Effect ofTHPOgene mutations on the composition of uORFs inTHPOmRNA. The main THPO mRNA transcribed from promoter 2 is shown. Boxes numbered in italic represent exons. The uORFs are drawn as thick red lines and are placed into one of the three reading frames (+1, 0, and −1). The THPO coding region is shown as a thick blue arrow. Numbers indicate the order in which the uAUGs appear in the full length THPO mRNA (uAUGs 1–4 are not shown). The eighth AUG is the physiological initiation codon. (A) Translation of normal THPO mRNA is physiologically almost completely inhibited by the presence of uORFs in the 5′-UTR. In particular, the uORF 7 is a potent inhibitor of translation, most likely because of its extension beyond the physiological start site. (B) A splice donor mutation in the Dutch HT family causes exon 3 skipping (DE3) that deletes uORF7 and shifts the THPO coding sequence into reading frame +1. Tpo translation now initiates from the fifth and sixth AUG. (C) The Japanese mutation I consists of a single G nucleotide deletion (DG) that shifts the THPO coding sequence into reading frame -1. Tpo translation now initiates from the seventh AUG. Note that both the Dutch and the Japanese mutation I create altered THPO signal peptides, but do not alter the sequence of the mature Tpo protein. Both signal peptides remain functionally active and promote secretion of a biologically active Tpo protein. (D) The Japanese mutation II creates a premature stop codon in uORF7. This allows re-initiation of translation at the physiological start site (the eighth AUG).

Figure reprinted from