THE 5'-PART OF THE HUMAN H19 RNA CONTAINS CIS-ACTING ELEMENTS HAMPERING ITS TRANSLATABILITY

H19 is an imprinted gene developmentally regulated in man and mouse an d implicated in various neoplasms. No corresponding protein product ha s yet been detected, although several open reading frames (ORFs) could be identified along its RNA. The largest ORF found in the human gene could encode a putative 26 kDa protein. We have isolated two H19 cDNAs (AP and ES) that contain this ORF4 and correspond to incomplete copie s of the unique 2.3 kb H19 RNA. In transient expression assays, AP was able to synthesize a 26 kDa protein whereas ES was not. With respect to ORF4, ES exhibits a 536 bp long GC-rich 5' untranslated region, whe reas AP contains the last 22 nucleotides of this 5'UTR. Using deletion s and point mutations, we have found that the length and probably the secondary structure of the 5'UTR strongly hampers the translatability of the RNA. In addition, a potential role of upstream ORFs (uORFs) was detected as stressed by the enhanced translation of a construct mutat ed in uORF3 overlapping ORF4. Interactions between H19 and proteins ar e indicated by a specific binding between 5'UTR derived RNA segments a nd two nuclear proteins of about 27 kDa. Our results favor a potential role of these particular structures and binding properties in general trans-regulation of RNA post-transcriptional processes rather than in normal control of H19 mRNA translation.