A simple screening test for variant transthyretins associated with familial transthyretin amyloidosis using isoelectric focusing

An ultra performance liquid chromatography method for transthyretin variants screening and heart failure assisting diagnosis

BACKGROUND

Transthyretin (TTR) gene mutations are associated with hereditary amyloidosis (ATTR) caused by mutant TTR protein dissociation, misfolding, aggregation, and insoluble fibrils deposition. Herein, we reported a chromatographic approach for quantification and identification of TTR tetramer in human blood serum by ultra performance liquid chromatography (UPLC).

METHODS

TTR proteins and serum were incubated with a fluorescent TTR tetramer sensor (A2). The A2 sensor specifically reacted with tetrameric TTR and released stoichiometric fluorescence that was detected by fluorescence detector coupled to UPLC. The external standard was used for quantification, the chromatographic peak parameters were used to identification certain mutation types.

RESULTS

UPLC correctly distinguished 18 types of mutant TTR proteins from wild type. The results were consistent with follow-up analysis of two ATTR patients' blood serum samples. In addition, the tetrameric TTR of 30 heart failure (HF) patients showed strongly correlation (r = -0.63, p < 0.00) with NT-proBNP, a HF clinical biomarker.

CONCLUSIONS

UPLC method has sufficient accuracy to eliminate the necessity of sequencing for certain types of TTR mutations and allows for facile initial screening of ATTR amyloidosis patients, carriers, and healthy individuals for time-saving and economical purposes. TTR tetramer may serve as a diagnostic biomarker to evaluate the risk of HF diseases.

An additive destabilising effect of compound T60I and V122I substitutions in ATTRv amyloidosis

BACKGROUND

The amyloidogenic transthyretin (TTR) variant, V122I, occurs in 4% of the African American population and frequently presents as a restricted cardiomyopathy. While heterozygosity for TTR V122I predominates, several compound heterozygous cases have been previously described. Herein, we detail features of ATTRv amyloidosis associated with novel compound heterozygous TTR mutation, T60I/V122I and provide evidence supporting the amyloidogenecity of T60I.

METHODS

A 63-year-old African American female presented with atrial fibrillation, congestive heart failure, autonomic and peripheral neuropathy. In vitro studies of TTR T60I and V122I were undertaken to compare the biophysical properties of the proteins.

RESULTS

Congophilic deposits in a rectal biopsy were immunohistochemically positive for TTR. Serum screening by isoelectric focussing revealed two TTR variants in the absence of wild-type protein. DNA sequencing identified compound heterozygous TTR gene mutations, c.239C > T and c.424G > A. Adipose amyloid deposits were composed of both T60I and V122I. While kinetic stabilities of T60I and V122I variants were similar, distinct thermodynamic stabilities and amyloid growth kinetics were observed.

CONCLUSIONS

This report provides clinical and experimental results supporting the amyloidogenic nature of a novel TTR T60I variant. In vitro data indicate that the destabilising effect of individual T60I and V122I variants appears to be additive rather than synergistic.

Hereditary Transthyretin-Related Amyloidosis: Genetic Heterogeneity and Early Personalized Gene Therapy

Point mutations of the transthyretin (TTR) gene are related with hereditary amyloidosis (hATTR). The number of people affected by this rare disease is only partially estimated. The real impact of somatic mosaicism and other genetic factors on expressivity, complexity, progression, and transmission of the disease should be better investigated. The relevance of this rare disease is increasing and many efforts have been made to improve the time to diagnosis and to estimate the real number of cases in endemic and non-endemic areas. In this context, somatic mosaicism should be better investigated to explain the complexity of the heterogeneity of the hATTR clinical features, to better estimate the number of new cases, and to focus on early and personalized gene therapy. Gene therapy can potentially improve the living conditions of affected individuals and is one of the central goals in research on amyloidosis related to the TTR gene, with the advantage of overcoming liver transplantation as the sole treatment for hATTR disease.

