Transthyretin asn90 variant: Amyloidogenic or non-amyloidogenic role

Hereditary transthyretin amyloidosis: a comprehensive review with a focus on peripheral neuropathy

Amyloidoses represent a group of diseases characterized by the pathological accumulation in the extracellular area of insoluble misfolded protein material called "amyloid". The damage to the tissue organization and the direct toxicity of the amyloidogenic substrates induce progressive dysfunctions in the organs involved. They are usually multisystem diseases involving several vital organs, such as the peripheral nerves, heart, kidneys, gastrointestinal tract, liver, skin, and eyes. Transthyretin amyloidosis (ATTR) is related to abnormalities of transthyretin (TTR), a protein that acts as a transporter of thyroxine and retinol and is produced predominantly in the liver. ATTR is classified as hereditary (ATTRv) and wild type (ATTRwt). ATTRv is a severe systemic disease of adults caused by mutations in the TTR gene and transmitted in an autosomal dominant manner with incomplete penetrance. Some pathogenic variants in TTR are preferentially associated with a neurological phenotype (progressive peripheral sensorimotor polyneuropathy); others are more frequently associated with restrictive heart failure. However, many mutations express a mixed phenotype with neurological and cardiological involvement. ATTRv is now a treatable disease. A timely and definite diagnosis is essential in view of the availability of effective therapies that have revolutionized the management of affected patients. The purpose of this review is to familiarize the clinician with the disease and with the correct diagnostic pathways in order to obtain an early diagnosis and, consequently, the possibility of an adequate treatment.

Association of Low Plasma Transthyretin Concentration With Risk of Heart Failure in the General Population

Importance

Several lines of evidence support low plasma transthyretin concentration as an in vivo biomarker of transthyretin tetramer instability, a prerequisite for the development of both wild-type transthyretin cardiac amyloidosis (ATTRwt) and hereditary transthyretin cardiac amyloidosis (ATTRm). Both ATTRm and ATTRwt cardiac amyloidosis may manifest as heart failure (HF). However, whether low plasma transthyretin concentration confers increased risk of incident HF in the general population is unknown.

Objective

To evaluate whether low plasma transthyretin concentration is associated with incident HF in the general population.

Design, Setting, and Participants

This study included data from 2 similar prospective cohort studies of the Danish general population, the Copenhagen General Population Study (CGPS; n = 9582) and the Copenhagen City Heart Study (CCHS; n = 7385). Using these data, first, whether low concentration of plasma transthyretin was associated with increased risk of incident HF was tested. Second, whether genetic variants in TTR associated with increasing tetramer instability were associated with lower transthyretin concentration and with higher risk of HF was tested. Data were collected from November 2003 to March 2017 in the CGPS and from November 1991 to June 1994 in the CCHS; participants from both studies were observed for survival time end points until March 2017. Data were analyzed from March to June 2019.

Exposures

Transthyretin concentration at or below the 5th percentile, between the 5th and 95th percentile (reference), and greater than the 95th percentile; genetic variants in TTR.

Main Outcome and Measure

Incident HF identified using the Danish National Patient Registry.

Results

Of 9582 individuals in the CGPS, 5077 (53.0%) were women, and the median (interquartile range [IQR]) age was 56 (47-65) years. Of 7385 individuals in the CCHS, 4452 (60.3%) were women, and the median (IQR) age was 59 (46-70) years. During a median (IQR) follow-up of 12.6 (12.3-12.9) years and 21.7 (11.6-23.8) years, 441 individuals (4.6%) in the CGPS and 1122 individuals (15.2%) in the CCHS, respectively, developed HF. Baseline plasma transthyretin concentrations at or below the 5th percentile were associated with incident HF (CGPS: hazard ratio [HR], 1.6; 95% CI, 1.1-2.4; CCHS: HR, 1.4; 95% CI, 1.1-1.7). Risk of HF was highest in men with low transthyretin levels. Compared with p.T139M, a transthyretin-stabilizing variant, TTR genotype was associated with stepwise lower transthyretin concentrations for wild-type TTR (-16.5%), p.G26S (-18.1%), and heterozygotes for other variants (p.V142I, p.H110N, and p.D119N; -30.8%) (P for trend <.001). The corresponding HRs for incident HF were 1.14 (95% CI, 0.57-2.28), 1.29 (95% CI, 0.64-2.61), and 2.04 (95% CI, 0.54-7.67), respectively (P for trend = .04).

Conclusions and Relevance

In this study, lower plasma and genetically determined transthyretin concentrations were associated with a higher risk of incident HF, suggesting a potential mechanistic association between low transthyretin concentration as a marker of tetramer instability and incident HF in the general population.

