NUCLEAR DEGRADATION OF NONSENSE MUTATED BETA-GLOBIN MESSENGER-RNA - APOSTTRANSCRIPTIONAL MECHANISM TO PROTECT HETEROZYGOTES FROM SEVERE CLINICAL MANIFESTATIONS OF BETA-THALASSEMIA

Nonsense mutations of the beta-globin gene are a common cause of beta- thalassemia. It is a hallmark of these mutations not only to cause a l ack of protein synthesis but also a reduction of mRNA expression. Both the pathophysiologic significance and the underlying mechanisms for t his surprising phenomenon have so far remained enigmatic. We report th at the reduction of the fully spliced mutant beta-globin mRNA already manifests itself within the nucleus. In contrast, the levels of mutant pre-mRNA are normal. The promoter and the 5'-untranslated region (5'- UTR) of the herpes simplex virus type 1 thymidine kinase (HSV1 Tk) gen e can independently circumvent this recognition/response mechanism in cis and restore nonsense mutated beta-globin mRNA expression to normal levels. These two genetic elements can thus exert a dominant influenc e on the post-transcriptional control of nonsense mutated beta-globin gene expression. While wildtype mRNA levels are restored by fusion of the HSV1 Tk 5'-UTR to the nonsense mutated beta-globin reading frame, translation of a wildtype reading frame in such a hybrid is precluded. In contrast, the HSV1 Tk promoter appears to efficiently deliver the mRNA to the translational apparatus. The 5'-UTR and the promoter seque nces therefore control the nuclear fate of nonsense mutated beta-globi n mRNA by separable pathways. The nuclear mRNA degradation mechanisms examined here may prevent the synthesis of C-terminally truncated beta -globin chain fragments and may protect heterozygotes from clinically relevant symptoms of beta-thalassemia.