Use of biomarkers to diagnose and manage cardiac amyloidosis

Amyloidoses are characterized by the tissue accumulation of misfolded proteins into insoluble fibrils. The two most common types of systemic amyloidosis result from the deposition of immunoglobulin light chains (AL) and wild-type or variant transthyretin (ATTRwt/ATTRv). Cardiac involvement is the main determinant of outcome in both AL and ATTR, and cardiac amyloidosis (CA) is increasingly recognized as a cause of heart failure. In CA, circulating biomarkers are important diagnostic tools, allow to refine risk stratification at baseline and during follow-up, help to tailor the therapeutic strategy and monitor the response to treatment. Among amyloid precursors, free light chains are established biomarkers in AL amyloidosis, while the plasma transthyretin assay is currently being investigated as a tool for supporting the diagnosis of ATTRv amyloidosis, predicting outcome and monitor response to novel tetramer stabilizers or small interfering RNA drugs in ATTR CA. Natriuretic peptides (NPs) and troponins are consistently elevated in patients with AL and ATTR CA. Plasma NPs, troponins and free light chains hold prognostic significance in AL amyloidosis, and are evaluated for therapy decision-making and follow-up, while the value of NPs and troponins in ATTR is less well established. Biomarkers can be usefully integrated with clinical and imaging variables at all levels of the clinical algorithm of systemic amyloidosis, from screening to diagnosis and prognosis, and treatment tailoring.

Race/ethnicity in systemic AL amyloidosis: perspectives on disease and outcome disparities

In marked contrast to multiple myeloma, racial/ethnic minorities are underrepresented in publications of systemic light-chain (AL) amyloidosis. The impact of race/ethnicity is therefore lacking in the narrative of this disease. To address this gap, we compared disease characteristics, treatments, and outcomes across racial/ethnic groups in a referred cohort of patients with AL amyloidosis from 1990 to 2020. Among 2416 patients, 14% were minorities. Non-Hispanic Blacks (NHBs) comprised 8% and had higher-risk sociodemographic factors. Hispanics comprised 4% and presented with disproportionately more BU stage IIIb cardiac involvement (27% vs. 4-17%). At onset, minority groups were younger in age by 4-6 years. There was indication of more aggressive disease phenotype among NHBs with higher prevalence of difference between involved and uninvolved free light chains >180 mg/L (39% vs. 22-33%, P = 0.044). Receipt of stem cell transplantation was 30% lower in Hispanics compared to non-Hispanic White (NHWs) on account of sociodemographic and physiologic factors. Although the age/sex-adjusted hazard for death among NHBs was 24% higher relative to NHWs (P = 0.020), race/ethnicity itself did not impact survival after controlling for disease severity and treatment variables. These findings highlight the complexities of racial/ethnic disparities in AL amyloidosis. Directed efforts by providers and advocacy groups are needed to expand access to testing and effective treatments within underprivileged communities.

New sequence variants in patients affected by amyloidosis show transthyretin instability by isoelectric focusing

The plasma protein transthyretin (TTR) can aggregate into insoluble amyloid fibrils causing systemic amyloidosis (ATTR amyloidosis) in patients carrying a variant TTR protein. If new variants arise, it is crucial to clarify whether they are disease-associated or benign. In this study, we further functionally characterize three new and unclassified TTR variants (Thr40Asn, Phe64Val and the described but not functionally assessed variant Leu12Val), using a simplified, fast isoelectric focusing (IEF) approach. After validating the system with known TTR variants, we assessed the sera of five patients carrying these new TTR variants in a heterozygous state. All three variants showed aberrant banding patterns that were similar to those of other well-characterized TTR variants, including the common Val30Met variant that causes ATTR amyloidosis. In addition to a clear band corresponding to monomeric wild-type TTR, we observed an additional variant band at the cathodal side of the IEF gel. These results indicate conformational instability of the new Thr40Asn, Phe64Val and Leu12Val variants. Together with the clinical and immunohistological data of these patients and affected family members, as well as the absence of these variants in human genetic mutation databases, our results strongly hint that these variants are amyloidogenic and therefore probably disease-associated. These findings have implications for patient therapy and for genetic counselling of family members.

Serum Proteomic Variability Associated with Clinical Phenotype in Familial Transthyretin Amyloidosis (ATTRm)

