Anti-βs-Ribozyme Reduces βs mRNA Levels in Transgenic Mice: Potential Application to the Gene Therapy of Sickle Cell Anemia

Abstract

ABSTRACT: Our current strategy for gene therapy of sickle cell anemia involves retroviral vectors capable of transducing “designer” globin genes that code for novel anti-sickling globins (while resisting digestion by a ribozyme), coupled with the expression of a hammerhead ribozyme that can selectively cleave the human βS mRNA. In this report, we have tested in vivo an anti-βS hammerhead ribozyme embedded within a cDNA coding for the luciferase reporter gene driven by the human β-globin promoter and hyper-sensitive sites 3 and 4 of the locus control region. We have created mice transgenic for this luciferase-ribozyme construct and bred the ribozyme transgene into mice that were already transgenic for the human βS gene. We then measured expression of the βS transgene at the protein and RNA levels by HPLC and primer extension. The presence of the ribozyme was associated with a statistically significant reduction in the level of βS mRNA in spleen stress reticulocytes (from 60.5±4.1% to 52.9 ± 4.2%) and in the percentage of βS globin chains in very young mice (from 44.5 ± 0.6% to 40.8 ± 0.7%). These results demonstrate that it is possible to decrease the concentration of βS chains and mRNA with the help of a hammerhead ribozyme. While the enormous amount of globin mRNA in reticulocytes is a challenge for ribozyme technology, the exquisite dependence of the delay time for formation of Hb S nuclei on the concentration of Hb S in red blood cells suggests that even a modest reduction in Hb S concentration would have therapeutic value.