In the context of the classification of MPNs and MPN/myelodysplastic syndrome with thrombocytosis and ring sideroblasts, V617F, JAK2, CALR and Exon 12 mutations are critical biomarkers. At present, general guidelines for JAK2 and CALR molecular testing in MPNs are unavailable. Several suggested indicators for these mutations’ molecular testing in screening and diagnosis involve the following work-up: (1) clinically suspected PV, PMF, ET, and MDS/MPN with thrombocytosis and ring sideroblasts, (2) unexplained leukocytosis, and (3) unexplained splanchnic vein thrombosis [20].
Mutational testing does not only play an important role in the diagnosis and prognosis of the diseases but it also provides significant prognostic and other critical information. For instance, type 1 CALR mutations in PMF have been associated with superior survival rates than type 2 CALR and JAK2 mutations [21]. Across all MPNs, JAK2 mutations are related with higher hemoglobin, older age, leukocytosis, increased thrombotic events, and lower platelets [21, 22]. The JAK2V617F, Exon 12 and CALR mutations are assessed by various molecular methods. This includes allele-specific (assessing for hotspot mutations) and sequencing-based methods.
We employed Sanger sequencing to detect JAK2V617F and CALR in this study, and allele-specific PCR was utilized to detect known mutations at Exon 12.
No mutations were detected amongst the 65 patients who were screened for exon 12 mutations. Although exon 12 mutations’ prevalence is uncommon, the method used for detection of mutations might not have the sensitivity margin to detect exon 12 mutations.
Allele-specific tests, which assess hotspot mutations, might fail to detect other relevant variants that are less common, for example, JAK2 Exon 12 mutations, and those that are found in approximately 3 percent of PVs, thereby giving false negatives. In addition, some allele‐specific methods might be affected by near variant interferences, wherein the hotspot mutation’s detection might be hindered by the interference of a second nearby single nucleotide variant.
The sensitivity of some techniques is less sensitive than that of others. Moreover, some MPNs (for example, ET) can present with low mutant allele fractions. Newer methods are more sensitive than Sanger sequencing, as it has the typical detection limit of 20 percent mutant allele fraction [23]. No universally accepted cutoffs are available for a positive result, even though some recommend the analytical sensitivity range between 1 and 3 percent mutant allele fraction for molecular assays [24, 25].
Mutation advent in the CALR gene changed the MPN landscape. Klampfl et al. in 2013 first recognized it as a somatic mutation in those patients who had MPNs but with no mutations in either MPL or JAK2 [19]. CALR is a protein found in cytoplasm, endoplasmic reticulum, or cell surface. It maintains calcium hemostasis and regulates cell proliferation, apoptosis, and phagocytosis while facilitate accurate glycoprotein folding [26].
CALR mutation was identified in eight out of the total 65 patients who presented with thrombocytosis and had been suspected to have MPN. Amongst patients who had the CALR mutation, six patients were diagnosed as suffering from essential ET based on the 2016 WHO classification and diagnostic criteria for MPNs. Consequently, the CALR mutation types 1 and 2 were equally identified amongst those patients who were diagnosed as suffering from ET. The other two patients in which CALR mutations were detected were diagnosed as suffering from systemic lupus erythematous (SLE) and chronic myelomonocytic leukemia (CMML). No discernible difference could be detected in the disease’s clinical phenotype amongst those with CALR and ET mutations.
Usually, the patients in whom heterozygous CALR-mutated PMF is detected are men whose age is comparatively younger than that of patients with JAK2-mutated cases. Furthermore, myeloproliferation in such cases is more specific to the megakaryocytic lineage and, therefore, presents with a more pronounced thrombocytosis. They usually have low white cell count and hemoglobin. Longer survival rates and low incidence of thrombotic complications are reported in this patient group. The prognostic impact of CALR on PMF is limited to type 1 mutation while the prognosis in case of type 2 is similar to that of JAK2-mutated PMF [27].
The widespread impact of CALR mutation on MPNs, baseline characteristics, disease outcome, a patient’s clinical behaviors, and benefits and risks in the long term warrants further exploration, as the impact of CALR mutation on MPNs is a recent scientific discovery. Prospective studies have to be conducted in order to outline the manner in which CALR mutation influences MPNs by following a detail-oriented approach toward the mutation’s homozygous pattern. Several publications have analyzed and elucidated the CALR mutation screening methods, and it has even been suggested by some authors that fragment analysis determination might sufficiently fulfill the needs of routine diagnosis and aid in the development of real-time PCR detection methods [28]. Such screening methods do not facilitate precise characterization. Therefore, determining the accurate size of insertion or deletion might sometimes prove to be difficult through fragment analysis. This is an important issue, as this study has showcased that in-frame indel polymorphisms could be misinterpreted as mutations in case they are improperly characterized. Sanger sequencing was employed in this study, as mutation characterization is a critical factor in not only determining whether the alteration belongs to clinically relevant types, namely, 1 or 2, or to the type-1/2-like but also differentiating polymorphisms from point/nonsense mutations, which might play an important role in diagnosis.
Rare nonsense mutations, which indicate the loss of a variable number of negatively charged amino acids of the C-terminus, have been reported. This includes p.E380X, p.E374X, and p.K391X [29]. We consider both the simultaneous assessment of multiple mutations by next-generation sequencing and sequential testing algorithms as valid testing approaches. NGS facilitates the holistic assessment of targeted genes that are relevant to myeloid neoplasms. The genes that were included in this regard differ from one another based on the specific panel. Moreover, larger insertions and deletions (notably, type 1 mutations in CALR exons 9) might be missed based on the data analysis types used for the panel [30]. NGS’ comprehensive nature will enable the reporting of less well‐characterized variants of uncertain significance at other loci as well as pathogenic variants.
Our understanding of the molecular landscape of PV, PMF and ET is rapidly evolving. Recent studies have implicated other genes and pathways in disease progression and MPN pathogenesis [22]. Although many amongst such mutations are not specific to MPNs, they represent additional biomarkers that can be utilized to provide additional prognostic-specific information or prove clonality [22]. Molecular testing is simultaneously becoming increasingly complex—in terms of the associated technologies as well as the produced information. The facilitation of strong and effective communication between pathologists, clinicians, and molecular diagnosticians is important to properly integrate molecular data with clinical and pathologic findings.