Blood, 1 January 2003, Vol. 101, No. 1, pp. 2-2
Thalassemic splice repair, delivered
The thalassemias constitute one of the most common
monogenic disorders and represent a heterogeneous group of anemias
characterized by an absence or reduction of one or more of the
globin-molecule subunits. The disorders result from a variety of
mutations that affect every step of globin biosynthesis. Vacek and
colleagues (page 104) describe a novel and innovative technique
employing modified U7 small nuclear RNAs (U7 snRNAs) to correct
aberrant splicing responsible for a sizable fraction of human
thalassemia. Whereas considerable effort has been placed on
gene replacement strategies, their method aims to repair, rather than
replace, the abnormality. The group has previously shown that antisense constructs targeting aberrant splice sites in the form of
oligonucleotides, morpholinos, or U7 snRNAs lead to increased levels of
correctly spliced 
-globin mRNA by effectively blocking the aberrant
splice site, yet delivery of these antisense constructs
remained a formidable obstacle to clinical application.
(Sierakowska et al, Proc Nat Acad Sci U S A. 1996;93:12840-12844; Gorman et al, Proc Nat Acad Sci U S A. 1998;95:4929-4934; Lacerra et al, Proc Nat Acad Sci U
S A. 2000;97;9591-9596). In order to circumvent the need for repeated administration, the group has now constructed lentiviral vectors to permanently deliver the modified U7 snRNAs and has tested
the delivery system in both cell lines and primary human cells carrying
a thalassemic intron 2 mutation. Two additional cell lines in which GFP
expression was interrupted by the mutated globin intron were also
employed, an ingenious system allowing monitoring of corrected splicing
by fluorescence detection. The lentiviral vector system resulted in
correction of abnormal splicing that ranged from 3% to nearly 100% in
these cell lines, expressing a variety of thalassemic pre-mRNAs. The
U7.623 lentivirus was then used to transduce blood cells from an
individual with thalassemia carrying the IVS2-623 mutation with a
23-fold increase in corrected mRNA, providing proof of principle in
human cells in vitro. Although correction rates were considerably lower
in the IVS2-654 cell line, the availability of a transgenic mouse
carrying the thalassemic 654 mutation should prove invaluable in
further refining this clever strategy.
John F. Tisdale
National Institutes of
Health
