Guillermo A. Herrera

Renal disease is a common cause of morbidity and mortality in patients with plasma cell dyscrasias. The serum-free light chain assay is used in patients, mostly older, with unexplained acute kidney injury to screen for potential myeloma cast nephropathy. This study consists of a systematic review of diagnostic features in myeloma cast nephropathy. The morphological features of tubular casts in patients with multiple myeloma have not been systematically analyzed. This study focuses on the morphology of these casts, emphasizing ultrastructural features, in a series of 23 patients with light chain ("myeloma") cast nephropathy and compared them with casts in 10 patients with various diseases. The immunofluorescence data were correlated with morphological findings to provide diagnostic assessments and practice guidelines. The ultrastructural features identified as diagnostic of casts associated with myeloma included: amyloid and crystals in the casts, multiple well-defined fracture planes forming a complex jigsaw puzzle arrangement of cast contents, indicative of the fragility of the immunoglobulin light chains involved, and reactive tubular cells lining the tubules with the casts. These features were seen in 95.2% of MCN cases and none of the casts in other renal conditions. Myeloma casts exhibited light chain monoclonality in a significant percentage of the MCN cases and often no staining for IgA or IgM. In contrast, the majority of non-myeloma casts stained for both kappa and lambda light chains, lgA, and lgM, and showed ultrastructurally a rather uniform finely to coarsely granular electron density occasionally admixed with cellular debris.

In this clinical case report, we present a rare subtype of amyloidosis, apolipoprotein CII (apo CII), which was diagnosed through a renal biopsy and subsequently confirmed by identifying the p.K41T mutation via germline DNA sequencing. Upon reviewing the literature, five patients exhibiting identical mutation were identified via renal biopsy, while an additional patient was diagnosed through biopsies of the fat pad and bone marrow. Notably, our patient is the youngest recorded case. We pioneered the application of immunofluorescence and immunogold electron microscopy techniques for apo CII evaluation. Our report provides a detailed description of this case, supplemented by an extensive review encompassing apo CII, documented instances of apo CII amyloidosis with renal or systemic involvement, and potential underlying mechanisms.

**Author Listing - Lead/ Submitting Author (we will be communicating with this individual): Name, Institution, City and State where the work was | POSTER

Primary mandibular telangiectatic osteosarcomas are very rare lesions, with only nine cases reported. Histologically, these lesions show multiple cystic blood‐filled cavities traversed by neoplastic bone in septa lined by high‐grade malignant cells. Here, we report an 81‐year‐old woman who presented with a mandibular mass, which was surgically resected and analyzed by histologic examination and whole exome DNA sequencing. A diagnosis of telangiectatic osteosarcoma was given. Comparative sequencing data analysis of paired benign and tumor DNA revealed 1577 variants unique to the tumor DNA, which clustered into several gene families, including those regulating DNA repair and apoptosis. Comparison of benign and tumor DNA revealed many shared gene polymorphisms associated with an increased cancer risk. These included polymorphisms in the ATM, p53, BRCA1, and BRCA2 and many other genes. Interestingly, the patient’s family history showed an unusually high cancer incidence, likely related to these cancer risk–associated polymorphisms. To our knowledge, this is the first‐time sequencing applied to a mandibular telangiectatic osteosarcoma. Our findings may shed light on the molecular origins of these rare tumors and how they may relate to other tumors in related kindreds.

