Relative impact of THPO mutation causing hereditary thrombocythemia.

Germline mutations of THPO were reported as causes of hereditary thrombocythemia. Six previously reported distinct sites of the mutation were clustered at the 5'-untranslated region or the exon 3 splicing donor site of the THPO gene. Each mutation was identified in an independent pedigree and the difference in the mutations were not compared. We cloned 6 distinct THPO mutations (THPO c.-47delG, THPO c.-31G>T, THPO c.13G>A, THPO c.13+1G>A, THPO c.13+2T>C, and THPO c.13+5G>A) and compared the molecular mechanisms that underlie the increased production of THPO protein. At the transcript level, all of the mutations except THPO c.-47delG showed an exon 3 skipping transcript including 2 mutations (THPO c.-31G>T and THPO c.13+5G>A) that were distant from the splicing donor site. THPO c.-47delG showed the same full-length transcript as that of the wild-type transcript. At the protein level, all mutations resulted in a higher level of production of THPO protein compared to wild-type THPO. There are only two distinct patterns of mechanisms for increased production of THPO: 1) exon 3 skipping that deleted upstream suppressive open reading frame (ORF) 7 and 2) one base deletion that shifted ORF7 to connect to the initial codon of THPO in-frame. The common mechanisms of hereditary thrombocytosis due to THPO mutation are unleashed THPO translation that is usually suppressed by upstream out-of-frame ORF7.