Mutation specific immunohistochemistry is highly specific for the presence of calreticulin mutations in myeloproliferative neoplasms

Principles of Analytic Validation of Immunohistochemical Assays: Guideline Update

CONTEXT.—

In 2014, the College of American Pathologists developed an evidence-based guideline to address analytic validation of immunohistochemical assays. Fourteen recommendations were offered. Per the National Academy of Medicine standards for developing trustworthy guidelines, guidelines should be updated when new evidence suggests modifications.

OBJECTIVE.—

To assess evidence published since the release of the original guideline and develop updated evidence-based recommendations.

DESIGN.—

The College of American Pathologists convened an expert panel to perform a systematic review of the literature and update the original guideline recommendations using the Grading of Recommendations Assessment, Development and Evaluation approach.

RESULTS.—

Two strong recommendations, 1 conditional recommendation, and 12 good practice statements are offered in this updated guideline. They address analytic validation or verification of predictive and nonpredictive assays, and recommended revalidation procedures following changes in assay conditions.

CONCLUSIONS.—

While many of the original guideline statements remain similar, new recommendations address analytic validation of assays with distinct scoring systems, such as programmed death receptor-1 and analytic verification of US Food and Drug Administration approved/cleared assays; more specific guidance is offered for validating immunohistochemistry performed on cytology specimens.

Production and Characterization of Peptide Antibodies to the C-Terminal of Frameshifted Calreticulin Associated with Myeloproliferative Diseases

Myeloproliferative Neoplasms (MPNs) constitute a group of rare blood cancers that are characterized by mutations in bone marrow stem cells leading to the overproduction of erythrocytes, leukocytes, and thrombocytes. Mutations in calreticulin (CRT) genes may initiate MPNs, causing a novel variable polybasic stretch terminating in a common C-terminal sequence in the frameshifted CRT (CRTfs) proteins. Peptide antibodies to the mutated C-terminal are important reagents for research in the molecular mechanisms of MPNs and for the development of new diagnostic assays and therapies. In this study, eight peptide antibodies targeting the C-terminal of CRTfs were produced and characterised by modified enzyme-linked immunosorbent assays using resin-bound peptides. The antibodies reacted to two epitopes: CREACLQGWTE for SSI-HYB 385-01, 385-02, 385-03, 385-04, 385-07, 385-08, and 385-09 and CLQGWT for SSI-HYB 385-06. For the majority of antibodies, the residues Cys¹, Trp⁹, and Glu¹¹ were essential for reactivity. SSI-HYB 385-06, with the highest affinity, recognised recombinant CRTfs produced in yeast and the MARIMO cell line expressing CRTfs when examined in Western immunoblotting. Moreover, SSI-HYB 385-06 occasionally reacted to CRTfs from MPN patients when analysed by flow cytometry. The characterized antibodies may be used to understand the role of CRTfs in the pathogenesis of MPNs and to design and develop new diagnostic assays and therapeutic targets.

Acute promyelocyte leukemia arose from CALR 1 mutated post essential thrombocythemia- myelofibrosis with splanchnic vein thrombosis: A case report

Major disease complications for patients with essential thrombocythemia (ET) include thrombosis and fibrotic or leukemic transformation. Calreticulin (CALR) mutation type 1 frequencies in ET are estimated between 7% and 11% and ET patients carrying CALR type 1 mutation are associated with lower risk of thrombosis but higher risk of myelofibrosis transformation compared to ET patients with JAK2 mutation. Leukemic transformation rates at 20 years are estimated at less than 5% for ET and risk factors for leukemic transformation are advanced age, thrombosis history, leukocytosis, and anemia. Amongst the subtypes of blast phase myeloproliferative neoplasms, acute promyelocytic leukemia is extremely rare. Herein, we present a case of a promyelocytic blast crisis of post-ET myelofibrosis with associated life-threatening splanchnic vein thrombosis. This case suggests that inflammation plays a key role in thrombotic events and fibrotic/leukemic transformation in ET patients, regardless the molecular landscape.

Megakaryocytes, erythropoietic and granulopoietic cells express CAL2 antibody in myeloproliferative neoplasms carrying CALR gene mutations

Testing for the CALR mutation is included in the updated WHO criteria for essential thrombocythaemia (ET) and primary myelofibrosis (PMF). We report on the application of the CAL2 monoclonal antibody, raised against the mutated CALR gene to myeloid cases. The immunostain was used on 116 acute myeloid leukaemias (AML) and 66 myeloproliferative neoplasms (MPN) or myelodysplastic syndromes/myeloproliferative neoplasms (MDS/MPN). None of AML cases was stained by the CAL2 antibody, while 20/66 MPNs and MDS/MPNs appeared positive. Fourteen of the latter cases were studied by molecular techniques, and all showed aberrations of the CALR gene. In addition, CAL2 positivity was found in some small‐sized elements besides megakaryocytes. By double staining, these elements corresponded to small megakaryocytes as well as both erythroid and myeloid precursors. This finding suggests possible occurrence of CALR gene abnormalities in a stem cell.

Mutant specific anti calreticulin antibody (CAL2) immunohistochemistry as a screening test for calreticulin (CALR) mutation testing

BACKGROUND

About 50 different CALR frameshift mutations have been identified in BCR-ABL1 negative MPN, all leading to the development of common new protein C terminus. Antibody targeting this terminal epitope can be useful to identify this driver mutation using immunohistochemistry.

