Safe Harbor? Leukemia, Gene Transfer, and Lentiviral Vectors

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A few further observations from the American Society of Gene Therapy Meeting…


A recurrent theme in this blog is the frequency with which novel research fields encounter safety problems that confound laboratory predictions. One presentation at the 2009 ASGT meeting brought this point home.

Recall my entry on May 12 discussing various refinements to retroviral gene transfer that are aimed at reducing risk of malignancy. Researchers have postulated that HIV-derived lentiviral vectors might not cause the same leukemia-inducing mutations as retroviruses, and RAC recently passed a favorable judgment on a lentiviral vector gene transfer protocol for X-SCID.

How confident can we be that lentiviruses will not trigger leukemias? Some indication is provided in a May 2009 review by John Rossi in Molecular Therapy. It concluded “overall, the results of these [safety] analyses [of lentiviral vectors] are highly encouraging…” but “clearly, more careful analyses… are warranted in appropriate animal models.”

At ASGT, researchers from France reported preliminary results from a phase 1 trial testing lentiviral vectors in patients with beta-thalassemia. The study involved two patients. Though no malignancies have been detected, tests in one patient showed signs that some cells were repopulating the patients blood much faster than others (what researchers call “clonal dominance”). This is a worrisome signal, as it might indicate a premalignant state.

The lessons here are not that lentiviral vectors are unsafe (we don’t know whether this will lead to a malignancy), or that such vectors shouldn’t be used in human beings (we can’t say anything yet about the risk-benefit balance). Instead, I think the lesson is: in novel research areas, be very wary of anyone who makes emphatic claims that their system provides safe harbor. Expect the unexpected. (photo credit: dark matter, 2005)

BibTeX

@Manual{stream2009-95,
    title = {Safe Harbor? Leukemia, Gene Transfer, and Lentiviral Vectors},
    journal = {STREAM research},
    author = {Jonathan Kimmelman},
    address = {Montreal, Canada},
    date = 2009,
    month = jun,
    day = 23,
    url = {http://www.translationalethics.com/2009/06/23/safe-harbor-leukemia-gene-transfer-and-lentiviral-vectors/}
}

MLA

Jonathan Kimmelman. "Safe Harbor? Leukemia, Gene Transfer, and Lentiviral Vectors" Web blog post. STREAM research. 23 Jun 2009. Web. 21 Sep 2017. <http://www.translationalethics.com/2009/06/23/safe-harbor-leukemia-gene-transfer-and-lentiviral-vectors/>

APA

Jonathan Kimmelman. (2009, Jun 23). Safe Harbor? Leukemia, Gene Transfer, and Lentiviral Vectors [Web log post]. Retrieved from http://www.translationalethics.com/2009/06/23/safe-harbor-leukemia-gene-transfer-and-lentiviral-vectors/


In Brugge / No Compassion (Part II)

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Further to the therapeutic outlook on first-in-human studies at the Brugge meeting was Adrian Thrasher’s thoughtful presentation on his own X-SCID study at Great Ormand Street Hospital. Thrasher’s study was able to restore immune function in nearly all volunteers. Recently, however, his team reported a lymphoproliferative disorder like those seen in a very similar Paris study.


Thrasher stated clearly “The purpose [of X-SCID protocol] is therapeutic effect; it is not a safety study.”  Fair enough: the study was in a pediatric population (standard research ethics requires clear therapeutic warrant for such risky studies), and Thrasher’s protocol did not range doses the way typical first-in-human studies do. And I should add, there is some grounds for thinking of the study as having therapeutic warrant, not the least because it was supported several unsuccessful X-SCID human studies and a successful one in Paris). Still, putting the therapy before the learning- this made me somewhat uncomfortable.  Therapy might have been his (and his hospital’s) intent, but to describe the study as ontologically “therapeutic” and not “research”?  Intent only gets us so far…

Thrasher revealed some unusual properties about the molecular events leading to this leukemia (see? told you it could be construed as a safety study).  And now, here’s the compassion part. Thrasher was circumspect about this particular leukemia, because the patient who developed the leukemia had originally been ineligible for the protocol because he had a matched unrelated bone marrow donor. The regulatory agency made a “one-time” exception to waive the normal risk-benefit balance.

Of course, one should be very careful generalizing from this one case where “compassion” seems to have led authorities astray. And presumably, the boy’s parents were thoroughly informed about the risks going in to the protocol. Still, the example is somehow instructive. (photo credit: missinguigga 2008).

BibTeX

@Manual{stream2008-120,
    title = {In Brugge / No Compassion (Part II)},
    journal = {STREAM research},
    author = {Jonathan Kimmelman},
    address = {Montreal, Canada},
    date = 2008,
    month = nov,
    day = 29,
    url = {http://www.translationalethics.com/2008/11/29/in-brugge-no-compassion-part-ii/}
}

MLA

Jonathan Kimmelman. "In Brugge / No Compassion (Part II)" Web blog post. STREAM research. 29 Nov 2008. Web. 21 Sep 2017. <http://www.translationalethics.com/2008/11/29/in-brugge-no-compassion-part-ii/>

APA

Jonathan Kimmelman. (2008, Nov 29). In Brugge / No Compassion (Part II) [Web log post]. Retrieved from http://www.translationalethics.com/2008/11/29/in-brugge-no-compassion-part-ii/


Just the FACS: Reprise on Insertional Mutagenesis

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I‘ve just returned from the annual European Society of Gene and Cell Therapy meeting in Belgium.  Lots of great material for upcoming posts. For now, I want to follow on the last posting on the leukemias in the X-SCID study.  A warning: those lacking a stomach for science geek-talk might want to skip this posting.