Transthyretin (TTR), normally a plasma circulating protein, can become misfolded and aggregated, ultimately leading to extracellular deposition of amyloid fibrils usually targeted to heart or nerve tissues. Referred to as TTR-associated amyloidoses (ATTR), this group of diseases is frequently life threatening and fatal if untreated. ATTR, caused by amyloid-forming variant TTR proteins (ATTRm) that arise from point mutations in the TTR gene, were classically referred to as familial amyloid cardiomyopathy (FAC) or familial amyloid polyneuropathy (FAP), reflecting the clinical phenotype. FAC and FAP are pathologies that can be challenging to diagnose as there are no definitive biomarkers of disease; moreover, disease-specific measures of progression are lacking, and treatment options are limited. Thus, the discovery of sensitive and specific indicators of disease has the potential to improve recognition, enable accurate measurement of amyloid progression and response to treatment, and reveal key information regarding FAC and FAP pathobiological mechanisms. In this study, the goal was to investigate serum proteomic features unique to FAC and FAP types of ATTRm. Multiple-reaction monitoring mass spectrometry (MRM-MS), a powerful technique in profiling proteomes, was used to measure the serum concentrations of 160 proteins in samples from FAC and FAP patients. Results were compared to data from healthy control sera obtained from individuals matched to age (≥60 years), gender (male), and race (Caucasian). Proteomic analyses of ATTRm (FAC and FAP) and control samples showed significant concentration differences in 107 of 192 (56%) of the serum proteins that were studied. In comparing FAC to FAP, differences in concentrations as well as interactions and functions of several proteins were identified as unique to each disease; significantly lower levels of TTR were specific to FAC, but not to FAP. Annotated functional clustering identified extracellular region, signal, and signal peptide as terms common to FAC and FAP. Conversely, disulfide bond was unique to FAC; secreted, glycosylation site: N-linked, glycosylation, glycoprotein, polymorphism, and sequence variant were associated solely with FAP. Predicted protein-protein associations in FAC were seen for reaction, binding, and activation processes; no associations were found in FAP. This study demonstrates significant proteomic differences between ATTRm patient and control sera, as well as ATTRm phenotype-associated variations in the circulating levels of several proteins including TTR. The identification of serum proteins unique to FAC and FAP may have diagnostic and prognostic utility and could possibly provide important clues about disease mechanisms.

Blood Proteomic Profiling in Inherited (ATTRm) and Acquired (ATTRwt) Forms of Transthyretin-Associated Cardiac Amyloidosis

Transthyretin-associated forms of cardiac amyloidosis are fatal protein misfolding diseases that can be inherited (ATTRm) or acquired (ATTRwt). An accurate diagnosis of ATTR amyloidosis can be challenging as biopsy evidence, usually from the affected organ, is required. Precise biomarkers for ATTR disease identification and monitoring are undiscovered, disease-specific therapeutic options are needed, and the current understanding of ATTR molecular pathogenesis is limited. The aim of this study was to investigate and compare the serum proteomes in ATTRm and ATTRwt cardiac amyloidosis to identify differentially expressed blood proteins that were disease-specific. Using multiple-reaction monitoring mass spectrometry (MRM-MS), the concentrations of 160 proteins were analyzed in serum samples from ATTRm and ATTRwt patients, and a healthy control group. Patient and control sera were matched to age (≥60 years), gender (male), and race (Caucasian). The circulating concentrations of 123/160 proteins were significantly different in patient vs control sera; TTR and retinol-binding protein (RBP4) levels were significantly decreased (p < 0.03) in ATTRm compared to controls. In ATTRm, 14/123 proteins were identified as unique to that group and found generally to be lower than controls; moreover, the concentrations of RBP4 and 6 other proteins in this group were significantly different (p < 0.04) compared to ATTRwt. Predicted interactions among the 14 proteins unique to ATTRm were categorized as reaction and binding associations. Alternatively, 27 proteins were found to be unique to ATTRwt with associated interactions defined as activation, catalysis, and inhibition, in addition to reaction and binding. This study demonstrates significant proteomic differences between ATTR patient and control sera, and disease-associated variations in circulating levels of several proteins including TTR and RBP4. The identification of serum proteins unique to ATTRm and ATTRwt cardiac amyloidosis may have diagnostic and prognostic utility, and may provide important clues about disease mechanisms.

Biochemical markers in early diagnosis and management of systemic amyloidoses

Systemic amyloid diseases are characterized by widespread protein deposition as amyloid fibrils. Precise diagnostic framing is the prerequisite for a correct management of patients. This complex process is achieved through a series of steps, which include detection of the tissue amyloid deposits, identification of the amyloid type, demonstration of the amyloidogenic precursor, and evaluation of organ dysfunction/damage. Laboratory medicine plays a central role in the diagnosis and management of systemic amyloidoses, through the quantification of the amyloidogenic precursor and evaluation of end-organ damage using biomarkers.

