Gene therapy and protein therapy of ectodermal dysplasias: A perspective†
How to cite this article: Gaide O. 2009. Gene therapy and protein therapy of ectodermal dysplasias: A Perspective. Am J Med Genet Part A 149A:2042–2044.
Ectodermal dysplasias (EDs) form a complex and heterogeneous group of diseases currently defined and classified according to their clinical symptoms. The characterization, for several EDs, of the molecular events underlying their development, not only challenges this classification but also opens the door to new therapeutic options such as gene or protein therapy. This article provides a concise overview of the most recent successes and failures of this new type of treatment and sets in perspective how the specificities of given EDs will influence their feasibility in the near future. It makes the case for the need of new classification of EDs that is based on our most recent knowledge of the molecular basis of these diseases. © 2009 Wiley-Liss, Inc.
Ectodermal dysplasias form a complex and heterogeneous group of diseases, for which a new classification is clearly needed. The NFED therefore initiated a conference aimed at laying the foundation of this new classification, inviting specialists from different background to give their point of view. Our first task was to find a consensual definition for ED, and this was not trivial. Indeed, each guest had a precise and slightly diverging idea on diseases that had to be included or excluded from EDs. One of the definitions proposed during the conference was the inheritable diseases of the epidermal appendages (IDEA). Although even this brilliant idea could not satisfy all, it highlighted the fact that most ED are due to mutations in a specific set of genes (whether inherited or due to a spontaneous mutation). In fact, recent advances in the genetic characterization of EDs have profoundly modified our understanding of their nature and justify an extensive revision of the ED classification [Itin and Fistarol, 2004; Irvine, 2006]. The overall clinical value of these findings is unfortunately undermined by the fact that gene identification remains elusive for most EDs, and by the limited correlation between genotype and phenotype. However, this newly generated knowledge may prove to have an important impact on the treatment modalities of EDs, which I will try to set here into perspective. However, EDs are a heterogeneous group of disease, and these concepts may not apply to all specific EDs.
STATE OF THE ART IN GENE THERAPY
Gene therapy is a very recent addition to our therapeutic arsenal. As with the advent of many new techniques, transplantation being a fine example, it should not be judged by it initial failures. Indeed, while these approaches resulted in failures more often that not, recent progress in molecular engineering bring them closer and closer to the bedside. Hence, a classification that does not take into account the molecular mechanisms underlying the development of EDs is unlikely to last very long. There are several ways to achieve gene transfer in cells or small rodents. However, in a clinically relevant setting, only virally based-gene transfer techniques have been used with striking success. These modified viruses are engineered to minimize the risk of infection in patients, but several safety concerns remain. In particular, the site of insertion of the transgene cannot be controlled, and this results in a potential modification of the expression of a nearby gene. Moreover, the limited size of the transferred DNA precludes the use of the entire gene promoter, and a fine regulation of the transgene expression level can therefore not be attained. Consequently, most gene therapy trials have been performed in high-risk clinical situations, inducing a bias in gene therapy trials targeting terminal cancers or heart disease. As a result, there are surprisingly few clinical trials aiming to cure genetic diseases [GeMCRIS, 2009]. Cystic fibrosis is the most prominent, having been the subject of at least 30 different clinical trials, none being active at the moment [GeMCRIS, 2009]. Cystic fibrosis is not an ED, although it shares some of the symptoms of hypohidrotic ectodermal dysplasia (HED). It is due to mutation in the cystic fibrosis transmembrane channel (CFTR), which codes for an ion channel essential for the correct water efflux in the mucus of the pulmonary and digestive tracts [Davies et al., 2007]. The last clinical trial was discontinued after the tragic death of a young patient, who suffered from a massive immune-reaction against the adenovirus used to deliver the CFTR gene to its lungs [Rosenecker et al., 2006]. This type of adenovirus is highly immunogenic, a characteristic that was thought to provide safety against the potential recombination of the virus that could have resulted in an infection. Another example is X-linked severe combined immunodeficiency type 1. In this disease, a subunit of the IL-2, -4, -7, and -15 receptors, the gamma chain, is deficient, leading to a severe combined immuno-deficiency [Fischer et al., 2005]. Children affected by the disease must be protected form human pathogens (hence the so-called bubble children) until they can benefit from a bone marrow graft [Fischer et al., 2005]. In children for whom no compatible donors could be found, bone marrow cells were extracted and transduced with the wild-type gene carried on a retroviral vector [Cavazzana-Calvo et al., 2000]. The viral transgene was found to restore the function of the white blood cells, a life-saving success [Cavazzana-Calvo and Fischer, 2004]. However, it preferentially inserted itself at an inopportune site and induced the transformation of these cells into cancerous cells [Deichmann et al., 2007]. Hence, both trials were stopped because of complications induced by the viral vectors, not because they were ineffective. Hence, at the moment, gene therapy technology is not ripe. However, the field is in constant evolution and once targeted transgene expression can be achieved, it will yield incomparable results in ED and other genetic diseases.
