The MPL p.S505N mutation is a recurrent mutation in some myeloproliferative and/or myeloproliferative/myelodysplastic disorders. It has also been reported in familial essential thrombocythemia. Biochemical studies of this variant have shown that it leads to moderate activation of downstream pathways including MEK-1/-2 and STAT5, which represent potentially targetable pathways.

This MPL variant has been previously reported in the COSMIC database (COSM28997) and has been reported to be an activating mutation (PubMed ID: 26423830, Milosevic Feenstra et al., Blood 2016).

TP53 encodes p53, a tumor suppressor protein that consists of transactivation domain, proline-rich domain, DNA-binding domain, oligomerization domain, and regulatory domain. p53 responds to diverse cellular stresses to maintain genomic stability and to induce cell cycle arrest, apoptosis, DNA repair and metabolic changes. TP53 mutations represent an important mechanism of resistance to DNA-damaging chemotherapeutic agents. Somatic TP53 mutations are found in a variety of cancers with various frequencies depending on cancer type; overall, TP53 is mutated in over one-half of human cancers. Missense mutations were the most frequent (~70-80%), followed by frameshift and nonsense mutations. Most TP53 mutations are clustered in the DNA-binding domain encompassing exons 5 and 8. These mutations either directly disrupt the DNA-binding domain of TP53 or cause conformational changes of the TP53 protein, thus leading to severely impaired TP53 function. Overall in myeloid malignancies, TP53 mutations are found in 5% to 15% of de novo MDS and AML but 20% of myelodysplastic syndrome with isolated del(5q) and ~50% of MDS/AML with complex karyotype. TP53 mutations are also more frequent in therapy-associated myeloid neoplasm (21-38%) compared to de novo MDS and AML. TP53 mutations are also found in 8% of blastic plasmacytoid dendritic cell neoplasm, and less than 5% in myeloproliferative neoplasms (ET, PV and PMF) and chronic myelomonocytic leukemia. TP53 mutations are independently associated with a poor prognosis in myelodysplastic syndrome (NCCN Guidelines for Myelodysplastic Syndromes) and is a poor risk factor in AML (NCCN Guildelines for AML). TP53 mutations are also associated with resistance to lenalidomide or relapse during lenalidomide treatment. TP53 mutations are independently associated with unfavorable outcomes and shorter survival after hematopoietic stem cell transplantation in patients with myelodysplastic syndrome and myelodysplastic syndrome/acute myeloid leukemia, but an increased response to decitabine in patients with myelodysplastic syndrome or acute myeloid leukemia.

U2AF1 encodes for the small subunit of the U2 auxiliary factor, which is a non-small nuclear ribonucleoprotein (non-snRNP) required for the binding of U2 snRNP to the pre-mRNA branch site and plays critical role in RNA splicing. U2AF1 is one of several spliceosome complex genes frequently mutated in a variety of hematologic malignancies. Two hotspot mutations (S34 in exon 2 and Q157 in exon 6) occur within the two zinc-finger domains of the U2AF1 protein. These mutations have been reported in approximately 4- 9% of chronic myelomonocytic leukemia, 8-11% of cases of myelodysplastic syndrome (typically without ring sideroblasts), 16% of primary myelofibrosis, 12% of blastic plasmacytoid dendritic cell neoplasm, 4% of acute myeloid leukemia and 1% of essential thrombocythemia. U2AF1 mutations are associated with an unfavorable prognosis in myelodysplastic syndrome (NCCN Guidelines for Myelodysplastic Syndromes) and essential thrombocythemia, and decreased response to lenalidomide in myeloid neoplasms with and without del(5q). U2AF1 mutations are also reported to be highly specific for secondary acute myeloid leukemia, and may also be helpful in identifying a subset of therapy-related AML or elderly de novo AML with worse clinical outcomes. U2AF1 mutations have been associated with altered splicing patterns in vitro and in vivo, and may play a significant role in the pathogenesis of myeloid malignancies due to selective mis-splicing of tumor-associated genes.