Expert consensus recommendations to improve diagnosis of ATTR amyloidosis with polyneuropathy

Amyloid transthyretin (ATTR) amyloidosis with polyneuropathy (PN) is a progressive, debilitating, systemic disease wherein transthyretin protein misfolds to form amyloid, which is deposited in the endoneurium. ATTR amyloidosis with PN is the most serious hereditary polyneuropathy of adult onset. It arises from a hereditary mutation in the TTR gene and may involve the heart as well as other organs. It is critical to identify and diagnose the disease earlier because treatments are available to help slow the progression of neuropathy. Early diagnosis is complicated, however, because presentation may vary and family history is not always known. Symptoms may be mistakenly attributed to other diseases such as chronic inflammatory demyelinating polyradiculoneuropathy (CIDP), idiopathic axonal polyneuropathy, lumbar spinal stenosis, and, more rarely, diabetic neuropathy and AL amyloidosis. In endemic countries (e.g., Portugal, Japan, Sweden, Brazil), ATTR amyloidosis with PN should be suspected in any patient who has length-dependent small-fiber PN with autonomic dysfunction and a family history of ATTR amyloidosis, unexplained weight loss, heart rhythm disorders, vitreous opacities, or renal abnormalities. In nonendemic countries, the disease may present as idiopathic rapidly progressive sensory motor axonal neuropathy or atypical CIDP with any of the above symptoms or with bilateral carpal tunnel syndrome, gait disorders, or cardiac hypertrophy. Diagnosis should include DNA testing, biopsy, and amyloid typing. Patients should be followed up every 6–12 months, depending on the severity of the disease and response to therapy. This review outlines detailed recommendations to improve the diagnosis of ATTR amyloidosis with PN.

A novel transthyretin variant p.H110D (H90D) as a cause of familial amyloid polyneuropathy in a large Irish kindred

Hereditary transthyretin (ATTR) amyloidosis is caused by inheritance of an abnormal TTR gene in an autosomal dominant fashion. In its native state, TTR is a homotetramer consisting of four identical polypeptides. Mutations in the TTR gene contribute to destabilization and dissociation of the TTR tetramer, enabling abnormally folded monomers to self-assemble as amyloid fibrils. Currently, over 120 TTR variants have been described, with varying geographic distributions, degrees of amyloidogenicity and organ involvement. We report here a large Irish family with familial amyloid polyneuropathy (FAP), consisting of multiple affected generations, caused by a novel TTR mutation; p.H110D (H90D). The demonstration, by immunohistochemistry and laser micro dissection-mass spectrometry (LMD/MS) that the amyloid fibrils were composed of TTR, in conjunction with a typical FAP phenotype, indicates that the novel TTR mutation was the cause of amyloidosis. We used a molecular visualization tool PyMOL to analyze the effect of the p.H110D (H90D) replacement on the stability of the TTR molecule. Our data suggest that the loss of two hydrogen bonds and the presence of an additional negative charge in the core of a cluster of acidic residues significantly perturb the tetramer stability and likely contribute to the pathogenic role of this variant.

Most recent common ancestor of TTR Val30Met mutation in Italian population and its potential role in genotype-phenotype correlation

INTRODUCTION

Transthyretin (TTR)-related amyloidosis is characterized by autosomal transmission of amyloidogenic mutated TTR. Val30Met is one of the most common amyloidogenic TTR mutations, showing a worldwide distribution with phenotypic heterogeneity among human populations. Multiple founder mutations for Val30Met foci have been hypothesized and the different origins may explain the phenotypic variability. The aim of our study is to determine the origin of Italian Val30Met and to analyze the genetic relationship of other Val30Met foci.

METHODS

We analyzed the origin of Italian Val30Met through 11 microsatellite markers around the TTR gene in 29 patients and 34 healthy controls.

RESULTS

Our genetic analysis showed an estimated age of origin of 34-36 generations ago for the Italian Val30Met. Comparing Italian Val30Met haplotypes with those from Sweden and Portugal highlights relevant differences that seem to be consistent with an independent origin of Italian Val30Met mutation. This genetic evidence agrees with the disease phenotypic variation in these populations.

DISCUSSION AND CONCLUSIONS

Italian Val30Met mutation should have originated before the Portuguese and Swedish Val30Met ones (which arose through independent mutational events). This indicates a genetic diversity in the surrounding regions of three different Val30Met mutations, supporting the hypothesis that TTR non-coding regions may contribute to phenotypic heterogeneity.