Circulating matrix metalloproteinases and tissue inhibitors of metalloproteinases in cardiac amyloidosis

BACKGROUND

Cardiac amyloidosis due to amyloid fibril deposition in the heart results in cardiomyopathy (CMP) with heart failure (HF) and/or conduction disturbances. Immunoglobulin light chain-related CMP (AL-CMP) features rapidly progressive HF with an extremely poor prognosis compared with a CMP due to the deposition of mutant (ATTR) amyloidosis or wild-type (senile systemic amyloidosis, SSA) transthyretin (TTR) proteins. Amyloid fibril deposition disrupts the myocardial extracellular matrix (ECM) homeostasis, which is partly regulated by matrix metalloproteinases (MMPs) and their tissue inhibitors (TIMPs). We therefore tested the hypothesis that circulating levels of MMPs and TIMPs in patients with AL-CMP and TTR-related CMP (TTR-CMP) are dissimilar and indicative of cardiac amyloid disease type.

METHODS AND RESULTS

Fifty AL-CMP patients were compared with 50 TTR-CMP patients (composed of 38 SSA and 12 ATTR patients). Clinical and laboratory evaluations including echocardiography were performed at the initial visit to our center and analyzed. Serum MMP-2, MMP-9, TIMP-1, and TIMP-2 levels were determined by ELISA. Compared with TTR-CMP patients, AL-CMP patients had higher levels of brain natriuretic peptide (BNP), troponin I (TnI), MMP-2, TIMP-1, and MMP-2/TIMP-2 ratio, despite less left ventricular (LV) hypertrophy and better preserved LV ejection fraction. Mortality was worse in AL-CMP patients than in TTR-CMP patients (log-rank P<0.01). MMP-2/TIMP-2 plus BNP and TnI showed the highest discriminative ability for distinguishing AL-CMP from TTR-CMP. Female sex (HR, 2.343; P=0.049) and BNP (HR, 1.041; P<0.01) were predictors for mortality for all patients with cardiac amyloidoses. Only BNP was a predictor of death in AL-CMP patients (HR, 1.090; P<0.01). There were no prognostic factors for all-cause death in TTR-CMP patients.

CONCLUSIONS

Circulating concentrations of MMPs and TIMPs may be useful in differentiating patients with AL-CMP from those with TTR-CMP, resulting in earlier diagnostic vigilance, and may add prognostic information. In addition to an elevated BNP level, female sex increased the risk of death in patients with cardiac amyloidoses.

Structure and assembly-disassembly properties of wild-type transthyretin amyloid protofibrils observed with atomic force microscopy

Transthyretin (TTR) is an important human transport protein present in the serum and the cerebrospinal fluid. Aggregation of TTR in the form of amyloid fibrils is associated with neurodegeneration, but the mechanisms of cytotoxicity are likely to stem from the presence of intermediate assembly states. Characterization of these intermediate species is therefore essential to understand the etiology and pathogenesis of TTR-related amyloidoses. In the present work we used atomic force microscopy to investigate the morphological features of wild-type (WT) TTR amyloid protofibrils that appear in the early stages of aggregation. TTR protofibrils obtained by mild acidification appeared as flexible filaments with variable length and were able to bind amyloid markers (thioflavin T and Congo red). Surface topology and contour-length distribution displayed a periodic pattern of ∼ 15 nm, suggesting that the protofibrils assemble via an end-binding oligomer fusion mechanism. The average height and periodic substructure found in protofibrils is compatible with the double-helical model of the TTR amyloid protofilament. Over time protofibrils aggregated into bundles and did not form mature amyloid-like fibrils. Unlike amyloid fibrils that are typically stable under physiological conditions, the bundles dissociated into component protofibrils with axially compacted and radially dilated structure when exposed to phosphate-buffered saline solution. Thus, WT TTR can form metastable filamentous aggregates that may represent an important transient state along the pathway towards the formation of cytotoxic TTR species.

Macroglossia - not always AL amyloidosis

AL amyloidosis and transthyretin (ATTR) amyloidosis are the most frequent forms of systemic amyloidosis diagnosed in the United States. Macroglossia is considered to be a pathognomonic feature of AL amyloidosis. We report on two cases of systemic amyloidosis with macroglossia that defied routine clinical diagnosis, in which the deposits were typed as ATTR in one case and AL in the other using immunoelectron microscopy. These cases highlight: (1) the difficulty of typing amyloidosis on clinical criteria alone; (2) the utility of immunoelectron microscopy and (3) that macroglossia, while occurring much more frequently in AL, can also accompany ATTR amyloidosis.

