Human TTRV30M localization within podocytes in a transgenic mouse model of transthyretin related amyloidosis: does the environment play a role?

Hereditary transthyretin amyloidosis: a myriad of factors that influence phenotypic variability

Hereditary transthyretin-related amyloidosis (ATTRv amyloidosis) is a rare and progressively debilitating disease characterized by the deposition of transthyretin (TTR) amyloid fibrils in various organs and tissues, most commonly in the heart and peripheral nerves. This pathological deposition can lead to significant organ dysfunction and, ultimately, organ failure. ATTRv amyloidosis exhibits a broad range of clinical presentations, from purely neurological symptoms to purely cardiac manifestations, as well as mixed phenotypes which result from both neurological and cardiac implications. This wide phenotypical spectrum realistically challenges disease diagnosis and prognosis, especially in individuals without or with an unknown family history. Multiple factors are thought to contribute to this variability, including genetic, epigenetic, and even environmental influences. Understanding these factors is crucial, as they can significantly affect disease expression and progression. This review aims to summarize each of these contributing factors, to help elucidate the current knowledge on the phenotypical variability of ATTRv amyloidosis.

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.

A mouse model of a novel missense mutation (Gly83Arg) in a Chinese kindred manifesting vitreous amyloidosis only

The purpose of this study is to establish a mouse model of transthyretin (TTR) Gly83Arg gene mutation by the technique of gene targeting for research on hereditary vitreous amyloidosis (HVA) and to confirm whether this point mutation is a genetic feature of HVA. A vector (pBR322-MK-TTR) was constructed to target ES cells. The successfully transfected ES cells were used for blastocyst injection, thus generating F0. F0 and Flp mice were mated to generate F1 (TTR^+/-, Flp ^+/-) mice that lacked the neo gene but carried the Flp gene. F1 mice were mated with C57BL/6N wild type mice to generate F2 (TTR^+/-) mice. F3 homozygous and heterozygous mice were generated by mating F2 mice with each other. PCR and sequencing were performed for F3 mice. Amyloid was detected using Congo red stain and polarized light. Immunohistochemistry was used to detect the expression of TTR in the tissues. Quantitative fluorescent PCR and Western blotting were used to detect the expression of TTR mRNA and TTR protein, respectively. Two F0-generation, 2 F1-generation and 15 F3-generation mice were obtained. The gene sequencing of F3 mice showed TTR Gly83Arg mutation. When examined with Congo red and polarized light, the vitreous of TTR Gly83Arg mutant mice tested positive for amyloid. The hearts, livers, brains and kidneys of the experimental group and control group were all negative by Congo red staining. Immunohistochemical staining showed that the vitreous of TTR Gly83Arg mutant mice and the livers of the control mice were positive, but the kidneys, hearts and brains of both groups were negative. Quantitative fluorescent PCR showed that the mRNA expression of mutant mice was significantly lower than that of wild-type mice (F = 0.295, P = 0.023). Western blotting showed that the expression of TTR protein was significantly lower in the model mice than in the wild-type mice (t = 3.224, P = 0.018). TTR gene mutation is indeed a molecular characteristic of HVA and manifest in the eye disease only. A C57BL/6 mouse line carrying the TTR Gly83Arg gene mutation was successfully established. This strain of mice can be used for the study of HVA.

Fibroblasts endocytose and degrade transthyretin aggregates in transthyretin-related amyloidosis

Transthyretin (TTR)-related amyloidosis is a fatal disorder characterized by systemic extracellular deposition of TTR amyloid fibrils. Mutations in the TTR gene cause an autosomal dominant form of the disease-familial amyloidotic polyneuropathy (FAP). Wild-type (WT) TTR can also form amyloid fibrils in elderly patients with senile systemic amyloidosis. Regression of amyloid deposits in FAP patients who undergo liver transplantation to remove the main source of mutant TTR suggests the existence of mechanisms for the clearance of TTR deposits from the extracellular matrix (ECM), but the precise mechanisms are largely unknown. Because fibroblasts are abundant, playing a central role in the maintenance of the ECM and because the skin is one of the major sites of soluble TTR catabolism, in the present study, we analyzed their role in clearance of TTR aggregates. In vitro studies with a fibroblast cell line revealed that fibroblasts endocytosed and degraded aggregated TTR. Subcutaneous injection of soluble and aggregated TTR into WT mice showed internalization and clearance over time by both fibroblasts and macrophages. Immunohistochemical studies of skin biopsies from V30M patients, asymptomatic carriers, recipients of domino FAP livers as well as transgenic mice for human V30M showed intracellular TTR immunoreactivity in fibroblasts and macrophages that increased with clinical status and with age in transgenic mice. Overall, the present in vitro and in vivo data show that fibroblasts endocytose and degrade TTR aggregates. The function or dysfunction of TTR clearance by fibroblasts may have important implications for the development, progression, and regression of TTR deposition in the ECM.