phenotypes in Sd mouse. Thus, Sd mutant mice will provide insight into the development of the spinal column, anus, and kidney.

Enhanced expression of human cDNA by phosphoglycerate kinase promoter-puromycin cassette in the mouse transthyretin locus

To produce a humanized mouse, it is critical to obtain a correct expression of a human gene/cDNA after insertion into a mouse locus. We previously generated a targeted allele in which the PGK-neo cassette, flanked by lox71 and loxP, was inserted into the first exon of the mouse endogenous transthyretin (Ttr) gene in ES cells. Using these ES cells, we showed that a human transthyretin (TTR) cDNA with the PGK-puro cassette can be efficiently inserted into this locus by Cre-mediated recombination, and that the human TTR cDNA was expressed in a tissue-specific manner under the control of the mouse endogenous Ttr promoter. To examine whether the PGK-puro cassette or IRES could affect the expression of human TTR cDNA, we generated four mouse lines using Cre and Flp-mediated recombination. The mouse line containing the PGK-puro cassette, but not IRES, exhibited quantitatively and temporally similar expression of human TTR cDNA. Removal of the PGK-puro cassette significantly downregulated the expression of the cDNA. The insertion of IRES sequence upstream of the human TTR cDNA resulted in decreased expression, even in the presence of the PGK-puro cassette. The mouse line containing IRES, but not PGK-puro, showed the lowest level of expression. These results suggest that the PGK-puro cassette is necessary to obtain the enhanced expression of a co-existing human cDNA in the mouse Ttr locus, even though the expression of co-existing cDNA was under the control of the mouse endogenous promoter.

Comparative analysis of right element mutant lox sites on recombination efficiency in embryonic stem cells

Background

Cre-mediated site-specific integrative recombination in mouse embryonic stem (ES) cells is a useful tool for genome engineering, allowing precise and repeated site-specific integration. To promote the integrative reaction, a left element/right element (LE/RE) mutant strategy using a pair of lox sites with mutations in the LE or RE of the lox sequence has previously been developed. Recombination between LE and RE mutant lox produces a wild-type loxP site as well as an LE+RE double mutant lox site, which has mutations in both sides and less affinity to Cre, resulting in stable integration. We previously demonstrated successful integrative recombination using lox71 (an LE mutant) and lox66 (an RE mutant) in ES cells. Recently, other LE/RE mutant lox sites showing higher recombination efficiency in Escherichia coli have been reported. However, their recombination efficiency in mammalian cells remains to be analyzed.

Results

Using ES cells, we compared six RE mutant lox sites, focusing on their recombination efficiency with lox71. All of the RE mutant lox sites showed similar recombination efficiency. We then analyzed the stability of the recombined product, i.e., the LE+RE double mutant lox site, under continuous and strong Cre activity in ES cells. Two RE mutants, loxJTZ17 and loxKR3, produced more stable LE+RE double mutant lox than did the lox66/71 double mutant.

Conclusion

The two mutant RE lox sites, loxJTZ17 and loxKR3, are more suitable than lox66 for Cre-mediated integration or inversion in ES cells.

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.