ALTERNATIVES TO GENE THERAPY
An interesting alternative to gene therapy is to correct the loss of function directly at the protein level. The pharmacological properties of these products can be determined with precision, allowing a well-controlled exposure of the patient to the drug. Hence, this is a simple(r) and safe(r) technique [Russell and Clarke, 1999]. Moreover, most clinicians have experience with it, as they routinely use recombinant proteins such as insulin, growth hormone, cytokines and, more recently, complex proteins such as enzymes and recombinant antibodies. The IL-1 receptor antagonist, for example, is now the golden standard for the treatment of genetic diseases due to mutations in the gene coding for the protein cryopirin [Ting et al., 2006]. However, for several EDs, an important limitation to this approach is the difficulty to target recombinant proteins to specific cellular compartments. Indeed, this is an easy deed for secreted proteins such as cytokines and antibodies, but a much harder task for intracellular proteins such as transcription factors, structural proteins or enzymes [Russell and Clarke, 1999]. HED is a good example to set these differences into perspective. In its most frequent form, the X-linked form, a secreted protein fails to activate its cognate receptor on distant cells [Zonana, 2008]. Hence, the target compartment is the extra-cellular space, which is easy enough to reach. However, a similar phenotype is found when either the receptor or its intracellular signaling partner is mutated, and targeting a recombinant (and functional) protein to these compartments remains technically very difficult, even in vitro [Zonana, 2008]. This illustrates the fact that molecular/genetic information is essential to discriminate between diseases with the same clinical outcome, as this influences not only genetic counseling but also the nature of the molecular treatment that must be considered.
It is important to note here that such limitations may be alleviated in the future. Gaucher disease type I is a genetic disease due to the absence of a glucocerebrosidase, resulting in the accumulation of glyco-lipids degradation products in macrophages [Russell and Clarke, 1999]. Injection of the recombinant enzyme in the blood would lead to no beneficial effect, as the enzyme would fail to reach its normal compartment, the lysosome. However, a cerebrosidase engineered to carry a modified glycosilation, acting as a signal for protein-degradation, is actively up-taken and transported to the lysosome [Niederau et al., 1998]. Hence, creative engineering may bring simple answers to apparently insoluble technical problems, thus yielding significant clinical successes.