Identification and quantitative analysis of human transthyretin variants in human serum by Fourier transform ion-cyclotron resonance mass spectrometry

Transthyretin (TTR) is a homotetrameric protein involved in thyroid hormone transport in blood and in retinol binding in the central nervous system. More than 80 point mutations in this protein are known to be associated with the formation of amyloid deposits and systemic amyloidotic pathologies. Age at onset varies according to the mutation but considerable variations also occur for subjects carrying the same mutation. Moreover, wild-type TTR forms amyloid deposits in systemic senile amyloidosis, a geriatric disorder. An accurate diagnostic and the choice of therapeutic options depend on the identification of the specific mutation. Previous characterization of TTR variants by mass spectrometry required the use of antibodies for sample enrichment. We developed a novel assay based on ultra high-resolution mass spectrometry to identify human TTR variants. The method, requiring a very low sample amount, is based on SDS-PAGE fractionation of human serum, followed by peptide mass fingerprinting by MALDI-FTICR-MS (matrix assisted laser desorption ionization coupled to Fourier transform ion cyclotron resonance mass spectrometry). Moreover, it is possible to perform a relative quantification of wild type and mutant TTR forms by mass spectrometry. The method was tested and validated with the V30M mutant, involved in familial amyloidotic neuropathy of Portuguese type.

Cardiac amyloidosis in African Americans: comparison of clinical and laboratory features of transthyretin V122I amyloidosis and immunoglobulin light chain amyloidosis

BACKGROUND

Transthyretin (TTR) mutations known to cause cardiac amyloidosis include V122I, found almost exclusively in African Americans at a prevalence of 3-3.9%. This retrospective study describes TTR V122I-associated cardiac amyloid disease (ATTR) in a major amyloid referral clinic population.

METHODS

Self-identified African Americans with amyloidosis (n = 156) were screened for TTR V122I by serum isoelectric focusing; mutant TTR was confirmed by DNA sequencing or mass spectrometry. Cardiac findings in ATTR V122I and immunoglobulin light chain (AL) amyloidoses were compared.

RESULTS

TTR V122I was identified in 36/156 (23.1%) of evaluated patients and included 5 homozygotes; the allele frequency was 0.013. One compound heterozygote (F44L/V122I) and 4 patients who had AL and the mutant TTR allele were characterized. In patients negative for V122I, AL was the most frequent diagnosis (86/120). Cardiomyopathy was present in 100% of patients with ATTR and 84% of patients with AL (P = .01). In patients with dominant cardiac involvement, better survival occurred in ATTR (n = 30) compared to AL (n = 31), (27 vs 5 months, P < .01) although the mean age in ATTR was higher (70.3 vs 56.2 years, P < .01). Congestive heart failure symptoms and electrocardiographic findings were similar in ATTR and AL, but significant differences in echocardiographic measurements were observed.

CONCLUSIONS

ATTR V122I and AL are equally prevalent as the cause of cardiomyopathy in African Americans referred for a diagnosis of amyloidosis. Available therapy for AL underscores the need for early and accurate determination of amyloid type.

Amyloid fibril composition is related to the phenotype of hereditary transthyretin V30M amyloidosis

Swedish familial systemic amyloidosis with polyneuropathy (FAP) depends on a mutation leading to a methionine-for-valine substitution in transthyretin. The disease appears with different clinical manifestations, including age of onset and involvement of the heart. Liver transplantation is currently the only curative treatment, but progressive cardiomyopathy may occur post-transplant. Two amyloid deposition patterns have previously been described in the heart. In one, the amyloid consists partially of transthyretin fragments and is weakly stainable by Congo red, while in the other, only full-length molecules are found and the fibrils have a strong affinity for Congo red. The present study aimed to see whether these morphological and biochemical variations have clinical implications. Subcutaneous adipose tissue biopsies were taken from 33 patients with Val30Met FAP and examined by microscopy, electrophoresis and western blot. Clinical data included age, sex, duration of disease and echocardiographic determination of the interventricular septum (IVS) thickness. It was found that fibrils composed of only full-length transthyretin were associated with early age of onset (44.8 +/- 12.9 years), no clinical cardiac involvement and a strong affinity for Congo red. In contrast, presence of transthyretin fragments in the amyloid was associated with late age of onset (67.3 +/- 7.0 years), signs of cardiac involvement and weak Congo red staining. For each individual, the same molecular type of amyloid was found in different organs. This is the first report showing that variations in clinical appearance of familial ATTR amyloidosis are associated with specific structural differences in the amyloid fibrils, and therefore may have a molecular cause. The molecular type of amyloid can be determined from a subcutaneous fat tissue biopsy.

