Immunoglobulin light-chain toxicity in a mouse model of monoclonal immunoglobulin light-chain deposition disease.

Light chain (LC) deposition disease (LCDD) is a rare disorder characterized by glomerular and peritubular amorphous deposits of a monoclonal immunoglobulin LC, leading to nodular glomerulosclerosis and nephrotic syndrome. We developed a transgenic model using site-directed insertion of the variable domain of a pathogenic human LC gene into the mouse immunoglobulin κ locus, ensuring its production by all plasma cells (PCs). High free LC levels were achieved after backcrossing with mice presenting increased PC differentiation and no immunoglobulin heavy chain production. Our mouse model recapitulates the characteristic features of LCDD, including progressive glomerulosclerosis, nephrotic-range proteinuria, and finally kidney failure. The variable domain of the LC bears alone the structural properties involved in its pathogenicity. RNA sequencing conducted on PCs demonstrated that LCDD LC induces endoplasmic reticulum stress, likely accounting for the high efficiency of proteasome inhibitor-based therapy. Accordingly, reduction of circulating pathogenic LC was efficiently achieved and not only preserved renal function but also partially reversed kidney lesions. Finally, transcriptome analysis of presclerotic glomeruli revealed that proliferation and extracellular matrix remodeling represented the first steps of glomerulosclerosis, paving the way for future therapeutic strategies in LCDD and other kidney diseases featuring diffuse glomerulosclerosis, particularly diabetic nephropathy.

Immunoglobulin variable domain high-throughput sequencing reveals specific novel mutational patterns in POEMS syndrome.

Polyneuropathy, organomegaly, endocrinopathy, monoclonal gammopathy, and skin changes (POEMS) syndrome is a rare multisystem disease resulting from an underlying plasma cell (PC) dyscrasia. The pathophysiology of the disease remains unclear, but the role of the monoclonal immunoglobulin (Ig) light chain (LC) is strongly suspected because of the highly restrictive usage of 2 λ variable (V) domains (IGLV1-40 and IGLV1-44) and the general improvement of clinical manifestations after PC clone-targeted treatment. However, the diagnostic value of Ig LC sequencing, especially in the case of incomplete forms of the disease, remains to be determined. Using a sensitive high-throughput Ig repertoire sequencing on RNA (rapid amplification of cDNA ends-based repertoire sequencing [RACE-RepSeq]), we detected a λ LC monoclonal expansion in the bone marrow (BM) of 83% of patients with POEMS syndrome, including some in whom BM tests routinely performed to diagnose plasma cell dyscrasia failed to detect λ+ monoclonal PCs. Twenty-four (83%) of the 29 LC clonal sequences found were derived from the IGLV1-40 and IGLV1-44 germline genes, as well as 2 from the closely related IGLV1-36 gene, and all were associated with an IGLJ3*02 junction (J) gene, confirming the high restriction of VJ region usage in POEMS syndrome. RACE-RepSeq VJ full-length sequencing additionally revealed original mutational patterns, the strong specificity of which might crucially help establish or eliminate the diagnosis of POEMS syndrome in uncertain cases. Thus, RACE-RepSeq appears as a sensitive, rapid, and specific tool to detect low-abundance PC clones in BM and assign them to POEMS syndrome, with all the consequences for therapeutic options.

Physiological and druggable skipping of immunoglobulin variable exons in plasma cells.

The error-prone V(D)J recombination process generates considerable amounts of nonproductive immunoglobulin (Ig) pre-mRNAs. We recently demonstrated that aberrant Ig chains lacking variable (V) domains can be produced after nonsense-associated altered splicing (NAS) events. Remarkably, the expression of these truncated Ig polypeptides heightens endoplasmic reticulum stress and shortens plasma cell (PC) lifespan. Many questions remain regarding the molecular mechanisms underlying this new truncated Ig exclusion (TIE-) checkpoint and its restriction to the ultimate stage of B-cell differentiation. To address these issues, we evaluated the extent of NAS of Ig pre-mRNAs using an Ig heavy chain (IgH) knock-in model that allows for uncoupling of V exon skipping from TIE-induced apoptosis. We found high levels of V exon skipping in PCs compared with B cells, and this skipping was correlated with a biallelic boost in IgH transcription during PC differentiation. Chromatin analysis further revealed that the skipped V exon turned into a pseudo-intron. Finally, we showed that hypertranscription of Ig genes facilitated V exon skipping upon passive administration of splice-switching antisense oligonucleotides (ASOs). Thus, V exon skipping is coupled to transcription and increases as PC differentiation proceeds, likely explaining the late occurrence of the TIE-checkpoint and opening new avenues for ASO-mediated strategies in PC disorders.