Amyloid Fibril Formation of Immunoglobulin Light Chain Peptide from IHC-Negative AL Amyloidosis and Its Inhibition By Doxycycline and Epigallocatechin

AL amyloidosis is the disease featuring deposition of insoluble amyloid fibril derived from immune light chain (LC). Although LC is known to be responsible for this disease, mechanisms regulating amyloid formation are not elucidated. Diagnosis of this disease depends on confirmation of LC within amyloid lesions by immunohistochemistry (IHC). However, LC cannot always be detected thus making the diagnosis difficult. We experienced four cases of AL amyloidosis without positive staining for LCs despite these cases had lambda type M-proteins in their serum. These cases were diagnosed as AL amyloidosis since immunoglobulin lambda light chain constant region 2 (IGLC2) was identified in amyloid lesions by analysis with mass spectrometry in all cases.

Nucleic acid sequence of mRNA extracted from purified plasma cells from these cases revealed that 2 out of 4 cases showed identical LC sequence. Based on the nucleic acid sequence, we synthesized peptide corresponding to variable and joining regions (V-J peptide) and tried to observe amyloid fibril formation in vitro. We found that V-J peptide formed amyloid fibril which is confirmed by thioflabin-T analysis and electron microscopy, suggesting variable and joining regions are capable to form amyloid fibril. Then we evaluated effect of doxycycline and epigallocatechin if these compounds could inhibit amyloid formation since these compounds are reported to potentially contribute improvement of survival of the disease. Interestingly these compounds exerted inhibitory effect in amyloid formation in a dose dependent manner.

In summary, these results showed that 1: amyloid fibril could be generated from variable and joining regions of LC although this peptide is not reactive to IHC technique, 2: Doxycycline and epigallocatechin have a potential in inhibiting amyloid formation. V-J peptide should be useful for understanding mechanisms regulating conversion of LC to amyloid fibrils and evaluating compounds targeting amyloid fibril formation.