Matrix metalloproteinases and their tissue inhibitors in cardiac amyloidosis: relationship to structural, functional myocardial changes and to light chain amyloid deposition

BACKGROUND

Cardiac amyloidosis is characterized by amyloid infiltration resulting in extracellular matrix disruption. Amyloid cardiomyopathy due to immunoglobulin light chain protein (AL-CMP) deposition has an accelerated clinical course and a worse prognosis compared with non-light chain cardiac amyloidoses (ie, forms associated with wild-type or mutated transthyretin [TTR]). We therefore tested the hypothesis that determinants of proteolytic activity of the extracellular matrix, the matrix metalloproteinases (MMPs), and their tissue inhibitors (TIMPs) would have distinct patterns and contribute to the pathogenesis of AL-CMP versus TTR-related amyloidosis.

METHODS AND RESULTS

We studied 40 patients with systemic amyloidosis: 10 AL-CMP patients, 20 patients with TTR-associated forms of cardiac amyloidosis, ie, senile systemic amyloidosis (involving wild-type TTR) or mutant TTR, and 10 patients with AL amyloidosis without cardiac involvement. Serum MMP-2 and -9, TIMP-1, -2, and -4, brain natriuretic peptide values, and echocardiography were determined. AL-CMP and TTR-related amyloidosis groups had similar degrees of increased left ventricular wall thickness. However, brain natriuretic peptide, MMP-9, and TIMP-1 levels were distinctly elevated accompanied by marked diastolic dysfunction in the AL-CMP group versus no or minimal increases in the TTR-related amyloidosis group. Brain natriuretic peptide, MMPs, and TIMPs were not correlated with the degree of left ventricular wall thickness but were correlated to each other and to measures of diastolic dysfunction. Immunostaining of human endomyocardial biopsies showed diffuse expression of MMP-9 and TIMP-1 in AL-CMP and limited expression in TTR-related amyloidosis hearts.

CONCLUSIONS

Despite comparable left ventricular wall thickness with TTR-related cardiac amyloidosis, AL-CMP patients have higher brain natriuretic peptide, MMPs, and TIMPs, which correlated with diastolic dysfunction. These findings suggest a relationship between light chains and extracellular matrix proteolytic activation that may play an important role in the functional and clinical manifestations of AL-CMP, distinct from the other non-light chain cardiac amyloidoses.

Genetic microheterogeneity of human transthyretin detected by IEF

Mutations of the human transthyretin (TTR) gene have attracted medical interest as a cause of amyloidosis. Recently, we have described in detail an electrophoretic procedure with PAGE followed by IEF in urea gradients for the study of the microheterogeneity of TTR monomers (Altland, K., Winter, P., Sauerborn, M. K., Electrophoresis 1999, 20, 1349-1364). In this paper, we present a study on 49 different mutations of TTR including 33 that result in electrically neutral amino acid substitutions. The aims of the investigation were to test the sensitivity of the procedure to detect TTR variants in patients with TTR amyloidosis and their relatives and to identify some common characteristics that could explain the amyloidogenicity of these variants. We found that all tested amyloidogenic mutations could be detected by our method with the exception of those for which the corresponding variant was absent in plasma samples. Most of the electrically neutral amyloidogenic TTR variants had in common a reduced conformational stability of monomers by the activity of protons and urea. For three variants, e.g. TTR-F64L, TTR-I107V and TTR-V122I, the monomers had a conformational stability close to that of normal monomers but we found experimental and structural arguments for a weakening of the monomer-monomer contact. All types of amyloidogenic mutations affected the stability of TTR tetramers.

A new transthyretin variant (Glu61Gly) associated with cardiomyopathy

We report the identification of a new transthyretin (TTR) gene mutation and variant protein, Glu61Gly, in a 55-year-old man with progressive cardiomyopathy, mild peripheral neuropathy and bilateral carpal tunnel syndrome. A diagnosis of TTR-associated familial amyloidosis (ATTR) was considered after an endomyocardial biopsy revealed amyloid deposits in the heart of a patient who had no family history of amyloidosis and no evidence of a plasma cell dyscrasia. Serum screening for a TTR variant by isoelectric focusing (IEF) was positive and prompted further studies to identify the genetic abnormality and to characterize the amyloidogenic protein. Direct DNA sequence analysis of all four coding regions in the TTR gene demonstrated heterozygosity in exon 3. Near equal amounts of guanine (G) and adenine (A) were observed at the second base position of codon 61. The wild-type (GAG) and mutated (GGG) sequences found in codon 61 correspond to glutamic acid (Glu) and glycine (Gly) residues, amino acids which differ in mass by -72 Da. Mass spectrometric analyses of TTR immunoprecipitated from serum showed the presence of both wild-type and variant proteins. The observed mass results for the wild-type and variant proteins were consistent with the predicted values calculated from the genetic analysis data.

