Two cases of late onset familial amyloid polyneuropathy with a Glu61Lys transthyretin variant

Clinical and biochemical characterization of hereditary transthyretin amyloidosis caused by E61K mutation

Objects: This study was intended to find out more about the clinical characterizations of patients carrying transthyretin (TTR) E61K (p.Glu81Lys) gene mutation and the biochemical characterization of this mutant protein.

Materials and methods: Five patients who had been diagnosed with hereditary transthyretin amyloidosis and two asymptomatic carriers carrying TTR E61K gene mutation were reported. Biochemical and biophysical tests were conducted to observe the thermodynamic and kinetic stability. Fibril formation tests measured by turbidity assay were performed to explore the pathogenicity of this mutation. Kinetic stabilizer responsiveness was measured to determine the inhibitory effect on protein aggregation.

Results: The average age of onset for the five patients was 62 years, and the course of the disease ranged from 2 to 10 years. Cardiac disease was prominent in this group of patients. Nerve pathology revealed a mildly to moderately reduced myelinated fiber density and muscle pathology showed predominant neurogenic impairment accompanied by possible myogenic impairment. E61K-TTR was characterized as a kinetically destabilized protein compared to WT-TTR but its thermodynamic stability was not compromised. In addition, the subunit exchange of E61K with WT-TTR further destabilized the heterozygous tetramer. Meanwhile, the E61K:WT heterozygous tetramer exhibited a poor response to kinetic stabilizers in the fibril formation assay. Finally, the serum TTR tetramer concentration was low in E61K-TTR symptomatic patients and in one asymptomatic gene carrier. Vyndamax (Tafamidis) could increase the TTR tetramer concentration.

Conclusions: Patients with E61K mutation tended to be late-onset. The concentration of TTR tetramer in the serum might serve as a biomarker to monitor disease progress, therapeutic window time, and therapeutic response to TTR kinetic stabilizer drugs.

A low amyloidogenic E61K transthyretin mutation may cause familial amyloid polyneuropathy

Patients with transthyretin (TTR)-type familial amyloid polyneuropathy (FAP) typically exhibit sensory dominant polyneuropathy and autonomic neuropathy. However, the molecular pathogenesis of the neuropathy remains unclear. In this study, we characterize the features of FAP TTR the substitution of lysine for glutamic acid at position 61 (E61K). This FAP was late-onset, with sensory dominant polyneuropathy, autonomic neuropathy, and cardiac amyloidosis. Interestingly, no amyloid deposits were found in the endoneurium of the four nerve specimens examined. Therefore, we examined the amyloidogenic properties of E61K TTR in vitro. Recombinant wild-type TTR, the substitution of methionine for valine at position 30 (V30M) TTR, and E61K TTR proteins were incubated at 37°C for 72 hr, and amyloid fibril formation was assessed using the thioflavin-T binding assay. Amyloid fibril formation by E61K TTR was less than that by V30M TTR, and similar to that by wild-type TTR. E61K TTR did not have an inhibitory effect on neurite outgrowth from adult rat dorsal root ganglion (DRG) neurons, but V30M TTR did. Furthermore, we studied the sural nerve of our patient by terminal deoxynucleotidyl transferase dUTP nick end labeling and electron microscopy. A number of apoptotic cells were observed in the endoneurium of the nerve by transferase dUTP nick end labeling. Chromatin condensation was confirmed in the nucleus of non-myelinating Schwann cells by electron microscopy. These findings suggest that E61K TTR is low amyloidogenic, in vitro and in vivo. However, TTR aggregates and amyloid fibrils in the DRG may cause sensory impairments in FAP because the DRG has no blood-nerve barrier. Moreover, Schwann cell apoptosis may contribute to the neurodegeneration.