The etiology of minimal change disease (MCD) remains a mystery as the only characteristic findings are the diffuse effacement of foot processes seen on electron microscopy (EM). Punctate IgG staining found floating outside glomerular capillary loops in MCD cases was recently identified as autoimmune antibodies against nephrin of podocytes. We hypothesized that the punctate IgG staining is located on budding ballooning clusters (BBC) of reactive foot processes in Bowman's space found on EM. We identified seven patients with MCD cases showing IgG staining that were subsequently evaluated for BBC on EM. We concurrently examined 12 negative controls, either unremarkable cases or tubulointerstitial diseases, by EM. Immunogold labeling was performed to confirm the presence of IgG and determine localization. In seven MCD cases, there were positive punctate IgG staining particles outside of the glomerular basement membranes (GBM) along with concurrent punctate staining for C3, kappa, and lambda. By EM, all seven (100%) MCD cases revealed BBC that was characterized by ballooning foot processes ranging from 1 to 6 mu m and was either budding or detached from the GBM in 3-7 clusters; no electron-dense materials were seen in BBC. BBC was also seen in only 1 of 12 (8%) negative controls. Immunogold labeling identified IgG particles within BBC of MCD by EM, but not in the negative control. Our data suggest that BBC are EM structures of reactive foot processes that are most likely correlated with punctate IgG staining seen in cases of MCD, supported by immunogold labeling for IgG.

The process of light-chain-associated amyloid (AL-Am) fibril formation in unique organelles (fibril-forming organelles) with lysosomal features has been documented in vitro in renal mesangial cells incubated with amyloidogenic light chains using electron microscopy and lysosomal gradient centrifugation to visualize intricate interactions between monoclonal light chains and endosomes/lysosomes. It is important to determine whether this process also occurs in vivo in the human renal mesangium. The present study analyzes 13 renal biopsies from patients with renal AL-amyloidosis and utilizes ultrastructural labeling techniques to define the nature and function of these organelles. Organelles were labeled for lysosomal-associated membrane protein (LAMP) and CD-68 (a macrophage marker). Furthermore, lambda was also localized inside these structures in transformed mesangial cells with a macrophage phenotype. These 11 cases from renal biopsies with a diagnosis of AL-amyloidosis (5 kappa and 8 lambda light-chain-associated) were examined ultrastructurally. All of the cases exhibited numerous fibrils forming organelles in approximately 40–50% of the remaining mesangial cells. All of the cases revealed mesangial cells engaged in active amyloidogenesis. Fibril-forming organelles are organelles with morphological/immunohistochemical and biochemical characteristics of lysosomes but with a unique, peculiar morphology. Five cases of other glomerular disorders used as controls were also carefully scrutinized for fibril-forming organelles and failed to show any. In the AL-amyloid renal cases, there was an intricate interaction between the fibril-forming organelles and lambda-/kappa-containing amyloid fibrils, supporting the notion that the monoclonal light chains participated in their formation.

Studies carried out in the last three decades have significantly advanced our knowledge about the structural factors that drive the amyloid aggregation of the immunoglobulin light chains. Solid-state nuclear magnetic resonance and cryo-electron microscopy studies have resulted in huge progress in our knowledge about the AL fibril structure. Now, it is known that the assembly of the light chain into AL fibrils implies an extensive conformational rearrangement that converts the beta-sandwich fold of the protein into a near flat structure. On the other hand, there has also been significant progress made in understanding the role that some cell types play as facilitators of AL formation. Such a role has been studied in glomerular amyloidosis, where mesangial cells play an important role in the mechanism of AL deposition, as well as for the pathogenic mechanisms that result in glomerular/renal damage. This review addresses what we currently know about why and how certain light chains are prone to forming amyloid. It also summarizes the most recent publications on the structure of AL fibrils and analyzes the structural bases of this type of aggregate, including the origin of its structural diversity. Finally, the most relevant findings on the role of mesangial cells in the amyloid deposition of light chains in the glomerular space are summarized.

For many years amyloidosis was considered an extremely rare, somewhat mysterious disease. However, in the last 2–3 decades its pathogenesis, particularly that of renal amyloidosis has been carefully dissected in the research laboratory using in-vitro and, to a lesser extent, in-vivo models. These have provided a molecular understanding of sequential events that take place in the renal mesangium leading to the formation of amyloid fibrils and eventual extrusion into the mesangial matrix, which itself becomes seriously damaged and, in due time, replaced by the fibrillary material. Amyloid, once considered to be an “inert” substance, has been proven to be involved in crucial biological processes that result in the destruction and eventual replacement of normal renal constituents. This review centers on mechanisms involved in the renal glomerular amyloidosis to understand its pathogenesis.