A de novo splice donor mutation in the thrombopoietin gene causes hereditary thrombocythemia in a Polish family

BACKGROUND

Hereditary thrombocythemia is an autosomal dominant disorder with clinical features resembling sporadic essential thrombocythemia. Germline mutations in families with hereditary thrombocythemia have been identified in the gene for thrombopoietin (TPHO) and its receptor, MPL.

DESIGN AND METHODS

Here we characterized a THPO mutation in a hereditary thrombocythemia pedigree with 11 affected family members.

RESULTS

Affected family members carry a G --> C transversion in the splice donor of intron 3 of THPO that co-segregated with thrombocytosis within the pedigree. We previously described the identical mutation in a Dutch family with hereditary thrombocythemia. Haplotype analysis using single nucleotide polymorphisms surrounding the mutation indicated that the mutations arose independently in the two families. MPL protein levels, but not mRNA levels, were low in platelets from affected family members. Bone marrow histology showed features compatible with those of essential thrombocythemia, but the megakaryocytes were unusually compact, as assessed by planimetric analysis. Impaired microcirculation resulting in brief episodes of fainting and dizziness that responded well to aspirin were the predominant clinical features in a total of 23 affected family members studied. Disease onset is earlier in patients with hereditary thrombocythemia than in those with essential thrombocythemia, but the frequencies of thrombotic, vascular and hemorrhagic events are similar in the two groups.

CONCLUSIONS

A mutation in THPO occurred de novo in the same position as in a previously described family with hereditary thrombocythemia. Patients with this mutation have elevated serum levels of thrombopoietin and a phenotype that responds to aspirin and does not require cytoreductive treatment.

Comparison of molecular markers in a cohort of patients with chronic myeloproliferative disorders

Decreased expression of c-MPL protein in platelets, increased expression of polycythemia rubra vera 1 (PRV-1) and nuclear factor I-B (NFIB) mRNA in granulocytes, and loss of heterozygosity on chromosome 9p (9pLOH) were described as molecular markers for myeloproliferative disorders (MPDs). To assess whether these markers are clustered in subgroups of MPDs or represent independent phenotypic variations, we simultaneously determined their status in a cohort of MPD patients. Growth of erythropoietin-independent colonies (EECs) was measured for comparison. We observed concordance between EECs and PRV-1 in MPD patients across all diagnostic subclasses, but our results indicate that EECs remain the most reliable auxiliary test for polycythemia vera (PV). In contrast, c-MPL, NFIB, and 9pLOH constitute independent variations. Interestingly, decreased c-MPL and elevated PRV-1 also were observed in patients with hereditary thrombocythemia (HT) who carry a mutation in the thrombopoietin (TPO) gene. Thus, altered c-MPL and PRV-1 expression also can arise through a molecular mechanism different from sporadic MPD.