SRSF2 is a member of the serine/arginine-rich family of pre-mRNA splicing factors, which constitute part of the spliceosome. It interacts with other spliceosomal components bound to both the 5- and 3-splice sites during spliceosome assembly. SRSF2 mutations typically occur as missense mutations at Pro95. SRSF2 mutations have been reported in approximately 40% of cases of chronic myelomonocytic leukemia, but they may not have prognostic significance in that entity. Comutation of TET2 and SRSF2 was highly predictive of a myeloid neoplasm characterized by myelodysplasia and monocytosis, including but not limited to, chronic myelomonocytic leukemia. In addition, SRSF2 mutations have been reported in approximately 15-20% of cases of myelodysplastic syndrome. SRSF2 mutations have also been described in 5-20% of patients with acute myeloid leukemia and appear to be enriched among AML patients with reduced blast counts. SRSF2 has been found to be mutated in approximately 10% of cases of primary myelofibrosis where mutations may occur together with mutations in JAK2, MPL, TET2, CBL, ASXL1, EZH2, IDH1/2. SRSF2 mutations are also present in 8% of blastic plasmacytoid dendritic cell neoplasm and 3% of polythemia vera. SRSF2 mutations tend to be (although are not entirely) exclusive of mutations in other splicing factor components. SRSF2 mutations are associated with a poor prognosis in myelodysplastic syndrome (NCCN Guidelines for Myelodysplastic Syndromes), primary myelofibrosis, polycythemia vera, and KIT D816V-mutated advanced systemic mastocytosis. SRSF2 mutations are also reported to be highly specific for secondary acute myeloid leukemia, and may also be helpful in identifying a subset of elderly patients with de novo acute myeloid leukemia and therapy-related AML with worse clinical outcomes.

TP53 is a well known tumor suppressor gene that is mutated in wide variety of cancers. Loss of function mutations (missense, nonsense and frameshift mutations) of TP53 have been described in 10-20% of CLL cases and TP53 gene defects tend to be enriched among cases with unmutated IGH variable regions; in some series, TP53 mutations have been reported in approximately 15%-18% of IGHV unmutated CLL cases . TP53 mutations appears to be less common in other types of CLL (eg, less than 5% of IGHV3-21-expressing CLL carried a TP53 defect according to one study). Mutations of TP53 in CLL have been found together with del17p and mutations in other genes such as NOTCH1 and SF3B1. Mutations and deletions of TP53 appear to represent adverse prognostic markers in chronic lymphocytic leukemia.

IDH2 is a mitochondrial enzyme involved in citrate metabolism. Mutations at Arg140 and Arg172 of IDH2 are typically heterozygous mutations and considered gain of function mutations that lead to increased levels of 2-hydroxyglutarate which is believed to alter epigenetic regulation (ie, DNA methylation) in AML. Mutations of IDH2 appear to be mutually exclusive of mutations in TET2, another gene involved in regulation of DNA methylation, and also exclusive of mutations in IDH1. Mutations of IDH2 have been shown to lead to increased DNA methylation in AML. IDH2 mutations have been reported in 10-20% of AML and are often associated with a normal karyotype. IDH2 mutations have been reported in less than 5% of cases of MDS and less than 10% of myeloproliferative neoplasms. The prognostic impact of IDH2 mutations in AML appears uncertain due to conflicting reports. In the setting of essential thrombocytosis, primary myelofibrosis and MDS, the presence of IDH2 mutations is associated with reduced survival. Therapeutic targeting with an FDA approved mutant IDH2 inhibitor (enasidenib (AG-221)) has been reported for patients with relapsed or refractory IDH2-mutated AML.

