A novel substitution of proline (P32L) destabilises beta 2-microglobulin inducing hereditary systemic amyloidosis

Background: beta 2-microglobulin amyloidosis was first described in the 1980s as a protein deposition disease associated with long-term haemodialysis. More recently, two inherited forms resulting from separate point mutations in the beta 2-microglobulin gene have been identified. In this report, we detail a novel beta 2M variant, P32L, caused by a unique dinucleotide mutation that is linked to systemic hereditary beta 2-microglobulin amyloidosis. Methods: Three family members from a Portuguese kinship featured cardiomyopathy, requiring organ transplantation in one case, along with soft tissue involvement; other involvements included gastrointestinal, neuropathic and sicca syndrome. In vitro studies with recombinant P32L, P32G, D76N and wild-type beta 2-microglobulin were undertaken to compare the biophysical properties of the proteins. Results: The P32L variant was caused by the unique heterozygous dinucleotide mutation c.154_155delinsTT. Amyloid disease featured lowered serum beta 2-microglobulin levels with near equal amounts of circulating P32L and wild-type proteins; amyloid deposits were composed exclusively of P32L variant protein. In vitro studies of P32L demonstrated thermodynamic and chemical instability and enhanced susceptibility to proteolysis with rapid formation of pre-fibrillar oligomeric structures by N- and C-terminally truncated species under physiological conditions. Conclusions: This work provides both clinical and experimental evidence supporting the critical role of P32 residue replacement in beta 2M amyloid fibrillogenesis.