In the previous posting, I stated that a recent paper provided evidence that retroviral integration in the genome (“insertional mutagensis”) had triggered leukemias in the X-SCID study rather than over-expression of the corrective gene (“transgene”), the gamma c-chain (hereafter, “gc”).  This was on the basis of data in the graphic above, which used cell sorting to show that levels of gc on the surface of T-cells was within a normal range.  In Belgium, Adrian Thrasher presented similar data for the fifth leukemia.

When I first encountered this figure, it bothered me: why did the authors measure gc expression by cell surface markers (a technique called “FACS”) rather than Western or Northern blotting, or quantitative PCR, or something along these lines?  It seemed a very indirect way of seeing whether gc expression levels are in fact normal. Here are two possibilities that this figure fails to rule out:  1- gc is expressed at very high levels, but not packaged and presented on the surface of T-cells, perhaps because of insufficiency of other receptor components; 2- some gc transgene is aberrantly spliced, such that surface levels are normal, but intracellular concentrations of the alternate splicing product are abnormal.

A few years back, one team of researchers presented data indicating that gc transgene overexpression contributes to T-cell transformation. Another team claimed it was unable to reproduce this. The jury seems to still be out on whether the gc product contributed to the X-SCID leukemias, and I’m not yet convinced that the latest round of data fully exonerates the gc chain. (Graphic: figure from Salima Hacein-Bey-Abina et al, J Clinical Investigation 2008; 108: 3132-42).

BibTeX

@Manual{stream2008-123,
    title = {Just the FACS: Reprise on Insertional Mutagenesis},
    journal = {STREAM research},
    author = {Jonathan Kimmelman},
    address = {Montreal, Canada},
    date = 2008,
    month = nov,
    day = 18,
    url = {http://www.translationalethics.com/2008/11/18/just-the-facs-reprise-on-insertional-mutagenesis/}
}

MLA

Jonathan Kimmelman. "Just the FACS: Reprise on Insertional Mutagenesis" Web blog post. STREAM research. 18 Nov 2008. Web. 21 Sep 2017. <http://www.translationalethics.com/2008/11/18/just-the-facs-reprise-on-insertional-mutagenesis/>

APA

Jonathan Kimmelman. (2008, Nov 18). Just the FACS: Reprise on Insertional Mutagenesis [Web log post]. Retrieved from http://www.translationalethics.com/2008/11/18/just-the-facs-reprise-on-insertional-mutagenesis/


Burst Bubbles

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Among the greatest traumas for gene transfer was the development of leukemias in several children participating in trials using retroviral vectors against X-linked Severe Combined Immune Deficiency (X-SCID– also known as “bubble boy syndrome”).  About 20 or so children have had their immune systems fully restored by this gene transfer strategy.  Tragically, however, five children in two X-SCID studies (one in Paris, the other, London) developed T-cell leukemias that were causally linked to the gene transfer approach.


In the September 2008 issue of Journal of Clinical Investigation, Salima Hacein-Bey Abina and 28 other authors characterize the molecular nature of four of the adverse events, and report the outcome.  Before this article, it was known that one of the children died. This article now reports that the other three children have “sustained remission” after chemotherapy. 

The authors report that the vector inserted itself at the same genetic locus (LMO2) in 3 of the 4 cases.  In one case, vector inserted at a different genetic locus (CCND2); in another, vector inserted itself at LMO2 as well as a second locus, BMI1.  This suggests that LMO2 disruption is not the only path to causing cancer for this vector.  One other finding stood out.  Since the first leukemia was detected, many have speculated that the cancer was partly caused by the gene (rather than just the vector). However, the authors present evidence that the gene was expressed at normal levels in the children who developed leukemia. This lends support to the theory that the leukemias were not caused by the gene, but rather by some combination of the vector, the cell types used, and perhaps some characteristic of the underlying disease. (photo credit: concretecandy, boy in the bubbles, 2006)

BibTeX

@Manual{stream2008-124,
    title = {Burst Bubbles},
    journal = {STREAM research},
    author = {Jonathan Kimmelman},
    address = {Montreal, Canada},
    date = 2008,
    month = nov,
    day = 6,
    url = {http://www.translationalethics.com/2008/11/06/burst-bubbles/}
}

MLA

Jonathan Kimmelman. "Burst Bubbles" Web blog post. STREAM research. 06 Nov 2008. Web. 21 Sep 2017. <http://www.translationalethics.com/2008/11/06/burst-bubbles/>

APA

Jonathan Kimmelman. (2008, Nov 06). Burst Bubbles [Web log post]. Retrieved from http://www.translationalethics.com/2008/11/06/burst-bubbles/


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