The 2008 guidelines for the WHO classification of tumours in haematopoeitic lymphoid tissues was revised in 2016. When specifically looking at myeloid neoplasms and acute leukemia there were several factors, which played a role in making these amendments:
BCR-ABL negative myeloid proliferative neoplasms (MPNs)
One of the main reasons for revisions of the guidelines for these disorders is due to the recent discoveries of mutations and other findings that have demonstrated to have both diagnostic and prognostic significance.
Specific molecular genetic changes are related to the diagnosis of eosinophilia-related proliferation. In 2016 the myeloid neoplasm with t(8;9) (p22;a24.1); PCM1-JAK2 will be incorporated as a new entity. It has been characterized by a combination of eosinophilia along with bone marrow morphology demonstrating left –shifted erythroid predominance, lymphoid aggregate and often myelofibrosis (at times mimicking primary myelofibrosis).
Chronic myelomonocytic leukemia (CMML)
The diagnosis of CMML needs the presence of persistent peripheral blood monocytosis ≥ 1 x 109/L and monocytes accounting for more than 10% of the white blood cell (WBC) differential count. Now with clinical and molecular markers, CMML can be split into two categories, proliferative (WBC ≥ 13 x 109/L) or dysplastic (WBC < 13 x 109/L).
Myelodysplastic/myeloproliferative neoplasm with ring sideroblasts and thrombocytosis (MDS/MPN RS-T)
With the discovery of refractory anaemia with ring sideroblasts associated with thrombocytosis (RARS T) being associated with mutations in the gene SF3B1 it has been considered that there is enough evidence to support to reclassify this disorder as MDS/MPNRS-T as a separate entity.
Myelodysplastic syndromes (MDS)
The 2016 guidelines have refined the interpretation of tissue morphology and assessment of cytopenia, taking into consideration the genetic factors linked with MDS diagnosis and classification. The terminology has therefore changed, replacing refractory anaemia and cytopenia with myelodysplastic syndrome.
Acute myeloid leukemia (AML)
The focus of AML continues on the genetic subgroups within the disease with the revisions of the guidelines focusing on refinements on gene names. The categories of the different subtypes of AML have been further expanded, on the prognostic significance of various mutations.
The 2016 guidelines are mainly a revision of the prior classification rather than creating a new classification system, integrating other clinical, prognostic and genetic information.
Philadelphia negative myeloproliferative neoplasms have a natural history of thrombo-haemorrhagic complications. Thrombosis results from an interplay of different disease related factors. Abnormalities in clonal proliferation and haematopoeitic stem cells lead to an increase in blood cell counts resulting in a pro-coagulant phenotype. Mutations such as JAK2, MPL and CALR have been categorized as phenotypic driver mutations since the mutated genes cause cytokine independent growth in primary cells in MPN patients. Other mutations have been found in epigenetic genes such as TET2, DNMT3A, IDH1/2, EZH2and ASXL1, which have been associated with disease progression of MPNs. With thrombosis in MPNs, it has been recorded that arterial thrombosis (particularly cerebrovascular thrombosis) is dominant vs. venous thrombosis. The risk of a pro-coagulant state is present in MPN patients and is associated with increased morbidity and mortality. In WHO defined ET, survival is considered to be ‘near to normal’ with 80% survival over a 15 year period. When looking at the 10 year survival rates for polycythaemia vera (PV) it has been shown to be more than 75%. Risk factors in the past that have been considered as important are age (≥ 60 years) and prior history of thrombosis. With more than 20 somatic mutations having been discovered over the past few years, the JAK2 mutation has been recently added as a prognostic risk factor to this list along with cardiovascular risk. This is demonstrated in the International Prognostic Score for Thrombosis in essential thrombocythaemia (IPSET-thrombosis), which has three risk categories of low, intermediate and high risk in ET. In PV there is a strong association with the JAK2 mutation, with possibly allele burden playing a role with increased risk of thrombosis. The CALR mutation is the second most frequent mutation after the JAK2 mutation, found in half of ET and MF patients not possessing the JAK2 orMPL mutations. In myelofibrosis the CALR mutation has demonstrated to be associated with lower risk of anaemia, thrombocytopenia along with marked leukocytosis compared to other MPNs. The majority of the ET patients with the CALR mutations are younger than patients with the JAK2 mutations and usually are placed in low or intermediate risk categories for thrombosis. Another driver mutation is the MPL mutation, which in ET patients has been associated with lower haemoglobin levels and higher platelet counts. TheJAK2 V617F mutations remains to have the strongest association with thrombotic risk with suggestions that the altered JAK2 V617F gene leads to changes in both megakaryocytes and platelets resulting in an increase in cell number. There seems to be a link between the severity of thrombotic complications andJAK2 V617F mutational status, which in turn has an affect on morbidity and mortality in patients with MPNs.
