Gene therapy 'caused T-cell leukemia'
Insertional mutagenesis pinpointed as cause of T-cell leukemia
in X-SCID gene therapy trial |
By Jo Lyford
An unanticipated complication of gene therapy has been confirmed
as the cause of T-cell leukemia in two boys receiving the pioneering
treatment for X-linked severe combined immunodeficiency (X-SCID).
In the October 17 Science,
an international research team confirms that inappropriate insertion
of the retroviral vector near the proto-oncogene LMO2 promoter
led to uncontrolled clonal proliferation of mature T cells (Science,
302:415-419, October 17, 2003).
“This is the nightmare scenario,” said Terry Rabbitts of the
Medical Research Council's
of Molecular Biology in Cambridge, whose previous work on
LMO2 played a key role in the investigation. “It's time
to step back and give ourselves some breathing space.”
The affected children were the youngest of 10 boys with X-SCID
enrolled in the gene therapy trial at Hôpital
Necker Enfants Malades under the direction of Alain Fischer.
Inherited X-SCID, the so-called “baby in the bubble” disease,
is characterized by a lack of both T and B cells. The only certain
cure at present is a bone marrow transplant from a histocompatible
sibling. For the vast majority of children, however, an allogenic
transplant is the best option, which carries a 5-year success
rate of just 65%.
Building on the discovery
that X-SCID was due to a defect in the common γ (γ c) chain of
the interleukin 2 receptor on chromosome Xq13, Fischer and colleagues
at the Necker Hospital developed an alternative
therapy based on the ex vivo transfer of the γc gene into
autologous hematopoietic precursor cells, employing a vector derived
from a defective Moloney murine leukemia virus.
Fourteen children with X-SCID—10 in France and four at Great
Ormond Street Hospital in London—were subsequently treated and
showed an impressive and sustained clinical response. “These children
are coping with lots of infections, have a fully functioning immune
system, and are effectively cured,” said Professor Rabbits.
In October of last year, the first serious side
effect in a trial subject who exhibited a “leukemia-like syndrome”
30 months after therapy was reported. In January, the development
of similar complications in a second participant in the French
trial triggered a moratorium and frantic investigations aimed
at discovering what had gone wrong.
The Science paper, a collaboration among Fischer's team,
the MRC, and experts in the United States and Australia, confirms
that insertional mutagenesis—originally perceived as a possible
but remote complication—caused both children's leukemias. Using
molecular techniques, Hacein-Bey-Abina et al. observed that clones
from the patients showed retrovirus insertion in proximity to
the promoter of LMO2, which encodes a transcription factor
required for hematopoiesis.
“Until this report, retroviral insertion in the context of gene
therapy has been considered an untargeted and largely random event,”
comment David A. Williams and Christopher Baum in an accompanying
article. Indeed, such insertional leukemogenesis without widespread
viral replication was reported in just
one of thousands of preclinical experiments.
What remains unclear is why the viral vector inserted near the
LMO2 promoter locus in just two of 14 children treated
with the same technique. Fischer believes that disease- and protocol-specific
issues may have played a part.
“These patients were the youngest to be treated—just 1 and 3
months—while the others were all at least 6 months old,” Fischer
told The Scientist. “They also received the highest number
of corrected cells. So, statistically speaking, the more you inject,
the higher the probability.”
Professor Hidde Haisma, from the Department
of Therapeutic Gene Modulation at the University of Gronigen,
agreed that the patients' age may be significant. “Very young
infants have a different subset of stem cells, with a higher proportion
of precursor T cells, so they may be at higher risk of insertional
mutagenesis,” he said. The Necker team is already testing the
“age” hypothesis in mice.
Another worrying aspect of the study is the apparent preference
of the retrovirus vector to insert in or near active genes. Professor
Christine Kinnon, head of the Centre
for Gene Therapy of Childhood Disease at Great
Ormond Street Hospital for Children, commented that gene therapy
in X-SCID children involves introducing a strong selective advantage,
which would inevitably carry a theoretical risk of leukemia.
“The insertional activation of the LMO2 locus in two independent
leukemias suggests an essential event that is required for the
initiation of a malignant cascade,” add Williams and Baum. “Given
the lack of similar side effects in previous studies, a combinatorial
process seems to be the likely culprit.”
In addition to identifying patient-related risk factors, attempts
are also underway to develop a vector with a reduced propensity
for insertional genotoxicity. “Retroviruses were the logical choice,
as they had a good track record and can relatively easily infect
bone marrow cells,” said Haisma. “But we could use lentiviruses
or adenoassociated viruses, or even artificial systems in which
DNA is combined with proteins to do the same job.”
Fischer said his team is already making progress toward the next
generation of highly selective vectors. “I expect that by next
year, we will be in a position where we have a safer therapeutic
approach and can start treating patients once again,” he said.
Kinnon agrees that vector modifications hold the key to success.
In the meantime, the Great Ormond Street trial is continuing to
recruit X-SCID children for gene therapy, albeit on a case-by-case
basis and only when authorized by the Gene
Therapy Advisory Committee.
“I still believe gene therapy has a relatively good risk–benefit
ratio,” Kinnon said, noting that all 14 children treated to date
are alive and well, and the two boys who developed leukemia responded
well to chemotherapy and are in complete clinical remission. “With
a mismatched bone marrow transplant, at least one third would
have died by now—that's four or five children versus none—so it's
still an huge improvement on current options. If it were my child,
I would give them this treatment.”
Links for this article
S. Hacein-Bey-Abina et al., “LMO2-associated clonal
T-cell proliferation in two patients after gene therapy for SCID-X1,”
302:415-419, October 17, 2003.
Medical Research Council (MRC)
MRC Laboratory of Molecular Biology
Hôpital Necker Enfants Malades
M. Noguchi et al., “Interleukin-2 receptor g chain
mutation results in X-linked severe combined immunodeficiency
in humans,” Cell
, 73:147-157, April 9, 1993.
M. Cavazzana-Calvo et al., “Gene therapy of human
severe combined immunodeficiency (SCID)-X1 disease,” Science
288:669-672, April 28, 2000.
B.A. Maher, “Gene therapy trials hit obstacle,”
, 16:26, October 28, 2002.
D.A. Williams, C. Baum, “Gene therapy: New challenges
302:400-401, October 17, 2003.
Z. Li et al., “Murine leukemia induced by retroviral
gene marking,” Science,
296:497, April 19, 2002.
Department of Therapeutic Gene Modulation
Centre for Gene Therapy of Childhood Disease
Great Ormond Street Hospital for Children
UK Gene Therapy Advisory Committee