Blood, 15 April 2003, Vol. 101, No. 8, pp. 2902-2902
Toward gene therapy for 
-thalassemia: new models, new
approaches
Gene therapy for hemoglobinopathies has suffered from problems
of vector instability, low viral titers, and variable expression for
over a decade. In a pioneering study, Sadelain's group had shown that
a lentiviral vector was able to stably transmit the human 
-globin
gene and its regulatory elements, resulting in therapeutic correction
of -thalassemia in Hbbth3/+ mice (May et al, Nature.
2000;406:82-86). But the Hbbth3/+ mice carry deletion of
the major and minor genes only on one
allele (Ciavatta et al, Proc Natl Acad Sci U S A. 1995;92:9259-9263;
Yang et al, Proc Natl Acad Sci U S A. 1995;92:11608-11612) and
resemble the human thalassemia intermedia phenotype. The degree of
correction accomplished by the TNS-9 vector (about 3 g/dL increase in
hemoglobin level per proviral copy) would be subtherapeutic in humans
with Cooley anemia (-thalassemia major). Homozygous deletion of both
the minor and major globins is embryonic
lethal in mice because, unlike in humans, the switch to adult globin
production occurs in utero.
In this issue, Rivella and colleagues (page 2932) have developed a
model of mouse thalassemia major by transplanting fetal liver stem
cells from thalassemia homozygous fetuses into lethally irradiated
healthy adults. They report a recapitulation of thalassemia major
phenotype observed in humans, starting as early as 6 weeks following
transplantation. Genetic correction of the thalassemia major bone
marrow with the TNS-9 vector followed by a transplantation rescues the
otherwise lethal anemia. But increases in hemoglobin level are no
higher than those previously reported by this group using the same
vector (May et al), predictably converting the thalassemia major
phenotype to that of severe thalassemia intermedia. Nevertheless, this
model of murine -thalassemia major is representative of human
disease and, eventually, may become the ultimate model to test
therapeutic strategies.
Using a different approach, Persons and colleagues (Blood.
2003;101:2175-2183) have used the human 
-globin gene in a lentiviral vector, driven by a minimal -globin promoter and the -globin locus control region (LCR) elements to correct murine thalassemia intermedia. They show therapeutic increases in fetal hemoglobin production with 2 or more copies of provirus per cell in
Hbbth3/+ mice. The levels of expression are less robust
than were levels of -globin or a mutant -globin
expression previously observed by the Sadelain (May et al) and Leboulch
(Pawliuk et al, Science. 2001;294:2368-2371) laboratories,
respectively, probably due to a smaller -globin gene promoter or LCR
fragments. Nevertheless, their vector produces the highest levels of
fetal hemoglobin protein reported in primary cells from an integrating
viral vector. Additionally, -globin vectors have the advantage of a
therapeutic potential both in thalassemia and sickle cell disease.
Persons et al have taken gene therapy for -thalassemia a step
further: they have used self-inactivating lentiviral vectors, where the
viral long-terminal repeat is deleted upon integration into cells,
inactivating viral transcription and improving their biosafety.
While both these studies represent important strides toward gene
therapy for thalassemia, they also highlight the obstacles yet to be
conquered. Both studies show presence of chromatin position effects and
underscore the need for better vectors, which would yield higher and
predictable increases in hemoglobin to be therapeutic in human
-thalassemia major.
Punam Malik
Children's Hospital Los
Angeles