MATERIALS AND METHODS

CALR mutation analysis was carried out in 51 JAK2V617F negative cases, PMF (n = 22) and ET (n = 29). PCR followed by fragment analysis was performed for molecular detection of CALR mutation. Bone marrow biopsy specimens of corresponding patients were subjected to IHC using mutation specific antibody CAL2. Staining pattern and intensity were observed. Staining of <2% of background nonmegakaryocytic (non- MK) cells were regarded as Pattern A, while staining of more than 2% of background nonmegakaryocytic (non-MK) was regarded as pattern B.

RESULTS

CALR mutation was noted in 40.9% (9/22) and 41.4% (12/29) of JAK2V617F negative PMF and ET, respectively. All CALR mutated cases, irrespective of the mutation type, showed a positive IHC staining in the megakaryocytes with moderate to bright intensity. All CALR wild-type cases were negative on IHC. (Concordance rate- 100%). Pattern A was noted in 40% cases, while pattern B was noted in 60% cases. Pattern A staining had significantly higher chances of having type 1 mutation as compared to pattern B. In contrast, pattern B had a nonsignificant trend toward higher bone marrow cellularity and marrow fibrosis.

CONCLUSION

CAL2 IHC detects all types of CALR mutation. This can act as a sensitive, specific, rapid, and cost-effective screening test for CALR mutation analysis.

Novel Detection of CALR-Mutated Cells in Myeloproliferative Neoplasm-Related Glomerulopathy With Interstitial Extramedullary Hematopoiesis: A Case Report

Myeloproliferative neoplasms (MPNs) are associated with somatic mutations of genes including JAK2, CALR, or MPL in hematopoietic stem cells. Various glomerular lesions are known to be involved in MPN-related glomerulopathy, including mesangial hypercellularity, segmental sclerosis, features of chronic thrombotic microangiopathy, and intracapillary hematopoietic cell infiltration. Renal extramedullary hematopoiesis (EMH) is uncommon, but it is reported to occur in the setting of MPN; however, to our knowledge, there have been no reports of renal EMH with pathologically verified mutations. We report the case of a 65-year-old woman with MPN who had a CALR mutation and developed nephrotic syndrome. Kidney biopsy showed the typical findings of MPN-related glomerulopathy. CALR mutation-specific immunostaining of the kidney revealed immunopositive cells in the EMH lesion of the interstitium, indicating that renal EMH was caused by CALR-mutated cells. Based on these findings, we diagnosed nephrotic syndrome caused by MPN-related glomerulopathy. After initiation of steroid therapy, the patient's proteinuria gradually decreased and she achieved an incomplete remission. Additionally, the patient was prescribed the JAK inhibitor ruxolitinib and maintained incomplete remission. There is no established treatment for MPN-related glomerulopathy; therefore, further studies are needed to elucidate its pathophysiology.

Calreticulin mutation specific CAL2 immunohistochemistry accurately identifies rare calreticulin mutations in myeloproliferative neoplasms

Mutations of the multifunctional protein calreticulin (CALR) are recognised as one of the main driver alterations involved in the pathogenesis of Philadelphia negative myeloproliferative neoplasms (Ph^- MPN) and also represent a major diagnostic criterion in the most recent World Health Organization classification of myeloid neoplasms. Nowadays, quantitative assessment of the driver mutations is gaining importance, as recent studies demonstrated the clinical relevance of the mutation load reflecting the size of the mutant clone. Here, we performed for the first time a manual and automated quantitative assessment of the CALR mutation load at protein level using CAL2, a recently developed CALR mutation specific monoclonal antibody, on a cohort of 117 patients with essential thrombocythemia (ET) or primary myelofibrosis (PMF) and compared the CALR protein mutation loads with the CALR mutation load values established by a molecular assay. Eighteen different CALR mutations were detected in the cohort of the 91 CALR mutant cases. Mutation loads of the CALR mutations were between 13% and 94% with mean value in PMF cases significantly higher than ET cases (49.94 vs 41.09; t-test, p=0.004). Cases without CALR mutation (n=26) showed no or only minimal labelling with the CAL2 antibody, while all 18 different types of CALR mutations were associated with CAL2 labelling. The CALR mutation load showed a significant correlation (p=0.03) with the occurrence of major thrombotic events, with higher mutation load in patients presenting with these complications. We report a 100% concordance between the mutation status determined by immunohistochemistry and the CALR molecular assay, and we extend the applicability of this approach to 16 rare CALR mutations previously not analysed at protein level.

Histological evaluation of myeloproliferative neoplasms

In 2017, the revised World Health Organization was published. Regarding myeloproliferative neoplasms, histological findings of bone marrow biopsy is becoming more important for diagnosis. This article highlights particularly the morphology of megakaryocytes and evaluation of myelofibrosis for pathological diagnosis, and immunohistochemistry which can detect somatic mutation.

Peptide Antibodies in Clinical Laboratory Diagnostics

Peptide antibodies, with their high specificities and affinities, are invaluable reagents for peptide and protein recognition in biological specimens. Depending on the application and the assay, in which the peptide antibody is to used, several factors influence successful antibody production, including peptide selection and antibody screening. Peptide antibodies have been used in clinical laboratory diagnostics with great success for decades, primarily because they can be produced to multiple targets, recognizing native wildtype proteins, denatured proteins, and newly generated epitopes. Especially mutation-specific peptide antibodies have become important as diagnostic tools in the detection of various cancers. In addition to their use as diagnostic tools in malignant and premalignant conditions, peptide antibodies are applied in all other areas of clinical laboratory diagnostics, including endocrinology, hematology, neurodegenerative diseases, cardiovascular diseases, infectious diseases, and amyloidoses.