Identification of Transthyretin Variants by Sequential Proteomic and Genomic Analysis

Background: Transthyretin-associated hereditary amyloidosis (ATTR) is an inherited disease in which variants in the primary structure of transthyretin (TTR; prealbumin) lead to the extracellular polymerization of insoluble protein fibrils, causing organ failure and ultimately death when major organs are involved. We have developed an integrated approach to molecular diagnosis with initial analysis of intact plasma TTR by electrospray ionization mass spectrometry (MS) and referral of positive samples for DNA sequence analysis and real-time PCR to confirm the common Gly6Ser polymorphism.

Methods: Samples from 6 patients previously diagnosed with ATTR and from 25 controls with (n = 15) or without (n = 10) polyneuropathy were analyzed in a blinded fashion for the presence of variant TTR. TTR protein was extracted with an immunoaffinity resin from 20 μL of archived plasma samples. The purified TTR was reduced with tris(2-carboxyethyl)phosphine and analyzed by MS. The appearance of two peaks (or a single peak shifted in mass indicative of a homozygous variant), including the wild-type mass of 13 761 Da, was indicative of the presence of a variant, and the individual was referred for DNA sequence analysis.

Results: MS analysis of intact reduced TTR correctly identified each of six samples known to contain variant TTR. These results were corroborated by subsequent DNA sequence analysis. Additionally, all Gly6Ser polymorphisms were correctly called based on the +30 mass shift and an equal relative abundance of the +30 polymorphism relative to wild-type TTR. No false-positive results were seen.

Conclusions: This referral method eliminates the necessity of sequencing most samples and allows screening for the familial forms of amyloidosis in a broad patient population in a timely fashion. This method correctly identified all previously known variants and also identified a novel variant, Val94Ala.

A rare transthyretin mutation (Asp18Glu) associated with cardiomyopathy

The identification of a rare transthyretin (TTR) gene mutation (Asp18Glu) in a middle-aged male with biopsy proven amyloid disease featuring cardiomyopathy is described. The more commonly occurring light chain amyloidosis (AL) was initially considered, but negative hematologic testing prompted screening for a pathologic TTR mutation. A differential diagnosis of familial transthyretin type amyloidosis (ATTR) was established using a combination of molecular genetic and biochemical techniques. Single-strand conformation polymorphism (SSCP) screening of exons 2, 3 and 4 of the TTR gene indicated the presence of atypical DNA. SSCP testing was performed using a new non-radioactive, silver stained minigel technique. The genetic abnormality was identified by direct DNA sequence analysis as a T to A transversion at the third base position in codon 18. This result was confirmed by restriction fragment length polymorphism (RFLP) testing. The presence of the variant protein, TTR Asp18Glu, in serum from the proband was confirmed by mass spectrometric analysis.

An on-line assay for clinical detection of amyloidogenic transthyretin variants directly from serum

We report here for the first time the on-line analysis of transthyretin genetic variants by mass spectral analysis. The use of mass spectrometry to analyze immunoprecipitated transthyretin has been previously described. However, the on-line analysis of TTR directly from serum reported here will allow for a fully automated high throughput analysis. Mutations in the plasma transport protein TTR are readily observed and distinguished from normal TTR. Free TTR as well as TTR-cysteine and TTR-cysteinylglycine adducts are clearly evident. The resulting assay from serum to final interpretation requires less than twenty minutes. The assay should be an effective first line discriminator of patients who are being considered to have Familial Amyloidotic Polyneuropathy (FAP) and an adjunct to definitive diagnosis by sequencing of the TTR gene or protein.

Identification of S-sulfonation and S-thiolation of a novel transthyretin Phe33Cys variant from a patient diagnosed with familial transthyretin amyloidosis

Familial transthyretin amyloidosis (ATTR) is an autosomal dominant disorder associated with a variant form of the plasma carrier protein transthyretin (TTR). Amyloid fibrils consisting of variant TTR, wild-type TTR, and TTR fragments deposit in tissues and organs. The diagnosis of ATTR relies on the identification of pathologic TTR variants in plasma of symptomatic individuals who have biopsy proven amyloid disease. Previously, we have developed a mass spectrometry-based approach, in combination with direct DNA sequence analysis, to fully identify TTR variants. Our methodology uses immunoprecipitation to isolate TTR from serum, and electrospray ionization and matrix-assisted laser desorption/ionization mass spectrometry (MS) peptide mapping to identify TTR variants and posttranslational modifications. Unambiguous identification of the amino acid substitution is performed using tandem MS (MS/MS) analysis and confirmed by direct DNA sequence analysis. The MS and MS/MS analyses also yield information about posttranslational modifications. Using this approach, we have recently identified a novel pathologic TTR variant. This variant has an amino acid substitution (Phe --> Cys) at position 33. In addition, like the Cys10 present in the wild type and in this variant, the Cys33 residue was both S-sulfonated and S-thiolated (conjugated to cysteine, cysteinylglycine, and glutathione). These adducts may play a role in the TTR fibrillogenesis.

