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 Jul 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/


Yellow Light on Gene Transfer Studies

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Among the greatest heartbreaks in the field of gene transfer have been problems encountered in trials involving a rare, hereditary immune disorder, X-SCID (known popularly as “Bubble Boy” syndrome).  As is well known, a team of researchers based in Paris– and then in London– successfully reversed severe immunodeficiencies in 20 or so children using retroviral gene transfer starting around year 2000.  Shortly thereafter, however, the Paris team began observing rare leukemic disorders that were causally related to the gene transfer. To date, the Paris team has reported 4 cases of leukemia, with one leading to death. The London team has reported one leukemia.


In response to these events, the U.S. Recombinant DNA Advisory Committee (RAC) recommended that investigators only use retroviral gene transfer in the most severe situations– namely, where patients are ineligible for even high risk alternative care options like haploidentical stem cell transplantation.  RAC’s recommendations were stricter than those in the U.K., which allowed children to enter a study even if they were candidates for haploidentical transplants.  

As reported in the current issue of Molecular Therapy, the RAC recently decided to liberalize its recommendations, allowing retroviral gene transfer in children who are eligible for halploidentical transplantation. RACs recommendations are still somewhat stricter than those of the UK, because the former recommends against retroviral gene transfer in children who are candidates for haploidentical transplantation but under 3.5 years age (children in this category respond better to haploidentical transplants). RAC additionally supported a similar trial involving a different vector that integrates its genome into the host’s (lentiviral vectors, which are derived from HIV).

Is this gentle liberalization of standards justified?  Some will argue that the benefits of haploidentical transplantation are variable and undependable, and that since initial leukemias have been reported, researchers have made progress in improving the safety of their vectors. All this might be true, if one were evaluating this as a clinical judgment.

However, the judgment is better viewed through the lens of research rather than therapy. Though laboratory testing indicates that new retroviral and lentiviral vectors are safer than the old ones, there remain substantial uncertainties. For example, current assays for determining the oncogenicity of integrating vectors are not well worked out. Neither the new retroviral vectors nor lentiviral vectors have been used in blood stem cell gene transfer in a pediatric population. The effect of lentiviral vectors on gene sequences near their integrating sites remains poorly understood. In short, the null hypothesis of new trials is that these new vectors are no better than the old ones.

What’s the safest way to refute this null hypothesis and confirm what many think, on laboratory evidence, will be the case? In my view, the safest approach– for patients as well as the field in general, which stands to lose much from another major toxicity– is to begin with the most narrow medical indication possible, which means excluding children who stand a chance of benefiting from standard (albeit suboptimal) care.  (photo credit: Jamelah 2007)

BibTeX

@Manual{stream2009-103,
    title = {Yellow Light on Gene Transfer Studies},
    journal = {STREAM research},
    author = {Jonathan Kimmelman},
    address = {Montreal, Canada},
    date = 2009,
    month = may,
    day = 12,
    url = {http://www.translationalethics.com/2009/05/12/yellow-light-on-gene-transfer-studies/}
}

MLA

Jonathan Kimmelman. "Yellow Light on Gene Transfer Studies" Web blog post. STREAM research. 12 May 2009. Web. 21 Jul 2017. <http://www.translationalethics.com/2009/05/12/yellow-light-on-gene-transfer-studies/>

APA

Jonathan Kimmelman. (2009, May 12). Yellow Light on Gene Transfer Studies [Web log post]. Retrieved from http://www.translationalethics.com/2009/05/12/yellow-light-on-gene-transfer-studies/


Departing Milano Stazione? ADA-SCID and Gene Transfer

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Greetings after a hiatus for teaching, grants, committees, book deadlines, wiping runny noses, and more. Much has happened since my last posting, and in the next two or three weeks, I hope to catch up.


First item on the agenda is a Jan 29 report in New England Journal of Medicine (NEJM) describing successful reconstitution of immune function in eight of ten children receiving gene transfer for adenosine deaminase severe combined immune deficiency (ADA-SCID). The paper follows on a previous report in Science, 2002, and almost certainly counts as gene transfer’s greatest clinical accomplishment to date.

I have previously argued in Lancet and Developing World Bioethics, as well as in my forthcoming book, that this study raised important justice concerns because it recruited volunteers from economically disadvantaged settings without clearly fulfilling the requirement, articulated in the Declaration of Helinki, of responsiveness. The NEJM article does not say where subsequent volunteers were recruited, though the fact that all but one new volunteer received PEG-ADA (a very expensive standard of care available only in high-income countries) suggests that later patients were not economically disadvantaged.

Rather than dwell on justice, I’d like to focus on the significance of this study. As indicated, eight of ten children with a life threatening immune disorder had their immune systems reconstituted. Five of these children had T-cell counts that were “above the lower limits of normal.” These children were able to enjoy normal social relations parents and other children.

There do not appear to have been any adverse events relating to the gene transfer vector. A major concern was the possibility that gene transfer might trigger a leukemia-like syndrome observed in two X-SCID studies. Blood tests of children in this ADA-SCID study, however, do not evidence of either the leukemia syndrome or its precursors– at least within the time frame of the study (median follow-up of 4 years; range: 1.8-8 years).

So is ADA-SCID gene transfer ready to leave Milan and conquer ADA-SCID?  For children lacking haplo-identical bone marrow donors, maybe so given the morbidity associated with marrow  transplantation. Still, there are lingering concerns. First, though these results are encouraging, risks of malignancy remain unquantified. Second, this gene transfer regime requires several ancillary treatments- like bone marrow conditioning- that expose patients to risk of infection until the gene transfer intervention kicks in. Several volunteers in this study developed infections and neutropenia, for example. In an accompanying editorial in NEJM, Donald Kohn and Fabio Candotti describe several ways that retroviral gene transfer to blood stem cells might be made safer. Last, it is important to remember that ADA-SCID is a multi-system disorder, with neurological, skeletal, and other effects. Though this approach seems to address what is by far the largest cause of morbidity and mortality in children with ADA-SCID, it does eradicate their condition.

The results of Aiuti et al have been widely celebrated in the gene transfer community.  Kohn and Candotti’s editorial, for example, is titled “Gene Therapy Fulfilling its Promise.”  More than any single gene transfer study I can think of, this one seems to have earned the vindicating headlines. (photo credit: Paolo Margari, Milano Sazione Centrale Ferrovi, 2008)

BibTeX

@Manual{stream2009-109,
    title = {Departing Milano Stazione? ADA-SCID and Gene Transfer},
    journal = {STREAM research},
    author = {Jonathan Kimmelman},
    address = {Montreal, Canada},
    date = 2009,
    month = mar,
    day = 10,
    url = {http://www.translationalethics.com/2009/03/10/departing-milano-stazione-ada-scid-and-gene-transfer/}
}

MLA

Jonathan Kimmelman. "Departing Milano Stazione? ADA-SCID and Gene Transfer" Web blog post. STREAM research. 10 Mar 2009. Web. 21 Jul 2017. <http://www.translationalethics.com/2009/03/10/departing-milano-stazione-ada-scid-and-gene-transfer/>

APA

Jonathan Kimmelman. (2009, Mar 10). Departing Milano Stazione? ADA-SCID and Gene Transfer [Web log post]. Retrieved from http://www.translationalethics.com/2009/03/10/departing-milano-stazione-ada-scid-and-gene-transfer/


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 Jul 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 Jul 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 Jul 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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