Germline MPLW515R Mutation in a Family with Isolated Thrombocytosis

Myeloproliferative neoplasms (MPNs) and related hereditary diseases form a group of chronic hematological diseases that represent a useful model of cancer development. Identification in 2005 of an activating somatic mutation (V617F) in the JAK2 gene in >50% MPN patients was of major importance in the understanding of these diseases. Later, somatic mutations of W515 in exon 10 of MPL were reported for 5–10% MPN patients with essential thrombocythemia (ET) and primary myelofibrosis (PMF), whereas certain germline mutations in the MPL and JAK2 genes (MPLS505N and JAK2V617I respectively) were associated with hereditary thrombocytosis. Here, we report the first cases of germline MPLW515 mutation (R515) in the context of familial, hereditary thrombocytosis.

A father and daughter presented with isolated thrombocytosis. Their JAK2, THPO and MPL genes were analyzed by direct sequencing on genomic DNA extracted from whole blood, CD3+ lymphocytes, buccal swabs and hair follicles. Whole blood DNA was also analyzed on Affymetrix 6.0 SNP arrays. Impact of MPLW515R mutation on thrombopoietin (Tpo) signalling was determined by comparison with wild-type and mutated (W515K/L/A) MPL in the Ba/F3-EpoR and K562 cell lines using transient transfection.

A 36 year old woman presented with chronic isolated thrombocytosis (628–780.10⁹ Plts/L), known for >10 years. There was a familial history of isolated thrombocytosis for the father (74 years old; 646.10⁹ Plts/L) and the father's mother (deceased). The JAK2V617F mutation was not detected in these patients; all other blood tests and bone marrow examination were normal. The two patients presented no evidence of MPN, no splenomegaly, no hepatomegaly, no cause of secondary thrombocytosis and no history of thrombosis. The daughter has two pregnancies without complication. A germline T1588C (W515R) mutation in exon 10 of MPL was found in all tested tissues; no other mutation was found in the JAK2, MPL and THPO genes. Affymetrix 6.0 SNP array analysis revealed no genomic abnormality except for a chromosome Y deletion for the father. Transient transfection in Ba/F3-EpoR and K562 cell lines showed that R515 was activating compared to wild type Mpl, as assessed by the constitutive and Tpo-induced tyrosine phosphorylation of Stat5, Stat3 and Akt. However, R515 was less activating than the K/L/A515 mutants.

We report the first germline MPLW515R mutation, found associated with isolated hereditary thrombocytosis. W515R increases Mpl activity but to a lesser extent than the somatic MPLW515K/L/A mutations observed in the context of MPNs. Thus the presence of a clearly activating JAK2 or MPL mutation is not sufficient for MPN development. Further investigation of this family might reveal why MPLW515R does not lead to MPN.

No relevant conflicts of interest to declare.