Familial amyloidotic polyneuropathy without familial occurrence: carrier detection by the radioimmunoassay of variant transthyretin

The relative amounts of plasma transthyretin forms in familial transthyretin amyloidosis: a quantitative analysis by Fourier transform ion-cyclotron resonance mass spectrometry

Familial transthyretin amyloidosis (ATTR) is a fatal autosomal dominant disease characterized by the formation of amyloid fibers, mainly composed of transthyretin (TTR). Protein aggregation and amyloid fiber formation are considered concentration dependent processes and since most ATTR patients are heterozygous it is crucial to determine the ratio between mutant and non-mutant TTR forms in human plasma. Using a high resolution mass spectrometry based approach we determined the ratio of TTR forms in ATTR patients, V30M mutation carriers, symptomatic and asymptomatic ones, as well as ATTR patients that received a wild type cadaveric liver transplant. Domino transplanted patients that received a liver from an ATTR patient were also investigated. We found that although wild type TTR is diminished in the plasma of non-transplanted ATTR patients comparatively to healthy subjects, the relationship with the V30M variant does not change with illness progression. Those who received a wild type liver showed no mutant protein while domino transplanted patients presented the same relative amount of V30M as found in asymptomatic and symptomatic individuals. The V30M to wild type TTR ratio in plasma is the same for all ATTR patients studied, showing no variation with disease clinical progression. Our results point to the involvement of additional non-genetic factors on the pathogenesis of this disease.

Beyond genetic factors in familial amyloidotic polyneuropathy: protein glycation and the loss of fibrinogen's chaperone activity

Familial amyloidotic polyneuropathy (FAP) is a systemic conformational disease characterized by extracellular amyloid fibril formation from plasma transthyretin (TTR). This is a crippling, fatal disease for which liver transplantation is the only effective therapy. More than 80 TTR point mutations are associated with amyloidotic diseases and the most widely accepted disease model relates TTR tetramer instability with TTR point mutations. However, this model fails to explain two observations. First, native TTR also forms amyloid in systemic senile amyloidosis, a geriatric disease. Second, age at disease onset varies by decades for patients bearing the same mutation and some mutation carrier individuals are asymptomatic throughout their lives. Hence, mutations only accelerate the process and non-genetic factors must play a key role in the molecular mechanisms of disease. One of these factors is protein glycation, previously associated with conformational diseases like Alzheimer's and Parkinson's. The glycation hypothesis in FAP is supported by our previous discovery of methylglyoxal-derived glycation of amyloid fibrils in FAP patients. Here we show that plasma proteins are differentially glycated by methylglyoxal in FAP patients and that fibrinogen is the main glycation target. Moreover, we also found that fibrinogen interacts with TTR in plasma. Fibrinogen has chaperone activity which is compromised upon glycation by methylglyoxal. Hence, we propose that methylglyoxal glycation hampers the chaperone activity of fibrinogen, rendering TTR more prone to aggregation, amyloid formation and ultimately, disease.

Mass spectrometric immunoassay for quantitative determination of transthyretin and its variants

Transthyretin (TTR, or prealbumin) is a tetrameric protein found in plasma and cerebrospinal fluid. Its major role is to transport thyroid hormones (thyroxin-T4) and retinol (through association with retinol-binding protein). TTR has been studied extensively due to the great number of point mutations that result in sequence heterogeneity. Many of these variants are associated with pathological conditions that result in extracellular deposition of amyloid fibers in tissues. In this work, we have developed a rapid mass spectrometric immunoassay for determination and quantification of TTR and its variants from human serum and plasma samples. The assay was fully characterized in terms of its precision, linearity and recovery characteristics. The new assay was also compared with a conventional TTR ELISA. Furthermore, we have applied the optimized method to analyze TTR and its modifications in 44 human plasma samples, and in the process optimized a method for TTR proteolytic digestion and identification of point mutations.

Transthyretin and familial amyloidotic polyneuropathy

Familial amyloidotic polyneuropathy (FAP) is an inherited autosomal dominant disease that is commonly caused by accumulation of deposits of transthyretin (TTR) amyloid around peripheral nerves. The only effective treatment for FAP is liver transplantation. However, recent studies on TTR aggregation provide clues to the mechanism of the molecular pathogenesis of FAP and suggest new avenues for therapeutic intervention. It is increasingly recognized that there are common features of a number of protein-misfolding diseases that can lead to neurodegeneration. As for other amyloidogenic proteins, the most toxic forms of aggregated TTR are likely to be the low-molecular-mass diffusible species, and there is increasing evidence that this toxicity is mediated by disturbances in calcium homeostasis. This article reviews what is already known about the mechanism of TTR aggregation in FAP and describes how recent discoveries in other areas of amyloid research, particularly Alzheimer's disease, provide clues to the molecular pathogenesis of FAP.

Transthyretin--an explanation of "anomalous" serum thyroid hormone values in severe illness?

The following serum analytes were measured in 464 patients with defined carcinomas and other tumours as well as those with chronic obstructive lung disease and under regular haemodialysis, and in 261 healthy controls: thyrotropin (TSH), thyroxine (T4), triiodothyronine (T3), free thyroxine (fT4), thyroxine binding globulin (TBG) and transthyretin (TTR). The following ratios were constructed: fT4 x TTR, defined as the thyroxine availability index fT4/TTR, defined as the thyroid hormone compensation index 100 x T3/TBG as the free T3 index (fT3I) and fT3I x TTR, defined as the triiodothyronine availability index. Significantly elevated thyrotropin values (p = 0.05) were only found in patients with breast cancer when compared with age matched controls, although elevated T4 and fT4 values were found in all experimental groups except the haemodialysis patients (p less than 0.01). The thyroxine availability index and triiodothyronine availability index values were not significantly different from the age matched controls (greater than 60 a) in the cancer groups, showing that the transthyretin concentrations compensated for changes in fT4 or vice versa. These findings are reflected in the euthyroid thyrotropin values. The T4 and fT4 values in the dialysis patient group were significantly lower than in the age matched controls (p less than 0.01), while the transthyretin values were significantly higher (p less than 0.01), which accounted for the normal thyroxine availability index and euthyroid thyrotropin values.

Amyloidosis of the nervous system

Various types of amyloid fibril deposits occur in the nervous system with unique clinical characteristics and pathogeneses. Genetic mutations cause the familial amyloidotic polyneuropathies and acquired polyneuropathies occurring particularly in patients suffering from hypernephromas and myelomas also result from the production of abnormal proteins. Amyloid fibril deposits in cerebral plaques and vessels consisting of beta-protein are seen in acquired and familial Alzheimer's disease and in Down's syndrome individuals over 40 years of age. This amyloid fibril deposition could result from a mutational, transcriptional or post-translational alteration in these pathologic processes with most evidence supporting the latter. Other diseases including hereditary cerebral hemorrhage of the Dutch type and Batten's disease involve beta-amyloid deposition. The features of the familial and transmissible forms of the spongiform encephalopathies are associated with the prion protein which comprises the amyloid fibril deposits in these conditions. This wide variety of nervous system disorders having amyloid deposits as their primary or subsidiary characteristic make studies of these conditions intriguing models for research workers in clinical, pathologic and molecular biologic fields.