SPECIFICITIES OF ED REGARDING MOLECULAR THERAPY
And what about ED? Are they good candidate for gene or protein therapy? They certainly are when the culprit gene and its target protein have been identified, a growing minority [Itin and Fistarol, 2004; Irvine, 2006]. However, there are some technical, ethical, regulatory and financial issues, depending on the ED, which are discussed below. In most EDs, the main issue with gene therapy is the necessity to act before birth. The developing embryo is both complex and delicate, and pregnancy is rightfully considered as a sensitive time when the latin adage “primum non nocere” (first do no harm) must be scrupulously followed. Consequently, no pre-birth gene therapy trials have been initiated yet, and few are likely to be authorized in the near future. Moreover, the molecular biology of some EDs can be extremely complex, as we have learned from p63 deficient mice, and would require a level of control of the expression of the target gene that is not yet mastered [Koster and Roop, 2004]. However, some EDs, such as plakophylin deficiency (McGrath syndrome), may instead require gene transfer mainly post-natally. In such cases, it may be possible in the future, to extract skin stem cells and transplant them after transduction, as proposed for epydermolysis bullosa junctionalis [Mavilio et al., 2006]. Moreover, a new concept emerges that proposes that cells derived from the bone marrow, which are easy to transplant and transduce, may take up the secreting role of keratinocytes, potentially eliminating the need to transplant wide skin areas [Lucky et al., 2008].
And what about protein therapy? If the protein must be given for years and reach the entire skin (as is likely to be the case for EDs), it may be difficult to produce the protein at a cost that makes it economically viable. If the protein acts during development, the safety profile for both the mother and the embryo or fetus will have to be excellent and well documented. This seems easier to achieve, compared to gene therapy, as the pharmacology of proteins is better understood and therefore easier to master. Moreover, some signaling pathway involved in ED may require stimulation for a very little amount of time, sometimes even after birth, to prime the cell into the right differentiation path. This could well be the case for XL-HED, which is due to the absence of the protein ectodysplasin A (EDA), a secreted protein that mainly exerts its effect during development [Gaide and Schneider, 2003]. Indeed, we could recently engineer a new form of EDA that is fused to the constant portion of an immunoglobulin to allow placental crossing and demonstrate that a single injection in pregnant mice was enough to cure its offspring [Gaide and Schneider, 2003]. Moreover, we could show, in mice and dogs, that it can alleviate the disease significantly, even when injected after birth [Gaide and Schneider, 2003; Casal et al., 2007].
However, a proof of concept in two animals is not enough to warrant its use in humans. A series of safety steps must first be taken, requiring a significant investment in both time and money. This task is best undertaken by Biotech Companies, which must however look for a return on their investment. Unfortunately, EDs are relatively rare. HED, which is the most frequent form of ED, is estimated to affect 17 births out of 100,000, a global market of about 500 birth per year in the industrialized country. As not all cases may be identified sufficiently early to warrant the efficacy of the treatment, this represent a very small market, as companies go. However, in the case of rare diseases, drug-licensing agencies such as the FDA may accord an orphan drug designation. This special status significantly decreases the investment necessary for a clinical trial, thus lessening this disadvantage. However, my personal experience is that companies and investors are reluctant to engage in clinical trials performed on embryos/fetuses/infants, maybe for fear of the potential negative image that a failure or unexpected side effect may create. In this respect, long term treatments based on the repeated administration of a drug, starting during infancy, may have a better chance of success. Taken together, the most difficult EDs to cure are likely to be those that may require both pre-natal and post-natal long-term treatment, such as connexin deficiencies (keratosis ichthiosis deafness syndrome).
IMPACT OF MOLECULAR THERAPY ON THE FUTURE CLASSIFICATION OF ED
So how does this sum up concerning the establishment of a new classification for ED? I think that the new ED classification will best serves its users by providing both as an intellectual and practical guide. Since molecular therapy is bound to become a reality for some EDs, a classification that ignores the molecular nature of ED is unlikely to last very long. Hence, the new classification should be based on our current understanding of the molecular events underlying these diseases, as, ultimately, one wants to know if a specific disease is a candidate for a cure, a clinical trial, or is still an orphan disease. Moreover, such a classification would also provide the type of information that is essential for optimal genetic counseling. Will it suit everyone's need? Probably not. But I am persuaded that parents of children suffering from an ED will rapidly become, thanks to the internet, semi-experts on the latest progresses in molecular engineering pertaining to the treatment of a genetic disease. Let the classification be a guide for the medical community, regarding the nature and the treatment of an ED, so that each parent/practitioner may answer this simple question: can my child benefit from a gene/protein therapy?