β2-microglobulin (β2m) and its truncated variant ΔΝ6 are co-deposited in amyloid fibrils in the joints, causing the disorder dialysis-related amyloidosis (DRA). Point mutations of β2m result in diseases with distinct pathologies. β2m-D76N causes a rare systemic amyloidosis with protein deposited in the viscera in the absence of renal failure, whilst β2m-V27M is associated with renal failure, with amyloid deposits forming predominantly in the tongue. Here we use cryoEM to determine the structures of fibrils formed from these variants under identical conditions in vitro. We show that each fibril sample is polymorphic, with diversity arising from a 'lego-like' assembly of a common amyloid building block. These results suggest a 'many sequences, one amyloid fold' paradigm in contrast with the recently reported 'one sequence, many amyloid folds' behaviour of intrinsically disordered proteins such as tau and Aβ. 

/pdf/disease-relevant-b2-microglobulin-variants-share-a-common-2fms19c1.pdf

Disease-relevant β2-microglobulin variants share a common amyloid fold

Abstract Background Cardiac amyloidosis is caused by the deposition of misfolded proteins in the myocardium. The majority of cases of cardiac amyloidosis is caused by misfolded transthyretin or light chain proteins. In this case report, we discuss a case of a rare form of cardiac amyloidosis related to beta 2-microglobulin (B2M) in a patient not on dialysis. Case summary A 63-year-old man was referred for workup of possible cardiac amyloidosis. Serum and urine immunofixation electrophoresis demonstrated no monoclonal bands, and the serum kappa/lambda light chain ratio was normal, excluding light chain amyloidosis. Bone scintigraphy imaging showed diffuse radiotracer uptake in the myocardium, and genetic testing of the Transthyretin gene was negative for variants. This workup was consistent with wild-type transthyretin cardiac amyloidosis. The patient, however, later underwent endomyocardial biopsy due to factors inconsistent with this diagnosis, including a young age of presentation and a strong family history of cardiac amyloidosis despite no variants in the Transthyretin gene. This showed B2M-type amyloidosis, and genetic testing of the B2M gene showed a heterozygous Pro32Leu (p. P52L) mutation. The patient underwent heart transplantation with normal graft function 2 years post transplant. Discussion While contemporary advancements allow for the non-invasive diagnosis of transthyretin cardiac amyloidosis with positive bone scintigraphy and negative monoclonal protein screen, clinicians should be aware of rarer forms of amyloidosis where endomyocardial biopsy is required to make the diagnosis.

/pdf/beta-2-microglobulin-case-report-of-a-rare-cause-of-cardiac-2e7ic23x.pdf

Beta 2-microglobulin: case report of a rare cause of cardiac amyloidosis

Diagnosis of myeloid neoplasm is currently performed according to the presence of a predetermined set of clinical, morphological, and molecular diagnostic criteria agreed upon by a consensus of experts. Even strictly adhering to these criteria, it is possible to encounter patients who present features that are not easily ascribable to a single disease category. This is the case, e.g., of patients with de novo myeloid neoplasms with features intermediate between primary myelofibrosis (PMF) and chronic myelomonocytic leukemia (CMML). In this study, we retrospectively searched the pathological database of IRCCS Humanitas Research Hospital to identify cases of chronic myeloid neoplasm with monocytosis with a driver mutation of classic myeloproliferative neoplasms (MPN) and showing morphological MPN features. For each case, we assessed all epidemiological, clinical, histopathological, and molecular data. Then, we carried out a literature review, searching for cases with features similar to those of our patients. We retrieved a total of 13 cases presenting such criteria (9 from the literature review and 4 from our institution); in all of them, there was a coexistence of clinical, histopathological, and molecular myelodysplastic and myeloproliferative features. To date, according to current classifications (World Health Organization and International Consensus Classification), given the presence/absence of essential features for PMF or CMML, these patients should be formally diagnosed as myelodysplastic/myeloproliferative neoplasm unclassified/not otherwise specified (U/NOS). This review aims to summarize the features of these difficult cases and discuss their differential diagnosis and their classification according to the novel classifications and the existing literature on overlapping myeloid neoplasms. 

Myelodysplastic/Myeloproliferative Neoplasms with Features Intermediate between Primary Myelofibrosis and Chronic Myelomonocytic Leukemia: Case Series and Review of the Entity

Abstract Background β2-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 β2-microglobulin gene have been identified. In this report, we detail a novel β2M variant, P32L, caused by a unique dinucleotide mutation that is linked to systemic hereditary β2-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 β2-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 β2-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 β2M amyloid fibrillogenesis.

A novel substitution of proline (P32L) destabilises β2-microglobulin inducing hereditary systemic amyloidosis

The MN1::ETV6 gene fusion resulting from t(12;22)(p13;q12) has been rarely reported in myeloid neoplasms. We describe a 69-year-old male with newly diagnosed acute myeloid leukemia (AML) with erythroid differentiation and t(12;22)(p13;q12) demonstrated by conventional chromosome studies. Subsequent fluorescence in situ hybridization studies demonstrated a balanced ETV6 gene rearrangement (at 12p13). To further characterize this translocation, whole-genome sequencing was performed which confirmed t(12;22) with breakpoints involving the MN1 and ETV6 genes. Herein, we describe our case and review the literature to summarize the clinical and laboratory findings in patients with this rare but recurrent MN1::ETV6 gene fusion observed in myeloid neoplasms. Importantly, this case expands the clinical spectrum associated with the MN1::ETV6 gene fusion to include AML with erythroid differentiation. Lastly, this case demonstrates the importance of moving toward more comprehensive molecular testing to fully characterize the driver events in neoplastic genomes.

/pdf/detection-of-an-mn1-etv6-gene-fusion-in-a-case-of-acute-1dfmqr78.pdf

Detection of an MN1::ETV6 Gene Fusion in a Case of Acute Myeloid Leukemia with Erythroid Differentiation: A Case Report and Review of the Literature

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A novel substitution of proline (P32L) destabilises β2-microglobulin inducing hereditary systemic amyloidosis

Detection of an MN1::ETV6 Gene Fusion in a Case of Acute Myeloid Leukemia with Erythroid Differentiation: A Case Report and Review of the Literature