Diagnostic tools for amyloidosis

Demonstration of amyloid deposits in biopsy specimens is the only means of confirming the diagnosis of amyloidosis. In experienced hands, nonsurgical biopsies of the rectal mucosa or, preferably, of the abdominal fat pad or labial salivary glands provide the diagnosis in 80 to 85% of cases. Immunolabeling studies help to determine the histological type of amyloidosis but are not performed routinely in everyday practice. In patients with a family history of amyloidosis, studies of the genome and amyloid protein can identify the protein variants capable of causing systemic amyloidosis. Once the diagnosis of amyloidosis is established, the extent of systemic involvement with amyloid should be evaluated by performing renal and hepatic function tests, a proteinuria assay, and an echocardiogram. Scintigraphy with radiolabeled serum amyloid P (SAP) component is a rapid and specific investigation that provides a map of the amyloid deposits. Deposits are usually seen in the liver and spleen. SAP component scintigraphy can provide support for the diagnosis of amyloidosis in patients with negative histological studies. Tissue retention of radioactivity predicts survival.

Transthyretin Val71Ala mutation in a Dutch family with familial amyloidotic polyneuropathy

A Dutch family with familial amyloidotic polyneuropathy associated with the transthyretin mutation Val71Ala is described. This is the third reported family with this mutation, causing at the protein level an unstable TTR monomer and at the clinical level progressive wasting, polyneuropathy, autonomic dysfunction and vitreous opacities.

[Diagnostic strategy in amyloidosis]

INTRODUCTION

The diagnosis of amyloidosis is a four-step strategy requiring: 1) a high degree of suspicion, as the clinical presentation of the disease is very polymorphous; 2) the biochemical nature of the disease; 3) the evaluation of the disease spread and 4) how it evolves.

EXEGESIS

Simple and noninvasive biopsies usually make it possible to diagnose amyloidosis. Available treatments of generalized amyloidosis require an exact characterization of the nature of amyloid deposits, which is based on the clinical context, immunohistochemical analysis of amyloid deposits, and genetic testing.

CONCLUSION

Management of amyloidosis should be improved by better characterization of amyloid deposits. This would result in epidemiological data which are currently lacking.

Detection of transthyretin variants using immunoprecipitation and matrix-assisted laser desorption/ionization bioreactive probes: a clinical application of mass spectrometry

In our continuing efforts to develop mass spectrometry-based methods for transthyretin (TTR) variant detection and characterization, we have sought to use matrix-assisted laser desorption/ionization (MALDI) bioreactive probes incorporating immobilized trypsin for screening purposes. These devices show good diagnostic potential as a clinical screening tool to detect amino acid substitutions in TTR. MALDI probes allow the on-probe generation of tryptic digests. The subsequent mass analysis of the on-probe digest yields the peptide map. The inherent advantages of this method include considerably reduced digestion times (minutes vs. hours), absence of autolysis products, minimized sample handling, and hence minimal sample loss. A further advantage is that the opportunity for loss of hydrophobic peptides is reduced because no sample transfer occurs. The method can be applied as a preliminary screen for TTR variants where TTR is isolated from patient serum through immunoprecipitation. This method should also be applicable to other proteins and suitable for automation.

A new transthyretin variant (Ser23Asn) associated with familial amyloidosis in a Portuguese patient

The detection and characterization of a new transthyretin (ATTR) variant, Ser23Asn, associated with cardiomyopathy in a Portuguese patient with familial amyloidosis is described. Isoelectric focusing (IEF) of serum from the propositus demonstrated heterozygosity for the presence of wild type and variant ATTR. A combination of mass spectrometric (MS) analyses, including electrospray ionization mass spectrometry (ESI MS), high performance liquid chromatography (HPLC)/ESI MS and matrix-assisted laser desorption/ionization mass spectrometry (MALDI MS) performed on the serum-derived TTR were used to identify and locate the amino acid replacement in the variant protein. Genetic mutation analysis by DNA sequencing and allele-specific PCR confirmed this finding.