FLT3 is a receptor tyrosine kinase important in hematopoietic cell proliferation and differentiation. In-frame, FLT3 internal tandem duplications (ITD), which show a wide range of number of nucleotides duplicated and/or inserted (eg, 18-204 bp), affect exons 14 and 15 in the FLT3 juxtamembrane/tyrosine kinase domain and have been reported in 20-30% of patients with acute myeloid leukemia. FLT3 ITDs usually occur in cases of AML with a normal karyotype and but may also occur in cases with chromosome abnormalities including t(15;17) or other cytogenetic groups. Individuals with FLT3 mutations are more likely to have AML than MDS and FLT3 mutations in MDS are associated with a poor prognosis. Functional studies have shown that FLT3 ITDs are ligand-independent, gain-of-function mutations. In addition, activating mutations at codon D835 in exon 20 (A-loop of the tyrosine kinase domain, TKD) of FLT3 have been reported in approximately 7-10% of AML. The FLT3 D835 mutations are also ligand-independent, gain-of-function mutations. FLT3 ITD and D835 mutations tend to occur in a mutually exclusive manner; however, the FLT3 D835 mutation or other mutations in the A-loop(eg, D839, Y842) may occur as an acquired resistance mutation in the setting of patients with FLT3 ITD mutations being treated with targeted therapy. In addition, variants at codon N676 and in exon 17 F691, G697 have been associated with resistance to FLT3 targeted therapy. The presence of FLT3 ITD mutation in young patients with AML and normal cytogenetics is thought to be associated with a poor prognosis. On the other hand, the prognostic significance of the FLT3 D835 and TKD mutations appears less clear. More recently, therapeutic targeting of FLT3 in combination with other chemotherapy is available in certain settings for acute myeloid leukemia with FLT3 ITD or TKD mutations. Lastly, in T cell acute lymphoblastic leukemia, up to 20% of cases have been reported to show a FLT3 mutation (TKD or ITD) and are often associated with an ETP(early T cell precursor) phenotype. In addition, FLT3 mutations have also been reported in up to 15% of B-ALL cases ("Ph-like phenotype") and may also be associated with hyperdiploidy and MLL rearrangement. Targetting FLT3 in acute lymphoblastic leukemia therapy may also be possible in some settings.

Mutations of NPM1 have been reported in approximately 25-35% of cases of acute myeloid leukemia (AML). The mutations of NPM1 are frameshift mutations in the C-terminus of the protein that alter the C-terminal amino acid sequence and are associated with aberrant cytoplasmic localization of the protein. NPM1 mutations in AML are typically associated with a normal karyotype and may co-exist with FLT3 mutations. The presence of NPM1 mutations has been associated with improved complete remission rates, but not necessarily overall survival, in multivariate analysis including assessment of the variety of more recently discovered mutations that may be present in AML. In addition, cytogenetically normal AML with mutated NPM1, without FLT3 ITD or mutated DNMT3A, has been considered to be a favorable genetic risk group according to some studies, although other studies suggest that coexistant mutations in IDH1 or IDH2 may be required for the favorable risk effect of NPM1.

KIT(cKIT) mutations are present in approximately 8-25% of cases of acute myeloid leukemia and have a higher prevalence in the favorable cytogenetic risk group including core binding factor (CBF) AMLs (ie, (t(8;21)(q22;q22)(RUNX1-RUNX1T1), inv(16)(p13q22)(CBFB-MYH11)) or normal karyotype AML. Mutations of KIT in AML are most common in KIT exon 17 (within the activation loop of the tyrosine kinase domain) but may also occur in KIT exons 8 (extracellular portion of the receptor implicated in dimerization), 9-11 (juxtamembrane/transmembrane domains). The presence of KIT mutations has been reported to be associated with a poorer survival and/or higher risk of relapse than expected for patients with the t(8;21)(q22;q22)(RUNX1-RUNX1T1), and to a lesser extent, in inv(16) AML. KIT mutations are also important in systemic mastocytosis and various mast cell disorders; over 90% of cases of systemic mastocytosis carry mutations in exon 17 of KIT (most commonly D816V or rarely D816H, D816Y or other variants). In patients with mastocytosis, the KIT mutations may be detectable in non-mast cell hematopoietic cells. The KIT D816V mutation has been shown to be resistant to imatinib; other KIT mutations may show variable responses to imatinib. The KIT D816V mutant has been reported to be sensitive to other tyrosine kinase inhibitors. In the context of core binding factor AMLs, the KIT mutation status can help direct therapeutic management.