The British Committee for Standards in Haematology (BCSH) guidelines or the World Health Organisation (WHO) guidelines are used for diagnosing essential thrombocythemia. There are however small discrepancies such as the WHO providing more importance to bone marrow morphology as one of the main diagnostic criteria, compared to BCSH guidelines which focus on exclusion of other types of myeloproliferative neoplasms (MPNs). This implies that with the BCSH guidelines bone marrow biopsy is recommended in cases where atypical features present themselves.
The aim of the study was to look at the clinical presentation and prognostic relevance of bone marrow morphology in ET diagnosis by comparing the criteria between the WHO and BCSH guidelines.
The study utilized a clinic-pathological database consisting of 626 patients who were diagnosed and treated for MPNs. Patients were included in the database were based on the following criteria:
Two cohorts were used from the MPN database with 232 patients meeting the WHO criteria and 238 meeting the BCSH criteria. Comparison of the cohorts was based on clinic-pathological findings, prognosis and adverse events during follow up.
With both cohorts it was shown that a higher proportion of ET defined patients were female. Patients who were diagnosed with the BCSH guidelines had higher levels of serum lactate dehydrogenase (p<0.001) and more frequent palpable splenomegaly (p=0.183) compared to patients diagnosed with the WHO diagnostic criteria. There was no difference in mutational status between the two groups.
The 238 BCSH diagnosed patients were reclassified with the WHO criteria which revealed a more hetereogenous population that consisted of:
Bone marrow biopsies carried out on the individuals with pre-PMF revealed grade 1 reticulin fibrosis which was not found in the cohort of patients that were initially diagnosed with the WHO criteria. During follow up, fibrosis free and overall survival were significantly more favourable in the WHO defined ET group (p=0.029/p=0.033 respectively). These results seemed most likely linked to the fact there were a few pre-PMF cases in the BCSH ET-defined group.
The first set of diagnostic criteria proposed by the BCSH guidelines does not seem to clearly differentiate between ‘True’ET and pre-PMF. Even though the second set of criteria in the BCSH guidelines use bone marrow morphology, there is a discrepancy upon recognizing other haematopoeitic cell lineages apart from megakaryocytes and fibres. The need to differentiate between ‘True’ ET and pre-PMF is very relevant when looking at clinical presentation, bleeding and prognosis, considering that treatment would have a different impact on each of these entities.
Risk of thrombosis in essential thrombocythaemia is currently stratified into 3 categories: low, intermediate or high risk. The prognostic factors used in this risk assessment are: JAK2V617F mutation, leukocytosis, age, previous history of thrombosis and CV risk. At the moment thrombocytosis is not considered a thrombotic risk due to the ‘haemostatic paradox’ (the lack of relationship between platelet counts and the occurrence of haemostatic complications).
Investigate the relationship between the clinical and biological characteristics at diagnosis of MPNs and thrombotic events that occurred before any confounding anti-thrombotc and/or cytoreductive treatment (Prior thrombosis, PrTh).
Final analysis used a population of 977 MPN patients who had been reclassified with the 2008 WHO diagnostic criteria. PrTh occurred in 194 patients with diagnosis taking place in up to 12 months for 114 patients and 13-46 months for 80 patients. Higher PrTh rates were seen to be directly related to white blood cell counts >10 x 109/L and haematocrit levels >45%. However there was an inverse relationship seen in patients with platelet counts ≤ 700 x 109/L that experienced more PrTh compared to patients with platelet counts ≥ 700 x 109/L . In addition patients that were positive for the JAK2 V617F mutation demonstrated a higher trend with PrTh rates compare with patients with the wild type gene (25.4% vs 15.7%, p=0.002).
The data from the study suggests that optimal control for cardiovascular risk factors, thrombocytosis, leukocytosis and erythrocytosis would be of importance for the prevention of future thrombosis and further trials would be needed to evaluate this aspect.